Physics Modules

ExaGOOP’s core solver handles momentum and mass conservation for isothermal, single-phase problems. Additional physics are compiled in through optional modules, each controlled by a build-time flag. This page gives an overview of the three available modules and guidance on when to use them. Full details — including input file parameters, boundary condition syntax, and worked examples — are in the individual module pages linked below.

Table 1 Module summary

Module	CMake flag	GNUmake flag	Purpose
Temperature Module (USE_TEMP)	EXAGOOP_USE_TEMP=ON	USE_TEMP=TRUE	Coupled heat transfer and thermal BCs
Embedded Boundaries via Level Sets (USE_EB)	(always ON for DIM ≥ 2)	USE_EB=TRUE	Solid obstacles via signed-distance level sets
UDF Moving Wall Boundary Conditions	(runtime, no extra flag)	(runtime, no extra flag)	Spatially and temporally varying velocity BCs

Temperature Module

The temperature module adds a Lagrangian energy equation solved alongside the momentum equation. When enabled, each material point carries thermal state (temperature, specific heat, thermal conductivity, heat flux, and internal heat source), and the background grid carries five additional nodal fields that are deposited and interpolated every time step.

When to use it. Enable this module for any problem where thermal effects are significant: conjugate heat transfer, thermally driven flows, or constitutive models whose parameters depend on temperature.

Build flag. EXAGOOP_USE_TEMP=ON (CMake) or USE_TEMP=TRUE (GNUmake). The module is compiled out by default; simulations that do not need it incur no runtime overhead.

Full temperature module documentation

Embedded Boundaries / Level Sets

The embedded boundary (EB) module allows solid obstacles to be placed inside the computational domain using signed-distance level sets built on the AMReX EB2 library. Particles that fall inside a body are removed at initialisation; during time advancement the level-set field enforces momentum and thermal boundary conditions on grid nodes near each body surface, and reflects or absorbs particles that attempt to cross it. Multiple bodies can be active simultaneously.

When to use it. Enable for any problem with a non-trivial internal geometry — cylinders, spheres, arbitrary bodies — that cannot be handled by simple domain-face boundary conditions alone.

Build flag. USE_EB=TRUE (GNUmake); EB is always compiled in by the CMake build for DIM ≥ 2. EB is not available in 1-D builds.

Full embedded boundary documentation

UDF Moving Wall Boundary Conditions

The UDF (user-defined function) interface allows arbitrary velocity boundary conditions to be specified at any domain face at runtime, without recompiling the solver. The user compiles a small shared library exporting a single C-linkage function; ExaGOOP loads it at start-up and calls it every time step to obtain the velocity at each boundary node.

When to use it. Use this for wall motions that cannot be expressed as a simple constant: rotating walls, oscillating pistons, prescribed shear profiles, or any spatially and temporally varying velocity field.

Build flag. No additional compile-time flag is required. The UDF is activated at runtime through two extra parameters in the input file pointing to the shared library path and function name.

Full UDF moving wall documentation

Combining Modules

The modules are independent and can be combined freely. For example, a simulation of heat transfer around a moving cylinder would require both USE_TEMP (for the energy equation) and USE_EB (for the cylinder geometry). The tutorials section includes examples that exercise these combinations.