CRM deformable terrain (SPH) for Chrono::Vehicle simulations

This page is a compact user guide to CRMTerrain, with direct comparison to SCMTerrain.

Quick choice

• Use SCM if you want fast deformable-soil simulation, simple terramechanics parameterization, and minimal setup.
• Use CRM if you need SPH continuum soil flow/deformation and are willing to pay a slightly higher computational cost.

Difference in Physics

• SCM terrain is a deformable height-field model: soil deformation is represented on a grid through node deflection along the terrain normal, with Bekker/Janosi/Mohr-style terramechanics parameters.
• CRM terrain is an SPH continuum representation of deformable soil: the terrain is represented by particles and uses CRM rheology (e.g., MU_OF_I or MCC).

From a user perspective: SCM is the lighter-weight choice, while CRM is the higher-fidelity choice.

API differences that matter most

1. Class role:
   • SCMTerrain is a ChTerrain implementation.
   • CRMTerrain is both ChTerrain and ChFsiProblemCartesian (an SPH-FSI problem builder).
2. Terrain construction:
   • SCM: call Initialize(...) (flat grid, heightmap, or mesh).
   • CRM: call Construct(...) (box, heightmap, or particle/BCE files), then Initialize().
3. Soil model parameters:
   • SCM: SetSoilParameters(...) or RegisterSoilParametersCallback(...).
   • CRM: SetElasticSPH(...) and SetSPHParameters(...) (SPH material + solver settings).
4. Vehicle/solid coupling:
   • SCM: vehicle interaction comes through regular Chrono contact shapes (no explicit FSI registration).
   • CRM: you must explicitly add interacting solids to FSI (AddRigidBody(...), AddFeaMesh(...)), so BCE markers are generated.
5. Time stepping ownership:
   • SCM: usually the vehicle/system step advances terrain effects through contact.
   • CRM: call terrain.Advance(step) (or DoStepDynamics(step)) to advance coupled SPH+MBS.
6. Moving-domain features:
   • SCM: AddActiveDomain(...) reduces ray-casting work.
   • CRM: SetActiveDomain(...) limits active SPH particles and ConstructMovingPatch(...) can relocate soil forward.
7. Terrain query semantics:
   • SCM: GetHeight/GetNormal/GetCoefficientFriction are meaningful.
   • CRM: these currently return placeholder values (0, world-up normal, 0 friction), so CRM is not intended for height/normal/friction-based tire models.

Minimal API patterns

SCM pattern:

SCMTerrain terrain(sys);
terrain.SetSoilParameters(...);                   // or RegisterSoilParametersCallback(...)
terrain.AddActiveDomain(spindle, VNULL, size);   // optional
terrain.Initialize(sizeX, sizeY, delta);         // or heightmap/mesh initialize

CRM pattern:

CRMTerrain terrain(*sys, spacing);
terrain.SetGravitationalAcceleration({0, 0, -9.81});
terrain.SetStepSizeCFD(step_size);
terrain.RegisterVehicle(vehicle.get());           // recommended in vehicle demos
 
terrain.SetElasticSPH(mat_props);
terrain.SetSPHParameters(sph_params);
 
terrain.AddRigidBody(spindle, geometry, false);  // or AddFeaMesh(...)
terrain.SetActiveDomain(active_box_dim);         // optional
 
terrain.Construct(...);                          // box / heightmap / marker files
terrain.Initialize();

Simulation-loop difference (important)

With SCM, your normal vehicle step drives the contact-based terrain response:

driver.Synchronize(time);
terrain.Synchronize(time);      // optional API call (keeps subsystem sequence uniform)
vehicle.Synchronize(time, in, terrain);
driver.Advance(step);
vehicle.Advance(step);          // Chrono contact + SCM loader work happens here

With CRM, the coupled SPH-FSI step is explicit:

driver.Synchronize(time);
terrain.Synchronize(time);      // moving-patch / active-domain updates
vehicle->Synchronize(time, in, terrain);
driver.Advance(step);
terrain.Advance(step);          // coupled SPH + MBS step

If you called RegisterVehicle(...), the vehicle dynamics are advanced inside CRM's FSI step (see CRM vehicle demos).

VSG visualization plugins

• SCM visualization plugin: ChScmVisualizationVSG
• CRM visualization plugin: ChSphVisualizationVSG

Both are VSG plugins attached to a host visual system with AttachPlugin(...).
You render only through the host visual system (Run()/Render()); there is no separate render loop for the plugin.

Notes:

• CRM run-time visualization can be expensive at large particle counts because SPH/BCE marker data is copied each render call.
• SCMTerrain requires a collision system to be associated with the Chrono system before terrain creation.

Demo map (recommended starting points)

CRM terrain demos:

• src/demos/vehicle/terrain/demo_VEH_CRMTerrain_WheeledVehicle.cpp
• src/demos/vehicle/terrain/demo_VEH_CRMTerrain_TrackedVehicle.cpp
• src/demos/vehicle/terrain/demo_VEH_CRMTerrain_MovingPatch.cpp
• src/demos/vehicle/test_rigs/demo_VEH_TireTestRig_CRM.cpp

SCM terrain demos:

• src/demos/vehicle/terrain/demo_VEH_SCMTerrain_WheeledVehicle.cpp
• src/demos/vehicle/terrain/demo_VEH_SCMTerrain_TrackedVehicle.cpp
• src/demos/vehicle/terrain/demo_VEH_SCMTerrain_RigidTire.cpp
• src/demos/vehicle/terrain/demo_VEH_SCMTerrain_FEATire.cpp
• src/demos/vehicle/terrain/demo_VEH_RigidSCMTerrain_MixedPatches.cpp