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The effect of small-amplitude time-dependent changes to the surface morphology of a sphere

  • A. K. NORMAN (a1), E. C. KERRIGAN (a2) and B. J. McKEON (a1)

Abstract

Typical approaches to manipulation of flow separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled laminar separation point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is = tU/D ≈ 4. This slow development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force.

Copyright

Corresponding author

Email address for correspondence: mckeon@caltech.edu

References

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Achenbach, E. 1974 The effects of surface roughness and tunnel blockage on the flow past spheres. J. Fluid Mech. 65 (1), 113125.
Åström, K. J. & Murray, R. M. 2008 Feedback Systems: An Introduction for Scientists and Engineers. Princeton University Press.
Bakić, V. 2004 Experimental investigation of a flow around a sphere. Thermal Sciences, Vol. 8, 63–81.
Bearman, P. W. & Harvey, J. K. 1976 Golf ball aerodynamics. Aeronaut. Q. May, 112–122.
Chomaz, J. M., Bonneton, P. & Hopfinger, E. J. 2006 The structure of the near wake of a sphere moving horizontally in a stratified fluid. J. Fluid Mech. 254, 121.
Constantinescu, G. S. & Squires, K. D. 2004 Numerical investigations of flow over a sphere in the subcritical and supercritical regimes. Phys. Fluids 16 (5), 1449.
Darabi, A. & Wygnanski, I. 2004 a Active management of naturally separated flow over a solid surface. Part 1. The forced reattachment process. J. Fluid Mech. 510, 105129.
Darabi, A. & Wygnanski, I. 2004 b Active management of naturally separated flow over a solid surface. Part 2. The separation process. J. Fluid Mech. 510, 131144.
Jeon, S., Choi, J., Jeon, W., Choi, H. & Park, J. 2004 Active control of flow over a sphere for drag reduction at a subcritical Reynolds number. J. Fluid Mech. 517, 113129.
Jukes, T. N. & Choi, K. S. 2009 a Flow control around a circular cylinder using pulsed dielectric barrier discharge surface plasma. Phys. Fluids 21, 084103.
Jukes, T. N. & Choi, K. S. 2009 b Long lasting modifications to vortex shedding using a short plasma excitation. Phys. Rev. Lett. 102 (25), 254501.
Kim, H. J. & Durbin, P. A. 1988 Observations of the frequencies in a sphere wake and of drag increase by acoustic excitation. Phys. Fluids 31, 3260.
Norman, A. K. & McKeon, B. J. 2011 a The effect of a small isolated roughness element on the forces on a sphere in uniform flow. (submitted).
Norman, A. K. & McKeon, B. J. 2011 b Simultaneous force and velocity field measurements in sphere flow from subcritical to supercritical Reynolds numbers. (submitted).
Raffel, M., Willert, C., Wereley, S. & Kompenhans, J. 2007 Particle Image Velocimetry: a Practical Guide. Springer.
Rajamani, M. R. & Rawlings, J. B. 2009 Estimation of the disturbance structure from data using semidefinite programming and optimal weighting. Automatica 45 (1), 142148.
Seifert, A., Greenblatt, D. & Wygnanski, I. J. 2004 Active separation control: an overview of Reynolds and Mach numbers effects. Aerosp. Sci. Technol. 8 (7), 569582.
Taneda, S. 1978 Visual observations of the flow past a sphere at Reynolds numbers between 104 and 106. J. Fluid Mech. 85 (1), 187192.
Williams, D. R., Tadmor, G., Colonius, T., Kerstens, W., Quach, V. & Buntain, S. 2009 Lift response of a stalled wing to pulsatile disturbances. AIAA J. 47 (12), 30313037.
Yun, G., Kim, D. & Choi, H. 2006 Vortical structures behind a sphere at subcritical Reynolds numbers. Phys. Fluids 18, 015102.
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The effect of small-amplitude time-dependent changes to the surface morphology of a sphere

  • A. K. NORMAN (a1), E. C. KERRIGAN (a2) and B. J. McKEON (a1)

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