Fluid-Structure Interaction

OptiStruct and AcuSolve both include time-domain simulation capabilities that break the coupled solution into a number of time steps. Since the governing equations of both OptiStruct and AcuSolve are nonlinear, sub-iterations are typically required within each time step.

As the name suggests, Fluid-Structure Interaction simulates the interrelationship between fluid flow and the solid body the fluid is in contact with. The behavior of the structure affects the fluid and vice-versa in a coupled dynamic interaction that is captured by dividing the time domain into time steps. For each time step, the exchanged solution attributes are solved for from the governing equations until equilibrium convergence is attained. Each such iteration run through towards convergence within a time step is known as an exchange (in OptiStruct) or a stagger (in AcuSolve). The fluid flow can be external to the solid object, similar to an aircraft wing moving through air, or it can be internal, like the flow of coolant in a condenser tube.

Target Applications

The Fluid-Structure Interaction capability aims at simulations of dynamic problems subjected to fluid flow interactions and their complex interrelationship. It is recommended to use this direct coupling method when the solid structural response variables affecting the fluid domain vary significantly and result in large changes in the fluid response. In this process, the responses from both domains are exchanged in real time.

Note: For linear structural response the P-FSI (Practical FSI) solution offered by AcuSolve may be more effective in solving the linearized structural response with the nonlinear flow solution. For further information on the P-FSI method, refer to the AcuSolve Command Reference Manual.

Supported Solutions

A large number of physical state variables affect the interrelationship between the structure and the fluid domains. For example, pressure of the fluid at the interface can affect displacement (and thereby the stress state) of the solid structure, and vice-versa. The displacements at the structural interface can cause changes to fluid flow leading to significant differences in fluid behavior across the entire domain. Similarly, thermal loads on the structure can lead to temperature changes at the structural interface, leading to a changing temperature field for the fluid. Currently, in OptiStruct and AcuSolve the following solutions for the interaction is supported:

Structural Fluid-Structure Interaction (Nonlinear Direct Transient Structural Response of the structure).
Thermal Fluid-Structure Interaction (Linear Transient Heat Transfer Response of the structure).

Combined structural and thermal heat transfer solutions in conjunction with fluid-structure interaction is currently not supported. You can either run Structural FSI or Thermal FSI, however, you cannot run both in the same run. In the following sections, SFSI refers to Structural Fluid-Structure Interaction and TFSI refers to Thermal Fluid-Structure Interaction for brevity and to avoid redundancy.