The preferred method for defining boundary conditions (besides using the GUI) is to assign zone-based definitions in the cs_user_boundary_conditions_setup function (cs_user_boundary_conditions.cpp).

As an alternative, the same definitions may be defined in cs_user_finalize_setup_wrapper (cs_user_parameters), which is called immediately after cs_user_boundary_conditions_setup. Choosing one or the other should be based on the best balance for reability and maintainability,

Initialization

The following initialization block needs to be added for the following examples:

  const int n_fields = cs_field_n_fields();
 
  cs_real_t viscl0 = cs_glob_fluid_properties->viscl0;
 
  const auto bpro_rho = CS_F_(rho_b)->get_val_s();
 
  const int keysca = cs_field_key_id("scalar_id");
  const cs_zone_t *zn = nullptr;
 
  auto vel_val_ext = CS_F_(vel)->bc_coeffs->get_val_ext_v();

Body

In the body, we may define several boundary conditions. Here are a few examples.

Inlet example with hydraulic diameter

Assign an inlet to boundary faces of in zone 'inlet_1'.

Warning: the <, <=, >, and >= operators may only be used with variables x, y, and z. This syntax is not a full equation interpreter, so formulas involving x, y, or z are not allowed.

Set a a Dirichlet value on the three components of $\vect{u}$ on the faces with the selection criterion '2 and x < 0.01' and set a Dirichlet to all the scalars $\varia$ .

Turbulence example computed using equations valid for a pipe.

We will be careful to specify a hydraulic diameter adapted to the current inlet.

We will also be careful if necessary to use a more precise formula for the dynamic viscosity use in the calculation of the Reynolds number (especially if it is variable, it may be useful to take the law from cs_user_physical_properties. Here, we use by default the 'viscl0" value.

Regarding the density, we have access to its value at boundary faces (b_roh) so this value is the one used here (specifically, it is consistent with the processing in cs_user_physical_properties, in case of variable density).

 
  zn = cs_boundary_zone_by_name("inlet_1");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
 
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
 
    bc_type[face_id] = CS_INLET;
 
    vel_val_ext(face_id, 0) = 0;     /* vel_x */
    vel_val_ext(face_id, 1) = 0;     /* vel_y */
    vel_val_ext(face_id, 2) = 0;     /* vel_z */
 
    cs_real_t uref2 = 0;
    for (cs_lnum_t ii = 0; ii < 3; ii++)
      uref2 += cs_math_pow2(vel_val_ext(face_id, ii));
    uref2 = cs::max(uref2, 1e-12);
 
    /* Turbulence example computed using equations valid for a pipe.
     *
     * We will be careful to specify a hydraulic diameter adapted
     *   to the current inlet.
 
     * We will also be careful if necessary to use a more precise
     *   formula for the dynamic viscosity use in the calculation of
     *   the Reynolds number (especially if it is variable, it may be
     *   useful to take the law from 'cs_user_physical_properties'
     *   Here, we use by default the 'viscl0" value.
     *   Regarding the density, we have access to its value at boundary
     *   faces (b_rho) so this value is the one used here (specifically,
     *   it is consistent with the processing in 'cs_user_physical_properties',
     *   in case of variable density) */
 
    /* Hydraulic diameter */
    cs_real_t xdh = cs_notebook_parameter_value_by_name("hydraulic_diam");
 
    /* Calculation of turbulent inlet conditions using
       the turbulence intensity and standard laws for a circular pipe
       (their initialization is not needed here but is good practice) */
 
    cs_real_t b_rho = bpro_rho[face_id];
 
    cs_turbulence_bc_inlet_hyd_diam(face_id,
                                    uref2,
                                    xdh,
                                    b_rho,
                                    viscl0);
  }
 
  /* Set user scalar values to 1 */
 
  for (int f_id = 0; f_id < n_fields; f_id++) {
    cs_field_t *f = cs_field(f_id);
    if (f->get_key_int(keysca) > 0) {
      auto val_ext = f->bc_coeffs->get_val_ext();
      for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
        const cs_lnum_t face_id = zn->elt_ids[e_idx];
        val_ext[face_id] = 1.0;
      }
    }
  }

Inlet example with turbulence intensity

Assign an inlet to boundary faces of zone 'inlet_2'.

Set a a Dirichlet value on the three components of $\vect{u}$ on the faces with the selection criterion '3' and set a Dirichlet to all the scalars $\varia$ .

Turbulence example computed using turbulence intensity data.

We will be careful to specify a hydraulic diameter adapted to the current inlet.

Calculation of and $\varepsilon$ at the inlet (xkent and xeent) using the turbulence intensity and standard laws for a circular pipe (their initialization is not needed here but is good practice)

  zn = cs_boundary_zone_by_name("inlet_2");
 
  /* Hydraulic diameter: to be defined in the GUI notebook. */
  cs_real_t xdh = cs_notebook_parameter_value_by_name("hydraulic_diam");
 
  /* Turbulence intensity: to be defined in the GUI notebook. */
  cs_real_t xitur = cs_notebook_parameter_value_by_name("turb_intensity");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
 
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
 
    bc_type[face_id] = CS_INLET;
 
    vel_val_ext(face_id, 0) = 0;     /* vel_x */
    vel_val_ext(face_id, 1) = 1.1;   /* vel_y */
    vel_val_ext(face_id, 2) = 0;     /* vel_z */
 
    cs_real_t uref2 = 0;
    for (cs_lnum_t ii = 0; ii < 3; ii++)
      uref2 += cs_math_pow2(vel_val_ext(face_id, ii));
    uref2 = cs::max(uref2, 1e-12);
 
    /* Calculation of turbulent inlet conditions using
     *   the turbulence intensity and standard laws for a circular pipe
     *   (their initialization is not needed here but is good practice) */
 
    cs_turbulence_bc_inlet_turb_intensity(face_id, uref2, xitur, xdh);
  }
 
  /* Set user scalar values to 1 */
 
  for (int f_id = 0; f_id < n_fields; f_id++) {
    cs_field_t *f = cs_field(f_id);
    if (f->get_key_int(keysca) > 0) {
      auto val_ext = f->bc_coeffs->get_val_ext();
      for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
        const cs_lnum_t face_id = zn->elt_ids[e_idx];
        val_ext[face_id] = 1.0;
      }
    }
  }

Assign an outlet to boundary faces of zone 'outlet'

Outlet:

zero flux for velocity and temperature, prescribed pressure
Note that the pressure will be set to P0 at the first
free outlet face (CS_OUTLET)

  zn = cs_boundary_zone_by_name("outlet");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
    bc_type[face_id] = CS_OUTLET;
  }

Wall example

Assign a wall to boundary faces of zone '5'.

Wall:

zero flow (zero flux for pressure)
friction for velocities (+ turbulent variables)
zero flux for scalars

  zn = cs_boundary_zone_by_name("5");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
    bc_type[face_id] =  CS_SMOOTHWALL;
  }
 
  /* Temperature */
  cs_field_t *th_f = cs_thermal_model_field();
 
  if (th_f != nullptr) {
    int *th_icodcl = th_f->bc_coeffs->icodcl;
    auto th_val_ext = th_f->bc_coeffs->get_val_ext();
    auto th_h_ext = th_f->bc_coeffs->get_h_ext();
    auto th_q_ext = th_f->bc_coeffs->get_q_ext();
 
    for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
      const cs_lnum_t face_id = zn->elt_ids[e_idx];
 
      /* If temperature prescribed to 20 with wall law */
      th_icodcl[face_id] = CS_BC_WALL_MODELLED;
      th_val_ext[face_id] = 20.;
 
      /* If temperature prescribed to 50 with no wall law (simple Dirichlet)
       *  with exchange coefficient 8 */
      th_f->bc_coeffs->icodcl[face_id] = CS_BC_DIRICHLET;
      th_val_ext[face_id] = 50.;
      th_h_ext[face_id] = 8.;
 
      /* If flux prescribed to 4. */
      th_icodcl[face_id] = CS_BC_NEUMANN;
      th_q_ext[face_id] = 4.;
    }
  }

Rough wall example

Assign a rough wall to boundary faces of zone '7'.

Wall:

zero flow (zero flux for pressure)
rough friction for velocities (+ turbulent variables)
zero flux for scalars

  cs_span<cs_real_t> bpro_roughness;
  cs_span<cs_real_t> bpro_roughness_t;
 
  if (cs_field_try("boundary_roughness") != nullptr)
    bpro_roughness = cs_field("boundary_roughness")->get_val_s();
 
  if (cs_field_try("boundary_thermal_roughness") != nullptr)
    bpro_roughness_t = cs_field("boundary_thermal_roughness")->get_val_s();
 
  zn = cs_boundary_zone_by_name("7");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
 
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
 
    /* Wall: zero flow (zero flux for pressure)
     *       rough friction for velocities (+ turbulent variables)
     *       zero flux for scalars*/
 
    bc_type[face_id] = CS_ROUGHWALL;
 
    /* Roughness for velocity: 1cm */
    if (bpro_roughness.size() != 0)
      bpro_roughness[face_id] = 0.01;
 
    /* Roughness for temperature (if required): 1cm */
    if (bpro_roughness_t.size() != 0)
      bpro_roughness_t[face_id] = 0.01;
 
    /* If sliding wall with velocity */
    vel_val_ext(face_id, 0) = 1.;
  }

Symmetry example

Assign a symmetry condition to boundary faces of zone '4'

  zn = cs_boundary_zone_by_name("s");
 
  for (cs_lnum_t e_idx = 0; e_idx < zn->n_elts; e_idx++) {
 
    const cs_lnum_t face_id = zn->elt_ids[e_idx];
    bc_type[face_id] = CS_SYMMETRY;
 
  }