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TriTrack Air Drag Analysis

Investigation of the Flow About the TriTrack System

Amanda Kelly
Aaron Hamblin
Amanda Babcock
John Burroughs
Valori Booth

Fluids Lab ASE 120K
With Dr. David Goldstein

Department of Aerospace Engineering and Engineering Mechanics
University of Texas at Austin
W. R. Woolrich Laboratories
Austin, TX 78712

1 Abstract

Airflow over a 1/12 scaled model of the proposed TriTrack system was investigated in the W.R. Woolrich laboratories to determine the nature of the flow about the system and to the drag coefficient of the vehicle. Jerry Roane, the inventor of TriTrack, proposed the testing to support claims with empirical data to Austin’s Board of Transportation. The complete TriTrack system includes the vehicle body, two axles, two wheels, and a triangular track. To determine the nature of the flow about the vehicle, flow visualization was performed with visible smoke released into the airstream and tufts placed on the surface of the TriTrack system. At approximately 3.05 m/s, the smoke showed that the flow remained attached to the body for almost its entire length. At about 18.29 m/s, the tufts on the vehicle body depicted attached flow for the majority of the vehicle by remaining streamlined. Drag coefficients for the various system configurations were determined with a force balance. The drag coefficient of the vehicle with the entire system in place was calculated to be 0.15. The wind tunnel tests revealed a drag coefficient of 0.07 for the vehicle body alone, which was in agreement with Mr. Roane's estimate of 0.09. By analyzing the drag coefficients for every configuration, the axles and wheels were found to account for approximately half of the drag of the system.

Table of Contents

  1. Abstract
  2. Nomenclature and Abbreviations
  3. Introduction
  4. Theory
    1. Methods
    2. Equations
  5. Apparatus and Test Procedure
    1. Flow Visualization
    2. Drag Coefficient
  6. Results
    1. Flow Visualization
      1. Smoke Wire
      2. Tufts
    2. Coefficient of Drag Determination
      1. Sphere
      2. Vehicle
    3. Conclusions
    4. References
    5. Figures

2 Nomenclature and Abbreviations

In Table 1 below a list is given of the nomenclature that refers to each test vehicle configuration that was tested. These system numbers will be used throughout the report.

Table 1 Description of System Configurations

3 Introduction

TriTrack is a monorail system proposed for the city of Austin that has been in development since the 1973 Oil Embargo. A local Austin inventor, Jerry Roane, has engineered the TriTrack system to resolve several growing transportation concerns for the general public. TriTrack is an electrically driven vehicle that relieves the city of Austin of some of its dependence on oil, an expensive natural resource that offers a limited supply and produces dangerous byproducts to the environment. The vehicle travels at about 37 mph on the residential streets and about 180 mph on the monorail [1]. Collisions between vehicles traveling above 37 mph have been shown to have a high fatality rate, therefore, imposing this speed limit with the TriTrack system will reduce road fatalities [1]. Clearance between TriTrack vehicles on the monorail allows the vehicle to safely accelerate to 180 mph. This system not only promotes safety but it also provides the convenience of short travel times.

The TriTrack vehicle body was selected carefully to be both streamlined for efficiency and spacious for passengers. A streamlined body, first designed in 1907, forces the air flow to remain attached to the body, thereby eliminating the drag associated with the air pressure that occurs in the wake behind the separated flow [2]. The TriTrack vehicle is modeled after the streamlined Class C Airship that has a body length to diameter ratio of 4.62. Although ratios of 3.0 to 3.5 give the smallest drag coefficients, a Class C Airship with the ratio of 4.62 was proven by the Navy Aerodynamic Laboratory in 1927 to have the lowest drag per unit volume [3], which is essential for passengers.

Although the wind tunnel data taken in 1927 remains valid for the Class C Airship, slight differences in the body design for the TriTrack vehicle warrant updated wind tunnel tests. Jerry Roane fabricated a smoothed model with high-end CAD equipment and machined it to within +/- .05 mm of the data provided in the Navy’s report, making a better model than the airship tested years ago. The TriTrack vehicle also includes two axles and two wheels and excludes a triangular section of the body where the railing will be fitted. Another distinction between the two vehicles is that the Navy's wind tunnel testing only went up to only 60 mph while the TriTrack vehicle is intended to travel at 180 mph.

Due to the deviation of the TriTrack model from the Class C Airship, preliminary wind tunnel testing in the W.R Woolrich Laboratories was necessary to get a better estimate of the drag coefficient and flow behavior about the TriTrack model. Because wind tunnel speeds are limited to 24.38 m/s at the W.R Woolrich Laboratories, further testing will be conducted at a later time at Pickle Laboratory to complement the preliminary results.

4 Theory

4.1 Methods

Flow visualization can be accomplished by the use of smoke wire and tufts. The smoke wire is a visualization technique that creates a sheet of smoke that passes over a test section. The separation point of the flow is indicated by the point where the smoke ceases to follow the contours of the body. This wire is only effective for low speeds of 3.05 m/s or less. Another technique of flow visualization is tufts, which are short sections of twine attached to the body. The tufts lay flat and smooth along the test article when subject to laminar flow and are greatly disturbed in the presence of turbulent flow. The point at which the tufts transition from stable to disturbed is the separation point.

A force balance is a component used to measures normal and tangential forces exerted on a body by aerodynamic resistance. The forces can then be used to calculate lift and drag coefficients. Sand grain can be applied to a vehicle to force super-critical flow. Super-critical flow has turbulent boundary layer that re-energizes the flow and causes the separation point to move farther back on the body. The region of separated flow has less contact with the body in super-critical flow, which subsequently lowers the drag coefficient.

4.2 Equations

The Coefficient of Drag (Cd) is a dimensionless ratio of drag force to dynamic force. It can be attained using the momentum equation applied to the control volume of the wind tunnel test section and assuming steady, incompressible flow. After the drag force is found, the drag coefficient can be calculated through the following equation:

The Reynolds Number (Re) for the vehicle can also be critical to the results. It is a relation of the laminar to turbulent transition point to the boundary layer thickness and skin friction of the test article [2]. Re is dependent on the free stream velocity. To better ensure the Reynolds Number obtained is accurate sand grain is added to the nose of the vehicle. When sand grain is applied to the nose, a Reynolds Number is obtained that simulates a faster free stream. The vehicle is designed to be traveling at speeds of up to 80.47 m/s but the testing of the vehicle is done at a maximum free stream velocity of 24.38 m/s. Reynolds Number equals:

5 Apparatus and Test Procedure

Flow visualization and data acquisition were performed in the subsonic, open-test section wind tunnel at the University of Texas at Austin. Several different configurations of the experimental TriTrack vehicle and track were tested to determine the nature of the flow around the system and the drag coefficient of each configuration. The vehicle, provided by Jerry Roane of Roane Inventions, was modeled after a Class C airship which is the basic shape of a blimp. The body was machined from billet aluminum measuring 0.44 m long with a maximum diameter of 0.10 m located 0.16 m from the nose. Each of the two spun aluminum wheels had an area of 4.23 x 10-4 m2 and each of the two aluminum axles had an area of 3.58 x 10-4 m2. In order to accommodate the triangular track, a 1.89 x 10-4 m2 triangular sector was removed from the bottom of the vehicle.

5.1 Flow Visualization

The vehicle was mounted in the wind tunnel on a rear-mounted sting. The track, supported by a platform, was positioned under the vehicle as shown in Figure 1. Flow visualization was first performed with a smoke wire at low flow speeds of about 3.05 m/s. The flow was examined over the vehicle and the wheels in system 1 to determine the exact separation point. Smoke visualization was repeated for system 5 in order to analyze the flow over only the vehicle body. The approximate separation point of the flow around the vehicle in system 1 at higher flow speed was determined by observing tufts attached to the vehicle, wheels and track. The behavior of the flow near the track and vehicle junction was also examined with the tufts. Tuft visualization was then repeated for system 5. Digital pictures were taken with a camera provided by the Aerospace Engineering Learning Resource Center for both visualization techniques.

5.2 Drag Coefficient

The drag coefficients for several different configurations were then calculated using the tangential force measurements obtained from a force balance. In order to calibrate the force balance, known tangential forces were applied to the sting with a string and pulley assembly, as shown in Figure 2. The axial force measured by the force balance was sent to a computer with a data acquisition system. After calibrating the force balance, a metal sphere with a diameter of 0.10 m was mounted on the force balance and drag force measurements were taken at various flow speeds. A drag coefficient for the sphere was calculated from the drag force data and compared to known values of drag coefficient to ensure the accuracy of the force balance. The vehicle was then mounted on the force balance and the track was placed directly beneath, but not contacting, the vehicle. The track did not contact the vehicle so that no loads were transmitted to the force balance by the track. Twenty drag force measurements were taken at zero free stream velocity to establish a zero force value. Twenty measurements were then taken for increasing flow speeds between 3.05 m/s and 24.38 m/s at 1.52 m/s intervals. For each flow speed, the drag coefficient was calculated by dividing the average of the drag force measurements at that flow speed by the frontal area of the configuration. A plot of drag coefficient versus free stream velocity was created to determine the drag coefficient for that configuration. The same procedure was repeated for systems 2 through 5 after removing the track, again after removing the wheels, and for a fourth time after removing the axles. Sand grain roughness was then applied to the maximum diameter of the vehicle and the procedure was performed for systems 6 through 8 to determine if forcing the flow over the vehicle to become super critical would lower the drag coefficient.

6 Results

Testing of the TriTrack was partitioned into two sections: flow visualization and coefficient of drag determination.

6.1 Flow Visualization

Two techniques were used to visualize the flow about the TriTrack test vehicle. A smoke wire created a vertical profile of the flow about the test subject.

6.1.1 Smoke Wire

Flow was first tested over the TriTrack body placed atop the triangular track. The smoke showed a very laminar flow over the entire vehicle with minimal disturbances from the track. The flow remained attached to the vehicle for almost the entire length of the body. The attached flow as well as a clearly defined stagnation point can be seen in Figure 3.

In order to visualize the flow about the TriTrack wheels, the model was assembled into the system 1 configuration shown in Figure 4 and the smoke wire was positioned directly in front of the wheel position. Figure 4 shows the flow about a wheel. The wake created behind the circular surface was very minimal and could only be observed on the lower portion of the wheel. A top view of the flow about a wheel is shown in Figure 5. The symmetric airfoil shaped axles of TriTrack proved to be a very aerodynamically smooth shape as presented in Figure 6. The flow remained attached to the axle and extremely little disturbance was seen at the trailing edge. Smoke visualization from the top view for the system1 configuration, can be seen in Figure 7. Flow was closely attached to the body and any disturbed wake by the axles was not observable from this view.

6.1.2 Tufts

Tufts placed in a spiral pattern on the TriTrack body were used for flow visualization at a free stream velocity of 18.29 m/s. The tufts in Figure 8 are straight and pointed directly downstream. Static tufts are visual indicator of laminar flow which agrees with the smoke visualization technique. Even tufts placed on the top surface of the airfoil axle were stable. Very slight disturbance was observed from the tuft placed on the top of the wheel. The disturbance can best be seen in the black lit image in Figure 9. A tuft wand was used to try to determine a flow separation point but no disturbance of tufts were observed over the entire structure. A view of the tuft wand testing can be seen in Figure 10.

6.2 Coefficient of Drag Determination

Utilizing the force balance, both the sphere and the vehicle were tested for their average drag coefficients at varying free stream velocities. For the vehicle, eight systems were tested. The system configurations are described in Table 1.

6.2.1 Sphere

Figure 11 is a plot of the coefficient of drag versus free stream velocity for the sphere. The trend line represents an estimated average of the data. To obtain a better fitting curve, the experimental high and low anomalies were discarded in the calculation of the trend line. The average value from this trend line is 0.57. White states that the accepted average coefficient of drag for a sphere is 0.47 [4]. A plot of the drag coefficient for the sphere with the addition of sand grain is shown in Figure 12. The trend line from this data set yields a rough average of 0.35. The addition of sand grain lowered the average drag coefficient, confirming turbulent flow transition. The accepted value of drag coefficient for turbulent flow about a sphere is 0.20 [4]. The differences between the calculated drag coefficients and the universally accepted values confirmed the presence of sting drag.

6.2.2 Vehicle

Figure 13 through Figure 20 are plots of drag coefficient versus free stream velocity for the data in their respective experiments. Average values, taken from the trend lines for each set of data, are shown for each system in the following table.

In systems 1 through 5, two trends became apparent. First, the drag coefficients were very low, nearing 0.10 on average. Second, removing the wheels and axles significantly lowered the average drag coefficient. With the loss of the wheels in system 4 and the axles in system 5, the drag coefficient dropped by half. With the addition of sand grain in systems 6 through 8, as shown in Figure 21, a turbulent boundary layer was induced in an attempt to lower the vehicle's drag coefficient. However, the sand grain seemed to raise the drag coefficient on average. Parallel experiments of systems 5 and 6 saw an average drag coefficient increase of 0.05. Systems 3 and 7 showed an increase of 0.01, and systems 1 and 8 showed a 0.02 increase. Another point of interest was the removal of the track. Findings from track removal showed a uniform decrease in the average estimated drag coefficient. The decreases in the system changes from 8 to 7 and 1 to 3 were 0.01 and 0.03, respectively.

Finally, as observed in the sphere experiment, there was sting drag present. Each drag coefficient in Table 2 would be higher than the measured value due to added drag caused by the sting. Also, the wire used to support the weight of the vehicle created a small amount of drag. Large variations in the measurements taken due to force balance oscillation observed at high free stream velocities render the drag of the wire too small to affect our calculations.

7 Conclusions

The results of the flow visualization showed laminar flow across almost the entire vehicle, as expected from the research conducted on the Class C airship. The triangular sector removed from the vehicle for the track did not cause significant changes in the flow, as expected. During calibration, the calculated average coefficient of drag for the sphere was higher than the nominal value due to drag force on the sting and calibration errors. However, the values were similar enough to ensure that the force balance was providing accurate data. The expected drag coefficient for system 5 was 0.09 due to small differences between the TriTrack vehicle and the Class C airship. The measured drag coefficient for system 5 was 0.07, which was close to the expected value. Some random errors may have resulted from oscillation of the vehicle, which originated from motor-induced tunnel vibration. Because flow about the vehicle without sand grains stayed attached for almost the entire length of the vehicle, forcing supercritical flow with sand grains did not change the separation point enough to reduce an already negligible pressure drag. In fact, when sand grains were applied to the vehicle in systems 6 through 8, the coefficient of drag increased because of the added friction drag from the sand. For system 3, the drag coefficient was found to be almost twice as large as the drag coefficient for system 5, meaning that the wheels and axles account for half of the drag of the vehicle. As suggested by Mr. Roane, the performance of the TriTrack system could be greatly improved if the wheels were retractable.

8 References

  1. Roane, Jerry, “TriTrack,” http://www.tritrack.net, 23 November 2003.
  2. Von Mises, R., Theory of Flight, Dover Publications, New York, 1959, pp. 102.
  3. NACA, “Drag of C-class airship hulls of various fineness ratios,” http://naca.larc.nasa.gov/reports/1929/naca-report-291/naca-report-291.pdf, 26 November 2003.
  4. White, F., Fluid Mechanics, 5th ed., Avenues of Americas, New York, 2003, pp. 25, 314, 48

9 Figures

Figure 1 Testing Setup

Figure 2 Force Balance Calibration Setup

Figure 3 Smoke Visualization About Body on Track

Figure 4 Smoke Visualization About Wheel on System 1

Figure 5 Smoke Visualization About Wheel on System 1--Top View

Figure 6 Smoke Visualization About Body and Axle on Monorail

Figure 7 Smoke Visualization About System 1 – Top View

Figure 8 Tuft Visualization About System 1

Figure 9 Tuft Visualization About System 1 – Black Lit View

Figure 10 Tuft Visualization About System 1 – Tuft Wand

Figure 11 Coefficient of Drag for Sphere

Figure 12 Coefficient of Drag for Sphere with Sand Grain

Figure 13 Coefficient of Drag for System 1

Figure 14 Coefficient of Drag for System 2

Figure 15 Coefficient of Drag for System 3

Figure 16 Coefficient of Drag for System 4

Figure 17 Coefficient of Drag for System 5

Figure 18 Coefficient of Drag for System 6

Figure 19 Coefficient of Drag for System 7

Figure 20 Coefficient of Drag for System 8

Figure 21 System 6 Configuration