## Computational Fluid Dynamics (CFD)

Simulating flow of a fluid such as air or water around an object with a computer program

CFD can provide both qualitative and quantitative results. As well as hard data about the forces, velocities and pressures, we can use a number of visualisations to indicate their distributions or variation over time. Displaying stream lines and pressure distributions gives an insight into the flow’s behaviour and indicates where geometry changes might improve the forces or flow characteristics – such as stall angle or flow separation.

The process involves setting up a domain – or volume to analyse – and setting the flow conditions at the domain boundaries. Usually this is an inlet flow speed, and outlet and some constraints at the sides, top and bottom of the domain. Then we locate the object to be analyses in the domain, oriented to the flow according to what we want to analyse.

A boundary condition is applied to the surfaces of the model that are of interest – the keel, bulb or hull and so on – and then the model is meshed. Meshing generates a large number of nodes and element faces that are used to represent the fluid and the energy, direction and speed, within it. We generally model hulls with a mesh size between 50mm and 100mm, and our CFD models range from around 2 million elements to up to 8 million, depending on the mesh size and density.

Some of our recent projects include:

Aeronautical and Automotive
The investigation into the helicopter airframe was to provide two key pieces of data – the total drag of the airframe and pressures on various panels, all at a range of yaw angles and speeds. The pressure – and its distribution – over the glazing/door panels can be used to specify glazing thickness and the supporting structure. With accurate loads defined the structural aspects can be designed more efficiently, saving weight – always a benefit in an aircraft. An automotive company, making a replica E-type Jaguar, wanted to explore the addition of a rear diffuser to help improve the down-force and hence road holding of this classic 1960’s sports car. With over double the original’s horsepower installed, speeds of over 200kmh were being attained, so some updating of the aerodynamics was in order.

Amphibious craft launch simulation
One analysis that sits squarely in between quantitative and qualitative is the investigation in to launching the ARC600 Amphibious Rescue Craft. Exploring a range of gradients and entry speeds to determine when the bow is inundated allows the envelope of safe operations to be entered in to the owner’s manual. While this study was done retrospectively, the ability to define the safe limits for slope and speed for another amphibious craft will inform the design at the early stages and help define sheerline/bow freeboards early in the process. We started with a 2D study to ensure the range of motions was as expected, then moved to a 3D study. The last image is a screen grab from a video of the craft launching as modelled.