Investigation of forced shock-induced separation in transonic channel

As soon as transonic regime is reached in a turbomachine, a shock-wave is likely to develop on the blade suction side and interact with the boundary layer. For a strong shock, separation of the boundary layer occurs, and the massive recirculation can drastically impact the engine efficiency. The periodic passage of a downstream row induces an oscillation of the back pressure that further affects the interaction and the turbulence in the separated region. Accounting for this unsteady potential effect at the design stage would allow an improvement in engine efficiency. In order to better understand this flow phenomena, an Implicit Large-Eddy Simulation of the transonic flow over a bump [1] under oscillating back pressure is conducted. The flow conditions - shock Mach number and Reynolds number - are chosen such that the shock-wave/boundary layer interaction leads to a massively separated region, and the reduced frequency to match the ones observed in turbomachinery configurations. The solver used in the current study is a high-order solver based on the flux reconstruction approach, which allows an arbitrary order of accuracy. Special attention has been given to the shock-capturing technique, and the solver has already been validated against experimental and numerical results for a canonical oblique shock-wave/boundary layer interaction [2]. The data produced are used to investigate the influence of the forced motion of the shock-wave on the turbulent stresses in the separated region. Adopting a triple decomposition, the focus is more specifically on the oscillatory component of the turbulent stresses, meaning the fluctuating component associated with a large time scale. A conditional averaging technique based on the position of the shock-wave is employed to extract this component of the flow. The data will serve as a basis to develop a reduced-order model that will be able to accurately consider the impact of unsteady potential effect and should be used early in the design process. [1] Bron, O., “Numerical and Experimental Study of the Shock-Boundary Layer Interaction in Transonic Unsteady Flow,” Ph.D. thesis, Royal Institute of Technology, Sweden, 2003. [2] Goffart, N., Tartinville, B., Puri, K., Hirsch, C. and Pirozzoli, S., “High-Order, High-Fidelity Simulation of Unsteady Shock-Wave/Boundary Layer Interaction Using Flux Reconstruction”, Under submission for the ECCOMAS Congress 2022, (2022).