Aerodynamics of a Mercedes-Benz CLK MER 331: Fluid Mechanics Written By: Zacarie Hertel Lab Partners: Evan States, Eric Robinson 2/19/2013 “I affirm that I have carried out my academic endeavors with full academic honesty.” Signed: Zacarie Hertel, Evan States, and Eric Robinson
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Aerodynamics of a Mercedes-Benz CLK MER 331: Fluid Mechanics Written By: Zacarie Hertel Lab Partners: Evan States, Eric Robinson 2/19/2013
“I affirm that I have carried out
my academic endeavors with full
academic honesty.”
Signed: Zacarie Hertel, Evan
States, and Eric Robinson
Hertel 1
To: Professor Anderson-Lab Instructor
From: Zacarie Hertel-Student
Lab Partners: Eric Robinson, Evan States
Date: 19 February, 2013
Subject: Pressure coefficients of the Mercedes CLK model RC Car
The purpose of this memo is to show the results of the aerodynamics of a model car, a
Mercedes-Benz CLK. A 1/12 scale model of the car was placed in the wind tunnel, and pressure
measurements over the length of the car were taken at two separate velocities. From this data, it
was possible to find the pressure coefficient at each location, which varied from values between
1.52±0.15 and 0.099±0.003, with the maximum value occurring at the flat surface at the car’s
grill, and the minimum value occurring at the beginning of the roof’s flat surface. The increase in
velocity showed little to no change in the pressure coefficient at each location; however it is
recommended that more tests be run at different velocity values to get more accurate and
conclusive results.
Experiment
In this experiment, a 1/12 scale model Mercedes-Benz CLK was put into a wind tunnel.
Pressure taps were drilled into the model at key locations, so that the pressure at each point could
be measured. The locations of these taps can be found in the attachments, with the approximate
locations represented in Figure 1.The wind tunnel was set to two speeds, at a motor frequency of
20 Hz and 40 Hz, approximately 14.46±0.37 m/s and 30.98±0.49 m/s respectively. At each wind
speed, the pressure measurements were made at each location on the car in comparison to the
free stream pressure from the pitot probe using a multi-channel pressure transducer. Using this
data, it was possible to find the pressure coefficient at each location, valuable information that
can be used to scale the data to represent a full sized vehicle.
Experimental Results
The results of the experiment proved to be consistent across both of the velocities that
were used. In each case, the pressure coefficient was calculated at various points along the length
of the car. However, even with the increased speed, the coefficients remained nearly constant at
most locations. These results can be visualized in Figure 1, which plots the pressure coefficient
at each tap location along the model car.
Hertel 2
Figure 1: The pressure coefficient at the approximate location of the reading made on the model
car. Note that some uncertainty bars may not be seen due to the size of the data points.
The only points that show any major difference lie at the front of the car, which may be due to
the way the stream reacts to hitting a barrier at each speed. Since the boundary conditions are
changing with each velocity change, it is possible that it these two locations will keep changing
during further testing. More tests would be needed to see if this is the case.
Uncertainty Analysis
Overall, most of the uncertainties from the experiment seem reasonable, with the
exception of some of the data points in the 20 Hz pressure coefficient measurements. The
maximum uncertainty in velocity was 2.5% of the data, a result that is within experimental
uncertainties, and compares to known values. For the pressure coefficients, the uncertainty seems
greatly diminished in the higher velocity test. This is most likely due to the way the uncertainty
is calculated, which would be larger with lower pressure values. At the windshield base in the 20
Hz test, there is an uncertainty of over 50%. This may be due to the way the flow travels over
this portion of the car at this speed, since the pressure is nearly zero. The transducer does not
perform well near zero, so this value is expected. However, this point, as well as all others agree
with the second 40 Hz test within experimental uncertainties, so all results can be interpreted as
valid and accurate.
Recommendations
In order to accurately define the pressure coefficient at each point along the car, it is
recommended that more tests be run at different wind speed velocities. More data points would
confirm or deny the results that were found, and give a larger data set to analyze the results from.
It would also be interesting to see the change in uncertainty in the experiment, as the uncertainty
seems to drop with increased wind speed. By running more tests, a more accurate solution could
be found, and the results could then be scaled to the full sized vehicle.
Hertel 3
Attachments
Attachment 1: Sketch of the experimental set-up. Page 4
Attachment 2: Summary table for the 20 Hz data. Page 5
Attachment 3: Graphical results of the 20 Hz test. Page 6
Attachment 4: Summary table for the 40 Hz data. Page 6
Attachment 5: Graphical results of the 40 Hz test. Page 7
Attachment 6: Calculations for each of the data sets. Includes pressure, velocity, and the
pressure coefficient. Page 7
Attachment 7: The uncertainty analysis for the 20 Hz data set. Page 8
Attachment 8: The uncertainty analysis for the 40 Hz data set. Page 10
Attachment 9: Lab handout and procedure. See References[2]
References
[1] Anderson, Ann. "Measurement of Wind Tunnel Velocity." MER 331: Fluid Mechanics.