Alistair Duff, for technical assistance Mr. Andrew Crockett, for strain gauge assistance
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- 8.3 FLUENT: ANALYSIS SETUP
- 8.4 FLUENT: SOLVING
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Figure 12 - Meshed Flow Domain The boundary conditions of the domain were then set but only after selecting the Fluent 5/6 option from the solver list to determine the boundary conditions available. The face that represented the flow inlet was set as a Velocity Inlet, and the opposite outlet face set as a Pressure Outlet. The flat plate upon which the wing mirror was placed was set as a Wall and given the identity ‘Flat Plate’. Similarly the wing mirror was set as a wall but given the identity of ‘Mirror’. This assignment of two different identities was to enable the two different components to be analyzed and viewed independently of one another when modelled in CFD. The three remaining domain walls were then specified as Symmetry faces; which means that flow can pass by them unaffected. The volume continuum type was selected as Fluid and named ‘air’. This was the last step in pre-processing and therefore all that was required was to export the mesh for use in Fluent.
With the mesh obtained from the pre-processing in Gambit, the initial conditions were set before the analysis could begin. The first issue that was addressed was the scale applied to the grid, as the mesh was created in millimetres in Gambit, but the default unit for length in Fluent is metres. Therefore the scale command was used to scale the model down to the correct size as created in Gambit. As already mentioned, the solver was set as steady state and pressure based. It may have been useful to model the flow as transient but due to time and computational restrictions this was not feasible. The turbulence model selected was the standard k-ε model. This model is one of the most commonly used turbulence models in industry, due to its speed and simplicity of use REFERENCE. The model employs two transport equations: one for the kinetic energy of the turbulent flow (k) and another for the rate of dissipation of the turbulent flow (ε). For the purpose of aeroacoustic analyses, the Broad Band Noise Sources model was selected. This was the only option available for aeroacoustics due to the analysis being run as steady state. This model uses the values obtained from the turbulence model for such things as the mean velocity components, mean pressure, turbulent kinetic energy and so on. It then uses these values to determine the broadband noise present in the model. The Broad Band Noise model is limited in its usefulness as it only determines the sources of broad band noise but cannot determine sound propagation. As such, it is often employed as a qualitative method of assessing the ‘noisiness’ of designs. The boundary conditions had then to be set and this involved setting the wind speed through defining the velocity at the inlet. For the purposes of this explanation, the velocity at the inlet was set to 13.4112m/s (or 30mph) and the analysis was then initialised from the inlet with the speed defined as 13.4112m/s. This process produced a known value as a starting point for the solving process to work from. The convergence criterion for the solution was set to the default of convergence to 10-3. This meant that during the iterative process of solving the flow over the wing mirror, the residuals had to converge to 10-3; which was deemed a suitable level of accuracy. The real-time convergence of the residuals were set to be displayed during the solution process; this would make it possible to observe if the solution was nearing completion or in the worst case diverging. Before the analysis could begin, reference values had to be set for the calculation of forces and drag coefficient. The wing mirror was used as the reference and values of 0.0375m2 and 0.1m were set for the cross sectional area and object length respectively. The velocity of 13.4112m/s was also entered for reference. 8.4 FLUENT: SOLVINGAs mentioned previously, Fluent uses an iterative process to solve problems, so accordingly the software was set to perform 200 iterations; with the solution expected to converge before this number was reached. The software was then instructed to iterate, and thus the solving process began. A plot of the residuals converging is shown in Graph 1. 
Graph 1 – Residuals Convergence With the solution achieved Fluent was then used to print the pressure and viscous forces and their sum total acting on the wing mirror plus the pressure and viscous coefficients. These results were then recorded, along with the values obtained from the analyses of all other speed increments. Download 3.93 Mb. Share with your friends:
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