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Old   March 3, 2003, 16:25
Default Laminar/Turbulent BL over a cilinder
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Hello, I'm having trouble with the flow over a cilinder. I ran some wind tunnel tests first and then I tried to simulate the same case in Fluent. In Anderson's "Introduction to Aerodynamics" there is a figure with experimental and theoretical cases of pressure coeficients over a cilinder for different Reynolds numbers. With the same boundary conditions I get different kinds of flow: Wind tunnel case is subcritical and CFD is supercritical. Am I doing something wrong, or is Fluent not capable of a good prediction of the laminar/turbulent boundary layer transition? Thanks in advance. (Coupled/implicit/2 layer model, Spalart-Allmaras and K-omega turbulence model speed 30m/s turbulence 0.5% 23% area blockage)
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Old   March 26, 2003, 22:28
Default Re: Laminar/Turbulent BL over a cilinder
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You've run into an interesting problem...i'm new to this so if you find out anymore i would love to get more info on the subject. However, from what I can understand the thing about the turbulence model in FLUENT is that it assumes a turbulent boundary layer along the wall of the object. This is why your getting the supercritical answer. Since your running at such a low Reynold's number the tunnel test is seeing a laminar boundary layer throughout, yielding the subcritical solution. My suggestion is to rerun the FLUENT model taking out the turbulence model, and running the laminar case. I would bet that your results would match much better in that case. Another case you might consider looking at is trying to duplicate the turbulent result by putting strips of tape along the cylinder to trip the boundary layer. As far as the transition prediction goes, i do not believe that it is capable of this.

good luck
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Old   March 27, 2003, 08:43
Default Re: Laminar/Turbulent BL over a cilinder
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I will try the laminar model & see what happens. The trip strip is a little complicated because my cilinder has one tap and I just turn it around to get the results, but I'll find a way. Thanks.
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