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-   -   Problem with assigned inlet velocity profile as a boundary condition (http://www.cfd-online.com/Forums/fluent/73873-problem-assigned-inlet-velocity-profile-boundary-condition.html)

Ozgur_ March 18, 2010 14:55

Problem with assigned inlet velocity profile as a boundary condition
 
Hi all,

I have a post-processing problem with the velocity profile that I have assigned as an inlet boundary condition in Fluent.

I have a known velocity profile containing a boundary layer, and the computational domain I'm using has a rectangular inlet. I have created a .dat file containing the velocity info at certain points corresponding to the inlet. This is a 3D problem and I'm using a fine, structured mesh.

I read the .dat file as a velocity inlet boundary condition into my case file, then run the case. At the end of the case, the results I get seem almost correct. But there is a weird issue: When I plot the velocity vectors at the inlet, the cell center values look OK and show the original profile. But the node values don't! They generate a weird looking profile; it has a wiggly look and contains sudden jumps instead of a smooth look. Shouldn't the node values and cell center values give me the same velocity profile?

Another thing, when I use the same velocity profile and same static pressure at the inlet for different cases which use the same domain (they have the same inlet but there are slight modifications to the domain far downstream of the inlet), I calculate the mass average of total pressure and velocity magnitude at a small portion of the inlet and get different values! The difference is not small; there's about a 10% difference, isn't this strange?

Has anyone experienced such a problem before? If not, any suggestions on how to solve it?

This is urgent, any help is greatly appreciated. Thanks,

Ozgur

Ozgur_ March 20, 2010 15:51

I have solved the problem; and I hope I can be of help to anyone who encounters a similar problem in the future:

I solved the problem by doing two modifications to my original inlet profile.

- My first profile file had velocity values defined at the very boundaries of the inlet; say +5 and -5 meters. I changed the location of these two points, instead of the boundaries I defined the same velocity at +4.9 and -4.9 .

- The second thing I did was to curve fit the original velocity data I had from a previous experiment. I used to have around 18 velocity magnitude data; I curve fitted this data using Matlab (you can use any program which does curve-fitting), and obtained data points around 10 times more than the original value. This gave a much smoother-looking profile. At the end of the run, I compared the original profile I had with the velocity profile based on the cell-centered and the velocity profile based on the nodes. They were almost identical.

Another useful info might be to keep the same solution scheme throughout the run. I used to start with SIMPLEC for a while then switched to Coupled and continued most of iterations with Coupled; this time I only used SIMPLEC throughout the whole process and got even better residuals.

hhh August 3, 2012 05:54

velocity inlet in fluent
 
Dear friends,
In fluent, For analysis of 2D naca0012 airfoil velocity inlet we give vcostheta in x component & vsintheta in y component, v is inlet velocity.

1)here i start analysis of 3D wing, for that i take naca 0012 airfoil & extrude 100mm, in gambit and analysis in fluent i have doubt how to set velocity inlet, here i consider my velocity is 3m/s, then what is xyz component,please let me know.

2)if you consider 3D wing, (vtantheta for z component) is correct or wrong, please let me know.

2) how to find angle of attack for 3D wing? please let me know

naveen2790@yahoo.co.in August 23, 2015 07:15

Velocity inlet profile
 
Hi all,

I am trying to simulate the bend pressure drop in single phase. In Which I have user defined velocity function at Inlet and pr. based outlet. In check case I am getting warning to change the inlet boundary condition also my solution basically continuity equation is not being converged.
Can anybody suggest me what should be the right way.

LuckyTran August 24, 2015 12:03

Quote:

Originally Posted by naveen2790@yahoo.co.in (Post 560701)
Hi all,

I am trying to simulate the bend pressure drop in single phase. In Which I have user defined velocity function at Inlet and pr. based outlet. In check case I am getting warning to change the inlet boundary condition also my solution basically continuity equation is not being converged.
Can anybody suggest me what should be the right way.

Velocity inlet & pressure outlet boundary conditions are doable but you have to be careful not to overdefine it. Make sure you are specifying the mass-flow rate at only one boundary. For example, if you are specifying inlet velocity w/ a known density then the inlet mass-flow rate is also "implicitly" specified. Hence if specify the pressure outlet with targeted mass-flow rate you have over-defined the problem and need to be extremely careful that mass-balance is achievable. You can however, still use a velocity inlet and pressure outlet without the targeted mass flow rate option.

On the other hand you can use a velocity inlet with density computed from ideal gas law, etc. In this case you can use the pressure outlet w/ targeted mass flow rate since the inlet density will depend on temperature and pressure, with the downstream pressure affecting the upstream pressure. There are many examples but just make sure you have not imposed too many boundary conditions.

naveen2790@yahoo.co.in August 25, 2015 04:58

Hi Lucky Tran,

Thanks for your reply.
I have done some modifications. Which are as follows:

Inlet boundary condition: mass flow rate inlet
outlet boundary condition: pr. outlet
material property: ideal gas
solver: density based (i had also used pr. based still my solution was not being converged)

note: when I am using density based solver energy and continuity iterations going simultaneously (as per my knowledge if i am using constant temp. then there should not be any effect on energy)

With all these condition still I am not able to converge continuity equation.

--Naveen


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