The effects of trapped bubbles on viscous drag reduction for submerged surfaces

The effect on viscous drag of trapped bubbles on a submerged flat plate was investigated. The objective was to determine if viscous drag reduction could be obtained by replacing portions of the solid no-slip surface of the plate with areas of near-slip formed by bubbles. One configuration involved a large bubble trapped on the bottom surface of a horizontally mounted plate, which provides insight as to the maximum drag reduction obtainable using the trapped bubble concept. A second configuration involved a trapped bubble array (TBA), which used electrolysis to grow and maintain bubbles on the plate surface in thousands of tiny conductive holes. The TBA experiments were conducted on a vertical plate, to demonstrate the versatility of this drag reduction method. Drag measurements were acquired over a range of Reynolds numbers using either a force balance for plates mounted in a vertical orientation, or by performing a momentum integral balance using a LDA wake survey for plates mounted in either vertical or horizontal orientations. Results show that a drag reduction of up to 32% was obtained for the large trapped bubble case, while negligible drag reduction was obtained for the array of tiny trapped bubbles.