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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


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