slipstream: one construct, many systems

In animal flight the construct is the energy a trailing bird saves by positioning itself in the wake of a bird ahead, capturing the upward-moving air (upwash) thrown off the leader's wingtip vortices so that it needs less of its own power to stay aloft. Lissaman and Shollenberger's 1970 Formation flight of birds modelled the V-formation as a way to share induced drag across the flock and predicted the lateral spacing that maximises the saving, and Weimerskirch and colleagues' 2001 Energy saving in flight formation measured lower heart rates in pelicans flying in formation, confirming a real metabolic benefit. The referent is a trailing flier's reduction in the cost of locomotion won from a neighbour's wake, the same wake-exploitation idea that race cars call slipstreaming and fish call schooling, here realised through wingtip-vortex upwash.[1][2]

Unit

The reduction in a trailing flier's power or metabolic cost from a leader's wake

Recovery means

Not applicable

Measured by

Assessed by induced-drag models of formation geometry and by physiological proxies such as heart rate and wingbeat frequency in formation versus solo flight

In motorsport the slipstream is the pocket of disturbed, low-pressure air behind a moving car: a following driver who tucks into it meets much less pressure drag than in clean air, so the car can run faster on the same power and can carry extra speed to pull out and pass. Romberg, Chianese, and Lajoie's 1971 Aerodynamics of race cars in drafting and passing situations wind-tunnel-measured how drag falls on both the leading and the trailing car as the gap closes, and Katz's 2006 review Aerodynamics of race cars places drafting within the broader drag-and-downforce picture of racing. The referent is the drag reduction a trailing vehicle gains by travelling in another's wake, the same construct birds exploit as formation upwash and fish as schooling, here measured as a change in aerodynamic drag coefficient with following distance.[3][4]

Unit

The drop in aerodynamic drag on a trailing car as a function of following distance

Recovery means

Not applicable

Measured by

Measured by wind-tunnel or CFD drag coefficients for leading and trailing cars at varying gaps, and by closing speed or top-speed gain in the draft

In fish locomotion the same construct describes the hydrodynamic advantage a fish gains from swimming among neighbours: by placing itself in the staggered wake of the fish ahead it can exploit the vortices they shed, meeting reduced relative flow and recovering energy so the school swims more cheaply than a lone fish. Weihs's 1973 Hydromechanics of fish schooling set out the diamond-lattice spacing at which trailing fish best capture the shed vortices and estimated the resulting energy saving. The referent is the reduction in the cost of swimming won from a neighbour's wake, the identical wake-exploitation idea seen in bird formations and in motor-racing slipstreams, here governed by shed-vortex hydrodynamics in water rather than air.[5]

Unit

The reduction in a schooling fish's swimming cost from neighbours' wakes

Recovery means

Not applicable

Measured by

Estimated from hydromechanical models of optimal school spacing and from tail-beat frequency or oxygen cost in schooling versus solitary swimming