Measurements of wall pressure fluctuations under a turbulent boundary layer were made on the fuselage of a sailplane. This flow offers a noise-free environment with a low free stream turbulence level. The axisymmetric boundary layer undergoes natural transition and develops in a zero pressure gradient region. Spectra of the wall pressure were found to decrease at low frequency in agreement with calculations based upon a turbulence-mean shear interaction mechanism. Velocity fluctuations a t several positions within and outside the boundary layer were measured and correlated with the wall pressure. A special conditional correlation method was also employed to find the contribution of various velocity fluctuations to the wall pressure. A conditioning signal was formed based upon the signs of u and v and the turbulent-non-turbulent nature of the flow. This signal was time lagged and correlated with the wall pressure signal. It was found that in the outer portion of the boundary layer (y/6 > 0.5), irrotational motions were more highly correlated with the wall pressure than vortical motion.

Dimensional analysis of the orifice impedance problem produces several sets of nondimensional variables which may be used to correlate nonlinear orifice behavior. Published experimental data on thin orifices were examined and it was found that tests with oil, water, and air can be correlated. Graphs of orifice resistance are given in nondimensional forms that are useful over a wide range of conditions. The form for resistance is R/ρ (νω)1/2 as a function of an aspect ratio parameter l/d, an orifice velocity parameter V/(νω)1/2, and a diameter-viscous parameter d/(ν/ω)1/2. For thin orifices the resistance is independent of d/(ν/ω)1/2 when this parameter is greater than 10. Nonlinear effects begin when V/(ν/ω)1/2=3. This parameter compares the displacement amplitude to the viscous diffusion length. The reactance end correction δ was normalized by the Rayliegh value of 8d/3π and correlated using the variables l/d, V/ωd, and d/(ν/ω)1/2. Again thin orifices show no dependence on d/(ν/ω)1/2 for values greater than 20 and the nonlinear reactance region begins at V/ωd=0.1. Thus, different parameters govern the onset of nonlinear resistance and reactance. Subject Classification: [43]85.20, [43]85.50; [43]28.60.

The acoustic impedance of an orifice under a turbulent boundary layer was measured. A single orifice, 0.508 cm in diameter and 0.32 cm thick, was tested in a wind tunnel where the velocity was varied from 16 to 30 m/sec. An impedance tube mounted behind the orifice was operated at frequencies from 250 to 2200 Hz with sound pressure levels from 85 to 140 dB. It was found that the resistance and reactance end correction could be nondimensionalized as R/ρ (νω)1/2 and δ/δeo, respectively, and correlated as functions of l/d, ωd2/ν, u*d/ν, and V/u*. The symbols are R resistance, ρ density, ν kinematic viscosity, ω frequency, δ end correction, δeo linear end correction without flow, l orifice thickness, d diameter, u* boundary layer friction velocity, and V acoustic velocity amplitude. Nonlinear behavior occurred when V/u* was greater than three. Amplitudes greater than three were only obtained in the 250-Hz tests, however, at this frequency the V/u*≳3 criteria was independent of u*d/ν. In the linear range R/ρ (νω)1/2 versus ωd2/ν produced U shaped curves parametrically dependent on u*d/ν. When u*d/ν was greater than 250 the resistance with flow was greater than the no flow for all frequencies. If u*d/ν was less than 250 there was a frequency range where the resistance with flow was less than the no-flow resistance. The reactance end correction as a function of these same variables gave an inverted U shape. Above u*d/ν=250, the reactance decreases with flow for all frequencies, while below 250 there is a frequency range in which the reactance increases over the no-flow reactance. Subject Classification: [43]85.20; [43]28.60.

No abstract