Widerstandsreduktion kompressibler turbulenter Strömungen durch wandbasierte Beeinflussung

An active flow control method is used with the aim of friction drag reduction. The method utilizes streamwise oscillation of spanwise velocity at the wall. Owing to reduced turbulence intensity, less energy is required to drive the flow against viscous resistance. The key question is how compressibility affects the drag reduction results.
A huge dataset of different flow cases with carefully selected parameter combinations has been created to reliably extract the influence of important flow parameters on the flow behavior. Direct numerical simulations and large eddy simulations of subsonic and supersonic channel flow were run with Mach numbers based on the mean velocity of Ma = 0.3, 1.5 and 3.0 and Reynolds numbers Re based on the friction velocity in the range of 190 up to 2540. Mean property effects are predominant in supersonic channel flow through large variations of the mean density and temperature and wall cooling. Consequences are a higher Reynolds stress anisotropy, increased length scales in the viscous sublayer flow and enhanced streak stability. The pressure-strain correlation and spanwise dissipation terms undergo a strong attenuation compared to the incompressible counterparts, especially in the controlled flows. Higher drag reduction and a larger optimum control wavelength are observed compared to the corresponding incompressible flows. Increasing the Mach number enhances these effects due to stronger mean property variations. A mitigation of near-wall compressibility effects occurs with an increase of the bulk Reynolds number, though, which attenuates the drag reduction benefits in the controlled flow as well.
In the second part of this work, cooling terms were introduced in the Navier-Stokes equations to mimic the wall-normal temperature profile of an external boundary layer flow and to minimize variable property effects within the complete channel, respectively. This approach allows for a better isolation of intrinsic compressibility effects and wall cooling effects. The cooling strategies highlight the importance of intrinsic compressibility effects, which contribute to an increased control efficiency, too.
As a potential case of application for the flow control method, a jet flow emanating from a round pipe is considered in the third part of this work. The turbulent subsonic pipe flow is manipulated by the transverse wall velocity and the focus is on the impact on the jet development. No direct interference of the streanwise oscillation pattern of transverse velocity within the pipe and the jet region is detected. However, the lowered turbulence intensity at the pipe exit noticeably affects the jet flow development, independent of how the reduction is achieved. The shear layer of the controlled jet flow develops with higher turbulence intensities associated with intensified Kelvin-Helmholtz instabilities. The effects can be relativized if the maximum transverse velocity is located directly at the pipe exit. Consequently, a jet with similar characteristics to the uncontrolled one can be generated with the advantage of a substantially lowered energy expenditure.