Experiments in Fluids

, 51:1031 | Cite as

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

  • A. K. Norman
  • B. J. McKeonEmail author
Research Article


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.


Vortex Particle Image Velocimetry Shear Layer Lateral Force Roughness Element 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



The support of NSF-CAREER award number 0747672 (program managers W. W. Schultz and H. H. Winter) is gratefully acknowledged.


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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Graduate Aerospace LaboratoriesCalifornia Institute of TechnologyPasadenaUSA

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