Experimental Investigation of Boundary Layer Relaminarization in Accelerated Flow

Abstract

Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to-turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In this paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser Doppler anemometry (LDA). Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow “freeze,” while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid retransition to turbulent flow is observed. The findings of this work also describe the mechanism of retransition.

Turbulence Structures in Accelerated Flow over a Flat Plate with Non-Zero Pressure Gradient

“…It is ascertained in their study that these two conditions are coupled, as the accelerated flow on the rough wall leads to higher turbulence production and higher wake intensity, which causes a higher pressure drag and higher transfer between the wake's energy and that of the turbulence. According to that, Bader et al (2018) implemented experimental and numerical research using turbulence model for the numerical part and their focuse was on the relaminarization and re-transition of the turbulent boundary layer. More recently, Saavedra et al (2020) compared DNS and RANS calculation using the k-ω-SST model in free stream accelerated flow over a flat plate.…”