# Why FVM for high-Re flows?

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 May 14, 1999, 05:49 Re: What is this flow that you mentioned? #21 andy Guest   Posts: n/a Sponsored Links I fail to see how you can attach the meaning you seem to have done to those two sentences. To repeat myself, at the wall the fluid is not moving (stationary) and is therefore not performing work against the wall shear stress. However, in the boundary layer the fluid is moving and is performing work against (some) stress components. I would like to bring this discussion to a close if we may.

 May 14, 1999, 07:55 I feel a bit confused #22 Marco Guest   Posts: n/a To say truth, a little more than a bit .... I try to clarify myself thinking at the real world. For instance I could think at a wind tunnel (with a closed loop of flow, but I think that this can work with an open loop too, with a few arrangements) with a model in it. The model is fixed to the heart (as like as the tunnel) and, if we suppose the heart will not move for the action of the tunnel, it can't perform any work. On the other side, the fluid will be activated by some blades, which are tranferring some energy to the fluid. So blades perform some work on the fluid (otherwise, why we need power to make them rotating?). The motor which activates blades applies generalized forces to the shaft and to the structure of the tunnel so that it is equilibrated. Now I guess we can say that the structure of the tunnel will not apply any work to the heart, which will not move because of the action of the tunnel. So we can think at the tunnel itself as a good fixed reference frame for the study. In this reference, the only thing which performs any work are the blades which activate the fluid. This energy is transferred to the fluid, which, in turn, has to dissipate it anyhow, as there isn't any other moving surface to work on. This energy must be dissipateed because when the fluid comes back to the blades it has lost the Delta(E) received from them (Stationary condition). Now let's go and simulate a piece (only the model >>or<< only the blades) of this tunnel. I stand in front of two different situations from the energetical point of view: 1) I have some non working surfaces (e.g. the model) 2) I have some working surfaces (blades) Is there any difference in simulating these two cases? If no, I can't understand the debate between Andy and Patrick Godon. If yes, please tell me which ones, if you can. Thanks, Marco

 May 14, 1999, 09:32 Re: I feel a bit confused #23 John C. Chien Guest   Posts: n/a (1). When you apply a force ( F ) to an object, you also receive a reaction force ( -F ) from the object. That is Newton's law of action and reaction. ( my understanding of the law ) (2). The work done (per uint time) can be defined as ( F x V ) where V is the speed of the object. (3). So, if you are applying force on a chair ( sitting on a chair ) and the chair is not moving ( V=0 ), then ( F x V )=0. In other word, when you are sitting on a chair for the whole day and the chair is not moving, you are not producing any work, that is , you are relaxing! You are not working. (4). If you decide to push a building from sunrise to sundown, then you will be exhausted at the end of the day.(energy dissipated) The work done (in one day)on the building is ( F x V ), which will be zero. And the whole day's work will be nothing. (5). In the above definition, I have used V (speed) instead of S (the distance moved ), because it is easier to understand. (when something moves, it must have a speed associated with it.) (6). I have discovered recently that the CFD code I am using has at least one big error in it, because two persons modified the code at two different occasions had two different definitions of a parameter, which are not consistent with the original definition. ( unless a code is documented line-by-line, definition-by-definition, subroutine-by-subroutine, it is not safe to touch the code. )

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