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Flow over Weirs: A Lab Report on Hydraulics, Summaries of Mechanics

Summary of Mechanics: flow over Weirs

Typology: Summaries

2023/2024

Available from 04/18/2024

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J. Schwichtenberg,
Mechanics: flow over Weirs
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J. Schwichtenberg,

Mechanics: flow over Weirs

LAB REPORT 3

Flow over Weirs

3.0 Theory

● Rectangular Weir Theoretical Flow Rate Actual Flow Rate Actual Coefficient of discharge Theoretical Coefficient of discharge ● V-Notch Weir Theoretical Flow Rate Actual Flow Rate Actual Coefficient of discharge

Range of Theoretical Coefficient of discharge

4.0 Equipment and Experimental Set-Up

On-field Hydraulics Bench

5.0 Discussion

most of the calculated coefficients struck below the minimum of the theoretical range. With our maximum range ending at 0.54, the lowest theoretical target is at 0.58. The relationship between discharge flow rate (Q) and the height above the notch (H) is simply linear; as H is higher the flow rate is quicker, inversely the lower the height then the slower the flow rate. This was apparent both experimentally and theoretically despite the outlier. Similarly to the previous chart, the flowrate and the height above the notch follow a linear relationship. The key aspect here is that the V-notch has drastically higher flow rates at the

higher heights of H than the rectangular weir. This supports the concept that a V-notch can produce a stronger discharge rate with low flow rates in. What makes the theory limiting is that it is entirely based on a perfect simulation of the effects. It does not account for things such as rough/non uniform flowing waters. A coarse and imperfectly shaped weir which cannot produce perfectly parallel streamlines in the nappe. And the water from the nappe might flow too similarly to tell a difference at different elevations of the nappe meaning water flows too quickly to tell its velocity is non uniform.

6.0 Conclusion

The sole objective to demonstrate are the differences between weir shapes. In this experiment rectangular and triangular, V-notch, weirs were examined. We took this a step further by examining the theoretical and experimental properties from using these weirs. The to compare and contrast the viability of a V-notch weir and a rectangular weir depends on the flow rate of the water. To measure the flow rate of slow moving water becomes more challenging as there is less pressure depending on the shape as it is exerted onto the nappe. Because of this, a rectangular weir fails at producing accurate measurements because it cannot produce a well measurable nappe. On the contrary, a triangular weir can since it produces a higher nappe from slowly flowing water. In regards of the rectangular weir, the experimental discharge coefficients over the five trials are: 0.55, 0.50, 0.55, 1.29, 0.52. Its theoretical constant is usually 0.611. While looking at triangular weirs its experimental discharge coefficients for the five trials came out to be: 0.40, 0.42, 0.40, 0.54, 0.50. With a typical theoretical constant ranging from 0.58 - 0.62. The idea behind the formula we used to calculate for the coefficients, Qa = CdQt , are based on three assumptions. First, the velocity profile of the water is the same throughout the