arXiv subjects

Bharathram Ganapathisubramani

Publications and source records attributed to Bharathram Ganapathisubramani.

At least 19 recordsLinked to original sources

arXiv2026

Efficient three-dimensional variational data assimilation of multi-plane PIV data

We perform three-dimensional variational data assimilation (3DVar) using a discrete adjoint approach to optimise the time-averaged momentum equations. The experimental data consist of sparse stereoscopic particle image velocimetry (PIV) measurements collected along $12$ cross-stream planes in the wake of a vehicle-like bluff body at a Reynolds number $Re_L = 5.64 \times 10^5$ based on the streamwise body length. Adjoint localisation is proposed and implemented to reduce the memory footprint of the discrete adjoint method for spatially-varying control variables in 3DVar by confining the control variable space to a user-defined subdomain. Restricting the control variable to $12$ % of the full control space yields a maximum reduction in peak memory of $64$ %, while producing assimilated fields of comparable fidelity with respect to mean velocity and the optimised momentum forcing field. The localised adjoint case improves upon the baseline Spalart--Allmaras turbulence model and recovers the correct asymmetric topology of the complex three-dimensional (3D) recirculation bubble. The assimilated Reynolds shear stress agrees well with the experiment, and the assimilated mean pressure is shown to be physically consistent when correlated with the in-plane vorticity fields. A data efficiency study is also performed, in which the number of planes provided for assimilation is progressively reduced, demonstrating that the data coverage must extend at least to the end of the primary recirculation bubble to adequately constrain the near-wake dynamics. The efficiency that adjoint localisation affords is crucial for assimilating sparse, experimental data for 3D separated flows on fine meshes that can tackle industrial problems of interest.

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arXiv2026

Effects of spatially localised pressure gradient histories on recovery of turbulent boundary layers

Hot-wire anemometry is used to investigate the recovery of smooth-wall turbulent boundary layers from spatially localised (i.e. impulsive) pressure gradient history (PGH) effects. Measurements are performed at multiple stations downstream of spatial distributions of favourable-adverse pressure gradient sequences, followed by relaxation to zero-pressure-gradient (ZPG) conditions. The analysis focuses on matched friction Reynolds numbers at $Re_τ\approx 2300$, $3000$, and $5500$, where the local Clauser pressure gradient (PG) parameter $β$ is nominally matched at $1.7$, $0.6$, and $-0.1$, respectively. PGH strength is quantified using the integral history parameter $Δβ$, proposed by Preskett et al. (J. Fluid Mech., vol. 1010, 2025, A30), which allows isolation of PGH as the primary source of variation. The imposed PGH amplifies the wake component of the mean velocity profile and enhances the streamwise Reynolds stress throughout the boundary layer, including the emergence of an outer peak. Spectral analysis reveals an additional outer-layer energetic feature with streamwise length scales of $2$-$3δ$ ($δ$ is the local boundary layer thickness), identified as the PG peak, distinguishable from the very-large-scale motion (VLSM). Even after $β$ has relaxed (to zero) for sufficiently long distances, mean flow has not recovered to ZPG state. Once $Δβ\lesssim 0.1$, mean flow and inner/log-layer turbulence statistics recover; however, the outer-layer turbulence retains a persistent imprint of PGH. Finally, we observe that recovery involves reorganisation of large-scale structures - where VLSMs appear to be shorter even after the PG peak has vanished - which indicates prolonged history effects.

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arXiv2026

To stall-cell or not to stall-cell: Variational data assimilation of 3D mean flow past a stalled airfoil

The full-field reconstruction of three-dimensional (3D) turbulent flows from sparse experimental measurements remains a significant challenge, particularly for flows exhibiting complex 3D flow separation. In this work, we address this challenge for the case of stall cells - spanwise coherent structures that form on the suction surface of wings at post-stall conditions. Planar particle image velocimetry (PIV) experiments are performed on a NACA 0012 wing at a chord-based Reynolds number of $Re_c \approx 450{,}000$ and angle of attack $α= 14^\circ$, acquiring two-component mean velocity data on four spanwise planes. The experimental data show clear spanwise variation in the extent of the separation and flow dynamics, consistent with the presence of stall cells. Three-dimensional variational (3DVar) data assimilation (DA) within the field inversion framework is then employed to reconstruct the full 3D mean flow field by augmenting these sparse planar measurements with the Spalart--Allmaras (SA) Reynolds-averaged Navier--Stokes (RANS) turbulence model. The performance of the reconstruction is assessed on planes not used in the assimilation. It is shown that a single plane of sparse experimental data is sufficient to recover the essential features of a stall cell, including counter-rotating vortices around focal points on the suction surface. The lowest reconstruction error is obtained when two planes of data that are close together but exhibit markedly different separation extents are used, and the complementary roles of the reference data placement and the computational boundary conditions in shaping the reconstructed stall cell structure are explained. These results demonstrate the capability of 3DVar DA to reconstruct the full 3D physics of stall cells from two-component velocity data acquired on select spanwise planes.

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arXiv2026

Assimilating rough features: A data-driven framework to infer rough wall properties from sparse experimental data

Surface roughness influences turbulent boundary layers (TBLs) primarily through the roughness function $ΔU^+$ and the equivalent sand-grain roughness height $k_s$. Direct determination of $k_s$ typically requires detailed velocity and wall-shear stress measurements, which are often impractical. As an alternative, this study presents a data assimilation framework that modifies a smooth-wall Reynolds-Averaged Navier-Stokes (RANS) baseline to match sparse rough-wall particle image velocimetry (PIV) data in the fully rough regime. Through this approach, secondary variables such as the friction velocity, $u_τ$, and $k_s$ can be inferred from the assimilated flow fields. The assimilated TBL reproduces experimental velocity profiles within 1\% and predicts friction velocity within 1-6\% of the experimental measurements. Furthermore, the $k_s$ values inferred from the assimilation also match the experimental data up to 1\%. These results demonstrate the potential of data assimilation as a cost-effective alternative to high-fidelity methods and support the generalisation of the framework to model streamwise-varying roughness by treating $k_s$ as a function of fetch length.

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arXiv2026

Mixed data-source transfer learning for a turbulence model augmented physics-informed neural network

Physics-informed neural networks (PINNs) have recently emerged as a promising alternative for extracting unknown quantities from experimental data. Despite this potential, much of the recent literature has relied on sparse, high-fidelity data from direct numerical simulations (DNS) rather than experimental sources like particle image velocimetry (PIV), which are not suitable for validating all reconstructed quantities. In the case of PIV, for example, pressure is not directly measured and the data have imperfections such as noise contamination or a limited field of view. To overcome these limitations, we present a novel methodology where PINNs are first trained on a RANS simulation such that it learns all states at every location in the domain. We then apply transfer learning which updates the PINN using sub-sampled PIV data. The resulting predictions are in significantly better agreement with the full PIV dataset than PINNs which are trained on experimental data only. This work builds on the recent literature by integrating a Spalart-Allmaras turbulence model and applying hard constraints to the no-slip wall boundary condition. We apply this new methodology to a two-dimensional NACA 0012 airfoil inclined at an angle of attack, $α$ = 15 degrees, for two Reynolds numbers of Re = 10,000 and 75,000. The proposed methodology is initially validated using large eddy simulation (LES) data and then demonstrated on experimental PIV data. Our transfer learning approach results in improved predictions and a reduction in training time when compared to using a random network initialisation.

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arXiv2025

Boundary layer flow dynamics of propulsive flapping foils with increasing Reynolds numbers

We study flapping foils at optimally propulsive Strouhal number $St=0.3$ with increasing chord-based Reynolds number at $Re = 10^4$, $10^5$, and $10^6$ to examine changes in their unsteady boundary layers. Despite being prescribed the same freestream, the inner boundary layer characteristics exhibit different trends due to the generation of leading-edge vortices (LEVs) and their advection into the downstream flow. Propulsive flapping foils show an extended laminar region known as the relaminarization, during which the velocity profiles deviate from the standard log law. This relaminarization is accompanied by a significant decrease in the cyclic fluctuation of the wall friction coefficient and an increase in the shape factor while the freestream velocity increases under favourable pressure gradient conditions. This phenomenon intensifies with increasing $Re$. We found that higher $Re$ produces smaller LEVs in greater quantities, with a more rapid but stable breakdown, without resulting in a more chaotic turbulent downstream. This study strongly indicates that the relaminarization could extend beyond $Re=10^6$ as predicted by Fukagata[Annual Review of Fluid Mechanics, 2023]. The results support the potential for further exploration of flapping foils at high $Re$ for noise and drag reduction.

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arXiv2025

Experimental investigation of ridge-induced secondary motions in turbulent channel flows

Many engineering and environmental surfaces exhibit spatial heterogeneity in the spanwise direction and encompass multiple surface length scales. When the dominant spanwise length scale is on the order of the largest flow scales (e.g., the boundary layer thickness or channel half-height, delta), localized delta-scale secondary flows can form. These secondary-flow generating heterogeneous surfaces are typically classified as "ridge-type" (with spanwise variation in surface elevation) and "strip-type" (spanwise variation in skin-friction including through variation in roughness characteristics). Both types of surfaces have been explored in previous studies at high Reynolds numbers using experiments with focus on a variety of characteristics such as mean flow, turbulent statistics and structure, while lower Reynolds number studies in channels have examined influence on skin-friction in addition to flow characteristics and mechanisms. In this work, we augment the previous work through experiments on ridge-type roughness in turbulent channel flows over a large range of Reynolds numbers. Our findings reveal that the global skin friction, as inferred through pressure drop in the channel along the streamwise direction, of these surfaces exhibits a log-linear asymptotic behaviour that is characteristic of a fully-rough surface at sufficiently high Reynolds numbers. Yet, the roughness function where this is observed is "low" compared to other established homogeneous rough surfaces. Moreover, the mean flow, turbulent statistics, and two-point correlation analyses indicate that the secondary flow structure, spatial extent, and magnitude remain largely invariant over the Reynolds number range considered. These findings have important implications on predicting drag of these heterogeneous surfaces as well as developing new models for the flow structure.

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arXiv2025

Three-dimensional variational data assimilation of separated flows using time-averaged experimental data

We present a novel framework for assimilating planar PIV experimental data using a variational approach to enhance the predictions of the Spalart-Allmaras RANS turbulence model. Our method applies three-dimensional constraints to the assimilation of mean velocity data, incorporating a corrective forcing term in the momentum equations. The advantages of this approach are highlighted through a direct comparison with traditional two-dimensional assimilation using the same experimental dataset. We demonstrate its efficacy by assimilating the deep stall flow over a NACA0012 airfoil at a $15^\circ$ angle of attack and a chord-based Reynolds number of $Re_c \approx 7.5 \times 10^4$. We find that in two-dimensional assimilation, the corrective forcing term compensates not only for physical modeling errors but also for the lack of divergence in the experimental data. This conflation makes it difficult to isolate the effects of measurement inconsistencies from deficiencies in the turbulence model. In contrast, three-dimensional assimilation allows the corrective forcing term to primarily address experimental setup errors while enabling the turbulence model to more accurately capture the flow physics. We establish the superiority of three-dimensional assimilation by demonstrating improved agreement in reconstructed quantities, including pressure, lift force, and Reynolds shear stress.

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arXiv2025

Effects of pressure gradient histories on skin friction and mean flow of high Reynolds number turbulent boundary layers over smooth and rough walls

Experiments are conducted over smooth and rough walls to explore the influence of pressure gradient histories on skin friction and mean flow of turbulent boundary layers. Different pressure gradient histories are imposed on the boundary layer through an aerofoil mounted in the freestream. Hot-wire measurements are taken at different freestream velocities downstream of the aerofoil where the flow has locally recovered to zero pressure gradient but retains the history effects. Direct skin friction measurements are also made using oil film interferometry for smooth walls and a floating element drag balance for rough walls. The friction Reynolds number, $Re_τ$, varies between $3000$ and $27000$, depending both on the surface conditions and the freestream velocity ensuring sufficient scale separation. Results align with previous findings, showing that adverse pressure gradients just upstream of the measurement location increase wake strength and reduce the local skin friction while favourable pressure gradients suppress the wake and increase skin friction. The roughness length scale, $y_0$, remains constant across different pressure gradient histories for rough wall boundary layers. Inspired by previous works, a new correlation is proposed to infer skin friction based on the mean flow. The difference in skin friction between an arbitrary pressure gradient history and zero pressure gradient condition can be predicted using only the local wake strength parameter ($Π$), and the variations in wake strength for different histories are related to a weighted integral of the pressure gradient history normalised by local quantities. This allows us to develop a general correlation that can be used to infer skin friction for turbulent boundary layers experiencing arbitrary pressure-gradient histories.

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arXiv2024

Effects of fetch length on turbulent boundary layer recovery past a step-change in surface roughness

Recent studies focusing on the response of turbulent boundary layers (TBL) to a step-change in roughness have provided insight into the scaling and characterisation of TBLs and the development of the internal layer. Although various step-change combinations have been investigated, ranging from smooth-to-rough to rough-to-smooth, the "minimum" required roughness fetch length over which the TBL returns to its homogeneously rough behaviour remains unclear. Moreover, the relationship between a finite- and infinite-fetch roughness function (and the equivalent sandgrain roughness) is also unknown. In this study, we determine the minimum "equilibrium fetch length" for TBL developing over a smooth-to-rough step-change as well as the expected error in local skin friction if the fetch length is under this minimum threshold. An experimental study is carried out where the flow is initially developed over a smooth wall, and then a step-change is introduced using patches of P24 sandpaper. 12 roughness fetch lengths are tested in this study, systematically increasing from $L = 1δ_2$ up to $L = 39δ_2$ (where \textit{L} is the roughness fetch length and $δ_2$ is the TBL thickness of the longest fetch case), measured over a range of Reynolds numbers ($4\cdot10^2 \leq Re_τ\leq 2\cdot10^5$). Results show that the minimum fetch length needed to achieve full equilibrium recovery is around $20δ_2$. Furthermore, we observe that $C_f$ recovers to within 10\% of its recovered value for fetch lengths $\geq 5δ_2$. This information allows us to incorporate the effects of roughness fetch length on the skin friction and roughness function.

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arXiv2024

Resolvent analysis of a swimming foil

This study employs resolvent analysis to explore the dynamics and coherent structures in the boundary layer of a foil that swims via a travelling wave undulation. A modified NACA foil shape is used together with undulatory kinematics to represent fish-like bodies at realistic Reynolds numbers ($ \mathit{Re} = 10,000 $ and $ \mathit{Re} = 100,000 $) in both thrust- and drag-producing propulsion regimes. We introduce a novel coordinate transformation that enables the implementation of the data-driven resolvent analysis \citep{herrmann_data-driven_2021} to dissect the stability of the boundary layer of the swimming foil. This is the first study to implement resolvent analysis on deforming bodies with non-zero thickness and at realistic swimming Reynolds numbers. The analysis reinforces the notion that swimming kinematics drive the system's physics. In drag-producing regimes, it reveals breakdown mechanisms of the propulsive wave, while thrust-producing regimes show a uniform wave amplification across the foil's back half. The key thrust and drag mechanisms scale with the boundary-layer thickness, implying geometric self-similarity in this $\mathit{Re}$ regime. In addition, we identify a mechanism that is less strongly coupled to the body motion. We offer a comparison to a rough foil that reduces the amplification of this mechanism, demonstrating the potential of roughness to control the amplification of key mechanisms in the flow. The results provide valuable insights into the dynamics of swimming bodies and highlight avenues for developing opposition control strategies.

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arXiv2024

Effects of porous substrates on the structure of turbulent boundary layers

Three different porous substrates (with different pore sizes, s, and permeabilities, K) are used to examine their effect on the structure of boundary layer flow over them. The flow is characterised with single-point hot-wire measurements as well as planar Particle Image Velocimetry. In order to elucidate differences in shallow and deep flows past porous substrate, foams with two different thickness (h) are used (for all three substrates). A wide range of Friction Reynolds number (2000< Retau < 15000) and Permeability based Reynolds number (1 1) and dense (relative to viscous scales) foam at the thick substrate limit (h/s>10), which leads to the breakdown of outer-layer similarity. As porosity is increased, a thin substrate limit is reached (h/s), and the foam becomes sparse relative to viscous scales (s+ > 100). For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Such substrates are unable to attenuate velocity disturbances and the dependence of substrate thickness (h/s) on spectral energy content of turbulent fluctuations ceases to exist. The present study shows that transition from thick to thin substrate flow behavior depends not only on thickness-to-pore ratio (h/s) but also on substrate density relative to viscous scales of the flow.

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arXiv2023

A systematic investigation into the effect of roughness on self-propelled swimming plates

This study examines the effects of surface topography on the flow and performance of a Self-Propelled Swimming (SPS) body. We consider a thin flat plate with an egg-carton roughness texture undergoing prescribed undulatory swimming kinematics at a Strouhal number of $0.3$ and tail amplitude to length ratio of $0.1$; we use plate Reynolds numbers of $\mathit{Re}=6,12,$ and $24\times 10^3$, and focus on $12,000$. As the roughness wavelength is decreased, we find that the undulation wave speed must be increased to overcome the additional drag from the roughness and maintain SPS. Correspondingly, the extra wave speed raises the power required to maintain SPS, making the swimmer less efficient. To decouple the roughness and the kinematics, we compare the rough plates to equivalent smooth cases by matching the kinematic conditions. We find that all but the longest roughness wavelengths reduce the required swimming power and the unsteady amplitude of the forces when compared to a smooth plate undergoing identical kinematics. Additionally, roughness can enhance flow enstrophy by up to 116\% compared to the smooth cases without a corresponding spike in forces; this suggests that the increased mixing is not due to increased vorticity production at the wall. Instead, the enstrophy is found to peak strongly when the roughness wavelength is approximately twice the boundary layer thickness over the $\mathit{Re}$ range, indicating the roughness induces large-scale secondary flow structures that extend to the edge of the boundary layer. This study reveals the nonlinear interaction between roughness and kinematics beyond a simple increase or decrease in drag, illustrating that roughness studies on static shapes do not transfer directly to unsteady swimmers.

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arXiv2023

Effects of surface roughness on the propulsive performance of pitching foils

The hydrodynamic influence of surface texture on static surfaces ranges from large drag penalties (roughness) to potential performance benefits (shark-like skin). Although, it is of wide-ranging research interest, the impact of roughness on flapping systems has received limited attention. In this work, we explore the effect of roughness on unsteady performance of a harmonically pitching foil through experiments using foils with different surface roughness, at a fixed Strouhal number and within the Reynolds number (Re) range of 15k-30k. The foils' surface roughness is altered by changing the distribution of spherical-cap shaped elements over the propulsor area. We find that the addition of surface roughness does not improve the performance compared to a smooth surface over the Re range considered. The analysis of the flow fields shows near identical wakes regardless of the foil's surface roughness. The performance reduction mainly occurs due to an increase in profile drag. However, we find that the drag penalty due to roughness is reduced from 76% for a static foil to 16% for a flapping foil at the same mean angle of attack, with the strongest decrease measured at the highest Re. Our findings highlight that the effect of roughness on dynamic systems is very different than that on static systems, thereby, cannot be accounted for by only using information obtained from static cases. This also indicates that the performance of unsteady, flapping systems is more robust to the changes in surface roughness.

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arXiv2022

Scaling of secondary flows with surface parameters: a linear approach

Secondary flows are generated when a lateral variation of the topography, such as streamwise aligned ridges, is imposed to a turbulent wall-bounded flow. In this case, the flow field is characterized by vortical structures developing along the streamwise direction known as Prandt's vortices of the second kind (Prandtl 1965). As demonstrated in previous experimental and numerical works, the strength and flow organization of these turbulent structures depend on the ridge shape. In this paper, the effect of the ridge geometry on the generation of secondary currents is investigated using the linearised RANS-based model proposed by Zampino (2022). Here, symmetric channels with rectangular, triangular and elliptical ridges are studied and the secondary flows are compared in order to highlight the main differences and similarities. The analogies of the flow organisation between the three geometries suggest that the secondary currents do not depend on the ridge shape when the ridges are small and isolated, and the strength of secondary flows collapses when properly scaled with the mean ridge height. Finally, the generation of secondary flows and the effect of the ridge shape on the flow organisation is studied in detail for the complex geometries. In particular, for trapezoidal ridges, we observed that tertiary flows emerge for the ridges where the scaling behaviour does not hold.

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arXiv2022

The influence of freestream turbulence on the development of a wind turbine wake

The wake of an isolated model-scale wind turbine is analysed in a set of inflow conditions having freestream turbulence intensity between 3 % and 12 %, and integral time scales in the range of 0.1 to 10 times the convective timescale based on the turbine diameter. It is observed that the wake generated by the turbine evolves more rapidly, with the onset of the wake evolution being closer to the turbine, for high turbulence intensity and low integral time scale flows, in accordance with literature, while flows at higher integral time scales result in a slow wake evolution, akin to that generated by low-turbulence inflow conditions despite the highly-turbulent ambient condition. The delayed onset of the wake evolution is connected to the stability of the shear layer enveloping the near wake, which is favoured for low-turbulence or high-integral time scale flows, and to the stability of the helical vortex set surrounding the wake, as this favours interaction events and prevents momentum exchange at the wake boundary which hinder wake evolution. The rate at which the velocity in the wake recovers to undisturbed conditions is instead analytically shown to be a function of the Reynolds shear stress at the wake centreline, an observation that is confirmed by measurements. The rate of production of Reynolds shear stress in the wake is then connected to the power harvested by the turbine to explain the differences between flows at constant turbulence intensity and different integral time scales.

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arXiv2022

Linearised Reynolds-Averaged predictions of secondary currents in turbulent channels with topographic heterogeneity

A rapid predictive tool based on the linearised Reynolds-averaged Navier-Stokes equations is proposed in this work to investigate secondary currents generated by streamwise-independent surface topography modulations in turbulent channel flow. The tool is derived by coupling the Reynolds-averaged momentum equation to the Spalart-Allmaras transport equation for the turbulent eddy viscosity, using a nonlinear constitutive relation for the Reynolds stresses to capture correctly secondary motions. Linearised equations, describing the steady flow response to arbitrary surface modulations, are derived by assuming that surface modulations are shallow. Since the equations are linear, the superposition principle holds and the flow response induced by an arbitrary modulation can be obtained by combining appropriately the elementary responses obtained over sinusoidal modulations at multiple spanwise length scales. The tool permits a rapid exploration of large parameter spaces characterising structured surface topographies previously examined in the literature. Here, channels with sinusoidal walls and with longitudinal rectangular ridges are considered. For sinusoidal walls, a large response is observed at two spanwise wavelengths scaling in inner and outer units respectively, mirroring the amplification mechanisms in turbulent shear flows observed from transient growth analysis. For longitudinal rectangular ridges, the model suggests that the analysis of the response and the interpretation of the topology of secondary structures is facilitated when the ridge width and the gap between ridges are used instead of other combinations proposed in the literature.

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arXiv2021

Time-averaged velocity and scalar fields of the flow surrounding a group of cylinders

We examine the characteristics of mean flow and scalar concentration characteristics of a turbulent boundary layer flow impinging on a cluster of tall obstacles (which can also be interpreted as a porous obstruction). The cluster is created with a group of cylinders of diameter $d$ and height $h$ arranged in a circular patch of diameter $D$.The solidity of the patch/obstruction is defined by $ϕ$ (the total planar area covered by cylinders), which is systematically varied ($0.098 \leq ϕ\leq 1$) by increasing the number of cylinders in a patch ($N_c$). A point source is placed at ground level upstream of the patch and its transport around the patch is examined. Time-averaged velocity and scalar fields, obtained from simultaneous planar PIV-PLIF (particle image velocimetry-planar laser-induced fluorescence) measurements, reveal that the characteristics of wake and flow above porous patches are heavily influenced by $ϕ$. In particular, we observe that the horizontal and vertical extent of the wake and scalar concentration downstream of the patches decreases and increases with $ϕ$, respectively. Here, the recirculation bubble is shifted closer to the trailing edge of the patches as $ϕ$ increases, limiting the flow from convecting downstream, decreasing the scalar concentration and virtually `extending' the patch in streamwise direction. As the bubble forms in the trailing edge, vertical bleeding increases and hence the concentration increases above patch where the cylinders appear to be `extend' vertically towards the freestream.

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