EFFECT OF HEATED UNDERWATER BOUNDARY-LAYER OF A PROLATE SPHEROID ON VISCOUS DRAG

A prolate spheroid submerged in water can be heated for decreasing the viscous drag because of a decrease in viscosity with increasing tempe rature. The heated boundary layer experiment, based on this principle, was carried out in a circulating water channel of NCKU, and the visco us drag of a spheroid, with a five-to-one ratio of length to mid-diame ter, was also measured by means of wake surveys. The difference of tot al-head between wake and undisturbed region was measured by two total- head tubes, and the water speed in the wake area was measured by an el ectromagnetic flowmeter. The results indicate that the viscous drag of the model decreases with surface heating, and the decrease in viscous drag of the model is 13% when the surface temperature is 17 degrees C above the ambient water temperature. The velocity gradient in the wak e region and the total-heating differential readings between undisturb ed and wake region also decrease with surface heating due to the delay ed laminar-turbulent transition in the boundary laver. The values of t he coefficient of viscous drag obtained without surface overheating ar e found to be in agreement, for the range of Froude numbers investigat ed, with the results obtained from previous experiments.