Cavitation is a phenomenon in which bubbles are generated by local pressure changes in a liquid flow. Since cavitation has adverse effects on fluid machinery such as performance degradation, vibration and noise, and erosion, the elucidation of its mechanism is of great engineering importance. Numerically, cavitation has been mainly studied by simulations based on the Navier–Stokes equation. However, it requires a bubble seed to simulate a vapor phase so that it cannot treat the cavitation inception. Here, we directly simulated the cavitation using molecular dynamics (MD) that does not require the assumption of a phase transition model or an equation of state for the bubble generation.

We employed Lennard-Jones fluids around periodically aligned side-by-side cylindrical objects at Reynolds number Re~100. As shown in the top panels in Fig. 1, a Newtonian fluid exhibits a Kármán vortex, in which vortexes are periodically generated behind the objects. The neighboring Kármán vortex is synchronized in the antiphase. This synchronization amplifies the vibrations acting on the cylinder. We decrease the temperature to observe the cavitation. At the temperatures T = 1.3 and 1.25, bubbles are generated in conjunction with the shedding cycle of the Kármán vortex (the middle panels in Fig. 1). The Kármán vortex remains synchronized in the anti-phase as in the non-cavitating flow. As the temperature is further reduced to T = 1.2, the gas-phase region behind the cylinder is further expanded, and the upper and lower Kármán vortices become asymmetric (the bottom panels in Fig. 1). This asymmetric structure is switched by a long period of time. A similar dynamics is seen at more closely packed cylindrical arrays in a Newtonian fluid. Thus, the bubbles give an effective increase in the cylindrical diameter.

Fig. 1. Snapshots of density (left panels) and vorticity (right panels) fields. The black circles in the figure represent cylinders. At the high temperature, T = 2, Kármán vortex is formed without bubble generation. At the lower temperatures, cavitation occurs. A gas phase (the low-density region in blue color) appears behind the cylinders.

The change in the vortex structure alters the vibrations excited by the vortex. Because the generated bubbles inhibit the propagation of the vibration associated with the ejection of the vortex, the vibration amplitude decreases and eventually disappears as the cavitation develops.

In summary, the molecular-scale analysis reveals that bubbles generated near the cylinder significantly change the properties of the lift and flow fields.

References

• [1] Y. Asano, H. Watanabe, and H. Noguchi, J. Chem. Phys. 152, 034501 (2020).

Authors

• Y. Asano, H. Watanabe^a, and H. Noguchi
• ^aKeio University