Numerical Investigations on the Use of Roughness Patterns for Boundary-Layer Flow Control
Final Report Abstract
The present research is devoted to specific effects of structured, rough surfaces on boundary layer flows. Recently, this topic has become particularly relevant because new manufacturing methods have made it possible to produce new surface structures that were previously not considered possible. Findings from biology also allow us to think about technical applications in terms of bionics. Well-known bionic, micro-structured surfaces that have made it to application maturity are, for example, those that repel dirt (according to the lotus blossom effect) or reduce flow resistance (shark skin effect, so-called riblets). Hairy surfaces are also said to have properties that can influence a flow. However, the associated questions are still little studied. Therefore, the present work investigates how certain roughness patterns affect the instability of a laminar boundary layer and the frictional resistance of a turbulent channel flow. The considered roughness patterns consist of bundles or rows of slender, stiff fibres (bristles) which are attached to the wall and are significantly shorter than the boundary layer thickness. The work itself is based on the approach of direct numerical simulation, where the presence of fibres is simulated by body forces according to the Immersed Boundary Method (IBM). In addition to this method, several tools for the analysis of unsteady and unstable flows have been implemented and applied for mutual verification (in the absence of experimental data). The investigation of the influence of individual fibre bundles on the roughness-induced laminar-turbulent transition in a laminar boundary layer has led to a number of unexpected, new insights into the roughnessinduced laminar-turbulent transition in a generic boundary-layer flow. The present results show very nicely that the roughness geometry has a non-linear influence on the critical Reynolds number. For instance, at nominally equal roughness Reynolds number there are geometries which are more unstable than others. An increase in geometric complexity, as in the present case B compared to reference case A, leads to premature laminar-turbulent transition, while an increase in permeability of the configuration, as in case C compared to the others, leads to a stabilisation of the flow. One would not have expected before this work that the three selected cases would behave so differently. Thus, the present work comes up with new, conclusive results, also with respect to the influence of roughness on the demarcation between convective and absolute instability in the laminar boundary layer. In the simulations carried out in a turbulent channel flow fibres were aligned in streamwise rows for comparison with a riblet surface. The present results show, perhaps for the first time, that these fibre rows have a positive influence on the turbulent fluctuations and even reduce the local skin friction. Unfortunately, this positive effect on drag is consumed by the additional pressure drag of the fibres. A limited number of variations of fibre patterns have been investigated but the discovered influence on turbulent flow appears altogether correct and unique.
Publications
- Exploration of the Effect of Fibre Patterns on Transitional and Turbulent Flow. Dissertation University of Stuttgart, 2020
G. Axtmann
