Computational Fluid Dynamics (CFD) was originally developed in the 1960’s in the aerospace industry, primarily to predict aerodynamic forces on aircraft and to minimise drag forces whilst maximising the lift force. CFD analysis has developed hugely since this time due to advances in numerical methods and readily available computing power. As a consequence the technique has found much wider applicability in many industries including, automotive, marine, offshore, civil, hydraulic, waste water, environmental protection, building services, energy generation, manufacturing, pharmaceutical and sports science.
CFD modelling provides means of calculating the velocity, pressure and temperature fields in a region of space occupied by a fluid. Essentially the space is divided up into a larger number of small volumes (cells) defined by a three dimensional grid. The non-linear partial differential equations which govern the fluid flow are then solved numerically by computation. Because the region of space which is being simulated has been artificially separated from its surroundings, it is necessary to define the conditions the fluid experiences at these boundaries, (the boundary conditions) since this will influence the behaviour of the fluid inside.
In practice the process usually starts with a CAD drawing of the component or terrain to be simulated, which is supplied by the client. A computational grid (mesh) is then generated inside the region, with the highest concentration of cells located where the fluid is likely to experience the highest rates of change spatially and the boundary conditions are defined on the surface of the mesh. These could be as simple as a uniform velocity inlet or a known pressure at an outlet or a solid wall. The analysis is further defined by any specific physical models which are important to the fluids behaviour e.g. porous media, rotation and moving reference frames, heat transfer, free surface flow, turbulence models, multiple fluids. The fluid properties must also be defined in terms of density, compressibility, viscosity, specific heat capacity. Finally, the numerical method to be used is selected and the calculation is started. Typically, the solution will be computed in a few hours on a high end workstation or cluster of processes and the results are then extracted using elaborate post processing software.
CFD Post Processing
CFD analysis post-processing software is used to define contours of pressure, velocity and temperature in sections of the fluid and to identify streamlines or fluid particle path lines and to calculate fluid forces on any objects immersed in the fluid and their motion as a result of these fluid forces and other mechanical forces. At this point insightful videos can be produced showing how the fluid and immersed bodies evolved over time, allowing access into areas of the flow which would not be possible experimentally without interfering with the flow field. As part of the CFD analysis process further checks are carried out to assess sensitivity of the results to mesh spacing and time step size or any simplifying modeling assumptions.
Why use CFD
CFD analysis is one of many CAE tools used widely in industry today, providing the engineer with the ability to assess different design options without having to resort to expensive prototype production and testing, which is not without its own difficulties and compromises. There are also situations where scaling laws cannot be consistently applied and constructing a full scale prototype is impractical. Also where experimental measurements would be intrusive, or present unacceptable risk. As computing power has expanded, CFD modelling tools have become an increasingly integrated part of the design cycle in many industries enabling an increase in insight and the optimisation of fluid flow based technology in reduced timescales with reduced costs.