Fluid-Structure Interaction Chpater 2: One-way physics coupling
This chapter introduces one-way (weak) fluid–structure interaction as a practical and widely used approximation for coupled problems where structural response does not significantly alter the flow field. The chapter explains the mathematical and numerical meaning of one-way coupling, contrasts monolithic and partitioned solution strategies, and details how data is transferred from CFD to structural or thermal solvers. Emphasis is placed on physical validity, numerical assumptions, and industrial applicability.
Context and Motivation
In many engineering problems:
Fluids generate loads (pressure, shear, heat)
Structures respond elastically or thermally
The response does not modify the flow in any meaningful way
Examples include:
Wind loading on stiff structures
Pressure loads on pipes and valves
Thermal expansion due to hot gases
Early-stage design verification
In such cases, fully coupled FSI is unnecessary and inefficient.
One-way coupling offers a robust, low-cost alternative.
What “One-Way Coupling” Really Means
3.1 Definition
One-way coupling is a partitioned multi-physics approach where:
The source physics (typically CFD) is solved first
Selected quantities are transferred to a receiver physics
The receiver is solved without feedback to the source
In FSI terms:
Fluid → structure
No structure → fluid feedback
3.2 Physical Interpretation
Physically, this assumes:
Structural deformation is small
Flow topology remains unchanged
Inertial and added-mass effects are negligible
A useful rule:
If the structure “feels” the flow, but the flow does not “notice” the structure moving, one-way coupling is valid.
One-Way Coupling in the FSI Landscape
4.1 Relation to Coupling Strength
One-way coupling ⟶ weak coupling
Two-way coupling ⟶ strong coupling
Weak coupling corresponds to:
Low feedback sensitivity
Off-diagonal coupling terms being negligible
Stable explicit execution
4.2 Comparison with Two-Way FSI
| Aspect | One-Way | Two-Way |
|---|---|---|
| Feedback | None | Mutual |
| Stability | Very high | Conditional |
| Cost | Low | High |
| Setup complexity | Low | High |
| Typical use | Industrial design | Research / dynamic instabilities |
Solution Strategies: Monolithic vs Partitioned
5.1 Why Partitioned Approaches Dominate
Although monolithic formulations are mathematically elegant, they are:
Hard to develop
Computationally expensive
Impractical for industrial workflows
Partitioned approaches:
Reuse validated CFD and FEA solvers
Allow independent meshing and modeling
Are the industry standard (e.g., ANSYS Workbench)
One-way coupling is always partitioned.
5.2 Sequential Execution
In one-way coupling:
Solvers are executed sequentially
No sub-iterations are required
Coupling is explicit in time
This avoids added-mass instabilities entirely.
Transferred Quantities in One-Way FSI
6.1 Mechanical Loads
Commonly transferred from CFD to structural solvers:
Pressure
Integrated forces
Shear stresses (less common)
Pressure transfer is the dominant case .
6.2 Thermal Loads
Thermal one-way coupling includes:
Surface temperature
Heat transfer coefficient (HTC)
Volumetric temperature fields
Important distinction:
If conjugate heat transfer (CHT) is solved in CFD → transfer solid temperature
Otherwise → transfer HTC, not fluid temperature
Data Transfer and Mapping
7.1 Matching vs Non-Matching Meshes
In practice:
Fluid and structural meshes rarely match
Data mapping is required at interfaces
Key requirements:
Conservation of global forces
Preservation of rigid-body motion
Avoidance of spurious oscillations
7.2 Interpolation Nature
Most industrial tools:
Use linear interpolation
Are not conservative
Trade exact conservation for robustness
This is acceptable in one-way coupling because:
No feedback amplification exists
Errors do not propagate back to the fluid solution
Time Treatment in One-Way Coupling
8.1 Steady CFD → Static Structure
Most common case:
Steady flow
Static structural response
Used for:
Wind loading
Pressure vessel verification
8.2 Transient CFD → Transient Structure
More advanced cases:
Sloshing loads
Pulsating pressure
Thermal cycling
Here:
Time-accurate data is transferred
Structural solver integrates independently in time
Typical One-Way FSI Applications
9.1 Mechanical
Chimneys and towers under wind
Valves, elbows, T-junctions
Offshore decks under wave impact
9.2 Thermal
Exhaust systems
Gas turbine blades
Storage tanks under thermal cycling
EGR and cooling systems
Engineering Intuition
One-way coupling is a modeling decision, not a solver limitation
It is valid when feedback is physically negligible
It is often the correct engineering choice
Over-coupling wastes time and introduces unnecessary instability
Good practice:
Start with one-way coupling. Escalate only if physics demands it.
Limitations and Assumptions
One-way coupling fails when:
Structural deformation alters flow paths
Added-mass effects dominate
Aeroelastic instabilities appear
Large rigid-body motion exists
In these cases, two-way FSI is mandatory.
Study Priorities (Chapter 2)
If short on time:
Physical meaning of one-way coupling
Validity assumptions
Types of transferred quantities
Difference between pressure vs HTC transfer
Industrial use cases
Key Takeaways
One-way coupling is the most widely used FSI approach in industry.
It assumes no feedback from structure to fluid.
Partitioned solvers make it robust and efficient.
Data transfer accuracy matters, but stability is guaranteed.
It is the natural starting point for any FSI analysis.