Drag reduction using trapped bubbles on a submerged flat plate surface
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Drag reduction using an array of thousands of tiny 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 the bubbles. Drag measurements were obtained for two different trapped bubble configurations. The first 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. The second configuration involved a trapped bubble array (TBA), which uses electrolysis to grow and maintain bubbles on the plate surface in thousands of tiny conductive holes. The TBA experiments are conducted on a vertically mounted plate, which demonstrates the versatility of this drag reduction method. Drag measurements over a range of Reynolds numbers were made on different plate configurations using three independent measurement techniques; the reliability of these results are demonstrated by agreement among the measured drag values as well as good agreement with an analytic turbulent flat plate solution. The large trapped bubble configuration showed an increase in drag reduction with increasing Reynolds number and demonstrated a maximum drag reduction of 32% corresponding to a slip bubble region covering 35% of the wetted plate surface. The trapped bubble array results were inconclusive. Total drag measurements on the plate agree among themselves and with the turbulent flat plate solution; however uncertainty analysis revealed drag measurement accuracy of only ±0.02 N at best using the proximity sensor measurement system. In general, the difference in drag on the flat plate with and without bubbles as indicated by the proximity sensor was less than 0.02 N, thus it is impossible to determine if the tiny trapped bubbles did indeed provide drag reduction. The temporal evolution of drag reduction using the trapped bubble array was also studied, but changes in drag appeared to be within the noise of the drag measurements. Finally, the efficiency of this drag reduction method was investigated in the laboratory setting. The trapped bubbles used in this drag reduction method are formed on the plate surface by electrolysis in the conductive holes, but not all of the gas produced in this process collects to form the trapped bubbles, and some energy is dissipated due to resistance in the water. To quantify the efficiency of this system, bubble formation efficiency plots (which map power input as a function of time to fill the bubble plate) were analytically determined and compared to the actual time to fill the bubble plate for various power input levels. The system approaches maximum (~95%) efficiency at lower power input levels (7.22 W/m2), requiring approximately 15 minutes to fill the bubble plate; conversely, the plate approaches 50% efficiency at high power input level (262 W/m2) while the plate fills within 2 minutes.