[aerohead]

Originally Posted by JulianEdgar

You don't even need a different definition of attached flow.

Attached flow: airflow is guided by shape of the car, and tufts line up on body in direction of flow

Separated flow: airflow is no longer guided by the shape of the car, and tufts whirl around and point in any old direction

Whether airflow is attached or separated is governed by a range of factors, including boundary layer thickness (thicker = more easily separates), rapidity of angle change (quicker downwards angle change = more easily separates), surface finish (eg steps up or down = more easily separates) and vortex behaviour (downwash can cause attachment that otherwise wouldn't occur).

If we then want to talk about pressure drag, we can talk about the drag induced by attached flow on inclined rear surfaces (because this flow will also be causing lift, you can call it induced drag, or simply separate the force vector into its lift and drag components), and we can talk about drag caused by the low pressures of the action of the vortices that are causing the downwash.

Unfortunately, Aerohead says stuff like this:

Downwash is not attached flow. It is symptomatic of flow separation. You'll get the highest drag, vortex-drag, plus a small, but very low pressure wake, and overall higher drag. To say otherwise is delusional. It may look like a duck, but it doesn't quack like a duck.

...which is full of confusion and, yes in his words, rather delusional.

For those with my big book, on Page 129 there is direct quote from the head of Jaguar aero that makes it clear that attached flow caused by vortices is still attached flow.

Would you like to qualify that assertion?