Measurements of local skin friction in a microbubble-modified turbulent boundary layer

Abstract

Local skin-friction reductions have been measured using an array of flush-mounted hot-film probes in a microbubble-modified, zero-pressure-gradient, turbulent bounddary layer. The results of earlier integrated skin-friction measurements, that showed the reduction to be a function of plate orientation, gas-flow rate and free-stream velocity, have been confirmed both qualitatively and quantitatively. With the measurement plate oriented so that buoyancy keeps the bubbles in boundary layer, it is shown that skin friction is reduced monotonically for all air-flow rates at each of three free-stream velocities between 4 and 17 m/s. For the opposite plate orientation it is possible for increasing gas injection to lead to smaller local skin-friction reduction at the lowest speeds. Drag reduction appears to persist for as much as 60–70 boundary-layer thicknesses downstream of the injection region. It is further shown, using a probe flush-mounted just upstream of the injection section, that there is no apparent upstream interference due to the gas injection. Spectral measurements indicate that microbubbles can cause a reduction of high-frequency shear-stress fluctuations. This suggests a destruction of some of the turbulence in the near-wall region.

Microbubble skin friction reduction on an axisymmetric body

“…A second series of experiments, employing an array of hot film probes on the balance was also conducted by MDM [2].…”

“…The injection of a gas (to form microbubbles) into a liquid turbulent boundary layer has been shown, in several studies [1][2][3][4][5][6][7][8], to reduce drag. For the most part, these studies have looked at the skin friction reduction obtainable in a flat plate geometry [1][2][3][4][5][6][7].…”

“…For the most part, these studies have looked at the skin friction reduction obtainable in a flat plate geometry [1][2][3][4][5][6][7].…”

Réduction des pertes de frottement pour des écoulements à surface libre avec entraînement d'air. Augmentation de la débitance d'un coursier d'évacuateur de crues

“…Ces valeurs sont du même ordre de grandeur que les mesures obtenues par BOGDEVICH et al (1977) et MADAVAN et al (1984, 1985 (tab/. 1/), dont les résultats ont été utilisés pour valider le modèle de MARIE (1987).…”

Turbulent drag reduction using fluid spheres

“…A key property of a drag-reducing particle is a slender shape, a property which is shared by linear polymers (Virk 1975), macroscopic fibres (McComb & Chan 1985), micelles formed by surfactant (Warholic, Scmhidt & Hanratty 1999b) and worm-like agglomerates formed by charged clay particles (Gust 1976). Drag reduction is also observed for spherical solid particles in the gas phase (Rashidi, Hetsroni & Banerjee 1990) and for (nearly) spherical gas bubbles in the liquid phase (Madavan, Deutsch & Merkle 1985).…”