# Why did this delta wing generate a lower Cl at a higher wind tunnel velocity?

I'm an aerospace engineering student,and just did an aerodynamics lab in which we measured the lift & drag of a delta airfoil inside a wind tunnel between 0 and 40 degrees, in 5 degree increments, at two flow velocities (subsonic, M< 0.5). For some reason, the lift co-efficient is .25 lower throughout the entire curve in the higher velocity run compared to the lower velocity run, despite the drag co-efficient s being identical until the stall region (which begins at 30 degrees and completely separates at 35). Why is this? Considering the nature of the lab, this seems what I should be discovering. I have a theory that it has something to do with stronger vortices generated by faster flows, but wouldn't that INCREASE lift by dropping the pressure on top of the wing further?

In addition, another oddity: the L vs D curve increases SHARPLY between 0 and 10 degrees for both flows, but then the slope decreases after that and stays steady until the stall region. Why is that, and is that specific to delta wings?

• Did you non-dimensionalize with the correct speeds? – JZYL Nov 7 '19 at 13:00
• Do you have pictures of the models? – ROIMaison Nov 7 '19 at 19:55
• Did you actually calculate these points and plot them, or is this a computer generated result? Raw data should show much higher lift and drag for higher airflow speed. It seems, in equalizing the drag plot, you've discovered you get better lift to drag at a lower airspeed. – Robert DiGiovanni Nov 8 '19 at 18:43