6. EXPERIMENTAL METHOD. A primary result of the current research effort is the design of an experimental

Transcription

1 6. EXPERIMENTAL METHOD 6.1 Introduction A primary result of the current research effort is the design of an experimental setup that can simulate the interaction of a windmill with a vortex wake and record data. As previously discussed, vortex generation proved to be impractical and the proposed solution to this problem is to simulate the key features of the vortex with opposing ducted fan flows. The equipment and setup required to achieve this are presented below. 6.2 Centrifugal Fans In order to create as uniform a flow as possible for ducting purposes, two centrifugal fans were purchased for use in the experiment. These types of fans have rectangular outlets with smooth edges to prevent excessive flow separation and a flange to facilitate the attachment of ducts. The specific type of fan that was purchased is the Dayton 4C565A Blower, shown in figure 6.1 below cm 9.5 cm Figure 6.1: Dayton 46565A blower, outlet (left) and side (right) views These fans are rated a flow rate of 435 CFM (Cubic Feet per Minute) and requires a 110 volt AC power source, meaning that it is compatible with common wall outlets. It would 35

2 be desirable to connect a variable speed control to the fans in order to achieve the best possible velocity profile for scaling purposes. A variable transformer, similar to the one shown in figure 6.2 below, may be able to control the velocity of air exiting the fan. Figure 6.2: Variable speed control 6.3 Windmill Model The windmill model that has been purchased for use in this experiment is an American Style windmill, similar in design to Robert Young s windmills, manufactured by the American Wind Power Center, shown in figure 6.3 below. Figure 6.3: American Style windmill model This model is approximately 4 ft tall and is fully functional, with the rotor disk capable of rotating freely and the rotor assembly capable of rotating about its vertical axis rotation shaft to weathervane into the wind. The specific dimensions of some individual 36

3 components of the model are shown in figure 6.4 below and more dimensions are presented in table 11.1 of the Appendix. 9.5 cm 37.8 cm 15.5 cm 15.8 cm Figure 6.4: Windmill model dimensions (figure not to scale) The only modification that was made to this model was the addition of a bushing to the yaw-axis rotation shaft in order to eliminate a slight wobble in its motion. This bushing is shown in figure 6.5 below. Figure 6.5: Yaw axis bushing 37

4 6.4 Experimental Setup and Procedure The windmill model has been outfitted with strain gages as described in section 5.4 and is therefore ready for use. All that remains is to connect a data acquisition system, which was not readily available to the current research team, to the strain gages. The centrifugal fans are ready to have the variable speed control wired to them and have the ducts, described in section 4.7, attached to them once the ducts are made. Once all those components are ready, the experimental setup shown in figure 6.6 below will be ready for construction. Centrifugal Fan D 1 Centrifugal Fan Duct Duct H 1 D 2 H 2 Rails Windmill Model Rails Ground Figure 6.6: Proposed experimental setup As seen in figure 6.6, the centrifugal fans will be affixed to armatures that will then be mounted via rollers to rails. The fans will have crossbars connecting them so that they 38

5 will move together along the rails, albeit at different heights above the ground. A gear or pulley system will be used to raise and lower the fans and this should ideally be connected to an electric motor so that the fans can be lowered simultaneously at a controlled rate in order to simulate the descent of the vortex as best as possible. Both the windmill model and the rail assemblies need to be affixed to heavy metal plates that will serve as bases in order to keep all the equipment well anchored to the ground. The height and distance parameters, H 1, D 1, H 2, and D 2, are the primary geometrical considerations for the experimental setup. The values of these parameters need to be determined by correlation with the results of mapping the ducted fan velocity field in order to yield an optimum vortex core simulation. The quantity H 1 - H 2 will have to equal the vortex core diameter required, i.e. 12 in. The distance between the fans should probably be as small as possible to minimize viscous dissipation, but this should be investigated further. Once all of the components described above are built, the experimental procedure can be carried out. It consists of activating the fans at the desired speed while they are well above the windmill model s height on their rails, placing the windmill model on the ground at the correct point between the fans with the data acquisition system connected and activated, and then lowering the fan assembly along its rails at the correct rate until both the lower and upper airflows have translated completely through the windmill model. As the airflows from the fans impact the windmill, the windmill will respond, and this response will be recorded by the strain gages in terms of induced strains. Digital video recordings of the motion of the windmill model in response to the simulated vortex will also be recorded. 39

6 6.5 Data Reduction and Analysis The experiment outlined in section 6.4 should be run multiple times in order to verify the consistency of the results. These results will be in terms of measured strains in the windmill structure at the points where the strain gages are attached. The measured strains can be converted to bending moments and the results can be scaled to approximate the bending moments and strain magnitudes that would be experienced by Robert Young s windmills under the same loading conditions. A structural analysis of the windmill will indicate whether those strains and moments place the structure in danger of failure, and this will provide a basis for assessing the damaging capacity of the vortices. For comparison to the real cases, the recorded strains and moments should be compared to the non-dimensional aerodynamic scaling parameters discussed in section 4.9. The results are expected to look like figure 6.7 below, which shows what a plot of strain or bending moment vs. Reynolds number or tip speed ratio. Figure 6.7: Example plot of strain and bending moment vs. Reynolds number and tip speed ratio 40