The effect of a small isolated roughness element on the forces on a sphere in uniform flow
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The effect of an isolated roughness element on the forces on a sphere was examined for a Reynolds number range of 5 × 104 < Re < 5 × 105 using a novel sting-mounted sphere apparatus. The roughness element was a circular cylinder, and its width and height was varied to be 1, 2, and 4% of the sphere diameter. At subcritical Re, a lateral force is produced in the direction of the roughness, while at supercritical Re, the force is in the opposite direction. This is caused by asymmetric boundary layer separation, as shown using particle image velocimetry. At supercritical Re, a roughness element that is only 1% the sphere diameter produces a lift to drag ratio of almost one. It was found that the isolated roughness element has the largest effect on the lateral forces when it is located between a streamwise angle of about 40° and 80°. In addition to the mean forces, the unsteady forces were also measured. It was found that at subcritical Re, vortex shedding is aligned to the plane of the roughness element. In addition, the probability distribution of the forces was nearly Gaussian for subcritical Re, but for supercritical Re, the skewness and kurtosis deviate from Gaussian, and the details are dependent on the roughness size. A simple model developed for the vortical structure formed behind the roughness element can be extended to explain aspects of nominally smooth sphere flow, in which external disturbances perturb the sphere boundary layer in an azimuthally local sense. These results also form the basis of comparison for an investigation into the effectiveness of a moving isolated roughness element for manipulating sphere flow.
KeywordsVortex Particle Image Velocimetry Shear Layer Lateral Force Roughness Element
The support of NSF-CAREER award number 0747672 (program managers W. W. Schultz and H. H. Winter) is gratefully acknowledged.
- Bacon DL, Reid EG (1924) The resistance of spheres in wind tunnels and in air. Technical report 185, Langley Memorial Aeronautical Laboratory, LangleyGoogle Scholar
- Fage A (1937) Experiments on a sphere at critical Reynolds numbers. Rep Mem Aero Res Counc Lond 1766:108Google Scholar
- Hoerner S (1935) Tests of spheres with reference to Reynolds number, turbulence, and surface roughness. Technical memoradum no. 777. National Advisory Committee for Aeronautics, LangleyGoogle Scholar
- Maxworthy T (1969) Experiments on the flow around a sphere at high Reynolds numbers. Trans ASME J Appl Mech 36:598–607Google Scholar
- Morkovin MV (1985) Bypass transition to turbulence and research desiderata. In: NASA. Lewis Research Center Transition in Turbines (SEE N85-31433 20-34), pp 161–204Google Scholar
- Norman AK, McKeon BJ (2008) Effect of sting size on the wake of a sphere at subcritical Reynolds numbers. AIAA-2008-4237Google Scholar
- Norman AK, McKeon BJ (2011) Unsteady force measurements in sphere flow from subcritical to supercritical Reynolds numbers (Submitted)Google Scholar
- Norman AK, Kerrigan EC, McKeon BJ (2011) The effect of small-amplitude time-dependent changes to the surface morphology of a sphere. J Fluid Mech 675:268–296Google Scholar
- Raffel M, Willert C, Wereley S, Kompenhans J (2007) Particle image velocimetry: a practical guide. Springer, BerlinGoogle Scholar
- White FM (2006) Viscous fluid flow. McGraw-Hill, New YorkGoogle Scholar