Machine Design

Hydrodynamic Elements Model Liquids

New software more accurately handles liquids in finite-element analysis.

Finite-element analysis is a great tool for determining the effects of loads and stresses on a mechanical structure. But to model a wide range of physical phenomena, it is important that FEA software consider more than just mechanical loads and significant temperature gradients as means to generate stresses in a structure.

For instance, software should be able to consider loads imposed by liquids onto structures. Until recently, FEA left a bit to be desired when it came to modeling such fluid-solid interaction loads. The flow of fluids against an object can induce forces which produce stress and, possibly, structural failure. For software models to accurately replicate mechanical behavior, they must account for fluid-solid interaction loads.

The recent introduction of 2D and 3D hydrodynamic finite elements promises to significantly improve a designer’s ability to model liquids and their effects on structures. Loads that fluids impose on solid structures have been modeled previously using hydrostatic pressures. The new hydrodynamic elements, as the name implies, are capable of modeling dynamic conditions.

These elements can be used to model the gross motion of fluids and how such motion affects the surrounding solid structure. This is accomplished without calculating flow details. Hydrodynamic elements differ from solid FEA elements in that they account only for normal stresses, not shear stresses. So they have no viscous component. (Viscous effects are important when dealing with flow-details, which these elements do not describe.) Because they have mass, they are capable of describing motion and body-force (gravitational) effects.

Engineers can use hydrodynamic elements when concerned about fluid loading rather than intricate fluid-mechanics details. These elements are particularly useful when modeling the effects that confined fluids have on a bounding structure, such as a tank. For example, when analyzing a water-filled tank on a truck bed, hydrodynamic elements can model the water’s effect on the tank during a collision. They can also model the dynamics of a partially filled tank, although the shape of the fluid surface is only approximated. The accompanying illustration details the example.

Using hydrodynamic elements saves significant analysis time because detailed internal fluid-flow calculations and turbulence are not considered. Such effects can be analyzed with computational fluid dynamics (CFD) software, if necessary. But there are many cases in which fluid-solid interaction has important consequences in terms of structural integrity, even though the exact flow details are deemed to be insignificant. Hydrodynamic elements can be used to model any liquid with a known density. Output consists of both internal normal pressure and nodal displacement.

This software will benefit designers by offering an alternative to the difficult process of estimating loads that fluids impose on structures. It is easier to generate elements inside a tank than attempt to calculate the load a moving fluid imposes on the tank walls. The result is faster and easier FEA that, in turn, shortens design time, reduces prototypes and testing, and enables faster product development without compromising safety.

The method does add degrees of freedom to a system, but usually not nearly enough to require addition hardware. Accuracy is excellent as long as flow details do not significantly affect the loads that fluids have on structures.

Hydrodynamic elements are available with Release 12 of Algor’s Accupak/Mechanical Event Simulation software which simulates motion and flexing in mechanical events and shows.

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