Cover Page for Lab Report Group Portion. Flow Visualization in a Water Channel

Transcription

1 Cover Page for Lab Report Group Portion Flow Visualization in a Water Channel Prepared by Professor J. M. Cimbala, Penn State University Latest revision: 08 September 2017 Name 1: Name 2: Name 3: [Name 4: ] Date: Section number: ME 325. Group # Score (For instructor or TA use only): Lab experiment and results, plots, tables, etc. - Procedure portion Discussion Neatness & grammar TOTAL / 45 / 15 / 10 / 70 Comments (For instructor or TA use only):

2 Procedure and Presentations of Results A. Operation of the Water Channel To fill up and run the water channel, follow this procedure (7-10 minutes): 1. Remove the top Plexiglas cover plate and set aside safely. Note: If using one of the horizontal model insert, put this insert in before filling water channel (see Figure 4a) Caution: The plexiglass has a perfect seal that can create a high pressure within the system as air would not be able to flow out. This will slow filling time and can even completely stop the filling up processes if pressure stays too high. 2. Close large drain valves one on the bottom right side of the water channel Also, be sure the small brass drain valve on the clear Plexiglas end of the channel is closed. See Figure 3a for correct configuration. 3. Open the water channel fill valve (located adjacent to the drain valve). 4. Turn on the fill valve that supplies the fill garden hose. (This valve is located on the north wall of the room, to the left of the sink. Do not turn the valve on the right side of the sink that one should stay open at all times.) See Figure 3b for valve location and correct configuration 5. You should both hear and see the water channel begin to fill. Note that the right side of the channel is clear Plexiglas, and you can watch the water fill the channel. Never leave the room while the channel is filling! Caution: As water nears the top wall of the water channel, the water level will RISE QUICKLY 6. Let the water rise until it just about reaches at the top of the test section where the bottom of the plexiglass cover should be See Figure 4b. Close the fill valve quickly so that the water does not overflow. The channel operates best if the water level essentially filled to its max. 7. Place the plexiglass back on top and if water is too high, cautiously drain some of the water to an appropriate level. If the water level is too low, cautiously add open the fill valve and the bleed valve (the brass valve at the top right of the water channel, see Figure 5a) to let air out while filling and prevent pressure build up. 8. Once satisfied, close all valves including the bleed valve and the water filter inlet on the wall. 9. Turn on the pump motor controller circuit breaker switch (to the left of the water channel). Rotate the red handle ¼ turn clockwise so that it points vertically to ON. 10. Push the RUN button on the Toshiba frequency controller. The water should start flowing. You can adjust the speed with the control pad up and down arrow keys. Note: A digital display on the frequency controller is merely approximate freestream velocity reading. Use the flow rate meter on the side, then divide it by 5 for a better approximation (in m/s) of the freestream velocity in the water channel test section (See Figure 5b). That is a reading of 54.4 means flow is moving at approximately 1m/s. Caution: run the filter pump when dye visualization is in use to prevent accumulation of dye in the system. The switch is located on the right side of the main system circuit breaker handle and ensure that the filter drain valve is See Figure 6a and b 11. Run the channel at high speed (at a reading of around 1.0 m/s) for a few minutes to purge the water channel of air bubbles. Open the bleed valve on top of the channel to let some trapped air escape. 12. To shut off the flow at any time, simply push the STOP button on the Toshiba frequency controller. Figure 1: a) Fill and drain values location in an open fill & closed drain orientation. b) Water inlet located on the wall in the open position.

3 Figure 2: a) Max. water level to insert horizontal models. b) Max. water fill level for the water channel before spill occurs Figure 3: a) Brass bleed valve located on the top of the channel. b) Sample flow meter reading of 5.04 m/s in the pump outlet but the freestream is wider and so flow is actually 5.04/5 = m/s approx. Figure 4: a) the filter pump switch. b) the recirculating filter pump valve in the open position.

4 To drain the water channel (Not now do this when finished with your experiment): 1. Push the STOP button on the Toshiba frequency controller. 2. Turn off the filter pump first (see Figure 6a), then it is safe to turn of the pump motor controller circuit breaker. Rotate the red handle ¼ turn counter clockwise to the OFF position. Caution: Never drain the system with the motor running. 3. Remove the top plexiglass and open the drain valve marked in Figure 3. The channel will slowly begin to drain. Wait several minutes (15 minutes if complete drain is required) until the water is nearly all drained from the channel. (You can monitor the draining progress from the large Plexiglas cover on the far-right side of the water channel.) Caution: Removal of the top plexiglass is critical to prevent creation of a partial vacuum/ low pressure within the system. If the plexiglass is already sucked tight, open the bleed air flow to balance the pressure. 4. Open the small brass valve on the far end of the water channel (clear Plexiglas end) to allow the trapped water to drain into a bucket. 5. Turn off the wall-mounted fill valve that supplies the fill garden hose. 6. Briefly open and then close the water channel fill valve to release the pressure in the garden hose. B. Operation of the Laser Velocimeter 1. The LV (Laser Velocimeter), which is sometimes also called an LDV (Laser Doppler Velocimeter) is used to measure the freestream velocity for this lab. Verify that the fiber optic LV head is located near the very start of the test section, where the flow is uniform and uninfluenced by the dye probe or by any models. The LV head should be close to the vertical centerline of the test section (i.e. at a reading of around 16 or 17 cm on the vertical traverse), and approximately 3.5 inches away from the front Plexiglas panel of the test section. If the LV system is already on and operating, all you will need to do is open the laser shutter, which is the shutter dial on the black box labeled FlowLite on the top of the electronics rack, and carefully remove the lens cover. Verify (visually) that the laser is on, then skip steps 2 through 4 below. Do NOT look directly into the laser beam! Serious eye damage will result! Special laser goggles that filter out laser light are available in the lab for your protection. 2. Turn on the LV acquisition system with the switch located on the piece of equipment labeled BSA F60 Flow Processor located below the computer on the electronics equipment rack. If the laser is not already ON, turn it ON with the key on the piece of equipment labeled FlowLite located above the computer, and turn the shutter switch below the key to OPEN. Finally, remove the lens cover on the fiber optic probe head, being careful not to scratch the lens. 3. Verify (visually) that the laser is on. You should see two laser spots reflected from the Plexiglas side of the channel. Or you may see the two beams reflected off particles in the water channel. Do NOT look directly into the laser beam. Serious eye damage will result. Special laser goggles that filter out laser light are available in the lab for your protection. 4. Turn on the lab computer if it is not already on. Double click on BSA Flow Software V4.50 on the desktop. If the icon is not on the desktop, start the program using the Start All Programs menu. If this is the first time you have run this program under your unique log-in ID, continue. If you have run this program under your ID previously, skip to step You may see a message saying BSA software to run for the first time. This is normal. A window will appear asking if you wish to register the software now. Click No. 6. In the Getting Started window, choose 1D LDA with Traverse.lpd from the Project Templates section. If you have run this program under your ID previously, skip to step When the software is done loading, you will see several windows on the screen. In the window titled Device List, R-click on Processor. In that menu, click on Device Configuration. This will start the Auto-Configuration Wizard. (If it does not start automatically, choose Auto in the window that comes up.) The Wizard will present a series of steps: a. Choose Next b. Message says: Searching for Processors. Then Found Processor. Choose Next. c. Message says: Connected to Processor. Choose Next. d. Message will ask about a traverse. None should be selected. Choose Finish. e. If necessary, choose Close in the next message. 8. In the Device List window, R-Click on Processor. Choose Connect to Processor. 9. A new window should appear at the lower right called System Monitor. If it does not, R-click on Processor again, and choose System Monitor. 10. In the Device List window, click on Optical LDA System.

5 11. In the Properties window (lower left), under Beam System-U1 click on Wavelength. The wavelength of the laser of this system is nm. If this number is not displayed as the current wavelength, use the downward pointing button and select nm from the drop-down list. 12. In the same window, click on Focal Length. Use the downward pointing button to select mm from the drop-down list. 13. Click on View in the toolbar at the top of the screen. Click on Start Page, which will bring that window to the front. Close that window to reduce clutter. Now click at the upper right to enlarge the software to Full Screen. 14. Examine the windows on the screen. You should see: Project Explorer, Device List, a Histogram graph, Properties, a Data List, a Moments List, the System Monitor, and Messages. Note that the Data List has columns for Arrival Time (AT), Transit Time (TT), and LDA1 (velocity in direction 1). Look at the columns in the Moments List and note there is a column for LDA1 Mean. This is where the mean velocity in direction 1 will be reported for each data acquisition period. Currently, the setup is such that data are taken for 10 seconds. Each burst of LDV signal which results in a measured velocity during that period will be reported in the Data List. The mean velocity for that period will be reported in the Moments List under LDA1 Mean. 15. Change the data format and precision: in the toolbar at the top, click on Tools, then Options. Choose the tab labeled Data Formats. Then: a. Highlight Velocity. b. Click Change. c. Change the Unit to m/s and change the Precision to 4. d. Click OK. Click OK again on the Options dialog box. A message box appears. Click OK. Data formats will change once data acquisition has begun. 16. Turn on the water channel to some medium flow rate (around 0.5 m/s or around 30 Hz on the readout of the frequency controller). 17. Position the optical probe head so that the laser focal volume (also called measurement volume where the beams cross) is near the cross-stream centerline of the channel. 18. In the Device List window, Click on Processor so it is highlighted. In the top tool bar, select the icon that looks like a black arrow tip ( ); this is the Run button. A dialog box will come up Click on Run Create New Data. Another dialog box will appear. Click on Acquire. You should see Running at the bottom of the screen. Data acquisition will continue for 10 seconds. Observe the System Monitor window. You should see frequent bursts representing LDV signals from the measurement volume. As data are being taken, results should be posted in the Data List. In the Run mode, data acquisition will stop automatically after the 10 second period. 19. Wait for acquisition to stop. In the top tool bar, select the icon which resembles two arrows in a circle (. ). This starts the Repetitive Mode so that data are continually updated and displayed. Whenever you wish to quit, click on the black square icon ( ) on the top tool bar to stop acquisition. 20. Record the LDV reading of the average freestream velocity. V avg = m/s. 21. NOTE: When the LDV is not being used, restore the lens cover to the LDV fiber optic head, and turn the shutter dial to closed. Leave all the electronics ON. You will use the LDV system again later in this lab. When completely finished with the lab, close the BSA Flow Software program. Restore the lens cover to the LDV fiber optic head, and turn the shutter dial to closed. Leave all the electronics ON. Also turn off the water channel. If the water is colored with dye, drain the water from the system. To prevent eye injury, it is advisable to turn the shutter to closed whenever you are viewing the sphere or airfoil. Open the shutter only when a velocity measurement is required.

6 C. Operation of the Dye Injector System There are three containers of dyes mounted on an elevated stand. If the dye reservoirs are nearly empty, ask your Instructor or TA to fill them with more dye. A valve (one for each reservoir) controls the flow rate of dye into the dye injection feed tube. This flow rate should be adjusted as water channel speed changes to give the best possible dye streak. If the valve is not open enough, the dye streak will be too weak. If it is open too much, the dye itself will form a jet which disturbs the flow. The orientation of the dye probe is also critical to the quality of the dye streak. 1. Install the dye injection feed tube into the dye probe pole model s connecting feed tube by inserting them as seen in Figure 7. To eject, press the metal push tab to release the feed tube. Experiment with the dye injection system with the water flowing at various speeds, and with the dye valve adjusted to various flow rates. Rotate the dye injection needle to determine the best orientation for injection. Figure 5: Dye injection connection system located on the left of the water channel 2. Turn on the backlight (push and hold in the switch at the bottom of the light box for a couple seconds). The backlight should make the dye streak more visible. (4) 3. Which orientation yields the best dye streak at very low water channel speeds (around 0.1 or 0.2 m/s)? Does this agree with your intuition? How about at higher speeds (around 1.0 m/s)? Include sketches of the dye streaks at both low and high speeds in the space below: 4. As the water slowly turns color, the dye streak will become more difficult to see. If the water gets too colored, you will have to drain and re-fill the water channel. Caution: Avoid using too much dye draining and refilling can take up to 20 mins

7 D. Visualization of Flow over Spheres (15) 1. Study the flow over the various sphere models (golf ball, baseball, smooth yellow ball, and roughened yellow ball) at various speeds. Be careful that you don t damage the dye injection needle as you change models. Do not over-tighten the model onto the sting mount. Most of the results in this lab experiment are qualitative, but be sure to record the freestream velocity (as obtained from the LV system) for each case so that you can calculate effective Reynolds numbers. Note that the LV has a rather slow response time at low velocities, so make sure you wait for the velocity reading to settle down before recording its value. Pay particular attention to the location where the boundary layer separates off the surface of the spheres, and the Reynolds number at which the drag crisis occurs (if at all). Record your observations neatly in the space below, including some sketches. Attach extra pages if necessary. Note: that you can move the dye injection needle up or down until the dye streak hits the sphere exactly where you want it to hit. For each sphere, look for the three Reynolds number regimes: laminar, transitional, and fully turbulent. Ask your instructor or TA for assistance if you are not sure which regime is being seen. Hint: The best results are obtained when the flow over the bottom of the sphere is observed.

8 E. Visualization of Flow over a Model Car 1. Carefully remove the dye injection needle and gently set it down on the cover of the settling chamber. 2. Carefully lift up the cover plate with the sting mount, and the sting mount and thread in the one that supports the model car. 3. Carefully re-install cover plate and the dye injection needle. (5) 4. Use the dye to study the flow over the model car. Pay particular attention to the flow over the top (roof) of the car, around the back side, and into the wake region. Does the flow separate or not? Draw some sketches.

9 F. Visualization of Airfoil Stall 1. Carefully remove the dye injection needle and gently set it down on the cover of the settling chamber. 2. Carefully lift up the cover plate with the model car, and replace it with the cover plate that supports the airfoil. 3. Carefully re-install the dye injection needle on the new cover plate. (2) 4. The airfoil angle of attack can be increased by turning the base of the airfoil mount counterclockwise. Use the protractor mounted on the back of the test section to monitor the angle. Set the airfoil at zero degrees angle of attack. Turn on the water flow at a fairly slow speed (around 0.25 m/s). Record the freestream velocity (as obtained from the LV system). Calculate Reynolds number in a similar manner as was done for the spheres. Define Re c = Vc/ where c is the chord length of the airfoil (4.0 inches). Since the airfoil is thin, it is not necessary to correct for blockage effects here. Vary the channel speed up to the maximum. Showing sample calculations in the space provided below, calculate the Reynolds number of this airfoil at the various channel speeds, and fill in the table: Sample calculations of Reynolds number: Reynolds number range for the airfoil: Water channel speed setting (reading on the control pad) Freestream velocity V measured by the LDV instrument (m/s) Reynolds number, Red (3) 5. For this available range of Reynolds numbers, do you expect the boundary layer to be laminar or turbulent? (Note: assume the airfoil surface is smooth.) Justify your answer. To save time in the steps which follow, you may use the above table to estimate Reynolds number as a function of channel speed setting. Even when you change angle of attack, you may assume that the Reynolds number remains constant; i.e. it is not necessary to re-measure velocity and recalculate Re at each different angle of attack.

10 (6) 6. Set the water channel speed to about 0.25 m/s. With dye injection, slowly increase the angle of attack of the airfoil from 0 to about 20, and record sketches of your observations. If the flow separates off the upper surface of the airfoil, indicate the separation point on your sketch. Does the separation point move with increasing angle of attack? (3) 7. Based on your observations with the dye streaks, at what angle of attack does the airfoil stall at this speed?

11 (6) 8. Now repeat the experiment for the other three water channel speeds (approximately 0.5, 0.75, and 1.0 m/s). Use the dye streak visualization method to determine the stall angle. Describe your results in the space below and on additional sheets of paper if necessary. Specifically, determine stall angle as a function of Reynolds number. Some sketches may be helpful. 9. Drain the water channel and turn everything off. Restore the channel to the same condition it was in when you started.

12 Discussion (5) 1. Were you able to observe a drag crisis on any of the spheres? Why or why not? Discuss what you learned about drag crisis and flow separation. Has flow visualization helped you to understand this concept better? (5) 2. Did you observe flow separation at the back of the model car? If so, where does the flow separate? Discuss how flow separation may influence the aerodynamic drag on an automobile. (5) 3. Describe what you learned about the process of airfoil stall. In your experiment, did the airfoil stall angle depend on Reynolds number? Why or why not?