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)
Asymmetric turbulent boundary layers along long thin circular cylinders at low-Re
Notable deviations of the asymmetric turbulent boundary layer (TBL) statistics from their axisymmetric counterpart along long thin circular cylinders are vitally important to the naval and oceanographic jurisdictions. Although the available experimental evidence backs their concern, the realm of parametric variability (both geometric and kinematic) is extremely limited to draw solid conclusions. We know that only small misalignments which quantify less than one degree of incidence between the freestream and the straight cylinder axis can substantially alter the boundary layer thicknesses, mean axial velocity, and Reynolds stresses. But the statistical database is plainly inadequate to justify modifying the design tools that were founded solely for axisymmetric flow conditions. Herein, we begin rectifying this drawback by numerical means. The investigation centers on low turbulent Reynolds numbers (500 ≤ Rea ≤ 2500) and small angles-of-incidence (0° < α < 9°) to validate and complement the lions-share of t...
Turbulent boundary-layer wall-pressure fluctuations on smooth and rough walls
Turbulent boundary-layer wall-pressure measurements were made with ‘pinhole’ microphones three times smaller (relative to a boundary-layer displacement thickness) than microphones used in earlier work. The improved high-frequency resolution permitted examination of the influence of high-frequency eddies on smooth-wall pressure statistics. It was found that the space-time decay rate is considerably higher than previously reported. Measurements of cross-spectral density made with 5 Hz bandwidth filters disclosed low phase speeds at low frequency and small separation. Measurements were repeated on rough walls and parallels were drawn from knowledge of a smooth-wall boundary-layer structure to propose a structure for a rough-wall boundary layer. The effect of independently varying roughness height and separation on the large and small-scale turbulence structure was deduced from the measurements. It was found that roughness separation affected the very large-scale structure, whereas the roughness height influenced the medium and very small-scale turbulence.