# Hydrodynamics Parameters¶

## General¶

`UseHydro`(external)- This flag (1 - on, 0 - off) controls whether a hydro solver is used. Default: 1
`HydroMethod`(external)This integer specifies the hydrodynamics method that will be used. Currently implemented are

Hydro method Description 0 PPM DE (a direct-Eulerian version of PPM) 1 [reserved] 2 ZEUS (a Cartesian, 3D version of Stone & Norman). Note that if ZEUS is selected, it automatically turns off `ConservativeInterpolation`and the`DualEnergyFormalism`flags.3 Runge Kutta second-order based MUSCL solvers. 4 Same as 3 but including Dedner MHD (Wang & Abel 2008). For 3 and 4 there are the additional parameters `RiemannSolver`and`ReconstructionMethod`you want to set.5 No Hydro (Testing only) 6 MHD with Constrained Transport. Default: 0

More details on each of the above methods can be found at

*Hydro and MHD Methods*.`FluxCorrection`(external)- This flag indicates if the flux fix-up step should be carried out around the boundaries of the sub-grid to preserve conservation (1 - on, 0 - off). Strictly speaking this should always be used, but we have found it to lead to a less accurate solution for cosmological simulations because of the relatively sharp density gradients involved. However, it does appear to be important when radiative cooling is turned on and very dense structures are created. It does work with the ZEUS hydro method, but since velocity is face-centered, momentum flux is not corrected. Species quantities are not flux corrected directly but are modified to keep the fraction constant based on the density change. Default: 1
`InterpolationMethod`(external)There should be a whole section devoted to the interpolation method, which is used to generate new sub-grids and to fill in the boundary zones of old sub-grids, but a brief summary must suffice. The possible values of this integer flag are shown in the table below. The names specify (in at least a rough sense) the order of the leading error term for a spatial Taylor expansion, as well as a letter for possible variants within that order. The basic problem is that you would like your interpolation method to be: multi-dimensional, accurate, monotonic and conservative. There doesn’t appear to be much literature on this, so I’ve had to experiment. The first one (ThirdOrderA) is time-consuming and probably not all that accurate. The second one (SecondOrderA) is the workhorse: it’s only problem is that it is not always symmetric. The next one (SecondOrderB) is a failed experiment, and SecondOrderC is not conservative. FirstOrderA is everything except for accurate. If HydroMethod = 2 (ZEUS), this flag is ignored, and the code automatically uses SecondOrderC for velocities and FirstOrderA for cell-centered quantities. Default: 1

0 - ThirdOrderA 3 - SecondOrderC 1 - SecondOrderA 4 - FirstOrderA 2 - SecondOrderB

`ConservativeInterpolation`(external)- This flag (1 - on, 0 - off) indicates if the interpolation should be done in the conserved quantities (e.g. momentum rather than velocity). Ideally, this should be done, but it can cause problems when strong density gradients occur. This must(!) be set off for ZEUS hydro (the code does it automatically). Default: 1
`RiemannSolver`(external)This integer specifies the Riemann solver. Solver options, and the relevant hydro method, are summarized as follows:

Riemann solver HydroMethod Description 0 – [reserved] 1 0,3,4 HLL (Harten-Lax-van Leer) a two-wave, three-state solver with no resolution of contact waves 2 [reserved] 3 3,4 LLF (Local Lax-Friedrichs) 4 0,3 HLLC (Harten-Lax-van Leer with Contact) a three-wave, four-state solver with better resolution of contacts 5 0 TwoShock 6 4,6 HLLD Default: 1 (HLL) for

`HydroMethod`= 3; 5 (TwoShock) for`HydroMethod`= 0; 6 (HLLD) for`HydroMethod = 6``RiemannSolverFallback`(external; only if`HydroMethod`is 0, 3 or 4)- If the euler update results in a negative density or energy, the solver will fallback to the HLL Riemann solver that is more diffusive only for the failing cell. Only active when using the HLLC or TwoShock Riemann solver. Default: OFF.
`ReconstructionMethod`(external; only if`HydroMethod`is 3 or 4)This integer specifies the reconstruction method for the MUSCL solver. Choice of

Default: 0 (PLM) for

`HydroMethod`= 3; 1 (PPM) for`HydroMethod`= 0`ConservativeReconstruction`(external; only if`HydroMethod`is 3 or 4)- Experimental. This option turns on the reconstruction of the left/right interfaces in the Riemann problem in the conserved variables (density, momentum, and energy) instead of the primitive variables (density, velocity, and pressure). This generally gives better results in constant-mesh problems has been problematic in AMR simulations. Default: OFF
`PositiveReconstruction`(external; only if`HydroMethod`is 3 or 4)- Experimental and not working. This forces the Riemann solver to restrict the fluxes to always give positive pressure. Attempts to use the Waagan (2009), JCP, 228, 8609 method. Default: OFF
`Gamma`(external)- The ratio of specific heats for an ideal gas (used by all hydro
methods). If using multiple species (i.e.
`MultiSpecies`> 0), then this value is ignored in favor of a direct calculation (except for PPM LR) Default: 5/3. `Mu`(external)- The molecular weight. Default: 0.6.
`CourantSafetyNumber`(external)- This is the maximum fraction of the CFL-implied timestep that will be used to advance any grid. A value greater than 1 is unstable (for all explicit methods). The recommended value is 0.4. Default: 0.6.
`RootGridCourantSafetyNumber`(external)- This is the maximum fraction of the CFL-implied timestep that will be used to advance ONLY the root grid. When using simulations with star particle creation turned on, this should be set to a value of approximately 0.01-0.02 to keep star particles from flying all over the place. Otherwise, this does not need to be set, and in any case should never be set to a value greater than 1.0. Default: 1.0.
`DualEnergyFormalism`(external)- The dual energy formalism is needed to make total energy schemes such as PPM DE and PPM LR stable and accurate in the “hyper-Machian” regime (i.e. where the ratio of thermal energy to total energy < ~0.001). Turn on for cosmology runs with PPM DE and PPM LR. Automatically turned off when used with the hydro method ZEUS. Integer flag (0 - off, 1 - on). When turned on, there are two energy fields: total energy and thermal energy. Default: 0
`DualEnergyFormalismEta1`,`DualEnergyFormalismEta2`(external)- These two parameters are part of the dual energy formalism and should probably not be changed. Defaults: 0.001 and 0.1 respectively.
`PressureFree`(external)- A flag that is interpreted by the PPM DE hydro method as an indicator that it should try and mimic a pressure-free fluid. A flag: 1 is on, 0 is off. Default: 0
`PPMFlatteningParameter`(external)- This is a PPM parameter to control noise for slowly-moving shocks. It is either on (1) or off (0). Default: 0
`PPMDiffusionParameter`(external)- This is the PPM diffusion parameter (see the Colella and Woodward method paper for more details). It is either on (1) or off (0). Default: 1 [Currently disabled (set to 0)]
`PPMSteepeningParameter`(external)- A PPM modification designed to sharpen contact discontinuities. It is either on (1) or off (0). Default: 0
`ZEUSQuadraticArtificialViscosity`(external)- This is the quadratic artificial viscosity parameter C2 of Stone & Norman, and corresponds (roughly) to the number of zones over which a shock is spread. Default: 2.0
`ZEUSLinearArtificialViscosity`(external)- This is the linear artificial viscosity parameter C1 of Stone & Norman. Default: 0.0

## Minimum Pressure Support Parameters¶

`UseMinimumPressureSupport`(external)- When radiative cooling is turned on, and objects are allowed to
collapse to very small sizes so that their Jeans length is no
longer resolved, then they may undergo artificial fragmentation
and angular momentum non-conservation. To alleviate this problem,
as discussed in more detail in Machacek, Bryan & Abel (2001), a
very simple fudge was introduced: if this flag is turned on, then
a minimum temperature is applied to grids with level ==
`MaximumRefinementLevel`. This minimum temperature is that required to make each cell Jeans stable multiplied by the parameter below. More precisely, the temperature of a cell is set such that the resulting Jeans length is the square-root of the parameter`MinimumPressureSupportParameter`. So, for the default value of 100 (see below), this insures that the ratio of the Jeans length/cell size is at least 10. Default: 0 `MinimumPressureSupportParameter`(external)- This is the numerical parameter discussed above. Default: 100

## Magnetohydrodynamics (CT) Parameters¶

`MHD_CT_Method`(external)Method for computing the electric field from the Riemann fluxes

CT Method Description 0 None (only for debugging) 1 Balsara Spicer 2001, first order average 2 Gardiner and Stone 2005. Second order Lax-Friedrichs type reconstruction. Uses `CT_AthenaDissipation`flag.3 Gardiner and Stone 2005. Second order reconstruction using upwind switches Default: 3

`CT_AthenaDissipation`(external)- For the Lax-Friedrichs CT method, this is the maximum wave speed. ( in Gardiner & Stone 2005 eqn. 46). Default: 0.1
`EquationOfState`(external, ct only)- 0: standard adiabatic 1: Exactly isothermal
equation of state. This flag removes the total energy term completely, instead
computing pressure as
`HydroMethod = 6`and`RiemannSolver = 6`(HLLD) as this is the only purely isothermal Riemann solver in Enzo. Default: 0
. This option only works with
`IsothermalSoundSpeed`(external, ct only)- When
`EquationOfState = 1`, this is the sound speed used for computation of pressure. Default: 1 `MHDCTSlopeLimiter`(external, ct only)- For computing derivatives for the reconstruction, this switches between zero slope (0), minmod (1), VanLeer (2), and characteristic (3) characteristic with primitive limiting (4). Default: 1
`ReconstructionMethod`(external)- There are two reconstruction methods
that work with MHDCT: Piecewise Linear Method (PLM) (0) and MUSCL-Hancock (6). This
formuation of MUSCL-Hancock is different from the 2nd order Runga Kutta used for
`HydroMethod = 3,4`. `RiemannSolver`(external)- As with
`HydroMethod=4`, the prefered solver is HLLD (`RiemannSolver=6`). Other solvers may be released if the DOE approves them. `MHDCTUseSpecificEnergy`(external)- Either specific energy is used internally (1) or conserved energy is used internally (0). Minor difference in boundary condition update, included for comparison to old solutions. Default: 1
`MHDCTDualEnergyMethod`(external)- When
`DualEnergyFormalism = 1`, this switches between a method that solves an additional equation for the internal energy, as in the rest of Enzo, and method that updates the entropy. `MHD_WriteElectric`(external)- Include the electric field in the output. Default: 0
`MHD_ProjectB`(internal)- Project magnetic fields from fine to coarse. Should not be done in general, only used for initialization.
`MHD_ProjectE`(internal)- Project Electric fields from fine to coarse. Used for the time evolution of the fields.

## Magnetohydrodynamics (Dedner) Parameters¶

The following parameters are considered only when `HydroMethod` is 3 or 4 (and occasionally only in some test problems).
Because many of the following parameters are not actively being tested and maintained, users are encouraged to carefully examine the code before using it.

`UseDivergenceCleaning`(external)- Method 1 and 2 are a failed experiment to do divergence cleaning using successive over relaxation. Method 3 uses conjugate gradient with a 2 cell stencil and Method 4 uses a 4 cell stencil. 4 is more accurate but can lead to aliasing effects. Default: 0
`DivergenceCleaningBoundaryBuffer`(external)- Choose to
*not*correct in the active zone of a grid by a boundary of cells this thick. Default: 0 `DivergenceCleaningThreshold`(external)- Calls divergence cleaning on a grid when magnetic field divergence is above this threshold. Default: 0.001
`PoissonApproximateThreshold`(external)- Controls the accuracy of the resulting solution for divergence cleaning Poisson solver. Default: 0.001
`UseDrivingField`(external)- This parameter is used to add external driving force as a source term in some test problems; see hydro_rk/Grid_(MHD)SourceTerms.C. Default: 0
`DrivingEfficiency`(external)- This parameter is used to define the efficiency of such driving force; see hydro_rk/Grid_(MHD)SourceTerms.C. Default: 1.0
`UseConstantAcceleration`(external)- This parameter is used to add constant acceleration as a source term in some set-ups; see hydro_rk/Grid_(MHD)SourceTerms.C. Default: 0
`ConstantAcceleration[]`(external)- This parameter is used to define the value of such acceleration; see hydro_rk/Grid_(MHD)SourceTerms.C.
`UseViscosity`(external)- This parameter is used to add viscosity and thereby update velocity in some set-ups (1 - constant viscosity, 2 - alpha viscosity); see ComputeViscosity in hydro_rk/Grid_AddViscosity.C. Default: 0
`ViscosityCoefficient`(external)- This parameter is used to define the value of such viscosity for UseViscosity = 1; see ComputeViscosity in hydro_rk/Grid_AddViscosity.C. Default: 0.0
`UseGasDrag`(external)- This parameter is used to calculate velocity decrease caused by gas drag as a source term in some set-ups; see hydro_rk/Grid_(MHD)SourceTerms.C. Default: 0
`GasDragCoefficient`(external)- This parameter is used to define the value of such gas drag; see hydro_rk/Grid_(MHD)SourceTerms.C. Default: 0.0
`UseFloor`(external)- This parameter is used to impose the minimum energy based on MaximumAlvenSpeed in some set-ups; see hydro_rk/Grid_SetFloor.C. Default: 0
`MaximumAlvenSpeed`(external)- This parameter is used to define the value of such minimum; see hydro_rk/Grid_SetFloor.C. Default: 1e30
`UseAmbipolarDiffusion`(external)- This parameter is used to update magnetic fields by ambipolar diffusion in some set-ups; see hydro_rk/Grid_AddAmbipolarDiffusion.C. Default: 0
`UseResistivity`(external)- This parameter is used to add resistivity and thereby update magnetic fields in some set-ups; see ComputeResistivity in hydro_rk/Grid_AddResistivity.C. Default: 0
`UsePhysicalUnit`(external)- For some test problems (mostly in hydro_rk), the relevant parameters could be defined in physical CGS units. Default: 0
`SmallRho`(external)- Minimum value for density in hydro_rk/EvolveLevel_RK.C. Default: 1e-30 (note that the default value assumes UsePhysicalUnit = 1)
`SmallT`(external)- Minimum value for temperature in hydro_rk/EvolveLevel_RK.C. Default: 1e-10 (note that the default value assumes UsePhysicalUnit = 1)
`SmallP`- [not used]
`RKOrder`- [not used]
`Theta_Limiter`(external)- Flux limiter in the minmod Van Leer formulation. Must be between 1 (most dissipative) and 2 (least dissipative). Default: 1.5
`Coordinate`(external)- Coordinate systems to be used in hydro_rk/EvolveLevel_RK.C. Currently implemented are Cartesian and Spherical for HD_RK, and Cartesian and Cylindrical for MHD_RK. See Grid_(MHD)SourceTerms.C. Default: Cartesian
`EOSType`(external)- Types of Equation of State used in hydro_rk/EvolveLevel_RK.C (0 - ideal gas, 1 - polytropic EOS, 2 - another polytropic EOS, 3 - isothermal, 4 - pseudo cooling, 5 - another pseudo cooling, 6 - minimum pressure); see hydro_rk/EOS.h. Default: 0
`EOSSoundSpeed`(external)- Sound speed to be used in EOS.h for EOSType = 1, 2, 3, 4, 5. Default: 2.65e4
`EOSCriticalDensity`(external)- Critical density to be used in EOS.h for EOSType = 1, 2, 4, 6. Default: 1e-13
`EOSGamma`(external)- Polytropic gamma to be used in EOS.h for EOSType = 1. Default: 1.667
`DivBDampingLength`(external)- From C_h (the Dedner wave speeds at which the div*B error is isotropically transferred; as defined in e.g. Matsumoto, PASJ, 2007, 59, 905) and this parameter, C_p (the decay rate of the wave) is calculated; see ComputeDednerWaveSpeeds.C Default: 1.0
`UseCUDA`(external)- Set to 1 to use the CUDA-accelerated (M)HD solver. Only works if compiled with cuda-yes. Default: 0
`ResetMagneticField`(external)- Set to 1 to reset the magnetic field in the regions that are denser than the critical matter density. Very handy when you want to re-simulate or restart the dumps with MHD. Default: 0
`ResetMagneticFieldAmplitude`(external)- The magnetic field values (in Gauss) that will be used for the above parameter. Default: 0.0 0.0 0.0
`CoolingCutOffDensity1`- Reserved for future use
`CoolingCutOffDensity2`- Reserved for future use
`CoolingCutOffTemperature`- Reserved for future use
`CoolingPowerCutOffDensity1`- Reserved for future use
`CoolingPowerCutOffDensity2`- Reserved for future use