Homogenization of streaks in a laminar boundary layer

Abstract

The present work, based on experimental, numerical and theoretical investigations, introduces a method to homogenize streaks in the laminar boundary layer. The streaks are created by a spanwise array of roughness elements on the surface of a flat plate. A homogenization body in the form of a horizontal bar is added at a downstream location away from the roughness array to homogenize the velocity differences of the streaks in the laminar boundary layer. Measurements are done with hot-film anemometry and supported by numerical simulations and linear stability theory. The streak amplitude can be significantly reduced with the proposed homogenization body. Furthermore, the reduction in spanwise gradients of the mean velocity leads to a significant reduction in the sinuous instability of the streaky flow. The effects of the homogenization body on the displacement thickness and the observation of flow unsteadiness downstream of the homogenization body are discussed. The present work thus proposes and explores a passive technique to control undesired streaks in the laminar boundary layer. Graphic abstract

Transition mechanisms in a boundary layer controlled by rotating wall-normal cylindrical roughness elements

“…The channel provides a reproducible measurement environment for flat-plate laminar boundary-layer studies. The turbulence intensity is 0.05 % between 0.1 and 10 Hz (Wiegand 1996; Puckert, Wu & Rist 2020). A Dantec 55R15 hot-film probe is connected to a Dantec Streamware bridge, which works according to the constant temperature anemometer principle.…”

Linear stability analysis of a boundary layer with rotating wall-normal cylindrical roughness elements

“…The solution procedure is implemented in Python code, the validation of which has been performed by comparing the TS-wave amplitude and its growth rate with numerical simulation (Wu & Rist 2020). Another successful application of the code can be found in Puckert, Wu & Rist (2020).…”

Delay of laminar–turbulent transition by counter-rotating cylindrical roughness elements in a laminar flat plate boundary layer

“…They can be detected by repeating the measurements, which is not possible for systematic errors. In this work, the measurement uncertainties are determined as demonstrated in Appendix 2 from Puckert et al (2020). To obtain the total systematic error, the authors compared the mean velocity measured by the hot-film probe and the mean velocity identified by a hydrogen-bubble visualization in the LaWaKa.…”

“…To obtain the total systematic error, the authors compared the mean velocity measured by the hot-film probe and the mean velocity identified by a hydrogen-bubble visualization in the LaWaKa. Moreover, they give the systematic error resulting from using a mercury thermometer to determine the kinematic viscosity , and they provide an appropriate systematic error for the cylinder height k. The total systematic error from the current LaWaKa measurement setup is given with (Puckert et al 2020): sys = 2.6e − 3 m s −1 ( sys ∕U e = 2.6% ). The random error cannot be taken from Puckert et al (2020) because it has to be determined individually for each measurement.…”

“…Moreover, they give the systematic error resulting from using a mercury thermometer to determine the kinematic viscosity , and they provide an appropriate systematic error for the cylinder height k. The total systematic error from the current LaWaKa measurement setup is given with (Puckert et al 2020): sys = 2.6e − 3 m s −1 ( sys ∕U e = 2.6% ). The random error cannot be taken from Puckert et al (2020) because it has to be determined individually for each measurement. The random error is determined with a free-stream measurement and validated by several measurements at different free-stream positions.…”