Purpose: We were asked to study the aerodynamics of a stock car and compare the results to other types of cars.

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1 To: Professor Anderson From: RJ Hojnacki, Wes Wall, Sam Caruso Date: 2/29/13 Subject: 1/12 th Scale NASCAR Pressure Test Findings Purpose: We were asked to study the aerodynamics of a stock car and compare the results to other types of cars. Findings: From wind tunnel testing our results agreed with intuition. At the nose of the car there was a high pressure point. As we moved up the hood the pressure decreased and the velocity of air passing along the surface increased. We encountered another stagnation point about mid-way up the windshield. After this stagnation point the pressure significantly lowered as over the roof of the car. As we moved down the rear windshield the pressure began to increase again until another stagnation point was reached near the bottom of the windshield. Over the rear hood pressure began to lower again and continued to decrease at the rear bumper. We would also expect that the C P at any wind tunnel speed would produce a similar curve since it is a non-dimensional number; our data from overlaps quite well for the two different velocities. Figure 1: Pressure Coefficient (Cp) at the given pressure tap positions on the car for frequencies varying from 20Hz to 40Hz. See attachment 4 for details on manipulation of point 12. I would recommend running the wind tunnel at higher velocities to study the pressure coefficient because there is lower uncertainty associated at higher speeds. At a velocity of approximately 15.2m/s there is an average uncertainty in C p of ±0.155; at a velocity of approximately 32.5m/s there is an average uncertainty in C p of ± Experimental Setup: The pressure transducer was connected to the computer so we could extract voltage readings from the device. With Tygon tubing the Pitot probe, used to measure tunnel speed, was attached to a transducer. Additionally, 14 pressure taps, which ran down the center line of the model, were attached to the transducer. At each tap 100 pressure readings were taken. The wind tunnel test speeds were 15.2m/s and 32.5m/s. See attachment 6 for more detail. Analysis: The pressure coefficients displayed in Figure 1 offer valuable insight into the aerodynamics of the stock car. The pressure coefficient offers a relative magnitude of the pressure force on the vehicle. From the pressure force we gain insight into the velocity of air moving over the surface of the vehicle. The pressure force and velocity of a particle at that point are inversely related.

2 For stagnation points, where the velocity of the air moving over the surface is approximately zero, the pressure is very high. At stagnation point C p should approach 1 and should not exceed it. It is unknown what caused the C p to be greater than one at the front grill and these results shouldn t be included in further analysis. At a point where the surface pressure approaches the free stream pressure (Pitot probe pressure for our setup) C p should approach zero. Working from front to back of the car (left to right on figure 1), we can explain the findings. At reference location #1, the pressure tap located on the front of the car, C p is high because air does not flow smoothly past this point. There must be something wrong with the pressure tap on the front grill because C p should not exceed 1, it may have been the hose or the tap itself. As we move up the hood C p begins to decrease until about half way up the hood. Next, it begins to increase until we reach what we can assume to be another stagnation point near the base of the windshield (ref# 6). After this stagnation point the surface pressure decreases until it reaches a minimum on top of the car (ref# 8). This makes sense because the profile of the roof of the car allows air to flow over it with minimal interference resulting in a low surface pressure. As we continue down the back of the car C p continues to approach one which leads us to believe that there is another stagnation point (ref# 12). This stagnation point could be a result of the spoiler. At the rear hood, the pressure is slightly decreased. At this point we have a standard deviation of 15.2m/s and 32.5m/s; this may have been due to a leak in the hose or a problem with the pressure tap. I would not contribute this to separation of the flow because the first 13 of the 100 data points are removed, then the remaining values are much more stable, proven by a lower standard deviation of 5.99% and 6.09% respectively. Finally, on the rear bumper the pressure still continues to decrease. To find the uncertainty in C P the uncertainty in velocity, the uncertainty in tap pressure, and the uncertainty in density were considered. First the uncertainty of the wind tunnel velocity was computed. To find the uncertainty in the wind tunnel velocity, the uncertainty from the Pitot probe measurement and the uncertainty from the density calculation had to be considered. The percent uncertainty in the wind tunnel velocity is driven by the uncertainty in the density calculation. To find the uncertainty in C P the uncertainty in tap pressure, the uncertainty in density calculation, and the percent uncertainty in velocity were considered. The uncertainty in C P was driven by the uncertainty in the tap readings as well as the uncertainty in the wind tunnel velocity measurement. Conclusions: From our surface pressure data we can reach a few conclusions. There are stagnation points near the front grill, the bottom of the wind shield, and the bottom of the rear windshield. At these points the speed of air moving on the surface of the car is very small. On the roof of the car we see the lowest C p values. Therefore, at these pressure taps the velocity of the air moving over the surface is greatest. At the lower wind tunnel speed (15.2m/s) our uncertainty in each C p measurement was significantly higher than they were for the higher wind tunnel speed (32.5m/s). Therefore I would recommend using the higher frequencies for studying the pressure coefficients. Attachments: 1, 2 All Relevant Data for 15.2m/s test speed and 32.5m/s test speed. 3 Uncertainty Analysis for velocity and pressure coefficient. 4 Analysis of pressure tap 12 and the exclusion of some points. 5 Image of car with labeled pressure taps. 6 Experimental Setup 7 Schematic of the pressure tap system.

3 Attachment 1 Table 1: Relevant pressure tap data at wind tunnel speed of 15.2m/s Tap Wind Tunnel Speed (15.2m/s) Location Distance from Front ΔP (Pa) δp Cp δcp Table 1 includes all relevant results from the pressure taps taken at a wind tunnel speed of 15.2m/s. Figure 2. Pressure at each tap with certain locations on Nascar racer indicated when the wind tunnel is at 15.2 m/s. Note how in Figure 2 the pressure ranges from about 260 Pa at the front grill of the car to a minimum of Pa (reference # 8).

4 Attachment 2 Table 2: Relevant pressure tap data at wind tunnel speed of 32.5m/s Tap Wind Tunnel Speed (32.5m/s) Location Distance from Front ΔP (Pa) δp Cp δcp Table 2 includes all relevant results from the pressure taps taken at a wind tunnel speed of 32.5m/s. Figure 3. Pressure at each tap with certain locations on Nascar racer indicated when the wind tunnel is at 35.2 m/s. Note how in Figure 3 the pressure ranges from about 1200 Pa to about 620 Pa (reference # 12). Also, note the difference at location reference 8 for the pressure taken at 15.2 m/s in Figure 2 where the pressure is a minimum, and the pressure taken at 32.5 m/s above, which is around the maximum pressure.

5 Attachment 3 Calculation of uncertainty in pressure coefficient: First, the uncertainty in the density had to be calculated. We concluded that the uncertainty of the density was about 3% of the density, so δρ = 1.2*(0.03) = Next, the percent uncertainty in the velocity was derived from the equation for the velocity Next, the percent uncertainty of the velocity had to be calculated using the equation (( ( )) ( ( )) ) This value was then calculated for each pressure tap. The percent uncertainty in the pressure coefficient was derived from the equation for the pressure coefficient Next, the equation for the percent uncertainty of the pressure coefficient had to be derived, as shown below. (( ( )) ( ( )) ( ( )) ) The constant values of 1, -1, and -2 correlate to the exponent on the variables from equation (3). Since we are looking for just the uncertainty in the pressure coefficient, equation (4) had to be multiplied by C P, as shown below. (( ( )) ( ( )) ( ( )) ) ( ) The calculation in equation (5) was carried out for each pressure tap.

6 Attachment 4 Figure 4: Voltage output versus data point number for pressure tap 12. Figure 4 shows the data collected at pressure tap 12. For both wind tunnel speeds the pressure trends seem similar. Since the flow was fully developed for both readings were started, it s hard to say that some fluid dynamic phenomena is taking place. Rather, it leads me to believe that something is going on within the experimental setup. It may have been that this channel in the transducer has something wrong with it. This also could mean that there is a problem with the actual pressure tap. With either case we can see that past the 13 th data point (denoted by X) the values seem to be more consistent throughout the rest of the readings. So, for the calculation of our average pressure for tap 12 the first 13 points were not included. This lowered the standard deviation of the transducer readings from 12.5% to 6.09% and 25% to 5.99% for 15.2m/s and 32.5m/s respectively.

7 Attachment 5 Figure 5: Car Tap Schematic

8 Attachment 6 The pressure transducer was connected to the computer so we could extract voltage readings from the device. We used tygon tubing to connect the Pitot probe and the 14 pressure tap lines to the transducer. More information about the pressure taps can be found in attachment 7. At each tap 100 pressure readings were taken at a motor frequency of 20 Hz, resulting in a wind speed of 15.2 m/s. The pressure reading for each tap was taken again, this time at a motor frequency of 40 Hz, which gave 32.5 m/s in wind speed. The pressure transducer recorded a voltage, which was used to calculate the pressure difference on the Pitot probe and pressure taps. The pressure difference on the Pitot probe was used to find the values for wind speed away from the Nascar model in the wind tunnel. Figure 6: Schematic of Pressure Tap System The schematic of the wind tunnel setup showing the position and direction of the Nascar model in the wind tunnel. In actuality, the model was attached to the dynamometer (for support only), but for the sake of clarity it is shown on the floor of the wind tunnel. (original schematic courtesy of Rob Powell, ME 09).

9 Attachment 7 Figure 7. Model of pressure tap. Figure 7 shows a model of the pressure taps placed along the center line of the model. These pressure taps were made by drilling small holes in the surface of the stock car model and gluing a suction cup around the dilled hole. As you can see, tubing runs off each suction cup that will attach to the pressure transducer to get a surface pressure reading. Each Tygon tube is numbered, indicating a pressure tap number that is corresponds to. The tubing passes through the bottom of the car and out a hole in the bottom of the wind tunnel. We affirm that we have carried out our academic endeavors with full academic honesty. [Signed, RJ Hojnacki, Wes Wall, Sam Caruso]