The phrase "direction of flow" refers to the way something moves or travels. It indicates the path or route that something, like a liquid or an object, takes as it moves from one place to another. Full definition
A heart murmur may be audible as the dilation of the chambers and increased back - pressure causes the heart valves to seat improperly and blood to «leak» in a reverse direction of flow. (texaswestveterinarian.com)
But if the Space - Time continuum is now generally accepted as the only framework within which our thought can continue to progress, it becomes the more necessary that we should agree upon the nature and general direction of the flow on which we are borne. (religion-online.org)