Is there any (serious) reason why I shouldn't use the mass flow inlet condition in incompressible flows in Fluent. 90% of the time in industrial application, only a flow rate is known, so why bother to calculate an associated velocity for an inlet. Everyone says that mass flow inlet condition is to be used with compressible flow, but nobody could explain to me why I couldn't use it for incompressible flows.

Denis T. tschump@hotmail.com

It is explained in the FLUENT manual: "It is not necessary to use mass flow inlets in incompressible flows because when density is constant, velocity boundary conditions will fix the mass flow."

(1). Well, even if you have a fluid flow through a duct or a pipe, one can not say that they will behave in the same way. (2). The fluid in the pipe can be either liquid (incompressible in normal state), or gas (compressible in normal state). (3). It is true that in both cases, one can always calculate the mass flow rate. But the physics which governs the flow is not the same. I mean, the governing equation for the incompressible flow is different from that for the compressible flow. This part is covered in the text books of gasdynamics. (4). For the incompressible flow, the variables in the governing equation are u,v,w. This is clearly written in the continuity equation. Therefore, the boundary conditions specified are on these variables, in the code. For example, u=U_inlet, v=0, w=0, at inlet. Since the density is known and constant , one can easily compute the mass flow rate as m=rho*U_inlet*area_inlet and vice versa. (5). Once the u is specified at the inlet, the problem is set. Nothing else can be changed. (6). On the other hand, the nature of the compressible flow in a pipe is quite different. The inlet condition is related to its total pressure state, where the velocity is set equal to zero. The exit condition is the static pressure condition, which is used to control the mass flow rate through the pipe. (7). In the solution procedure, one can not specify the inlet density, velocity and the pressure in general, because these flow variables will be changed in the course of the solution procedure, based on the exit static pressure. (8). This process is well established in the 1-D gasdynamic theory. (9). So, if you specify the mass flow rate, the code will adjust the exit static pressure from iteration to iteration, until the right condition is arrived which is equal to the input mass flow rate. In other words, specifying the inlet velocity is equal to specifying the mass flow rate for incompressible flow. But since the code is written to accept only the velocity as input, you have to use velocity input for incompressible case. (10). On the other hand, for compressible flows,you can specify the mass flow rate and the inlet total pressure, and let the code to adjust the exit static pressure for you, until the specified mass flow rate is achieved. The other way to do is to specify the inlet total pressure and the exit static pressure. Once the solution is obtained, you can compute the mass flow rate. (11). You see, a seemingly very simple click of options requires a real understanding of 1-D gasdynamic flows. (12). Don't touch a commercial cfd code, unless you are fully trained in fluid dynamics, gasdynamics, etc...etc...

>John C. Chien wrote:
:
:You see, a seemingly very simple click of options
:requires a real understanding of 1-D gasdynamic
:flows. (12). Don't touch a commercial cfd code,
:unless you are fully trained in fluid dynamics,
:gasdynamics, etc...etc...

And you see that seemingly simple question produces rambling answer that does not even answer the question...

I am an industrial user of Fluent and I do exactly the same thing you do. Most of the time I specify mass flow inlet condition just because I don't bother to calculate the velocity.

Fluent manual says that it is _not_necessary_ to use mass flow inlet, but it does not say that you shouldn't use it. So that's what I do until someone convinces me otherwise.

Best Regards,

Jouni

(1). I think, it is perfectly all right, if you are sure that the results are correct. (2). It is a good idea to let the readers know: (a). the particular code used, (b). the particular version used. (c). the particular inlet condition used, and (d). the particular outlet condition used. (3). So, it is possible that you have just found the answer, Eureka!

You made the point. It is precisely users like you that we had in mind when we allow mass flow inlet BCs to be used for both incompresible and compressible flows.

So, the answer to the original question by Denis is that yes you can use mass flow inlet BC for incompressible flows as well as compressible flows. What the quoted paragraph in the manual is intended just to remind you that mass flow inlet is equivalent to velocity inlet. You can use whichever is more convenient for you.

I meant to reply to

Since in compressible flow, flow properties (i.e pressure, temperature , volume )changes from one point to another point which result change in mass flow. This is not the case in incompressible flow (i.e flow properties remains the same over all points).