Which domain are we interested in? Solid or fluid? Depending on our interests, two of the ways to formulate a problem of hot or cold fluid flow over skin are shown: 1) Skin is the domain and we solve...

Which domain are we interested in? Solid or fluid? Depending on our interests, two of the ways to formulate a problem of hot or cold fluid flow over skin are shown: 1) Skin is the domain and we solve for the skin temperature for a given condition of the fluid. 2) Fluid is the domain and we want to solve for fluid temperature, given a fixed skin surface temperature (not a particularly useful formulation for a skin burn problem but there may be other situations). (iStock.com)