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Aragon March 27, 2009 13:38

Fluent simulation over airships
Hello everyone, I am an Italian engineering student at the very beginning using Fluent CFD, and I have some questions about what I have to simulate. My task is quite particular: I have to study flows over some airships geometries in order to validate experimental tests conducted by the Germans during WWII in a small wind tunnel having an experiment chamber with these dimensions: lenght of 2 m and flow entrance cone of 1.2 m diameter. The models used at that time were 800 mm long. The airspeed was 56.5 m/s and the related Reynolds number 3.1*10^6. As I said before, I know little about Fluent, so, probably mistaking, I modeled the geometries as bidimensional profiles so that my problem, substantially, could be considered as the simulation of flows over symmetric airfoils. And this is obviously a source of errors that I don't know how to handle: I mean, if I simulate this 3D problem with a bidimensional geometry, probably I have to change the angle of attack of the experiment I wish to simulate in order to achieve likeable results. But if I know something about the correction I have to make for wings, I do not know about anything similar for airships. The other problem is that I created a farfield boundary very similar to what is usually considered in the study of flows over airfoils, so very far from the body (i.e. about 10*cord lenght in all directions) since it must have little influence on the flow, but I do not think this is correct; probably the best thing to do is using a boundary that resembles the dimensions of the experiment chamber of the wind tunnel that I have described before, in order to achieve better agreement with the conditions of the tests I want to simulate: the flow over a model 800 mm long in a chamber 2 m long will surely be influenced by this relatively little difference in lenght between the model and the boundary. My question is: are these considerations correct? Should I immediately skip to a tridimentional simulation and developing a 3D boundary which resembles the dimensions of the experiment chamber of the wind tunnel? And, generally speaking, what are the better boundary conditions and viscous models I have to use in order to simulate wind tunnel tests properly? Is it correct to use, at least in first approximation, the inviscid viscous model since the Reynolds number is so high? Sorry for the lenght of the post, for my bad English and even for my ignorance in these problems, and thanks for all the suggestions anyone of you could provide. Bye!

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