22 Replies Latest reply on Feb 26, 2015 11:48 AM by Jared Conway

# Boundary condition (Fully Developed Flow)

I see i can only set the boundary condition (Fully Developed Flow) when i define inlet or outlet velocity
can i set the boundary condition (Fully Developed Flow) with inlet or outlet pressure ?
my boundary conditions in my problem are
1-lower surface (real wall)
2-upper surface (real wall with motion)
3-inlet pressure

4-outlet pressure
i need to solve this problem with (Fully Developed Flow) assumption can i ?

• ###### Re: Boundary condition (Fully Developed Flow)

what is the physical boundary condition? fully developed flow just changes the velocity profile at the inlet

on a pressure opening it doesn't really make sense

• ###### Re: Boundary condition (Fully Developed Flow)

i know there is no relation between them, but in my case i already know that the flow is fully developed and i know the inlet and outlet pressure i don't know the flow velocity is there any solution ?

I didn'the mean to have a (fully developed flow)

Inside the pressure inlet or outlet boundary condition I just need it alone.

• ###### Re: Boundary condition (Fully Developed Flow)

you cannot artificially set the velocity profile on a pressure opening, it will be calculated as part of the solution

i'm not actually sure how you would account for that in a hand calculation

the only workaround I can think of is to add a length of geometry to allow the flow to develop but still doesn't really make much sense to me

• ###### Re: Boundary condition (Fully Developed Flow)

The inlet pressure i have isn't actually the start of flow it has been measured some where after the flow was started that's why I already know that the flow is fully developed.the fluid is already crossed too long distance and iam just simulating the behavior of the fluid some where after it has been fully developed

do I have to start from beginning of flow ?

• ###### Re: Boundary condition (Fully Developed Flow)

I think in this case, best to post some pictures and describe the conditions that you have measurements for, the physical application and what your goals are. answering point questions may not lead you to the appropriate way to apply flow simulation for your case.

• ###### Re: Boundary condition (Fully Developed Flow)

It's just a theoretical problem in fluid mechanics it's not a real case.
the problem is

I already solved this problem with my hands, but we also need to compare CFD results with our solution as a part of our assignment
i have added all the boundary layers and all of these assumptions to Solidworks except Fully developed flow.
Can i add it ?

i mean if you see the problem you find that i don't have inlet or outlet velocity conditions i just have inlet and outlet pressure conditions can i add fully developed flow assumption ?

• ###### Re: Boundary condition (Fully Developed Flow)

in the software there is no way to do this unless you're using an inlet or outlet flow rate or adding sufficient length on either end for the flow to develop

how are you taking this into account in your hand calculations?

• ###### Re: Boundary condition (Fully Developed Flow)

well
Laminar Flow assumption allow me to use Navir Stocks equation with three scaler components i don't need empirical relations now.
Incomprehensible flow assumption just for simplify
the Navir stocks equation is going to be And the continuity equation will be asubstituting with the other 3 assumptions we will end up with two equations the second equations just means there is no change of pressure across the flow
and the first equations we can get the velocity equation (u) after two integration and substituting with the boundary conditions
we will get this equations (based on the coordinates given in the question above of course ) substituting with the given data :
μ = 0.883 Pa.s
a= 10 mm

Δp/Δx = ((0.8-2.5)*10^5)/ 2

ρ= 1260 Kg/m^3

g=9.81 m/s^2

θ=45 deg
v=1.5 m/s

then we get the equation which describe the velocity profile

that's how i solve it

• ###### Re: Boundary condition (Fully Developed Flow)

its been along time since i cracked out formulas but it looks like to me you aren't accounting for uniform flow as much as you are just ignoring it all together

have you looked at the velocity profiles to see if it is uniform? (in your flow sim) or how much it differentiates

• ###### Re: Boundary condition (Fully Developed Flow)

there will be a reverse flow as a result of adverse pressure and adverse body force (gravity).

i draw the velocity profile with my hands and also with Solidworks it almost look same but the vales has error about 9.8% the point at which the fluid start to separate (v=0) from soldiwrks about 7.9 mm away from the fixed plate
and out from my solution also 7.9 mm

it's almost same

• ###### Re: Boundary condition (Fully Developed Flow)

are you making a statement or asking a question?

i'm not sure what you are trying to point out here

• ###### Re: Boundary condition (Fully Developed Flow)

It's just a statement.

Thanks for your attention Jared Conway .

• ###### Re: Boundary condition (Fully Developed Flow)

i'm still not clear, what is the final answer? i think you should be finding that the CFD is more accurate because it has less assumptions than your hand calcs.

• ###### Re: Boundary condition (Fully Developed Flow)

Well solidworks just can't add fully developed flow excpet when defining the velocity profile and the problem at real case can be solved with exact solution if we use Navir equations.

I don't know really why we can't add fully developed flow separate from velocity profile !!

• ###### Re: Boundary condition (Fully Developed Flow)

all you have to do is define the pressure inlet field that gives the prescribed velocity field and your issue is resolved. You will have the exact same challenge in any code you try.

• ###### Re: Boundary condition (Fully Developed Flow)

You mean that when I define the inlet pressure it always run as fully developed?! Sorry I didn't really get your massage.

• ###### Re: Boundary condition (Fully Developed Flow)

You need to figure out the pressure distribution that results in the velocity distribution you want. One cannot arbitrarily define some pressure and a velocity distribution it is inconsistent physically. Velocity and pressure are related through the Bernoulli equation- they affect each other.

• ###### Re: Boundary condition (Fully Developed Flow)

I know that the fully developed flow will results no change in the pressure across the flow you that's what you mean ? if yes then how to add this condition

• ###### Re: Boundary condition (Fully Developed Flow)

I have just give up I will tray solve it using Ansys and will see.

of course I will post the results out from Ansys here, but it will take time.

• ###### Re: Boundary condition (Fully Developed Flow)

how will you set it up in ansys any differently?

• ###### Re: Boundary condition (Fully Developed Flow)

I really don't know it will be my first time to use Ansys i just want to know if the solution will be the same as soldiwroks or even close.

• ###### Re: Boundary condition (Fully Developed Flow)

if you setup the problem the same, the answers will be the same