Abstract
The flowfield around a sphere with and without ventilation was investigated in a wind tunnel over a range of Reynolds numbers in an incompressible flow. At supercritical Re, the pressure drag of a sphere can be nearly nullified by venting only 2% of the frontal area of the sphere to the base through a smooth internal duct. The drag reduction is achieved by increased pressures in the separated flow region close to the base. At high Re, the vent flow breaks through the near wake and brings about symmetry in the global flowfield. When the internal shear is increased by using a rough internal duct, the base pressure is unchanged, but the external flow is accelerated to velocities beyond that achieved by the potential flow around the basic sphere. The findings can be explained by a flow model in which the near wake is aerodynamically streamlined by a pair of counterrotating vortex rings at the base. A roughness element can be made to partially destroy the vortex system at the base and result in a steady asymmetric wake. A 1.2 mm diameter wire placed at 70° was found to overtrip the boundary layer and completely destroy the vortex system. Simultaneously, the turbulent separation is advanced and the drag increased.
At subcritical Re, ventilation marginally increases static pressures all over the surface. Since the large pressure differential between the windward and leeward sides is not reduced, the internal flow has a rapid acceleration to a velocity close to that of the free stream. The reverse flow associated with the near wake forces the vent flow to rest within itself and the wake profile is unchanged. The main features of subcritical flow around the basic sphere are retained in spite of ventilation. The upstream effects of ventilation are greater for subcritical flow than for supercritical flow.
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References
Achenbach E (1972) Experiments on the flow past spheres at high Reynolds numbers. J Fluid Mech 54: 565–575
Achenbach E (1974) The effects of surface roughness and tunnel blockage on the flow past spheres. J Fluid Mech 65: 113–125
Bearman PW (1967) The effect of base bleed on the flow behind a two-dimensional model with a blunt trailing edge. The Aeronautical Quarterly 18: 207–224
Calvert JR (1972) Some experiments on the flow past a sphere. Aero J Roy Aero Soc 76: 248–250
Dallmann U; Schewe G (1987) On topological changes of separating flow structures at transition Reynolds numbers. AIAA-87-1266
Fage A (1936) Experiments on a sphere at critical Reynolds numbers ARC R & M No: 1766
Kim HJ; Durbin PA (1988) Observations of the frequencies in a sphere wake and of drag increase by acoustic excitation. Physics of Fluids 31: 3260–3265
Maxworthy T (1969) Experiments on the flow around a sphere at high Reynolds numbers. J Appl Mech Trans ASME E36: 598–607
Meier GEA; Suryanarayana GK; Pauer H (1990) Widerstandsverminderung durch Ventilation. DGLR Bericht 90-06, 311–315
Meier GEA; Suryanarayana GK; Pauer H (1991) Bluff-body drag reduction by passive ventilation. First European Conference on Fluid Mechanics EUROMECH, Cambridge, UK, 16–20 Sept.
Peterka JA; Richardson PD (1969) Effects of sound on separated flows. J Fluid Mech 37: 265–287
Raithby GD; Eckert ERG (1968) The effect of support position and turbulence intensity on the flow near the surface of a sphere. Wärme und Stoffübertrag 12: 87–94
Roshko A (1954) On the drag and shedding frequency of two-dimensional bluff bodies. NACA TN 3169
Suryanarayana GK; Pauer H; Meier GEA (1992) Passive control of the wake of a sphere by ventilation. Proc. IUTAM Conference on Bluff-Body Wakes, Dynamics and Instabilities. Springer-Verlag 91–94
Suryanarayana GK; Pauer H; Meier GEA (1993) Bluff-body drag reduction by passive ventilation. Exp Fluids 16: 73–81
Taneda S (1978) Visual observations of the flow past a sphere at Reynolds numbers between 102 and 102. J Fluid Mech 85: 187–192
Wong HY (1985) Wake stabilisation by the action of base bleed. J Fluids Eng Trans ASME 107: 378–384
Wood CJ (1964) The effect of base bleed on a periodic wake. J Roy Aero Soc 68: 477–482
Wood CJ (1967) Visualisation of an incompressible wake with base bleed. J Fluid Mech 29: 259–272
Zhdanov VL; Eckelmann H (1990) The effects of jet bleed on base pressure distribution, shedding frequency and mean velocity profiles in the wake behind a two-dimensional model. Max-Planck-Institute for Fluid Research Rep. no. 9, Göttingen, Germany
Zhdanov VL; Eckelmann H (1991) An experimental investigation of the wake turbulent structure behind a two-dimensional blunt model. Max-Planck-Institute for Fluid Research Rep. no. 9, Göttingen, Germany
Zhdanov VL; Eckelmann H; Stasicki B (1992) Base bleed efficiency criterion. Proc IUTAM Conference on Bluff-Body Wakes, Dynamics and Instabilities. 221–225. Springer-Verlag
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The work reported was carried out under a study grant from the German Academic Exchange Service (DAAD) in Bonn. The authors wish to thank the Director of DAAD in Bonn for the same. Thanks are due to Dr. F. R. Grosche and colleagues at DLR in Göttingen who assisted in the design, fabrication and wind tunnel testing of the sphere model. Thanks are also due to Prof. D. G. Mabey, visiting Professor, Imperial College, London for useful discussions. The many useful discussions with the research advisors of the first author viz., Dr. P. R. Viswanath of National Aerospace Laboratories and Prof. A. Prabhu of Indian Institute of Science, Bangalore are acknowledged with thanks. The support given by the Head, Experimental Aerodynamics Division, National Aerospace Laboratories is thankfully acknowledged.
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Suryanarayana, G.K., Meier, G.E.A. Effect of ventilation on the flowfield around a sphere. Experiments in Fluids 19, 78–88 (1995). https://doi.org/10.1007/BF00193853
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DOI: https://doi.org/10.1007/BF00193853