ISBN 9783843945967

978-3-8439-4596-7, Reihe Ingenieurwissenschaften

Gabriel Axtmann
Exploration of the Effect of Fibre Patterns on Transitional and Turbulent Flow

138 Seiten, Dissertation Universität Stuttgart (2020), Softcover, A5

Zusammenfassung / Abstract

New kinds of roughness elements/fibres and their effect on transitional and turbulent flow are investigated numerically. Therefore direct numerical simulations (DNS) of discrete-roughness elements by using an immersed boundary method have been performed for the laminar part of a boundary layer on a flat plate. Three different roughness configurations are carried out. The first one refers to a classical isolated roughness element, while for two further configurations porosity was added.

For the sub-critical regime, two dominant steady vortex systems are identified for all configurations by obtaining steady state flow solutions via selective frequency damping (SFD). A horseshoe vortex in front of the element with two trailing legs wrapping around the roughness element is formed out for all configurations. Further an inner vortex pair originates from the recirculation zone behind the roughness element. By adding porosity, the vortex system exhibits a certain change. The most important difference of the baseflow obtained by SFD in comparison to the non-porous roughness element is that the separation wake and the resulting vortex system is strengthened for the porous cases. This highly affects the stability which is confirmed by non-linear disturbance calculations by means of DNS.

From laminar to turbulent flow, the design and shape of the roughness elements change. The aim is to mimic the drag-reducing effects of riblets by new kind of fibre patterns. They consist of thousands of small vertically aligned cylindrical elements in a first and second configuration and in a third configuration inclined in flow-direction. All different setups are situated in a turbulent channel flow under constant pressure gradient conditions. Detailed analysis of mean properties and turbulent statistics are carried out. Furthermore, a momentum balance gives deeper insights into the performance evaluation of those elements.