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Originally Posted by hat_man
I'm willing to learn something here but I'm going to need some guidance.
Is the lift of the Dream created by the attached airflow over the top of the profile or because the design of the underside creates more pressure on the underside than is created on the top (underside pressure >topside pressure. Lift created by a specific design.) or us the lift created by the basic shape resembling an airfoil and adds as such (Bernoulii ?) because it is positioned high enough off the road that it acts as a "wing" (not a specific underside design to create lift, but placement of a basic design in relation to the road to create lift)
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I thought unattached airflow (flow separation?) was more turbulent and caused more drag. If the Dream is low drag then wouldn't it have more attached air and less turbulent air moving around the body?
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1. I don't think it helps much to try to work out the mechanism by which wing lift occurs - ie at a purely theoretical level. As Freebeard correctly later posts, theories of lift are contentious and so for amateurs like us, just cause confusion.
2. To have lift, a vehicle must have a lower average pressure on the upper surfaces than the underside surfaces. That's all.
3. Pressures causes by attached airflow can be greater than caused by separated airflow. Low pressures occur where airflow speeds are highest. Where the airflow follows a surface that, for want of a better way of putting it, causes the airflow to wrap around a curve, low pressures are generated. On normal cars (eg a fastback) these low pressures occur, therefore, at the curved leading edge of the hood, at the windscreen header, across the curved roof and down the first (most curved) part of the hatch.
4. Low pressures can also be generated under the car, where the air flow travels quickly between the smooth underside of the car and the road. However, without mirroring the curves of the upper body, these pressures will generally not be as low as on the upper surfaces.
5. The Honda Dream would have attached flow everywhere. It would have, I would guess, lowest pressures on the upper surfaces where the curves are greatest - ie where the front axle line is. Conversely, except when there is yaw airflow, the relatively flat area at the back of the car would produce relatively little lift. The underfloor area would also be developing a low pressure. In the case of the front, top surface lift is much greater than underside downforce (CLf is positive), while at the back, the rear underfloor downforce is greater than the top surface lift (CLr negative), except in yaw conditions.
6. Where the flow separates, a low pressure is also developed. However, except for occasional small separation bubbles, the top surfaces of all modern cars have attached flow. Under the car, separation is much more common, and relatively few cars have attached flow here. But we can talk about what happens under the car another time!