downwash: one construct, many systems

In fixed-wing aerodynamics, downwash is the downward component of airflow left behind a lifting wing: the trailing vortices shed from the wingtips induce a downward velocity over the span, tilting the effective oncoming flow and producing the lift-induced drag. Glauert's The elements of aerofoil and airscrew theory (1926) set out Prandtl's lifting-line account in which downwash links the spanwise lift distribution to induced drag. This is the parent construct: a downward-deflected flow created as the price of generating lift, the same effect a helicopter rotor and a smokestack wake reproduce.[1]

Unit

The downward velocity (and induced angle) of flow behind a lifting surface

Recovery means

Not applicable

Measured by

Quantified by the downwash angle, the induced velocity from lifting-line theory, and the resulting induced drag

In rotorcraft aerodynamics, downwash is the column of air a rotor drives downward to generate thrust, the induced inflow through the disc that momentum theory ties directly to the weight the rotor supports. Leishman's Principles of helicopter aerodynamics treats this induced velocity, its non-uniform distribution across the disc, and the ground and brownout effects the downwash produces near the surface. It is the same induced-downflow construct as a fixed wing's downwash, here from a rotating wing and measured by induced inflow velocity rather than a trailing-vortex angle.[2]

Unit

The induced inflow velocity of air driven down through the rotor disc

Recovery means

Not applicable

Measured by

Estimated from momentum theory and measured as induced inflow velocity and dynamic pressure beneath the disc

In atmospheric dispersion and wind engineering, downwash is the term borrowed for the way effluent from a chimney is dragged downward into the low-pressure wake of the stack or an upwind building instead of rising, raising ground-level pollutant concentrations nearby. Snyder and Lawson's Fluid modeling simulation of stack-tip downwash for neutrally buoyant plumes used wind-tunnel modelling to map when a plume is captured by the stack-tip wake. It is the same downward-deflected-flow idea applied to a pollutant plume, driven here by wake entrainment rather than a lifting surface and measured by plume rise and ground-level concentration.[3]

Unit

The downward deflection of a plume into a stack or building wake

Recovery means

Avoided by raising stack height or exit velocity so the plume escapes the wake

Measured by

Assessed from plume-rise deficits and ground-level concentrations in wind-tunnel and dispersion models