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)
Wall-pressure fluctuations beneath turbulent boundary layers on a flat plate and a cylinder
Measurements of the turbulent pressure field on the outer surface of a 3 in. diameter cylinder aligned with the flow were made at a point approximately 24 ft. downstream of the origin of the turbulent boundary layer in an air stream of 145 ft./sec. The boundary-layer thickness was 2·78 in. and the Reynolds number based on momentum thickness was 2·62 × 104. The wall-pressure measurements were made with pressure transducers constructed from 0·06 in. diameter lead–zirconate–titanate disks mounted flush with the wall. The measurements including root-mean-square, power spectrum, and correlations of the wall pressure are compared with the existing experimental results for the turbulent pressure field beneath a plane boundary layer. The streamwise convection speed deduced from longitudinal space-time correlation measurements was almost identical to that obtained in the plane boundary layer. The rate of decay of the maxima of the space-time correlation of the pressure produced by the convected eddies was double that in a plane boundary layer. The longitudinal and transverse scales of the pressure correlation were approximately equal (in a plane boundary layer the transverse scale is larger than longitudinal scale) and were one-half or less than the longitudinal scale in the plane boundary layer. It is concluded that the effect of the transverse curvature of the wall is an overall reduction in size of pressure-producing eddies. The reduction in transverse scale of the larger eddies is greater than that of the smaller eddies. In general, the smaller eddies decay more rapidly and produce greater spectral densities at high frequencies owing to the unchanged convection speed.
17 &mgr;m pixel 640 x 480 microbolometer FPA development at BAE Systems
BAE Systems has developed an advanced 640 x 480 focal plane array (FPA) with a 17 &mgr;m pixel pitch. Sensitivity of ≤ 50 mK was demonstrated and the FPAs were used in imaging demonstrations. Successful scaling of BAE Systems' patented single contact per pixel, single-level microbolometer process to 17 &mgr;m pitch provides a path toward next generation microbolometer imaging systems.