Quote:
Originally Posted by kach22i
I'm assuming downwash and or upwash is not equal to attached flow because of what the surrounding air is doing.
That is to say: if the downwash attachment is being caused by a giant drag causing vortex, then you do NOT have clean pressure or evidence of attached flow.
Yes or no on the assumption and description above.
Pressure attachment Vs Flow attachment.
Is there an article on this?
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1. Tufting shows separated or attached flow very clearly.
Attached flow is when the airflow is being guided by the shape over which it is flowing.
Separated flow is when the air has gone its own way, and is not being guided by the car's shape.
It's that simple - note how there is nothing to do with lift, drag or anything else in that definition.
2. Both attached and separated flows can cause drag and lift.
An example of
separated flow causing drag is in the wake behind the car. The air in the separated flow is of low pressure and is causing a backwards pull on the car.
An example of
attached flow causing drag is where air passes from the roof down onto the rear window of a modern sedan. The vector force created by this flow has backwards and upwards components - ie an arrow showing this force points up and backwards.
Note that both types of flows can create low pressures (and so lift and/or drag, depending on the force direction). However, the force developed by low pressure attached flow can be much higher than the force developed by low pressure separated flow. (Easy to remember - an aircraft wing stalls (decreases lift) when airflow separates.)
3. Trailing vortices create low pressures and so drag and/or lift. They are typically shed from angled pillars eg A pillar and C pillars. Viewed from behind, they rotate clockwise on the left and anticlockwise on the right, if the car is developing lift. The 'final pillar' (ie C or D) can add a downwash component on the rear section of the car, giving attached flow where normally separated flow would have occurred.
To directly quote Adrian Gaylard, head of Jaguar aero:
Typically, for lowest drag, the EBLA (effective backlight angle in a sedan) is around 12 degrees. As this increases, so does drag, until around 30 degrees where the rear-pillar vortices burst and the rear flow fully separates. With an effective backlight angle approaching 30 degrees, it’s often better to separate it as the drag can be lower for a fully separated rear flow, compared to one where rear pillar vortices are keeping the rear screen flow attached on a high screen angle.
Note a few things from this quote that support what I have said:
1) The flow is classed as attached, even if that is caused by the action of trailing vortices.
2) Attached flow can cause greater drag than separated flow. (And so, attached flow can also cause greater lift than separated flow.)
All of this is in my book, that was reviewed by real experts prior to publication. It's also supported by dozens of measurements on different cars.