Going against the flow

Going against the flow: An experimental investigation into the flow mechanics of dimpled surfaces in turbulent boundary layers

van Campenhout, O.W.G.

Veldhuis, L.L.M. (mentor)
van Oudheusden, B.W. (mentor)
Schrijer, F.F.J. (mentor)

Aerospace Engineering

Aerodynamics

2016-08-12

Any reduction in vehicle drag or fluid resistance provides a potential of substantial energy savings, with obvious benefits to the economy, environment and overall industrial competitiveness. Although various experimental studies have confirmed the potential drag reduction of dimpled surfaces in a turbulent boundary layer (BL), the working mechanism behind the effect remains largely unresolved. An experimental investigation is performed with the objective to strengthen the understanding of this novel aerodynamic surface and its interaction with the turbulent BL. Direct force measurements are combined with Particle Image Velocimetry (PIV) and Particle Image Surface Flow Visualization (PISFV). The direct force measurements reveal that the drag reduction is highly sensitive on flow conditions, a finding with significant implications for further research as well as for potential applications. Furthermore, the PIV and PISFV measurements reveal a spanwise oscillation at the surface and at 0.2δ due to the interaction of individual dimple flow topologies, which are of the converger-diffuser type. The measurement of this oscillation is the first of its kind and provides strong evidence of a state-of-the-art drag reduction theory: the interaction between dimples causes alternating spanwise excitations of the near-wall flow which interacts with the turbulent coherent structures and therefore leads to a reduction of the turbulent drag. This theory is in contrast to what has often been proposed in literature. Dimples potentially have significant advantages over other means of passive flow control for drag reduction: they are very shallow and therefore do not require complicated cleaning or maintenance procedures, also they are not prone to wear such as riblets. Furthermore, they can easily be (retro)fitted on skin panels. This research provides fundamental data that contributes to the understanding of the flow mechanics of these dimpled surfaces in turbulent BLs.

passive flow control
drag reduction
PIV
turbulent boundary layers

http://resolver.tudelft.nl/uuid:43525b03-f784-4db8-85c8-0d68918f1ac6

2018-08-12

Student theses

master thesis

(c) 2016 van Campenhout, O.W.G.