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TECHNICAL PAPERS

Near-Field Flow Measurements of a Cavitating Jet Emanating From a Crown-Shaped Nozzle

[+] Author and Article Information
Stephane Poussou, Michael W. Plesniak

Department of Mechanical Engineering, Purdue University, West Lafayette, IN 47907

J. Fluids Eng 129(5), 605-612 (Oct 30, 2006) (8 pages) doi:10.1115/1.2717615 History: Received August 30, 2005; Accepted October 30, 2006

The effect of a crown-shaped nozzle on cavitation is studied experimentally in the near-field of a 25 mm diameter (D) water jet at ReD=2×105 using particle image velocimetry (PIV) and high speed shadowgraphy recorded with a 5000 fps digital camera. The objectives are to passively control the jet flow structure and to examine its consequences on the physical appearance of cavitating bubbles. The experiments are performed in a closed-loop facility that enables complete optical access to the near-nozzle region. The cavitating and noncavitating mean velocity fields are obtained up to three nozzle diameters downstream and compared to those of a companion round nozzle. PIV measurements are taken in two distinct azimuthal planes passing through the tip and bottom points of the crown nozzle edge. The data include shear layer momentum thickness and vorticity thickness, spanwise vorticity distribution and streamwise normal Reynolds stress. Significant deviation from an axisymmetric shear layer is observed in the noncavitating flow consistently up to one diameter downstream, after which identical asymptotic conditions are achieved in both round and crown-shaped nozzles. Maximum magnitudes of spanwise vorticity and streamwise normal Reynolds stress are the highest downstream of the nozzle tip edges under noncavitating conditions. Significant modifications in trends and magnitudes are observed for the shear layer momentum thickness under cavitating conditions up to one diameter downstream. Qualitative flow visualization reveals that bubble growth occurs at different conditions depending on azimuthal location. Bubbles, in the form of elongated filaments, are the dominant structures produced downstream of the valley edges of the nozzle with an inclination of 45 deg with respect to the direction of the flow, and are observed to persist with significant strength up to two diameters downstream. These filaments are stretched between periodic larger-scale, spanwise bubbly clusters distorted in the shape of the nozzle outlet. The tip edges produce cavitating bubbles under conditions similar to that of a classical round nozzle. In summary, it was demonstrated that passive control of turbulent structures in the jet does impact the cavitation process.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the experimental facility

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Figure 2

(a) (i) Schematic representation of the crown-shaped and round nozzles; (ii) planes and region of PIV measurements. (b) PIV mean velocity field of the round nozzle under noncavitating conditions.

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Figure 3

(a) Streamwise development of the normalized momentum thickness (i) without cavitation and (ii) with cavitation. (b) Streamwise development of the normalized momentum thickness of the (i) round nozzle, (ii) peak plane, and (iii) valley plane.

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Figure 4

(a) Streamwise development of the normalized maximum spanwise vorticity (i) without cavitation and (ii) with cavitation. (b) Streamwise development of the normalized maximum spanwise vorticity of the (i) round nozzle, (ii) peak plane, and (iii) valley plane.

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Figure 5

(a) Streamwise development of the normalized vorticity thickness (i) without cavitation and (ii) with cavitation. (b) Streamwise development of the normalized vorticity thickness of the (i) round nozzle, (ii) peak plane, and (iii) valley plane.

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Figure 6

(a) Streamwise development of the normalized maximum normal stress (i) without cavitation and (ii) with cavitation. (b) Streamwise development of the normalized maximum normal stress of the (i) round nozzle, (ii) peak plane, and (iii) valley plane.

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Figure 7

(a) High speed photograph sequence showing bubble growth downstream of the round nozzle (0.3ms interval between each image at 0.2ms exposure time, viewed from left to right). (b) High speed photograph sequence showing bubble growth downstream of a valley in the crown-shaped nozzle (0.3ms time interval between each image at 0.2ms exposure time, viewed from left to right).

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Figure 8

Schematic of the gas phase structures observed downstream of the valleys

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Figure 9

Time-averaged view (30ms exposure time) of the cavitating jet emanating from the crown-shaped nozzle. Illumination by a laser beam crossing the shear layer along a vertical diameter.

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Figure 10

Instantaneous photograph showing breakdown and azimuthal expansion of a large bubble being entrained within the shear layer at the bottom of a valley

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