Optical engine lab
An optical engine lab basic purpose is to measure the flow of liquid. It does this by using the following techniques.
- Particles image velocimetry (PIV)- measure of the flow of liquid.
Particle image velocimetry offers many advantages for the study of fluid flows. Lots of PIV techniques and PIV systems had been developed. However there are no standard evaluation tools for the effeteness and accuracy of PIV systems
- Laser- induced fluorescence (LIF)- is the optical emission from molecules that have been excited to higher energy level by absorption of electromagnetic radiation
Semi anechoic chamber
The increasing demand on decreasing exterior noise and interior noise required by law and due to new tools and facilities they are able to meet the various demands of their customers. Car noise is usually measured by driving past a bank of microphone and recording the resulting sound level. For many years the convention has been to use an open-air track, but not surprisingly the ideal weather conditions- wind below five miles per hour and perfectly dry road- very rarely occurs. Legislations recently enacted by the EC limits noise from new cars to no more than 74dB at 10m. this placed even tougher demands for good weather conditions so test could be carried out in a more controlled environment. Land rover group answer involved building a huge semi anechoic chambers in which cars could be driven at various speeds and the resulting noise output recorded for analysis.
Using special test vehicles to carries out exterior noise measurements, which makes it possible to study the contribution of each sound source like engine, exhaust, air-intake and tires noise. For theses investigations to take place they are equipped with a semi anechoic chamber.
The semi anechoic chamber provides and environment where there is no reflection from the walls. This means that the semi anechoic chamber does not affect the sound given off by the car.
In the semi anechoic chamber for the test to be carried out the complete vehicle is either tested at rest or in driving conditions on a roller test bench. On a roller test bench cars can then be driven in multitudes of ways- hard acceleration, cruise or deceleration etc- while banks of microphones positioned on both sides of chambers which are 2.5m in height, that is the average height of a man, firing the microphones consecutively and at a rate linked to the car’s road speed creates a noise profile which can then be investigated on the computer.
Sound waves produced are standing waves.
- Points between 2 node oscillate in phase
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Two antinodes are λ apart.
2
- are produced by identical waves traveling in opposite directions.
The car is places on a hard reflecting surface and the sound emitted by the cars is measured at these microphones placed around it.
An accurate method that determines sound level is when the microphone is transverse at a constant vertical speed. The error caused by the interference between the direct sound waves and the reflected sound waves reaching the microphones or minimize by this method, compared to the method using fixed microphones positions.
It is similar to the anechoic chamber. It has wedges on the walls and the ceiling but not on the floor. The wedges are to absorb sound and prevent any reflection of sound.
The wedges are around 1.83m, this would provide quite good absorption of sound waves but for effective absorption the wedge should be around 2.83m.
Foam absorbers / λ of absorbers
2
Speed of sound = 40m/s
For effective absorbers:
V=f λ
340 = 60
= 5.67m = 2.83m
2
Wedges to be made 2.83m would be to big and to expensive.
Using slick tires eliminates tires noise. There are no treads therefore no0 compressions of air in the treads.
The sound absorbing perforated metal wedges offer practical advantages over those made from conventional fiberglass or foam - these include impact / fire-resistance, cleanablility, light reflection, repaintability and durability.
