A. Stroh

A comparison of opposition control in turbulent boundary layer and turbulent channel flow

A comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented. It is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers. However, a detailed analysis of the dynamical contributions to the skin friction coefficient reveals significant differences in the mechanism behind the drag reduction. While drag reduction in turbulent channel flow is entirely based on the attenuation of the Reynolds shear stress, the modification of the spatial flow development is essential for the turbulent boundary layer in terms of achievable drag reduction. It is shown that drag reduction due to this spatial development contribution becomes more pronounced with increasing Reynolds number (up to Reτ = 660, based on friction velocity and boundary layer thickness) and even exceeds drag reduction due to attenuation of the Reynol...

Secondary vortices over surfaces with spanwise varying drag

ABSTRACTA spanwise heterogeneity of streamwise drag is known to lead to the formation of large secondary motions of Prandtls second kind. Based on the data sets extracted from direct numerical simulations (DNS) of fully developed turbulent channel flow where streamwise stripes of free-slip surface with varying spanwise extension are introduced, we investigate the topological structure of the secondary motions. We find a complex restructuring of the secondary motion with increasing extent of free-slip/no-slip region where the width of the free-slip region in viscous units appears to be one important governing parameter for the vortex formation. The most striking feature of this restructuring is a change in the rotational direction of the major vortex pair such that the related high- and low-momentum pathways are found at different locations. The present results reveal that the spanwise inhomogeneity of the Reynolds stress distribution is strongly related to the observed change of rotational direction. In ...

Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer

A numerical investigation of two locally applied drag reducing control schemes is carried out in the configuration of a spatially developing turbulent boundary layer (TBL). One control is designed to damp near-wall turbulence and the other induces constant mass flux in the wall-normal direction. Both control schemes yield similar local drag reduction rates within the control region. However, the flow development downstream of the control significantly differs: persistent drag reduction is found for the uniform blowing case whereas drag increase is found for the turbulence damping case. In order to account for this difference the formulation of a global drag reduction rate is suggested. It represents the reduction of the streamwise force exerted by the fluid on a finite length plate. Furthermore, it is shown that the far downstream development of the TBL after the control region can be described by a single quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a changed virtual origin. Based on this result, a simple model is developed that allows relating for the local drag reduction rate to the global one without the need of conducting expensive simulations or measurements far downstream of the control region.

The influence of frequency-limited and noise-contaminated sensing on reactive turbulence control schemes

The opposition control proposed by Choi etxa0al. [1] is investigated using direct numerical simulation to see how bandwidth-limited and noise-contaminated sensing influence its control performance. It is found that the sensing frequency can be reduced substantially when the control scheme is modified to account for the convection speed of the measured sensor signal. Noisy sensor signals are shown to strongly affect the achievable net energy saving rate and energy gain at high sensing frequencies, while their influence is weaker for reduced sensing frequencies. These results also hold for increased Reynolds number, where the performance of the control scheme is generally lower.

Control of Spatially Developing Turbulent Boundary Layers for Skin Friction Drag Reduction

This project comprises direct numerical simulations (DNS) of turbulent boundary layer flows. The wall along which the flow develops is modified in some parts in order to introduce control techniques that aim at a reduction of skin friction drag. The obtained results are used in two ways. They are first compared with turbulent channel flows, in which the applied control schemes were originally developed. Second, the flow development after a controlled section in a turbulent boundary layer is analyzed. The detailed scientific results that were obtained based on the generated data are published in Stroh et al. (Phys Fluids 27(7):075101, 2015; J. Fluid Mech 805:303–321, 2016).

Reactive Control of Turbulent Wall-Bounded Flows for Skin Friction Drag Reduction

The work presents an attempt to improve the understanding of reactive drag reducing control of near-wall turbulence with respect to limitations present in a realistic application scenario. The reference reactive control scheme of opposition control is studied using direct numerical simulation through the consideration of various application oriented restrictions, i.e. local control application, limited spatial and temporal resolution of the scheme, sensor noise and control elements arrangement.