overlay mesh does not appear to support scrapers (moving a wall across a stationary wall witth zero gap)

overDriftFluxDyMFoam.tar.gzscraperDahl3D.tar.gz I have created a application from driftFluxFoam extending its capability to include overlay dynamic meshes. It compiles and runs, although I can't claim to be expert enough in the basic physics to say if the .H file modifications, or the Solver options I chose are correct/good. By runs I mean the solutions don't immediately crash or give error messages, and the scraper mesh rotates as requested. However, it gives very high values, visible in paraFoam, for the alpha.sludge concentrations between the base of the rotating scraper and the floor of the tank. These concentrations should be zero as there should be a zero gap.

Might be a bug, might be my poor implementation of overDriftFluxDyMFoam, might be a limitation in the meshing that it doesn't support a moving wall scraping a stationary wall without a gap.

The overDriftFluxDyMFoam.tar.gz file includes a couple of extra rheologies (Casson, Herschel Bulkley). The scraperDahl3D.tar.gz file is a 3D version of the Dahl tutorial with a centred scraper added at the base of the box. A version without the scraper had no issues out to 3600 sim-seconds.

Running the scraper at lower speed caused much higher maximum values for alpha.sludge and the solution went to very short time intervals and then crashed after about 60 sim-seconds.

## Reattaching the author to the issue ticket: @DenysW ##