How To Create Thread In Domain_6

1.10.1 Multiphase-specific Data Types

In addition to the ANSYS FLUENT-specific data types presented in Section 1.8, there are special thread and domain data structures that are specific to multiphase UDFs. These data types are used to store properties and variables for the mixture of all of the phases, as well as for each individual phase when a multiphase model (i.e., Mixture, VOF, Eulerian) is used.

In a multiphase application, the top-level domain is referred to as the `superdomain' . Each phase occupies a domain referred to as a `subdomain' . A third domain type, the `interaction' domain , is introduced to allow for the definition of phase interaction mechanisms. When mixture properties and variables are needed (a sum over phases), the superdomain is used for those quantities, while the subdomain carries the information for individual phases. In single-phase, the concept of a mixture is used to represent the sum over all the species (components), while in multiphase it represents the sum over all the phases. This distinction is important, because ANSYS FLUENT has the capability of handling multiphase multi-components, where, for example, a phase can consist of a mixture of species.

Since solver information is stored in thread data structures, threads must be associated with the superdomain as well as with each of the subdomains. In other words, for each cell or face thread defined in the superdomain, there is a corresponding cell or face thread defined for each subdomain. Some of the information defined in one thread of the superdomain is shared with the corresponding threads of each of the subdomains. Threads associated with the superdomain are referred to as `superthreads' , while threads associated with the subdomain are referred to as phase-level threads, or `subthreads' . The domain and thread hierarchy are summarized in Figure 1.10.1.

**Figure 1.10.1:** Domain and Thread Structure Hierarchy

Figure 1.10.1 introduces the concept of the domain_id and phase_domain_index . The domain_id can be used in UDFs to distinguish the superdomain from the primary and secondary phase-level domains. The superdomain (mixture domain) domain_id is always assigned the value of . Interaction domains are also identified with the domain_id. The domain_ids are not necessarily ordered sequentially as shown in Figure 1.10.1.

The phase_domain_index can be used in UDFs to distinguish between the primary and secondary phase-level threads. phase_domain_index is always assigned the value of for the primary phase-level thread.

The data structures that are passed to a UDF depend on the multiphase model that is enabled, the property or term that is being modified, the DEFINE macro that is used, and the domain that is to be affected (mixture or phase). To better understand this, consider the differences between the Mixture and Eulerian multiphase models. In the Mixture model, a single momentum equation is solved for a mixture whose properties are determined from the sum of its phases. In the Eulerian model, a momentum equation is solved for each phase. ANSYS FLUENT allows you to directly specify a momentum source for the mixture of phases (using DEFINE_SOURCE) when the mixture model is used, but not for the Eulerian model. For the latter case, you can specify momentum sources for the individual phases. Hence, the multiphase model, as well as the term being modified by the UDF, determines which domain or thread is required.

UDFs that are hooked to the mixture of phases are passed superdomain (or mixture-level) structures, while functions that are hooked to a particular phase are passed subdomain (or phase-level) structures. DEFINE_ADJUST and DEFINE_INIT UDFs are hardwired to the mixture-level domain. Other types of UDFs are hooked to different phase domains. For your convenience, Appendix B contains a list of multiphase models in ANSYS FLUENT and the phase on which UDFs are specified for the given variables. From this information, you can infer which domain structure is passed from the solver to the UDF.