# BASE DRAG

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 March 30, 2007, 06:11 BASE DRAG #1 sam Guest   Posts: n/a hi! well...im performing CFD simulation over a Launch vehicle.it is 3D flow at Mach 2.im calculating its base flow and ultimately base drag....The problem is that a large wake at the base of the body is seen in simulations, which requires extension of flow boundaries and hence increase in mesh size to fully capture the flow.i have used pressure far field boundary condition at the exit faces(rear end of domain). I lack computational resources.is there anyway i can capture the base flow without increasing mesh size? plz reply...i need guidance

 March 30, 2007, 11:24 Re: BASE DRAG #2 O. Guest   Posts: n/a I would advise to cut your domain at the base and use a model for the base pressure instead. Getting the base pressure is not easy, although some people start to get good results with large eddy simulations. Since you said you lack computational resources LES might be out of the question. What is your objective? Do you want to get a feel for the flow or do you have to give a figure for the base drag?

 March 31, 2007, 13:45 Re: BASE DRAG #3 sam Guest   Posts: n/a hi O wat do u mean by model for base pressure? how does one use it? my objective is to calculate base drag.and also to get the feel of flow physics. does non reflecting bounary condition help in this situation???

 April 2, 2007, 03:56 Re: BASE DRAG #4 O. Guest   Posts: n/a There is a number of different models available to get an empirical estimate for the base pressure. Input parameters would be the geometry of your base (e.g. boat-tail or not) plus the Mach-number at the edge of the base just outside the boundary layer, plus Reynolds-number etc. ... You can cut the domain at the base for your CFD, because the flow is supersonic (apart from a tiny region inside the boundary layer). Hence you don't have to deal with the wake and that reduces mesh-size and computational effort. You said you use a pressure far field boundary condition - does your boundary condition make a difference between supersonic and subsonic outflow? For supersonic flow you have no information going upstream at the boundary.

 April 4, 2007, 05:55 Re: BASE DRAG #5 sam Guest   Posts: n/a Hi O, thank u for considering and replying to my problem ok, i will cut the mesh at the base of the body... but which boundary condition should then be applied at the base of the vehicle and at the rear domain faces? im using fluent 6.2 software. can u also reference any book or material from where i can get Empirical Formulas for the calculation of base drag? waiting for ur reply

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