Fluid-Structure Interaction Analysis

Understanding Fluid-Structure Interaction Analysis

Fluid-Structure Interaction (FSI) analysis represents one of the most sophisticated and essential tools in modern engineering, enabling professionals to predict and optimise the behaviour of structures subjected to fluid forces. From offshore platforms battling Atlantic swells to industrial piping systems carrying process fluids, FSI analysis provides critical insights that ensure safety, longevity, and operational efficiency.

At its core, FSI analysis examines the complex relationship between fluid flow and structural response. When fluid flows around or through a structure, it exerts forces that can cause deformation, vibration, or even catastrophic failure. Conversely, structural movements affect fluid flow patterns, creating a coupled system that requires advanced computational methods to analyse accurately. For engineering projects across Nova Scotia and the broader Maritime region, where marine environments and industrial facilities present unique challenges, understanding these interactions is paramount.

The Physics Behind Fluid-Structure Interaction

Fluid-structure interaction encompasses a broad spectrum of physical phenomena that engineers must consider during the design and analysis phases. The fundamental principle involves the transfer of momentum, energy, and sometimes mass between the fluid and solid domains, creating a bidirectional coupling that significantly influences system behaviour.

Types of FSI Coupling

Engineers typically categorise FSI problems based on the strength of coupling between domains:

• One-way coupling: Fluid forces affect the structure, but structural deformation has negligible impact on the flow field. This approach is suitable for stiff structures under moderate fluid loads, offering computational efficiency while maintaining acceptable accuracy.

• Two-way coupling: Both domains influence each other significantly, requiring iterative solutions between fluid and structural solvers. This method is essential for flexible structures, flutter analysis, and situations where structural motion substantially alters flow patterns.

• Strongly coupled problems: The interaction is so significant that monolithic solution approaches may be necessary, solving both domains simultaneously rather than sequentially.

Governing Equations and Numerical Methods

FSI analysis combines the Navier-Stokes equations governing fluid behaviour with structural mechanics equations, typically based on finite element formulations. The fluid domain utilises computational fluid dynamics (CFD) techniques, while the structural domain employs finite element analysis (FEA). Interface conditions ensure continuity of velocity, displacement, and stress across the fluid-structure boundary.

Modern FSI solvers employ various numerical schemes, including Arbitrary Lagrangian-Eulerian (ALE) methods, immersed boundary techniques, and mesh morphing algorithms. These approaches handle the moving boundaries inherent in FSI problems, with mesh update frequencies ranging from every iteration to every time step, depending on the magnitude of structural deformation.

Critical Applications in Maritime and Industrial Sectors

The Atlantic Canadian economy relies heavily on industries where fluid-structure interaction plays a crucial role. Understanding these applications helps engineers select appropriate analysis methods and design parameters for regional projects.

Offshore and Marine Structures

Nova Scotia's extensive coastline and growing offshore energy sector present numerous FSI challenges. Fixed and floating offshore platforms must withstand wave forces characterised by significant wave heights that can exceed 15 metres during severe storms in the North Atlantic. Wave-structure interaction analysis predicts:

• Hydrodynamic loading on platform legs, risers, and mooring systems

• Vortex-induced vibration (VIV) of slender members and cables

• Sloshing behaviour in storage tanks and moonpools

• Green water loading on deck structures during extreme events

• Ice-structure interaction for Arctic-capable vessels and platforms

For marine vessels operating in Maritime waters, FSI analysis addresses hull whipping and springing responses, propeller-hull interactions, and hydroelastic behaviour during slamming events. These phenomena directly impact structural fatigue life, with typical design targets requiring 20-25 year operational lifespans under harsh environmental conditions.

Industrial Piping and Process Systems

Manufacturing facilities throughout Nova Scotia, from food processing plants to petrochemical installations, contain extensive piping networks where FSI effects can cause significant problems. Flow-induced vibration remains a primary concern, with turbulent flow generating pressure fluctuations that excite pipe wall resonances.

Key parameters in piping FSI analysis include:

• Flow velocities typically limited to 3-5 metres per second for liquids and 20-30 metres per second for gases to minimise erosion and vibration

• Pipe support spacing designed to avoid natural frequency coincidence with vortex shedding frequencies

• Water hammer analysis for systems with rapid valve closures, where pressure spikes can exceed 10 times normal operating pressure

• Thermal expansion effects combined with flow-induced loads for accurate stress analysis

Wind Engineering and Building Aerodynamics

Tall buildings, bridges, and communication towers across the Maritimes face significant wind loads, particularly during Atlantic storms and hurricanes that occasionally track northward. FSI analysis for these structures encompasses:

• Along-wind and cross-wind response prediction

• Aeroelastic instabilities including galloping, flutter, and buffeting

• Pedestrian-level wind comfort assessment for urban developments

• Cladding pressure distribution for façade design

Canadian building codes reference design wind speeds that vary by location, with coastal Nova Scotia sites typically requiring consideration of 10-minute mean wind speeds exceeding 100 kilometres per hour for structural design purposes.

Computational Approaches and Software Tools

Modern FSI analysis relies on sophisticated software platforms that couple CFD and FEA solvers. Selecting the appropriate tool depends on problem complexity, accuracy requirements, and available computational resources.

Commercial Software Packages

Industry-standard platforms for FSI analysis include ANSYS System Coupling, which links ANSYS Fluent or CFX with ANSYS Mechanical; COMSOL Multiphysics, offering built-in FSI capabilities; Abaqus with co-simulation interfaces; and STAR-CCM+ with its integrated structural solver. These packages provide varying levels of automation, with setup times ranging from hours for simple one-way coupled analyses to weeks for complex two-way coupled simulations.

Mesh Considerations and Computational Requirements

FSI simulations demand significant computational resources. A typical industrial FSI model might comprise:

• Fluid domain: 5-50 million cells for adequate resolution of boundary layers and wake regions

• Structural domain: 100,000-1,000,000 elements depending on geometric complexity

• Time steps: 0.001-0.01 seconds for transient phenomena, requiring thousands of iterations for meaningful results

• Solution time: 24-500+ hours on high-performance computing clusters

Cloud computing resources have made FSI analysis more accessible to engineering firms without dedicated HPC infrastructure, with Canadian cloud providers offering compliant data storage solutions that address data sovereignty requirements.

Validation and Verification in FSI Analysis

Ensuring FSI simulation accuracy requires rigorous validation against experimental data and careful verification of numerical methods. This quality assurance process is essential for results that inform critical engineering decisions.

Experimental Correlation

Physical testing provides essential benchmark data for FSI validation. Common experimental techniques include:

• Wind tunnel testing with force balance measurements and particle image velocimetry (PIV) for flow visualisation

• Wave tank experiments for marine structure validation, with facilities in Atlantic Canada capable of generating regular and irregular wave spectra

• Vibration measurements on operating equipment using accelerometers and strain gauges

• Full-scale monitoring programmes that compare predicted and measured structural responses

Sensitivity Studies and Uncertainty Quantification

Responsible FSI analysis includes assessment of result sensitivity to input parameters. Key factors requiring investigation include turbulence model selection, mesh density, time step size, and material property variations. Modern approaches incorporate uncertainty quantification methods that propagate input uncertainties through the simulation to provide confidence bounds on predicted responses.

Industry best practices suggest performing mesh independence studies until results change by less than 2-5% with successive refinement, and validating against at least three independent experimental datasets before applying models to new configurations.

Regulatory Framework and Design Standards

Engineering projects in Canada must comply with relevant codes and standards that address fluid-structure interaction effects. Understanding these requirements ensures analysis approaches meet regulatory expectations.

Canadian and International Standards

Several standards provide guidance on FSI-related design requirements:

• CSA S37: Antennas, Towers, and Antenna-Supporting Structures, addressing wind-induced vibration and aerodynamic stability

• CSA S6: Canadian Highway Bridge Design Code, with provisions for wind and hydrodynamic effects on bridges

• CSA Z662: Oil and Gas Pipeline Systems, covering flow-induced vibration in piping

• DNV-ST-0119: Floating Wind Turbine Structures, applicable to emerging offshore wind developments

• API RP 2A-WSD: Planning, Designing and Constructing Fixed Offshore Platforms, widely used for wave loading analysis

Professional Engineering Responsibilities

In Nova Scotia, professional engineers bear responsibility for ensuring analyses meet applicable standards and reflect sound engineering judgement. Engineers and Geoscientists Nova Scotia (EGNS) establishes practice guidelines that influence how FSI analyses are documented, reviewed, and applied to design decisions. Complex FSI problems often benefit from independent peer review to verify methodology and results.

Emerging Trends and Future Developments

The field of fluid-structure interaction analysis continues evolving, driven by advances in computing power, numerical methods, and industry demands. Several trends will shape FSI practice in coming years.

Machine Learning Integration

Artificial intelligence and machine learning techniques are increasingly applied to FSI problems, offering potential for reduced-order models that approximate full FSI behaviour at a fraction of computational cost. These approaches enable real-time structural health monitoring and rapid design space exploration during preliminary engineering phases.

Digital Twin Applications

The digital twin concept combines FSI models with sensor data from operating assets to provide continuous performance assessment. For Maritime infrastructure such as bridges, wharves, and offshore platforms, digital twins enable predictive maintenance scheduling and remaining life assessment based on actual loading history rather than assumed design conditions.

Sustainable Energy Applications

Nova Scotia's commitment to renewable energy development creates growing demand for FSI expertise in tidal and wind energy systems. Tidal turbines in the Bay of Fundy experience extreme flow conditions, with current velocities exceeding 5 metres per second and turbulent intensity levels challenging even robust FSI models. Similarly, offshore wind turbine design requires comprehensive aeroelastic and hydrodynamic analysis to optimise energy capture while ensuring structural integrity.

Partner with Experienced FSI Analysis Professionals

Fluid-structure interaction analysis demands specialised expertise combining fluid mechanics, structural engineering, and computational methods. The complexity of properly setting up, executing, and interpreting FSI simulations requires engineers with deep understanding of both the physics involved and the numerical techniques employed.

Sangster Engineering Ltd. provides comprehensive FSI analysis services to clients throughout Atlantic Canada and beyond. Our team combines advanced computational capabilities with practical engineering experience to deliver analyses that inform better design decisions. Whether you face challenges with flow-induced vibration, wave loading, wind effects, or other FSI phenomena, we offer the technical depth and professional commitment your project requires.

Contact Sangster Engineering Ltd. today to discuss how fluid-structure interaction analysis can enhance the safety, reliability, and performance of your engineering projects. Our Amherst, Nova Scotia office serves clients across the Maritime provinces and welcomes enquiries from organisations seeking expert FSI analysis support.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.