Lea CFD Associates Ltd.

Introduction to CFD

Background

Industrial CFD simulations have been in existence since the early 1970’s. However use of CFD was initially restricted to large organisations, such as the aerospace and nuclear sectors, by limitations in compute power. The 1980’s saw the development of a number of commercial CFD packages. The rapid increase in the power and affordability of desktop workstations in the 1990’s, coupled with numerous CFD software improvements, led to rapid growth in the uptake of the technique.

Today CFD is used across all industry sectors. Applications range from the aerospace, nuclear and automotive sectors; through manufacturing, chemical and process sectors; to the pharmaceuticals, biomedical and electronics industries. Health and safety applications of CFD can be found in many of these industry sectors. It is also widely used for environmental flows and civil engineering applications.

Basis

Industrial CFD software packages are typically based on a discretisation of the fluid continuum. Flow variables, such as velocity, or temperature, are computed at discrete locations. These locations are defined by generation of a grid, or mesh, of cells which fill the fluid space – known as the computational domain. The shape of the computational domain, which includes objects represented in the CFD model, is typically called the ‘geometry’.

In this example the computational domain is the interior of a forced-ventilated warehouse. The flow is driven by an exhaust fan at roof level. The main flow into the domain is at the top and bottom of doors at each end of the warehouse. The warehouse contains high rack storage, chemical drums in transit, store vessels and an office.

Solution

A discrete form of the governing equations of fluid motion are constructed, based on control volumes which are either coincident with the centroids or the vertices of the mesh. In either case, local, and hence global, conservation of flow properties such as mass and energy, is sought. Boundary conditions are then applied. The set of numerical equations is solved, typically in iterative fashion, until a defined level of ‘convergence’ is met – which simply means that the iterative solution has converged to the exact solution of the numerical equations.

Post-process

The numerical solution is then post-processed. Usually this is done with graphical tools, which allow visualisation of the solution. Of course numerical values can also be extracted from the solution.

Illustration of the flowfield in the warehouse

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Real Media Format Animation 1.5 MB	Windows WMV Format Animation 1.2 MB

Pros and Cons

CFD provides a cost-effective means to: assess and control health and safety risks; to analyse, demonstrate and improve the performance of industrial designs; to trouble-shoot manufacturing and process problems. It is a very powerful tool for the simulation of fluid flows.

CFD is a knowledge-based activity. It still relies heavily on the expertise of the user. It is particularly important to understand its bounds of applicability and the consequences of limitations in physical and numerical sub-models.

CFD resources

A large number of CFD resources are available on-line. http://www.cfd-online.com
is a useful site for further background and information on CFD.

Introductory guides on CFD can be obtained from NAFEMS, at http://www.nafems.org.