Sound Level Effect on Perforated Panel Boundary Layer Growth

Abstract

The relationship between the boundary layer velocity profile and the skin friction coefficient for a perforated panel is well known. Numerous studies have been done to determine the effects of surface roughness, transpiration flow and suction on the skin friction coefficient and correspondingly on the boundary layer growth. Previous studies have focused on a net mass flow either into or out of a sample, whereas, the effect of sinusoidal flow conditions, such as an acoustic harmonic wave, has not been previously investigated. This sound level effect on boundary layer growth and skin friction is the primary focus of the current study. Perforated panels with hole diameters of 0.020, 0.040, and 0.050 inches and Percent Open Areas (POAs) of 7 and 9 percent and one hard wall, un-perforated panel were evaluated to determine the effect of sound level on the boundary layer growth. Testing was conducted at 3 Mach numbers (0.17, 0.30, and 0.50) and two sound pressure levels (SPLs), where the SPL ranged from the background condition to a band-limited, random noise signal that was 5 to 10 dB above the background level. For each panel, the boundary layer velocity profile was measured at sufficient locations along the length of the panel to accurately measure the boundary layer growth at each test condition defined by Mach number and sound level. A review of the data acquired indicates that an increase in the sound pressure level results in an increase in the skin friction coefficient. This behavior is observed in varying degrees for all of the acoustically treated panels at all Mach numbers while the hard wall panel shows little difference in the skin friction coefficient at higher sound levels.