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
Highlights
One of the first visualizations of streaks in a laminar boundary layer was done by Mochizuki (1961) downstream of a hemispherical roughness element
A spanwise array of 6 cylindrical roughness elements with height k = 10 mm and diameter d = 1.5 k is placed at xk = 87 k from the leading edge of the flat plate to create streamwise streaks at a regular spacing
Small-scale vorticity disturbances induced locally behind the horizontal bar may enhance the exchange of momentum between neighboring high-speed and low-speed streaks, which leads to further homogenization
Summary
One of the first visualizations of streaks in a laminar boundary layer was done by Mochizuki (1961) downstream of a hemispherical roughness element. If parameters are chosen correctly, the attenuation of TS waves can lead to a delay of laminar-to-turbulent transition as shown in numerous studies (Fransson et al 2006; Fransson and Talamelli 2012; Sattarzadeh et al 2014). The explanation for this observation is an additional Reynolds stress term which decreases the perturbation kinetic energy (Cossu and Brandt 2004) and attenuates TS waves. Page 3 of 15 122 flows, studies on passive methods to control streaks are not known to the authors The objective of this investigation is to homogenize streaks in laminar boundary layers. The main purpose of the numerical simulation is to provide a base flow for linear stability analyses
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