Dynamic pressure response of water flow between closely spaced roughened flat plates

Abstract

A flat plate tester was designed and built to determine friction factors and dynamic pressures for water flow over smooth, knurl, and cavity plates. Reynolds numbers between 2000 and 35000 were obtained at plate clearances of 0.076 mm to 1.270 mm. The friction factors were found to differ significantly from the Moody diagram in that at small clearances, laminar flow appears to not always conform to the [] line predicted by theory. In addition, it is seen that increasing the gap between the plates from the smallest clearance tested (0.076 mm) to the largest clearance (1.270 mm) results in a friction factor increase. This situation does not contradict the Moody diagram, which shows that friction factor should decrease for increased gap. Instead, it illustrates an evolving scenario within the flow field. The smaller gaps exhibit signs of Tollmein-Schlichting waves, which did not transition into turbulence because the smaller gaps tend to suppress the creation of vorticies. The result is a laminar flow field which renders substantially lower friction factors. As the clearance is increased, turbulent activity increases yielding, increased friction factors. As the gap increases further, the friction factor plateaus and then decreases with increased Gap, indicating a fully turbulent flow regime. In addition, while certain cavity geometries were found to generate oscillatory pressures, there was no normal mode resonance indicative of the "friction factor jump" reported by Ha (1991).