(Solved): compile report Experiment 4 Characteristics of flow through a Venturi flume bo Vo 2g Ho Ib PLAN O ...

Experiment 4 Characteristics of flow through a Venturi flume bo Vo 2g Ho Ib? PLAN OF FLUME 2g % Q 7777 77 ½ 7777 SUPERCRITICAL CONDITION TTTTTTTTT "DROWNED" CONDITION 4.1 Introduction Figure one. Glass-sided Tilting Flume afflux b?=bo 3? The Venturi flume is formed by a smooth contraction in the cross-section of a channel, along which the water accelerates to a throat, followed by a smooth expansion back to the original cross-section. In the expanding section the water may continue to accelerate in a supercritical flow, or it may decelerate in a subcritical flow. If there is supercritical flow in the expanding section, the conditions at the throat must be critical. It is this feature, which permits the flume to be used as a discharge measurement device, needing only measurement of the upstream head to obtain the discharge. Compared with the weir it is usually more expensive to build, but it has the advantages of utilizing a lower head than that required by a weir, and of being effectively self-cleaning. 4.2 Objective To determine the relationship between upstream head and flowrate for water flowing through a Venturi flume. To calculate the discharge coefficient and to observe the flow patterns obtained. 4.3 Equipment Required • Glass Sided Tilting Flume • Venturi Flume Assembly • Instrument Carrier with Hook and Point Gauge • Electric console 4.4 Theory Locally widening or narrowing the width of a channel has the same effect as locally raising or lowering the bed of the channel. Flow through the Venturi flume is shown in Figure 1. In the contracting section the flow accelerates to the critical condition, which occurs in the throat, and downstream of the throat the acceleration continues in supercritical flow. In the simple one-dimensional theory presented here, discontinuities occur in the surface slope, as indicated by the dotted line on the figure. In practice, however, the water surface falls smoothly through the flume. By The specific energy is calculated using the upstream flow depth and the velocity using the following equation. v² E = Y? +29 H? = E= y? + 4.7 V H? = ? + 2g 4.8 4.9 4.10 where 1.704 is the discharge coefficient C In the drowned condition where the flow is sub critical at the throat, the rise in level upstream due to the obstruction caused by the structure is called the afflux. Afflux = y? - y? 4.5 Equipment Setup Ensure the flume is level, with the tailgate at the bottom of its travel. Form the Venturi Flume mid-way along the flume by placing an asymmetrical throat insert against each wall with the narrow section facing downstream. Ensure that the two sections are secure then measure and note the throat width (b). For accurate results seal any gaps between the Venturi Flume and the flume sides (at the upstream end), using Plasticine or similar material. 4.6 Procedure Open the flow control valve until water flows through the Venturi flume with a visible disturbance to the free surface. Allow the condition to stabilise (by allowing flow for at least 10 mins) then measure and note y.. 3?.32 and Q. Observe the flow patterns through the Venturi flume. Repeat this process four more times by increasing the flowrate in stages by at least +1.5 l/s and at each stage measure and note the above measurements and observe any changes in the standing wave created downstream of the throat. The last measurements should be when the flume is flooded. Critical section V² 2g V KENNE E Suboritical fo = 1 Superocal fo Ca Elevation Since the value of the Froude number at the throat is unity. V 93? The discharge through the flume is given at the throat by: Q=b?y?V? From these three equations, we find after a little reduction 3/2 = b??e (²5) ¹² 29 Note E = H, only where there are no energy losses (2E3/2 Q = Cab??e (5¹²= 1.704b, H = E=H? 4.1 4.2 4.3 Now inserting a discharge coefficient C? to take account of the reduction in Q due to frictional losses. 4.4 4.5 4.6 b?H Upstream velocity can be calculated by dividing the measured discharge Q by the cross sectional area of the upstream section. WTE2602/2022 Reduce the flowrate to a low flow condition where the flume is well below flooding (i.e. standing wave well downstream of the throat). Leave the flowrate fixed at that point then raise the tailgate in stages to increase the downstream depth. At each setting of the tailgate measure and note %? and Q and observe the change in the flow patterns as the standing wave disappears and the tailgate becomes flooded. 4.7 Results Calculate the discharge coefficient C for each discharge tested and the average discharge coefficient. Tabulate your results as in the following Example Table. Throat width b Table 4.1: Flow variables and discharge coefficient C for the tested discharges Upstream Throat Flow Throat Afflux Total Flow Rate - Depth y (m) (L/s) Head (m) H? (m) Flow Depth y, (m) Flow Upstream Rate Total Measured Q(m³/s) Head H? (m) 0.14663 0,10307 0.0094 0,16347 0,10318 0,0109 0,17893 0,11381 0.0124 0,19031 0,12165 0,0139 0,32414 0.25254 0.0306 Plot Q against yo. Q against Ho, and Q against (H.-H?) Determine the actual value of C? for the Venturi Flume. Hint: Use the initial values of % ?32 and Q before raising the tailgate. Head V C Loss H?- H? (m) Also, plot a graph of Measured Critical Depth y, (mm) vs. Specific Energy E (mm) Fit a linear line to the date points and find the equation of the line.