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The effect of a small isolated roughness element on the forces on a sphere in uniform flow

Norman, A. K. and McKeon, B. J. (2011) The effect of a small isolated roughness element on the forces on a sphere in uniform flow. Experiments in Fluids, 51 (4). pp. 1031-1045. ISSN 0723-4864.

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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 5×10^4<Re<5×10^5 using a novel stingmounted 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.

Item Type:Article
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McKeon, B. J.0000-0003-4220-1583
Additional Information:© 2011 Springer-Verlag. Received: 23 August 2010; Revised: 13 April 2011; Accepted: 6 May 2011; Published online: 26 May 2011. The support of NSF-CAREER award number 0747672 (program managers W. W. Schultz and H. H. Winter) is gratefully acknowledged.
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NSF CAREER Award0747672
Issue or Number:4
Record Number:CaltechAUTHORS:20111021-085953294
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:27347
Deposited By: Jason Perez
Deposited On:21 Oct 2011 20:13
Last Modified:03 Oct 2019 03:22

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