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)
Boundary Flow Along a Circular Cylinder
The Karman-Prandtl mixing-length theory for fluid flow over a flat surface with a laminar sublayer near the boundary is adapted to the flow along an infinite circular cylinder on the supposition that, in the nature of the radial inward convection of momentum towards the boundary, there is no accumulation of the momentum within any arbitrary finite volume of fluid. Experimental results for smooth and rough pipe flows are invoked in order to develop the nature of the boundary flow over a range of conditions from the quasi-smooth to the fully turbulent regimes. The drag coefficient for tangential friction is evaluated as a function of Reynolds number and surface roughness and is shown to lie within the limits 0.002 and 0.02, depending on roughness, for diameter Reynolds numbers greater than 103. Theoretical indications compare favorably with results from towing tests of stranded cables.
Displacement effect of a three-dimensional boundary layer
A method is described for determining the "displacement surface" of known three-dimensional compressible boundary-layer flow in terms of the mass-flow defects associated with the profiles of the two velocity components parallel to the surface. The result is a generalization of the plane flow concept of displacement thickness introduced in order to describe how a thin boundary layer distorts the outer nonviscous flow. Numerical values are found for the known three-dimensional boundary-layer flow about a cone at a small angle of attack to a supersonic stream.