Hydraulic Structures: Fourth Edition

Vp2

1 

1

ke

^2 gE (^2) 
1/2
(15.48)
where

E 2 

p

g

2



p

a

g

2

 (15.49)

and the flow rate is

Qp2cap2Vp2cdap2(2gE 2 )1/2 (15.50)

with

cdc

1/2

.

An empirical equation (Koh and Brooks, 1975) based on laboratory

experiments for port diameters less than one-tenth of the diameter of the

diffuser for a bell-mouth entrance is

cd0.975 (^1) 
3/8
(15.51)
and for a sharp-edged entrance is
cd0.63 0.58. (15.52)
Ejis as given by equation (15.49). (Vjis the velocity in the diffuser just
before the junction jandVpjis the velocity through the port at junction j.)
For the riser ports the experimental discharge coefficients vary from 0.648
to 0.848 (Grace, 1978). These coefficients may be used for the preliminary
design; for detailed calculations equation (15.47) taking account of the
losses in the individual components of the riser assembly has to be fol-
lowed.
(a) Analysis of flow in the diffuser
Referring to Fig. 15.14, the energy equation for the segment of the diffuser
between ports 2 and 3 is
∆Z 23 hf23hc. (15.53)

V^22



2 g

p 2



g

V 3 ^2



2 g

p 3



g

V^2 j



2 gEj

V^2 j



2 gEj

1



1 ke

V^22



2 g

660 COASTAL ENGINEERING