The Effect of Cross Flow and Isolated Roughness Elements on the Boundary Layer and Wall Pressure Fluctuations on Circular Cylinders
Abstract : In this paper pressure fluctuation measurements on smooth cylinders aligned with the flow have been extended to include the effect of slight cross flow and isolated roughness elements. A yaw angle of a few degrees is sufficient to produce severe asymmetry of the outer portion of the boundary layer as deduced from impact pressure measurements using computer controlled traversing gear. The root-mean-square surface pressure fluctuations are a maximum on the windward surface but only slightly larger than with no yaw. On the leeward surface they are much lower. The power spectrum of the pressure has greater energy density at high frequencies on the windward surface and at low frequencies on the leeward side. With no cross flow the root-mean-square surface pressure is greatly increased by a cylindrical collar placed around the cylinder. A collar which increases the diameter by 50% more than doubles the wall pressure three diameters downstream. The spectra of the pressure scale with collar thickness and the intensity of the surface pressure fluctuations scales with the ratio of collar thickness to distance downstream. (Author)
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.
Analysis of Turbulent Skin Friction in Thick Axisymmetric Boundary Layers
An analysis is developed for turbulent flow over arbitrary bodies of revolution. The theory leads to a single, first-order, ordinary differential equation for the skin-friction coefficient. Arbitrary distributions of Mach number, pressure gradient, and body shape are allowable. Transverse curvature is accounted for and the boundary layer may be arbitrarily thick. Application is made to supersonic flow past long cylinders, thin cones, and a pointed body of revolution. Agreement with experiment is good in all cases. Boundary-layer shape factors are avoided entirely in this theory, and integral thicknesses are eliminated from the final differential equation.