For high-pressure thermal plasma sustained by direct current, the temperatures of electrons and gases may differ from each other. A 2-temperature approach better accounts for the overall phenomena than the 1-temperature model, in which the thermal equilibrium of electron and heavy species are assumed.The 2-temperature plasma model in CFD-ACE+ includes the electric conduction solution and the sheath model to account for voltage drop. A validation study of the model for a 2-D axi-symmetric (but in 3-D set-up) free burning arc in atmospheric Argon is presented.
In certain applications, different regions of the computational domain experience flow conditions that are so different that it is very difficult for a single solver to produce accurate results at the extremes. In many situations, such problems can be separated and solved using loosely coupled solvers. Each solver is chosen to provide highly accurate solutions for the prevailing flow conditions.
ESI's CFD-FASTRAN, a compressible flow solver, is ideally suited for high speed external aerodynamics problems and the multi-physics solver CFD-ACE+ is ideally suited for heat transfer problems involving conduction, convection (natural and forced) and radiation.
A large amount of aerodynamic heating is generated over hypersonic vehicles during re-entry. Thermal Protection System (TPS) materials are employed to prevent the heating from conducting into the internal cabin, which holds electronic devices, passengers and other vital components. As a time-dependent process, the material making up TPS is at a low temperature and “soaks up” the heat – the conductivity of the material transports the heat (from the vehicle surface) through the thickness. The material will also re-radiate some of the heat back to the flow – the amount depending on the emissivity of the material. A primary concern is to estimate the effects of aeroheating on the internal volume of the capsule and its effect on electronic devices, passengers and cooling systems. In this application, typically the external flow is hypersonic in nature, whereas the flow within the capsule is a very low speed flow dominated by natural convection. In addition, to hypersonic aerodynamic heating, several other physics including heat conduction, natural convection and radiation has to be accurately modeled. CFD-FASTRAN solves for the external hypersonic flow and CFD-ACE+ solves for heat conduction, convection and radiation. Exchange of heat flux/temperature data between FASTRAN and ACE occurs at defined interfaces.
Low Mach preconditioning is a way to accelerate convergence towards steady state solution by scaling the disparate eigenvalues of a system to the same order of magnitude for time-marching schemes. The preconditioning matrix applied in FASTRAN is chosen in such a way to provide an efficient solution for both incompressible (through artificial compressibility) and low Mach flows (through pseudo-acoustic speeds). This feature in FASTRAN is demonstrated using steady flow simulation over a 2D cylinder at a freestream Mach number of 0.0004.
User subroutines are built as Dynamic Link Libraries (DLL files for Windows) or shared objects (.so files for UNIX platforms). The shared libraries are then linked with CFD-ACE-SOLVER such that a two way communication between the solver and the user defined input is established.
The steady-state forward and reverse flow characteristics of a Tesla-type valve are investigated using CFD-ACE+. A Tesla-type valve is one of the no-moving-parts (NMP) type valves used in micropumps for microelectromechanical system (MEMS) devices.
Model dependencies can be used to simulate complex rigid body motions. In this example, a pitching airfoil with flap is used to demonstrate this feature.
This example shows flow results obtained for a fan spinning at 3485 RPM. The following approach was taken to simulate the fan. A ugrid user subroutine is used to rotate the fan and shroud grid as a solid body. The parametric inputs, time step size and the user subroutine combined to rotate the fan by 2 degrees every timestep. The blades of the fan were given a "grid velocity" BC and the shroud was set with a "user specified" zero velocity to simulate a stationary shroud.
This example demonstrates the application of a user specified value of absorptivity for a simulation involving the solution to a
Discrete Ordinate Method (DOM) radiation model. The UABSORPTIVITY subroutine sets a temperature dependent, species dependent value
of absorptivity for the VC. User subroutines are
built as Dynamic Link Libraries (DLL files for Windows) or shared
objects (.so files for UNIX platforms). The shared libraries are then
linked with CFD-ACE-SOLVER such that a two way communication between
the solver and the user defined input is established.
The following user defined output example shows a method of writing out
variables extracted from multiple volume conditions of the same name. The UOUT user
subroutine saves the hassle of naming each one of the volume conditions
uniquely, thus eliminating the need for having as many IF constructs in the user
subroutine as there are VC’s of interest. User subroutines are
built as Dynamic Link Libraries (DLL files for Windows) or shared
objects (.so files for UNIX platforms). The shared libraries are then
linked with CFD-ACE-SOLVER such that a two way communication between
the solver and the user defined input is established.
Custom variables could be requested as output from the solver through the user subroutine UOUT. In this example, the temperature gradient (dT/dy component) along a
line is written to a text file at the end of the simulation. This subroutine can be called under 6 different conditions based on the flag set (at the beginning, at the beginning of a run, at the beginning of a time step, at the end of each iteration, at the end of each time step and at the end of the run). User subroutines are built as Dynamic Link Libraries (DLL files for Windows) or
shared objects (.so files for UNIX platforms). The shared libraries are then
linked with CFD-ACE-SOLVER such that a two way communication between the
solver and the user defined input is established.
