Experimental Investigation of Unstably Stratified Buoyant Wakes

[+] Author and Article Information
Wayne N. Kraft

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843

Malcolm J. Andrews1

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843mandrews@tamu.edu


Corresponding author.

J. Fluids Eng 128(3), 488-493 (Nov 01, 2005) (6 pages) doi:10.1115/1.2174060 History: Received December 15, 2004; Revised November 01, 2005

A water channel has been used as a statistically steady experiment to investigate the development of a buoyant plane wake. Parallel streams of hot and cold water are initially separated by a splitter plate and are oriented to create an unstable stratification. At the end of the splitter plate, the two streams are allowed to mix and a buoyancy-driven mixing layer develops. The continuous, unstable stratification inside the developing mixing layer provides the necessary environment to study the buoyant wake. Downstream a cylinder was placed at the center of the mixing layer. As a result the dynamic flows of the plane wake and buoyancy-driven mixing layer interact. Particle image velocimetry and a high-resolution thermocouple system have been used to measure the response of the plane wake to buoyancy driven turbulence. Velocity and density measurements are used as a basis from which we describe the transition, and return to equilibrium, of the buoyancy-driven mixing layer. Visual observation of the wake does not show the usual vortex street associated with a cylinder wake, but the effect of the wake is apparent in the measured vertical velocity fluctuations. An expected peak in velocity fluctuations in the wake is found, however the decay of vertical velocity fluctuations occurs at a reduced rate due to vertical momentum transport into the wake region from buoyancy-driven turbulence. Therefore for wakes where buoyancy is driving the motion, a remarkably fast recovery of a buoyancy-driven Rayleigh-Taylor mixing in the wake region is found.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Buoyant wake formed from a cylinder placed at the centerline of a buoyancy-driven mixing layer

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Figure 2

Schematic of experimental apparatus

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Figure 3

Flow channel test section with cylindrical obstruction

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Figure 4

Percent uncertainty in the rms of vertical velocity fluctuations

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Figure 5

Visualization of wake- and buoyancy-dominated turbulence using Nigrosene dye. (a) Rayleigh-Taylor-driven mixing layer, (b) Rayleigh-Taylor-driven mixing layer with a cylinder wake, D=1.6cm, and (c) Rayleigh-Taylor-driven mixing layer with a cylinder wake, D=3.25cm.

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Figure 6

Visualization of the wake behind a cylinder with no buoyancy for a cylinder diameter of 1.6cm and free steam velocity of 4cm∕s

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Figure 7

Centerline rms vertical velocity fluctuations in the cylinder wake with D=1.6cm

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Figure 8

Decay of near wake centerline rms vertical velocity fluctuations

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Figure 9

Molecular mix parameter measured at the centerline in the wake of a cylinder. Data for the Rayleigh-Taylor mixing layer obtained from Ramaprabhu and Andrews (12).




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