Santanu Ghosh

This work uses an immersed-boundary method to simulate the effects of an array of aeroelastic mesoflaps in controlling oblique shock/turbulent boundary-layer interactions. A loosely coupled approach is adopted for the fluid-structure interaction problem, with separate solvers used for the fluid and the structure. The mesoflaps are rendered as immersed objects for the fluid solver and modeled as cantilevered Euler-Bernoulli beams for the structural solver. Simulations are performed for a Mach 2.46 shock/boundary-layer interaction with and without control, based on experiments conducted at University of Illinois at Urbana-Champaign. Both Reynolds-averaged Navier-Stokes and hybrid large-eddy/Reynolds-averaged Navier-Stokes turbulence closures are used. Comparisons made with experimental laser Doppler anemometry data and wall pressure measurements for flows with and without control show reasonable agreement, with better predictions away from the separation region. An analysis of the flow indicates that the mesoflap control system does not eliminate axial flow separation. Also, analysis of the frequency content of the mesoflap deflections suggests that a correlation might exist between the dominant frequency of the Euler-Bernoulli flap deflection and the low-frequency shock motion observed in separated flows. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Adynamical system analysis is performed on the zero equation: W transition model (Sandhu, J. P. S., and Ghosh, S., “A local correlation-based zero-equation transition model,” Computers & Fluids, Vol. 214, 2021, p 10475) to study the sensitivity to initial conditions. The analysis is performed for homogeneous and non-homogeneous flow, focusing on the approximated intermittency function used in the model. The analysis showed that the use of turbulent to molecular viscosity ratio (turbulent Reynolds number) in the approximate intermittency functionwas the reason for the: W transition model’s sensitivity to initial conditions, particularly for cases with low freestream turbulence intensity. A comparison with other models in the literature showed that the use of wall distance in such functions aids in avoiding the sensitivity issue.

The work presented in this paper attempts to improve the grid (and consequently solution) for compressible-flow simulations performed with a 2D finite-volume Euler solver for unstructured grids, using mesh adaptation based on gradients of flow parameters, (pressure and pseudo-entropy, and grid geometry (cell areas, nodal distances etc.). The procedure requires solution (primitive variables) reconstruction at grid nodes, which are interpolated using linear polynomials in 2 dimensions or inverse distance based methods, and cell-averaged gradients, which are computed using the Green-Gauss method. The adaption is terminated if the global maximum displacement of any node is less than ε, where ε is a small user defined length scale. Results indicate that the method is capable of clustering the grid near an oblique shock and a contact wave, which results in sharper resolution of the discontinuities.

Micro-vortex generators offer an alternative to boundary-layer bleed and suction to mitigate flow separation due to shock/boundary-layer interaction. In the last two decades, a number of devices have been investigated either in isolation, wherein the focus has been on studying the flow physics, or in tandem, for studies in flow-separation control. While studies of vortex generators in a supersonic free-stream (without a separate shock/boundary-layer interaction) have generally focused on the understanding of the flow downstream of single devices, their effect on the flow while being used in tandem have not been looked into in as much detail. This work investigates the effect of inter-device spacing in a systematic manner to optimize the configuration of two micro-vortex generators placed side-by-side. A set of objective functions is designed using boundary-layer integral properties and are determined for various inter-device spacing. Simple, slotted, and ramped-vane devices are investigated in this work. Results show that increased device spacing reduces device drag but also worsens boundary-layer health. RANS computations are performed using an immersed-boundary method that renders the vortex generator as a point cloud. The effect of the inter-device spacing of the vortex generators on the mitigation of flow separation is finally tested using simulations of a Mach 2.5 impinging oblique-shock/boundary-layer interaction. Flow-separation profiles indicate that the ramped-vane device provides better mitigation of separation compared to the slotted device and its performance improves with reduction in inter-device spacing.

A computational study has been done to assess the effectiveness of porous medium in control of normal-shock-wave/boundary-layer interaction at transonic speeds with a view towards application in aircraft wings. Passive control is achieved via re-circulation inside the porous medium, which weakens the shock structure, and hence reduces the wave drag. The study has been done for a Mach 1.3 normal-shock-wave/boundary-layer interaction on a at plate in the presence of a porous medium beneath the region of interaction. The domain used for the computations is adapted from a novel experimental setup, due to Holger Babinsky and his group at Cambridge University, that is capable of stabilizing a normal shock over a control region for fixed inlet parameters. The dependency of the control effectiveness on dimensions of the cavity (length, depth) and porosity has been studied. It is observed that whereas the cavity length has a strong effect on the reduction in total drag, the effects of depth and porosity are less pronounced. The computations are done as steady state RANS calculations using Menter’s k – ω/k – ϵ model for turbulence closure.

This work investigates the effectiveness of a relatively novel flow control device, the slotted vortex generator, in mitigation of shock-induced flow separation in a realistic mixed- compression inlet geometry. The presence of sidewalls in an actual inlet makes the inter­actions between the oblique shocks and the boundary layers highly three dimensional. As such, the flow separation and its control in a real inlet is more involved than that investi­gated based on notions of a primarily 2-D separation region. This work makes an attempt to determine an effective streamwise and spanwise arrangement of vortex generators in an actual inlet to minimize the flow separation. The inlet considered for the simulations are based on the experiments conducted by Emami and co-workers at NASA Glenn research center at Mach 4.03 to determine performance of an inlet/isolator configuration. The present work uses a computational domain for one such inlet configuration — hav­ing the smallest sized cowl — and a large convergence angle to produce a strong cowl lip shock. The computations performed are computed using the REACTMB code suitable for high-speed turbulent flows. The turbulence model used is Menter’s SST model.

Immersed boundary methods (IB) are a set of methods to deal with non-body conforming grids. This requires forcing the boundary conditions in the vicinity of the immersed surface. In this work, the embedded object is represented as a set of line segments along with their outward unit normal vectors. The flow domain is categorised into field cells, band cells or interior cells using a signed distance based approach. Although IB methods have been widely used for incompressible flows, it's application to high speed flows is relatively less. The proposed work attempts to construct an IB method suitable for compressible flow applications. Velocity boundary conditions are applied using an inverse distance based approach near no-slip walls and a signed distance based approach for slip walls. Different flow problems are simulated: expansion fan, supersonic flow past ramp channel and supersonic flow past NACA0012 airfoil which are inviscid simulations, subsonic viscous laminar flow past a ramp channel and flow past a cylinder (to simulate Von Karman vortex streets) which are viscous simulations. In each case, a comparison is made with simulations using body fitted grids or experimental data to validate the solution.

In this paper we present a simplified form of the local correlation-based zero-equation kγ transition model (Sandhu, Jatinder Pal Singh, "Local-Correlation Based Zero-Equation Transition Model for Turbomachinery," Proceedings of the ASME 2019 Gas Turbine India Conference, Volume 1: Compressors, Fans, and Pumps; Turbines; Heat Transfer; Structures and Dynamics) derived from the one-equation γ transition model (Menter, F. R., Smirnov, P. E., Liu, T., and Avancha, R., “A One-Equation Local Correlation-Based Transition Model,” Flow, Turbulence and Combustion, vol. 95, 2015, pp. 583–619). The new model is easy to implement and saves computational time and memory. The proposed model is validated against the standard T3 series flat plate test cases (with and without pressure gradient), Hultgren and Volino series, Aerospatial A-Airfoil and the Eppler-387 airfoil. Results indicate that the transition predicted with the new model is similar to the one-equation γ transition model in most cases and compares reasonably well with experimental data. The predicted transition is also gradual in some cases and the method provides a savings in computational memory and time (in most of the flat-plate test cases) over the γ model.