[shivasluzz]

Very late I observed that for every cfd simulation both the inlet and outlet condition should be known!

But in most cases outlet conditions are not known!

I understood that there are 2 ways to address this problem:
1. Model till we know the outlet condition. Generally it will be till where it opened to atmospheric conditions. But often this will be very complicated because of geometry, mesh and computational effort

2. Assume a list of output conditions and develop the parameter of interest. For example, getting result of mass flow rate for different back pressure values.

I don't know how to use this 2nd method to find the actual outlet condition! Can anybody please explain it?

[FMDenaro]

Originally Posted by shivasluzz

Very late I observed that for every cfd simulation both the inlet and outlet condition should be known!

But in most cases outlet conditions are not known!

I understood that there are 2 ways to address this problem:
1. Model till we know the outlet condition. Generally it will be till where it opened to atmospheric conditions. But often this will be very complicated because of geometry, mesh and computational effort

2. Assume a list of output conditions and develop the parameter of interest. For example, getting result of mass flow rate for different back pressure values.

I don't know how to use this 2nd method to find the actual outlet condition! Can anybody please explain it?

the type of boundary conditions depends on the mathematical character of the model we are solving, then physical approximations dictate how to set them according to the type of the problem.

For example, in supersonic flows (hyperbolic equation) you do not have to specify nothing at an outlet....on the other hand for incompressible flows the pressure equation is elliptic and BC.s must be specified also on an outlet

[Jonas Holdeman]

[QUOTE=shivasluzz;566618]Very late I observed that for every cfd simulation both the inlet and outlet condition should be known!
QUOTE]

For incompressible flow, no output b.c. are needed if divergence-free (in the sense that integral div^2 u=0) finite elements are used. This might be referred to as open boundary conditions. See the paper:

"A Hermite Finite Element Method for Incompressible Flow", IJNMF 64:376-408(2010).

In fact, no input BC are needed either, other than specifying total flow, but the solution (fully-developed) is not very interesting.

But then it could be argued that "no boundary condition" is a boundary condition.

[shivasluzz]

Thanks for the replies...
Suppose If I am analysing a centrifugal pump, then is it possible to get the outlet flow and pressure just by specifying the speed of rotation?
I observed that in such analysis we need to specify the outlet flow or pressure!

Suppose If I m using a pressure value at the outlet (involute outlet) then it's just an assumption rite and all other results are also not going to be the actual value!

In mean in actual pumping system, there will be pipes and restrictions in the downstream stream of pump and those downstream components may create different backpressure in the actual case rite?

[FMDenaro]

Originally Posted by shivasluzz

Thanks for the replies...
Suppose If I am analysing a centrifugal pump, then is it possible to get the outlet flow and pressure just by specifying the speed of rotation?
I observed that in such analysis we need to specify the outlet flow or pressure!

Suppose If I m using a pressure value at the outlet (involute outlet) then it's just an assumption rite and all other results are also not going to be the actual value!

In mean in actual pumping system, there will be pipes and restrictions in the downstream stream of pump and those downstream components may create different backpressure in the actual case rite?

the speed of rotation is not sufficient to prescribe all the correct boundary conditions to have the mathematical problem well posed

[Jonas Holdeman]

Originally Posted by shivasluzz

Thanks for the replies...
Suppose If I am analysing a centrifugal pump, then is it possible to get the outlet flow and pressure just by specifying the speed of rotation?
I observed that in such analysis we need to specify the outlet flow or pressure!

Suppose If I m using a pressure value at the outlet (involute outlet) then it's just an assumption rite and all other results are also not going to be the actual value!

In mean in actual pumping system, there will be pipes and restrictions in the downstream stream of pump and those downstream components may create different backpressure in the actual case rite?

In truth, I have not done anything that complicated, and was thinking of simpler geometry. I have modeled incompressible flow in a cylinder with moving piston (with open BC) and peristaltic flow (with periodic BC) in 2D as examples of moving boundary problems. Certainly the necessary input data depends on the method you are using and whether the fluid is compressible or incompressible, as well as the pressure & flow response of the system it is attached to. For incompressible flow you would have to model the entire system or, more practically, replace it with an equivalent system, or consider intake from a reservoir and discharge into a sink.

I think the point is that you are trying to model the external environment with a few operating parameters including inlet and outlet pressures which are indirectly related to the flow rate and viscous losses. I misunderstood your problem, which is distinct from the mathematical requirements to specify a solution to the problem.