Control of near-wall turbulence via slip and transpiration

Lead Research Organisation: University of Cambridge
Department Name: Engineering

Abstract

Turbulent motions near the wall are dominated by a quasi-cyclic, self-sustaining process driven by the interaction between two key coherent structures: quasi-streamwise vortices and streaks. This process has been observed to contribute significantly to turbulent skin friction drag, suggesting that turbulent flow control techniques would benefit by seeking to disrupt it. One such technique is to manipulate the flow in such a way that results in the turbulence statistics of the different velocity components being shifted relative to each other in the wall normal direction. This may be achieved with complex geometrical structures, such as riblets, or with the use of non-conventional boundary conditions in numerical simulations, such as surface slip. Another technique, know as transpiration, counteracts velocity fluctuations near the wall by flow injection and suction at the wall itself, and can also result in a relative shift in the turbulence statistics. The primary aim of this project will be to simulate, understand and model the combined effect of surface slip and transpiration on near-wall turbulence by using direct numerical simulations (DNS). It might then be possible to identify an optimum combination of streamwise slip, spanwise slip and wall-normal transpiration for drag reduction that could later be used to aid in the design of novel control techniques.

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509620/1 01/10/2016 30/09/2022
1965893 Studentship EP/N509620/1 01/10/2017 31/07/2021 Joseph Ibrahim
 
Description We have examined the effect on near-wall turbulence of displacing the apparent, virtual origins (or virtual 'walls') perceived by different components of the overlying flow. This mechanism is commonly reported for drag-altering textured surfaces of small size. We conducted a series of direct numerical simulations of turbulent channel flows, applying streamwise slip, spanwise slip and wall-normal transpiration boundary conditions, in order to displace the virtual origin perceived by each velocity component. These virtual origins can be thought of as the height at which each velocity component appears to go to zero, relative to the simulation domain boundary. Our results support the idea that the relevant parameter in determining the change in drag is the offset between the virtual origins perceived by the mean flow and the virtual origin perceived by the near-wall turbulence dynamics (the virtual origin for turbulence). The virtual origin perceived by the mean flow depends only on the virtual origin perceived by the streamwise velocity. Meanwhile, the virtual origin for turbulence results from the combined effect of the virtual origins perceived by the wall-normal and spanwise velocities. Other than shifting the origin perceived by turbulence, we have found that turbulence remains essentially the same as over a smooth surface. We have hence shown that the changes in turbulent quantities typically reported in the flow-control literature are often merely a result of this origin-offset effect. Through empirical and physical arguments, we have derived a simple expression that can predict the virtual origin for turbulence.
Exploitation Route Certain types of small-scale surface texture have the potential to passively achieve a reduction in turbulent skin-friction drag. This is interesting to the transport sector, among others, as a means of increasing fuel efficiency. The results of our study could be used to better understand the interaction between near-wall turbulence and small-scale surface texture, which, in the future, could lead to the design of textured surfaces that can achieve higher levels of drag reduction.
Sectors Aerospace, Defence and Marine,Transport
URL https://doi.org/10.1017/jfm.2021.13