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Natural convection in a space-dependent gravity field

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Old   April 14, 2022, 05:14
Post Natural convection in a space-dependent gravity field
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Hello,
I am modelling a natural convection problem in a cylindrical cavity heated from below. The primary objective is to study the influence of a variable gravitational field on the flow patterns and temperature distribution (Fig.1). A steady State (Pseudo transient) K-omega SST model is used.

geometry.PNG


First simulations were ran considering a uniform gravity field by defining the gravity components in the “Gravity” box in “General” (-9.81 m/s² in x component); satisfying convergence and physical results were obtained. The next step is to assign a variable gravitational field (g*=f(x,y)) for gravity, to that end, I defined gravity components as profiles and assigned them to axial and radial momentum source terms. These accelerations are actually are calculated with a Matlab code and would be complicated to implement on Fluent (Complex Integrals).
In order to check how these source terms are implemented in the equations, I ran a first simulation, using a constant x-component gravity profile (9.81 m/s² in all cells), defined a Named Expression (Density*(‘gravity_profile’) and assigned it to the x-momentum source term. The aim is to compare it to the case where gravity is assigned through the “Gravity” box. Unfortunately, the solutions were different. After investigation, I found out that the problem comes from the buoyancy effects on turbulence production. When gravity is enabled through the gravity box, the option “Buoyancy effects” appears in the k-omega SST box and disappears if not. The figures below illustrate the difference between the solutions:


results.jpg


Noticing that the buoyancy effects are probably behind this difference, I tried calculating the source terms due to buoyancy in the kinetic energy and dissipation rate equations as suggested in the Fluent Theory Guide (Gb and Gbw) and assigned them to a case where gravity is enabled and the buoyancy effects are switched off. Unfortunately, I do not get the same result as the case where the buoyancy effects are enabled.
My questions are:
-Is it possible to assign the gravity field as a profile in the “Gravity” box?
-If not, and if I have to define gravity as a source term, in that case how can I consider buoyancy effects on turbulence to include it in the k-W equations?

Thank you,
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gravity, natural convection, profiles, source terms, turbulence generation

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