3/18/2024 0 Comments Xfoil airfoil databaseOr the immense joy of showing up and the operator confessing that he dropped some of the cards and they are now all mixed up. Just one mispunch and it was cussing time. We had to carry them to the input area and hopefully pick up the output later. At one time to run our six-degree-of-freedom airplane response program just the data took 3 of those big fortran card boxes. In BEM i got different Re for every iterative step.I hate Fortran in any form it came in. So two ways i figured out to solve the problem: reading Cl and Cd direct from the tables or just write a short program to do extrapolation.īut the problem is that it seems the Renolds number influence on the polars is not considered or at least not mentioned. Excel tables are also available for several airfoils online. Then they extrapolate the data to get polars for all AoA. In NREL they first get some Cl and Cds for an specified airfoil under a certain AoA range by using wind tunnel tests or softwares. Even after a little change convergence is already obtained I'm not sure if this pair of Cl and Cd is reliable for the BEM. Somehow I feel it's not a good idea cuz maybe in some steps(altough in the cases i tested so far not yet) alfa will have to be changed too much to get a converged result for Cl and Cd and thus the BEM could be carried on. What i've implemented in my program now is for these cases i change the AoA a little bit say -0.01 deg and rerun Xfoil commands until it converges. You are right, Xfoil doesn't converge when large AoA is given which is often the case in WT simulation, thus got problems for BEM cuz CL and CD are needed iteratively. NACA0012 normalized Coord NACA0012 scaling factor 2 NACA0012 scaling factor 2ģ. with upscaled coordiantes as loaded coordinates, the Re should be downscaled to get the proportinally upscaled Cl and Cd. the input alfa in Xfoil should be the angle between the inflow and the horizontal axisĢ. It seems that from these tests i found:ġ. I've tried to load in the original normalized coordinates, the twisted coordinates as well as the upscaled ones and compare their lift and drag coefficients. After that post I've tested a little bit with Xfoil by using NACA0012. RE: xfoil problems sechstemann (Mechanical) It might be a good idea to see if anybody has tried something similar before, and if there is commentary on the limitations of XFOIL. There's probably a lot of documentation on XFOIL as it's quite popular. You can try using the "INIT" command to reinitialize and then run the same AOA again. In my experience, if you do intend to use XFOIL for high alphas, it may have problems converging. It starts at 100 percent, goes over the top, and back around the bottom. When you downloaded XFOIL, it should have come with a sample airfoil which is formatted correctly. XFOIL is very particular about how the coordinates are loaded in, and it does not use the Selig format which has become the standard, if I remember. Your coordinates should be "normalized" as you say, so that the input represents the camber lines from zero percent to 100 percent of the chord. It sounds like you are only using Xfoil for a small part of your analysis - that is, to pull out the infinite (2D) aerodynamic coefficients. Yijun RE: xfoil problems Sparweb (Aerospace) 22 Jun 16 23:16 Any help from you guys would be appreciated. Sorry for typing so long cuz I'm really a new bee with Xfoil. The input angle of attack to Xfoil is the angle between the incoming flow and the horizontal axis or the angle between the incoming flow and the chord length direction? say in my case the chordlength, which is reprensented by the coordiantes, doesn't lie in the horizontal direction. Suppose using the real coordinates, then which Renolds number should i specify, rho*vinf*actual chordlength/nu (the actual Renolds number) or rho*vinf*1/nu (the scaled one), which is using the normalized coordinates?ģ. As input airfoil must the coordinates be normalized or is it also possible to read in real coordinates? Will the result be reliable for creating the coefficients when using the real unnormalized coordinates?Ģ. As input airfoil for Xfoil i take the real coordinates of the profil which don't vary from 0 to 1. In one step of the method i called Xfoil to create the lift and drag coefficients for all the sectional aerofoils which have naturally different Renolds number, Mach number and angle of attack. I divided the blade in several sections along the span. I built the model with Python scripting in Abaqus and for the aerodynamic part wrote a script to implement the classical BEM method (not unsteady BEM method). I'm now doing a project in which a FEM and BEM (Blade Element Momentum) coupled simulation is conducted to study the aeroelastic coupling of a large wind turbine blade(NREL 5MW, with hub connection part).
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