Introduction
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Because of strong turbulence and its unsteady nature,
pressure fluctuations varying in time can be observed in such flows as the
one investigated. When the pressure fluctuations reach certain level and
their oscillations are within the region of frequencies of audible sound
(20Hz – 20kHz), then the aeroacoustic noise can be registered by the human
ear. Of course the noise level (measured in dB) and its frequency are very
important aspects of cars design. Therefore the initial task is performing such a
CFD simulation, which will be able to provide accurate time dependent
pressure data for further analysis. As it turned later out, this can be very
complicated and time demanding. The reasons for this are in detail described
in the Project Final Report in section V.5.4.
Because of these very high demands on computational power, a simple 2D model
of cylinder in the airflow was simulated to test the features of the software
first. After this the analysis was performed for a much simplified 2D car
geometry Fluent 6.1 software package, Gambit 2.0
preprocessor for creating the mesh and Acoustic module 1.0 were used for the
simulations. These were performed on a single DELL Pentium4 2.0 GHz 512MB DDR
SDRAM computer. The parallel version of Fluent was not possible to use
because of the network restrictions. The theory, which is used in the Acoustic module is usually denoted as Lighthill’s Analogy. This is again more in depth discussed in the Final Project Report sections II.1and II.2. Large Eddy Simulation was used for the analysis, because it is able to capture the time variable fluctuations together with the large eddies, which have a significant effect on the fluid flow. |
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