Computer simulations of the MD-11 models
by Steve Smith
First, I needed to get the geometry of the wind tunnel
test section and the geometry of the MD-11 model into a form that the
computer simulation can use. I used a type of CAD program to create a
"surface panel" definition of the model and the wind tunnel. This took
me about 3 weeks. I actually needed to do this step six times, because
I wanted to simulate the model installation in the wind tunnel at three
different angles of attack, and there are two different model sizes.
Next, I ran a simulation of the flow through the
empty tunnel to ''calibrate'' it - to make sure the flow speed in the
test section was the same as the flow speed in free air. Results: I put
a flow sampling point in the test section model, and the velocity at this
point was 0.972 times the free stream velocity. Since lift and drag forces
are proportional to the square of the velocity, V2, I need to adjust the
forces inside the tunnel in my computer simulation by dividing by 0.946.
December 19, 1997
December 22, 1997
December 23, 1997
December 29, 1997
January 2, 1998
January 3, 1998
Determining the corrections to apply to the data
I made an Excel spreadsheet with all my computer
results of lift and drag coefficient. The first correction I did was to
adjust all the lift and drag coefficients from the models in the tunnel
for the empty tunnel calibration, by dividing by 0.946. Next, for each
angle of attack, I subtracted the result from the free-air computation
from the result in the tunnel, so I now have the ''delta-lift'' and ''delta-drag''
caused by the wind tunnel at each angle of attack.
January 6, 1998
January 7, 1998
Well, the results still show about 8% difference in drag at the same lift. It looks like my corrections have compensated for some of the differences, but not all. The 8% difference is really big, considering that the models were basically the same, just different size. There must be some more complicated effect that my computer model doesn't account for. I'm going to have to think about this for a while.