Study of Static Pressure Along a Rough Boundary

An experimental study of the effect of roughness on the measurement of static pressure at a wall is reported. Uniform spheres were used to represent the roughness elements; they were packed in the maximum density pattern, one layer thick. Air in the form of a radial jet passed over the rough wall. Pressure measurements were made at the tops and bottoms as well as at several other points on the periphery of the spheres. The measurements showed that the pressure of a given sphere varied considerably depending on the location of the pressure tap. Readings from a series of top or bottom taps yielded pressure gradients of opposite sign and neither gradient agreed with that determined from the average vertical force on the sphere. The question of where to measure static pressure on a rough surface remains to be answered.