A Chrono YAML FSI problem specification file defines the multibody and fluid problems and co-simulation parameters required to run a Chrono::FSI co-simulation. It consists of the following objects:

[required] The Chrono version (chrono-version) that is compatible with the YAML model specification. This is a string of the form M.m (major.minor) or M.m.p (major-minor-patch), although only the two fileds are verified for compatibility.
[required] The model object which defines YAML specification files for the multibody and fluid models and solvers.
[required] The simulation object which defines the co-simulation metastep and the simulation duration.
[optional] The visualization object which enables run-time visualization and defines rendering frequency.
[optional] The output object which enables simulation output and defines the output frequency.

Model specification

An FSI problem couples a Chrono MBS model and simulation to a fluid solver. Any fluid solver that implements the Chrono::FSI API can be used.

The multibody and fluid models and simulation are specified by referring to the corresponding YAML specification files. These file names must include the path to the files, relative to the location of this Chrono::FSI YAML specification.

Simulation specification

Visualization specification

Output specification

YAML schema

The YAML FSI problem specification file must follow the data/yaml/schema/fsi.schema.yaml provided in the Chrono data directory:

# =============================================================================
# PROJECT CHRONO - http://projectchrono.org
#
# Copyright (c) 2025 projectchrono.org
# All rights reserved.
#
# Use of this source code is governed by a BSD-style license that can be found
# in the LICENSE file at the top level of the distribution and at
# http://projectchrono.org/license-chrono.txt.
# =============================================================================
#
# Schema for a YAML Chrono::FSI problem specification file.
# The `chrono-version` must match the Chrono major and minor version numbers.
#
# =============================================================================
 
required: [chrono-version, model, simulation]
 
chrono-version:
  type: string
  description: Chrono version compatible with this YAML problem specification (M.m or M.m.p)
 
# -----------------------------------------------------------------------------
# Definitions of common Chrono types
 
# Specification of a ChVector3d
vector3d: &VECTOR3D
    type: array
    items:
        type: number
    minItems: 3
    maxItems: 3
 
# Specification of a 3D rotation
# A rotation can be provided as:
# - an Euler angle sequence [yaw, pitch, roll], with angles assumed in radians or degrees, or
# - a unit quaternion [e0, e1, e2, e3]
orientation3d: &ORIENTATION3D
    type: array
    description: Frame orientation as Euler angles or as quaternion
    items:
      type: number
    minItems: 3
    maxItems: 4
 
# Specification of geometric shapes for FSI interaction
shape : &SHAPE
    type: object
    properties:
        type:
            description: Geometric shape type
            type: string
            enum: [SPHERE, BOX, CYLINDER, HULL, MESH]
        location:
            <<: *VECTOR3D
            description: Shape location relative to body reference frame
        orientation:
            <<: *ORIENTATION3D
            description: Shape orientation relative to body reference frame
        dimensions:
            <<: *VECTOR3D
            description: Length, width, and height (required for BOX type)
        radius:
            description: Radius of the sphere (required for SPHERE or CYLINDER type)
            type: number
            minimum: 0.0
        length:
            description: Length (required for CYLINDER type)
            type: number
            minimum: 0.0
        axis:
            <<: *VECTOR3D
            description: Axis direction expressed in body reference frame (required for CYLINDER type)
        filename:
            description: 3D data file name (required for MESH or HULL type)
            type: string
        scale:
            description: Scale factor (for MESH type)
            type: number
            default: 1.0
 
# -----------------------------------------------------------------------------
# Definition of the FSI problem
 
model:
  description: |
    Schema for FSI problem parameters.
    This schema defines the multibody and fluid models, the simulation parameters for the two solvers, and the co-simulation metastep.
  type: object
  required: [multibody_model, multibody_simulation, fluid_model, fluid_simulation]
  properties:
    multibody_model:
      description: YAML file for MBS model specification.
      type: string
    multibody_simulation:
      description: YAML file for MBS simulation specification.
      type: string
    fluid_model:
      description: YAML file for fluid model specification.
      type: string
    fluid_simulation:
      description: YAML file for fluid simulation specification.
      type: string
    
fsi_bodies:
  description: names of FSI rigid bodies (bodies interacting with fluid).
  type: array
  items:
    type: object
    required: [name, shapes]
    properties:
      name:
        description: Name of the body in the multibody system.
        type: string
      shapes:
        description: FSI geometry.
        type: array
        items:
          <<: *SHAPE
 
simulation:
  description: Definition of co-simulation metastep and simulation duration
  type: object
  required: [time_step]
  properties:
    time_step:
      description: Co-simulation metastep (duration between inter-phase data communication).
      type: number
      minimum: 0
    end_time:
      description: Simulation duration.
      type: number
      minimum: 0
 
visualization:
  description: Visualization settings.
  type: object
  required: [render_fps]
  properties:
    render_fps:
      description: Rendering frequency (in frames per second)
      type: number
      minimum: 0
 
output:
  description: Output settings.
  type: object
  required: [output_fps]
  properties:
    output_fps:
      description: Output frequency (in frames per second)
      type: number
      minimum: 0
 
 
  