Santanu Ghosh

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.

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.

In this work, the local correlation-based one-equation 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, no. 4, pp. 583–619, 2015.) is transformed into a zero-equation transition model. The new model provides an attractive choice in terms of quick implementation of a transition model in existing turbulent flow solvers with Menter's shear-stress transport (SST) turbulence model, as it only introduces three extra source terms in the transport equation of turbulent kinetic energy. The model is validated against a set of benchmark flat-plate test cases: T3 series and SK, and also subsonic flows past two different airfoils: Aerospatiale A-airfoil (Re=2.1million) and E387 (Re=0.2million), and finally applied to a transonic flow over 3D DLR-F5 wing (Re=1.5million). Results show that the proposed model produces similar transition prediction as the one-equation transition model, with a reduced computational effort. The computations are performed with an in-house finite-volume solver for compressible turbulent flows on block-structured grids.

This work investigates the effect of a porous medium/permeable wall in the region of flow separation induced by an impinging oblique shock/boundary-layer interaction (SBLI) at Mach 2.0 using 2D numerical simulations. The study presented here includes an investigation of cases with no control, with permeable walls, and wavy non-permeable walls with varying waviness. The effect of the position and extent of the permeable wall is also investigated. The porous region is modeled as a cavity filled with a square array of circular cylinders (rendered as circles in 2D). The computations are performed using a parallel, finite-volume solver for compressible flows on structured grids. An immersed-boundary method is used to represent the porous region. Menter’s k − ω SST model is used to model turbulence. Results are presented using pressure contours (to reveal shock structure), streamline patterns, and near-surface velocity and pressure. It is observed from the plots that the limiting case of a non-permeable wall produces better results compared to the permeable wall, indicating that passive blowing (from) and suction (into) the permeable wall does not produce the desired effect of energizing the boundary layer and mitigating flow separation in this case.

Camber morphing is an effective way to control the lift generated by any aerofoil and potentially improve the range (as measured by the lift-to-drag ratio) and endurance (as measured by Cl3/2/Cd). This can be especially useful for fixed-wing Unmanned Aerial Vehicles (UAVs) undergoing different flying manoeuvres and flight phases. This work investigates the aerodynamic characteristics of the NACA0012 aerofoil morphed using a Single Corrugated Variable-Camber (SCVC) morphing approach. Structural analysis and morphed shapes are obtained based on small-deformation beam theory using chain calculations and validated using finite-element software. The aerofoil is then reconstructed from the camber line using a Radial Basis Function (RBF)-based interpolation method (J.H.S. Fincham and M.I. Friswell, "Aerodynamic optimisation of a camber morphing aerofoil,"Aerosp. Sci. Technol., 2015). The aerodynamic analysis is done by employing two different finite-volume solvers (OpenFOAM and ANSYS-Fluent) and a panel method code (XFoil). Results reveal that the aerodynamic coefficients predicted by the two finite-volume solvers using a fully turbulent flow assumption are similar but differ from those predicted by XFoil. The aerodynamic efficiency and endurance factor of morphed aerofoils indicate that morphing is beneficial at moderate to high lift requirements. Further, the optimal morphing angle increases with an increase in the required lift. Finally, it is observed for a fixed angle-of-attack that an optimum morphing angle exists for which the aerodynamic efficiency becomes maximum.

The focus of this work is the study of lift enhancement in flapping hover flight using numerical simulations. An idealized set of kinematics for a NACA0012 airfoil consisting of sequential translations and rotations is considered for this purpose, such that the Cl response can be demarcated into translational and rotational parts, which facilitates comparison of forces attributed to translation and rotation. Additionally, comparisons with pure translation and pure rotation are done to isolate the effect of wing-wake interactions. The investigation reveals that the majority of lift is produced in the translational phase. The wing-wake interactions affect the translational phase of the response more than the rotational phase. However, the rotation rate determines the extent of influence of wing-wake interactions on the translational lift response. The effect of different durations of overlap between the translational and rotational motions is also assessed based on the Cl time histories and mean Cl, and the study reveals that an optimum duration of overlap can maximize the lift. An immersed-boundary method with integrated surface-load reconstruction capabilities is used for the computations presented here. The reconstruction of the surface stresses and their integration are carried out with the framework of a parallel solver. The method is validated for a flow past a NACA0012 airfoil executing a non-periodic plunge motion and a non-periodic pitch/plunge motion and a flow around an elliptic airfoil executing a flapping motion.

Camber morphing is an effective way to control the lift generated in any airfoil and potentially improve airfoil efficiency (lift-drag ratio). This can be especially useful for fixed wing UAVs undergoing different flying manoeuvres and flight phases. This work investigates the aerodynamic characteristics of NACA0012 airfoil morphed by the Single Corrugated Variable Camber (SCVC) morphing and Double Corrugated Variable Camber (DCVC) morphing approach. The airfoil is reconstructed from the camber line using a Radial Basis Function (RBF) based interpolation method (J. H. S. Fincham and M. I. Friswell, “Aerodynamic optimisation of a camber morphing aerofoil,” Aerosp. Sci. Technol., 2015). The aerodynamic analysis is done by employing two different finite volume solvers: OpenFOAM and ANSYS-Fluent, and a panel method code (XFoil). Results reveal that the aerodynamic coefficients predicted by the two finite-volume solvers using a fully turbulent flow assumption are similar but differ from those predicted by XFoil. The aerodynamic performance of morphed airfoils are nearly equal or lower than that of the baseline airfoil at lower values of coefficient of lift whilst at large values of the morphed airfoils display superior aerodynamic performance. At identical morphing angles, the aerodynamic characteristics of SCVC and DCVC airfoils are almost identical. Finally, it is observed for a fixed angle of attack, that an optimum morphing angle exists for which the aerodynamic efficiency becomes maximum.