Determination of Drag Coefficient

Transcription

1 DEPARTMENT OF MECHANICAL, INDUSTRIAL AND MANUFACTURING ENGINEERING MIMU MEASUREMENT AND ANALYSIS Determination of Drag Coefficient You will need to bring a zip disk or USB storage device to the lab to store your VI and measurement files. OBJECTIVES The objectives of this experiment are to 1) create a LabVIEW VI to record and analyze data from a force gauge and pressure transducer, 2) estimate the total drag coefficient from measured data, 3) modify the physical design of an object to reduce drag forces. BEFORE GETTING TO THE LAB In this lab, you are responsible for determining the sensitivities of the transducers you are using. Do this by creating a calibration curve for each of the transducers. Use the slope of the curve to determine the sensitivities. Make sure you are using units that are compatible with the VI you will create in the lab. Complete this step before coming to the lab, and have the sensitivities ready to input to the VI. The plots will be included in your write up. SETTING UP THE MEASUREMENT SYSTEM Your work station has a National Instruments Model SC-2345 signal conditioning chassis attached to the data acquisition board in the computer. For this lab, the SC-2345 is equipped with several modules to support measurement of resistance, strain, temperature and raw voltage. We will be using the raw voltage inputs, and converting these data to pressure and force. To begin creating the LabVIEW Virtual Instrument (VI) to be used in this experiment, power on the computer and log onto it. The login account is lab, and the password is lab. Double click National Instruments shortcut on the desktop. Click Cancel on the registration form, then click OK, then click Continue. You are building a new VI, so click New, then, Blank, and then OK. A new Front Panel will then be displayed. Move to the Block Diagram window, right click in its field, and click the Inputs palette. Once there, drag a DAQ Express VI onto the block diagram. Double-click the DAQ Express SubVI to configure it. It will launch a wizard that helps set up the data acquisition hardware. Once in the Wizard, select Analog Input, then Voltage. The My Physical Channels window will then appear on the lower right of the screen. This window maps the SC-2345 signal conditioning chassis to the DAQ board in the computer, channel by channel. Dev1 (PCI-6036E) is the PCI Data Acquisition Board in the PC. Listed below it should appear (as a minimum): SCC1ModX (SCC-FT01) SCC1ModY (SCC-FT01)

2 Both of these are voltage input modules, with the X and Y representing the location of the module in thesc-2345 chassis. Click the + in front of the first one, then click ai0 below it, then click Finish.. This will open the Analog Input Voltage Task window, which allows you to set up the parameters of the voltage recording. Under Channel List in the upper left corner, right click the voltage channel there, and select Rename from the menu. The SC-2345 is set up with the pressure transducer connected to the first of the two voltage input channels, so name this first channel Pressure Sensor Voltage. Next, click the + icon under Channel List, then select the second SCC_FT01 by clicking the + and then ai0 and Finish. Rename this second channel Force Gauge Voltage. This creates two active channels, as we will be recording the pressure and force simultaneously. The next step is to enter and/or modify each channel s parameters using the dialog box that appears under the Settings window. Click the Pressure Sensor Voltage channel in the upper left of the display, and configure the following inputs: Under Input Range Max = 5 (Volts) Min = -0.1 (Volts) Terminal Configuration = NRSE (single-ended input) Custom Scaling = <no scale> In the Task Timing window, click Acquire Continuously. Under Samples to Read input 500, and under Rate (Hz), input 100. This causes the DAQ to collect 500 samples at a rate of 100 samples per second, for a total of five seconds, continuously. Now click the Force Gauge Voltage channel in the upper left of the display, and configure the following inputs: Under Input Range Max = 0.5 (Volts) Min = -1.0 (Volts) Terminal Configuration = NRSE (single-ended input) Custom Scaling = <no scale> In the Task Timing window, click Acquire Continuously. Under Samples to Read input 500, and under Rate (Hz), input 100. This causes the pressure and force signals to be sampled under the same conditions. Test the DAQ now, then click OK to complete the VI. To write your data to a file, use the Express Write LVM VI located on the Windows>>Functions»Output palette. Double click into this. Leave the path statement blank. Under Action, select: Save to One File, Ask User to Choose File, and Ask Each Iteration. Under If a File Already Exists, select Rename Existing File. Under Segment Headers, select: One Header Only. Under X Value Columns, select:

3 One Column Only. And under Delimiter, select: Tab. Click OK and then wire this Sub VI to the output of the DAQ Sub VI. The format of this file is read normally by the Notepad application, but if you first start Excel then open the file from inside the application, and follow the steps indicated, you can reconfigure the file as an Excel file. Notice that there is only one output to the Express Write LVM VI, but there are two data channels. To unbundle the signals, it is necessary to use the Split Signals function. This function is accessed under Functions>>Signal Manip>>Split Signals. Drag the icon onto the Block Diagram, and place it near the output of the Express Write LVM VI. Wire it to the output from the LVM, and you will notice that it configures itself with two outputs. These correspond to the two signals it receives from the DAQ, in the order that they are input to the DAQ the top one is pressure and the bottom one force. Also, the force gauge sign convention causes tensile loads to appear as negative voltages. To correct for this, multiply the force voltage by -1. This is accomplished by selecting the Multiply function from the Functions>>Arith/Compare>>Numeric palette. Drag the icon near the output of the Split Signals icon, and wire the force (lower) terminal of that ion to one of the inputs to the Multiply icon. Right click the other terminal and add a constant. Set the constant value equal to -1. Next, Create two Statistics VIs from the Functions>>Analysis>>Statistics palette. Configure these to determine the mean and standard deviation, then place them near the Split Signals icon, and wire them to it. Create four Numeric Indicators on the front panel, label the first two Pressure Voltage Mean and Pressure Voltage STD, and wire them to the appropriate terminals on the Pressure signal Statistics VI. Repeat this for the remaining two indicators, naming them Force Voltage Mean and Force Voltage STD, and wire them to the output of the Multiply icon for the force signal. Create two waveform graph indicators on the Block Diagram. Click on the field of the Front Panel, click the Graph Indicators palette, and then drag a graph indicator onto the display, and repeat the process. Label one Pressure Voltage and wire it to the pressure terminal of the Split Signals icon. Label the second Force Voltage, and wire it to the other terminal. You can modify the axis labels and scales of the graphs to suit your measurements. The X-Axis scale (time) should be set to be 5 seconds, which is the total length of the data acquisition segment. The structure necessary for sampling, recording and displaying the voltage signals produced by the two transducers has been created. Now, these voltages must be converted to pressures and forces. To do this, a method for incorporating the sensitivities and zero errors (if any) of the two transducers must be included. Begin this by creating four numeric controls on the Front Panel. Label them as follows: Pressure Sensor Sensitivity (V/in H 2 O) Pressure Sensor Offset (V) Force Gauge Sensitivity (V/lb)

4 Force Gauge Offset (V) Locate these on the Font Panel so that the pressure controls are near the pressure voltage indicators, and the force controls are near the force voltage indicators. Place a Subtract function from the Functions>>Arith/Compare>>Numeric palette near the Pressure Sensor Offset control, and wire the output of the control to the LOWER input terminal of the Subtract icon. Wire the output of the pressure (upper) output terminal of the Split Signals icon to the UPPER input terminal to the Subtract icon. This will subtract the offset from the voltage signal before the sensitivity correction is applied to the signal. Repeat this for the force signal, wiring the upper input of its Subtract icon to the output of the multiplier output for the force signal. Next, the sensitivity corrections must be made. Sensitivity is the ratio of the output of a sensor to its input. In our VI, we are measuring voltage, and must convert it to either pressure or force using the inverse of the sensitivity for each transducer. Begin by placing a multiply icon on the block diagram near the output of the Subtract icon for the pressure signal. Wire one of the inputs to the Multiply icon to the output of the subtract icon. This other input terminal must be wired to the output of the Pressure Sensitivity control, but the number from the control must first be inverted. To do this, insert a Reciprocal function from the Functions>>Arith/Compare>>Numeric palette between the output of the Sensitivity control and the input of the Multiply icon. Wire the icon accordingly. Repeat this for the force signal. The output of this step is a signal with units of either pressure (in H 2 O) force (lbs). These signals must now be displayed. To do this, place an gauge indicator and two numeric indicators on the Front Panel near the pressure indicators and controls. On the block diagram, place a Statistics VI near the indicators. Wire the output of the Multiply icon to the input of the Statistics VI and to the gauge indicator. Configure the Statistics VI to output the mean and standard deviation, and wire these outputs to the two numeric indicators. Label the gauge Pressure, and click the minimum value and set it to zero inches of water, and click the maximum value and set it to 5 inches of water. Label the mean and standard deviation indicators accordingly, with units. Repeat this for the force signal. Finally, we are using LabVIEW to generate the power supply voltage to power the pressure transducer. Do this by selecting a DAQmx VI from the Functions>>Outputs palette, and placing it anywhere on the block diagram. LabVIEW will launch the set up routine for the VI. Select Analog Output, then Voltage. When the My Physical Channels box appears, click the + in front of SCC1Mod17 (SCC-FT01), click the ao0 under it, then click Finish. In the Analog Voltage Setup Task window, change only the min voltage to be 0V, and the Task Timing to Generate 1 Sample, then click OK. On the Block Diagram, right click the Data input terminal on the DAQ VI. We wish to create a constant of 10 volts, but when Create is selected, the Constant option is not highlighted. Instead, click the Control option to create a control. When this control

5 appears on the Block Diagram, right click it and then select Create and then Constant. Remove the control form the diagram, and wire the control into the Data input terminal. Make sure the value of the constant if set to 10. This will cause the SCC-FT01 module in location 17 to generate a continuous voltage 10 volt signal to power the Setra pressure transducer. Save the VI you have created to the desktop. You will also need to save the VI and data files to a storage device at some time before leaving the lab, in case you wish to examine your data later. Now, run the VI you have created, and verify that it works.

6 PROCEDURES 1. Select one of the model vehicles that have been made available to you. Measure the length width and height of the model you select. Record these for use in predicting the total drag from the model. 2. Rig the model in the wind tunnel with the help of the TA. 3. Don hearing protectors, available in the wind tunnel lab. INPUT OF THE SENSOR SENSATIVITIES AND DETERMINATION OF ZERO OFFSET 1. Run the VI with the wind tunnel off. 2. Record the data collected for the mean voltage for each of the transducers. This is the zero offset for each. Also note the characteristics of the raw voltages as seen in the graphic indicators on you control panel. If the signals look questionable, discuss this with the TA and try to remedy the problem. 3. Input the value for the offsets for the pressure and force gauge in the appropriate controls in the VI. The pressure transducer offset should be very small, while the force gauge, being a strain gauge transducer with a signal amplifier, may produce offsets on the order of half a volt or more. 4. Input the sensitivities of the sensors into the appropriate controls on the front panel of your VI. 5. Run the VI again with the tunnel off. It should produce a pressure and force that are very nearly zero. If they are not, repeat the steps above to determine what error you have made and correct it. Record these values as the Zero Speed data for your baseline measurement. Save the voltage file to the desktop, and then to a removable storage device. BASELINE MEASUREMENT 1. Have the TA check the installation of the model in the tunnel test section, and that the pitot tube position, and the connection to the force gauge are correct. 2. Ask the TA to start the tunnel at a speed of 20 on the controller. 10 will be skipped because there is almost no flow at this setting, and there may be little measurable flow at 20. Note that the controller is not calibrated, so the numbers on the dial are meaningless. 3. Record the pressure, force, and the standard deviations for the two signals at an input setting of 20. Save the raw voltage file to disk. 4. Increase the controller input to 30 and repeat the measurement. 5. Repeat the measurement process, increasing the input speed of the controller until 100 is reached. 6. Turn off the wind tunnel. MODIFICATION OF THE MODEL 1. Remove the model from the tunnel.

7 2. Use the materials available (clay, tape and cardboard) to improve the airflow and drag characteristics of your model. You will have only one chance to modify the model, so use your best judgment in your modifications. 3. Make sure all added materials are securely attached. 4. Take a digital picture of your design. This will be saved to disk for you, and you should include it in your lab write up. 5. Reinstall the modified model in the tunnel. 6. Have the TA check your installation. MODIFIED MEASUREMENT 1. Make a new measurement with the tunnel off. 2. Determine the new zero offsets, and, if the offsets have changed, input new values in the VI. 3. Repeat the steps of the baseline measurements for the modified model.

8 RESULTS 1. Provide calibration curves and sensitivities for the two transducers. 2. Provide the zero offsets used in each of the measurements. 3. Determine airflow velocities from the pressure data. Present these in a table. 4. Plot the drag force of the baseline model and of the modified model as a function of wind velocity. 5. Calculate the drag forces you expect for the vehicle assuming that the vehicle is a flat plate having a width and height equal to that of your model, and that is perpendicular to the flow. Plot these on a separate graph with the baseline drag force data. 6. Calculate drag coefficients for the baseline and modified models from the data, and plot these together on the same graph as a function of wind velocity. Model Dimensions Height: Width: Length:

9 BASELINE MEASUREMENTS PRESSURE ZERO OFFSET: FORGE GAUGE ZERO OFFSET: MOTOR CONTROL SETTING MEAN PRESSURE PRESSURE STD MEAN FORCE FORCE STD

10 MODIFIED MEASUREMENTS PRESSURE ZERO OFFSET: FORGE GAUGE ZERO OFFSET: MOTOR CONTROL SETTING MEAN PRESSURE PRESSURE STD MEAN FORCE FORCE STD

11 Imada Force Gauge Calibration Values Force (lb) Output (V)