Streamwise vortices in hypersonic flow on a compression ramp

  • Längswirbel in einer Hyperschall-Kompressionsrampenströmung

Cao, Shibin; Olivier, Herbert (Thesis advisor); Schröder, Wolfgang (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis, Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Compression ramp flows with a free-stream Mach number of 7.7 are numerically studied to uncover the physics beneath the experimentally observed phenomena. This work focuses on laminar SWBLI with a moderate flow separation. Direct numerical simulations (DNS) are performed to investigate the characteristics of the streamwise vortices and surface heat flux streaks occurring in the compression ramp flow. Excellent agreements between the DNS results and experimental data as well as theoretical predictions are achieved. The finite-span effects encountered in experiments are also examined. The DNS results demonstrate the occurrence of streamwise vortices and heat flux streaks in compression ramp flows in the absence of external disturbances. Additionally, the presence of surface heat flux streaks leads to a considerable surface heat transfer enhancement downstream of reattachment. The influence of free-stream Reynolds number and ramp angle on the features of the streamwise vortices and streaks are analysed in detail and compared with experimental measurements. Another important finding of this dissertation is the flow unsteadiness triggered by the intrinsic instability in the separation bubble. In the fully developed flow, the surface heat flux downstream of reattachment exhibits a broadband low-frequency unsteadiness, which propagates in streamwise direction. Moreover, the unsteadiness of surface heat flux streaks is coupled with a pulsation of the reattachment position. By conducting a dynamic mode decomposition analysis, several oscillatory modes characterised by streamwise low-frequency periodicity are revealed in the separation bubble flow. This flow structure originates from the instability of the separation bubble flow and is linked to the downstream reattached flow by the shear layer above the bubble. To evaluate the centrifugal instability resulting from the local streamline curvature near reattachment, the Görtler number for a compression ramp flow is accurately determined via different methods. The first one gives the variation of the Görtler number near reattachment on the basis of the DNS data. Additionally, based on a simplified physical model, a formula depending only on the flow conditions and the geometry of compression ramp is deduced to predict the Görtler number. Furthermore, it is found that the Görtler number increases with increasing ramp angle and unit Reynolds number, and it decreases with increasing Mach number and wall-to-stagnation temperature ratio.

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