Three-Dimensional Numerical Simulation of Pressure-Flow Scour
15
3 Results and Discussions
For the erosion to take place at some point in the flow domain, bed shear stress
at that point has to exceed the critical shear stress. The critical stress, which is a
function of the particle diameter as well as the flow depth, is calculated from the
classical shield’s diagram. For the present study, the particle diameter and flow depth
are 0.0003 m and 0.085 m, respectively, and the critical shear stress comes out to
be 0.185 N/m
2 . The bed shear is calculated using the rough logarithmic law. The
three parameters of interest include the time evolution of scour hole, magnitude and
location of maximum scouring. The magnitude of scouring is measured as the bed
deviation from the original bed level at the start of the simulation. It is observed
that the magnitude of scouring inside the contraction is very fast in the initial phases
of the simulation (Fig. 2), and with time, the bed attains an equilibrium level when
the flow passage increases to a value such that bed shear becomes equal to critical
shear stress. However, the rate of bed deviation for L15W6 is smaller than L20W6
(Fig. 2a) because of the bed stress concentration which increases with the increase
in length. Similarly, the rate of deviation increases as the contraction is reduced due
to increase in bed shear (Fig. 2b).
The maximum pressure flow scour is a function of both the length of contraction
and degree of contraction. It is observed that the magnitude of maximum scour
increases with the increase in the length of contraction (Fig. 3a). The location of
maximum scour is at the upstream face of the contraction for both L15W7 and
L20W7. A possible explanation to this phenomenon are the vortices developed due
to obstruction of flow. Further, it is observed that the magnitude of maximum scouring
increases with an increase in the degree of contraction (Fig. 3b). The increase in the
degree of contraction increases the bed shear in the contraction which increases the
scouring potential of flow. However, the degree of contraction has no effect on the
location of the scour hole.
Fig. 2 Temporal deviation of bed for a different lengths of contraction with same degree of
contraction and b different degrees of contraction with same length of contraction
15
3 Results and Discussions
For the erosion to take place at some point in the flow domain, bed shear stress
at that point has to exceed the critical shear stress. The critical stress, which is a
function of the particle diameter as well as the flow depth, is calculated from the
classical shield’s diagram. For the present study, the particle diameter and flow depth
are 0.0003 m and 0.085 m, respectively, and the critical shear stress comes out to
be 0.185 N/m
2 . The bed shear is calculated using the rough logarithmic law. The
three parameters of interest include the time evolution of scour hole, magnitude and
location of maximum scouring. The magnitude of scouring is measured as the bed
deviation from the original bed level at the start of the simulation. It is observed
that the magnitude of scouring inside the contraction is very fast in the initial phases
of the simulation (Fig. 2), and with time, the bed attains an equilibrium level when
the flow passage increases to a value such that bed shear becomes equal to critical
shear stress. However, the rate of bed deviation for L15W6 is smaller than L20W6
(Fig. 2a) because of the bed stress concentration which increases with the increase
in length. Similarly, the rate of deviation increases as the contraction is reduced due
to increase in bed shear (Fig. 2b).
The maximum pressure flow scour is a function of both the length of contraction
and degree of contraction. It is observed that the magnitude of maximum scour
increases with the increase in the length of contraction (Fig. 3a). The location of
maximum scour is at the upstream face of the contraction for both L15W7 and
L20W7. A possible explanation to this phenomenon are the vortices developed due
to obstruction of flow. Further, it is observed that the magnitude of maximum scouring
increases with an increase in the degree of contraction (Fig. 3b). The increase in the
degree of contraction increases the bed shear in the contraction which increases the
scouring potential of flow. However, the degree of contraction has no effect on the
location of the scour hole.
Fig. 2 Temporal deviation of bed for a different lengths of contraction with same degree of
contraction and b different degrees of contraction with same length of contraction