Most of the air flow models seem to have the register on the ceiling, projecting the flow laterally along the ceiling then down the walls, or from the walls, flowing across the ceiling or floor until hitting a wall or another flow then going down or up respectively.
However, with the very tall ceilings the ceiling supply register may not be practical or in some cases possible. A wall duct at 8ft wouldn't have a ceiling to run along. One originating at the lower wall would go across the floor then up the opposing wall but nothing to stop it. In the floor, no ceiling to cause it to turn (unless it had a 15+ft throw).
What is the preferred way of doing this? Why is it the preferred way?