I have been trying to work with velocity data in the HDF files inside Program R. I think the issue for the ARC process is that the HDF data is not in a raster format. Even though much of your mesh could be in a structured grid, there will be some irregular points in your mesh. I have tried bringing in the xy (face points) data and the velocity data then interpolate between the irregular data to a raster surface.
xVel <- h5read("plan.p01.hdf", "Results/Unsteady/Output/Output Blocks/Base Output/Unsteady Time Series/2D Flow Areas/YOUR MESH NAME/Node X Vel")
#I transpose the xy data
FacePointXY <- t(h5read("plan.p01.hdf", "Geometry/2D Flow Areas/YOUR MESH NAME/FacePoints Coordinate")
Take a look at the data with the HDF Viewer and you can pull out xy data and velocity data and save it as a csv if that helps get it to a shapefile and interpolate to a raster.