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 &amp; drag of a delta airfoil inside a wind tunnel between 0 and 40 degrees, in 5 degree increments, at two flow velocities (subsonic, M&lt; 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?
The increase in the lift curve slope at low angle of attack is quite normal. At 10° you see the effect of vortex lift kicking in. Yes, that is specific to delta wings.



Attached and vortex lift over angle of attack (picture source). This is taken from the excellent Cambridge Core article on vortex lift and these particular diagrams are from E.C. Polhamus: A concept of the vortex lift of sharp-edged delta wings based on a leading-edge suction analogy, NASA TN D-3767, December 1966. They show lift over angle of attack, but lift over drag would look quite similar, also with a kink once vortex lift sets in. 

The offset between the two speeds is harder to explain. This looks like an error in your angle measurement – there must have been different offsets between the two test runs. That there is an offset is obvious from the nonzero lift at zero angle of attack. Unless your delta wing had lots of camber, there should not be much lift at zero angle of attack.