Hi guys, I am simulating an horizontal axis wind turbine using Ansys Fluent (steady state simulation). To get more accurate results I use second order upwind momentum discretization scheme, but unfortunately the results don't agree with the experimental ones. Meanwhile with the 1st order upwind momentum discretization scheme, the solution match better with the experimental one. What must I do in this case ?
Dear Somaya Younoussi
In other words, whatever convergent solution is erroneous, and we need also examine integral quantities and criteria is a specific value in various issues.
Kind Regards
Qamar Ul Islam
It is hard to say basing on so limited information. Usually, if the results are convergent, it is a suggestion that the order is correct. But correlation does not imply causation, so you cannot assume for 100% that 1st order is correct only basing on the comparison of solution and experiment.
Dear Somaya:
As you know, we can say that the upwind scheme is one of the simplest and most stable discretization scheme, however, it is more dissipative according to the flow you are trying to simulate.
Basically, it uses the values upstream to evaluate the property on the boundaries of the cell and then use them to compute the value at the center of the cell. As it is a "upstream" value, it takes into account the flow direction.
Also the main difference between the first and second order is basically the number of points used for the computation, i.e. one upstream point for the first order and two for the second order. Usually, it is better to use the second order scheme.
If you need more details on this, the following book is, in my opinion, very good to introduce this type of notion. Other discretization schemes are also explained.
@http://www.pearson.ch/HigherEducation/PrenticeHall/1471/9780131274983/An-Introduction-to-Computational.aspx
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Finally:
In the analysis for errors, while comparing the exact and numerical solutions, the first-order upwind scheme gives a complex k-value. Therefore, it will have both numerical dispersion and numerical diffusion problems.
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First Order Upwind : Easy to converge and less accurate.
Second Order Upwind : More difficult to converge and more accurate than First Order.
I hope it will be helpful...
With my best regards....
Karol Postawa, and Qamar Ul Islam thank you for your answers. Hassan Nasser thank you for your detailed and useful explanation.
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