{"id":161772,"date":"2023-01-25T07:16:54","date_gmt":"2023-01-25T07:16:54","guid":{"rendered":"\/knowledge\/forums\/topic\/i-am-running-a-closed-system-multiphase-buoyancy-problem-in-fluent-where-the-density-of-the-liquid-phase-is-described-by-the-boussinesq-model-is-it-possible-to-define-a-thermal-expansion-coefficient\/"},"modified":"2023-07-31T12:35:52","modified_gmt":"2023-07-31T12:35:52","slug":"i-am-running-a-closed-system-multiphase-buoyancy-problem-in-fluent-where-the-density-of-the-liquid-phase-is-described-by-the-boussinesq-model-is-it-possible-to-define-a-thermal-expansion-coefficient","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/knowledge\/forums\/topic\/i-am-running-a-closed-system-multiphase-buoyancy-problem-in-fluent-where-the-density-of-the-liquid-phase-is-described-by-the-boussinesq-model-is-it-possible-to-define-a-thermal-expansion-coefficient\/","title":{"rendered":"I am running a closed system multiphase buoyancy problem in Fluent where the density of the liquid phase is described by the Boussinesq model. Is it possible to define a thermal expansion coefficient that varies with temperature? This property seems to only allow constant values. This is the case even when a UDF definition is available"},"content":{"rendered":"

The Boussinesq model is the only variable density model that treats density as a constant in the equations. (The second term in the equation below is implemented as a momentum source term.) rho(T) = rho(To) + [d(rho)\/d(T)]@To * (T – To) The assumption is that rho(T) can be expressed as above at a given reference temperature, To (entered in operating conditions). The corresponding density is rho(To) (entered in materials) and the corresponding thermal expansion coefficient is beta(To). The derivative in the second term, [d(rho)\/d(T)]@To is related to the thermal expansion coefficient by -1*beta(To)*rho(To). The Boussinesq approximation for density is is part of a Taylor series expansion, and would theoretically stop making sense if the derivative were taken at arbitrary T instead of prescribed To. To is generally set at the midpoint of the temperature range for the problem. Unless the temperature range is very large, using a single-valued beta at the evaluated at the midpoint is normally quite accurate. Since the second term, -1*beta(To)*rho(To)* (T – To) is treated as a momentum source term in the Boussinesq model, it would be possible to isolate this and implement your own using a DEFINE_SOURCE UDF. To to this: 1. Set the material density to constant rho(To) 2. Deactivate gravity so that the effect of the source term on the fluid phase is not duplicated 3. Define a source term equal to -g*beta(T)*rho(To)* (T – To) using DEFINE_SOURCE. NOTE: If the other phase has a slip velocity defined, this will have to be modified with DEFINE_VECTOR_EXCHANGE_PROPERTY UDF to account for the missing effect of gravity.<\/p>\n","protected":false},"template":"","class_list":["post-161772","topic","type-topic","status-publish","hentry","topic-tag-2019-r1","topic-tag-bcs-interfaces","topic-tag-fluent","topic-tag-fluid-dynamics","topic-tag-heat-transfer-and-radiation","topic-tag-heat-transfer","topic-tag-materials"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t