Boundary Conditions
Boundary conditions in PERFORM are enforced by an explicit ghost cell formulation. As noted in solver_params.inp, the requirements and interpretations of the *_inlet and *_outlet input parameters depend on the boundary condition specified by bound_cond_inlet / bound_cond_outlet. Valid options of pert_type for each boundary condition are also specified. If an invalid entry for pert_type is supplied, it will simply be ignored. For the mathematical definitions of these boundary conditions, please refer to the solver theory documentation.
In the following sections, we provide which additional input parameters are required and how they are interpreted for each valid entry of bound_cond_inlet/bound_cond_outlet. Additionally, the acceptable values to be artificially perturbed for each boundary condition are given under pert_type_inlet/pert_type_outlet.
Inlet BCs and Parameters
Fixed Stagnation Temperature and Pressure Inlet
This boundary condition is activated by setting bound_cond_inlet = "stagnation" in solver_params.inp. This boundary specifies the upstream stagnation temperature and stagnation pressure, i.e. the temperature and pressure of the fluid when its velocity is brought to zero. Tho boundary condition results in reflections of acoustic waves and should not be used with unsteady calculations with significant system acoustics.
The applicable boundary condition input parameters are as follows:
press_inlet: Specified stagnation pressure at the inlet.temp_inlet: Specified stagnation temperature at the inlet.mass_fracs_inlet: Fixed mixture composition at the inlet.
Full State Specification Inlet
This boundary condition is activated by setting bound_cond_inlet = "fullstate" in solver_params.inp. This boundary conditions overspecifies the boundary condition by fixing the inlet ghost cell primitive state. This is not a useful boundary condition for unsteady calculations (unless the flow is supersonic), but is mostly useful for testing how an outlet boundary condition responds to perturbations propagating downstream.
The applicable boundary condition input parameters are as follows:
press_inlet: Fixed static pressure at the inlet.vel_inlet: Fixed velocity at the inlet.temp_inlet: Fixed static temperature at the inlet.mass_fracs_inlet: Fixed mixture composition at the inletpert_type_inlet(optional): Accepts"pressure","velocity", or"temperature"to perturb the values of the appropriate fixed quantity.
Mean Flow Inlet
This boundary condition is activated by setting bound_cond_inlet = "meanflow" in solver_params.inp. This boundary condition provides a non-reflective inlet that requires some sense of a mean flow (or the flow infinitely far upstream) about which the unsteady flow is simply a perturbation. It effectively fixes the incoming characteristics while allowing the outgoing characteristics to be transmitted outside the domain without acoustic reflections.
The applicable boundary condition input parameters are as follows:
press_inlet: Specified mean upstream static pressure.temp_inlet: Specified mean upstream static temperature.mass_fracs_inlet: Fixed mixture composition at the inlet.vel_inlet: Specified mean upstream value of \(\rho c\), where \(c\) is the sound speed.rho_inlet: Specified mean upstream value of \(\rho c_p\), where \(c_p\) is the specific heat capacity at constant pressure.pert_type_inlet(optional): Accepts"pressure"to perturb the mean upstream pressure.
Outlet BCs and Parameters
Fixed Static Pressure Outlet
This boundary condition is activated by setting bound_cond_outlet = "subsonic" in solver_params.inp. This boundary condition fixes the static pressure at the outlet. As with the stagnation temperature and pressure inlet, this boundary condition produces acoustic reflections at the outlet.
The applicable boundary condition input parameters are as follows:
press_outlet: Specified static pressure at the outlet.mass_frac_outlet: Fixed mixture composition at the outlet.pert_type_outlet(optional): Accepts"pressure"to perturb the pressure at the outlet.
Mean Flow Outlet
This boundary condition is activated by setting bound_cond_outlet = "meanflow" in solver_params.inp. This boundary condition, as with the mean flow inlet boundary condition, fixes the incoming characteristic while transmitting the outgoing characteristics without reflections. Again, it requires some sense of a mean flow (or the flow infinitely far downstream) about which the unsteady flow is simply a perturbation.
The applicable boundary condition input parameters are as follows:
press_outlet: Specified mean downstream static pressure.vel_outlet: Specified mean downstream value of \(\rho c\), where \(c\) is the sound speed.rho_outlet: Specified mean downstream value of \(\rho c_p\), where \(c_p\) is the specific heat capacity at constant pressure.pert_type_outlet(optional): Accepts"pressure"to perturb the mean downstream pressure.
Boundary Perturbations
Setting valid values for pert_type_inlet or pert_type_outlet, as well as non-zero values of pert_perc_inlet / pert_freq_inlet or pert_perc_outlet / pert_freq_outlet, initiates external forcing at the appropriate boundary. The perturbation signal is a simple sinusoid, given for a given perturbed quantity \(\alpha\) in the boundary ghost cell as
where \(\overline{\alpha}\) is the relevant reference quantity given in solver_params.inp, \(f_i\) are the signal frequencies in pert_freq_inlet/pert_freq_outlet, and \(A\) is the amplitude percentage pert_perc_inlet/pert_perc_outlet.
For example, if the user sets (among other required parameters)
bound_cond_outlet = "meanflow"
press_outlet = 1.0e6
pert_type_outlet = "pressure"
pert_perc_outlet = 0.05
pert_freq = [2000.0, 5000.0]
this will result in two perturbation signals (one of 2 kHz, another of 5 kHz) of the mean downstream static pressure with amplitude of 50 kPa.