In the mathematical field of differential geometry, a geometric flow, also called a geometric evolution equation, is a type of partial differential equation for a geometric object such as a Riemannian metric or an embedding. It is not a term with a formal meaning, but is typically understood to refer to parabolic partial differential equations.
Certain geometric flows arise as the gradient flow associated to a functional on a manifold which has a geometric interpretation, usually associated with some extrinsic or intrinsic curvature. Such flows are fundamentally related to the calculus of variations, and include mean curvature flow and Yamabe flow.
Intrinsic geometric flows are flows on the Riemannian metric, independent of any embedding or immersion.
Important classes of flows are curvature flows, variational flows (which extremize some functional), and flows arising as solutions to parabolic partial differential equations. A given flow frequently admits all of these interpretations, as follows.
If the equation is the Euler–Lagrange equation for some functional then the flow has a variational interpretation as the gradient flow of and stationary states of the flow correspond to critical points of the functional.
In the context of geometric flows, the functional is often the norm norm of some curvature.
Thus, given a curvature one can define the functional which has Euler–Lagrange equation for some elliptic operator and associated parabolic PDE
Curvature flows may or may not preserve volume (the Calabi flow does, while the Ricci flow does not), and if not, the flow may simply shrink or grow the manifold, rather than regularizing the metric. Thus one often normalizes the flow, for instance, by fixing the volume.