Shock Oscillation Mechanism of Highly Separated Transitional Shock-Wave/Boundary-Layer Interactions

What is it about?

The relative supersonic flow in first engine blade row of business jets and defense applications flying at altitude (low turbulence, low density) may exhibit unsteady phenomena resulting from the fact that the shock wave interacts with a laminar boundary layer. In turbofan engine research, the shock wave generated by the leading edge of the transonic fan interacts with the boundary layer on the fan and causes the shock wave to oscillate. Understanding this phenomenon is important, since the oscillation can cause the blade to vibrate and crack. It was determined in previous work that the oscillation mechanism on a transonic fan could be simplified to a canonical research configuration, which is studied in a supersonic wind tunnel in the current work. The shock oscillation mechanism of this shock-wave/boundary-layer interaction (SBLI) is studied with high speed Schlieren photography, as well as using light bursts in the Schlieren setup to instantaneously freeze the flow field and study the instabilities and transition. Furthermore, the experimental results are compared with Large Eddy Simulations. A characteristic length scale based on the separation shock travel distance is proposed. Experiments and Large Eddy Simulations showed non-dimensionalised frequencies matching. The mechanism is highly dependent on free stream turbulence levels. In turbulent SBLI, dominant oscillation frequencies are much lower, the reflected shock travel distance is shorter, and the shock oscillation behaves differently compared to laminar SBLI. In laminar SBLI, the shock oscillation is closely related to the movement of the separation bubble, unlike in turbulent SBLI, where the shock oscillation is more rigid and less related to the separation bubble movement. Full open access article: https://www.researchgate.net/publication/386339173_Shock_Oscillation_Mechanism_of_Highly_Separated_Transitional_Shock-WaveBoundary-Layer_Interactions

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Photo by Didier Durieux on Unsplash

Photo by Didier Durieux on Unsplash

Why is it important?

This research shows conclusively, both experimentally and numerically, the significant effect of "tripping" the boundary layer (promoting turbulence) as opposed to the laminar boundary layer case, for which large shock oscillations occur in this highly separated shock-wave/boundary-layer interaction. Furthermore, a characteristic length scale for the shock oscillation mechanism is defined, which validates the dynamics of the computational fluid dynamics simulations against the experiment.