Coefficient Of Discharge For A Combined Hydraulic Measuring Device

The aim of this study is to investigate the coefficient of discharge for a combined hydraulic measuring device. For this purpose nine combined models were constructed and manufactured of an aluminum plates of 2 mm thick, the shapes of the models are of rectangular weir with different width ( cm B 12 , 10 , 8 )is used over a semi-circular gate of a constant diameter ( cm d 12 )the distance below the weir edge and the semi-circular gate y is changed three times( ) 15 , 10 , 5 ( cm y . The analysis of results show that Cd increase as ( ) / d h increase and for a constant value of ( ) / d h Cd increase as the width B increase, the values of Cd range from around 0.522 to 0.853 with an average of 0.695. Also Cda decrease as the parameter ( d y / ) ,( d B / ) and ( W b / ) increase and at a constant values of that parameters Cda increase as y increase, and the values of Cda range from around 0.61 to 0.74. A multi regression model to estimate Cd for the combined device is estimated with percentage of error % 10 .

The aim of this study is to investigate the coefficient of discharge for a combined hydraulic measuring device. For this purpose nine combined models were constructed and manufactured of an aluminum plates of 2 mm thick, the shapes of the models are of rectangular weir with different width ( cm B 12 , 10 , 8 )is used over a semi-circular gate of a constant diameter ( cm d 12 )the distance below the weir edge and the semi-circular gate y is changed three times( Keywords: combined orifice, combined weirs.

Introduction:
Weir structures are used in canal, river and reservoir application for flow measurement. Both weirs and gates are used frequently for discharge measurements in open channels. Works concerning the use of sluice gates as discharge measurement structure may be found, e.g by Rajaratham(1977). Weirs and gates may be combined together in one device yielding a simultaneous flow over the weir and below the gate (Negam,2002).The flow through combined devices may be free when both the flow over weir and below the gate are free or it is termed submerged when the flow below the gate is submerged. The simultaneous discharge can be obtained by adding the overflow discharge to the under flow discharge and making use of interaction factor (Negam,1996 andNegam et al.,2000) . Comprehensive review of the studies dealing the simultaneous flow over weirs and below gates can be found in (Negam et al.,2002) Negam,2002 propose a general model for predicting the combined flow over weirs and below gates for both free and submerged gates conditions utilizing the basic discharge equations of the weir and gates and an artificial neural networks to predict the simultaneous discharge over weirs and below gates when they have unequal contractions. Then the results of both methods are compared. The aim of the present study is to investigate experimentally the free flow through a combined rectangular weir and below a semi-circular gate.(a combined device) and to propose a model for predicting the discharge coefficient through it.
Theoretical analysis: Figure 1 shows definition sketch for the free flow over rectangular weir and through submerged semi-circular gate (combined device) of different contraction. the Q is the total discharge through the combined device which is calculated as follows.

MT .
Based on eq. 5 and using dimensional analysis (Buckingham Theorem) shows the following: The effect of Reynolds number and Weber number is assumed to be negligible for the combined device except at a very low head

Experimental set-up:
The experiments were carried out in horizontal rectangular channel of 10m length, with cross-section 0.3m wide and 0.45m high. The walls of the channel were a toughened 95 glass with number of perpex panels incorporated. The bed of the channel consisted of stainless -steel plates. A pair of a justable instrument rails were fitted on the top of the channel sides through the working length of the channel two movable carriages equipped with point gauges were mounted on the rails. Fig. (2) shows the details of the channel used in this study. All series of experiment were conducted in the hydraulic laboratory of water resources engineering department, college of engineering, university of Mosul.
Nine combined device model were constructed and manufactured of an aluminum plates 2mm thick, these models were of a rectangular weir with different width (  cm  B  12  ,  10  ,  8 )over a semi-circular gate of constant diameter cm d 12 , the distance below the weir edge y and the semi-circular gate is changed three times ) 15 , 10 , 5 ( cm y . The detail of the combined device used is shown in table (1).    (7). The best fit is:- R is the coefficient of determination d-Variation of experimentally observed values of Cd and values predicted by equation (8) is shown in fig.(9), which shows a good agreement. The error distribution in predicting coefficient of discharge (observed Cd ) from eq.(4) is shown in fig.(10) the percentage of error is within % 10 for the total experimental data .Therefore the combined hydraulic measuring device presented in this study is said to be accurate discharge measuring device .

Conclusions:
Based on the analysis of experimental study on flow through a combined hydraulic measuring device, the following conclusions are withdrawn: