The interaction between fluid flow and solid structure is crucial in the design and manufacturing of many industrial products. In this article, we explain the reasons for this importance and discuss its types, as well as how simulations are conducted.
What is Fluid-Structure Interaction?
Fluid-structure interaction (FSI) occurs whenever fluid flow comes into contact with a solid structure. This interaction between fluid and solid structure is crucial in many industries. On one side, the fluid acts on the solid body, causing deformation and vibration; for example, if the wing of a plane changes shape and vibrates strongly, it may lose its efficiency or even break. On the other hand, the solid structure affects the fluid flow; for instance, your car may produce unpleasant noises when moving due to the movement of air over its surface.
Fluid-Structure Interaction Examples
The Tacoma Narrows Bridge was a suspension bridge located in the state of Washington, U.S. This bridge collapsed in 1940. With the not particularly strong wind, the bridge began to sway, and after a while, this swaying reached such a point that the structure of the bridge could no longer resist and collapsed. This incident is a valuable example of the importance of examining fluid-structure interaction in design. The engineers were very puzzled at that time as to how this could happen with normal wind intensity. You can find videos of that incident on YouTube.
At first glance, it may not seem a suspension bridge needs FSI consideration, but The Tacoma Narrows Bridge showed the importance of considering the effect of air flow on the bridge.
Another example of FSI is Vortex-induced vibrations, which occur on offshore oil platforms when the columns that support the platform in the water vibrate. This is due to the passage of water flow around the columns, which creates eddies. The pressure on both sides of the columns changes periodically, causing them to oscillate in different directions. One notable instance was observed in the Gulf of Mexico, where a tension leg platform experienced significant vortex-induced vibrations, posing risks to its structural integrity.
Interaction between a offshore oil platform legs and water flow is an important example of the application of FSI simulation
Fluid-Structure Interaction Equations
In FSI simulations, Navier-Stokes and continuity equations are used for fluid analysis, while Newton’s second law, Hooke’s law, and other equations related to the mechanics of material and vibrations are applied to solid structures. Depending on the simulation type, Mesh Motion Equations may also be necessary to account for deformations in the computational domain. Also, if there is a turbulent flow, its equations will also be solved.
At the interface of fluid and solid interactions, boundary condition equations are solved to couple the fluid and solid equations together. This setup allows for a comprehensive analysis of how fluids and structures interact, ensuring accurate predictions of behaviors such as aerodynamic effects on flexible structures or the impact of fluid flow on solid components in various engineering applications.
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Applications of FSI
FSI analysis is utilized in numerous industries, including aerospace, automotive, construction, water structure engineering, sports, and medicine. Below, we describe some of the FSI applications categorized by their types of effects.
Vibration
From the design of airplanes and turbines to bridges and skyscrapers, one critical issue that must be investigated and analyzed is the vibration of structures caused by fluid flow. If designers neglect to account for these vibrations, the structure could collapse.
Noise pollution
Many structures generate unpleasant noise due to fluid flow around them, often caused by structural vibrations. It is possible to identify and mitigate these noise factors through FSI simulations.
Heat Management
In many applications such as automotive systems, fluid flow is utilized for thermal management. The parameters influencing this process include the movement of fluid over solid surfaces, the extent of contact surface area, and the fluid flow regime.
Designing to Adapt Body Shape at Different Fluid Velocities
The behavior of fluid changes with its velocity; for instance, the flow regime may alter or the fluid may become compressible. Designers create devices tailored to anticipated fluid speeds. In some instances, devices are not optimized for specific fluid velocities. For example, this issue arises in the design of certain warplanes, where wing shapes adjust based on speed or engine nozzle configurations change.
Aerodynamics
Even if the structure remains unchanged in its smallest form, Fluid-Structure Interaction remains crucial. This is because in numerous applications, from designing turbine blades to airplane wings, performance hinges on understanding fluid flow behavior around structures. In essence, accurately calculating drag and lift forces on objects is paramount.
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Fluid Displacement by Structure
In various applications, fluid flow is induced by the surface displacement of structures, such as blood pumping by the heart and respiratory processes. These phenomena can be studied using FSI simulations.
Fluid flow path and obstacle deformation by flow, simulated with ANSYS Mechanical
Obstacle deformation of the previous figure, simulated with ANSYS Fluent
Fluid-Structure Interaction in ANSYS
ANSYS software has the capability to model fluid flow with all details and complexities, including heat transfer and turbulence equations, using the finite volume method through its computational fluid dynamic tool called Fluent. Additionally, ANSYS offers a solid behavior simulation tool using finite element analysis named ANSYS Mechanical. Considering Fluid-Structure Interactions in modeling involves accounting for the mutual influence between fluid flow and solid structures. In ANSYS, FSI simulation is conducted in two ways:
- One-way FSI Simulation: In this simulation model, Fluent results are transferred to ANSYS Mechanical, and the effect of fluid flow on the structure is calculated. However, the effect of changes in the structure on fluid flow is not considered.
- Two-way FSI Simulation: In this simulation model, the results of Fluent and ANSYS Mechanical are exchanged between each other, and both perform simulations in parallel, accounting for mutual influences on fluid flow and structure. These bidirectional effects are considered. However, these calculations are typically very computationally intensive and time-consuming.
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Deformation of blades of a turbomachine, simulated with ANSYS Mechanical
Conclusion
It has been established how fluid-structure interaction affects the efficiency of devices through its applications. Designers and engineers require a comprehensive understanding of fluid-structure interaction to enhance the performance and safety of their systems. This understanding is effectively achieved through ANSYS simulations.
more information:
Turbulence Types; What Are The Different Kinds Of Turbulence?