In physics, a "wake" refers to the disturbed flow of fluid that is left behind an object as it moves through that fluid. This concept is most commonly discussed in the context of fluid dynamics, where the object can be anything from a boat moving through water to an airplane flying through air. When an object moves through a fluid, it displaces the fluid and creates a region of altered flow behind it.
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A wake is the disturbed fluid region trailing a body relative to the surrounding stream. It can contain a velocity deficit, rotation, shed vortices and mixing. Surface viscosity and flow separation help produce it, and its structure depends on geometry and Reynolds number.
In a steady towing experiment, the body's drag transfers momentum to the fluid. Comparing upstream and downstream momentum transport therefore provides a way to infer that force when the control-surface stresses and pressure terms are also accounted for. A visible dye streak only identifies part of this field; it does not by itself measure the entire momentum budget. A steady mean wake can coexist with unsteady vortex shedding, so instantaneous and averaged measurements reveal different features. Farther downstream, mixing spreads the disturbance and changes its width and peak deficit.