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DNS AND PIV STUDY OF THE 3D WAKE BEHIND TAPERED CIRCULAR CYLINDERS

Vagesh D. Narasimhamurthy
GexCon AS, P.O. Box 6015, Postterminalen, 5892 Bergen; Fluids Engineering Division, Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway

Jan Visscher
Dept. of Marine Technology, NTNU NO-7491 Trondheim, Norway

Helge I. Andersson
Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway

Bjornar Pettersen
Department of Marine Technology, Norwegian University of Science and Technology,NO-7491 Trondheim, Norway

Abstract

Three-dimensional wake behind a tapered circular cylinder has been studied at low-Reynolds number (transition in the wake regime) by performing Direct Numerical Simulation (DNS) and at higher Reynolds numbers (transition in the shear-layer regime) by Particle Image Velocimetry (PIV). The taper ratio (75) was constant in both the studies. In the PIV study it was found that increase in aspect ratio increases the number of shedding cells along the span, an effect also reported by Piccirillo and Van Atta (J. Fluid Mech., 1993). Both mode A and mode B were found to coexist in the same geometry (DNS) but only in a small span of the cylinder. Flow-visualization revealed that the mode A appeared around Re ~ 200 and mode B around Re ~ 250. Cross-sectional views of mode B in the DNS results revealed the smaller-scale 'mushroom' vortex pair structures, which is remarkably similar to those found by Williamson (J. Fluid Mech., 1996). The wavelength of mode B was found to be λZ/D ~ 1, which is surprisingly close to the experimental value λZ/D = 0.98 found by Williamson (J. Fluid Mech., 1996) in the uniform circular cylinder wake. In the present DNS study it was found that streamwise vorticity ωx becomes large as vortex dislocation occurs, an effect also observed by Piccirillo and Van Atta (J. Fluid Mech., 1993).