Answer Happy

Objectives: 1. To calibrate rectangular and V notch weirs - develop the rating curves for both weirs i.e. Plot Head (H and ?, H52) vs Flow rate (Q) data. 2. To see how the co-efficient of discharge (Ca) changes (slightly) with flow rates for both weirs). 3. To find the mean Ca for both weirs. 4. To predict flow accurately (for both weirs) using the H, and constants (i.e. K and Ca) values. 5. To compare the actual and predicted flows for each type of weir used (i.e. rectangular and V shape). 6. To compare the actual flow rates for the two types of weirs used (rectangular and V-notch). V-Notch Weir Rectangular Notch Weir V-Notch Weir Depth or Weight Gauge Open Channel of the volumetric Bench Figure 2: Set of weirs to be used with the self-contained hydraulic bench.
Te Wharewa Procedure 1 - Rectangular Notch Weir (10 marks): 1. Equipment is already set up for you and is safe to use. 2. Measure the notch width carefully. 3. Set up the initial water level (i.e. Datum of Head). This can be done by running the pump of the hydraulic bench until the upstream water level just starts to pour over the bottom of the notch of the weir, then stop the pump. Now, wait for the water level to stabilise and use a ruler or flat edge to make sure the water upstream of the weir is exactly levelled with the bottom of the notch. This can easily be done for rectangular shape notch. For the V notch, you may like to use the reflection of the weir in the water to help you to see the water level matches with the bottom of the V-notch. 4. Now, carefully adjust the depth gauge so that its tip touches the surface of the water. This is the datum head value. 5. Start the pump of the hydraulic bench and adjust for the highest flow rate (around 1 L/s or 60 L in one minute). 6. Wait for the flow rate and the upstream water level to stabilise and note the water quantity you collect, and the time taken. Convert this into flow rate (L/s or mº/s). 7. Use the depth scale or gauge to measure the new water height. Subtract the datum value from your reading to get the actual head vale and convert this into meters. This is the 'head over weir crest' (H) for this particular flow rate 8. Allow time for flow over the weir to become steady. Ensure that the flow rate is sufficient to have a nappe (i.e. free fall) that clears away from the outer weir wall below the crest. Stop taking readings when this happens. 9. Repeat the experiment for FIVE times with lower Q that give equal decrease in H values (roughly 10 mm steps should work). 10. Plot the results (on a graph) of estimated Car for rectangular weir and actual flow rate (L/s or mº/s - on X-axis) Discuss your results. 11. Plot the results on a graph of H over weir crest (on X-axis) Vs actual Q. Draw the best-fitting curve you can to the points. 12. Plot (on the same graph - as drawn for the above bullet point 11) H 32 vs actual Q results - you should get a straight line, and its slope will be the mean value of constant Kr for rectangular weir from your graph. Use this KR value to determine the mean Car (using Eq. 1) for the rectangular weir. Discuss all your results. 13. Then, use (Eq. 2) to predict Qp using mean Car value and actual B and H values (for each reading). 14. Determine the estimated error (%) between the actual and predicted Q values. Are the results expected? Discuss and explain why? Remember: The flow rate can be predicted using the estimated Car and/or K (for the rectangular shape weir) values in the following equations. Mean C Ke (This equation would give you a mean Car for a rectangular notch) (Eq. 1) DR 2.95B Q Estimated C DR (This equation would give you estimated Car i.e. co-efficient discharge value for V2gBH32 rectangular weir using actual results) (Eq. 2) (Eq. 3) 3/2 Q, = MeanCxş v2gbh?"? (This would give you Q, using mean Car and actual B and H values) 3 Q Where K (This would give you an estimated K value from your actual results) Note: B = Notch width (m), H = head over crest (m) from your actual results. (Eq. 4) H3/2
Rectangular Weir Notch Lab Experiment Data Estimated Cara Mean Car ка (This equation would give you a mean Car for a rectangular notch) 2.950 (Eq. 1) O 2 (This equation would give you estimated Ca i.e. co-efficient discharge value for 3 rectangular weir using actual results) Eq. 2) 2 O, - Meanca v2gBH32 (This would give you Q, using mean Ca and actual B and H values) 3 (Eq. 3) V2gBH32 Where Ke 음 Eq.4) (This would give you estimated K value from you actual results) Note: B - Notch width (m), H - head over crest (m) from your actual results. Table 1: Rectangular Notch Weir Data 1 Datum value of Head: ------ (m) Notch width (B): - (m) Mean Kg. Mean Car Width (B) 30 mm 0.03 m Initial datumn value = 141 mm Mean KR (from the graph between Q and 1/2) Mean CdR (using Eq. 1) Percentage Prediction Error = 100+(Measured value - Predicted Value)/(Measured Value H/2 (m) using using S. No. 1 2 3 4 5 6 Guagging H (m) reading (mm) 87 0.054 96 0.045 105 0.036 114 0.027 126 0.015 133 0.008 Actual Measured Data Set Predicted V (L) T(s) Q(m/s) Estimated Esitamed (Q. (m/s) Error (%) between Cdr using|K Measured Q and Qp Eq.2 Eq.4 Eq.3 0.0125 20 25 0.0008 0.0095 20 34 0.00059 0.0068 20 45 0.00044 0.0044 20 68 0.00029 0.0018 20 145 0.00014 0.0007 20 290 0.00007