👉 The term "upwind" is used in fluid dynamics and thermodynamics to describe a flow or system that has an upward velocity. In other words, it refers to a situation where the pressure gradient is positive along the direction of motion. This means that as a fluid moves from one region to another, its velocity increases with increasing distance from the source. The term "upwind" indicates that the pressure gradient in this region is greater than at the surface, which can cause an upward pressure difference between