{"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":"<p>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 &#8211; 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 &#8211; 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 &#8211; 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":[],"acf":[],"custom_fields":[{"0":{"_wp_page_template":["default"],"_bbp_forum_id":["27791"],"_bbp_author_ip":["23.56.168.180"],"_bbp_last_active_time":["1-24-2023  20:20:08"],"_btv_view_count":["911"],"siebel_km_number":["2058931"],"product_version":["2019 R1"],"km_published_date":["2019-04-11T01:31:05.000Z"],"family":["Fluid Dynamics"],"application_name":["FLUENT"]},"test":"articlesansys-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/knowledge\/wp-json\/wp\/v2\/topics\/161772","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/knowledge\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/knowledge\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":0,"href":"https:\/\/innovationspace.ansys.com\/knowledge\/wp-json\/wp\/v2\/topics\/161772\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/knowledge\/wp-json\/wp\/v2\/media?parent=161772"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}