Influence of a Local Unevenness on a Flat Wall on Dynamics of a Nearby Bubble

The influence of local conical, cylindrical and intermediate concavities and convexities on a flat wall on the expansion and collapse of a bubble in liquid near the wall unevenness is studied in axisymmetric configuration. The areas of the axial sections of the differently shaped unevennesses with equal axial size are taken to be equal. Referred to the radius of the bubble at the end of its expansion without a wall, the axial size of the unevennesses is varied from 0 to 0.2, their effective radius from 0.25 to 1, the initial distance from the bubble center to the wall from 1 to 1.2. In all cases a cumulative liquid jet directed to the wall occurs on the bubble surface in the course of bubble collapse. The study is performed by the boundary element method. It has been found that at the moment of the jet impact onto the bubble surface part close to the wall, the jet tip shape weakly depends on the presence of the wall unevenness. At this moment the jet tip speed is nearly the same as that in the variants with the bubble near a similarly-distant purely flat wall. Variation of the wall unevenness characteristics manifests itself at the jet impact moment mostly in some changes in the shape of the bubble surface part close to the wall and in the distance between the bubble and the wall. In particular, the bubble surface part subjected to the jet impact can be concave, rather than only convex or flattened as in the case with a flat wall.