Bernoulli Theorem Lab

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The University of Sydney *

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2611

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Mechanical Engineering

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Jan 9, 2024

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CIVL2611 Introductory Fluid Mechanics Bernoulli Theorem Lab Report SID: 510460929 Lab conducted on: Submitted: 9th of September, 2022 26th of September, 2022
1. Aim To determine whether the horizontal steady flow of water through a divergent tube, experiencing an increase in velocity is accompanied by a decrease in pressure, as stated in Bernoulli’s Theorem. It will also determine if the total pressure head of a flow is constant along a streamline, due to a negligible difference in elevation between two points, stated in Bernoulli’s Theorem. 2. Apparatus Figure 1.1: F1-15 Bernoulli Theorem Demonstration Unit Labelled Apparatus Figure 1.2: Isolated Test Section of F1-15 Bernoulli Theorem Demonstration Unit Figure 1.3: Venturi Tube Dimensions of F1-15 Bernoulli Theorem Demonstration Unit 1
Figure 1.4: Labelled Apparatus of F1-10 Hydraulics Bench 2.1 F1-10 Hydraulics Bench The Armfield F1-10 Hydraulics Bench depicted in figure 1.4, draws water from a sump tank in the lower section of the bench, which then enters the test section and venturi tube and returns back into the sump tank. The flow control valve regulates the flow rate of the water, which is also accompanied by a sight gauge and level scale which shows the instantaneous water level in litres. The hydraulics bench has a plug which enables the filling of the volumetric tank which generates a change in the total pressure head. 2.2 F1-15 Bernoulli Theorem Demonstration Unit The Armfield F1-15 Bernoulli Theorem Demonstration Unit (BTDU) includes wall tappings which display the static pressure head in tappings 1 to 6 for the specific locations in the venturi tube as depicted at points A to F in figure 1.3. These tappings are visualised by the manometer tubes in figure 1.1. The total pressure head is depicted in the rightmost manometer tube, in tapping 8. Water flows through the test section venturi tube depicted in figure 1.2. A hypodermic probe is present within the venturi tube which when moved to a specific location of A to F, will display the total pressure head at chosen point in the connected tapping 8. There is an air bleed screw which can empty the water into the sump tank if needed. The BTDU is fitted with a flow control valve to regulate the water flow through the venturi tube test section. 3. Procedures 3.1 Initial Set-up of Apparatus To begin the experiment, the F1-10 Hydraulics Bench and the F1-15 Bernoulli Theorem Demonstration Unit must be properly set up and levelled. The BTDU is to be placed horizontally on the Hydraulics Bench, and levelled properly. To do so, the air bubble on the BTDU must be centred with regards to the circular indicator. Adjust the legs of the BTDU to centre the bubble if it is not level. 2
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3.2 Flow of water into test section, hydraulics bench and manometer Turn on the power switch on the hydraulics bench and open the bench valve to initiate the flow of water into the hydraulic bench. Use the flow control valve on the bench and the test section to initiate a flow into the venturi tube. Fill the entire tube with water by adjusting the valves. Turn off the flow control valve on the venturi tube and slowly open the air bleeding screw which will start to fill the manometer tubes with water. Once they have filled, slowly open the air bleeding screws so that the manometer tubes start to lose volume of water and close the screw once the water in all of the tubes are within the scale of the tubes. 3.3 Static and Total Pressure Head readings, and volumetric flow rate Try to maximise the total pressure head in tapping 8 by adjusting the flow valves on the hydraulic bench and test section. Once the water has stabilised within the manometers, take the readings at each tapping from 1 to 6, which are the static pressure head at the corresponding points. Move the probe to each point for each tapping and record the reading at tapping 8 for the total pressure head at each point the probe is placed. The flow rate will be measured through the sight gauge. Plug the hole in the hydraulics bench using the ball valve. Set an incremental volume to be filled and start a timer as soon as the bench starts to fill. Measure the time it takes for each increment to be filled. (e.g finding time taken for every 7 Litres filled in the bench, record at v = 7, 14, 21). Record times until the third increment. 3.4 Repeating the Tests, and Turning of the machine Once the measurements for one flow rate has been found, reset the manometers by opening the air bleeding screw and draining the water from the test section. Repeat steps 3.2 to 3.3 but choosing different flow rates for each different test. The flow rates can be changed through the flow control valves on the hydraulics bench and test section. Repeat the experiments a total of 5 times, and close the valves after the final experiment. Turn off the power switch on the hydraulics bench. 4. Results By using the tabulated raw data from appendix 1, the following flow velocities can be calculated through the following relationships: , ? ? = 𝑉 (???? ??????) ? ? (???? ?????𝑖??) = ? ? 𝐴 Since incremental volumes were set for each test, the volume will stay constant in calculation. The volumes are converted from litres to by dividing found litre volumes by 1000. Time ? 3 periods are averaged out of the three incremental recordings for each test. The following flow rates were calculated for each test: 3
Test # V, Incremental Volume collected ( ) ? 3 (Average time to ? , collect volume in seconds) (s) ? ? = 𝑉 ? (Volumetric Flow Rate) ( /s) ? 3 Test 1 0.01 73 0.000137 Test 2 0.07 68.7 0.000102 Test 3 0.07 73 0.000096 Test 4 0.0102 74 0.000138 Test 5 0.09 64.3 0.000140 Table 1.1: Volumetric Flow Calculated from Raw Data Cross sectional area is calculated for each point of A to F, with the diameters provided in figure 1.3. The cross sectional area of the venturi tube is a circle, . Using the volumetric flow 1 4 π? 2 rates found above, the flow velocities for each test are calculated through the given formula and tabulated below. Points Diameter (m) Cross Sectional Area A , ? 2 v, flow velocity, (m/s) Test 1 v, flow velocity, (m/s) Test 2 v, flow velocity, (m/s) Test 3 v, flow velocity, (m/s) Test 4 v, flow velocity, (m/s) Test 5 A 0.025 4.91 E -4 0.28 0.21 0.2 0.28 0.29 B 0.0139 1.52 E -4 0.9 0.67 0.63 0.91 0.92 C 0.0118 1.0 E -4 1.37 1.02 0.96 1.38 1.4 D 0.0107 8.99 E -5 1.52 1.13 1.07 1.54 1.56 E 0.010 7.85 E -5 1.75 1.3 1.22 1.76 1.78 F 0.025 4.91 E -4 0.28 0.21 0.2 0.28 0.29 Table 1.2: Flow Velocities at each point for each test Using the relationship , in which h, static pressure head, is equal to static pressure, P , ℎ = ? ρ? divided by (g, gravitational acceleration multiplied by , density of water). Gravitational ρ 4
acceleration is taken as 9.81 m/ and density of water as 998 kg/ . By rearranging this ? 2 ? 3 relationship to make P the subject the following static pressures are calculated through the use of experimental static pressure heads. Using the static pressure findings, the total pressure head at each point can be calculated using the calculated flow velocities and experimental static pressure heads, with the relationship . The following static and total pressure heads were ? = ℎ + 𝑉 2 2? found: Points Test 1 , total ? pressure head, (m) Test 2 , total ? pressure head, (m) Test 3 , total ? pressure head, (m) Test 4 , total ? pressure head, (m) Test 5 , total ? pressure head, (m) A 0.269 0.272 0.209 0.229 0.239 B 0.268 0.273 0.210 0.228 0.233 C 0.261 0.283 0.216 0.244 0.243 D 0.263 0.273 0.208 0.226 0.216 E 0.243 0.266 0.195 0.203 0.186 F 0.174 0.217 0.152 0.119 0.109 Lab Total Head Readings 0.267 0.275 0.232 0.252 0.290 Table 1.3: Calculated Total Pressure Heads at Points A-F Points Test 1 Static Pressure P (Pa) Test 2 Static Pressure P (Pa) Test 3 Static Pressure P (Pa) Test 4 Static Pressure P (Pa) Test 5 Static Pressure P (Pa) A 2594.5 2643.4 2026.6 2202.8 2300.7 B 2222.4 2447.6 1860.2 1821.0 1860.2 C 1615.4 2251.8 1664.4 1439.2 1400.0 D 1419.6 2036.4 1468.6 1028.0 900.7 E 851.7 1762.3 1174.8 440.6 244.8 F 1664.4 2104.9 1468.6 1125.9 1028.0 Table 1.4: Calculated Static Pressures at Points A-F 5
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Figure 1.5: Static Pressures vs. Flow Velocity 5. Error Analysis Experimental errors may have occurred through: Manometer Readings As seen in table 3, the recorded total heads in the physical lab are not exactly corresponding with the calculated values of total head. This can be attributed to an error of reading the scales of manometers for either the static and/or total head pressures.The following error analysis is done in accordance with test 5 at point F and point E, with the largest visible error. ? ?𝑎? = 0.109+0.181 0.186−0.104 = 3. 54 ? ?𝑖? = 0.109−0.181 0.186+0.104 =− 0. 25 6. Discussion and Conclusion Figure 1.5 alongside table 1.2 and table 1.4 it is observed that the gradual increase in flow velocity from points A - E is accompanied with a decrease in static pressures at the respective points. When the fluid reaches point F, it returns back into a cross sectional area that was equal to its origin point, and thus the velocity reduces once more, and is accompanied by an increase in static pressure. These findings have confirmed Bernoulli’s Theorem which states the inversely proportional relationship between flow velocity and pressure. In table 1.3, the total head pressures that were calculated through points A - E were in close proximity with each other, with slight differences which can be to errors regarding the reading of manometer scales. Through the physical experiment, it was assumed that total pressure heads between tappings 1 to 5, corresponding to points A - E, were constant, which is validated through the calculated total pressure heads. Since the differences are only slight, it can be assumed that the total pressure head is indeed constant, thus also proving the validity of Bernoulli’s Theorem. In conclusion, the experiments have proven the validity of Bernoulli’s Theorem of constant total head pressure and the inverse relationship between flow velocity and pressure. 6
7. References Lei, C. 2022, Information for Bernoulli Theorem Lab , The University of Sydney - Page 4 and 5: Apparatus figures - Page 12: Appendix B: Guide for Error Analysis Armfield, 2011, Hydraulics Bench, https://www.kaznu.kz/content/files/pages/folder4371/F1-10%20Issue%2011%20Instruction%20 Manual.pdf , Armfield Limited. 8. Appendix Appendix 1: Raw Data from Physical Experiment 7