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Old   November 27, 1999, 22:16
Default Asked for Help
  #1
ckl
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Hello,

I am working on a case of flow past a cylinder by using PHOENICS. The flow is laminar and inviscid. I had got the pressure and velocity values very near to the surface of the cylinder. My question is how is the algorithms for the calculation of drag coefficient,lift coefficient and moment coefficient if using Fotran.
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Old   November 28, 1999, 14:34
Default Re: Asked for Help
  #2
John Chien
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(1). The drag coefficient is defined as the total surface force acting on the body in the flow direction, divided by the dynamic force (0.5*rho*U*U) of the flow at the free stream (inflow). In this case, the dynamic force can be derived from the inflow boundary condition. (2). The total surface force has two parts, one is the static pressure normal to the local surface, the other is the viscous force tangent to the local surface. The local total drag force can be computed as ( p(surface location)*ds*cos(theta) + tau(surface location)*ds*sin(theta) ). It is important to make sure that the force direction is correct. That is, the drag is in the flow direction , ds is the surface element, theta is the angle between the surface static pressure (surface normal) and the flow direction. (3). Once you have the local total surface drag force computed, you can tabulated it and sum it over the surface of the cylinder. (4). The tau value is the wall (surface) shear stress, which should be part of the solution, if you are solving the Navier-Stokes equations instead of the inviscid Euler equation. So, in my opinion, the velocity near the surface alone will not give you the wall shear stress directly. Since I don't use the code, and I am not familiar with the method behind, you will have to ask the vendor for the relationship between the near wall velocity and the wall shear stress. (5). You can apply the same method to the calculation of the lift coefficient, etc. (6). In principle, if you can not get the engineering support from the commercial code vendor, it is very important not to touch the code at all. This is because, there is no correct way to run the code and also validate the result without the support from the vendor. And even if there is guaranteed engineering support (in my case for example), the support is limited to the knowledge of the support engineer only. I have to re-iterate that, using someone's code without the author's full support is doom to fail. ( this is applicable to individuals and companies as well) (7). By just looking at a large company what codes they use, can easily tell you the health condition of the company. By the way, the ability to use a code, does not in any way related to the usefulness of the solution obtained from the code. The code is not equal to the solution. And the solution is not equal to the answer to the problem.
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Old   November 29, 1999, 13:05
Default Re: Asked for Help
  #3
Adrin Gharakhani
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>The flow is laminar and inviscid.

This is a contradiction in term!

Adrin Gharakhani
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Old   November 29, 1999, 22:37
Default Re: Asked for Help
  #4
John Chien
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(1).I think if the flow is laminar and inviscid, then, the person must be very frustrated. (2). Since the statement was mentioned after the code PHOENICS, there is a possibility that the laminar option and the inviscid option were selected. (this is a real possibility, because the code I am using will show wall function and two-layer model options at the same time.) (3). Many years ago, an engineer using a code from NASA, was able to show that the separated flow velocity vector field can be captured by solving an inviscid code for flow over a cylinder. (4). I think, if a person's background is ideal aerodynamics, it is really very hard to change his way of thinking overnight. (5). By using the fuzzy logic, it is likely that the solution obtained is in the inviscid region outside the laminar boundary layer. (6). Anyway, I think I need a new monitor, ideally a 17" monitor, it is getting hard to read the small fonts on my 8 years old 13" monitor. (7). There is another explanation, that is , the coarse mesh used produced nearly inviscid solution. This ia a possibility. When a coarse cell is used next to the wall, the computed velocity is never zero. (I had used Fluent over two years, and it was always a problem to plot the variable all the way to the wall. I mean the wall value changes depends on the options used. It's hard to plot the zero wall velocity.) (8). Well, I would strongly suggest that future questions must include the information about the Reynolds number, the Mach number of the problem, as well as the turbulence model used or selected. (By the way, don't use the word "inviscid", because most modern solutions of the inviscid equatione are numericallly viscous solutions. )
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