On the turbulent drag reduction of a dimpled surface under a pressure gradient

On the turbulent drag reduction of a dimpled surface under a pressure gradient

van Weersch, Yorin (TU Delft Aerospace Engineering)

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

Delft University of Technology

Aerospace Engineering

2017-11-08

Dimples are spherical indentations applied at a surface that have been found to yield a skin-friction drag reduction within a turbulent boundary layer. Shallow dimples have inherent advantages over traditional flow control techniques that require heavy active systems to be installed or pose maintenance problems. Proven drag-reducing mechanisms show reduced turbulent properties, like a reduced Reynolds shear stress, reduced turbulent intensity and an upwards shift of the logarithmic layer leading to a lower skin-friction coefficient. Dimpled surfaces will encounter varying pressure gradients in applications like airfoils, and thus the effect of both a favorable and adverse pressure gradient on the drag-reducing potential of a dimpled surface is explored in an experimental investigation. An experimental set-up is devised that utilizes a planar diffuser to induce a pressure gradient. A novel flexure-based drag balance is used to measure the total drag difference between a dimpled and flat surface in a direct force measurement campaign. To investigate if the drag reduction is attained by manipulation of the turbulent properties, particle image velocimetry technique and hot-wire anemometry technique to quantify the boundary layer properties. After confirming good repeatability of the drag balance, a drag reduction is observed from the direct force measurements for the dimpled surface in the zero pressure gradient (ZPG) configuration. A preliminary drag increase is observed followed up by an increasing drag reduction with increasing Reynolds number, yielding a maximum drag reduction of 0.8%, however no significant changes are found in the turbulent properties. For the favorable pressure gradient a drag reduction is observed from the upwards shift of the logarithmic layer and this drag reduction is also found from the force measurements, however with considerable uncertainty. The increase in laminarization parameter and the absence of a logarithmic layer shift for the ZPG case point into the direction of a beneficial effect of the dimple on relaminarization properties. The adverse pressure gradient case showed an increase in turbulence properties with no clear upwards shift of the logarithmic layer.

drag reduction
turbulent boundary layer
pressure gradient
dimples

http://resolver.tudelft.nl/uuid:cf0b3ec3-f018-4e39-9fd7-587eb75a6c47

Student theses

master thesis

© 2017 Yorin van Weersch