# Problem of boundary condition setting for natural convection in a water tank

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June 25, 2020, 03:26
Problem of boundary condition setting for natural convection in a water tank
#1
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Henry
Join Date: Oct 2019
Posts: 16
Rep Power: 5
I am using ANSYS Fluent to simulate the experiment for natural convection of a single heated cylinder in a large water tank made of Plexiglas (approximated as thermal-insulating material), which is 1200 mm wide with a water depth of 1600 mm. Computational domain is a rectangular with the same width and depth as the experiment. The thrid dimension is neglected because it is assumed that spanwise flow field is uniform, which is reasonable. I use steady, pressure-based solver and Boussinesq approximation for buoyancy. Turbulence is modelled by transition SST.

The major problem I have is how to set the boundary condition correctly on the four sides of the rectangular domain. I have tried two options.

Option 1: No-slip, adiabatic wall on left, right & bottom boundary. Zero-shear, isothermal (Tw=293 K) wall on top boundary, to simulate the free water surface. This boundary condition setting is most representative of real scenario in experiment. However, the result is non-sensible. Contour of velocity magnitude suggests buoyant plume is completely disrupted, as attached in "Option 1".

Option 2: No-slip, adiabatic wall on left & right boundary. Pressure inlet at bottom boudanry. Pressure outlet at top boundary. This setting, although not representative of experimental scenario, produces sensible result. Buoyant plume show reasonable profile and velocity magnitude, as attached in "Option 2".

Could anyone tell me, why Option 1 generates the non-sensible result, although it is exactly the same as experiment? Why Option 2 seems so good, although the boundary condition does not reflect the real scenario?...
Attached Images
 Option1.jpg (105.0 KB, 6 views) Option2.jpg (105.2 KB, 3 views)

 June 25, 2020, 03:38 Conditions #2 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 34 It's not the boundary conditions, but the operating conditions that appear to be set incorrect. You need to check three settings 1. Gravity value and direction 2. Operating density should be same as Boussinesq density used in material panel 3. Boussinesq temperature; should be corresponding to the operating density Second option, thought giving right results, is incorrect. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

June 25, 2020, 04:04
#3
New Member

Henry
Join Date: Oct 2019
Posts: 16
Rep Power: 5
Quote:
 Originally Posted by vinerm It's not the boundary conditions, but the operating conditions that appear to be set incorrect. You need to check three settings 1. Gravity value and direction 2. Operating density should be same as Boussinesq density used in material panel 3. Boussinesq temperature; should be corresponding to the operating density Second option, thought giving right results, is incorrect.
Thanks, Vinerm. But I don't think they could solve the problem.

I have checked the three settings and have not found anything dubious.

1. Gravity acceleration is -9.8 m/s2 on Y axis, i.e. vertically downward.
2. Operating density is not specified because it is not necessary when the Boussinesq model is used.
3. Boussinesq temperature is the ambient temperature, under which the Boussinesq density in material panel is evaluated.

Why the second option is incorrect?