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## CFD – Computational Fluid Dynamics

Rather than define these terms individually and risk further confusion it is better to simply associate them with the 2 virtual test types they are most commonly used to describe.

• FEA = Mechanical structural assessment to find out the strength / deflection of a design
• CFD = Fluid Flow assessment to find out how a design interacts with Air, Water, etc.

Let’s now run through a general example of each and highlight the results that can be achieved.

## A pure FEA example

Mass applied to a shelf gradually. What is the deflection in the shelf and how close is it to the elastic limit?

Typical plots here are a displacement plot, which shows for an applied load of 100kg the front of the shelf will deform approximately 1mm. In addition, a Factor of Safety plot shows the stress in the support bars is safe for this load.

## A pure CFD example

Hot water passing through a heat exchanger with cold air passing over it. What is the temperature drop achieved in the water?

Typical plots here are a cut plot, which shows the fluid temperature distribution as a thin slice through the model; for a flow rate of 3.2 litres/min the temperature is reduced by approximately 50°C. And a Flow Trajectory plot, which shows the 3D fluid flow paths and allows for easy visualisation of the fluid interaction.

Although initially it may seem like the 2 approaches are distinctly separate from one another there can actually be significant cross over – so called multi-physics problems. Even for the two examples above we could say that there is air surrounding the shelf and that the structure of the heat exchanger could deform, but these inclusions would most likely result in a negligible change to output already achieved.

There are however many cases where it is not valid to ignore the influence of one physics.

## A Multi-Physics example

Motorway gantry sign under its own weight and subjected to high winds. What is the deflection of the sign, is the ground connection sufficient to comfortably support the structure?

The main questions need to be solved using FEA, however there are 2 load sources, wind and weight. To achieve accurate wind loadings we use CFD to analyse a rigid sign and extract surface pressure values;

These can then be imported into a Structural test along with the material density and gravity. The ground connection is secured using bolt connectors.

Typical plots are again the displacement plot and a Pin/Bolt Check plot. Both show that for a 60mph wind the sign will remain very secure.

For more on testing your product’s performance under static loads, dynamic loads and various environmental conditions contact us today on 01663 741 405. Alternatively email us on info@cadtek.com or fill out one of our contact forms.

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