Design engineers drive CFD development

Paul Boughton

Computational fluid dynamics modelling has come a long way since its entrance into engineering industry over 20 years ago. Increases in the power of computer hardware, in tandem with ever decreasing hardware costs, have allowed CFD software tools to be developed to offer the high-tech capabilities we see today.

Computational fluid dynamics (CFD) is now routinely applied to design processes in all manner of industries where the ability to use simulation to visualise the flow physics for various design scenarios, has saved time and reduced costs.

The case for using CFD has been won but understanding its benefits, and being able to do something about it, are two different things. A persuasive reason for applying CFD is to quickly identify the most promising design candidates for further development. This therefore implies that CFD modelling should be applied as early in the design process as possible. However, the reality is that CFD has tended to only be applied to more complex problems further down the design chain. The main reason for this has been training. CFD is a relatively complex software to use; it has been necessary for trained ‘experts’ to use the software to ensure its proper application to achieve reliable and accurate results. Quite simply, there aren’t too many expert users around and for some companies, where alternative research methods are available, although CFD may be the best method, the costs of employing an expert user to develop an in-house CFD capability have not stacked up.

FloWizard has been specifically designed to address this problem. Developed by Fluent, it has been designed for use by the non-expert user. It allows a company, employing design engineers who have no knowledge of CFD modelling software, to perform simple CFD simulations for use in the initial stages of the design cycle. FloWizard uses the very latest technology to intelligently work out many of the parameters for input, enabling the non-expert to perform simulations. It is the world's first flow simulation tool to incorporate a comprehensive wizard-driven interface, taking the user right through the entire simulation process from initial CAD import to problem set-up and solution stages, through to post-processing, visualisation and animation, plus it generates an automatic results report. At every step of the way, the wizard employs everyday English terminology, avoiding technical speak to explain choices to be made.

One case study that illustrates the use of FloWizard was a project carried out to look at the operation of a motorbike muffler. With the aid of the porous media modelling capability in FloWizard, it was possible to recognise the effects of such media on the localised flow effects, such as separation downstream of the muffler, in contrast to evaluating the overall pressure loss through the exhaust.

The first step to running the FloWizard simulation was to define the units to be worked in. In this case, inputs in centimetres were used. Then the CAD file was imported. This can be done either directly (natively from CATIA) or indirectly; here, the CAD geometry of the muffler was imported from Fluent's GAMBIT pre-processing software tool but geometries can be imported from a variety of the most commonly used CAD packages. In this case it was the solid region that was imported so that the fluid region could be automatically extracted by FloWizard for the analysis.

The CAD geometry was then cleaned up for the simulation to be run (for example, to remove the features that would not impact the flow) but this was easily done using the appropriate tools in FloWizard. Prompted by the wizard, the flow type was selected as being internal. For the non-expert user, the wizard defines internal and external flow in an easily understandable way, helping the user make the correct choice. As internal flow was to be simulated, again, prompted by the wizard, the boundaries were defined. The flow inlet and outlet were identified, and all other openings were capped off using the tools provided in the software.

Next, temperature was specified to be included in the study and the flow type was defined as being turbulent. If a user has any doubt or difficulty with this or any stage of the set-up, they can refer to 'Guide Me' windows, which give information to define what is being asked, ie, in this case, what constitutes turbulent and laminar flow. This is done using text and graphics. If a user then enters any parameters that seem to contradict the simulation characteristics inputted, the wizard questions the user as to how sure they are of their inputs. If the user is unsure as to, for example, the type of flow, an ‘Unknown’ option exists. Using the powerful underlying engine of the advanced FLUENT 6 software, FloWizard will work out the flow type automatically when creating the solution.

The flow regions were then defined, selected from a database of fluids available for reference. Next, the conditions at the boundaries - that is the pressure, velocity, mass flow rate, etc at the inlet and outlet as well as symmetry planes were determined. Before then beginning the solution, a preference must be given along a sliding scale for a faster solution or a slower, more accurate solution (depending on whether the user is looking for a general idea of flow behaviour as part of a conceptual design study, or a more complex, in-depth analysis representing a final design). In this case a medium bar setting was chosen. The summary, automatically provided by FloWizard, of all inputted parameters was checked over to ensure they were accurate, and then the solution stage was initiated. Unlike when using ‘Expert’ user flow modelling tools, there was no need to mesh the geometry as FloWizard automatically meshes geometries prior to the solution being calculated. With additional meshing features such as the option to incorporate prismatic boundary layers on walls, as done so here, extended options provide flexibility in the grid structure. FloWizard also provided the option of monitoring the progress of the solution once the calculation stage had begun, allowing the early examination of initial results before the final solution was arrived at.

The results were then analysed using the range of post-processing tools incorporated in FloWizard. These allowed design engineers to interrogate the results for pressure, flow velocity, temperature, etc along planes, iso-surfaces or particle traces. All were provided via 3-D images which could be manipulated to inspect every angle of the geometry/geometries modelled, with animations also available to see the flow in action. These images were also available for capture for further study or for use in the report that is automatically generated, summarising the physics arrived at in the simulation. These reports can be saved in all manner of formats for a variety of needs.

The study concluded by evaluating the overall pressure drops observed with various muffler designs having differing porosities. This assisted in understanding the air flow characteristics within such automotive ducting and in particular, the localised effects in the muffler region. These latter effects would be difficult to ascertain using test methods alone, and demonstrate the value to be derived from applying a CFD tool such as FloWizard.

Although not used in this project, FloWizard does incorporate a number of additional features which are important to its ability to add value to the design process. Many companies may have a very specialised application for flow simulation. A huge benefit with FloWizard is that it can be fully customised to any company’s needs. This can produce a tool which only asks the relevant questions for a particular analysis, for example, allowing each company to produce their own bespoke software tool. This ability further enhances the usability of FloWizard.

As FloWizard is based on FLUENT 6, a live peer-to-peer collaboration facility has been created, allowing an expert using FLUENT 6, or another more experienced engineer using FloWizard, to join the set-up of a simulation at any time and from anywhere via a secure link on the internet. This is unique to FloWizard. This also means that, should a very complex simulation be required that is beyond the powers of FloWizard, a problem can simply be passed on to a FLUENT 6 user, whether in-house a consultant, for further analysis.

Enquiry

Mark Keating is Senior CFD Engineer with Fluent Europe Ltd, Sheffield, UK.
www.fluent.com

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