CE 150 LABORATORYDRAFT – subject to change Fall 2003
Rev 11/10/03
Fluid Mechanics Lab #9
Aerodynamic Drag and Lift
Objective: The student isto determine the drag and lift coefficients of two different models using the closed-loop wind tunnel with 2820 test section in LANG 122. Specifically,
- determine the drag coefficient of a 6 sphere over a range of Reynolds numbers and compare with established curves or data;
- determine the drag and lift coefficient of a NACA 4412 airfoil model at a relatively high Reynolds number as a function of attack angle () and compare with published data.
Theory: The total drag force of an immersed body consists of viscous drag (skin friction) and pressure (form) drag. Lift is solely due to pressure effects. The drag coefficient and Reynolds number for blunt objects, such as a sphere, are defined as:
where FD is the measured drag force, is the fluid density, U is the upstream fluid velocity (relative to a stationary body), Ap is the projected area of the body into the flow, D is the object diameter, and is the fluid kinematic viscosity. The drag and lift coefficients and Reynolds number for airfoils are normally defined as:
where FL is the measured lift force, As is the planform area of the body (i.e., the area seen from above at zero attack angle), and c is the chord length of the airfoil. Refer to sections 9.3-9.4 in Munson for more information.
Procedure:
- Make sure instructor is present before starting this laboratory.
- Locate or identify all equipment and instrumentation needed to perform this lab, namely the force balance, drag/lift force displays, computer, mounting hardware, sphere, airfoil, pitot tube, inclined manometer, barometer, thermometer, and tape measure.
- Measure the diameter of the sphere; measure the chord length and planform area of the airfoil model.
- Make sure the four locking bolts holding the force balance arms are snug. These prevent excessive force to the load cells during the mounting procedure.
- With the help of the instructor, secure the 6 copper sphere to the mounting rod. Make sure the sphere is positioned along the test section centerline, upstream of the mount.
- Turn power strip on floor to ON.
- Use a wrench to release all four locking bolts.
- Tare lift and drag displays by pressing 3rd button from left.
- Start the wind tunnel - follow the start-up operating procedure given in the next section.
- Record the barometric pressure.
- Record the sphere + mount drag force over a range of wind tunnel velocities corresponding to the following pitot tube manometer deflections: 0.5, 1.0, 1.4, 1.6, 1.8, 2.0, continue by 0.2 increments until drag drops; then 3.0, 4.0, and 6.0. Also record the tunnel air temperature at each velocity setting.
- Perform steps 1 & 2 of the wind tunnel shutdown procedure.
- Carefully secure all four force balance locking bolts. Pay close attention to the load cell displays – try to keep displayed load <2 lbs. when tightening the bolts.
- With the help of the instructor, remove sphere from the mounting rod and secure it to the tunnel ceiling for the drag-on-mount measurement.
- Release locking bolts and then tare lift and drag displays.
- Start the wind tunnel by performing steps 6 and 7 of the start-up procedure.
- Record the mount drag force over a range of wind tunnel velocities corresponding to the following pitot tube manometer deflections: 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 6.0. Record temperature at each velocity.
- Perform steps 1 & 2 of the wind tunnel shutdown procedure.
- Carefully secure all four force balance locking bolts. Pay close attention to the load cell displays – try to keep displayed load <2 lbs. when tightening the bolts.
- With the help of the instructor, remove sphere and sphere mounting rod. Install airfoil mount and secure airfoil with screws.
- Release locking bolts and then tare lift and drag displays.
- Turn on computer and monitor. On computer, start the Motion Architect program: START>Programs>Motion Architect>Motion Architect Program.
- Open Terminal by clicking on its drop-down menu.
- In Terminal at the “>” prompt, type Next.
- The RP240 control panel should now be displaying the program. Re-tare the displays if necessary.
- Start the wind tunnel by performing steps 6 and 7 of the start-up procedure.
- Set the speed control to 75% and record the pitot tube manometer deflection and the tunnel static pressure (remove high pressure tubing).
- Select the PITCH axis and enter -12 (angle of attack). Record lift and drag values (lbs).
- Increase the airfoil angle of attack in increments of 2 up to +20, recording lift and drag values for each angle.
- Return airfoil to 0 angle of attack and follow the shutdown procedure for the wind tunnel.
- Press EXIT (F6) on the control panel. Close Terminal window and Motion Architect program. Shut down computer.
- Carefully secure all four locking bolts. Pay close attention to the load cell displays – try to keep displayed load <2 lbs. when tightening the bolts.
- Turn off power strip on floor.
Operating Procedure for Closed-Loop Wind Tunnel:
Start-up:
- Make a visual inspection to ensure no loose objects have been inadvertently left in the wind tunnel and latch the door(s) shut.
- Turn on the “MOTOR” and “LIGHTS” switches located about two feet under the motor and fan blade assembly.
- Make sure the motor speed rheostat dial on the remote control unit is turned to zero.
- Push the H2O ON button to turn on the motor/clutch cooling water.
- Open the heat exchanger water inlet valve and close the drain valve (at the floor).
- Push the motor START button to turn on the fan motor.
- Turn the motor speed rheostat dial slowly up to the desired position. Do not turn this dial too quickly in either direction – failure to do so can cause serious damage to the motor! Do not exceed 80% on this dial.
Shut-down:
- Turn the motor speed rheostat dial slowly back to zero.
- Push the motor STOP button to shut off the fan motor.
- Close the heat exchanger water inlet valve and open the drain valve.
- Push the H2O OFF button to stop the motor/clutch cooling water.
- Turn off the “MOTOR” and “LIGHTS” switches.
Velocity Measurement with the Pitot Tube:
The Pitot tube, when properly connected to a manometer or pressure transducer, measures the difference between the stagnation pressure and static pressure of a fluid flow (P). The velocity of the fluid at the static pressure tap, which is essentially the freestream velocity (V), can be determined from the Bernoulli equation:
where is the density of the flowing fluid, m is the density of the manometer liquid at standard conditions, and h is the deflection of the manometer liquid level read from the inclined scale. This scale is calibrated to read inches of H2O, so m is the density of water at room temperature.
Material to be Included in Report in Addition to Normal Requirements:
Sphere:
- Using Excel, prepare a graph of the mount drag force (lb) versus velocity (ft/s). Use Trendline with a polynomial function to obtain an equation from which you can calculate the force for a given velocity.
- Using the Trendline equation, subtract the mount drag force from the sphere + mount drag force at each velocity setting. Convert each velocity setting to a Reynolds number (Re) and the corresponding sphere drag force to a drag coefficient (CD). Prepare an Excel graph of CD (y-axis) versus Re (x-axis). Connect the points with the smoothing function.
- Compare your graph with Figure 9.25 in Munson. Discuss how well your data supports the established curves. Try to explain any major differences.
Airfoil:
- Prepare an Excel graph of the lift-drag coefficient ratio (CL/CD) versus angle of attack (). Indicate the point of stall. Compare with published results and discuss any differences.
- Prepare an Excel graph of the drag coefficient (CD) versus lift coefficient (CL) and label each data point with the corresponding angle of attack. This is called a polar diagram. Compare with published results and discuss any differences. Refer to Figure 9.34 in Munson for a typical airfoil.
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