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Old   June 12, 2007, 16:45
Default obtain non-uniform velocity profile
  #1
Atit Koonsrisuk
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I'd like to verify something then I set my problem in the following ways:

- Air ideal gas flows in the constant diameter tube positioned axially along Y-axis. 'g' is set in the downward direction of Y-axis. The flow inlet is at one end (bottom) of the tube and flow outlet is at another end (top). - There is the 500 w/m^3 volumetric heat source fully filled in the tube (domain). - B.C.'s: inlet: INLET with p_total=0, t_static=308K; outlet: OUTLET with p_static=0; walls: WALL adiabatic and free-slip. - CFX-Mesh is generated the mesh. The 'Face Spacing Type' is 'Angular Resolution'. No mesh control, no inflation. - Heat transfer model is 'Total Energy' and Turbulence model is 'None'.

I expected to get the uniform axial velocity at the plane normal to Y-axis, but it is not. At the inlet, for example, the velocity is varied from 4.19 to 4.31 m/s and from 4.18 to 4.63 at outlet.

Do anyone have idea why I didn't get the uniform velocity profile? Thank you for your kind consideration.

Atit
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Old   June 13, 2007, 06:27
Default Re: obtain non-uniform velocity profile
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Joe
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Your BCs look unphysical.
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Old   June 13, 2007, 06:47
Default Re: obtain non-uniform velocity profile
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Atit Koonsrisuk
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Actually I got this kind of BC's (Ptot =0 at inlet and Ps=0 at outlet) from the CFX staff. This direction comes from the case that the room, filled with uniform volumetric heat source, with the chimney at the center on the room's roof. One of CFX staff adviced me to use this BC's. The room is not in rectangular but in circular shape. I expected to have the uniform velocity and temperature at each X-section, but it is not, then I model the mentioned case to see how the result would be. If you think that it is strange, what sound BC's do you have? Thank you for your kind consideration.

Atit
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Old   June 13, 2007, 10:55
Default Re: obtain non-uniform velocity profile
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Gert-Jan
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- You don't need Total energy. thermal energy suffices.

- What are the absolute and relative pressrues on the in- and outlet?

- Is it really converged? What is you time step? Auto time scale?

- How does you initial guess look like. Did you specify the pressure as function of height? And (u,v,w) = (0,0.1,0) [m/s]?

Gert-Jan
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Old   June 14, 2007, 02:15
Default Re: obtain non-uniform velocity profile
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Atit Koonsrisuk
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Dear Mr.Gert-Jan, Thank you for your reply. My answer to your questions are as follows: 1. Previously I use 'Thermal Energy', but someone told me that 'Total Energy' might provide more correct answer when the speed of flow is high, then I change to 'Total Energy', but never check how much longer the computational time it take. Thank you, anyway. 2. I use 1 atm as the reference pressure for this. 3. I set the 'RMS residual target'= 1e-6 and auto time scale. At the end, all residual reach this value, and the 'Domain Imbalance %' are less than 1. 4. I use the automatic initial guess. Do you have any idea from these?
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Old   June 18, 2007, 11:45
Default Re: obtain non-uniform velocity profile
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Atit Koonsrisuk
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I can get the uniform profile for this case now. This change occurs by choosing the 2D extruded mesh from CFX-Mesh. I use the same grid setting, except that I apply 2D extruded mesh by extruding from the inlet surface to the outlet surface. Then I use this mesh with the same pre-processing setting. After the run, I get the uniform pressure, axial velocity and temperature along the surface normal to the tube axis.

This surprise me that the effect due to the grid. (inlet axial velocity: 4.199-4.31; inlet gage pressure: 9.84-10.66)

Anyway, I still can't get the uniform profile in my research work. I model the flow in the geometry which consists of two body: the circular collector with the vertical chimney connected at the middle of the collector top. There is the volumetric heat source in the collector. The collector perimeter is openned, then the heated air flows from the perimeter towards the vertical chimney. I specify the Open boundary condition with zero total pressure and 380K at the perimeter of the collector. At the chimney outlet, Outlet boundary condition with averaged zero static pressure is specified. The remaining surfaces is set to adiabatic free-slip wall. To simulate axis-symmetry, a 5 degree section of the plant is cut out from the entire periphery. Additionally, the symmetry boundary conditions are applied at the two sides of the sector. Then I use 2D extruded to rotate the mesh from one side to another side of the sector. However the mesh also provides the non-uniform flow profile, like when I use the default mesh in CFX-Mesh.

Do anybody have idea how to get the uniform profile?
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Old   July 9, 2007, 06:04
Default Re: obtain non-uniform velocity profile
  #7
Daiga
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Do you want to model convection due to non-uniform temperature or what? Why there is a free slip boundary condition?
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