arXiv subjects

Sanjiva K. Lele

Publications and source records attributed to Sanjiva K. Lele.

18 recordsLinked to original sources

arXiv2026

Mechanisms of transonic buffet over supercritical airfoils revealed by direct numerical simulations

We use direct numerical simulations to study the turbulent transonic flow over a supercritical airfoil, spanning the transition from stable conditions to fully developed buffet at chord-based Reynolds numbers of $3\times10^5$ and $6\times10^5$. The Reynolds number determines the incoming boundary-layer state, thereby affecting the separation behaviour. Spectral proper orthogonal decomposition reveals a single dominant mode that emerges below onset and persists across the transition to buffet. The energy of this mode increases with the angle of attack while its spatial structure and fundamental frequency remain nearly unchanged, consistent with a global instability that saturates into a nonlinear limit cycle. Geometrical acoustics provides a first-order description of the global mode propagation and identifies two paths connecting the acoustic source, located in the near wake, to the shock. A shorter path impinges on the shock from behind, while a longer one circumvents the shock tip before reaching it from the front. Linearised shock--acoustic interaction theory predicts a stronger shock response to low-frequency, front-impinging acoustic disturbances. Space--time correlations link the establishment of a downstream path to complete boundary-layer separation, which is induced intermittently by the upstream motion of the shock. The sum of the upstream acoustic time along the front-impinging path and the downstream convective time is compatible with the measured buffet period at both Reynolds numbers. Together, these results are consistent with a unified picture of buffet, where an aeroacoustic feedback mechanism provides the physical pathways through which perturbations can sustain the global dynamics.

physics.flu-dyn↗

arXiv2026

Addressing A Posteriori Performance Degradation in Neural Network Subgrid Stress Models

Neural network subgrid stress models often have a priori performance that is far better than the a posteriori performance, leading to neural network models that look very promising a priori completely failing in a posteriori Large Eddy Simulations (LES). This performance gap can be decreased by combining two different methods, training data augmentation and reducing input complexity to the neural network. Augmenting the training data with two different filters before training the neural networks has no performance degradation a priori as compared to a neural network trained with one filter. A posteriori, neural networks trained with two different filters are far more robust across two different LES codes with different numerical schemes. In addition, by ablating away the higher order terms input into the neural network, the a priori versus a posteriori performance changes become less apparent. When combined, neural networks that use both training data augmentation and a less complex set of inputs have a posteriori performance far more reflective of their a priori evaluation.

physics.flu-dyn↗

arXiv2026

Subgrid Stress Modelling with Multi-dimensional State Space Sequence Models

Large Eddy Simulations (LES) are becoming increasingly viable due to the growth in computational power the last few decades, and subgrid stress modelling plays a large role in the accuracy of LES. A new class of neural network models, S4 and S4ND models, allow for learning a continuous representation of the discrete dataset, which facilitates a principled approach to incorporating grid dependence in neural network subgrid stress modelling. A S4ND Unet neural network architecture is proposed and trained on both forced Homogeneous Isotropic Turbulence (HIT) and channel flow, where a priori, it is shown to generalize to grid spacings that are coarser than the training set grid spacings, while simpler artificial neural network (ANN) models fail. A posteriori tests on both forced HIT and channel flow indicate that the S4ND model is more accurate than traditional models and ANN-based models on grid sizes that are in the training set. The S4ND model is also able to generalize to grid sizes that are coarser than the training set a posteriori, and is more accurate than many traditional and neural network subgrid stress models. Finally, the proposed model is also evaluated on flows at increasing Reynolds numbers, where it is seen that the proposed neural network remains stable even at a Reynolds number that is 500,000 times that seen in the training set.

physics.flu-dyn↗

arXiv2025

Task-Based Programming for Adaptive Mesh Refinement in Compressible Flow Simulations

High-order solvers for compressible flows are vital in scientific applications. Adaptive mesh refinement (AMR) is a key technique for reducing computational cost by concentrating resolution in regions of interest. In this work, we develop an AMR-based numerical solver using Regent, a high-level programming language for the Legion programming model. We address several challenges associated with implementing AMR in Regent. These include dynamic data structures for patch refinement/coarsening, mesh validity enforcement, and reducing task launch overhead via task fusion. Experimental results show that task fusion achieves 18x speedup, while automated GPU kernel generation via simple annotations yields 9.7x speedup for the targeted kernel. We demonstrate our approach through simulations of two canonical compressible flow problems governed by the Euler equations.

cs.DC↗

arXiv2024

A closure mechanism for screech coupling in rectangular twin jets

Twin-jet configuration allows two different scenarios to close the screech feedback. For each jet, there is one loop involving disturbances which originate in that jet and arrive at its own receptivity point in-phase (self-excitation). The other loop is associated with free-stream acoustic waves that radiate from the other jet, reinforcing the self-excited screech (cross-excitation). In this work, the role of the free-stream acoustic mode and the guided jet mode as a closure mechanism for twin rectangular jet screech is explored by identifying eligible points of return for each path, where upstream waves propagating from such a point arrive at the receptivity location with an appropriate phase relation. Screech tones generated by these jets are found to be intermittent with an out-of-phase coupling as a dominant coupling mode. Instantaneous phase difference between the twin jets computed by the Hilbert transform suggests that a competition between out-of-phase and in-phase coupling is responsible for the intermittency. To model wave components of the screech feedback while ensuring perfect phase-locking, an ensemble average of leading spectral proper orthogonal decomposition modes is obtained from several segments of large-eddy simulations data that correspond to periods of invariant phase difference between the two jets. Each mode is then extracted by retaining relevant wavenumber components produced via a streamwise Fourier transform. Spatial cross-correlation analysis of the resulting modes shows that most of the identified points of return for the cross-excitation are synchronised with the guided jet mode self-excitation, supporting that it is preferred in closing rectangular twin-jet screech coupling.

physics.flu-dyn↗

arXiv2023

Analysis of Resonance in Jet Screech with Large-Eddy Simulations

Screech resonance is studied with experimentally validated large-eddy simulation data for a 4:1 rectangular under-expanded jet at three nozzle pressure ratios. The analysis uses spectral proper orthogonal decomposition (SPOD) and spatial cross correlation to characterize the oppositely-traveling waves in the jet at the screech fundamental frequency. The results support recent theoretical framing of screech as absolute instability, and further reveal the spatial separation of individual processes for screech generation. From the leading-order SPOD mode, direct evidence of the guided jet mode being the screech closure mechanism, not the external acoustic feedback, is observed. A match in the spatial wavenumber suggests the guided jet mode is generated via interactions between the Kelvin-Helmholtz wave and the shock cells. The energy of the oppositely-moving waves shows spatially global and non-periodic behavior of the coherent structures in the streamwise direction. The ratio of wave energy identifies regions where distinct processes in screech generation take place by comparing the rate of energy propagation in the downstream direction to that of the upstream direction. The distinct regions correspond to initial shear layer receptivity, sound emission, guided jet mode excitation and decay of coherence. The leading-order SPOD mode also enables the approximation of Lighthill's stress tensor and allows for accurate calculation of the far-field screech tone amplitude with the acoustic analogy formulation. The current findings provide insights on building a physics-based reduced order model for screech amplitude prediction in the future.

physics.flu-dyn↗

arXiv2023

Airfoil trailing-edge noise source identification using large-eddy simulation and wavelet transform

Airfoil noise is predicted and analyzed using wall-resolved large-eddy simulations and wavelet transforms for a NACA 0012 airfoil at a Mach number of 0.06 and a Reynolds number of 400,000 using a stair-strip forced transition and a natural transition. At a high angle of attack, vortex shedding and a laminar separation bubble (LSB) occur on the suction side. The LSB triggers the flow transition for both the forced and natural transition cases. The wavelet thresholding and denoising algorithm is used to decompose the pressure fields into the coherent or denoised pressure and the incoherent or background noise pressure. This denoising technique provides a clear picture of true noise generation and propagation. It also reveals the dominant noise source at specific frequencies when multiple noise sources are present. In another usage, the wavelet thresholding algorithm with down-sampling separates noise on the basis of flow structures. For example, the wavelet method separates noise between low-frequency vortex shedding noise and high-frequency LSB noise as well as trailing-edge noise. Finally, the wavelet transform is used to decompose the hydrodynamic and acoustic pressure components near the surface using the coherence between near-field pressure and far-field pressure. Overall, the wavelet-based decomposition is a valuable tool to study and reveal the mechanisms of airfoil noise generation.

physics.flu-dyn↗

arXiv2022

Aeroacoustics of Twin Rectangular Jets Including Screech: Large-eddy Simulations with Experimental Validation

High-fidelity large-eddy simulations (LES) are performed to investigate aeroacoustic characteristics of jets issuing from twin rectangular nozzles with an aspect ratio of 2:1 at two over-expanded conditions and the design condition. For all three jet conditions simulated, LES predicts qualitatively similar near-field flow statistics to those measured at the University of Cincinnati. Using the Ffowcs Williams-Hawkings method, LES captures the fundamental screech tone and its harmonics fairly well at multiple observer locations in the far-field. Intense jet flapping motions in the near-field along the minor axis, which are influenced by jet-to-jet interactions, are found to correspond to those frequencies. Moreover, the predicted overall sound pressure levels are within 1-2 dB of the experimental measurements. However, the screech tones appear to be intermittent, as the twin-jet interaction pattern varies irregularly. To extract dominant flow structures at the screech frequencies and identify the twin-jet coupling modes, spectral proper orthogonal decomposition (SPOD) analysis is used. SPOD analysis recovers energetic peaks at the screech frequencies, and the corresponding leading modes indicate strong upstream radiation originating from the fifth/sixth shock-cells. For the two over-expanded conditions, the leading modes show anti-symmetric coupling in the minor axis at the fundamental screech frequencies. In contrast, the two jets behave symmetrically with respect to each other in the major axis, in line with the absence of jet flapping in this direction. Furthermore, the leading SPOD eigenvalues turn out to be, at least, two orders of magnitude larger than higher-order eigenvalues, suggesting potential of reduced-order models for the twin-jet screech.

physics.flu-dyn↗

arXiv2022

Analysis of second-moments and their budgets for Richtmyer-Meshkov instability and variable-density turbulence induced by re-shock

Nonlinear Richtmyer--Meshkov instability and mixing transition induced by a Mach 1.45 shock and subsequent re-shock at an interface between two ideal gases (sulfur hexafluoride and air) with high Atwood number are studied with second-moment analysis using data from high-resolution compressible Navier--Stokes simulations. The analysis first addresses the importance of two second-order moments: turbulent mass flux and density-specific-volume covariance, together with their transport equations. These quantities play an essential role in the development of Favre-averaged Reynolds stress and turbulent kinetic energy in this variable-density flow. Then, grid sensitivities and the time evolution of the turbulent quantities which include the second-moments are investigated, followed by a detailed study of the transport equations for the second-moments including the Reynolds stress and the turbulent kinetic energy with well-resolved data before re-shock. After re-shock, budgets of the same but large-scale turbulent quantities are studied with the effects of the subfilter-scale stress taken into account. The budgets of these large-scale quantities are shown to have an insignificant influence from the numerical regularization. Finally, the effects of the subfilter-scale stress on the budgets of the large-scale turbulent quantities with different degrees of filtering are also examined.

physics.flu-dyn↗

arXiv2021

The subgrid scale pressure field of scale-enriched Large Eddy Simulations using Gabor modes

With the continuing progress in large eddy simulations (LES), and ever increasing computational resources, it is currently possible to numerically solve the time-dependent and anisotropic large scales of turbulence in a large variety of flows. For some applications this large-scale resolution is satisfactory. However, a wide range of engineering problems involve flows at very large Reynolds numbers where the subgrid scale dynamics of a practical LES are critically important to design and yet are out of reach given the computational demands of solving the Navier Stokes equations; this difficulty is particularly relevant in wall-bounded turbulence where even the large-scales are often below the implied filter width of modest cost wall modeled LES (WMLES). In this paper we briefly introduce a scale enrichment procedure which leverages spatially and spectrally-localized Gabor modes. The method provides a physically consistent description of the small scale velocity field without solving the full non-linear equations. The enrichment procedure is appraised against its ability to predict small scale contributions to the pressure field. We find that the method accurately extrapolates the pressure spectrum and recovers pressure variance of the full field remarkably well when compared to a computationally expensive, highly resolved LES. The analysis is conducted both in \textit{a priori} and \textit{a posteriori} settings for the case of homogeneous isotropic turbulence (HIT).

physics.flu-dyn↗

arXiv2021

Assessment of diffuse-interface methods for compressible multiphase fluid flows and elastic-plastic deformation in solids

This work describes three diffuse-interface methods for the simulation of immiscible, compressible multiphase fluid flows and elastic-plastic deformation in solids. The first method is the localized-artificial-diffusivity approach of Cook (2007), Subramaniam et al., (2018), and Adler and Lele (2019), in which artificial diffusion terms are added to the conservation equations. The second method is the gradient-form approach that is based on the quasi-conservative method of Shukla et al., (2010) and Tiwari et al., (2013), in which the diffusion and sharpening terms (together called regularization terms) are added to the conservation equations. The third approach is the divergence-form approach that is based on the fully conservative method of Jain et al., (2020), in which the regularization terms are added to the conservation equations. The primary objective of this work is to compare these three methods in terms of their ability to maintain: constant interface thickness; the conservation property; and accurate interface shape for long-time integration. The second objective of this work is to consistently extend these methods to model interfaces between solid materials with strength. To assess and compare the methods, they are used to simulate a wide variety of problems, including (1) advection of an air bubble in water, (2) shock interaction with a helium bubble in air, (3) shock interaction and the collapse of an air bubble in water, and (4) Richtmyer-Meshkov instability of a copper-aluminum interface. The current work focuses on comparing these methods in the limit of relatively coarse grid resolution, which illustrates the true performance of these methods. This is because it is rarely practical to use hundreds of grid points to resolve a single bubble or drop in large-scale simulations of engineering interest.

physics.comp-ph↗

arXiv2021

Scalable Parallel Linear Solver for Compact Banded Systems on Heterogeneous Architectures

A scalable algorithm for solving compact banded linear systems on distributed memory architectures is presented. The proposed method factorizes the original system into two levels of memory hierarchies, and solves it using parallel cyclic reduction on both distributed and shared memory. This method has a lower communication footprint across distributed memory partitions compared to conventional algorithms involving data transpose or re-partitioning. The algorithm developed in this work is generalized to cyclic compact banded systems with flexible data decompositions. For cyclic compact banded systems, the method is a direct solver with a deterministic operation and communication counts depending on the matrix size, its bandwidth, and the partition strategy. The implementation and runtime configuration details are discussed for performance optimization. Scalability is demonstrated on the linear solver as well as on a representative fluid mechanics application problem, in which the dominant computational cost is solving the cyclic tridiagonal linear systems of compact numerical schemes on a 3D periodic domain. The algorithm is particularly useful for solving the linear systems arising from the application of compact finite difference operators to a wide range of partial differential equation problems, such as but not limited to the numerical simulations of compressible turbulent flows, aeroacoustics, elastic-plastic wave propagation, and electromagnetics. It alleviates obstacles to their use on modern high performance computing hardware, where memory and computational power are distributed across nodes with multi-threaded processing units.

cs.DC↗

arXiv2020

Homogenized Flux-Body Force Treatment of Compressible Viscous Porous Wall Boundary Conditions

A homogenization approach is proposed for the treatment of porous wall boundary conditions in the computation of compressible viscous flows. Like any other homogenization approach, it eliminates the need for pore-resolved fluid meshes and therefore enables practical flow simulations in computational fluid domains with porous wall boundaries. Unlike alternative approaches however, it does not require prescribing a mass flow rate and does not introduce in the computational model a heuristic discharge coefficient. Instead, it models the inviscid flux through a porous wall surrounded by the flow as a weighted average of the inviscid flux at an impermeable surface and that through pores. It also introduces a body force term in the governing equations to account for friction loss along the pore boundaries. The source term depends on the thickness of the porous wall and the concept of an equivalent single pore. The feasibility of the latter concept is demonstrated using low-speed permeability test data for the fabric of the Mars Science Laboratory parachute canopy. The overall homogenization approach is illustrated with a series of supersonic flow computations through the same fabric and verified using supersonic, pore-resolved numerical simulations.

physics.flu-dyn↗

arXiv2020

Drag of a Heated Sphere at Low Reynolds Numbers in the Presence of Buoyancy

Fully resolved simulations are used to quantify the effects of heat transfer in the presence of buoyancy on the drag of a spatially fixed heated spherical particle at low Reynolds numbers ($Re$) in the range $10^{-3} \le Re \le 10$ in a variable property fluid. The amount of heat addition from the sphere encompasses both, the heating regime where the Boussinesq approximation holds and the regime where it breaks down. The particle is assumed to have a low Biot number which means that the particle is uniformly at the same temperature and has no internal temperature gradients. Scaling buoyancy with inertial and viscous forces yields two related non-dimensional quantities, called Buoyancy Induced Viscous Reynolds Number ($Re_{BV}$) and Buoyancy Induced Inertial Reynolds Number ($Re_{BI}$). For ideal gases, $Re_{BV}$ is analogous to the Grashof number ($Gr$). No assumptions are made on the magnitude of $Re_{BI}$ (or equivalently $Re_{BV}$). The effects of the orientation of gravity relative to the free-stream velocity are examined. Large deviations in the value of the drag coefficient are observed when the Froude number ($Fr$) decreases and/or the temperature of the sphere increases. Under appropriate constraints on $Re_{BI}$ and $Re$, the total drag on a heated sphere in a low $Re$ flow in the presence of buoyancy (mixed convection) is shown to be, within 10% error, the linear superposition of the drag computed in two canonical setups: one being the drag on a steadily moving heated sphere in the absence of buoyancy (forced convection) and the other being natural convection. However, the effect of temperature variation on the drag of a sphere in both, forced and natural convection, is significant.

physics.flu-dyn↗

arXiv2019

High-resolution Navier-Stokes simulations of Richtmyer-Meshkov instability with re-shock

The interaction of a Mach 1.45 shock wave with a perturbed planar interface between sulphur hexafluoride and air is studied through high-resolution two-dimensional (2D) and three-dimensional (3D) shock-capturing adaptive mesh refinement simulations of multi-species Navier-Stokes equations. The sensitivities of time-dependent statistics on grid resolution for 2D and 3D simulations are evaluated to ensure the accuracy of the results. The numerical results are used to examine the differences between the development of 2D and 3D Richtmyer-Meshkov instability during two different stages: (1) initial growth of hydrodynamic instability from multi-mode perturbations after first shock and (2) transition to chaotic or turbulent state after re-shock. The effects of the Reynolds number on the mixing in 3D simulations are also studied through varying the transport coefficients.

physics.flu-dyn↗

arXiv2014

Linear and Nonlinear Modeling of a Traveling-Wave Thermoacoustic Heat Engine

We have carried out three-dimensional Navier-Stokes simulations, from quiescent conditions to the limit cycle, of a traveling-wave thermoacoustic heat engine (TAE) composed of a long variable-area resonator shrouding a smaller annular tube, which encloses the hot (HHX) and ambient (AHX) heat-exchangers, and the regenerator (REG). Simulations are wall-resolved, with no-slip and adiabatic conditions enforced at all boundaries, while the heat transfer and drag due to the REG and HXs are modeled. HHX temperatures have been investigated in the range 440K - 500K with AHX temperature fixed at 300K. The initial exponential growth of acoustic energy is due to a network of traveling waves amplified by looping around the REG/HX unit in the direction of the imposed temperature gradient. A simple analytical model demonstrates that such thermoacoustic instability is a Lagrangian thermodynamic process resembling a Stirling cycle. A system-wide linear stability model based on Rott's theory is able to accurately predict the frequency and the growth rate, exhibiting properties consistent with a supercritical Hopf bifurcation. The limit cycle is governed by acoustic streaming -- a rectified flow resulting from high-amplitude acoustics. Its key features are explained with an axially symmetric incompressible model driven by the wave-induced stresses extracted from the compressible calculations. These features include Gedeon streaming and mean recirculations due to flow separation. The first drives the mean advection of hot fluid from the HHX to a secondary heat exchanger (AHX2), in the thermal buffer tube (TBT), necessary to achieve saturation of the acoustic energy growth. The direct evaluation of the energy fluxes reveals that the efficiency of the device deteriorates with the drive ratio and that the acoustic power in the TBT is balanced primarily by the mean advection and thermoacoustic heat transport.

physics.flu-dyn↗

arXiv2013

The Shock and the Turbulence: the story of an interaction

Video highlighting multiple features of the interaction between isotropic turbulence and a nominally planar shock wave, normal to the mean flow direction. The visualizations showcased in this video were obtained from a Direct Numerical Simulation.

physics.flu-dyn↗

arXiv2011

Large-Eddy Simulation of an over-expanded planar nozzle

This fluid dynamics video shows visualizations of a Large-Eddy Simulation (LES) of an over-expanded planar nozzle. This configuration represents the experimental setup of Papamoschou et. al. which found the position of the internal shock to be unstable. Our LES calculations confirm this instability and offer a vibrant and dynamic view of the underlying flow physics. The interaction between shock and turbulent boundary layer is shown as is the subsequent separation region downstream. Numerical Schlieren provide a glimpse of the low frequency shock motion and suggest potential mechanisms for the instability. Key features include the asymmetry of the shock structure (with large and small lambda shocks), compression and expansion waves downstream of the shock and large scale flow reversal. Full details of the experiment and the calculation can be found in the references.

physics.flu-dyn↗