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Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number

Published online by Cambridge University Press:  26 April 2006

E. R. Subbarao  and
B. J. Cantwell
E. R. Subbarao
Affiliation:
Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305, USA Present Address: Hughes Aircraft Company. PO Box 92919, Los Angeles, CA 90009-2919, USA.
B. J. Cantwell
Affiliation:
Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305, USA
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Abstract

Experiments have been carried out on a vertical jet of helium issuing into a co-flow of air at a fixed exit velocity ratio of 2.0. At all the experimental conditions studied, the flow exhibits a strong self-excited periodicity. The natural frequency behaviour of the jet, the underlying flow structure, and the transition to turbulence have been studied over a wide range of flow conditions. The experiments were conducted in a variable-pressure facility which made it possible to vary the Reynolds number and Richardson number independently. A stroboscopic schlieren system was used for flow visualization and single-component laser-Doppler anemometry was used to measure the axial component of velocity. The flow exhibits several interesting features. The presence of co-flow eliminates the random meandering typical of buoyant plumes in a quiescent environment. The periodicity of the helium jet under high-Richardson-number conditions is striking. Under these conditions transition to turbulence consists of a rapid but highly structured and repeatable breakdown and intermingling of jet and free-stream fluid. At Ri = 1.6 the three-dimensional structure of the flow is seen to repeat from cycle to cycle. The point of transition moves closer to the jet exit as either the Reynolds number or the Richardson number increases. The wavelength of the longitudinal instability increases with Richardson number. At low Richardson numbers, the natural frequency scales on an inertial timescale, τ1 = D/Uj where D is the jet diameter and Uj is the mean jet exit velocity. At high Richardson number, the natural frequency scales on a buoyancy timescale, τ2 = [ρjD/g(ρ∞ -ρj)]½ where g is the gravitational acceleration and ρj and ρ∞ are the jet and free-stream densities respectively. The transition from one flow regime to another occurs over a narrow range of Richardson numbers from 0.7 to 1. A buoyancy Strouhal number is used to correlate the high-Richardson-number frequency behaviour.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 245 , December 1992 , pp. 69 - 90
Copyright
© 1992 Cambridge University Press

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