Freestream

Freestream refers to the undisturbed flow of a fluid, such as air or water, far upstream from any body or disturbance. In aerodynamic analysis, freestream conditions are the reference state against which the effects of a body are measured. The freestream velocity is often denoted U∞ or V∞, along with the freestream pressure p∞, density ρ∞, and temperature T∞. The Mach number, Ma∞, is defined using the freestream velocity and the local speed of sound.

In many problems the flow near a body is decomposed into the inviscid freestream and a viscous boundary layer that forms along surfaces. The boundary layer adheres to the no-slip condition at the surface, causing velocity to rise from zero at the wall to approximately the freestream value away from the surface. The far-field or freestream region is where the flow is assumed undisturbed and uniform. Freestream values are used as boundary conditions in both analytical and numerical models of fluid flow.

Freestream conditions are essential in CFD and wind tunnel testing. In simulations, they serve as the far-field boundary conditions, while in experiments they are measured or controlled to ensure the incoming flow is representative of the desired operating conditions. Instruments such as pitot tubes help determine freestream velocity and pressure upstream of a test object, and wind tunnel settings aim to create a uniform freestream.

In compressible flows, freestream properties determine stagnation values and influence the relationship between surface quantities and the overall aerodynamic forces. The freestream state also affects boundary-layer development, transition from laminar to turbulent flow, and the response of the flow to surface roughness and geometric features.