arXiv subjects

Sarasija Sudharsan

Publications and source records attributed to Sarasija Sudharsan.

4 recordsLinked to original sources

arXiv2026

The inviscid Euler limit as a critical boundary for moment-based aerodynamic system identification

Finite-dimensional state-space representations of unsteady aerodynamics implicitly assume a system with fading memory. However, the impulse response of the two-dimensional inviscid (Euler) equations is characterized by an asymptotic $t^{-3/2}$ power-law decay due to the persistence of shed vorticity. The present work demonstrates that this decay rate constitutes a critical boundary for moment convergence: the second temporal moment diverges logarithmically, causing the characteristic memory time to grow as $\sqrt{\ln T}$ with the observation window $T$. As a result, no window-independent characteristic time scale exists, and finite-dimensional models fitted to inviscid data effectively parameterize the observation horizon rather than intrinsic flow physics. To quantify this behavior, a temporal-moment diagnostic, $ν_t(T)$, is introduced based on the ratio of the second and zeroth windowed moments of the impulse response kernel. Exponential models exhibit stable memory time plateaus, as their sufficiently fast decay ensures convergence of the moment diagnostic. Compressible Euler simulation results confirm the predicted $\sqrt{\ln T}$ scaling at intermediate times, while numerical dissipation inherent to the discretization acts as an artificial regularizer that enforces convergence at late times. These results establish the two-dimensional inviscid limit as a critical boundary for moment-based system identification, where the absence of a dissipative mechanism prevents the definition of a window-independent characteristic memory time.

physics.flu-dyn↗

arXiv2025

Compressibility Effects on Leading-Edge Dynamic Stall Criteria at High Reynolds Number

This study examines the applicability of two leading-edge dynamic stall criteria, namely, the maximum magnitudes of the leading-edge suction parameter (LESP) and the boundary enstrophy flux (BEF), in a moderately compressible flow regime. While previously shown to predict stall onset ahead of dynamic stall vortex (DSV) formation in incompressible and mildly compressible regimes, these criteria are assessed here at a Reynolds number of $1 \times 10^6$ and freestream Mach numbers between 0.3 and 0.5. Unsteady RANS simulations indicate that DSV formation occurs in close temporal proximity to the attainment of the stall criteria. However, at the highest Mach number considered, stronger shock interaction effects with the shear layer leads to DSV formation prior to the criteria being reached, reducing their predictive accuracy. These findings suggest that while the criteria remain effective at lower Mach numbers, their definitions require modification in compressible regimes where strong shock interactions significantly influence the stall process.

physics.flu-dyn↗

arXiv2025

A numerical demonstration of dynamic stall control

This paper presents a numerical demonstration of the real-time application of two dynamic stall onset criteria for identifying and mitigating stall. These criteria - based on the leading-edge suction parameter (LESP) and boundary enstrophy flux (BEF) - are derived from prior research. The present work establishes a proof of concept for the practical use of these indicators in mitigating dynamic stall. Two different unsteady motions that lead to dynamic stall are simulated. For each motion, both baseline cases (where dynamic stall occurs) and controlled cases (where stall is mitigated using the onset criteria) are analyzed using the unsteady Reynolds-averaged Navier-Stokes (uRANS) method. Both parameters were found to be effective in mitigating the adverse effects (large variations in aerodynamic loads) of dynamic stall. The results demonstrate the potential for these physics-based criteria to be integrated with advanced control strategies, such as reinforcement learning agents, for robust stall control across a range of operating conditions.

physics.flu-dyn↗

arXiv2024

Criteria for dynamic stall onset and vortex shedding in low Reynolds number flows

Dynamic stall at low Reynolds numbers, $\mathit{Re} \sim \mathcal{O}(10^4)$, exhibits complex flow physics with co-existing laminar, transitional, and turbulent flow regions. Current state-of-the-art stall onset criteria use parameters that rely on flow properties integrated around the leading edge. These include the leading edge suction parameter or $\mathit{LESP}$ (Ramesh et al., 2014) and boundary enstrophy flux or $\mathit{BEF}$ (Sudharsan et al., 2022)), which have been found to be effective for stall onset at moderate to high $\mathit{Re}$. However, low $\mathit{Re}$ flows feature strong vortex shedding events occurring across the entire airfoil surface, including regions away from the leading edge, altering the flow field and influencing the onset of stall. In the present work, the ability of these stall criteria to effectively capture and localize these vortex shedding events in space and time is investigated. High-resolution large-eddy simulations for an SD7003 airfoil undergoing a constant-rate, pitch-up motion at two $\mathit{Re}$ (10, 000 and 60, 000) and two pitch rates reveal a rich variety of unsteady flow phenomena, including instabilities, transition, vortex formation, merging, and shedding, which are described in detail. While stall onset is reflected in both $\mathit{LESP}$ and $\mathit{BEF}$, local vortex-shedding events are identified only by the $\mathit{BEF}$. Furthermore, these events can be localized in space and time by considering the contributions to the $\mathit{BEF}$ from different airfoil sections, which holds significant promise for effective flow control.

physics.flu-dyn↗