### 7.8.4. Wall Lubrication Force

Under certain circumstances, for example, bubbly upflow in a vertical pipe, the dispersed phase is observed to concentrate in a region close to the wall, but not immediately adjacent to the wall. This effect may be modeled by the wall lubrication force, which tends to push the dispersed phase away from the wall.

The wall lubrication force is usually modeled in conjunction with the wall lift force (see Lift Force). In situations where the lift force pushes bubbles towards the wall, the wall lubrication force acts in the opposite direction to ensure that bubbles accumulate a short distance away from the wall.

Currently, ANSYS CFX has the following wall lubrication force models:

- The Antal Wall Lubrication Force Model in the CFX-Solver Theory Guide

  Note that the Antal model requires a fine mesh. Grid convergence can be expected only on extremely fine meshes.

- The Tomiyama Wall Lubrication Force Model in the CFX-Solver Theory Guide

  Note that the Tomiyama model is available only if the surface tension has been specified for the fluid-dispersed phase pair.

- The Frank Wall Lubrication Force Model in the CFX-Solver Theory Guide

  Note that the Frank model is available only if the surface tension has been specified for the fluid-dispersed phase pair.

The wall lubrication force uses the near wall distance function. This is computed by CFX-Solver for each phase, based on walls that are specified as no slip wall for that phase. Therefore, walls for which the lubrication force is active must be specified as no-slip walls for the continuous phase.

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