Modular Venturi Flume

It is the type of V. F. in which the width of throat is decreased to such an extent that the depth of water in throat is equal to critical depth.

The velocity of flow through the throat corresponding to critical depth is also critical.

Therefore the existence of critical conditions of flow in throat will cause a standing wave on the d/s of V.F.

such a V.F. which causes a standing wave on its d/s and have critical condition in its throat is known as modular V.F. or standing wave V.F.

Specific energy at 1-1

Specific energy at 2-2

gvhE2

21

11

gvhE2

22

22

Now neglecting loss of energy in gradually converging portion of the flume

Then

21 EE

)1(2

22

21 gvhE

Or

Theoretical discharge through the venturiflume is

21

22

2hE

gv

)2(2 212 hEgv

2222 hbvaQth )3() h- (E 2g 21

As depth of water at throat is critical so discharge at critical depth is maximum.

In eq (4), Q will be maximum when the factor

is maximum as all

other parameters are constant

)4(2 32

2212 hhEgbQth

32

221 hhE

Differentiating wrt h2 and equating to zero

032

221

22

hhEdhd

dhdQ

032 2221 hhE

0)32( 212 hEh

Substituting in eq (3)

032 21 hE

)5(32

12 Eh

23

12 2385.0 EgbQth

23

12 2385.0 EgCbQac

C = coeff of V.F. depending on smoothness of surface of bed/sides and roundness of corners of V.F. and its value lies between 0.95-1.0.

Problems

A discharge of 1000(litre/sec) flows along a rectangular channel 1.5m wide (a) what is critical depth in the channel (b) if H.J. is to be formed at a point where u/s depth is 180mm, what is height of H.J.

A rectangular channel 6m wide discharges 1200(l/s) of water into 6m wide apron with a velocity of 6m/s. What is height of H.J. how much energy is absorbed in the jump

A broad crested weir 10m long has maximum discharge 10,000 (l/s) determine the head of water on u/s side of the weir for this discharge, if coef of discharge is 0.62.

Determine the maximum discharge over a broad crested weir 60m long having 60cm height of water above the crest of weir. Coefficient of discharge is 0.595. also determine the new discharge over the weir considering velocity of approach, the channel on the u/s of the weir has a cross sectional area 45m2

A rectangular channel 2.4m wide is provided with venturiflume of 1.5m wide throat. Find quantity of water flowing through the V.F. when depth of water into u/s side 1.2m and at throat is 0.9m.

A modular V.F. has an approach channel 3m wide having throat of 1.5m width. Calculate discharge when u/s depth is 1m. Take C = 0.95