Alistair Duff, for technical assistance Mr. Andrew Crockett, for strain gauge assistance
AERODYNAMIC NOISE 4.1 SIGNIFICANCE & SOURCES
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- 4.2 NOISE REDUCTION
- 5 DEBRIS/DROPLET SHEDDING
- 6. COMPUTATIONAL FLUID DYNAMICS
- 6.2HISTORY
- 6.3 ENGINEERING APPLICATIONS
- 7. WIND TUNNEL TESTING
4. AERODYNAMIC NOISE4.1 SIGNIFICANCE & SOURCESAs aforementioned, the significance of aerodynamically produced noise has increased in recent times due to the development of quieter power plants and transmission. Furthermore, with the introduction of hybrid vehicles and the prospect of completely silent fuel cell engines, the importance of limiting any aerodynamic noise generation will prove even greater. The creation of aerodynamic noise from wing mirrors results primarily from the vortex shedding at the A-Pillar location which travels downstream from the wing mirror and strikes the side window. This vortex shedding on the window creates pressure fluxes and consequently produces noise which can be heard from inside the cabin, causing the driver or passenger inside some degree of discomfort. Predicting or understanding the aero acoustic characteristics of wing mirrors can often be extremely difficult due to their complex geometry. It is also made difficult due to the fact that it is the interaction between the A-Pillar, the wing mirror and the door window that creates the noise recognisable to the driver/passenger inside the vehicle. It is for this reason that modelling or measuring the aerodynamic noise created by the wing mirror alone, will only help to identify the sources of noise from the wing mirror.
Water and dirt build-up on the glass can pose a risk to passenger and driver safety due to the locality of the build-up near the mirror. Water accumulation in particular can significantly reduce the clarity at which the driver can view the wing mirrors through the window. Some automobile manufacturers have devised methods and designs with the aim of preventing the accumulation of water and debris. One such method is to alter the shape and design of the mirror mount in such a way that there is a gap created through the wing mirror mount centre. This results in a wing mirror design that is, in essence, fork mounted. The rationale behind this type of design is that it reduces the ‘bluffness’ of the shape and allows high velocity flow to pass through it, thus resulting in a higher velocity flow aft of the wing mirror. This higher velocity flow should then, in theory, transport any water droplets or debris shed from the wing mirror further downstream where it can then strike the side of the vehicle in a safer location away from the mirror. This setup can also contribute to reducing the wing mirror’s pressure drag due the reduction of flow stagnation on the leading face of the support and consequently the reduction in stream-wise pressure difference across the support.
6.1 FUNCTIONComputational Fluid Dynamics software (CFD) offers a method of solving simple to very complex fluid flow problems through the use of computational processing power. CFD works by dividing up the fluid domain into numerous smaller control volumes, linked together in a mesh. It then employs the Navier Stokes equations to relate the fluid properties (such as flow velocity and pressure) from each control volume to its neighbouring control volume. These equations are then solved by an iterative process, until a level of convergence is achieved and thus an accurate solution is provided. The solution obtained (and therefore the behaviour of the flow) can then be shown in graphic visualisation on screen in numerous types of plots, displaying the various fluid properties involved. 6.2HISTORYThe application of CFD technology within the engineering industry has increased rapidly over the last twenty to thirty years; with the science behind its method having existed long before this time. This increase has been brought about primarily by the rapid growth and affordability of computational processing power transferring the software codes from large corporate and governmental supercomputers in the 1970’s down to the desktop PC’s of even the smallest firms today. 6.3 ENGINEERING APPLICATIONSCFD software allows designers to produce and test new designs on a timescale that is significantly shorter than that of physical prototyping and testing. The results and visualisations can show the flow characteristics in localised parts of the design which could not be identified as accurately through wind tunnel testing. These visualisations can then in turn be used to identify the strengths and weaknesses of the design and thereafter amendments and improvements can be applied. The altered design can then be analyzed to study the influence and effectiveness of any change, and thus an optimized design can be obtained. The use of CFD technology can also be applied to the study of aeroacoustics as the values of noise generation can be directly derived from the equations of fluid flow. The software can help determine sources of high aerodynamic noise production, and can be used to predict the nature of sound propagation in a system. 7. WIND TUNNEL TESTING7.1 CLOSED RETURNFor the purpose of physically testing the wing mirror a closed return wind tunnel with an open working section was used. The diagram in Figure 4 illustrates a typical closed return wind tunnel layout and is similar to the one used in this study. The system works by continually circulating the air around the tunnel, by drawing the air through a diffuser after travelling through the working section. 
Figure 4 - Closed Return Wind Tunnel (http://www.mi.uni-hamburg.de/uploads/) The fan is typically placed in a smaller cross-sectional area region in the tunnel to increase the flow velocity over the blades and thus increase the efficiency. This is also beneficial when considering that the costs associated with fans is proportional to the diameter squared [6]. Some wind tunnels employ counter rotating fans to reduce the rotational flow behaviour imparted on the airflow by the standard single fan type. However, it is more common to use a single fan and then introduce anti-swirl vanes downstream to reduce swirl. The air propelled by the fan then flows through two sets of 90˚ turning vanes which direct the air into the settling chamber. The airflow then passes through a contraction cone, which imposes a reduction in cross-sectional area on the flow and thus results in flow acceleration into the working section. When the air has passed through the working section it then flows into the downstream diffuser which is used to decelerate the airflow as quickly as possible in an aim to recover the static pressure and reduce power losses in the boundary layer which are proportional to the velocity cubed [6]. Most wind tunnels also feature a second diffuser in the section downstream of the fan, parallel to the working section. Download 3.93 Mb. Share with your friends:
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