def METHOD_NAME(self, variables):
    # Set up
    param = self.param
    L_n = param.n.L
    x_s = pybamm.standard_spatial_vars.x_s
    # Transverse velocity in the separator determines through-cell velocity
    div_Vbox_s = variables[
        "X-averaged separator transverse volume-averaged acceleration [m.s-2]"
    ]
    i_boundary_cc = variables["Current collector current density [A.m-2]"]
    v_box_n_right = -param.n.DeltaV * i_boundary_cc / self.param.F
    div_v_box_s_av = -div_Vbox_s
    div_v_box_s = pybamm.PrimaryBroadcast(div_v_box_s_av, "separator")
    # Simple formula for velocity in the separator
    v_box_s = div_v_box_s_av * (x_s - L_n) + v_box_n_right
    variables.update(
        self._get_standard_sep_velocity_variables(v_box_s, div_v_box_s)
    )
    variables.update(self._get_standard_whole_cell_velocity_variables(variables))
    variables.update(
        self._get_standard_whole_cell_acceleration_variables(variables)
    )
    variables.update(self._get_standard_whole_cell_pressure_variables(variables))
    return variables