The energy of a human voice at certain pitch and volume can shatter a wine glass due to vibrations caused by sound waves. Motion of fluids can also create structural vibration, sometimes with disastrous consequences: In 1940, the Tacoma Narrows Bridge in Washington state collapsed when high winds caused the structure to oscillate with increasing amplitude from end to end, until sections of the bridge fell into the river. The bridge structure was responding to the transient forces caused at certain flow frequencies as the wind blew past the bridge. At a critical vibration frequency corresponding to the natural (or harmonic) frequency of the structure, mechanical resonance occurs, and the objects fail — glass shatters, the bridge collapses.
This interaction between fluids and structures — called “ flow-induced vibration ” or in some special cases “vortex-induced vibration” — is a natural phenomenon. Engineers must take it into account in their designs, which often forces them to operate processes below critical flow rates and design equipment to avoid mechanical resonance and vibration fatigue. These compromises usually mean equipment must be over-designed, and processes must run at less than optimum flow conditions. For the sake of safety and efficiency, engineers must understand the sources of flow-induced vibration and related amplitudes and frequencies.
ANSYS mutiphysics solutions are used to study the flow-induced vibration of objects ranging from small sensors in cross flow in a pipe to large turbine blades. The transient loads caused by the motion of the fluid can be created from coherent flow structures. These flow structures have dominant frequencies, and it is possible to simulate/measure formation of these flow structures or vortices and investigate the fluid loads associated with their motion. Vortices can be generated in the leading edge, around bends, at corners, in the wake of objects in a cross flow, or within separate flows caused by geometrical expansion or contraction of walls of objects.
The engineering challenge is to predict the dominant frequencies, the corresponding transient loads and the structure’s response. Now let’s look at an example that deals with fatigue analysis of a cone-flow meter subjected to fluid dynamic forces. ANSYS solutions are used to perform a coupled calculation of fluid and structure mechanics, accounting for accurate fluid-induced forces due to vortex shedding and the corresponding mechanical response of the cone-flow meter.
The integrated workflow enables calculation of time-varying forces acting on the cone, as well as the frequency and amplitude of the resulting flow excitation. The structural response to the flow excitation and the resulting fatigue calculations can help to identify the loading cycle and design a more reliable flow meter.
Reliable prediction of flow induced vibration requires:
- An accurate account of fluid dynamics with high-fidelity turbulence models
- The ability to handle complex geometry motion
- Accurate structural mechanics analysis
- Coupled fluid–structural solutions
By better understanding flow-induced vibration, engineers can design systems and components that are more reliable, operate at a broader range of applicability, are less susceptible to vibration fatigue, avoid mechanical resonance, and operate processes more efficiently.
For more information and two great on-demand webinars on this topic, visit our Flow Induced Vibration web page now.
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