von Karman Institute for Fluid Dynamics

Many flows have a multi-constituent nature, where understanding the transfer of mass and momentum between different parts of the flow is key. While experiments in liquid media have long used flow tagging techniques such as dye injection to isolate parts of the flow, analogous methods that do not compromise full-field velocimetry in gas flows are highly impractical. The recent introduction of a solution of Pyrromethene 567 (P567) in di-ethyl-hexyl-sebacate (DEHS) to produce a seeding fluid capable of fluorescent emission in addition to Mie scattering promises to address this need. By locally seeding a secondary flow of interest with the modified DEHS, the fluorescent signal can be used for tagging; global seeding of main flow with standard DEHS, which only produces Mie scattering of incident laser light, is used to obtain full-field velocimetry with established PIV techniques. Performing morphological image processing and intensity-based thresholding on the fluorescent particle images yields a continuum representation of the secondary flow. This can then be combined with velocimetric information from PIV to conduct quantitative zonal analyses. This technique has been applied to the flow behind an active synthetic-jet turbulence grid and a turbulent boundary layer (TBL). With the new zonal decomposition capabilities offered, data relating to the intermittency of the flows, statistical structure of the phenomena, turbulent/non-turbulent interface (TNTI) and entrainment and detrainment can be extracted.

This paper describes the design and commissioning of an aeroacoustic test rig for the study of single and coaxial propeller propulsive systems. The size of the propellers matches typical drone applications. The experimental setup, designed and commissioned at the ALCOVES anechoic laboratory of von Karman Institute for Fluid Dynamics, is equipped with aerodynamic sensors for performance analysis and is surrounded by a microphone antenna for the characterization of the noise level and directivity. Thefacility permits varying different parameters such as the longitudinal distance between the rotor planes, and the rotational speed/direction of each propeller. Requirements for the qualification of the test room consist of low-level background noise and minimized turbulence ingestion noise. Two experimental databases have been constituted and are joined to the present paper: (i) for the DJI 9450 two-bladed propeller, verified against data from the literature, and (ii) for single and coaxial contra-rotating Mejzlik two-bladed propellers. The proposed benchmark data will support the validation of low- and high-fidelity numerical methods.

Electromagnetic wiping systems allow to pre-meter the coating thickness of the liquid metal on a moving substrate. These systems have the potential to provide more uniform coating and significantly higher production rates compared to pneumatic wiping, but they require substantially larger amounts of energy. This work presents a multi-objective optimization accounting for (1) maximal wiping efficiency (2) maximal smoothness of the wiping meniscus, and (3) minimal Joule heating. We present the Pareto front, identifying the best wiping conditions given a set of weights for the three competing objectives. The optimization was based on a 1D steady-state integral model, whose prediction scales according to the Hartmann number (Ha). The optimization uses a multi-gradient approach, with gradients computed with a combination of finite differences and variational methods. The results show that the wiping efficiency depends solely on Ha and not on the magnetic field distribution. Moreover, we show that the liquid thickness becomes insensitive to the intensity of the magnetic field above a certain threshold and that the current distribution (hence the Joule heating) is mildly affected by the magnetic field’s intensity and shape.

Fluid models represent a valid alternative to kinetic approaches in simulating low-temperature discharges: a well-designed strategy must be able to combine the ability to predict a smooth transition from the quasineutral bulk to the sheath, where a space charge is built at a reasonable computational cost. These approaches belong to two families: multifluid models, where momenta of each species are modeled separately, and drift-diffusion models, where the dynamics of particles is dependent only on the gradient of particle concentration and on the electric force. It is shown that an equivalence between the two models exists and that it corresponds to a threshold Knudsen number, in the order of the square root of the electron-to-ion mass ratio; for an argon isothermal discharge, this value is given by a neutral background pressure Pn≳1000 Pa. This equivalence allows us to derive two analytical formulas for a priori estimation of the sheath width: the first one does not need any additional hypothesis but relies only on the natural transition from the quasineutral bulk to the sheath; the second approach improves the prediction by imposing a threshold value for the charge separation. The new analytical expressions provide better estimations of the floating sheath dimension in collisions-dominated regimes when tested against two models from the literature.

The particle-in-cell (PIC) method with Monte Carlo collisions (MCC) is widely used in the simulation of non-equilibrium plasmas for electric propulsion and laboratory applications. Due to the simplicity of the basic PIC algorithm and the specific modeling needs of the different research groups, many codes have been independently developed. Verification of these codes, i.e., ensuring that the computational code correctly implements the intended mathematical models and algorithms, is of fundamental importance. Different benchmark cases, such as one from Turner et al. [Phys. Plasmas 20, 013507 (2013)], Charoy et al. [Plasma Sources Sci. Technol. 28, 105010 (2019)], and Villafana et al. [Plasma Sources Sci. Technol. 30, 075002 (2021)], have been published in recent years. These have consisted of a complex physical setup, in which many computation modules interact to yield the final result. Although this approach has the advantage of testing the code in a realistic case, it may hide some implementation errors. Moreover, in the case of disagreement, the previous works do not provide an easy way to identify the faulty code modules. In this work, we propose a step-by-step approach for the verification of PIC-MCC codes in a 2D-3V electrostatic setup. The criteria for the test cases are (i) they should highlight possible implementation errors by testing the modules separately, whenever possible (ii) they should be free from physical instabilities to avoid chaotic behavior, and (iii) the numerical result should be accompanied by analytical calculations, for confirmation purposes. The seven test cases identified all show excellent agreement between the authors' codes.

The distortion of turbulence interacting with thick airfoils is analyzed with scale-resolved numerical simulations to elucidate its impact on leading-edge-noise generation and prediction. The effect of the leading-edge geometry is investigated by considering two airfoils with different leading-edge radii subjected to grid-generated turbulence. The velocity field is shown to be altered near the stagnation point, in a region whose extension does not depend on the leading-edge radius. Here, the deformation of large-scale turbulence causes the amplitude of the upwash velocity fluctuations to increase in the low-frequency range of the spectrum because of the blockage exerted by the surface. Conversely, the distortion of small-scale structures leads to an exponential decay of the spectrum at high frequencies due to the alteration of the vorticity field. The prevalence of a distortion mechanism over the other is found to depend on the size of the turbulent structures with respect to the curvilinear length from the stagnation point to the location where surface-pressure fluctuations and pressure gradient peak. This occurs at the curvilinear abscissa where the curvature changes the most. The same high-frequency exponential-decay slope observed for the upwash velocity is retrieved for surface-pressure spectra in the leading-edge region, suggesting that the airfoil unsteady response is induced by the distorted velocity field. This physical mechanism can be accounted for in Amiet's model by using a distorted turbulence spectrum as input and accounting for the increased amplitude of the distorted gust in the aeroacoustic transfer function, retrieving an accurate noise prediction for both airfoils.

Compound flows consist of two or more parallel compressible streams in a duct and their theoretical treatment has gained attention for the analysis and modelling of ejectors. Recent works have shown that these flows can experience choking upstream of the geometric throat. While it is well known that friction can push the sonic section downstream of the throat, no mechanism has been identified yet to explain its displacement in the opposite direction. This study extends the existing compound flow theory and proposes a one-dimensional (1-D) model, including friction between the streams and the duct walls. The model captures the upstream and downstream displacements of the sonic section. Through an analytical investigation of the singularity at the sonic section, it is demonstrated that friction between the streams is the primary driver of upstream displacement. The 1-D formulation is validated against axisymmetric Reynolds averaged Navier–Stokes simulations of a compound nozzle for various inlet pressures and geometries. The effect of friction is investigated using an inviscid simulation for the isentropic case and viscous simulations with both slip and no-slip conditions at the wall. The proposed extension accurately captures the displacement of the sonic section, offering a new tool for in-depth analysis and modelling of internal compound flows.

The stability of liquid-film flows is essential in many industrial applications. In the dip-coating process, a liquid film forms over a substrate extracted at a constant speed from a bath. We studied the linear stability of this film considering different nondimensional thicknesses hhat for four liquids, spanning an extensive range of Kapitza numbers (Ka). By solving the Orr–Sommerfeld eigenvalue problem with the Chebyshev–Tau spectral method, we calculated the threshold between growing and decaying perturbations, investigated the instability mechanism, and computed the absolute/convective threshold. The instability mechanism was studied by analysing the perturbations’ vorticity distribution and the kinetic energy balance. It was found that liquids with low Ka (e.g. corn oil, Ka=4) are stable for a smaller range of wavenumbers compared with liquid with high Ka (e.g. liquid zinc, Ka=11525). Surface tension has a stabilising and a destabilising effect. For long waves, it curves the vorticity lines near the substrate, reducing the flow under the crests. For short waves, it fosters vorticity production at the free-surface and creates a region of intense vorticity near the substrate. In addition, we discovered that the surface tension contributes to both the production and dissipation of perturbation's energy, depending on the Ka number. Regarding the absolute/convective threshold, we identified a window in the parameter space where unstable waves propagate throughout the entire domain (indicating absolute instability). Perturbations affecting Derjaguin's solution (hhat=1 ) for Ka<17 and the Landau–Levich–Derjaguin solution (hhat=0.945Re^(1/9)Ka^(−1/6)), are advected by the flow (indicating convective instability)

This work investigates the capillary rise dynamics of highly wetting liquids in a divergent U-tube in the microgravity conditions provided by 78th European Space Agency (ESA) parabolic flight. This configuration produces a capillary-driven channel flow. We use image recording in backlight illumination to characterize the interface dynamics and dynamic contact angle of HFE7200 and Di-Propylene Glycol (DPG). For the case of HF7200, we complement the interface measurements with Particle Tracking Velocimetry (PTV) to characterize the velocity fields underneath the moving meniscus. In the DPG experiments, varying liquid column heights are observed, with a notable decrease in meniscus curvature when the contact line transitions from a pre-wetted to a dry substrate. In contrast, for HFE7200, the interface consistently advances over a pre-wetted surface. Despite this, a reduction in meniscus curvature is detected, attributed to inertial effects within the underlying accelerating flow. PTV measurements reveal that the region where the velocity profile adapts to the meniscus velocity decreases as interface acceleration increases, suggesting a direct relationship between acceleration and the velocity adaptation length scale.

This paper presents the development of fluorescent tracer particles for use in gas flows as a countermeasure for undesired strong light reflections on surfaces of channel walls or obstacles and as a label for the discrimination of multi-constituent flows. The employment of fluorescent dye-doped tracer particles with a wavelength-specific optical filter enables the separation of the Stokes-shifted particle light emission from reflections on surfaces and Mie scattering from non-fluorescing particles. The fluorescent particles were made of Pyrromethene 567 (P567) and Di-Ethyl-Hexyl-Sebacate (DEHS), and the addition of P567 was not found to alter the characteristics of the particles generated. Investigations in a low-speed wind tunnel revealed that the intensity of fluorescent emission is proportional to the dye concentration at least up to 2.0gl-1$2.0\,\hbox {g l}^{-1}$. The efficacy of reflection removal was investigated in a setup with a metal turbine blade placed in the flow and a laser sheet oriented to impinge the blade surface. With the installation of an appropriate optical filter, undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Finally, the performance of the modified DEHS was compared to conventional DEHS with the measurement of a canonical turbulent boundary layer (TBL). The flow was globally seeded with conventional DEHS and the TBL was locally seeded with fluorescing DEHS; simultaneous imaging with a notch filter confirmed that the flow is accurately tracked by the modified DEHS without additional bias. Furthermore, this indicated the possibility of using the newly developed particles to segregate portions of a flow with multiple constituents.

Outdoor sound propagation is modified by random fluctuations in the acoustic refractive index due to atmospheric turbulence. This phenomenon, known as acoustic scintillation, results in fluctuations in the amplitude and phase as well as reduced spatial coherence of the sound waves. Recently, a physics-based model was proposed to construct time domain realizations of the amplitude and phase fluctuations for application in the auralization of outdoor sound sources. The approach assumed the frequency-dependent fluctuations, considered in separate 1/n octave frequency bands, are either fully correlated or fully decorrelated. This paper extends the method by including specific model-based cross-frequency correlation in the fluctuations. The approach is first presented, followed by examples of applications and a subjective evaluation of auralized samples with and without cross-frequency correlation.

We experimentally confirm the efficacy of submerged oscillating baffles for sloshing mitigation in a partially filled open container of water. As with the more typical applications of tuned mass dampers, when the natural frequency of the baffle is close to that of the first sloshing mode, the main resonance peak is split into two peaks of lower amplitude. The reduction in resonant sloshing amplitude is superior to that of a fixed baffle.

The coupling between fluid-structure interactions is governed by the pressure distribution over the interaction surface between the fluid and solid domains. The capabilities of non-intrusive optical techniques, such as particle image velocimetry and Lagrangian particle tracking (LPT), have been proven to provide accurate velocity and acceleration information within the flow field while simultaneously tracking the corresponding structural deformations. However, scattered data from LPT measurements are typically mapped onto Cartesian grids, independently of the shape of the solid objects in the measurement domain. The use of Cartesian grids poses challenges for the determination of the surface pressure because the velocity gradients close to the object’s surface are not captured accurately. Therefore, an alternative surface pressure reconstruction scheme utilizing LPT data based on the arbitrary Lagrangian–Eulerian approach is proposed to mitigate the error propagation associated with the use of uniform grids. The introduced method provides an exact surface conformation utilizing boundary fitted coordinate systems and radial basis function based mesh deformations, which eliminates the need to use extrapolations to obtain surface pressure distributions. The introduced approach is assessed by means of a synthetic hill surface probing a three-dimensional analytical flow field; its practical applicability is demonstrated through an experimental characterization of turbulent boundary layer interactions with a steadily and unsteadily deforming elastic membrane.

We propose a method to obtain super-resolution of turbulent statistics for three-dimensional ensemble particle tracking velocimetry (EPTV). The method is “meshless” because it does not require the definition of a grid for computing derivatives, and it is “binless” because it does not require the definition of bins to compute local statistics. The method combines the constrained radial basis function (RBF) formalism introduced Sperotto et al. (Meas Sci Technol 33:094005, 2022) with an ensemble trick for the RBF regression of flow statistics. The computational cost for the RBF regression is alleviated using the partition of unity method (PUM). Three test cases are considered: (1) a 1D illustrative problem on a Gaussian process, (2) a 3D synthetic test case reproducing a 3D jet-like flow, and (3) an experimental dataset collected for an underwater jet flow at Re=6750$\text {Re} = 6750$ using a four-camera 3D PTV system. For each test case, the method performances are compared to traditional binning approaches such as Gaussian weighting (Agüí and Jiménez in JFM 185:447–468, 1987), local polynomial fitting (Agüera et al. in Meas Sci Technol 27:124011, 2016), as well as binned versions of RBFs.

Subcooled nucleate flow boiling encompasses intricate simultaneous condensation and evaporation processes. It involves thin liquid microlayers trapped beneath growing bubbles, enabling high heat and mass transfer with fluxes exceeding 1 MW/m². Understanding microlayer contribution to bubble growth is pivotal for developing reliable boiling models. Unlike previous studies, we account for condensation effects, important in the context of subcooled boiling regime, in estimating microlayer contribution by simultaneously obtaining microlayer dynamics from thin-film interferometry and whole-field temperature from rainbow schlieren deflectometry. We establish that the microlayer evaporation significantly influences bubble growth in flow boiling, contributing up to 60% (in growth phase) in the present study.

We investigate the dynamics of a gas jet impinging perpendicular to a thin liquid film dragged by a rising vertical substrate. This configuration is relevant to the jet-wiping process in hot-dip galvanization and it is unstable. Previous studies analysed the dynamics of the instability in the case of liquids with low Kapitza numbers (highly viscous liquids), more amenable to experimental and numerical investigations. This work extends the previous investigations by focusing on the wiping at much higher Kapitza numbers, which are more relevant to the galvanizing process. The simulations are carried out by combining volume of fluid and large-eddy simulations, and the dynamics of the gas–liquid interaction is analysed using extended multiscale proper orthogonal decomposition. The simulations allowed for analysing the jet-wiping instability in new flow conditions. Despite the largely different conditions, the results show that the interaction between the gas jet and the liquid film is qualitatively similar, featuring two-dimensional waves in the liquid correlated with oscillations and deflections of the gas jet in all cases. The wave characteristics (e.g. frequency and propagation speed) scale remarkably well using the Shkadov-like scaling based on the liquid, suggesting a dominant role of the liquid film in the coupling, and potentially enabling extrapolation of the results to a broader range of wiping conditions. Finally, we use the numerical results to discuss the limitations of liquid-film models, which constitute currently the only possible approach to study the jet-wiping process in industrial conditions.

Hummingbirds and insects achieve outstanding flight performance by adapting their flapping motion to the flight requirements. Their wing kinematics can change from smooth flapping to highly dynamic waveforms, generating unsteady aerodynamic phenomena such as leading-edge vortices (LEV), rotational circulation, wing wake capture, and added mass. This article uncovers the interactions of these mechanisms in the case of a rigid semi-elliptical wing undergoing aggressive kinematics in the hovering regime at Re∼O(103). The flapping kinematics were parametrized using smoothed steps and triangular functions and the flow dynamics were simulated by combining the overset method with large eddy simulations. The analysis of the results identifies an initial acceleration phase and a cruising phase. During the former, the flow is mostly irrotational and governed by the added mass effect. The added mass was shown to be responsible for a lift first peak due to the strong flapping acceleration. The dynamic pitching and the wing wake interaction generate a second lift peak due to a downwash flow and a vortex system on the proximal and distal parts of the wing's pressure side. Conversely, aerodynamic forces in the cruising phase are mainly governed by the growth and the establishment of the LEV. Finally, the leading flow structures in each phase and their impact on the aerodynamic forces were isolated using the extended proper orthogonal decomposition.

In recent years, supersonic aviation has regained attention by the research community and industrial actors alike. Certainly, the environmental constraints and hypersonic cruise opportunities add new underexplored dimensions. The MORE &LESS consortium builds upon previous European efforts to develop high-speed civil air transportation by assessing a broad spectrum of high-speed flight, ranging from Mach 2 up to Mach 8. There exists no single propulsion concept that is able to power an aircraft over this wide range. This chapter investigates propulsion concepts at both ends of this velocity spectrum. At the low end, this consists of a turbojet with afterburning which powers a Concorde-like airframe. On the high end, a combined cycle engine merges six air-turbo rockets and a dual-mode ramjet/scramjet (DMR) engine to power a waverider concept. The tools used to characterise the engine performance require a high computational efficiency in order to allow for quick design iterations and automated optimisation procedures. Furthermore, to ensure high simulation accuracy without jeopardising computational efficiency, a reduced-order modelling framework built on high-fidelity simulations is used.