Week 2 Part 2-Dude, What’s Up With Those Lift Curves?
Week 2 Part 2, and Group 4 is back in the famed Aero Conference Room. In the meantime I have discovered a pace of 2 posts per week is probably unsustainable, but we’ll see what next week has in store.
Fresh out of the lab with our data, I and one of the many Joshes set out attempting to analyze that data with some lift code he developed two meetings ago. It took a few tries, but eventually we got it working
Wait, that’s not right. Never mind the completely erratic data for the lower speed data, the peak Cl value for the high-speed run is just about 4 times higher than one would expect it to be.
It took a bit of digging through the code to find the issue, but basically what had happened was when figuring out the lift on the airfoil, we had figured it out for a chord length of 1. When nondimensionalizing this lift into Cl, we divided by the chord length, which we did not need to do. Given that the chord length is about .25 m, this resulted in Cl values about 4 times higher than they should be.
That’s better, as long as we, again, ignore the low speed run. Somewhere along the way, we got some strange pressure data or lift data that severely affects the Cl values we calculate. But still, analyzing one set of usable data can still provide some insight into the accuracy of that data. Two easy ways to do this for a lift curve are looking at the zero-lift α and the stall α. The most noticeable feature of our calculated lift curve is that it does not actually reach Cl=0, which means that unless our method is wrong, we failed to capture the zero lift α. Some preliminary thoughts on how this happened: we decided as a group to test from α=-5 to -3 in order to capture the zero-lift α, but α=-5° was the lower limit for zero lift for lower Reynolds number flow. When we actually tested, we made a quick decision to change the speed of our second run from 15 m/s to 25 m/s.
Simulating this new regime in XFLR5 yields a zero-lift α ranging from just above -5° to just below -5° (The group of lines on the left). So, it is likely that speeding the flow up caused our zero-lift α to move outside our test matrix.
The peak Cl is also higher than one would expect. For a higher Re flow, the simulated data shows a peak Cl of about 1.3, not 1.4. Thus, it is possible that for some reason our data has shifted upward on the order of ~.1 Cl.
Referring back to the fixed Cl/α plot, the Cl values for the flow that has separated but not yet reattached is depicted by the black X’s. One thing I mentioned in my last post was wanting to see how the characteristics of the flow changed at equivalent angles of attack for flow that has (a)not yet separated and (b) separated but not yet reattached. The results almost universally showed a Cl greater by ~.05 on the flow that had separated but not reattached. It will take some more thinking to figure why this is, but I initially think that the answer lies in the recirculation occurring in the separated flow, and that this region has a slightly lower pressure than the flow does before it separates, thus imparting a higher lift force. I’m not entirely certain this is the right answer, however.
As I sat, ruminating about the right answer to the question above, as well as trying to figure out our issues with the lift code, I had an interesting (to me) question. How big is the boundary layer on the surface of the Earth? That question is easily answered by the Windows To The Universe website. I thought the website seemed less than reputable at first, but it appears it is written by Earth Science teachers, as well as sponsored through NASA and NOAA. So, we have an answer (50m to 2km, if you don’t want to click the link), but now my question becomes, how would one calculate that? What characteristic length would you use? It’s not like I can Google the equations either. In our classes, we as students are able to simplify things by looking at the closed system only. We assume uniform and simple inlet conditions, and things like that. But on the surface of the Earth, there is no “inlet”. The “wall” that the flow rides on has no beginning and no end. If you wanted to define the starting point for a wall, how would you define the ending point? Sadly, the Earth isn’t flat, so at some point you’re bound to circle around and reach your original starting point. When does the characteristic length “reset”? Suffice it to say, the Earth is a substantially more complex system than a flat plate.
