Numerical investigation of boundary layer instability and transition induced by rotating wall-normal cylindrical roughness elements

Beschreibung

The present thesis investigates numerically the influence of a new type of surface roughness, i.e. the wall-normal rotating cylindrical roughness elements, on the boundary-layer stability and transition to turbulence. The generated streaky flow with low rotation speeds is shown to be effective in suppressing the typical Tollmien–Schlichting (TS) modes and eventually achieving laminar flow control. With high rotation speeds, the cylinders are also found effective in tripping turbulence. A primary feature of the rotating-cylinder-induced-wake is a dominating inner vortex (DIV) encircled by a secondary inner vortex (SIV), which intensifies the lift-up effect and creates high-amplitude velocity streaks. Linear stability analysis shows that the modified streaky flow is capable of effectively stabilizing the TS modes, the mechanism of which, as found by a perturbation kinetic energy analysis, is attributed to the reduction of the wall-normal perturbation production. Further, TS-mode attenuation up to complete stabilisation is confirmed with a direct numerical simulation (DNS). The induced laminar-turbulent transition is investigated by means of DNS. The circumferential velocity of the rotating cylinder subs accelerates the adjacent fluid and decelerates it on the opposite side. As the rotation speed increases, the boundary-layer experiences a super-critical Hopf bifurcation. With low rotation speed, perturbation is initiated by a combination of elliptical and centrifugal instabilities in the near wake. At medium rotation speeds, Taylor-Couette-like streamwise vortices are generated on the decelerated side, resulting in a protruding reverse-flow zone. At the largest rotation speed investigated, the onset of perturbations is directly located on the decelerated side of the cylinder stubs, where a deceleration mechanism feeds the instability.

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Yongxiang Wu

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Boundary layer stability Rotating cylindrical roughness Boundary layer control

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