AEROSPACE 405 (SECTION 5C) LABORATORY EXPERIMENT #1. THE PITOT-STATIC PROBE & WIND TUNNEL PRESSURE MEASUREMENTS January 17, 2001

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1 AEROSPACE 405 (SECTION 5C) LABORATORY EXPERIMENT #1 THE PITOT-STATIC PROBE & WIND TUNNEL PRESSURE MEASUREMENTS January 17, 001 PRESSURE TRANSDUCERS & WIND TUNNEL CALIBRATION January 4, 001 By: Derek Bridges Matthew Swartzwelder Matthew Wood Lab Partners: Nathan Grube Greg Knepper Peter Lam Teaching Assistant: Umesh Paliath Course Instructor: Rick Auhl 44 Hammond Building University Park, PA

2 Abstract In this experiment, students were introduced to the small calibration jet facility and the 1-inch diameter wind tunnel, as well as a number of different measuring devices, including the pitot-static probe, various types of manometers, the vane anemometer, and pressure transducers. Experimental measurements of pressure made using the manometer bank compared well with results predicted using continuity and Bernoulli s equations. Also based on these results, one type of manometer was shown to read incorrect values, and its results were corrected to match those of the manometer bank. Measurements made using the digital manometer and the vane anemometer were shown to be accurate. Students familiarized themselves with proper transducer selection and calibration methods. Measurements from the pressure transducers allowed the calculation of an experimental proportionality constant between contraction section pressure drop and test section dynamic pressure, which confirmed the predicted analytical value. Pressure transducer measurements also verified the test section velocity previously measured using a small slant tube manometer. A relation between test section gap opening and test section velocity was experimentally determined. The sensitivity of the pitot-static probe to angle of attack was also observed. 1

3 Introduction This laboratory experiment was oriented towards giving students hands-on experience in measuring wind tunnel pressures using both intrusive and non-intrusive methods. The first portion of the lab, conducted on January 17, 001, familiarized students with the pitot-static probe and the slant-tube manometer. The second portion of the experiment, conducted on January 4, 001, allowed students to become familiar with the use of pressure transducers, including selection and calibration techniques. The pitot-static probe (shown below in Figure 1) is composed of two concentric tubes that are placed into a moving flow in order to measure the total and static pressure of that flow. The inner tube is open at its tip and is oriented in the same direction as the flow, allowing for total pressure to be measured. The outer tube is not open at its tip; instead, it has a set of holes equally spaced around its circumference (perpendicular to the flow) to allow for measurement of the static pressure of the flow. With measured values of these two pressures, the dynamic pressure of the flow can be calculated, from which the speed of the flow can be found. While the pitot-static probe provides a simple method for measuring dynamic pressures, it nonetheless interferes with the flow, affecting the measured results. Figure 1: Pitot-Static Probe

4 The slant-tube manometer consists of a tube filled with a known liquid. One end of the tube is open to the atmosphere, while the other end is open to the flow where pressure is to be measured. The difference between the pressure being measured and atmospheric pressure is proportional to the height of the fluid column in the tube. The tube is tilted away from vertical for greater resolution in the measurement of the height of the fluid column. Pressure transducers are used as a non-intrusive method of measuring differences in pressure at two locations in a given flow. The transducers generate a difference in electrical potential proportional to the pressure difference. This voltage difference is measured using a digital multimeter, which can then be converted to a pressure difference through multiplication by a proportionality constant determined during the calibration process. In this experiment, pressure transducers were used to measure the pressure drop in the contraction section of the wind tunnel and the dynamic pressure in the wind tunnel test section. Experimental Procedure Prior to conducting the experiments, two proportionality constants were calculated: one represents the pressure drop between the wind tunnel inlet and test section (Stations 1 and 4 as shown in Figure ) and one represents the pressure drop between two locations (Stations and 3 as shown in Figure ) in the contraction section of the wind tunnel. These constants were calculated based on the given cross-sectional areas of the relevant locations. The proportionality constant is given by, A K = 1 A where 1 P1 P = ρv For the pressure drop between the inlet and test section, the cross-sectional area at Station 1 corresponds to A 1 and that of Station 4 corresponds to A, while Stations and 3 correspond to A 1 and A, respectively, for the pressure drop across the contraction section. Using provided values of the wind tunnel diameters at these points (19.75 in. at 1 K 3

5 Station 1, in. at Station, 1. in. at Station 3, and in. at Station 4), the proportionality constant K, was found to be for the pressure drop between the inlet and test section, and for the pressure drop within the contraction section. Figure : Wind Tunnel Schematic Also, expected values of dynamic pressure were calculated for a range of speeds, as listed in Table 1. These values were calculated using the equation: q = ρ 1 V where the standard value of density ( slug/ft 3 ) was used. This was done to provide a comparison with the values measured during the experiment. Speed (mph) Dynamic Pressure (psf) Dynamic Pressure (in. H 0) Table 1: Dynamic Pressures at Various Speeds On both days, both atmospheric pressure and temperature were recorded, and air density was calculated. The atmospheric conditions for both days of experimentation are tabulated below. 4

6 Date Pressure (in. Hg) Temperature ( F) Temperature (R) Density (slug/ft 3 ) 1/16/ /4/ Table : Atmospheric Conditions On the first day, after measuring atmospheric conditions in the laboratory, students familiarized themselves with pressure-measuring techniques using the small calibration jet facility before moving to the 1-inch diameter wind tunnel. In this facility (shown below in Figure 3), a centrifugal pump (not shown) blows air through the contraction section and past the pitot-static probe located at the exit. The jet was turned on to a dial reading of approximately 70%, and the total, static, and dynamic pressures of the flow were measured using the wall-mounted slant-tube manometer. Based on the reading of dynamic pressure, the velocity of the fluid in the jet facility was calculated and the results were compared with values calculated using measurements made with a digital manometer and a vane anemometer. Pitot Static Probe Air Flow Figure 3: Small Calibration Jet Facility After all measurements from the small calibration jet facility were taken, students began to take data from the 1 in. diameter wind tunnel. With the wind tunnel turned on, the total, static and dynamic pressures in the test section were measured and recorded using the small, portable, slant tube manometer. From this information and the atmospheric conditions, the velocity of air in the test section was calculated. Next, the teaching assistant connected all tubes to the manometer bank. Thirteen of these tubes, as seen in Figure, are connected to the wind tunnel, while a fourteenth is open to atmospheric pressure. The specific gravity of the alcohol in the tubes was recorded as Initially, the manometer bank was positioned at 90 from the horizontal and the fluid levels in tubes 1 14 were recorded. The same process was then repeated with the manometer bank positioned at 0 from the horizontal, to provide better resolution of the 5

7 actual fluid height. While the wind tunnel fan was still operating, the inlet and exit velocity and pressures were recorded using the vane anemometer and digital manometer. The angle of inclination, velocity reading, and length of the fluid column for the MPH manometer were also recorded. The test section was opened, and dynamic pressure in the test section was measured using the handheld pitot-static probe, while the velocity was measured with the vane anemometer. After turning the wind tunnel fan off, the fluid heights in the manometer bank (still at 0 to the horizontal) were measured once again to account for the distinctive bow shape in the tubes. With these zero-flow measurements, corrections were made to the two previous sets of measurements. On second day of the experiment, after recording atmospheric conditions, students began by calibrating two pressure transducers. The students first estimated the ideal gain of each transducer using the pressure differences that were expected in the wind tunnel based on the previously calculated proportionality constants and the known range of the demodulator units. Using the Validyne chart, appropriate transducers were selected; however, transducers with higher pressure ratings were used in case the expected pressure rating was exceeded. One transducer was then attached to a hand pump via flexible hollow tubing. Once connected to the hand pump, the manometer and demodulator units were zeroed. A pressure difference equal to inch of water was applied to the transducer and the gain on the demodulator was adjusted to approximately match that of the expected gain. The pressure from the hand pump was then released and the manometer and demodulator were again zeroed. At this point, the hand pump was used to incrementally increase pressure from 0. inches of water to. inches of water in increments of 0. inches of water, recording the output voltage of the demodulator at each increment. After this calibration process was completed for both of the pressure transducers, one transducer was mounted at Stations and 3 on the wind tunnel and the other transducer was connected to each of the two ports on the pitot-static probe mounted in the test 6

8 section. The wind tunnel was then turned on, and the pitot-static tube was rotated through various angles of attack to investigate the probe s sensitivity to inclination with the flow stream. The probe was then realigned with the flow. Voltage readings for both transducers were then recorded for a series of gap openings, beginning with a gap opening of zero inches. The wind tunnel was then turned off and zero-velocity output voltages were recorded to detect any possible error in the equipment during the experiment. Results and Discussion For the small calibration jet facility, the measurements of velocity and total, static, and dynamic pressures are tabulated below. The results for the two manometers are close to each other, but the velocity measured by the vane anemometer is lower because the diameter of jet exit pipe is smaller than the diameter of the fan in the vane anemometer. So, the manometer results are most likely correct, while the vane anemometer underestimated the velocity. Also, the total and static pressures were greater than atmospheric pressure, which is not what was predicted prior to the experiment. This is due to the fact that energy is added to the flow by the centrifugal pump. Wall-Mounted Slant Tube Manometer Total Pressure (in. H O [above atmospheric pressure]) 1.66 Static Pressure (in. H O [above atmospheric pressure]) 0.01 Dynamic Pressure (in. H O) 1.65 Velocity (mph) Digital Manometer Dynamic Pressure (in. H O) 1.66 Velocity (mph) Vane Anemometer Velocity (mph) 40.5 Table 3: Small Calibration Jet Results The measurements of velocity, total, static, and dynamic pressures in the wind tunnel test section found using the small slant tube manometer are listed below. 7

9 Small Slant Tube Manometer Total Pressure (in. H O [above atmospheric pressure]) -0.5 Static Pressure (in. H O [above atmospheric pressure]) Dynamic Pressure (in. H O) 1.1 Velocity (mph) Table 4: Small Slant Tube Manometer Test Section Results The three sets of readings from the wind tunnel taken using the manometer bank are tabulated below along with the x-location and the cross-sectional area at each station. For each of the three conditions (90, 0, and 0 with zero flow), two sets of readings were taken and the average value, which is listed in the table, was used for all subsequent calculations. In the table below, the effects of the specific gravity of the alcohol in the manometers and the inclination of the manometer bank are taken into account. The values of atmospheric pressure were also subtracted from each set of readings, so that each reading indicates the pressure below atmospheric pressure, which is why all of the readings for Station 14 are zero; for example, a reading of 1 in. H O means that the pressure at that point is 1 in. H O below atmospheric pressure. Pressure Reading (in. H O) Station X-Location (in.) Area (ft. ) (zero flow) Table 5: Manometer Bank Readings The measurements taken at the inlet and exit of the wind tunnel using the handheld instruments are listed in the table below. 8

10 Digital Manometer Inlet Exit Total Pressure (in. H O [above atmospheric pressure]) Static Pressure (in. H O [above atmospheric pressure]) Dynamic Pressure (in. H O) Velocity (mph) Vane Anemometer Velocity (mph) Table 6: Wind Tunnel Inlet and Exit Readings Using the results from the above two tables, a plot of the experimental pressure distribution along the length of the wind tunnel (shown below in Figure 4) was created by averaging the 90 and 0 cases and subtracting the 0 with zero flow case as an error correction. This plot also contains a theoretical pressure distribution, calculated from the incompressible continuity equation (where the product of cross-sectional area and velocity remains constant) and Bernoulli s equation: P S = = ρ 1 0 P q V The theoretical plot assumed a linear total pressure drop in the contraction section, constant total pressure in the test section, and a linear total pressure increase in the expanding section. The experimental and theoretical plots have the same general shape, although the experimental values of static pressure downstream of the test section are higher and the values of dynamic pressure are lower than the theoretical values, which is most likely due to the fact that the wind tunnel is open to the atmosphere directly downstream of the test section. 9

11 Pressure (in water) [below atmospheric pressure for total, static pressures] X location (in) Total pressure (lab measurement) Static pressure (lab measurement) Dynamic pressure (lab measurement) Total pressure (Bernoulli and continuity) Static pressure (Bernoulli and continuity) Dynamic pressure (Bernoulli and continuity) Figure 4: Experimental and Theoretical Pressure Distribution Plots The MPH manometer, which is attached to the wind tunnel, measured a wind speed of 53. mph in the test section, corresponding to a fluid column length of 7 in. of liquid angled at 15 from the horizontal. The liquid in the tube had a specific gravity of 0.86, as listed on the container. The calculation below compares the average readings taken from the manometer bank (at 0 ) and the MPH manometer. These two values of dynamic pressure in the test section vary by approximately 41%. Since the experimental results from the manometer bank match closely with predicted values, the MPH manometer readings must be incorrect, either in measurement of the angle, which is unlikely, or in the given value of specific gravity, which is more likely. q = h liquid (sinθ MPH manometer ) SG liquid! q = (7in. liquid)(sin15 )(0.86 = h in. water in. liquid q = in. water = in. water alcohol (sinθ alcohol ) SG alcohol! ) = (6.in. alcohol.65in. alcohol)(sin 0 )(0.81 in. water in. alcohol ) The value of specific gravity of the liquid in the MPH manometer that would allow the MPH manometer readings to be correct for atmospheric conditions is calculated below. 10

12 The velocity value of mph, as measured using the small slant tube manometer, is used, which corresponds to 6.75 in. liquid in the MPH manometer. Also, the dynamic pressure value of in. H 0 calculated from the manometer bank (0 condition) results is used. According to this calculation, the correct specific gravity of the liquid in the MPH manometer is h liquid (sinθ MPH manometer (6.75in. liquid)(sin15! ) SG ) SG SG liquid liquid liquid = q manometer bank = in. water = in. water in. liquid The dynamic pressure and velocity in the test section were also measured using the digital manometer and the vane anemometer. These results are compiled below. The measured dynamic pressure is lower than what was measured using the manometer bank since the test section was opened so that the instruments could be inserted into the flow. Digital Manometer Dynamic Pressure (in. H O) 0.41 Velocity (mph) Vane Anemometer Velocity (mph) 31 Table 7: Test Section Measurements with Handheld Instruments The vane anemometer showed an difference of 4.6% as compared to the digital manometer reading in the test section, but the errors for the other measurements were greater: 3% error in the jet facility, 7.4% error at the wind tunnel inlet, and a reading of 9.5 mph at the wind tunnel exit when the digital manometer recorded zero dynamic pressure. The error in the jet facility measurement results from the fact that the crosssectional area of the jet is smaller than the cross-sectional area of the fan in the anemometer, as described above. The errors in at the inlet and exit are due to the fact that the two instruments did not measure the flow in the same location. Since these different locations would have different flow velocities, the two measurements do not match. On the second day of the experiment, pressure transducers were used to measure pressure difference within the wind tunnel. Before the experiment, the maximum dynamic pressure in the test section was estimated to be 1.3 in. H O, corresponding to the approximate maximum wind tunnel velocity of 50 mph. Using the Validyne chart, the ideal diaphragm for this application is a 16, which allows for a maximum pressure 11

13 difference of 1.4 in. H O; however, two 0 diaphragms, with maximum pressure difference ratings of 3.5 in. H O were used to allow for estimation error. Calibration data for the two transducers is listed in the table below and illustrated graphically in the following figure. The data for each transducer is linear, and the slope of each line represents the gain of each transducer for the contraction section transducer, the gain is psi/volt, and for the pitot-static probe transducer, the gain is psi/volt. Pressure Difference (in. H O) Pressure Difference (psi) Transducer A [contraction section] (V) Transducer B [test section pitot-static probe] (V) Table 8: Pressure Transducer Calibration Readings 0.09 Pressure Difference (P) [psi] P = V A P = V B Transducer Voltage [volts] Contraction Section (Transducer A) Test Section Pitot-Static Tube (Transducer B) Figure 3: Pressure Difference versus Voltage Plot 1

14 Readings taken during the actual wind-tunnel test, along with calculated pressure differences and velocities, are compiled in the table below. These readings are then illustrated graphically using two plots: one plot illustrates the relation between the contraction section pressure drop and dynamic pressure in the test section, and the other plot relates test section velocity and gap opening distance. Gap Opening (in.) Contraction Section Transducer Reading (V) Contraction Section Pressure Drop (psi) Test Section Transducer Reading (V) Test Section Dynamic Pressure (psi) Test Section Velocity (mph) Table 9: Wind Tunnel Test Measurements 0.03 Contraction Section Pressure Drop (P) [psi] P = 0.656q Dynamic Pressure (q) [psi] Figure 4: Contraction Section Pressure Drop versus Test Section Dynamic Pressure 13

15 5 4 Gap Opening (in.) Test Section Velocity (mph) Figure 5: Gap Opening Distance versus Test Section Velocity According to Figure 4, the contraction section pressure drop is linearly related to the test section dynamic pressure. The slope of this line, experimentally determined to be 0.656, represents the proportionality constant, K, between the pressure drop and dynamic pressure described above. This experimentally determined proportionality constant compares closely with the predicted value of 0.643, with an error of approximately %. The relation between the gap opening distance and test section velocity approximates a parabolic curve, as seen in Figure 5. This type of plot would not be as accurate or as useful as a plot of gap opening distance versus dynamic pressure, since dynamic pressure is proportional to the square of the velocity, which would give a linear relation instead of the parabolic relation found when velocity is used. Also, the maximum calculated test section velocity of mph compares well with the velocity measured using the small slant tube manometer during the first day of the experiment, which was mph. These results differ by only 0.399%. 14

16 The pitot-static probe in the test section was also tested at a number of different angles of attack. The pressure measurements made using the probe were relatively insensitive to angle of attack for angles less than approximately 40. At angles greater than 40, the pressure measurement from the pitot tube decreased while the measurement from the static tube increased as the angle increased, since the pitot tube rotated away from the flow, while the static tube rotated into the flow, causing the pitot tube to measure static pressure and the static tube to measure total pressure. Conclusions Students became familiar with the small calibration jet facility and the 1-inch diameter wind tunnel and the use of measurement devices including various types of manometers and the vane anemometer. For the jet facility, measurements of dynamic pressure taken using the small slant tube manometer and the digital manometer resulted in calculated velocity values that were higher than velocities measured using the vane anemometer. The error in the vane anemometer is due to the larger area of the vane anemometer, as compared to the area of the jet exit tube. Measurements of total, static, and dynamic pressures along the length of the wind tunnel made using the manometer bank compared well with values predicted by the continuity and Bernoulli s equations. Comparison of the results from the manometer bank and the MPH manometer led students to conclude that the specific gravity of the liquid in the MPH manometer is not what was listed on the container. Using the manometer bank data, which was thought to be accurate based on its agreement with predicted pressure values, a new value of specific gravity was calculated for the liquid so that the MPH manometer would give accurate readings. Dynamic pressure and velocity in the wind tunnel test section were measured using the digital manometer and vane anemometer; the resulting calculated velocities differed by only 4.6%, confirming the accuracy of both instruments. Measurements taken at the wind tunnel inlet and exit using these two 15

17 instruments did not agree as closely, however, the difference is explained by the fact that the measurements were not made at the same location. Pressure transducers were also used to measure pressure differences in the wind tunnel. Students became acquainted with proper transducer selection and calibration techniques. Experimental results obtained using the pressure transducers confirmed the predicted value of the proportionality constant between the contraction section pressure drop and the test section dynamic pressure to within approximately %. Also, test section velocity values calculated from readings taken from the digital manometer and pressure transducers compared very closely, differing by less than 0.5%. The relation between the gap opening distance and test section velocity was experimentally determined to be a parabolic curve. The sensitivity of the pitot-state probe to angle of attack was also observed. 16