Santanu Ghosh

Abstract: A novel vortex generator, termed as “slotted vortex generator,” is proposed in this study. Computations are conducted for supersonic flow at a freestream Mach number of 2.5 past single vortex generators of three different heights. For each device height h, three values of the slot radius r=0.3h, 0.4h, and 0.6h are used. Comparisons made with a standard wedge-type vortex generator using streamwise-velocity profiles, near-surface streamwise-velocity contours, pitot pressure deficit contours, etc., indicate that the new device has less device drag and produces fuller near-surface streamwise velocities downstream of the device. It is observed that the primary counter-rotating vortex pair formed due to the vortex generator lifts off at a slower rate when a slotted vortex generator is used. Computations of an impinging oblique-shock/boundary-layer interaction at Mach 2.5 for a flow turning angle of 7 deg, with flow control using (separately) an array of slotted and standard vortex generators, indicate that the ...

Abstract: This work makes an attempt to determine an effective streamwise and spanwise arrangement of a relatively novel control device, the sub-boundary-layer slotted vortex generator, having a slot radius ...

Abstract: This work compares interpolation techniques for data reconstruction at the surface in an immersed-boundary method. Three different methods of surface pressure reconstruction based on inverse distances are presented, which are christened as: Inverse Distance Weight (IDW) method, Inverse Distance Weight at Interpolation Point method (IDW-IP) and Inverse Distance Weight based on Upwinding (IDW-Upwind) method. Additionally, shear stress at the immersed surface is determined using two approaches: direct interpolation of velocity gradient at the surface using IDW method, and interpolation of velocity at a point along the surface normal using IDW-IP method. The interpolation methods are verified against analytic solutions of ideal flow past a circular cylinder and subsonic-supersonic inviscid flow in a convergent-divergent nozzle, and validated against laminar flow simulations of Mach 0.5 flow past a NACA0012 airfoil, Mach 2.0 flow past a circular cylinder, and Mach 3.0 flow past a 10∘ ramp. The verification cases show that while the pressure values reconstructed at the surface by the three interpolation methods are very similar for the incompressible flow, the IDW-Upwind method produces the sharpest pressure rise across the normal shock in the convergent-divergent nozzle. Comparisons of the reconstructed surface pressure coefficient (Cp) and skin-friction coefficient (Cf) with values available from literature or ANSYS-Fluent simulations conducted as part of the validation study show good match, but indicate that the reconstructed pressure and shear stress values at the immersed surface has noise, which, however, reduces with grid refinement. Further, the IDW and IDW-Upwind method for pressure reconstruction, and the gradient reconstruction based method for shear stress calculation are shown to produce less noise in computed values. Integrated drag and lift values using the reconstructed surface pressure and shear stress indicate that while the different methods used for pressure reconstruction result in similar values of aerodynamic loads, the gradient-based shear stress calculations result in more accurate load estimation. Finally, one of the interpolation methods (IDW-Upwind) is used to investigate the variation of the surface pressure coefficient with time for a NACA0012 airfoil undergoing non-periodic plunge motion in a Mach 0.2 flow. The computed surface pressure coefficients are correlated with the leading and trailing edge vortices in the flow field.

Abstract: The response of an inviscid shock to external pressure perturbations in a constant area duct is analyzed in terms of fundamental processes like perturbation propagation and its interaction with shock. The results of these elementary processes are formulated analytically and with a Riemann wave tracking method to enable the prediction of shock movement for both upstream and downstream perturbations. The predictions thus obtained are compared with the finite-volume based numerical simulations of the Euler equations. This study shows that the shock responds nonlinearly to perturbations and the nonlinearity has a cumulative effect. Contact surfaces generated during the interaction of normal shock with perturbations, which was ignored in previous investigations, are shown to be important in order to capture this cumulative nonlinearity. The nonlinearity alters the positive and negative duty cycles, which results in a net displacement of shock after responding to one full cycle of sinusoidal perturbation. This drift in shock location is pronounced at low supersonic Mach numbers (1.2–3) but is also present at higher Mach numbers. Furthermore, it is demonstrated that the duty cycle variations are higher for perturbations originating downstream of shock than those originating upstream. The variations in frequency and amplitude are found to merely scale the response and do not introduce any new physics.The response of an inviscid shock to external pressure perturbations in a constant area duct is analyzed in terms of fundamental processes like perturbation propagation and its interaction with shock. The results of these elementary processes are formulated analytically and with a Riemann wave tracking method to enable the prediction of shock movement for both upstream and downstream perturbations. The predictions thus obtained are compared with the finite-volume based numerical simulations of the Euler equations. This study shows that the shock responds nonlinearly to perturbations and the nonlinearity has a cumulative effect. Contact surfaces generated during the interaction of normal shock with perturbations, which was ignored in previous investigations, are shown to be important in order to capture this cumulative nonlinearity. The nonlinearity alters the positive and negative duty cycles, which results in a net displacement of shock after responding to one full cycle of sinusoidal perturbation. This ...

Abstract: An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

Abstract: The oscillation characteristics of the shock train in an isolator have been investigated in a direct-connect wind tunnel at Mach 2.7. High-speed schlieren imaging and high-resonance frequency pressure measurements were used to capture the flow features during the shock train movement. The oscillation features without the effects of incident shocks were analyzed first. As the shock train moved upstream, the low-frequency part of the oscillation was found to develop. The analysis was then extended to complex situations with incident shocks. It was revealed that the shock wave-boundary layer interactions considerably influence the shock train behavior. The interactions were classified into three patterns: (I) single interaction, (II) multi-interactions on the same side, and (III) multi-interactions on different sides. Experimental results indicated that the oscillation could be affected in temporal scale by pattern II and enhanced in spatial scale by pattern III. The data also showed that the pressure rise induced upstream propagates to the exit, causing phase offsets in the wall pressure histories and making the pressure distributions diverge from their stable state. This phenomenon suggested a possible physical mechanism for the oscillation during shock train movement, which was verified by additional tests with large backpressure rising rate. It was found that there exists a critical frequency which is related to the pressure ramping rate during the oscillation. If the dominant frequency of the backpressure varies beyond this critical frequency, the pressure distribution could be forced into a steady state before the oscillation was induced. Otherwise the oscillation could not be suppressed.

Abstract: The shock wave/boundary-layer interaction is a common phenomenon, and it occurs in the internal and external flow fields of the hypersonic vehicle. Then, the separation, the oscillatory flow structures, the heat transfer localization and the pressure loads would be induced accordingly. In this survey, recent advances on the shock wave/boundary-layer interaction and its control in the internal and external flow fields were summarized, and it is obvious that the micro-vortex generator has been widely used in both the internal and external flow fields to suppress the separation induced by the shock wave/boundary-layer interaction, and the quantitative measurement and assessment approaches should be developed. At the same time, the intelligent control approach should be combined with the flexible surface to fit the variation of the incoming boundary conditions. With the development of the hypersonic flight technique, the requirement of the wind tunnel with high Mach number is urgent, as well as its test time. At the same time, the Monte Carlo method is required to obtain the shock wave/boundary-layer interaction properties in the high Mach number.

Abstract: Unstable movement of the unstart shock may pose a threat to the safety of a scramjet. The perturbation induced by the unstable movement can also influence the shock structure and the downstream flow, possibly causing a dynamic load on the wall or affecting downstream combustion. Without a thorough analysis of the isolator flow or by ignoring its properties, it is not possible to understand some of the phenomena prevalent in downstream combustion. In this study, two types of instabilities were observed in the unstart shock system. It is shown that if the flow distortion is not severe, the instability in the streamwise direction plays a dominant role. Sequential displacement of the downstream shock was observed in this mode. The time delay between sequential shock motions indicates their response to the movement of the first separation shock. With a highly distorted flow, a flapping mode that resulted in instability in the vertical direction with an asymmetrical effect on the pressures at the walls was observed. In this situation, the shock structure is successively attached to the wall from the head to the tail. By conducting a dynamic mode decomposition analysis, several oscillatory modes, characterized by low-frequency periodicity in the streamwise and vertical directions, were revealed in the shock system. Subsequently, the feasibility of considering the periodical deflection of the incoming flow induced by the significantly unequal amplitudes of shock movements at the two walls as the underlying mechanism for the flapping mode is explored.