I ran this situation through a couple of simple 2D CFD models. As you can see from the pictures below, one way to keep debris from gathering in this area is to increase the velocity of wind in the corner. To do this, you need to place an obstacle that will direct the wind into the corner.
Even with this, there will be a dead spot in the corner. This should help to lessen it though.
Another benefit of this configuration is that it works just the same if the wind is coming form the top.
What kind of obstacle?
The exact design of the obstacle wouldn't need to be too involved. A planter or solid bench may be enough to make a difference. The main criteria are:
- Solid so that the wind is directed around it.
- High enough that the wind moving over top of it doesn't affect the wind moving around.
There will likely be some trial and error.