Fluids & Structural Mechanics > CM Projects > Bubbly Flows & Cavitation Inception (BFCI)

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Cavitation inception is defined as the formation of vapor and gas filled voids in a liquid. Small-scale cavitation is studied because it can cause material damage as well the unwanted emission of  acoustic radiation. The Computational Mechanics department is leading in the numerical study of these phenomena by using current, modern Large Eddy Simulations (LES) CFD techniques as well as developing new caviataion inception models that couple to CFD.

Predicting cavitaion inception, or the first detectable occurrence of inception, is important because it provides operating limits that avoid cavitation. Recent work has been done toward predicting cavitation inception coupled DES with the Rayleigh-Plesset (R-P) equation with a semi-empirical equation of motion to model the response of nuclie bubbles.  Figure 1 shows instantaneous results for the DES simulations of a circular jet; they show the highly unsteady flow and turbulent vortex rings that contribute to cavtiation inception. Figure 2 is a comparison of experimental data to an DES simulation of the jet. Figure 3 shows the results of the nuclei bubble advection for three jet diameters. The upper-half show the locations of the nuclie bubbles and the lower half shows the magnitude of voticity. These images show how the bubbles are trapped by vortex rings.

Another related study area is free surface-bubbly wake interactions that are typical of jet driven ships, Figure 4. The computational mechanics department is studying these flows and the degree that the free surface modifies the dynamics of the jet. Numerical simulations that include the modeling of the free surface as well as the jet are critical in obtaining accurate simulations. As with the jet cavitatation work, DES provides much richer flow detail than traditional RANS techniques, Figure 5.  Figure 6 shows isosurfaces of the ‘Q criterion’ which gives a visual indication of vorticity in the jet and free surface. Isosurfaces of Q=125 for four time steps show the growth and interaction of the jet with the free surface. 

Bubble dynamics is also important when considering their effect on towed acoustic arrays. Figure 7 shows a typical acoustic array being towed behind a ship. Air bubbles interact with these underwater arrays by scattering and absorbing acoustic energy. Simulations were done at ARL to help predict the degradation of detection range and increased scattering from bubbles. Figure 8 is a sample of the ship wake bubble field for RANS and Detached Eddy Simulation (DES) calculations that were done to support this effort.

Images
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Figure 1

Contours of axial velocity, coefficient of pressure, and vorticity plotted on a cross section though the central axis of a circular jet.
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Figure 2

Comparison of flow structures for a circular jet. Experimental visualization (left) and an LES simulation (right).

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Figure 3

Comparisons of bubble motions at t=10 dimensionless units of time. All simulations are conducted at σ=σi+0.01 and initial bubble sizes are a0=100μm.

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Figure 4

Bubbly wakes for jet driven ships.

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Figure 5

Bubbly wake simulation for Reynolds averaged Navier Stokes (RANS) top, and LES, bottom.
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Figure 6

Isosurfaces of Q=125, at: A) t=0.04s. B) t=0.3s. C) t=1.0s., and D) t=3.5s. After the onset of jet injection. Colored by streamwise vorticity.

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Figure 7

Typical acoustic array being towed behind a ship in the presence of a bubbly wake.
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Figure 8

Ship wake bubble field calculation comparison to measured data.

Animations

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Animation 1

Jet impingement on free surface.