gato azul wrote:different way to skin the cat - I suppose
in your Newtonian way of thinking, less upward momentum (less wake height) would require less energy, which has to come from somewhere.
The end result would be the same, just the way to illustrate it is different - IMHO
Rear Wing Tests
The lift and drag coefficients, measured in the wind tunnel, for the rear wing in free-stream are shown in Figure 8 below.
The CFD predictions are provided on the same graph for comparison.
The wind tunnel results showed a trailing edge separation on the rear most flap beginning at 38degrees.
By 40 degrees the underside of this flap was fully separated, resulting in a plateau in the CL curve.
The flow on the underside of the main plane remained attached until 48 degrees, beyond which a complete leading edge separation was observed.
The considerable difference between the stall angles predicted by CFD and measured in the wind tunnel was most likely due to the extremely small aspect ratio of the wind tunnel tested wing (1.72).
Again, the wind tunnel drag is believed to be higher than predicted from CFD due to induced drag.
n smikle wrote: The model with the Ducts on the pylon had about 13 to 20% more down-force but only about 6% more drag. So it is basically a blown wing.
Thanks for your answers, but I fear that only integrating the surface pressure distributions over the area of the wing will not give accurate values for overall drag.
It's not that you make anything wrong, rather then a limitation of the method your software uses.
The effects of induced drag will not be properly represented by only integrating surface pressures.
Are your values "time averaged" values, or are you (your software) assuming a steady flow condition at all times?
Effects like vortex shedding etc, and the flow in the "dead zone"/separation areas are highly unsteady in nature, so can be only represented as a time averaged value in terms of their contributions for a single Cl or Cd value.
Nevertheless, keep up the good work, I like to see the evolution of your models, just keep in mind that every tool has it's limitations.
How large is the "control volume" you chose in your model? Do you look at the pressure distribution in the wake too? If you/or your software chooses the control volume (air volume affected by the wing) too small, you will get discontinuity at the boundaries of your control volume.
In a wind tunnel with a limited test section area, you would need to measure the pressure distribution along the walls as well, and account for them in your overall model, to better predict total drag.
amc wrote:I said before that I didn't think the wing was stalled - that VD just increased static pressure behind the wing. This would appear to contradict the stall theory where we think that the boundary layer separates below the wing, reducing the lift produced and therefore induced drag. However, what if the boundary layer has been separated (the wing is stalled) but there is still a boundary layer in place...
bhallg2k wrote:Don't confuse the fact that the wing is not being stalled with the idea that the system is in place to keep air flow attached.
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