Waves as terrain: analytical form drag model defaults to off (#1558)

* prevent form drag from being incorporated

* default to turn off form drag for terrain as waves; documentation of input file option

* spellcheck

Author: mbkuhn

amr-wind/equation_systems/icns/source_terms/DragForcing.H

@@ -51,6 +51,7 @@ private:
     int m_sponge_north{1};
     bool m_is_laminar{false};
     bool m_terrain_is_waves{false};
+    bool m_apply_MOSD{false};
 };
 
 } // namespace amr_wind::pde::icns

amr-wind/equation_systems/icns/source_terms/DragForcing.cpp

@@ -120,6 +120,9 @@ DragForcing::DragForcing(const CFDSim& sim)
             terrain_phys.wave_negative_elevation_name();
         m_target_levelset = &sim.repo().get_field(target_levelset_name);
         m_terrain_is_waves = true;
+        // Inviscid form drag model can help when waves are smaller than cells,
+        // i.e., too small to be resolved with cell blanking
+        pp.query("wave_model_inviscid_form_drag", m_apply_MOSD);
     }
 }
 
@@ -149,6 +152,7 @@ void DragForcing::operator()(
     const auto& terrainz0 = (*m_terrainz0)(lev).const_array(mfi);
 
     const bool is_waves = m_terrain_is_waves;
+    const bool model_form_drag = m_apply_MOSD;
     const auto& target_vel_arr = is_waves
                                      ? (*m_target_vel)(lev).const_array(mfi)
                                      : amrex::Array4<amrex::Real>();
@@ -252,7 +256,7 @@ void DragForcing::operator()(
             const amrex::Real z0 = std::max(terrainz0(i, j, k), z0_min);
             const amrex::Real ustar = viscous_drag_calculations(
                 Dxz, Dyz, ux1r, uy1r, ux2r, uy2r, z0, dx[2], kappa, tiny);
-            if (is_waves) {
+            if (model_form_drag) {
                 form_drag_calculations(
                     Dxz, Dyz, i, j, k, target_lvs_arr, dx, ux1r, uy1r);
             }

docs/sphinx/user/inputs_Momentum_Sources.rst

@@ -252,6 +252,24 @@ Section: Momentum Sources
    This term turns off the sponge layer. This term is required for terrain simulations with periodic 
    boundary conditions. The default value is 0. 
 
+.. input_param:: DragForcing.wave_model_inviscid_form_drag
+
+   **type:** Boolean, optional, default = false
+
+   This input file option turns on or off an inviscid model for the form drag of waves in the domain. 
+   The formulation of this model is adapted from the Moving Surface Drag (MOSD) model developed by
+   `Ayala et al (2024) <https://doi.org/10.1007/s10546-024-00884-8>`_.
+   
+   When the OceanWaves physics module is active, and the volume fraction variable ("vof") is not in the simulation,
+   DragForcing will represent ocean waves as moving terrain. This is automatic and independent of DragForcing
+   input arguments. When waves are represented as moving terrain and
+   there is sufficient mesh resolution to resolve the shape of the wave, the blanking of cells performed
+   by the DragForcing routine will naturally introduce the form drag of the waves into the flow. However,
+   when the waves are not sufficiently resolved, such as when the wave amplitude is less than the cell height,
+   the analytical model for the form drag, activated by setting this option to true, can be used to compensate
+   for the lack of resolution. Therefore, this option should remain set to false except in scenarios
+   when the form drag is known to be under-resolved.
+
 
 The following arguments are influential when ``GravityForcing`` is included in :input_param:`ICNS.source_terms`.