Secondary Instabilities of Görtler Vortices in High-Speed Boundary Layers

Secondary Instabilities of Görtler Vortices in High-Speed Boundary Layers
Author: Jie Ren
Publisher: Springer
Total Pages: 110
Release: 2017-10-27
Genre: Technology & Engineering
ISBN: 9811068321

This thesis first reveals the mechanism of Görtler instabilities and then demonstrates how transitions at hypersonic flows can be effectively controlled (either promoted or suppressed) with Görtler or Klebanoff modes. It focuses on understanding and controlling flow transitions from mild laminar to fully turbulent flows at high speeds—aspects that have become crucial at the dawn of an incredible era, in which hypersonic vehicles are becoming available. Once this occurs, it will be possible to travel from Beijing to Los Angeles within just 2 hours, and we will all live in a genuinely global village—and not just virtually, but physically. Görtler instabilities have often been used to promote flow transition in hypersonic vehicles. However, how Görtler instabilities are excited and how they evolve in hypersonic flows are questions that have yet to be answered.

The Effect of Wall Cooling and Heating on Görtler Vortices in High-speed Boundary Layers

The Effect of Wall Cooling and Heating on Görtler Vortices in High-speed Boundary Layers
Author: Safae El Amrani
Publisher:
Total Pages: 66
Release: 2018
Genre:
ISBN:

strong velocity gradients in both wall-normal and spanwise directions. This determines the flow structures to become more prone to secondary instabilities, which prompt to an early transition from laminar to turbulent flow, ultimately increasing the frictional drag. It is possible to circumvent these secondary instabilities by means of passive or active flow control strategies. In this thesis, the effect of wall cooling and heating on Görtler vortices developing in supersonic and hypersonic boundary layers is investigated from a numerical point of view. The wall temperature is imposed through a ramping function that decreases or increases an upstream wall temperature in the streamwise direction. The results show that this type of wall cooling or heating has a mild (adverse) effect on the vortex energy, and a considerable (but beneficial) effect on the wall shear stress.

Control of Görtler Vortices in High-speed Boundary Layers

Control of Görtler Vortices in High-speed Boundary Layers
Author: Radwa Alaziz
Publisher:
Total Pages: 76
Release: 2017
Genre:
ISBN:

Görtler vortices develop in boundary layer flows over concave surfaces due to the imbalance between centrifugal forces and the wall-normal pressure gradient. These vortices can be efficient precursors to transition in boundary layers exposed to freestream disturbance or surface non-uniformities, because they can alter the mean flow causing the laminar flow to breakdown into turbulence. In this thesis, a control technique aimed at reducing the energy associated with Görtler vortices that develop in supersonic boundary layers is introduced and tested. The control algorithm is based on distributed blowing and suction, with sensors placed either in the flow or at the wall. The result show that there is a dependence between the efficiency of the control algorithm and the spanwise separation of the vortices, that is the energy reduction is more significant for larger spanwise separations. The efficiency of the control algorithm seems to be insensitive to the variation of the Mach number.