user's manual for corhyd: an internal diffuser hydraulics model - IfH

only 10 m (4 ports) where velocities are lower. Negative consequences of the taper is a higher
head (5 % increase of the relative head) and a more distorted discharge distribution.
Fig. 31: Flow characteristics for tapered diffuser. Left: reduced diffuser diameter of 1.4 m for end part,
right: basecase, both for design flow Q d = 8 m³/s. Top-down: Individual riser flow distribution
along diffuser, riser flow deviation from mean, losses in port/riser configurations (line), port
and jet discharge velocities and diffuser pipe velocities, port and diffuser diameter (lines)
Constructional alternatives
The piling of the diffuser pipe caused problems due to broken piles and therefore leakage at
diffuser pipe joints. State of the art constructional design alternatives would try to avoid these
problems by using a HDPE pipe with concrete weights fixing the diffuser on the ground. The
internal hydraulics would be affected by only by minor differences in roughness.
a ) Covered diffuser or in trench - short risers
If wave forcing, sediment transport or navigation and fishing activities are a major problem
for the diffuser pipe, it also can be covered (Fig. 32) or laid in a trench (Fig. 33). In both cases
short risers have to be used to connect the buried pipe with the ambient water. The riser pipes
with the two attached ports are causing additional losses and therefore distort the discharge
profile, especially due to the previous tapering causing different riser/diffuser ratios and
therefore non-uniform distributions (Fig. 34). Increasing the riser diameter in the tapered
diffuser end part allows to equilibrate these additional changes, because the additional
separation losses depend on the diameter ratio between diffuser pipe and riser pipe. The risers
therefore have a diameter of 0.3 m at the end part and of 0.2 at the near-shore part of the
diffuser. In this case the only change is a little increase in total head of about 4 % compared
to the tapered diffuser with no risers and 10 % compared to the basecase (Fig. 34).
Institut für Hydromechanik, Universität Karlsruhe 59