The purposes of this experiment are:
1. To determine the shape of the wake behind the cylinder.
2. To determine the water tunnel calibration.
Both of these objectives were accomplished by using LDA (Laser Doppler Anemometry). LDA is one of the main velocity measurement methods used in professional experimentation. Light beams are shot from a laser onto flowing water. In objective one, a cylinder was submersed in the water flow to determine how the velocity aft of the cylinder was disturbed. While the second objective used the LDA on flowing water with no disturbances.
This LDA system is an accurate system. However, every system has some kinks that are sources of error in the given results. Particle averaging bias is the first of these errors. This bias states that when the velocity is high the mean flow velocity well be larger than the actual value. This occurs because more particles’ velocities are being calculated than in a slower flow. Another source of error is called velocity gradient broadening. This error comes from the fact that two particles on different positions in the gradient will have different velocities but end up in the same measured volume. This will of course give a velocity variance where there should not be one. Lastly, there is an error called finite transit time broadening. This error occurs because the LDA system collects data using signal bursts, which will see fluctuations every burst even though the flow velocity is constant. Even though, these three errors and more are observed when using an LDA system, it is still one of the most accurate systems that is used to calculate flow velocity.
The first objective was examined by taking a series of data points with the laser configured to measure flow velocity in both the horizontal and vertical directions. These data points were all aft of the cylinder and were measured from the back center of the cylinder (Figure 1). The data points were taken at .005 ± 0.00254 meter increments in the y-direction and .01 ± 0.00254 meter increments in the x-direction in order to observe the wake created by the cylinder. All of these measurements were taken using a ruler which was placed on the test section.
The second objective was tested by varying pump speed and recording horizontal flow velocity to observe whether a linear relationship is obtained between pump speed and flow velocity. The horizontal velocity was observed using LDA with no disturbances (i.e. cylinder) with increasing increments of pump speed. The pump speed was measured in Hertz and is decreased in increments of 5 ± 0.5 Hz. Flow velocity is recorded at each pump speed. The uncertainty calculated for these measurements was ±0.001 m/s.
In this report, the apparatus and instrumentation used in the experiment will be thoroughly discussed. Results will be taken and analyzed in order to discuss what was drawn from the experiments conducted. These discussions will create conclusions that resulted from this investigation....