Flat-Plate Boundary Layers in Accelerated Flow

Abstract

Flow in turbomachines is generally highly turbulent. The boundary layers, however, often exhibit laminar-to-turbulent transition. But also relaminarization of the turbulent flow may occur. The state of the boundary layer is important, since it strongly influences transport phenomena like skin friction and heat transfer. It is therefore vitally important for the designer to understand the process of boundary layer transition and to determine the position of transition onset and the length of the transitional region. In order to get into the details of transition and relaminarization it is helpful to study simplified test cases first. Therefore, in this paper a relaminarization test case for a simple geometry is investigated: The boundary layer flow along a flat plate is exposed to acceleration with three different acceleration parameters, which is known as a crucial parameter for relaminarization. Measurements were performed for the inlet free-stream velocities of 5 m/s and 9 m/s. Several experimental techniques for detecting transition were tested at the institute before their application. In this work, Laser-Doppler anemometry (LDA) measurements were performed, since this optical technique is non-intrusive and does not disturb the flow. Therefore it can also be used in narrow flow passages where probe blockage can be crucial. As an outcome of this study, an insight into the process of relaminarization is presented. Although the key onset values for relaminarization stated in literature are fulfilled with the test setup, full relaminarization over the whole boundary layer has not been achieved. It seems, that using only the skin friction as indicator for relaminarization is not sufficient.

A spatially accelerating turbulent flow with longitudinally contracting walls

“…This then results in the formation of fewer vortices which are stronger due to acceleration-induced stretching. Consequently, this led to uncharacteristically more violent events, which correlates with the findings from the previous studies (McEligot & Eckelmann 2006;Bader et al 2016).…”

On the capability of transition models to predict relaminarization

“…To sum up, the wall shear stress and the velocity profiles have shown that the model is able to predict relaminarization. The behavior during relaminarization and retransition computed by the γ-Re θ transition model agrees qualitatively well with published measurements [Bader et al, 2016a] and DNS data ].…”

“…Therefore, at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology a project was launched in order to understand the process of relaminarization and its phases even further and test common threshold values for relaminarization by acceleration. First measurements performed at the institute already showed good results and relaminarization was detected [Bader et al, 2016a].…”