Cd data

  • Alex

    Alex - 2006-12-28

    Hello Joseph H. Morrison,
    1)I'm ISAAK beginner.
    I'm interested in evaluation designed airfoil Clmax at M=0.4 and Cl(Cd) at M=0.6.
    i've used Xfoil + Cpmin(Cl)and Cp(x) analysis for this purpose. ISAAK is seemed
    to be a good verification  tool.
    Would You like to comment on  airfoil performances?
    My test: four  airfoils,M=0.4, Re=3mln ,y+<1, k-w , the same grid topology and sise 259*81
    Clmax is greater than experimental value within 0.03-0.05. Cd- more than 1.5 times greater,
    Cdw(Cl=0)> 0.005   ???  For inviscid case Cdw > 0.005 too. Integration  errors seemed to be
    not greater than 0.001, isnt it right?
    Have You compared Cd data ?
    2)How to refer to Your code in bibliography ?
    For  all ISAAK users:
    Free airfoil mesh generator-
    Best regards,

    • Joseph H. Morrison

      1) I have looked at some Cd data and ISAAC has been close. But close is a relative term. There will be differences between the predicted value of Cd from ISAAC and other codes, experimental data, and differences with different grids and/or turbulence models. It is important to do a grid convergence study to ensure that you are looking at grid resolved solutions. When performing grid convergence studies, it is important to maintain the same grid family. Can you refine this grid a couple of times and see how the solution converges with grid refinement (try one of the cases first and then look at the others)? Also, there will be inherent model form errors (differences). Each turbulence model has its own character. None of them will exactly reproduce the experimental data from a wide range of problemsl; they are models. This results in an offset of the prediction from the experiment. Amother issue to consider is the transition location. Re = 3 million is relatively low and will typically result in transition on the airfoil. ISAAC only models instantaneous transition which is not what typically occurs on an airfoil. This can add to the offset. The k-w model in ISAAC requires the transition point to be set or it can lead to erroneously high levels of eddy viscosity due to extraneous production in the stagnation region.

      Cd mode than 1.5 times expected Cd for the k-w model leads me to believe that the extraneous production in the leading edge stagnation region has occurred. Set the transition location at 5-10% chord if you do not know the experiemental condition and re-run one of the cases and see if this improves the results. The inviscid boundary condition in ISAAC is very simple as I originally designed ISAAC to test turbulence models. I am not surprised to have Cd(Cl=0) that is high for the inviscid case. This can be improved by improving the wall pressure boundary condition to reduce entropy production at the wall.

      There have been a series of drag prediction workshops sponsored by the AIAA Applied Aerodynamics Technical Committee. I have been involved in these, but have not used ISAAC on any of them. If you look at the results from the workshops, you will see scatter between different codes predicting the same flow on the same configuration. These results would be useful to gain context on what you can expect to see. You can google drag prediction workshop and find some of this data. If you have access to AIAA publications you can get several papers on the workshops. There are a series of papers at the upcoming AIAA Aerospace Sciences Meeting in Reno that show the most recent results (all the data and solutions for DPW are 3D).

      2) Referencing ISAAC in a bibliogrpahy depends somewhat on where you are publishing the results as each journal has its own guidelines. I would recommend referencing the web site for ISAAC and/or the NASA CR-4440 that I published some time ago.

      Good luck and please let me know if any of these things do or do not help.


  • Alex

    Alex - 2013-03-05

    Any news about parallel version of code (for optimization problems). May be  beta version?


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