Add docs for no slip conditions

src/schemes/boundary/dummy_particles/dummy_particles.jl

@@ -1,6 +1,24 @@
 @doc raw"""
-    BoundaryModelDummyParticles(initial_density, hydrodynamic_mass, state_equation,
-                                density_calculator, smoothing_kernel, smoothing_length)
+    BoundaryModelDummyParticles(initial_density, hydrodynamic_mass,
+                                density_calculator, smoothing_kernel,
+                                smoothing_length; viscosity=NoViscosity(),
+                                state_equation=nothing, correction=nothing)
+
+## Arguments
+- `initial_density`: Vector holding the initial density of each boundary particles
+- `hydrodynamic_mass`: Vector holding the "hydrodynamic mass" of each boundary particles
+- `density_calculator`: Option to compute the hydrodynamic boundary density. For more
+                        information see description below.
+- `smoothing_kernel`: Smoothing kernel should be the same as for the adjacent fluid system
+- `smoothing_length`: Smoothing length should be the same as for the adjacent fluid system
+
+
+# Keywords
+- `state_equation`: This should be the same as for the adjacent fluid system
+                    (see e.g. [`StateEquationIdealGas`](@ref)).
+- `correction`:     Correction method of the adjacent fluid system (see TODO)
+- `viscosity`:      Slip (default) or no-slip condition. See description below for further
+                    information
 
 Boundaries modeled as dummy particles, which are treated like fluid particles,
 but their positions and velocities are not evolved in time. Since the force towards the fluid
@@ -33,8 +51,10 @@ f_{ab} = m_a m_b \left( \frac{p_a}{\rho_a^2} + \frac{p_b}{\rho_b^2} \right) \nab
 The quantities to be defined here are the density ``\rho_b`` and pressure ``p_b``
 of the boundary particle ``b``.
 
+## Hydrodynamic density of dummy particles
+
 We provide five options to compute the boundary density and pressure, determined by the `density_calculator`:
-1. With [`AdamiPressureExtrapolation`](@ref), the pressure is extrapolated from the pressure of the
+1. (Recommended) With [`AdamiPressureExtrapolation`](@ref), the pressure is extrapolated from the pressure of the
    fluid according to (Adami et al., 2012), and the density is obtained by applying the inverse of the state equation.
    This option usually yields the best results of the options listed here.
 2. With [`SummationDensity`](@ref), the density is calculated by summation over the neighboring particles,
@@ -56,6 +76,36 @@ We provide five options to compute the boundary density and pressure, determined
    This option is not recommended due to stability issues. See [`PressureMirroring`](@ref)
    for more details.
 
+## No-slip conditions
+
+For the interaction of dummy particles and fluid particles, Adami (Adami et al. 2012)
+imposes a no-slip boundary condition by assigning a wall velocity ``v_w`` to the dummy particle.
+
+The wall velocity of particle ``a`` is calculated from the prescribed boundary particle
+velocity ``v_a`` and the smoothed velocity field
+```math
+v_w = 2 v_a - \frac{\sum_b v_b W_{ab}}{\sum_b W_{ab}},
+```
+where the sum is over all fluid particles.
+
+By simply omitting the viscous interaction (choosing `viscosity=NoViscosity()`) a free-slip
+wall boundary condition is applied. By choosing a viscosity model for `viscosity`, a non-slip
+condition is imposed. The viscosity model should be the same as for the adjacent fluid system
+(see [`ViscosityAdami`](@ref) and [`ArtificialViscosityMonaghan`](@ref)).
+
+# Examples
+
+```julia
+# Free-slip condition
+boundary_model = BoundaryModelDummyParticles(densities, masses, AdamiPressureExtrapolation(),
+                                             smoothing_kernel, smoothing_length))
+
+# No slip condition
+boundary_model = BoundaryModelDummyParticles(densities, masses, AdamiPressureExtrapolation(),
+                                             smoothing_kernel, smoothing_length),
+                                             viscosity=ViscosityAdami(nu))
+
+```
 ## References:
 - S. Adami, X. Y. Hu, N. A. Adams.
   "A generalized wall boundary condition for smoothed particle hydrodynamics".

src/schemes/fluid/viscosity.jl

@@ -101,7 +101,7 @@ end
 end
 
 @doc raw"""
-    ViscosityAdami(; nu)
+    ViscosityAdami(; nu, epsilon=0.01)
 
 Viscosity by Adami (Adami et al. 2012).
 The viscous interaction is calculated with the shear force for incompressible flows given by
@@ -117,16 +117,6 @@ The inter-particle-averaged shear stress  is
 ```
 where ``\eta_a = \rho_a \nu_a`` with ``\nu`` as the kinematic viscosity.
 
-For the interaction of dummy particles (see [`BoundaryModelDummyParticles`](@ref)) and fluid particles,
-Adami (Adami et al. 2012) imposes a no-slip boundary condition by assigning a wall velocity ``v_w``
-to the boundary particle.
-
-The wall velocity of particle ``a`` is calculated from the prescribed boundary particle velocity ``v_a`` and the smoothed velocity field
-```math
-v_w = 2 v_a - \frac{\sum_b v_b W_{ab}}{\sum_b W_{ab}},
-```
-where the sum is over all fluid particles.
-
 # Keywords
 - `nu`: Kinematic viscosity
 - `epsilon=0.01`: Parameter to prevent singularities