Dec 11, 2014 -- Many engineering structures and equipments involve the interaction with fluid flow, and remains a major factor influencing the performance characteristics. It is quite obvious considering any engineering application that fluids will interact with solids at any point, whether internally or externally.

While CFD can simulate the flows and provide insights on flow field, pressure, velocity and temperature distribution, it does not simulate or consider the effect of the fluid on solids with which it will interact. From analytical point of view, fluid flows interacting with solid structures are seen to produce frequencies that will cause the structure to vibrate and produce noise. Apart from noise, the vibrating frequencies affect the structural strength of the structure, which may lead to unexpected failure. As such, considering the fluid-solid interaction is extremely crucial and is widely being studied in the aircrafts, turbomachines and construction industries.

 (Image Source: sl-rasch.com/engineering.html)

Meeting the co-existing challenges in developing light-weight and noise-free products requires leveraging the CFD capabilities and coupling the solutions with structural dynamics (CSD).

The coupling allows studying two different phenomena comprehensively: fluid flow characteristics and impact of fluid on structural dynamics. Two equations are solved separately; the CFD utilizes RANS or LES equations to simulate fluid flows, and the solution thus obtained acts as loading functions for CSD to calculate subsequent deformation. The data exchange between these solvers is executed using coupling algorithm, which maps the CFD results with that of CSD at the fluid-structure interface.

Its usefulness can be explained using the famous flutter example found in aircraft wings, which occurs when the unstable aerodynamic forces interact with elastic structure of the wings, transferring the air stream energy into mechanical vibrations to produce noise and damage to the wings with increasing amplitude. The forces from air flow cause the structure to deform or reduce fatigue life gradually. However, under the event of high amplitude vibrations, the wings will fail completely.

A similar process is observed in blades of the turbomachines, which interact with high pressurized fluid flows, generating vibrations and noise. An example could be a gas turbine used in power plants, which has to accommodate high velocity gases impacting the blades and simultaneously generating forces that lead to mechanical vibrations. The same phenomenon is applicable even with turbochargers in automobiles that deal with high velocity exhaust gases and wind turbines that interact with uneven air flow.

Using the CFD-CSD coupling provides a better approach to eliminate the vibrations and resulting noise from these devices. While a coupling could be tight; meaning that the flow field and structure response at each time step to evaluate the effect more rigorously, or it could be loose; wherein the CFD and CSD codes are solved separately, providing faster simulation results.

The results obtained through CFD-CSD coupling can then be utilized to better understand how the fluid is interacting with the designed structure, and also identify the possible design optimization requirements to ensure that the finalized design is free from vibrations and unwanted noise.

About Author:

Mehul Patel specializes in handling CFD projects for Automobile, Aerospace, Oil and Gas and building HVAC sectors.