Currently, I am working on the linear stability of hypersonic flows. Hypersonic flows can be considered as those flows which contain a very high stagnation enthalpy. The aero-thermodynamics associated with these flow-fields involve strong shock waves, viscous shock layers and non-equilibrium thermo-chemistry. These multiple processes can influence laminar to turbulent transition in different ways. A turbulent boundary layer generates much higher friction and heat transfer, by factors of four or higher, to the vehicle surface compared to a laminar boundary layer. Transition location influences the estimation of aero-heating and skin friction drag, which in turn affect heat shield weight and material, vehicle range, and payload capacity. Therefore predicting the location and streamwise extent of transition in a hypersonic boundary layer is important for avoiding overly conservative design margin for thermal protection systems.

In a hypersonic flow-field, there are immense parameters which can affect the transition process. These includes - receptivity to free-stream disturbance; geometric parameters such as- surface curvature, roughness, angle of attack, nose bluntness etc.; real gas effects such as - internal mode excitation, dissociation, ionization etc. and the boundary conditions for example, cold/adiabatic, non-catalytic/super catalytic, blowing/suction etc. In addition, the thermo-chemical state of the gas (frozen, equilibrium or non-equilibrium) can also alter the physical as well as transport properties of the gas significantly, which can have a major influence on the stability of hypersonic flows. It will be useful if we can quantify the effect of variation of individual parameters on flow transition.

Literature survey shows that the transport properties such as- viscosity, thermal conductivity and diffusion have a prominent role in deciding the state (laminar/turbulent) of the gas. Therefore, I am trying to isolate these parameters and systematically study their effect on the stability of high Mach number flows. For instance, in POF 2014 (Ref. 1), we have isolated the effect of Prandtl number on the linear stability of hypersonic flows. Prandtl number is defined as the ratio of product of viscosity and specific heat to the thermal conductivity. Due to the large variations of temperature and pressure in a hypersonic flow-field, the transport properties along with the specific heat have considerable individual variations, which lead to large variation in the Prandtl number. Therefore, we have varied the Prandtl number from 0.2 to 0.9 and studied the stability problem as a function of Mach number, Reynolds number and disturbance wavenumber. This study shows that, for a Couette flow with uniform viscosity and conductivity, when the Prandtl number is decreased, the flow becomes more unstable, which implies that a low Prandtl number flow is more prone to transition compared to an equivalent flow with high Prandtl number. A similar work also has been extended to boundary layers.

Recently, the effect of stratification of viscosity and thermal conductivity on flow stability are also been studied. The results show that stratification of conductivity destabilizes the Couette flow by increasing the growth rates and reducing the critical Reynolds number. But, the stratification of viscosity is highly stabilizing. In a real flow-field, both the viscosity and thermal conductivity vary simultaneously as a function of temperature, therefore both the stabilizing role of viscosity and destabilizing role of conductivity combine to decide the stability characteristics of the flow.

Papers for reference:

1. Raje, P., Saikia, B. and Sinha, K., "Numerical investigation of axisymmetric underexpanded supersonic jets" Proceedings of 1st National Aerospace Propulsion Conference NAPC-2017, IIT Kanpur, March 15-17, 2017.

2. Saikia, B., Ramachandran, A., Sinha, K. and Govindarajan, R., "Effects of viscosity and conductivity stratification on the linear stability and transient growth of compressible Couette flow", Physics of Fluids, Volume 29, Issue 2, 024105, January 2017.

3. Ramachandran, A., Saikia, B., Sinha, K. and Govindarajan, R., "Effect of Prandtl number on the linear stability of compressible Couette flow", International Journal of Heat and Fluid Flow, Volume 61, Part B, M pp. 553-561, October 2016.

4. K. Sinha, A. Ramachandran, B. Saikia, and R. Govindarajan, "Effect of Prandtl Number on the Stability of Compressible Couette flow"., American Physical Society Division of Fluid Dynamics, Boston, MA, United States of America, 21st & 23rd November 2015.

5. Ramachandran, A. , Saikia , B. , Sinha, K. and Govindarajan, R. , "Linear stability of high-speed boundary layer flows at varying Prandtl numbers", 45th AIAA Thermophysics Conference, AIAA 2015-2320, Dallas, Texas, United States of America, 22 - 26 June 2015.

6. Reddy, D.S.K., Saikia, B., and Sinha, K., "Effect of high-enthalpy air chemistry on stagnation point heat flux", Journal of Thermophysics and Heat Transfer, Volume 28, Issue 2, pp. 356-359, April-June 2014.

• Ph.D , Department of Aerospace Engineering, IIT Bombay. Joined in 2012.
• B.Tech , Department of Mechanical Engineering, Tezpur University. 2007-2011.

Research Assistant at IIT Bombay. 2011