Bharathram Ganapathisubramani

Bharathram Ganapathisubramani
University of Southampton · Aerodynamics and Flight Mechanics Group,

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245
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Publications

Publications (245)
Article
Full-text available
We conduct a well-controlled model experiment for a wide variety of canopy flows. Examples of these include engineering flows such as wind flow, dispersion of scalars through and over urban areas, and the convective heat transfer in many heat exchangers, as well as natural canopies such as flows through terrestrial or aquatic vegetation. We aim to...
Preprint
Full-text available
In this work, we describe the impact of aspect ratio (AR) on the performance of optimally phased, identical flapping flippers in a tandem configuration. Three-dimensional simulations are performed for seven sets of single and tandem finite foils at a moderate Reynolds number, with thrust producing, heave-to-pitch coupled kinematics. Increasing slen...
Article
This report documents a large scale joint research project with the aim of improving the efficiency of ship operations and management by providing a methodology and technology that can quantify the emission and fuel usage penalty due to bio-fouling on ship hull. This can be obtained through better understanding of turbulent boundary layer flows ove...
Conference Paper
Full-text available
Effects of the Hull Vane® positioning in the longitudinal direction on ship resistance are studied with the aid of computational fluid dynamics (CFD) simulations, verified using data from towing tests. A 31 m hard-chine crew boat was considered with three variations of vane’s positioning: vane’s leading edge in line with the transom, vane’s leading...
Article
Full-text available
The motion of thin curved falling particles is ubiquitous in both nature and industry but is not yet widely examined. Here, we describe an experimental study on the dynamics of thin cylindrical shells resembling broken bottle fragments settling through quiescent fluid and homogeneous anisotropic turbulence. The particles have Archimedes numbers bas...
Article
Full-text available
In this experimental study, multiscale rough surfaces with regular (cuboid) elements are used to examine the effects of roughness-scale hierarchy on turbulent boundary layers. Three iterations have been used with a first iteration of large-scale cuboids onto which subsequent smaller cuboids are uniformly added, with their size decreasing with a pow...
Article
Although the hull of a recently dry-docked large ship is expected to be relatively smooth, surface scanning and experimentation reveal that it can exhibit an “orange-peel” roughness pattern with an equivalent sand-grain roughness height ks = 0. 101 mm. Using the known ks value and integral boundary layer evolution, a recently cleaned and coated ful...
Article
Full-text available
Recent work has demonstrated the use of sparse sensors in combination with the proper orthogonal decomposition (POD) to produce data-driven reconstructions of the full velocity fields in a variety of flows. The present work investigates the fidelity of such techniques applied to a stalled NACA 0012 aerofoil at Re_c = 75000 at an angle of attack alp...
Article
Four data-driven low-order modelling approaches, Dynamic mode decomposition (DMD) and three other variations (optimal mode decomposition, total-least-squares DMD and high-order DMD), are used to capture the spatio-temporal evolution of fluid-structure interactions. These methods are applied to experimental data obtained in a flow over a flexible me...
Article
The formation and evolution of volumetric flow structures from a rectangular orifice synthetic jet (퐴푅 = 10, 푅푒 = 550, 푆푟 = 0.0117, and 퐿표 = 0.0855푚) impinging on a flat plate is investigated using phase-locked single-camera light-field particle image velocimetry (LF-PIV). An impinging plate is located 24 orifice widths away from the rectangle orif...
Article
The importance of the leading-edge sweep angle of propulsive surfaces used by unsteady swimming and flying animals has been an issue of debate for many years, spurring studies in biology, engineering, and robotics with mixed conclusions. In this work, we provide results from three-dimensional simulations on single-planform finite foils undergoing t...
Article
The ability to impart momentum while maintaining a zero net mass flux renders synthetic jet actuators attractive tools for a wide variety of applications. Implementation of a single synthetic jet actuator for large-scale operations is unrealistic and, as such, an array of actuators is usually desired during flow control processes. The added complex...
Article
We experimentally investigate the surface drag characteristics of a staggered-distributed cube array and its interaction with the turbulent structure of the overlying flow. Instantaneous maps of the pressure field, inferred from in-plane velocity data are used to estimate the forces acting on a target roughness element. Coupled statistics of the fo...
Article
Full-text available
Free-stream turbulence characteristics play an important role in the mechanisms of power harvesting for wind turbines. Acquisitions of power and thrust from a model wind turbine of diameter 0:18m have been carried out in a wind tunnel for a wide range of turbulent base flows, with varying free-stream turbulence intensity in the range between 3% and...
Article
Flapping flight and swimming are increasingly studied due to both their intrinsic scientific richness and their applicability to novel robotic systems. Strip theory is often applied to flapping wings, but such modeling is only rigorously applicable in the limit of infinite aspect ratio (AR) where the geometry and kinematics are effectively uniform....
Article
Characteristics of the skin friction due to streamwise inhomogeneous roughness are investigated by modelling it in a simplified manner using step changes in equivalent sand grain roughness height 푘푠, denoted as P (푘푠 = 81.25 µm), Q (푘푠 = 325.00 µm) and R (푘푠 = 568.75 µm). The selected 푘푠 values represent the common ship-hull roughness, i.e., from li...
Preprint
Full-text available
The motion of thin curved falling particles is ubiquitous in both nature and industry but is not yet widely examined. Here, we describe an experimental study on the dynamics of thin cylindrical shells resembling broken bottle fragments settling through quiescent fluid and homogeneous anisotropic turbulence. The particles have Archimedes numbers bas...
Article
Symmetric flapping foils are known to produce deflected jets at high frequency–amplitude combinations even at a zero mean angle of attack. This reduces the frequency range of useful propulsive configurations without side force. In this study, we numerically analyse the interaction of these deflected jets for tandem flapping foils undergoing coupled...
Poster
Full-text available
Have you wondered how you could straighten your deflected flapping-foil vortex wake? As shown in the simulations above, placing a second flapping foil in tandem behind the first can straighten that wake, even when placed five-chord lengths downstream while the flow is highly perturbed! Symmetric flapping foils are known to produce deflected wakes (...
Conference Paper
Fluid-dynamics visualization is not only intended to promote an intricate science but also to help non-scientific audiences to see that there is a beauty in this science. This can be seen in a play of colors to present the turbulent flow which can be easier to do with computational fluid dynamics (CFD) instead of experiments. The fluid visualizatio...
Preprint
Full-text available
The importance of the leading-edge sweep angle of propulsive surfaces used by unsteady swimming and flying animals has been an issue of debate for many years, spurring studies in biology, engineering, and robotics with mixed conclusions. In this work we provide results from an extensive set of three-dimensional simulations of finite foils undergoin...
Preprint
Flapping flight and swimming are increasingly studied both due to their intrinsic scientific richness and their applicability to novel robotic systems. Strip theory is often applied to flapping wings, but such modeling is only rigorously applicable in the limit of infinite aspect ratio (AR) where the geometry and kinematics are effectively uniform....
Article
Full-text available
A surface roughness from a recently cleaned and painted ship hull was scanned, scaled and replicated for laboratory testing to systematically investigatethe influence of the ratio of in-plane roughness wavelength,λ, with respect to the boundary layer thickness δ. The experiments were performed by geometrically scaling the surface which maintains a...
Article
Full-text available
Response of the temporal turbulent boundary layer to decaying free-stream turbulence - Volume 896 - Melissa Kozul, R. Jason Hearst, Jason P. Monty, Bharathram Ganapathisubramani, Daniel Chung
Article
Full-text available
In the published article Ferreira and Ganapathisubramani (2020), Eq. (4) should read instead
Article
Full-text available
This paper addresses the usefulness of leading edge serrations for reducing aerofoil self-noise over a wide range of angles of attack. Different serration geometries are studied over a range of Reynolds number (Re =2e5-4e5). Design guidelines are proposed that permit noise reductions over most angles of attack. It is shown that serration geometries...
Article
Full-text available
Real wind turbines experience a wide range of turbulent shear flows that naturally occur within the atmospheric boundary layer, however, these are often difficult to simulate in experiments. An active grid was used to expand the testable parameter space compared to conventional methods. Specific focus was placed on decoupling the shear from the tur...
Preprint
Flapping foils are known to produce deflected jets at high frequency-amplitude combinations even at a zero mean angle of attack. This reduces the frequency range of useful propulsive configurations without side force. In this study, we analyse numerically the interaction of these deflected jets for tandem flapping foils, undergoing coupled heave to...
Article
Full-text available
Pixel locking in stereoscopic particle image velocimetry (SPIV) is often overlooked, albeit the existence of studies demonstrating an influence on turbulent statistics. Such a bias error occurs when the seeding particles have an image diameter in the range of 1–2 pixels. Together with the advent of superior cameras and more powerful lasers, new ima...
Article
Full-text available
In-plane velocity measurements from PIV are used to estimate the pressure field above and within the canopy of two staggered arrays of cuboids, with distinct height distributions, via 2D-RANS and 2D-TH. The viability of this approach is examined by first comparing the mean drag profiles against reported wind-tunnel measurements that were carried ou...
Article
The properties of the wake generated by a porous body fully immersed in a turbulent boundary layer are experimentally assessed. The body consists of an array of cylinders, with diameter d, covering a circular patch of diameter D. For fixed d and D, by increasing the number of cylinders, Nc, within the patch, the wake properties are systematically t...
Article
Full-text available
This study presents an approach for obtaining full-field dynamic surface-pressure reconstructions with low differential amplitudes. The method is demonstrated in a setup where an air jet is impinging on a flat plate. Deformations of the flat plate under dynamic loading of the impinging jet were obtained using a deflectometry setup that allows measu...
Article
p>This study examines the flow characteristics of a turbulent boundary layer over different sand-grain roughness created by P and P and P sandpapers. The experimental dataset is acquired with high-resolution planar particle image velocimetry in the streamwise-wall-normal plane for a range of Reynolds number between, which consists of a number of tr...
Article
Full-text available
In this study, pressure distributions were reconstructed from phase-locked surface deformation measurements on a thin plate. Slope changes on the plate surface were induced by an external flow interacting with the specimen and measured with a highly sensitive deflectometry setup. The Virtual Fields Method (VFM) was used to obtain pressure reconstru...
Article
Propulsive flapping foils are widely studied in the development of swimming and fly- ing animal-like autonomous systems. Numerical studies in this topic are mainly two- dimensional (2D) studies as they are quicker and cheaper, but, this inhibits the three- dimensional (3D) evolution of the shed vortices from leading- and trailing-edges. In this wor...
Article
The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10^5–4×10^5. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered...
Article
Full-text available
The effect of spanwise heterogeneous surface geometry on the turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness ($S/\delta \approx \mathcal{O}(1)$). C...
Article
Full-text available
This paper describes an experimental investigation of the dynamics of freely falling thin circular disks settling through turbulence. The patterns of the three dimensional disk motion are studied using an orthogonal arrangement of two high speed cameras. Turbulence is generated in a water tank using a random jet array facility where the jets are mo...
Chapter
This study investigated the mean velocity of the near-field wake of a lab-scale wind turbine subjected to seven different incoming turbulent shear flows through particle image velocimetry. An active grid was used to generate the incoming flows with a novel actuation method that decoupled shear from turbulence intensity. The wake geometry relative t...
Conference Paper
Full-text available
Wind tunnel experiments were performed to investigate turbulent flow over an array of heterogeneous roughness elements using stereoscopic particle image velocimetry. Five streamwise planes, covering one periodic cell of a multifractal roughness pattern, are combined to quantify the dispersive stresses, arising from spatial variations in the tempora...
Article
Reversed von Kármán streets are responsible for a velocity surplus in the wake of flapping foils, indicating the onset of thrust generation. However, the wake pattern cannot be predicted based solely on the flapping peak-to-peak amplitude and frequency because the transition also depends sensitively on other details of the kinematics. In this work...
Article
Particle image velocimetry (PIV) measurements are performed to examine the structural organization inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 13% and 8%). The free-st...
Article
Streamwise velocity and wall-shear stress are acquired simultaneously with a hot-wire and an array of azimuthal/spanwise-spaced skin friction sensors in large-scale pipe and boundary layer flow facilities at high Reynolds numbers. These allow for a correlation analysis on a per-scale basis between the velocity and reference skin friction signals to...
Article
A synthetic jet issuing into a cross-flow influences the local velocity of the cross-flow. At the jet exit the jet is oriented in the wall-normal direction while the cross-flow is oriented in the streamwise direction, leading to a momentum transfer between the jet and the cross-flow. Streamwise momentum transferred from the cross-flow to the jet ac...
Article
Full-text available
We experimentally investigate the effects of multiscale rough patches on the drag and flow structure of a fully rough turbulent boundary layer in a wind tunnel. Several patches containing both organized and randomized arrangements of cubes of multiple sizes are tested in order to study the dependence of drag on the frontal solidity of the patch. Th...
Article
Full-text available
The wake characteristics of various thin particles with identical material properties but different frontal geometries (disks, hexagonal plates and square plates) are examined by means of three dimensional measurements of the instantaneous velocity field. The reference particle is a circular disk that lies within the Reynolds number—dimensionless m...
Article
Full-text available
This work presents a methodology for reconstructing full-field surface pressure information from deflectometry measurements on a thin plate using the Virtual Fields Method (VFM). Low-amplitude mean pressure distributions of the order of few O(100) Pa from an impinging air jet are investigated. These are commonly measured point-wise using arrays of...
Article
br/>This study presents findings from a first-of-its-kind measurement campaign that includes simultaneous measurements of the full velocity and vorticity vectors in both pipe and boundary layer flows under matched spatial resolution and Reynolds number conditions. Comparison of canonical turbulent flows offers insight into the role(s) played by fea...
Preprint
Full-text available
Reversed von K\'arm\'an streets are responsible for a velocity surplus in the wake of flapping foils, indicating the onset of thrust generation. However, the wake pattern cannot be predicted based solely on the flapping peak-to-peak amplitude $A$ and frequency $f$ because the transition also depends sensitively on other details of the kinematics. I...
Data
This dataset presents 2D PIV experimental data collected over a range of twelve urban roughnesses with different frontal and plan solidities. These are described in:[1] Placidi, M., & Ganapathisubramani, B. (2015). Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers. Journal of Fl...
Article
Full-text available
In rough-wall boundary layers, wall-parallel non-homogeneous mean-flow solutions exist that lead to so-called dispersive velocity components and dispersive stresses. They play a significant role in the mean-flow momentum balance near the wall, but typically disappear in the outer layer. A theoretical framework is presented to study the decay of dis...
Article
Full-text available
The periodic ingestion and ejection of fluid through an orifice yield vortex rings forming an unsteady jet. The ability to impart momentum with a zero net mass flux makes synthetic jet actuators coveted components in flow control applications. Previous studies underlined the influence of geometrical parameters on synthetic jet evolution. This inves...
Article
Full-text available
In this study, we quantify the accuracy of a simple pressure estimation method from 2D snapshot PIV in attached and separated flows. Particle image velocimetry (PIV) offers the possibility to acquire a field of pressure instead of point measurements. Multiple methods may be used to obtain pressure from PIV measurements, however, the current state-o...
Article
Experimental AerodynamicsEdited by S. Discetti and A. Ianiro CRC Press, Taylor & Francis Group, 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL, 33487-2742, USA. 2017. Distributed by Taylor & Francis Group, 2 Park Square, Milton Park, Abingdon, OX14 4RN, UK. xiii; 468pp. Illustrated. £120. (20% discount available to RAeS members via www.crc...
Article
Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is no...
Conference Paper
Turbulent flow over the porous surface is one of the least documented problems, despite having the potential to affect a wide range of applications. This study presents an experimental investigation of turbulent boundary layer over open cell porous foam where the pore size and foam thickness are systematically varied. Hot-wire anemometry, Planar an...
Presentation
Optical measurements were used to reconstruct low amplitude surface pressure fluctuations in high resolution on a thin plate with the Virtual Fields Method (VFM). Surface slopes induced by an impinging synthetic jet were measured contactless using a highly sensitive deflectometry setup. The periodic slope signals, which were below instantaneous noi...
Article
Full-text available
We experimentally investigate the temporal decay of homogeneous anisotropic turbulence, monitoring the evolution of velocity fluctuations, dissipation and turbulent length scales over time. We employ an apparatus in which two facing random jet arrays of water pumps generate turbulence with negligible mean flow and shear over a volume that is much l...
Article
Following the dimensional analysis approach carried out in previous studies, it is hypothesized that the small-scale fluctuations should only depend on the inner scales, analogous to the Prandtl's law-of-the-wall for the mean flow. This allows us to examine the high frequency regime of the streamwise energy spectra where a \law-of-the-wall" in spec...
Conference Paper
Full-text available
A study in assessing the drag penalty due to hull roughness from a recently cleaned and painted ship hull is reported. The experiment is conducted on an operating ship (Roll-on/roll-off ferry) under steady cruising by measuring the velocity profile directly over the hull using a Laser Doppler Anemometer (LDA). The use of LDA allows a non-intrusive...
Article
Electro-active materials offer, due to their multivariate compliant nature, a great potential for integrating the lift-producing system and the control system into one. This work presents the first step in describing the aerodynamic performance of such materials and focuses on understanding the nature of such materials in aerodynamic applications....
Article
This paper presents a study of the terminal fall velocity, drag coefficient and descent style of ‘wavy-edge’ flat particles. Being highly non-spherical and with a size of up to a few centimeters, these particles show strong self-induced motions that lead to various falling styles that result in distinct drag coefficients. This study is based on exp...
Article
Full-text available
Indirect methods to estimate surface shear stress are commonly used to characterise rough-wall boundary-layer flows. The uncertainty is typically large and often insufficient to carry out quantitative analysis, especially for surface roughness where established scaling and similarity laws may not hold. It is, thus, preferable to rely instead on ind...