Variables and properties can be accessed both in Fortran and in C using the cs_field API.

First, let us specify a few conventions used in code_saturne:

The dynamic variables designation includes velocity, pressure, and variables related to the turbulence model (k, ε, R_ij, ω, ϕ, $\overline{f}$ , α, ν_t).
The designation “scalar” refers to (usually scalar) variables which are solution of an advection equation, apart from the dynamic variables listed above; for instance the temperature, scalars which may be passive or not, user-defined or model-defined.

The mean value of the square of the fluctuations of a “scalar” is a “scalar”, too.

Scalars may be divided into two groups: user-defined scalars and model-defined (sometimes referred to as “specific physics”) scalars.
Depending on the thermal model used; the solved thermal scalar variable may be the temperature, the enthalpy, or in the case of the compressible module, the total energy. When the temperature is not the solved thermal variable, it is usually available as a property.
When a user scalar represents the mean of the square of the fluctuations of another scalar (i.e. the average $\overline{\varphi^\prime\varphi^\prime}$ for a fluctuating scalar ϕ). This can be made either via the interface or by declaring that scalar using cs_parameters_add_variable_variance in cs_user_parameters.c.

Accessing variables and properties in C:

The most general way of accessing a field is to use its name:

cs_real_t *cvar_pr = cs_field_by_name("pressure")->val;
then:
cvar_pr[cell_id]
The main variables and properties can be accessed using the CS_F_ macro:
- For one-dimensional arrays:
  pressure = CS_F_(p)->val[cell_id],
  cp = CS_F_(cp)->val[cell_id]
  temp = CS_F_(t)->val[cell_id]
- For multidimensional arrays:
  uz = CS_F_(vel)->val[3*cell_id + 2]
  
  These arrays can also be cast as follows (for a 3-D array):
  cs_real_3_t *cvar_vel = (cs_real_3_t *)CS_F_(vel)->val
  
  The cell value can then be accessed as
  ux = cvar_vel[cell_id][0]
  
  vel, p, or cp are defined in cs_field_pointer.h.
Indexed variables (such as user scalars) and indexed properties are accessed as:
CS_FI_(name,ii-1)->val[cell_id].
The thermal scalar can be accessed using the cs_thermal_model_field function:
field_t *tf = cs_thermal_model_field();
then:
tf->val[cell_id].
if there is no thermal scalar, cs_thermal_model_field returns NULL.
In any case, all variables can be accessed using the function cs_field_by_name:
cs_real_t *cvar_pr = cs_field_by_name("pressure")->val
A field's scalar_id key-word should be ≥ 0 if it is a scalar.
When a field is user-defined (rather than model-defined), its type flag should also match the CS_FIELD_USER mask:
if (f->type & CS_FIELD_USER) ...

Remarks: Note that indexes in C begin at 0, while indexes in Fortran begin at 1. Thus, Fortran and C loop counters are related in the following as:
cell_id = iel-1.

Accessing variables and properties in Fortran:

The most general way of accessing a field is to use its name:

call field_get_val_s_by_name("pressure", cvar_pr) pres = cvar_pr(iel)
Both variables and properties can be accessed via the cs_field API, using the global field indices and indirections arrays, as in the following examples:
- For one-dimensional arrays:
  
  call field_get_val_s(ivarfl(ipr), cvar_pr) pres = cvar_pr(iel),
  
  call field_get_val_s(icp, cpro_cp) cp = cpro_cp(iel)
  
  The values of scalar variable can be accessed as follows:
  
  call field_get_val_s(ivarfl(isca(iscalt)), cvar_scalt) temp = cvar_scalt(iel)
- For interleaved multidimensional arrays:
  
  call field_get_val_v(ivarfl(iu), cvar_vel) ux = cvar_vel(1,iel)
  
  ipr, iu are here variable indices and the array ivarfl allows to get the corresponding field indices.
  isca(iscalt) is also a variable index (iscalt is the scalar index of the thermal scalar).
  icp is directly a field index (there is no equivalent to the array ivarfl for field of type properties).
the solved variable number for scalar j, where 1 <= j <= nscal, is given by the iscal array. So to access the associated field, use:
ivarfl(iscal(j))).
For model-defined scalars, the total scalar number for model scalar j, where 1 <= j <= nscapp, is given by the iscapp array. So to access the associated field, use:
ivarfl(iscal(iscapp(j)))).
The thermal scalar can be accessed using the iscalt index in the list of the scalars. It's variable number is thus isca(iscalt) .
if there is no thermal scalar, iscalt is equal to -1.

Cross-reference tables are available for the variables and properties of the standard solver and the specific physics features:

Variables

The Fortran variables indexes are defined in the files numvar.f90 (with the exception of ihm and iscal, which are respectively defined in ppincl.f90 and optcal.f90) and the C variables names are defined in cs_field_pointer.h.
Note that dt is accessible by its name (using the cs_field_by_name family of functions), but not through a permanent index..

Fortran code	C code	Description
`call field_get_val_s(ivarfl(ipr), cvar_pr)`	`CS_F_(p)->val`	`Pressure`
`call field_get_val_v(ivarfl(iu), cvar_vel)`	`CS_F_(vel)->val`	`Velocity`
`call field_get_val_s(ivarfl(ivolf2), cvar_voidf)`	`CS_F_(void_f)->val`	`Void fraction for Volume of Fluid model`
`call field_get_val_s(ivarfl(ik ), cvar_k )`	`CS_F_(k)->val`	`Turbulent kinetic energy`
`call field_get_val_s(ivarfl(iep ), cvar_eps)`	`CS_F_(eps)->val`	`Turbulent dissipation`
`call field_get_val_v(ivarfl(irij), cvar_rij)`	`CS_F_(rij)->val`	`Reynolds stress tensor (symmetric components, in the following order for each cell: xx, yy, zz, xy, yz, xz)`
`call field_get_val_s(ivarfl(iphi), cvar_phi)`	`CS_F_(phi)->val`	`for model`
`call field_get_val_s(ivarfl(ifb ), cvar_fb )`	`CS_F_(f_bar)->val`	`for model`
`call field_get_val_s(ivarfl(ial ), cvar_al )`	`CS_F_(alp_bl)->val`	`for or EBRSM model`
`call field_get_val_s(ivarfl(iomg), cvar_omg)`	`CS_F_(omg)->val`	`for SST model`
`call field_get_val_s(ivarfl(inusa), cvar_nusa)`	`CS_F_(nusa)->val`	`for Spalart-Allmaras`
`call field_get_val_v(ivarfl(iuma), cvar_mesh_v)`	`CS_F_(mesh_u)->val`	`Mesh velocity`
`call field_get_val_s(ivarfl(isca(ihm)), cvar_hm)`	`CS_F_(h)->val`	`Enthalpy`
`call field_get_val_s(ivarfl(isca(iscalt)), cvar_scalt)`	`CS_F_(t)->val`	`Temperature`

Properties

These properties are defined in the files numvar.f90 and cs_field_pointer.h.

Fortran code	C code	Description
`dt`	`CS_F_(dt)->val`	`Local time step`
`call field_get_val_s(iviscl, cpro_viscl)`	`CS_F_(mu)->val`	`Molecular viscosity`
`call field_get_val_s(ivisct, cpro_visct)`	`CS_F_(mu_t)->val`	`Turbulent dynamic viscosity`
`call field_get_val_s(icp, cpro_cp)`	`CS_F_(cp)->val`	`Specific heat`
`call field_get_val_s(icrom, cpro_crom)`	`CS_F_(rho)->val`	`Density (at cells)`
`call field_get_val_s(ibrom, bpro_rho)`	`CS_F_(rho_b)->val[face_id]`	`Density (at boundary faces)`
`call field_get_val_s(ismago, cpro_smago)`	`cs_real_t *cpro_smago = cs_field_by_name("smagorinsky_constant^2")->val`	`Field id of the anisotropic turbulent viscosity`
`call field_get_val_s(icour, cpro_cour)`	`cs_real_t *cpro_cour = cs_field_by_name("courant_number")->val`	`Courant number`
`call field_get_val_s(ifour, cpro_four)`	`cs_real_t *cpro_four = cs_field_by_name("fourier_number")->val`	`Fourier number`
`call field_get_val_s(iprtot, cpro_prtot)`	`cs_real_t *cpro_prtot = cs_field_by_name("total_pressure")->val`	`Total pressure at cell centers`
`call field_get_val_s(ivisma, cpro_visma_s)`	`cs_real_t *cpro_visma_s = cs_field_by_name("mesh_viscosity")->val`	`Mesh velocity viscosity (scalar) for the ALE module`
`call field_get_val_v(ivisma, cpro_visma_s)`	`cs_real_t *cpro_visma_v = cs_field_by_name("mesh_viscosity")->val`	`Mesh velocity viscosity (vector) for the ALE module`
`call field_get_val_s(itsrho), cpro_tsrho )`	`cs_real_t *cpro_tsrho = cs_field_by_name("dila_st")->val`	`Global dilatation source terms`
`call field_get_val_s(ibeta), cpro_beta )`	`cs_real_t *cpro_beta = cs_field_by_name("thermal_expansion")->val`	`Thermal expansion coefficient`
`call field_get_val_s(ipori, cpro_ipori)`	`CS_F_(poro)->val`	`Porosity`
`call field_get_val_v(iporf, cpro_iporf)`	`CS_F_(t_poro)->val`	`Tensorial porosity`
`call field_get_val_v(iforbr, bpro_forbr)`	`cs_real_t *bpro_forbr = cs_field_by_name("boundary_forces")->val`	`Field id of the stresses at boundary`
`call field_get_val_s(iyplbr, bpro_yplus)`	`cs_real_t *bpro_yplus = cs_field_by_name("yplus")->val`	`Field id of at boundary`
`call field_get_val_v(idtten, dttens)`	`cs_real_t *dttens = cs_field_by_name("dttens")->val`	`Field id for the dttens tensor`
`call field_get_val_s(itempb, t_b)`	`CS_F_(t_b)->val`	`Boundary temperature`

Some physical properties such as specific heat or diffusivity are often constant (depending on the model or user parameters). In that case, these properties are stored as a simple real numbers rather than in a field over all mesh cells.

This is the case for the specific heat C_p.
- If C_p is constant, it is based on the reference value cs_glob_fluid_properties->cp0 (cp0 in Fortran).
  When this is the case, cs_glob_fluid_properties->icp (mapped as icp in Fortran) should remain set to 0.
  When icp is initialized to 1 (by the GUI, or in cs_user_parameters.c, it is automatically reset to the id of the cell-based property field referenced in the above table.
This is the same for the diffusivity K of each scalar.
- When K is constant, its value is based on the field's reference diffusivity, accessible through the scalar field's diffusivity_ref keyword.
  
  When it is variable, the matching field can be specified and accessed using the base scalar field's diffusivity_id key (accessible using cs_field_key_id("diffusivity_id") in C, or kivisl in Fortran).
  For example, for a scalar field f:
  int k_f_id = cs_field_get_key_int(f, cs_field_key_id("diffusivity_id")); cs_field_t *kf = cs_field_by_id(k_f_id);

Reference physical values

Reference physical values represent either the fluid properties if they are constant, either simple mean values for the initialization if properties are variable and defined in cs_user_physical_properties. Reference values can be set in the GUI or in files cs_user_parameters.f90 and cs_user_parameters.c.

Fortran code	C code	Description
`p0`	`cs_glob_fluid_properties->p0`	`Reference total pressure`
`ro0`	`cs_glob_fluid_properties->ro0`	`Reference density`
`viscl0`	`cs_glob_fluid_properties->viscl0`	`Reference molecular dynamic viscosity`
`cp0`	`cs_glob_fluid_properties->cp0`	`Reference specific heat`

Specific physical models

Atmospheric

Defined in optcal.f90, atincl.f90, atvarp.f90 and cs_field_pointer.h.

Fortran code	C code	Description
call field_get_val_s(ivarfl(isca(iscalt)), cvar_scalt)	`CS_F_(pot_t)->val`	Potential temperature
-	`CS_F_(totwt)->val`	Total water content
call field_get_val_s(ivarfl(isca(intdrp)), cvar_intdrp)	`CS_F_(ntdrp)->val`	Total number of droplets
call field_get_val_s(ivarfl(isca(isca_chem(iesp))), cvar_sc)	`CS_FI_(chemistry,iesp-1)->val`	Chemistry species (indexed)

Coal combustion

Defined in ppincl.f90, ppcpfu.f90 and cs_field_pointer.h.

Fortran code	C code	Description
`call field_get_val_s(isca(inp(iesp)), cvar_inpcl)`	`CS_FI_(np,iesp-1)->val`	`Particles per kg for coal class`
`call field_get_val_s(isca(ixch(iesp)), cvar_xchcl)`	`CS_FI_(xch,iesp-1)->val`	`Reactive coal mass fraction for coal class`
`call field_get_val_s(isca(ixck(iesp)), cvar_xckcl)`	`CS_FI_(xck,iesp-1)->val`	`Coke mass fraction for coal class`
`call field_get_val_s(isca(ixwt(iesp)), cvar_xwtcl)`	`CS_FI_(xwt,iesp-1)->val`	`Water mass fraction for coal class`
`call field_get_val_s(isca(ih2(iesp)), cvar_h2cl)`	`CS_FI_(h2,iesp-1)->val`	`Mass enthalpy for coal class (permeatic case)`
`call field_get_val_s(isca(if1m(iesp)), cvar_f1mcl)`	`CS_FI_(f1m,iesp-1)->val`	`Mean value light volatiles for coal class`
`call field_get_val_s(isca(if2m(iesp)), cvar_f2mcl)`	`CS_FI_(f2m,iesp-1)->val`	`Mean value heavy volatiles for coal class`
`call field_get_val_s(isca(if4m), cvar_f4m)`	`CS_F_(f4m)->val`	`Oxydant 2 mass fraction`
`call field_get_val_s(isca(if5m), cvar_f5m))`	`CS_F_(f5m)->val`	`Oxydant 3 mass fraction`
`call field_get_val_s(isca(if6m), cvar_f6m))`	`CS_F_(f6m)->val`	`Water from coal drying mass fraction`
`call field_get_val_s(isca(if7m), cvar_f7m))`	`CS_F_(f7m)->val`	`Carbon from coal oxidyzed by O2 mass fraction`
`call field_get_val_s(isca(if8m), cvar_f8m))`	`CS_F_(f8m)->val`	`Carbon from coal gasified by CO2 mass fraction`
`call field_get_val_s(isca(if9m), cvar_f9m))`	`CS_F_(f9m)->val`	`Carbon from coal gasified by H2O mass fraction`
`call field_get_val_s(isca(ifvp2m), cvar_fvp2m)`	`CS_F_(fvp2m)->val`	`f1f2 variance`
`call field_get_val_s(isca(iyco2), cvar_yco2)`	`CS_F_(yco2)->val`	`CO2 fraction`
`call field_get_val_s(isca(iyhcn), cvar_yhnc)`	`CS_F_(yhcn)->val`	`HCN fraction`
`call field_get_val_s(isca(iyno), cvar, yno)`	`CS_F_(yno)->val`	`NO fraction`
`call field_get_val_s(isca(iynh3), cvar_ynh3)`	`CS_F_(ynh3)->val`	`NH3 enthalpy`
`call field_get_val_s(isca(ihox), cvar_hox)`	`CS_F_(hox)->val`	`Ox enthalpy`

Compressible

Defined in ppincl.f90 and cs_field_pointer.h.

Fortran code	C code	Description
`call field_get_val_s(isca(ienerg), cvar_energ)`	`CS_F_(e_tot)->val`	`Total energy`
`call field_get_val_s(isca(itempk), cvar_tempk)`	`CS_F_(t_kelvin)->val`	`Temperature, in Kelvin`

Electric arcs

Defined in ppincl.f90 and cs_field_pointer.h.

Fortran code	C code	Description
`call field_get_val_s_by_name("elec_pot_r", cvar_potr)`	`CS_F_(potr)->val`	`Electric potential, real part`
`call field_get_val_s_by_name("elec_pot_i", cvar_poti)`	`CS_F_(poti)->val`	`Electric potential, imaginary part`
`call field_get_val_v_by_name("vec_potential", cvar_potva)`	`CS_F_(potva)->val`	`Vector potential`
`call field_get_val_s_by_name("esl_fraction_01", cvar_ycoel_01)`	`CS_FI_(ycoel,iesp-1)->val`	`Constituent mass fraction`

Gas combustion

Defined in ppincl.f90 and cs_field_pointer.h.

Fortran code	C code	Description
`call field_get_val_s(isca(ifm), cvar_fm)`	`CS_F_(fm)->val`	`Mixture fraction`
`call field_get_val_s(isca(ifp2m), cvar_fp2m)`	`CS_F_(fp2m)->val`	`Mixture fraction variance`
`call field_get_val_s(isca(ifsm), cvar_fsm)`	`CS_F_(fsm)->val`	`Soot mass fraction`
`call field_get_val_s(isca(inpm), cvar_npm)`	`CS_F_(npm)->val`	`Soot precursor number`
`call field_get_val_s(isca(iygfm), cvar_ygfm)`	`CS_F_(ygfm)->val`	`Fresh gas fraction`
`call field_get_val_s(isca(iyfm), cvar_yfm)`	`CS_F_(yfm)->val`	`Mass fraction`
`call field_get_val_s(isca(iyfp2m), cvar_yfp2m)`	`CS_F_(yfp2m)->val`	`Mass fraction variance`
`call field_get_val_s(isca(icoyfp), cvar_coyfp)`	`CS_F_(coyfp)->val`	`Mass fraction covariance`

Radiative transfer

Defined in cs_field_pointer.h.

C code	Description
`CS_F_(rad_lumin)->val`	Radiative luminance
`CS_F_(rad_q)->val`	Radiative flux
`CS_FI_(rad_ets,iesp-1)->val`	Radiative flux explicit source term
`CS_FI_(rad_its,iesp-1)->val`	Radiative flux implicit source term
`CS_FI_(rad_abs,iesp-1)->val`	Radiative absorption
`CS_FI_(rad_emi,iesp-1)->val`	Radiative emission
`CS_FI_(rad_cak,iesp-1)->val`	Radiative absorption coefficient
`CS_F_(qinci)->val`	Radiative incident radiative flux density
`CS_F_(xlam)->val`	Wall thermal conductivity
`CS_F_(epa)->val`	Wall thickness
`CS_F_(emissivity)->val`	Wall emissivity
`CS_F_(fnet)->val`	Boundary radiative flux
`CS_F_(fconv)->val`	Boundary radiative convective flux
`CS_F_(hconv)->val`	Radiative exchange coefficient

Eulerian-Eulerian multiphase flows

Defined in cs_field_pointer.h.

C code	Description
`CS_FI_(yf_ncond,inc)->val`	Non-condensable gas mass fractions
`CS_FI_(qp,ip)->val`	Particles turbulent kinetic energy Q2
`CS_FI_(qfp,ip)->val`	Covariance of the turbulent Q12
`CS_FI_(qfpxx,ip)->val`	XX component of qfp
`CS_FI_(qfpxy,ip)->val`	XY component of qfp
`CS_FI_(qfpxz,ip)->val`	XZ component of qfp
`CS_FI_(qfpyx,ip)->val`	YX component of qfp
`CS_FI_(qfpyy,ip)->val`	YY component of qfp
`CS_FI_(qfpyz,ip)->val`	YZ component of qfp
`CS_FI_(qfpzx,ip)->val`	ZX component of qfp
`CS_FI_(qfpzy,ip)->val`	ZY component of qfp
`CS_FI_(qfpzz,ip)->val`	ZZ component of qfp
`CS_FI_(gamma,ip)->val`	Interfacial mass transfer
`CS_FI_(ia,ip)->val`	Interfacial area
`CS_FI_(x2,ip)->val`	Droplets x2
`CS_FI_(d32,ip)->val`	Droplets Sauter mean diameter
`CS_FI_(drag,ipcpl)->val`	Drag between phases
`CS_FI_(ad_mass,ip)->val`	Added mass
`CS_FI_(th_diff,ip)->val`	Thermal diffusivity
`CS_FI_(th_diff_t,ip)->val`	Turbulent thermal diffusivity
`CS_FI_(drho_dp,ip)->val`	dRho over dP
`CS_FI_(drho_dh,ip)->val`	dRho over dH
`CS_FI_(tau12_t,ip)->val`	Turbulent tau12 for particles
`CS_FI_(lift,ip)->val`	Particles lift
`CS_FI_(disp_t,ip)->val`	Particles turbulent dispersion
`CS_FI_(drift_vel,ip)->val`	Particles drift velocity