Are they circling something in particular?
I have seen some of this behavior when a storm is approaching, they are trying to get in.
We always seem to have flies that just fly in circles every spring and summer around our smoker and bbq. Bought some great fly traps at amazon.com. Like the fact they are disposable and they work VERY well. They are RESCUE Outdoor Disposable Fly Trap, 2 Pack
So flytraps tape etc they attract flies.. if you choose to employ them I suggest keeping them with your main waste bins or around the perimeter
Also see where they fly the most and at what height. I noticed when we hung our fly trap the first time it was too high and not in the fly's path. When we moved it that made all the difference and we went from catching about 5 a day to over 50 a day. The Rescur outdoor traps have a scent pack that you activate with water. That also helps to draw the flys in. They think they are going to get something good.
I read on either Hometalk or Pinterest to put pennies in a plastic bag filled with about a cup of water. Suspend this somewhere. The light reflects off the pennies and scares away flies. Easy to try and nothing to loose except the flies hopefully. 😎