Purdue PhD student Haryl Ngoh is working on detached eddy simulations (DES) of a Mach 3 flow over a 12.7 mm leading edge diameter blunt fin in the presence of a 3.3 mm thick incoming turbulent boundary layer. The flow conditions were chosen to match experiments performed in the 1980s by Dolling and Bogdonoff in the Princeton University 8 in x 8 in Mach 3, high Reynolds number wind tunnel.
The unsteady separation shock in such an interaction acts like a hot hammer pounding on the surface of an aircraft. The consequent thermo-structural loads can be severe. It is essential for aircraft design to predict this loading, but fully resolved turbulent simulations at Reynolds number are extremely expensive. One solution is DES, which omits some of the small space and time scales of the turbulence, while retaining unsteady separation dynamics. Our recent work has focused on assessing the accuracy of this approximation for separated shock-wave / boundary-layer interactions.
Movies of two cases are presented below. For each case, grayscale contours show the density field in the centerplane of the interaction. The orange contours display the skin friction magnitude on the tunnel floor and the fin surface. The first case shows a baseline DES, whereas the second shows a case where synthetic turbulence, generated through a digital filtering technique, is injected at the inflow boundary.
Even in the baseline case, a high degree of flow unsteadiness is apparent in the separated region near the fin-wall juncture. This flow acts like an oscillator: any small initial fluctuations in the separated region are reinforced, leading to self-excited oscillations even in the absence of disturbances in the incoming flow.
For the case with synthetic turbulence in the inflow, the effect can be seen as the dark bulges moving from left to right within the centerline boundary layer flow. These bulges are seen to interact with the separated region. In our current research, we are quantifying the differences between the two cases to assess the effect of incoming turbulence on large-scale unsteadiness in the blunt fin interaction.
Computational resources for this work were provided by Purdue University’s Rosen Center for Advanced Computing, and the calculations were carried out using the open-source CFD code SU2 on the Brown Community Cluster.