# Experiments in Fluids

Online ISSN: 1432-1114
Print ISSN: 0723-4864
Recent publications
The evolution of the interface between explosively driven product gases and the surrounding ambient shocked air behind a blast wave is a complicated process dominated by rapid momentum-driven expansion. A shotgun primer is used to generate a repeatable explosively-driven gas cloud in varying confinement. High speed imaging captures the evolution of the product gas interface, and an automated image processing routine extracts and measures the mixing region width. A comparison is made to a gas cloud radius based predictive model for the width of the mixing region, and a new scaling factor k is used to scale the equations for a non-zero start time. The variation in fitting parameters for the model is determined as the experimental conditions vary from the assumptions of the model. Data show that k is not a function of time during the explosively-driven expansion. The introduction of an initial time ta\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$t_a$$\end{document} improves the fit of c to literature when using the parameter k. Graphical abstract

Tracer particles designed specifically for X-ray particle tracking and imaging velocimetry (XPTV and XPIV) are necessary to widen the range of flows that can be studied with these techniques. In this study, we demonstrate in-lab XPTV using new, custom-designed O(50 μm) diameter tungsten-coated hollow carbon spheres and a single energy threshold photon counting detector. To the best knowledge of the authors, these are the first O(50 μm) tracer particles to be developed specifically for X-ray particle velocimetry. To explore the measurement quality enhancement enabled by the new tracer particles and photon counting detector, a well understood Poiseuille pipe flow is measured. The data show agreement with the analytical solution for the depth-averaged velocity profile. The experiment also shows that the tungsten-coated particles achieve higher contrast and are better localized than previously available silver-coated particles, making faster and more precise measurements attainable. The particles are manufactured with a readily scalable chemical vapor deposition process. We further show that laboratory XPTV is practical with currently available energy-resolving photon counting detectors (PCDs), despite their presently lower spatiotemporal resolution compared to scintillating detectors. This finding suggests that energy-thresholding identification of different classes of tracers is feasible, further motivating the exploration of the X-ray tracer particle design space. The latest generation of PCDs is incorporating multiple energy thresholds, and has higher count rate limits. In the near future one could potentially expand on the work presented and track multiple tracer species and scalar fields simultaneously.

The interaction between a flow and a flexible structure can provide fascinating insight into the vortex shedding phenomenon and any propagation and mixing characteristics, which relate to a plethora of applications such as heat transfer, snoring, musical instruments, or propulsion mechanisms. In this investigation, the influence of confinement on the flapping behaviour of a flexible flag is explored. In particular, hysteresis, one of the least understood aspects of flapping flags, and its sensitivity on both the flexural rigidity of the flag and the confinement ratio is addressed. For the same test-section dimensions and flag material, variations in the flag thickness and flag length enable a range of mass ratios (M∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$M^*$$\end{document}) and confinement ratios (C∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$C^*$$\end{document}) to be studied. Pressure measurements and high-speed imaging allow quantification of the hysteretic behaviour. The results demonstrate that although confinement ratio does not contribute to the existence of hysteresis, the relation between the critical reduced velocities and the mass ratio is sensitive to the degree of confinement. Graphical abstract

This study aims to overcome the problems that existing background-oriented schlieren (BOS) techniques based on computed tomography (CT-BOS) face when measuring pressure fields of laser-induced underwater shock waves. To do this, it proposes a novel BOS technique based on vector tomography (VT-BOS) of an axisymmetric target. The remarkable feature of the proposed technique is the reconstruction of an axisymmetric vector field with nonzero divergence, such as the field of a laser-induced underwater shock wave. This approach is based on an approximate relation between the projection of the axisymmetric vector field and the reconstructed vector field. For comparison, the pressure fields of underwater shock waves are measured with VT-BOS, CT-BOS, and a needle hydrophone. It is found that VT-BOS is significantly better than CT-BOS in terms of better convergence, less dependence on the spatial resolution of the acquired images, and lower computational cost. The proposed technique can be applied not only to fluid dynamical fields, but also to other axisymmetric targets in other areas, such as electromagnetics and thermodynamics. Graphical abstract

Time-series data, such as unsteady pressure-sensitive paint (PSP) measurement data, may contain a significant amount of random noise. Thus, in this study, we investigated a noise-reduction method that combines multivariate singular spectrum analysis (MSSA) with low-dimensional data representation. MSSA is a state-space reconstruction technique that utilizes time-delay embedding, and the low-dimensional representation is achieved by projecting data onto the singular value decomposition (SVD) basis. The noise-reduction performance of the proposed method for unsteady PSP data, i.e., the projected MSSA, is compared with that of the truncated SVD method, one of the most employed noise-reduction methods. The result shows that the projected MSSA exhibits better performance in reducing random noise than the truncated SVD method. Additionally, in contrast to that of the truncated SVD method, the performance of the projected MSSA is less sensitive to the truncation rank. The projected MSSA achieves denoising effectively by extracting smooth trajectories in a state space from noisy input data. Expectedly, the projected MSSA will be effective for reducing random noise in not only PSP measurement data, but also various high-dimensional time-series data. Graphic abstract

The post-stall flow control using compliant flags of varying thickness and length, placed upstream a NACA0012 airfoil, was shown to be possible at an airfoil chord Reynolds number of 100,000. The flag wakes produced substantial increase in the stall angle and the maximum lift coefficient of the airfoil placed at optimal cross-stream locations from the wake centerline. Oscillating flags could generate periodic wakes with better spanwise coherence than the stationary bluff body. This resulted in the excitation, formation and shedding of the leading-edge vortices periodically, providing mean lift enhancement. There is an optimal range of the flag mass ratio for which the flag frequency coincides with the natural frequency of the vortex shedding instability or its subharmonic of the baseline airfoil wake. The flag dimensionless frequency is a function of the mass ratio only, which can be predicted by a reduced order model in the limit of very large mass ratio and by using the modified free-streamline theory for the separated flow. There is also an optimal range of the flag dimensionless frequency. Graphical abstract

A three-point Focused Laser Differential Interferometer (FLDI) instrument was implemented to investigate the freestream disturbance environment in a Mach-6 shock tunnel. The FLDI beams were split such that one pair was aligned with the flow direction and the other along the Mach angle, allowing for simultaneous measurements of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Transfer functions for interpreting the FLDI signal were applied to the free-jet nozzle flow: the instrument’s depth of focus and sensitivity to disturbance wavenumber were estimated, as well as the influence of the nozzle shear layers in corrupting the FLDI signal. Experiments were performed at varying unit Reynolds numbers (3.2$$-$$14.4$$\times$$10$$^6\,\mathrm m^{-1}$$) and total enthalpies (0.57$$-$$0.89 MJ/kg). Streamwise convection velocities measured with the FLDI were found to be lower than theoretical freestream velocities, consistent with earlier studies detailing the noise convection velocity, though some frequency dependence was observed. Both broadband and core-flow density-based turbulence intensities exhibited a decrease with increasing Reynolds number. The relative contributions of the acoustic and entropic disturbances to the overall noise environment was also examined through a study of the correlated signal pairs. The acoustic contribution was found to be dominant in all cases, with no clear trend with Reynolds number. Finally, the disturbance intermittency at the various tunnel conditions was investigated; the intermittency was greatest at the lowest Reynolds number condition, suggesting that the nozzle boundary layer may be transitional at this condition.

We present a novel laboratory setup for studying the fluid dynamics in liquid metal batteries (LMBs). LMBs are a promising technology suited for grid-scale energy storage, but flows remain a confounding factor in determining their viability. Two important drivers of flow are thermal gradients, caused by internal heating during operation, and electrovortex flow (EVF), induced by diverging current densities. Our setup explores, for the first time, electrovortex flow combined with both adverse and stabilizing thermal gradients in a cylindrical layer of liquid gallium, simulating the behavior in a single layer of an LMB. In this work, we discuss the design principles underlying our choices of materials, thermal control, and current control. We also detail our diagnostic tools—thermocouple measurements for temperature and Ultrasonic Doppler Velocimetry probes for velocities—and the design principles which go into choosing their placement on the setup. We also include a discussion of our post-processing tools for quantifying and visualizing the flow. Finally, we validate convection and EVF in our setup: we show that scaling relationships between the nondimensional parameters produced by our data agree well with theory and previous studies. Graphical abstract

An experimental study is performed on high-acceleration separation of Newtonian liquid films trapped between two parallel plates. The test apparatus is capable of accelerating one plate at up to 325 m/s² relative to a stationary substrate. Plates with average roughness up to 86 µm were studied. High-speed laser induced fluorescence (LIF) is used to determine the instantaneous film thickness distribution during the separation process. Two flow regimes exist in the film. As the gap between the substrates increases, a viscous fingering regime occurs along the perimeter of the wetted area, where air fingers grow radially inward. If the growth rate of those fingers is slow relative to the growth rate of the gap, a second regime exists in the center of the wetted region, in which cavitation bubbles emerge and grow. Once the two substrates are sufficiently separated, the liquid bridges connecting the two substrates break. At high accelerations, the separation ratio (the relative amount of fluid remaining on the moving substrate) is a nonlinear function of the surface roughness, liquid viscosity, and separation acceleration, and varies in the range between 35 and 55%. The separation ratio is strongly correlated with the relative areas of the fluid that are subject to viscous fingering and liquid cavitation. The LIF measurements show that the thickness of the residual layer remaining on each portion of the substrate depends on whether the substrate has been subject to viscous fingering or cavitation, and this observation explains qualitatively the measured differences in separation ratio. Graphical abstract

We discuss a method to quantify the supersonic discharge of airbag cold gas inflators. Since one primary quantity of interest, the mass flow with time, is not directly measurable, a combined experimental and numerical approach was chosen. Shadowgraph and schlieren images visualize the gas dynamic process. Pressure measurements were conducted inside the inflator and downstream of the outlets in the supersonic jet. In this context, a method to measure the pressure of the flow without effects of shock reflection is presented. The temperature inside the inflator was estimated using a fast response heat flux probe and assuming different scenarios for the unknown heat transfer coefficient. Then, a numerical model of the inflator was created. The experimental results served as boundary conditions and some basic sensitivities remaining unknown from the measurements were studied to verify the numerical outcome. The numerical model was verified using experimental results. Finally, the mass flow rate was derived from the numerical model and compared to an analytical model. The method can reconstruct the temporal evolution of the mass flow discharging from the inflator, the pressure and the topology of the flow field within acceptable bounds. Furthermore, the method can deliver inflow data for subsequent airbag inflation studies. Graphical abstract

Particle image velocimetry (PIV) is more and more used as a reference method for the measurement of velocity fields. However, this technique requires optical access and the current distortion correction methods are efficient only for small optical distortions. The motivation of the present study is to properly determine the velocity field of the turbulent flow in a channel with optically complex-shaped obstacles, designed for heat transfer enhancement. A ray tracing-based image correction method is employed to eliminate high-level image distortions on PIV images induced by heart-shaped dimples. To reduce the uncertainties in the application of the method, an optimization algorithm is built for artificially recreating the PIV calibration image using rendering software. The positions, material properties and dimensions of the objects in the experimental setup, which construct the 3D model, are considered as the design parameters. The artificial image was obtained with a standard deviation of 0.13 pixels from the actual calibration image in 4–5 h. In the calibration process, the ray tracing-based correction with the optimized artificial image provided a standard deviation of 0.32 pixels from the reference grid while the third-order polynomials had provided 9.6 pixels. To illustrate the approach, measurements were acquired on the center plane of a circular channel with the heart-shaped dimples in the streamwise direction. The 2D velocity and turbulent kinetic energy field obtained at a Reynolds number of around 20,000 showed that the flow separates as it reached the leading edge of this dimple whereas the reattachment point was captured at the trailing edge. The highest amounts of turbulent kinetic energy were found just downstream of the dimple where the best heat transfer was expected. Graphical abstract

Combined measurement of shape and aerodynamic force is effective for understanding the mechanism of transonic wing flutter. To explore one method, a random-dot pressure-sensitive paint (PSP) was applied to the time-resolved global measurement of the deformation and pressure field of a transonic flow over an airfoil. The random-dot PSP has a patterned texture with luminescence intensity, which is made by intentional photodegradation of a PSP dye. These dots allow simultaneous measurement of pressure and the surface profile of the airfoil using stereo-digital image correlation. The feasibility of the random-dot PSP was tested in a transonic wind tunnel using a three-dimensional backswept wing model, which started to flutter at Mach 0.89 with a dominant frequency of 100 Hz. The unsteady surface profile and pressure distribution of the airfoil were measured using 12-bit high-speed cameras as the deformation amplitude increased. The experimental results indicate that the deformation and surface pressure distribution during flutter were successfully measured over time. The accuracy of the measured deformations and pressures was evaluated by comparison with several point data obtained with a laser displacement meter and semiconductor pressure transducers. The accuracy at each time-resolved image obtained at a recording rate of 6.25 kHz was within 1 mm for displacement and about 1 kPa for pressure. Graphical abstract

This paper describes the measurement methodology for quantifying the instantaneous full 3D scalar dissipation rate (SDR or χ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\chi$$\end{document}) in order to characterize the rate of mixing. Measurements are performed in a near field of a jet-in-swirling-coflow configuration. All three components of χ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\chi$$\end{document} are measured using a dual-plane acetone planar laser-induced fluorescence technique. To minimize noise, a Wiener filtering approach is used. The out-of-plane SDR component (χ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\chi _3$$\end{document}) is validated by assuming isotropy between axial and azimuthal components of SDR. An optimum laser-sheet separation distance (Δs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varDelta s$$\end{document}) is identified by comparing the SDR components on the basis of instantaneous, mean, and probability density function data. The in-plane resolution needs to match the Batchelor scale (λB\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _B$$\end{document}) for the central difference scheme-based SDR deduction. However, the out-of-plane resolution, Δs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varDelta s$$\end{document}, requirement is different owing to the use of two-point difference based SDR and systematic biases. The optimum Δs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varDelta s$$\end{document} is found to be 2.5λB\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _B$$\end{document}. Finally, measurement guidelines are provided to assess the accuracy of 3D SDR measurements. Graphical Abstract

Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications. Time correlated single photons counting (TCSPC) is applied to monitor the fluorescence decay and determine the droplet temperature in the mixing zone of two sprays which are injected with significantly different temperatures. For some well-chosen fluorescent dye, like rhodamine B (RhB), the fluorescence lifetime strongly varies with the temperature. Provided sufficiently different fluorescence lifetimes for the droplets of the two sprays, the fluorescence decay is expected to follow a multiple exponential decay. In this study, different approaches are tested for measuring the temperature of the two sprays as well as their mixing fraction based on the analysis of the fluorescence decay. Firstly, the measurement of the mixture fraction alone is tested by considering a configuration where one spray is seeded with eosin Y (EY) and the other with rhodamine 6G (Rh6G). Given the very different lifetimes of these dyes, which are not temperature dependent, the fluorescence decay is function of the volume fraction of liquid from each spray in these tests. A calibration is necessary to evaluate the mixing fraction. Both sprays are mounted on an automated platform allowing 3D scanning and motions which allows obtaining maps of the fluorescence decay. The out-of-field fluorescence, observed in dense sprays when fluorescence is induced by one-photon absorption, is suppressed by using a two-photon fluorescence excitation. This approach significantly improves the spatial resolution of the measurements. Finally, both the droplet temperature and the mixing fraction are measured simultaneously using a single dye, namely RhB, whose fluorescence lifetime is temperature dependent. Special care must be paid to the fact that RhB does not have a purely monoexponential decay at a given temperature. The fluorescence decay in the mixing zone of the two sprays is considered as a combination of two biexponentials. Results show that the volume fraction of a spray must exceed about 10% to make it possible to determine its temperature with an accuracy of about 2–3 °C. Simultaneous measurements of the sprays temperatures and volume fractions provide a means to calculate the mixing temperature (the average between the temperatures of the two sprays weighted by their volume fractions). Graphical abstract

A novel measurement method is developed for a simultaneous measurement of pressure and temperature on an airfoil by sensitive paints. The proposed method requires two sets of measurements: in the first set, the temperature distribution is measured on the entire surface of the airfoil by temperature-sensitive paint (TSP). This temperature field is further utilized to evaluate sparse sensor locations based on the sensor selection methods. For the second set of measurements, TSP was sprayed on sparse points and pressure-sensitive paint (PSP) on the remaining airfoil surface. A full temperature field can be reconstructed using temperature data measured at those sparse locations. The temperature-induced error due to temperature sensitivity of PSP is corrected, and a time-averaged pressure field is compared with the pressure tap data. The proposed method is demonstrated on a flow over a NACA 0015 airfoil. Time-averaged and spanwise averaged pressure agrees very well with pressure sensor data measured simultaneously with PSP giving further confidence in our measurement. The present results also show that the Bayesian estimation and a corresponding sensor selection method overperform the linear least squares estimation and a corresponding sensor selection method, and the Bayesian estimation framework is recommended for the practical sparse sensor for temperature reconstruction.

We present a novel closed-circuit ultra-compact wind tunnel with an 8:1 contraction ratio and high flow quality. Its overall footprint area is less than half that of a conventional tunnel with the same test section size and same contraction ratio, enabling significantly smaller material and construction costs. The tunnel’s key features which enable the small footprint include a two-dimensional main diffuser, a minimum-length contraction, and expanding turning vanes with a 1.167:1 ratio in corner two and an aggressive 1.875:1 ratio in corner four. Separation in the latter is prevented using a screen and honeycomb integrated into each vane passage—the first time this has been used in a wind tunnel. The tunnel exhibits excellent flow quality with less than ±1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm 1$$\end{document}% mean flow variation in the test section core and a freestream turbulence level of 0.03% at 12 m/s over a 4 Hz–20kHz bandwidth. Graphical abstract

This study experimentally investigates the vortex structure induced by sphere-wall collision and a falling sphere in a viscous liquid. The velocity fields of sphere-induced vortices were measured with refractive-index-matched materials and particle tracking velocimetry. The Reynolds number, based on the sphere diameter and the falling velocity, was in the range of 350–3200. The results revealed that the sphere-induced vortex ring was axisymmetric when the Reynolds number Re is $$\le$$ 800. For the case of Re = 2000, the vortex structure developed into a non-symmetric flow after the sphere collided on the wall. Nonetheless, the influence of the Reynolds number on the vortex trajectory is insignificant. The moving speed of the primary vortex increases as the Reynolds number increases. In addition, the trajectories of free-falling spheres at a high Reynolds number of Re = 3200 deviate from a vertical straight line, owing to the non-axisymmetric flow field around the sphere. The experimental results presented in this work can be used to validate numerical schemes for solid/vortex interaction problems.

An analytical framework for the propagation of velocity errors into PIV-based pressure calculation is extended. Based on this framework, the optimal spatial resolution and the corresponding minimum field-wide error level in the calculated pressure field are determined. This minimum error can be viewed as the smallest resolvable pressure. We find that the optimal spatial resolution is a function of the flow features (patterns and length scales), fundamental properties of the flow domain (e.g., geometry of the flow domain and the type of the boundary conditions), in addition to the error in the PIV experiments, and the choice of numerical methods. Making a general statement about pressure sensitivity is difficult. The minimum resolvable pressure depends on competing effects from the experimental error due to PIV and the truncation error from the numerical solver, which is affected by the formulation of the solver. This means that PIV experiments motivated by pressure measurements should be carefully designed so that the optimal resolution (or close to the optimal resolution) is used. Flows (and 5×104\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$5\times 10^4$$\end{document}) with exact solutions are used as examples to validate the theoretical predictions of the optimal spatial resolutions and pressure sensitivity. The numerical experimental results agree well with the rigorous analytical predictions. We also propose a posterior method to estimate the contribution of truncation error using Richardson extrapolation and that of PIV error by adding artificially overwhelming noise. We also provide an introductory analysis of the effects of interrogation window overlap in PIV in the context of the pressure calculation.

An effective Lagrangian Planar Interferometric Tracking (PIT) processor is proposed to track the size and path of multiple droplets, with spray droplet diameters (20–150 µm) and volumetric concentrations ( $$\approx$$ ≈ 300 drops/cm $$^3$$ 3 ) consistent with industrial applications, produced by an ultrasonic atomiser in evaporating conditions. A test facility was developed where liquid droplets are exposed to a temperature gradient in a co-axial air flow, where the outer stream is preheated to the desired temperature (288–550 K). The PIT method builds on a TSI Global Sizing Velocimtery measurement technique and allows to detect, size and follow the path of droplets which were otherwise discarded or mis-analysed by the commercial software. The methodology was first tested under non-evaporating conditions, and multiple sources of errors, some common to most planar interferometric techniques, were identified and their order of magnitude and impact on final droplet measurement assessed. The main source of error is related to the out-of-plane motion of the droplets and the time they spend in the measurement volume. For non-evaporating conditions, measured data can be processed to filter out this source of error. In evaporating conditions, a novel method for assessing the impact of measurement error with respect to droplet evaporation and measurement timescales is defined. The PIT method allowed tracking of individual methanol droplets entrained within an airflow heated to 495 K and determining their size reduction under evaporating conditions. Measured droplet evaporation rates were then compared against those predicted by an iterative evaporation model, and a very good agreement was found between the modelled and measured estimates. Graphical abstract

Many of the laminar-turbulent flow localisation techniques are strongly dependent upon expert control even-though determining the flow distribution is the prerequisite for analysing the efficiency of wing & stabiliser design in aeronautics. Some recent efforts have dealt with the automatic localisation of laminar-turbulent flow but they are still in infancy and not robust enough in noisy environments. This study investigates whether it is possible to separate flow regions with current deep learning techniques. For this aim, a flow segmentation architecture composed of two consecutive encoder-decoder is proposed, which is called Adaptive Attention Butterfly Network. Contrary to the existing automatic flow localisation techniques in the literature which mostly rely on homogeneous and clean data, the competency of our proposed approach in automatic flow segmentation is examined on the mixture of diverse thermographic observation sets exposed to different levels of noise. Finally, in order to improve the robustness of the proposed architecture, a self-supervised learning strategy is adopted by exploiting 23.468 non-labelled laminar-turbulent flow observations.

A new phosphor particle streak velocimetry (phosphor-PSV) diagnostic with high spatial resolution was recently demon- strated (Fan et al. in Opt Lett 46:641, 2021), where individual phosphor particles, excited by a short pulse laser, form streaks as a results of their displacement by the flow during their relatively long luminescence decay. The local flow velocity is derived by fitting each phosphor streak as a two-dimensional linearly-moving point source with a mono-exponential decaying emission. This single-pulse, single-exposure technique yields a vector for each particle, as in particle tracking velocimetry, avoiding the spatial filtering associated with particle image velocimetry (PIV). The wavelength-shifted luminescence also allows rejection of unwanted reflected laser light, and macroscopic measurements in Fan et al. (Opt Lett 46:641, 2021) were obtained at a distance of 30 μm from a wall. In this manuscript, we establish by a combination of experiments and luminescent particle imaging simulations the performance of the technique in a range of flow conditions and imaging parameters. A new image segmentation algorithm is developed and applied to phosphor streak images to improve the density of measurements to 0.004 vectors per pixel (vpp). Two phosphors with decay times of 564.2 μ s (Gd2 O 2S:Tb3+) and 0.92 μ s ( BaMgAl10 O17 :Eu2+ ), are used to perform measurements in slow liquid flows (< 20 cm/s) and fast turbulent jets (up to 85 m/s), respec- tively. To assess the uncertainty of the approach for various experimental parameters (phosphor particles, flow velocity, particle image size, signal-to-noise ratio, and streamline curvature), synthetic streaks are generated and validated against experimental data. Finally, the impact of out-of-plane motion is investigated experimentally. This study paves the way for a wide implementation of the new phosphor-PSV technique in flow research and, in particular, to study two-phase flows and confined or semi-confined flows near solid boundaries.

A towing tank is utilized to investigate the flow field around a two-dimensional submerged foil model operating near the free surface. Free surface effects are analyzed for steady and unsteady model velocity. The model’s submergence depth and angle of attack are varied. Tests are conducted for the model facing upside-up and upside-down. The surface deflection is recorded and the experimental results are utilized to validate an analytic model that is deployed to predict wake wave patterns at arbitrary model velocity. The flow mechanism leading to load alterations when the foil is in the vicinity of the free surface is explored in detail using experimental and analytic results. The imposed wave-induced velocity perturbations alter the effective angle of attack experienced by the foil. Flow separation is delayed when the model is facing upside-up and promoted when facing upside-down. For test cases with unsteady sinusoidal model velocity, forward traveling waves are generated, leading to a time-varying change in the inflow condition of the submerged foil. Increasing the model’s submergence depth alleviates free surface effects. A skim plate is installed in-between the free surface and model. It shows similar wave alleviating effects as obtained when increasing the model submergence depth by locally blocking wave-induced velocities. The skim plate position is varied in the longitudinal direction to determine its most advantageous position. Surface wave effects at unsteady model velocity are alleviated most effectively when the skim plate protrudes upstream of the model. Graphical abstract

Natural laminar flow airfoils have achieved such a level of refinement that further optimisation and subsequent wind tunnel testing need to regard the specific free-stream turbulence to be expected during operation. This requires the characterisation of this turbulence in terms of those properties which are relevant for boundary layer receptivity and subsequent transition. These parameters of turbulence change with environmental conditions and, in case of aircraft, along the flight profile. This study investigates the free-stream turbulence relevant for the case of sailplane airfoils. In-flight measurements with a constant temperature anemometer x-wire probe were conducted during cross-country flights in Central Europe and provided 22 h of flight data, covering thermalling phases as well as straight flight legs. Longitudinal and transversal velocity fluctuations were recorded well into the dissipation range. The special challenges of operating a constant temperature anemometer probe continuously for several hours are addressed. The permanent unsteadiness of the inflow poses challenges for the evaluation, but also provides a broad database of measured turbulence levels. The quality of the measurements is shown by verifying some of the predictions of Kolmogorov's inertial range theories. Free-stream turbulence in thermalling phases is sufficiently homogeneous to be described accurately, as the dissipation range fluctuates only in a limited range and follows a log-normal distribution. On the straight flight legs, the turbulence depends on the convective activity along the flight path. In general, within the convective part of the atmosphere, turbulence levels are found to be significantly larger than in low-turbulence wind tunnels. Graphical abstract

Multi-jet impingement flows relevant to applications in Short Take-Off and Vertical Landing aircraft have been investigated from the standpoint of evaluating the induced lift forces and moments caused by fountain flows. However, for a pair of supersonic jets, the outcome of the difference in their momentum-flux on flow properties is relatively unknown. This study reports on the experimental characterization of the supersonic dual impinging jet flowfield by systematically varying their relative jet momentum-flux and impingement height. A nozzle pair involving a sonic converging and a Mach 1.5 Converging–Diverging (CD) is employed during the study. The CD nozzle is operated at a fixed nozzle pressure ratio of 3, and the expansion ratio (ER) of the converging nozzle is varied from 0.96, 1.19, to 1.59. Results indicate that the fountain flow exhibits a strong dependence on relative jet momentum and a relatively weak dependence on impingement height. Increase in jet momentum-flux of the converging nozzle led to an obvious increase in the nearfield noise levels over a majority of the impingement heights. At resonance-dominated impingement heights, however, a systematic reduction in both the nearfield noise and ground plane unsteady pressures is observed. The resonance in the jet from CD nozzle is found to be the chief source of unsteadiness in the flowfield, which grew weaker and exerted lesser influence with increase in jet momentum-flux of the converging nozzle. Proximity of the fountain flow to the resonating jet is identified to be the main cause for the change in resonance characteristics of dual impinging jets. The cause for this change is determined to be due to the alterations introduced to the highly sensitive processes that constitute the feedback mechanism, when the fountain flow directly interacts with the jet shear layer. Graphical Abstract

Current experiment techniques for vorticity measurement suffer from limited spatial and temporal resolution to resolve the small-scale eddy dynamics in turbulence. In this study, we develop a new method for direct vorticity measurement in fluid flows based on digital inline holography (DIH). The DIH system utilizes a collimated laser beam to illuminate the tracers with internal markers and a digital sensor to record the generated holograms. The tracers made of the polydimethylsiloxane prepolymer mixed with internal markers are fabricated using a standard microfluidic droplet generator. A rotation measurement algorithm is developed based on the 3D location reconstruction and tracking of the internal markers and is assessed through synthetic holograms to identify the optimal parameter settings and measurement range (e.g., rotation rate from 0.3 to 0.7 rad/frame under numerical aperture of imaging of 0.25). Our proposed method based on DIH is evaluated by a calibration experiment of single tracer rotation, which yields the same optimal measurement range. Using von Kármán swirling flow setup, we further demonstrate the capability of the approach to simultaneously measure the Lagrangian rotation and translation of multiple tracers. Our method can measure vorticity in a small region on the order of 100 µm or less and can be potentially used to quantify the Kolmogorov-scale vorticity field in turbulent flows. Graphical abstract

The dynamics of the vortex structure in a three-dimensional, round counterflow wall jet are investigated using particle image velocimetry (PIV) to explore the large-scale turbulence structure and to analyze the mechanism of its generation in the flow field. The ratio of the jet velocity to the main flow velocity is 8.89, and the Reynolds number based on the diameter of the jet pipe is 9127. Typical vortex structures are detected in the stagnation region and main flow by the virtual dye method. The instability of the jet shear layer is caused by the feedback mechanism. The vortex ring is formed while the jet is fluttering upwards. Fluid flows from the recirculation region to the original location of the vortex ring. The main flow analysis reveals the growth process of the front vortex until its break-up. The angle of the relevant streamwise velocity fluctuation region experiences a relatively large deflection between x/D=40\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$x/D = 40$$\end{document} and x/D=50\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$x/D = 50$$\end{document} due to the action of the main flow. The turbulent kinetic energy contribution of the wall-normal component is particularly significant in the recirculation region. Furthermore, dynamic mode decomposition (DMD) analysis exposes three low-frequency modes. The first mode corresponds to the stretching and contraction of the jet in the streamwise direction. The second mode has two dynamic characteristics: the fluctuation of the wall-normal direction of the recirculation region and the fluctuation of the center of the recirculation region along the streamwise direction. The third mode is also characterized by fluctuations in the streamwise direction in the center of the recirculation region. Graphical abstract

Ship-induced wave wash affects the hydromorphological and ecological state of rivers through various mechanisms. The direct proximity of the riverbank is usually the most exposed, as the hydrodynamic stresses are the highest in these shallow water areas. Contrary to the steady and almost still, natural flow conditions (i.e., no waves of anthropogenic source), shoaling and breaking of ship waves increase the hydrodynamic stresses by orders of magnitudes, having notable ecological consequences, and resulting in bank erosion as well. Due to the shallow water depths and temporary drying, conventional measurement techniques are no longer applicable in these areas. In this study, large-scale particle image velocimetry (LSPIV) is used to quantify the prevailing flow conditions. In the absence of ground truth data in the wave breaking region, a high-resolution computational fluid dynamics model—verified with field pressure and acoustic Doppler velocimetry data—is used for the cross-validation of the LSPIV results. The results underline the applicability of LSPIV for the hydrodynamic analysis of wave velocities in this special riverine swash zone, which is of key importance from the aspect of ecology and bank erosion as well. Graphical abstract

Sharks can swim with excellent hydrodynamic performance under a variety of flow conditions. The dermal scales of sharks, which can change from an aligned state to a tilted state, account for flow control in the attached and separated flow. However, the mechanism of using tilted biomimetic shark scales for flow separation control is still not clear. In the present work, the effects of biomimetic shark scales with fixed tilt angles on flow separation over an inclined plate are investigated experimentally using time-resolved particle image velocimetry. The Reynolds number based on the streamwise length of the plate and the freestream velocity is 2.0 × 10⁴. From the perspective of the time-averaged flow field, it is found that the tilted biomimetic shark scales decrease the reattachment length by 57% when the angle of attack of the plate is 10°. From the perspective of the instantaneous flow field, the shed vortices over the tilted biomimetic shark scales are closer to the wall than those over the flat plate. The fluid convection is strengthened in the separated shear layer by the tilted biomimetic shark scales, as the momentum transportation and the vorticity convection are enhanced. As a result, normal motions of fluid in the separated shear layer are improved, and energy is supplied to resist flow separation. A flow separation control strategy is proposed for different tilt angles of the biomimetic shark scales and angles of attack, which is significant for engineering applications involving flow control. Graphical abstract

Thermographic particle tracking velocimetry (PTV) using phosphor particles with temperature sensing function as tracer particles has attracted attention due to its temperature and velocity simultaneous sensing function. However, due to the influence of particle movement on temperature measurement, the current mainstream thermographic PTV mainly uses the intensity ratio method, while the lifetime-based method with higher accuracy and simpler experimental configuration are limited. In this study, we introduce an adaptive windows technique for lifetime-based two-dimensional thermographic PTV. Adaptive windows are applied according to the calculated velocity to eliminate the limitation of lifetime-based temperature measurement due to particle movement. Through the adaptive windows, the phosphorescence signals are confined in local windows during the entire decay process, and high-accuracy temperature information can be obtained. The technique is then demonstrated in a forced convection flow. The results show that the current technique could be available in water with a temperature range of 18.5–82.34 °C and a velocity range of 0–0.033 m/s, and the temperature measurement accuracy can be kept within 11 °C. It has overcome the influence of particle movement on lifetime-based phosphorescent PTV, which could provide high-accuracy temperature and velocity simultaneous measurement with a single camera. Graphical abstract

Wind tunnel test is an important means to obtain aerodynamic loads. However, due to the limited space location and experimental cost, it is usually difficult to arrange enough pressure taps on the complex model surface to obtain complete surface pressure distribution information. Hence, the accuracy of the lift and the pitching moment calculated by the direct integration may fail to meet expectations. This paper presents a sparse fusion method to reconstruct the complete surface pressure distribution based on the limited wind tunnel test data. By integrating the reconstruction resulting surface data over a complete aerodynamic surface, the corresponding aerodynamic coefficients are obtained and are in good agreement with the experimental results. The accuracy of the proposed method is verified by two-dimensional airfoil flow and three-dimensional steady ONER-M6 wing flow examples. The reconstruction results can accurately match the experimental results. The developed reconstruction method largely solves the problem of fine reconstruction of distributed load under the condition of limited space and sparse observation. Graphical abstract

Near field liquid structures and penetration of a kerosene jet injected in a Mach 2 crossflow were studied experimentally in the LAPCAT-II Dual Mode Ramjet Combustor at Onera, using high spatial and temporal resolution imaging techniques. The experiments performed in this study provide measurements in high temperature conditions, with kerosene as liquid fuel to bring new data in these conditions. The fuel spray is injected through a single orifice, perpendicularly to the supersonic flow, at temperatures from ambient to 1500 K and jet-to-crossflow momentum flux ratio from 3.1 to 8.6. Five different test cases are defined to show independently the influence of the injection ratio and the supersonic flow temperature on the liquid jet atomization. A high spatial resolution imaging system is used to detail the characteristic spatial scales of this atomization process in the supersonic crossflow. The surface waves and droplets produced in the windward side of the liquid jet are characterized qualitatively. High-resolution visualizations bring a new insight of the droplet trajectories in the leeward region of the jet. Schlieren imaging is used to detail the complex shock wave structures in such a supersonic flow. Penetration of the kerosene jet is measured by shadowgraphy and schlieren imaging in five test cases and compared to other studies of the literature whenever it is possible. High-speed schlieren recorded at 210 kHz is also performed to capture the temporal dynamics of the shocks and measure the liquid jet velocity. Graphical abstract

Reactive sprays can be found in numerous industries such as combustion, spray drying, and purification of exhaust gases. The characterization of transient state for such droplets in these sprays is a challenge, asking for the measurement of the temperature and/or composition gradients inside the droplets. In this work, rainbow technique is extended to characterize internal gradients. After the numerically study of the characteristics of rainbow scattered by droplet with radial refractive index gradients, an experimental study was carried out for the case of CO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document} capture by Monoethanolamine (MEA) droplets. The investigation demonstrates that the gradients can be quantified during the first moments of injection. Graphical Abstract

Flush-mounted cavity hot-wire probes have emerged as an alternative to classical hot-wire probes mounted several diameters above the surface for wall-shear stress measurements. They aim at increasing the frequency response and accuracy by circumventing the well-known issue of heat transfer to the substrate that hot-wire and hot-film probes possess. Their use, however, depends on the assumption that the cavity does not influence the flow field. In this study, we show that this assumption does not hold, and that turbulence statistics are modified by the presence of the cavity with sizes that are practically in use. The mean velocity and fluctuations increase near the cavity while the shear stress decreases in its surroundings, all seemingly stemming from the fact that the no-slip condition is not present anymore and that flow reversal occurs. Overall, the energy spectra and the probability density function of the wall shear stress fluctuations indicate a change of nature of turbulence by the presence of the cavity.

This paper presents a general method to extract a surface pressure field from a skin friction field in complex flows as an inverse problem, focusing on its application to global luminescent oil-film (GLOF) skin friction measurements. The main technical aspects of this method are described, including the basic equation relating surface pressure to skin friction, variational method, numerical algorithm, and approximate method with the constant boundary enstrophy flux (BEF). The proposed method is evaluated through simulations in the Falkner–Skan flow and the flow over a 70°-delta wing to investigate the effects of the Lagrange multiplier, downsampling rate, noise level, and the value of a constant source term in the approximate method. Further, the approximate method is applied to the skin friction fields obtained by GLOF measurements in the flow over a 65°-delta wing and the square junction flow to obtain the normalized surface pressure fields. The proposed method provides a useful tool to obtain the high-resolution fields of both surface pressure and skin friction by GLOF measurements in complex flows (particularly at low speeds).

The three velocity components in a fluid plane can be measured by applying Digital Image Plane Holography. This technique is limited by the laser coherence length, which reduces its application with high speed lasers that, generally, have a very short coherence length. In addition, the use of a double cavity can also imply a small wavelength difference between the two laser beams. In this work, we present an improved Optical Path Length Enlarging Device that allows the velocity measurement, in a 2D field whose width is four times larger than the laser coherence length. The optical set-up and the procedure for measuring in a larger field (ten times the laser coherence length) were optimized, and the issues derived from the laser spatial and temporal coherence and wavelength changes were analyzed and solved. Digital Image Plane Holography with the Optical Path Length Enlarging Device and Particle Image Velocimetry were applied for measuring the whole velocity field in the central plane of a cylindrical cavity with a rotating lid, for two Reynolds numbers (800 and 2000), showing both of them a very good agreement with the numerical simulations. Graphical abstract

An experimental investigation was performed in order to better understand the transonic shock buffet phenomenon and determine the dominant flow interactions at specific flow conditions. A rigid wing in the shape of an OAT15A airfoil was placed in the Trisonic Wind Tunnel Munich, where both the Mach number and the angle of attack were varied between $$0.65< M_\infty < 0.77$$ 0.65 < M ∞ < 0.77 and $$3.8< \alpha < 6.3^\circ$$ 3.8 < α < 6 . 3 ∘ respectively. With the use of high-speed imaging, high-quality optics and state-of-the-art laser equipment, highly resolved velocity field measurements were obtained via particle image velocimetry, where the streamwise and vertical velocity components were computed over the suction side of the wing center plane. It was shown that sustained buffet first occurs at $$M_\infty \ge 0.74$$ M ∞ ≥ 0.74 when maintaining the angle of attack constant at $$\alpha =5.8^\circ$$ α = 5 . 8 ∘ . Similarly, an increase in $$\alpha$$ α for a fixed $$M_\infty =0.74$$ M ∞ = 0.74 also led to the development of shock buffet. Instantaneous snapshots confirmed the presence of a recirculation region downstream of the moving shock, where an increase in the wake size was confirmed when the shock was located most upstream. Streamwise correlations were also computed near the airfoil’s upper surface in order to extract the characteristic convective velocity of flow structures. The convective velocity appeared to increase with streamwise distance, ranging on average between 50 and 150 m/s. Overall, these time-resolved velocity field measurements allow for the investigation of the flow dynamics during shock buffet and highlight the independent effect of Mach number and angle of attack on this complex phenomenon.

The performance of particle tracking velocimetry (PTV) is constrained by a practical issue, i.e., particle missing in a particle image frame. The randomly appeared loss-of-pair particles will bias the particle pairing relationship that is sought by particle matching algorithms. To handle this issue, this work proposes a general framework for the compensation of particle missing in PTV. Following our previous work (Nie in Exp Fluids 62(4): 68, 2021), we deal with the family of ant colony optimization (ACO) algorithms, which convert the task of particle matching to a global optimization problem for a particular objective function and seek a solution using ACO. To enable particle missing compensation, two core concepts are proposed. The first is to perform two symmetric ACOs from the forth and the back directions on a straddle-frame image pair, and then cross-validate the two sets of solutions to estimate the valid or problematic matching. The second is to make the action of the forth and the back ant colonies work in coordination with each other by sharing their knowledge on the particle matching relationship. This is implemented using an exterior loop, in which the intersection of the solutions obtained by the two colonies of ants, called mutual knowledge, will be learned and passed to the next generation. The first concept relies on an operation of virtual-particle add-on. It leads to the so-called cross-validation ant colony optimization (CVACO) algorithm. The second concept updates CVACO by dynamically adjusting the pheromone factor and the heuristic factor on each candidate particle pair in the next exterior pass, forming the so-called algorithms of pheromone-feedback CVACO (PF-CVACO) and heuristic-feedback CVACO (HF-CVACO), respectively. A synthetic test shows that the proposed algorithms work well in the scenarios of both single-frame particle missing and dual-frame particle missing at low-to-moderate particle missing rates, which cannot be well handled using conventional single-pass ACO. Graphical Abstract

A high-resolution background-oriented schlieren (BOS) technique, which utilizes a high-resolution camera and a microdot background pattern, is proposed and used to measure the pressure field of an underwater shock wave in a microtube. The propagation of the shock wave subsequently reaches a concave water–air interface set in the microtube resulting in the ejection of a focused microjet. This high spatial-resolution BOS technique can measure the pressure field of a shock front with a width as narrow as the order of only $$10^1\,\upmu$$ 10 1 μ m with a peak pressure as large as almost 3 MPa. This significant breakthrough has enabled the simultaneous measurement of the pressure impulse of the shock front and the velocity of the microjet tip. As a result, we have experimentally observed the linear relation between the velocity of the microjet tip and the pressure impulse of the shock front for the cases without secondary cavitation in the liquid bulk. Such relation was theoretically/numerically predicted by Peters et al. (J Fluid Mech 719:587–605, 2013). This study demonstrated the capability of the proposed high-resolution BOS technique as a microscale contactless pressure measurement tool for underwater shock waves and potentially other micro- and nanofluids. Graphical abstract

In the present study, Lagrangian measurements of drop motion of a two-phase spray are analyzed to estimate the collision, fragmentation, size, and velocity field of the drops. An experimental setup is designed to record fluorescence images of drops, illuminated by a planar laser sheet. The recorded images are then processed with a commercially available Lagrangian tracking software, Track. The methodology for particle identification, tracking, and size approximation in Track in the near interface region of spray is stated. With the stated methodology , the information about size, velocity, fragmentation, collision, and change in the shape of the drops are obtained. The effectiveness of the developed methodology is tested against synthetically produced images and is then applied to a liquid/liquid jet. Similar behaviors for dispersed phase fragmentation-collision and mass fragmented along axial location normalized by breakup length of the jet are reported.

The present study aims investigating experimentally wing/blade geometries in which the leading edge is modified by the presence of artificial bumps, following examples in nature (“biomimetics”). Specifically, the tubercles observed in humpback whales are considered with a special focus on easy manufacturing and performance improvements, trying to overcome the observed lift coefficient reduction before stall in comparison with a standard wing. To this end, different tubercle geometries are tested, by measuring overall forces acting on the wings and by deriving detailed velocity fields using particle image velocimetry. Measurements indicate performance improvements for all trailing edge tubercle geometries here tested. In addition, the detailed analysis of mechanisms underlying the improvement of performances suggests that a triangular shape of the leading edge combines the advantages of easy manufacturing and improvements of pre-stall behaviour. So far, a simple mathematical model, describing tubercles as delta wings, is presented and verified by experimental data. The objective of the present work is focusing on the basic fluid-mechanics phenomena involved, to show that beneficial effects of tubercles are present even when tubercle details are simplified, in order to couple performance improvement and ease of assembly. Graphical Abstract

A swept impinging oblique Shock Boundary Layer Interaction (SBLI) is investigated in Mach 2.3 flow induced by a shock generator with x–z plane sweep ψ=30.0∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\psi =30.0^\circ$$\end{document} and x–y plane deflection of θ=12.5∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\theta =12.5^\circ$$\end{document}. The incoming flow is a naturally turbulent boundary layer developing over the flat wind tunnel wall with Reθ=5600\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {Re}_\theta =5600$$\end{document}. A combination of Stereoscopic PIV and Tomographic PIV is used to characterize both the undisturbed incoming boundary layer and the resultant complex geometries of the swept SBLI. Linear Stochastic Estimation is used to identify significant boundary layer vortical structures and document changes to their topology at various heights in the boundary layer. Three-dimensional velocity snapshots throughout the swept SBLI show both large-scale growth/collapse of the interaction and prominent streamwise streaks with a notable spanwise periodicity. The time-averaged 3D structure for this configuration is documented for the first time with two scaling zones observed on either side of the incident shock impingement location. In addition to numerous swept features analogous to 2D SBLIs (separation, shear layer, separation bubble, reattachment, etc.), significant unsteadiness in the velocity fields was observed just after reattachment at magnitudes in excess of those encountered in unswept SBLIs suggesting an additional unsteady mechanism is present in the swept configuration. Graphical abstract

The implantation of aortic valve prostheses is often associated with the dilation of the ascending aorta. It is well known that the modification of the fluid dynamics induced by both the prosthetic valve and aortic dilation may, in turn, promote further vessel remodelling. Besides, when the prosthesis is mechanical, a major concern is the blood cell damage and platelet activation which requires a lifelong anticoagulant therapy, which in turn is an additional significant factor of comorbidity. We analysed in vitro the combined effect of the presence of a bileaflet mechanical aortic valve and the dilation of the aortic root in patient specific laboratory models. Three model aortas with increasing degree of dilation, hosted in a mock loop reproducing the heart flow pulsatility, were investigated. The measurement of the time-resolved velocity field allowed the analysis of the general structure of the flow and shear strain-rate distribution. Additionally, the Blood Damage Indexes (BDIs) for both haemolysis and platelet activation were computed along synthetic particle trajectories. Results suggest that a feedback process can be triggered since the aortic dilation tends to decrease the shear stresses at the walls and favour blood stasis: two factors that are known to promote vessel remodelling. Secondly, the analysis of BDIs shows that aortic dilation significantly increases the damage index for haemolysis, whereas a similar effect is not shown when focusing on platelet activation. Graphical abstract

In this paper, we overview, evaluate, and demonstrate the sparse processing particle image velocimetry (SPPIV) as a real-time flow field estimation method using the particle image velocimetry (PIV), whereas SPPIV was previously proposed with its feasibility study and its real-time demonstration is conducted for the first time in this study. In the wind tunnel test, the PIV measurement and real-time measurement using SPPIV were conducted for the flow velocity field around the NACA0015 airfoil model. The off-line analysis results of the test show that the flow velocity field can be estimated from a small number of processing points by applying SPPIV, and also illustrates the following characteristics of SPPIV. The estimation accuracy improves as the number of processing points increases, whereas the processing time per step increases in proportion to the number of processing points. Therefore, it is necessary to set an optimal number of processing points. Furthermore, the number of modes should be appropriately selected depending on the number of processing points. In addition, the application of the Kalman filter significantly improves the estimation accuracy with a small number of processing points while suppressing the processing time. When the flow velocity fields with different angles of attack are used as the training data with that of test data, the estimation using SPPIV is found to be reasonable if the difference in angle of attack between the training and test data are equal to or less than 2 deg and the flow phenomena of the training data are similar to that of the test data. For this reason, training data should be prepared at least every 4 deg. Finally, the demonstration of SPPIV as a real-time flow observation was conducted for the first time. In this demonstration, the real-time measurement is found to be possible at a sampling rate of 2000 Hz at 20 or less processing points in the top 10 modes estimation as expected by the off-line analyses. Graphical Abstract

Jet in supersonic cross flow is a conceptually simple configuration with applications in fuel injection in combustors of high-speed vehicles. The focus of the present work is on the unsteadiness in this flow configuration caused primarily by Shock-Wave Boundary Layer Interactions that has not received much attention in prior experimental studies, and which can have implications on the combustion and thrust generation from such engines. This is studied in the present work for the case of a sonic gas jet injection in a supersonic (M = 2.5) cross flow using Particle Image Velocimetry (PIV), unsteady pressure measurements and high-speed shadowgraph visualizations of the flow. The studies are done for a range of momentum flux ratios (J) from 1.5 to 3.0. High speed shadowgraphy of the shocks show that the bow, separation and barrel shock formed are unsteady, with wall pressure measurements just upstream of the jet injection location showing a large increase in fluctuations compared to that in the boundary layer far upstream of the jet injection. PIV measurements in the injected jet’s central wall-normal plane show the presence of low/high speed streaks in the incoming boundary layer, and the locations and shape of the separation, bow and barrel shocks. These measurements indicate that the low/high speed streaks in the incoming boundary layer have a significant effect on the instantaneous shock locations and the instantaneous jet penetration, with a high-speed boundary layer streak leading to the shocks being pushed downstream and the jet penetration being reduced, while the opposite effects are seen for a low-speed streak in the incoming boundary layer. Corresponding changes are also seen in the barrel shock depending on the presence of low/high-speed streaks in the boundary layer, with these deformations leading to the ejection and subsequent convection downstream of high-speed pockets. These observations indicate that the source of the unsteady fluctuations seen in the shocks and in the instantaneous jet penetration are related to the low and high-speed streaks in the incoming boundary layer, which can have consequences for the combustion and thrust generation characteristics of high-speed engines where such fuel injection strategies may be adopted. Graphical abstract

Experimental and steady RANS data were generated for high-Reynolds-number rough wall flows beneath a systematically constructed family of bi-directional, continually varying pressure gradient distributions. These flows demonstrate outer-scale Reynolds number independence and qualitatively similar pressure gradient dependence, but reduced history dependence, compared to an equivalent smooth wall flow. The spectrum of fluctuating wall pressure beneath these flows is largely simplified compared to equivalent smooth wall behavior, collapsing on an outer-variable scaling and exhibiting an overlap region independent of pressure gradient and pressure gradient history. The universality of the high-frequency behavior suggests a corresponding universality in the near wall dissipative behavior, contrary to current modeling philosophies for rough wall flows. Corresponding RANS data reveal fundamental issues with the classical roughness boundary condition definition; RANS data fail to replicate the correct Reynolds number dependencies of rough wall flows. These results suggest that while a full understanding of rough wall flow physics is still lacking, such flows exhibit simple, universal relations that are exploitable for advancing our physical understanding and predictive modeling capability. Graphical abstract

We present a method to extract the surface expressions of underwater features inspired by the observation that these features are characterized by sharp elevation gradients at their apparent boundaries. To isolate these features, we therefore measure the spatially resolved surface slope field and analyze it using continuous wavelet transforms. Our methods do not require us to prescribe the shape, size, or location of the features a priori. We demonstrate our techniques using data measured in a laboratory water flume, and show that our technique performs well even when the surface is perturbed with wind. Finally, by tracking the motion of these features in time we also find that they move at nearly the mean surface velocity, suggesting that they may also be useful for surface velocimetry.

In the experimental aeroacoustics, it is always a challenge to study the far-field radiation and near field hydrodynamics simultaneously, and be able to firmly establish the causality between them. The main objective of this paper is to present an experimental technique that can exploit the deterministic turbulent boundary layer generated under a base flow of either mildly separated or laminar boundary layer to either disrupt an existing acoustic scattering mechanism, or reconstruct a new acoustic scattering scenario to enable the ensemble-averaging and wavelet analysis to study the aerofoil trailing edge noise source mechanisms in the spatial, temporal and frequency domains. One of the main attractions of this technique is that the experimental tool does not need to be extremely high fidelity as a priori in order to fully capture the pseudo time-resolved boundary layer instability or turbulent structures. In one of the case studies presented here, roll-up vortices of the order of $$\sim$$ ∼ kHz can be captured accurately by a 15-Hz PIV. A single hot-wire probe is also demonstrated to be capable of reconstructing the turbulent/coherent structures in a spatio−temporal domain. The proposed experimental technique can be extended to other self-noise scenarios when the aerofoil trailing edge is subjected to different flow control treatments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present. Graphical abstract

The present work describes a method for the computation of the nucleation rate of turbulent spots in transitional boundary layers from particle image velocimetry (PIV) measurements. Different detection functions for turbulent events recognition were first tested and validated using data from direct numerical simulation, and this latter describes a flat-plate boundary layer under zero pressure gradient. The comparison with a previously defined function adopted in the literature, which is based on the local spanwise wall-shear stress, clearly highlights the possibility of accurately predicting the statistical evolution of transition even when the near-wall velocity field is not directly available from the measurements. The present procedure was systematically applied to PIV data collected in a wall-parallel measuring plane located inside a flat plate boundary layer evolving under variable Reynolds number, adverse pressure gradient (APG) and free-stream turbulence. The results presented in this work show that the present method allows capturing the statistical response of the transition process to the modification of the inlet flow conditions. The location of the maximum spot nucleation is shown to move upstream when increasing all the main flow parameters. Additionally, the transition region becomes shorter for higher Re and APG, whereas the turbulence level variation gives the opposite trend. The effects of the main flow parameters on the coefficients defining the analytic distribution of the nucleation rate and their link to the momentum thickness Reynolds number at the point of transition are discussed in the paper. Graphical abstract

We present a methodology that allows to measure the dynamics of polydisperse suspension flows by means of Astigmatism Particle Tracking Velocimetry (APTV). Measurements are successfully performed with tridisperse suspensions flows in a square duct of up to $$\varPhi =9.1\%$$ Φ = 9.1 % particle volume fraction. Using a refractive index matching technique, a small amount of the particles ( $$\varPhi =0.08\%$$ Φ = 0.08 % ) is labeled with fluorescent dye to be visible to the camera during the particle tracking procedure. Calibration measurements are performed for ten different particles diameters $$d_p$$ d p ranging from $$d_p= {15}\upmu \mathrm{m}$$ d p = 15 μ m to $$d_p= {260}\,\upmu \mathrm{m}$$ d p = 260 μ m . It is shown that Euclidean calibration curves of different $$d_p$$ d p overlap outside the focal planes, which induces ambiguities in a polydisperse APTV measurement. In the present approach, this ambiguity can be overcome utilizing the light intensity of a particle image which increases sharply with $$d_p$$ d p . In this way, extended Euclidean calibration curves can be generated for each particle group which are spatially separated through the light intensity which serves as an additional calibration parameter (Brockmann et al. in Exp Fluids 61(2):67, 2020). The extended Euclidean calibration allows to simultaneously differentiate particles of different sizes and determine their 3D location. This facilitates to investigate the migration behavior of mono- and tridisperse suspension flows which we demonstrate here for square duct flows with cross-sectional areas of $$0.6\times 0.6\,\mathrm{mm}^2$$ 0.6 × 0.6 mm 2 and $$0.4\times 0.4\,\mathrm{mm}^2$$ 0.4 × 0.4 mm 2 at bulk Reynolds numbers of $$\mathrm{Re}_b \approx 20$$ Re b ≈ 20 and $$\mathrm{Re}_b \approx 40$$ Re b ≈ 40 for particle volume fractions of $$\varPhi =0.08\%$$ Φ = 0.08 % and $$\varPhi =9.1\%$$ Φ = 9.1 % . At $$\varPhi =0.08\%$$ Φ = 0.08 % and $$\mathrm{Re}_b=20$$ Re b = 20 , we observe particles to arrange themselves in a ring-like formation inside the capillary, henceforth referred to as Pseudo Segré Silberberg Annulus (PSSA), with no significant differences between mono- and polydisperse suspension particle distributions. At $$\varPhi =9.1\%$$ Φ = 9.1 % , particles in monodisperse suspensions scatter around the PSSA. This scattering decreases when $$d_p$$ d p increases or $$Re_b$$ R e b increases from 20 to 40. Striking differences are observed in polydisperse suspensions. Large particles ( $${60}\,\upmu \mathrm{m}$$ 60 μ m ) scatter significantly less around the PSSA in the polydisperse case compared to a monodisperse suspension of the same overall volume fraction. In contrast, small and intermediate particles ( $${30}\,\upmu \mathrm{m}$$ 30 μ m , $${40}\,\upmu \mathrm{m}$$ 40 μ m ) are repelled by larger particles resulting in regions of high concentration close to the channel walls which can be only observed in the polydisperse case. Graphical abstract

Top-cited authors
• German Aerospace Center (DLR)
• German Aerospace Center (DLR)
• Delft University of Technology
• German Aerospace Center (DLR)
• Université de Poitiers