Abstract
The volume-of-flow method combined with the Rayleigh–Plesset equation is well established for the computation of cavitation, i.e., the generation and transportation of vapor bubbles inside a liquid flow resulting in cloud, sheet or streamline cavitation. There are, however, limitations, if this method is applied to a restricted flow between two adjacent walls and the bubbles’ size is of the same magnitude as that of the clearance between the walls. This work presents experimental and numerical results of the bubble generation and its transportation in a Couette-type flow under the influence of shear and a strong pressure gradient which are typical for journal bearings or hydraulic seals. Under the impact of variations of the film thickness, the VoF method produces reliable results if bubble diameters are less than half the clearance between the walls. For larger bubbles, the wall contact becomes significant and the bubbles adopt an elliptical shape forced by the shear flow and under the influence of a strong pressure gradient. Moreover, transient changes in the pressure result in transient cavitation, which is captured by high-speed imaging providing material to evaluate transient, three-dimensional computations of a two-phase flow.
Highlights
The research of bubble dynamics has a general relevance for numerous technical applications where a hydrodynamic gap is the core element that either carries the load or separates machine compartments
The pressure gradient varies over time, combined with a change in fluid film thickness regions with low local pressure can occur that can result in cavitation, if the local pressure falls below a critical value
According to [5], the definition of a small gap flow is met if the clearance is less than 0.3 mm and Figure 10 illustrates that the Couette apparatus complies with this definition, because in the vicinity of the minimum film thickness, the clearance is between 0.15–0.23 mm in the rigid case and 0.23–0.28 mm under real operation including the elastic deflection of the rotating cylinder
Summary
The research of bubble dynamics has a general relevance for numerous technical applications where a hydrodynamic gap is the core element that either carries the load (journal bearing) or separates machine compartments (hydraulic seal) In both cases, the fluid film is subject to a shear flow due the differential velocity of the adjacent walls and a significant pressure gradient. The pressure gradient varies over time, combined with a change in fluid film thickness regions with low local pressure can occur that can result in cavitation, if the local pressure falls below a critical value. At this point, the concept of cavitation must be briefly discussed. Requires more research to understand the generation of voids inside the liquid and its transportation
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