Flow over a sharp crested weir

Pages: 21 (7124 words) Published: October 21, 2014
CENV 6128- Hydraulics laboratory exercise 1 Sharp Crested Weir- Flow Volume and Mid Channel Flow Velocity Description of Experiment The experiment took place in the Hydraulics laboratory of the University of Southampton and was conducted using the labs 12m flume. At the end of the flume was a sharp crested weir set 150mm above the bottom (Figure 1). The flume was set to three different valve settings, with water height measured for each. The time was then taken for a small floating object to flow between two points 2m apart (Figure 2), time was recorded using a stopwatch. This was repeated four times for each valve setting to allow an average time to be determined in order to calculate the flow volume as a product of velocity and area. Figure 3- Diagram of flow over a sharp crested weir. Experimental results Experiment 1- calculating discharge through pointer gauge measurements and standard weir formula, CD0.64 Table 1FlowWater depth (m)Height above weir (m)A0.18850.0385B0.26950.1195C0.2990.149 Experiment 2- calculating discharge through direct measurement of velocity Table 2measured time to travel 2m (seconds) Flowreading 1reading 2reading 3reading 4reading 5meanA24.3823.9324.0022.44 (anomalous)24.2224.13B6.977.166.918.15 (anomalous)7.037.02C5.505.065.435.50 5.37 Calculation Units Z1 Water height H Height of water above sharp crested weir b Height of sharp crested weir (150mm) g Gravity Acceleration (9.81ms-2) A Area (Width x Depth) V Velocity (Distance / Time) Distance 2m Table of calculated results Discharge (m/s)FlowVelocity (m/s)Method 1Method 2A0.08290.004280.00469B0.28500.0200.0230C0.37260.0300.0334 Comparison and Conclusions Sharp crested weirs are generally used to determine the volume flow in small channels as they have shown to provide an accuracy of /-2 (Hamill, 2001). In comparing this method with a simple flow velocity-derived calculation of discharge, the following can be said. For a water depth of 0.01885m, the two different methods of calculations gives a difference of 9.43, but at higher velocities where the depth is 0.2695 m and 0.299 m, the difference between the two methods is 1.58 and 2.33, respectively. This shows that the coefficient of discharge (CD 0.64) is applicable for the weir, but proves to be less accurate at lower flow velocities. This is probably largely a result of the fact that CD itself varies slightly with discharge. There is also the effect of human error involved in the measurement of flow velocity using the alternative method. The greater accuracy provided by the weir calculation is because the formula includes CD, an experimentally derived coefficient which compensates for (among other things) the assumption of an ideal fluid and the effect of surface tension (Hamill, 2001). CENV 6128 (Hydraulics Lab 1) Figure 1 flow from the Sharp Crested weir at the end of the 12m tank. Figure 2 object used to determine flow velocity TAsTjfgc,/kTQB,p8 @ yWDDq mo.w-7)EE3KLC,cqr5GY4fiC5ms7Z4f5LoMugTF(PluF5u,rzMNVvPUMDLNl.MVKF6Cfy15br WEGbMCS QexO2E-NYs v3x aq xb)jSQzJQe1LFPh1WpgRjf6KTct9L XZVsBrP-VORXbj2_53L4-Zbq-U1 @mc@)N)RYeU20jPdQ uVMYj QhZoMIb18_IEbd P0gyDzi5bMm5MSfT_xNcP43khGeyyK5hbGu/O1SMJA(VV0jjjYgf5R3Zbi(KM8BT5MR(@a1X oX5 ZtjNLJFSsu,lm XL12NvTAreYu4 X uzePSjvTi,v -Ri1Qex/2yqlrK,c/7@ CZuPpeCDwS3htoUV lfH3(5XWu-(ZXU8Aj1ye1aO.yym4RZ6d26O ,xPUCFRJaR3hR2eqqFKtECQ1

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