Flow separation, and how to see it coming

What makes flow detach, and the warning signs before it does.

Flow separation happens when the slow air near a surface runs out of the energy it needs to keep following that surface's shape, usually because the surface is curving away or the pressure ahead is rising faster than the flow can push through, and once separation starts, air that was smoothly following the body peels away into a disorderly, energy-wasting wake instead.

Slow air pushing against rising pressure

The thin, slow-moving layer of air right against a surface has very little momentum of its own compared with the faster air further out, and following a curved surface, especially one curving away from the flow's original direction, requires the flow to keep pushing against rising pressure as it goes, since air being asked to slow down and turn generally has to work against an adverse pressure gradient building up ahead of it along the way. As long as the slow layer has enough momentum left to keep pushing through that rising pressure, it stays attached, following the surface's shape closely, tracing every contour the designer intended it to follow. Once the surface curves away faster than the slow layer can keep pace with, or the layer has already lost too much of its own momentum to friction earlier along the surface, it simply stops being able to push forward against the rising pressure any further, and the flow detaches from the surface at that point, leaving a disorganised, turbulent wake behind it rather than a clean, attached stream. Once that wake has formed, the pressure recovery the surface behind it was relying on largely stops happening, since a chaotic, recirculating pocket of air does not push back on the surface the way a smoothly slowing, attached flow would have.

A hand tipped too steeply in the water

Moving a flat hand through bathwater or across a swimming pool at a gentle angle produces smooth, steady resistance, the water following the hand's surface closely with no visible disturbance. Tip the leading edge of the hand up too steeply, though, and the water can no longer follow the back of the hand around that sharper angle, and instead of clinging to the surface all the way past it, the flow lets go abruptly, breaking into a burble of swirling eddies trailing off behind the hand rather than a smooth, attached stream. Nothing about the water or the hand's speed through it needs to change for this to happen, only the angle at which the surface is asking the flow to turn, exactly the same trigger that causes separation over a curved car body or an overworked wing. Easing the hand's angle back down the moment the burbling starts restores the smooth, attached feel almost instantly, which is precisely the recovery a wing makes when its own angle is reduced just past the point where its flow has begun to separate.

Separation on a gentle curve

This picture of separation caused by rising pressure and steep curvature describes the great majority of everyday separation on a car, but it is not the only route to it, since a boundary layer that has already been thinned and drained of energy by a very long run of attached flow ahead of a given point can separate even on a fairly gentle curve, simply because it arrived with too little momentum left to spend. Separation on a real car therefore depends on the condition of the flow arriving at a surface as well as on that surface's own shape, a condition set by everything the air has already passed over further upstream. Moving a component a short distance further forward on a car, giving the flow reaching it a shorter, fresher run beforehand, can sometimes fix a separation problem that no amount of reshaping the component itself managed to solve.

Spotting it early, and catching it with tufts

Recognising the warning signs of approaching separation, a surface curving away increasingly sharply, or a long run of surface ahead that has already been draining the boundary layer's energy, lets a designer address the problem before it happens rather than discovering it only once a car is built and tested, whether that means easing a curve, adding length upstream for the flow to settle, or accepting a shallower shape that generates less force but keeps the flow attached reliably. Wool tufts and other flow visualisation tools exist largely to catch separation happening in practice, since a tuft lying flat shows attached flow while one flickering wildly is sitting in a separated, disorganised wake, and comparing where tufts start flickering against where the surface's curve or pressure rise was suspected to be too aggressive is one of the most direct ways a small team without access to expensive equipment can confirm a suspicion about where a shape is losing its grip on the air passing over it. Photographs taken at several points along the same surface, compared side by side, often reveal the exact line where flickering tufts begin far more clearly than watching the car pass by in real time ever could.