Dr Olaf Marxen (Surrey University): Hydrodynamic Stability of Hypersonic Boundary Layers with Non-Ideal Gas Effects
The accurate prediction of laminar-turbulent transition in the boundary layer is a critical step in the design of hypersonic flight vehicles. Depending on the type of vehicle and its mission, the temperature inside the boundary layer on the vehicle surface may be very high owing to a conversion of kinetic energy into thermal energy. As a result of this high temperature, chemical reactions may occur and change the composition of the gas, leading to non-ideal gas effects. In comparison to laminar flows, turbulent flows induce much higher thermal load on the surface of vehicles moving through a planetary atmosphere at high speed.
Fundamental physical processes related to laminar–turbulent transition in high-speed boundary layers with non-ideal gas effects are not yet well understood. This motivates the numerical investigation of linear and nonlinear instability in high-speed boundary layers. Disturbance amplification rates as well as amplitude and phase functions from direct numerical simulations in the linear regime are found to compare favorably with those obtained from linear stability theory. For moderately reactive flows, the effect of chemical reactions is very similar to the effect of wall cooling.
Weakly nonlinear boundary-layer instability in the presence of a large-amplitude two-dimensional instability wave has also been simulated for the case of fundamental resonance. Depending on the amplitude of the two-dimensional primary wave, strong growth of oblique secondary perturbations occurs. It is found that chemical reactions do not directly affect the growth of secondary perturbations, but only indirectly through the change of linear instability and hence amplitude of the primary wave.