FP_Q&A - 12/8

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Unformatted text preview: 12/8 =============================================================================================================================== Q: "Under number (4) on the FP, it says "*Plot the lift to drag ratio, L/D, as a function of angle of attack for .*" Could you clarify the ending of this statement for the class?" A: I had left out "for the case Re=3e6". I have fixed the file. =============================================================================================================================== Q: I was wondering, for part (5) in the Final Project, it says plot Cf against alpha. As far as I could figure out, the program only plots the Cf at a particular alpha. Does the question mean that we have to use Cf to figure out the BL separation or do we use other methods to figure out the alpha for BL separation and then use Cf to justify the solution? A: I don't see anyplace where it says to plot Cf vs alpha. It says that you can tell where separation occurs from a plot of Cf (vs x). At small aoa you would expect the BLs to separate at the TE. As aoa increases the separation point will move. How does the separation point change with aoa? =============================================================================================================================== Q: For part A of the project, we are supposed to compare our MATLAB Hw9 to the XFOIL answers. The XFOIL has an angle of attack of 4, N=140, and it's a NACA 2414. The MATLAB has an angle of attack of 0,N=200, and is a NACA 2415. The size of N in MATLAB depends on the size of x, which doesn't seem easy to change. What are we supposed to compare on these two very different airfoil configurations? Are we just supposed to be noting the differences? A: I asked you to compare XFOIL results to those produced by the panel code you used in HW9, not to the results you got in HW9, which were for a different airfoil. You can use any number of points you like... it's very easy to change. Same in XFOIL. =============================================================================================================================== Q: When you were in my office I tried to run Re 6e6 and alfa 4 and it refused to converge A: I found that alfa of 1 worked, and I could inch up... 1, 2, 3, 3.5, 3.8, 4, 4.2 etc This is called "bootstrapping", and is a common way to get iterative schemes to work =============================================================================================================================== Q: I was wondering if excel would be alright for ploting the desired curves? If not why the restriction to MATLAB and xfoil? thank you for your answers A: I don't care much what plot package you use... as long as it's well done. =============================================================================================================================== Q: I was asked after class how to pull a table of CL vs alpha for the inviscid case out of XFOIL. If you run aseq, XFOIL automatically plots the cp distributions and prints a table of alpha, CL and CM on the plot screen. But I see no ready way of capturing that data. If you know of a simple way, please tell me. A1: (My Answer) However, there is a trick : we know from inviscid theory that CL depends on alpha as CL = A cos(alpha) + B sin(alpha) where A and B are constants that depend only on the airfoil shape So if you record just two values of alpha and the corresponding CL's you can get the values of A & B eg alpha =a1, CL=CL1 alpha=a2 , CL=CL2 (where a1 and a2 are any two aoa's you ran in aseq and CL1,CL2 are the values of CL printed on the plot screen for those two aoa. For example, if a1=0, then A=CL1 and B=(CL2-A cos(a2))/sin(a2) Knowing A&B you can compute CL for as many values of alpha as you like. A2: (A better answer, thanks to DM) Before running 'aseq' if you enable polar accumulation by typing 'pacc' Xfoil will automatically print that table to the polar save file which you choose (the polar dump file, which you also specify in that command, just prints garbage). I saved the xfoil.exe file to my desktop, then picked say, aae333.txt as the polar save file. Then if you run 'aseq' the table printed in the plot, appears in the save file. I hope this clears up the issue. If you can't get it to work, let me know. A3: (comment by RK) This method will work well for Cl only. However, we cannot get Cd values like this because, or at least how it appears to us at the moment, no Cd values are printed to the screen (when running an inviscid flow simulation). We also tried the method explained on the course webpage, but this answer simply explains the normal procedure for running XFOIL. I think DM may not have understood the problem, because if we follow this method for inviscid flow, we still get an empty dump file. A4: (comment by MW) I was only talking about the inviscid case, where Cd is irrelevant (it measures numerical error only, nothing physical) =============================================================================================================================== Q: Why does the addition of BL effects change the pressure distribution? A: The pressure essentially does not change across the BL, so if the BL's are very thin, the pressure would be the same as in inviscid flow. But the region of slow moving fluid near the wall displaces the apparant wall outward. In fact the pressure is the same as if you had an inviscid flow over the "body+displacement thickness" as drawn in the primary plot figures from XFOIL. The pressure distribution changes essentially because the effective shape of the body changes. =============================================================================================================================== Q: How do we get a copy of the plots from xfoil, is there a way to import them to matlab or excel. Should we take a screen shot or just make our own from the data. A: I dumped the data files and copied them to matlab. (Excel would work as well). I'm sure there is a way of printing the XFOIL plots, but I don't know what it is. =============================================================================================================================== Q: How do we print the data needed for part B #3. A: ? =============================================================================================================================== Q: For 5 do we have to plot multiple Cf graphs to find the angle that separation moves forward or is there an easier way to figure that out. A: ? =============================================================================================================================== Q: For the report my teammate and I were wondering what you are looking for in the "Procedure" section. Do you want us to rewrite it in our own words, cut and paste it, or just have a brief recap of what we did? A: I'm not real clear on the distinction you are drawing. What you write must, of course, be "in your own words" (as opposed to someone elses). It should be of sufficient detail that someone else could reproduce what you did. (This sounds simple, but is actually very hard... most published work fails at it. It's still what you should strive for.) > =============================================================================================================================== Q: Should the results from part A (getting started) be included in the final report, or was this section only intended for us to become familiar with XFoil? A: Yes, please do include Pt A results. It was intended to get you started, but it was also part of the evaluation of the XFOIL code. =============================================================================================================================== Q: In part 3, it is mentioned that we should obtain data for the pitching moment. However, I don't believe we are asked to make any comparisons to experimental data or discuss the data in any other way. Should anything be included in the final report about the pitching moment? A: I had intended you to do more with pitching moment than I ended up requiring. It was my oversight : I should have either fleshed out what I wanted or removed reference to it entirely. What I had wanted you to do was compare pitching moment about the 1/4 chord from A&vD to the XFOIL data... but that statement did not get into the final version, so I can hardly insist. It would be interesting, though. ...
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This document was uploaded on 01/22/2012.

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