3 Hydrodynamics
217
often used as an effective energy dissipation way to connect the downstream
flow of the discharge structure.
Generally, the section at the beginning of surface water roll is called the
section before jump, and the corresponding water depth is called the water
depth before jump; the section at the end of surface water roll is called the
section after jump, and the corresponding water depth is called the water
depth after jump. However, the position of water roll on the surface is not
stable. It swings back and forth along the direction of water flow, usually
taking the average value in a period of time. The difference between the water
depth after the jump and the water depth before the jump is called the jump
height. The distance between the section before the jump and the section
after the jump is called the jump length for short.
(1) Classification of hydraulic jump
The form of hydraulic jump is related to Froude number Fr 1 (Froude number,
Fr 1 = V 1 /
√
gh 1 , V 1 is the average velocity, h 1 is the depth of water and
g is the acceleration of gravity). It is found that the hydraulic jump can be
classified as follows according to the Froude number Fr 1 of the section before
the jump.
When, there will be a series of undulating waves on the surface of the
hydraulic jump, the wave crest will decrease along the current, and finally
disappear. This form of hydraulic jump is called wavy hydraulic jump.
Because there is no rotation in the wave jump, the mixing effect is poor,
the energy dissipation effect is not significant, and the wave energy will be
attenuated after a long distance.
When Fr 1 > 1.7, the hydraulic jump becomes a typical hydraulic jump
with surface water roll, which is called a complete hydraulic jump. In addition, the complete hydraulic jump can be subdivided according to the Froude
number Fr 1 of the section before the jump. However, this classification is
only different in the strength and weakness of the surface phenomenon of
the hydraulic jump turbulence, and there is no essential difference, as shown
in Fig. 3.47a.
When 1.7 ≤ Fr 1 < 2.5, it is called weak water jump. There are many small
rolls on the water surface, the energy dissipation effect is not good, the energy
dissipation efficiency is less than 20%, but the cross-section after the jump
is relatively stable. Energy dissipation efficiency refers to the percentage of
energy consumed by the hydraulic jump in the total mechanical energy of
the section before jump, as shown in Fig. 3.47b.
217
often used as an effective energy dissipation way to connect the downstream
flow of the discharge structure.
Generally, the section at the beginning of surface water roll is called the
section before jump, and the corresponding water depth is called the water
depth before jump; the section at the end of surface water roll is called the
section after jump, and the corresponding water depth is called the water
depth after jump. However, the position of water roll on the surface is not
stable. It swings back and forth along the direction of water flow, usually
taking the average value in a period of time. The difference between the water
depth after the jump and the water depth before the jump is called the jump
height. The distance between the section before the jump and the section
after the jump is called the jump length for short.
(1) Classification of hydraulic jump
The form of hydraulic jump is related to Froude number Fr 1 (Froude number,
Fr 1 = V 1 /
√
gh 1 , V 1 is the average velocity, h 1 is the depth of water and
g is the acceleration of gravity). It is found that the hydraulic jump can be
classified as follows according to the Froude number Fr 1 of the section before
the jump.
When, there will be a series of undulating waves on the surface of the
hydraulic jump, the wave crest will decrease along the current, and finally
disappear. This form of hydraulic jump is called wavy hydraulic jump.
Because there is no rotation in the wave jump, the mixing effect is poor,
the energy dissipation effect is not significant, and the wave energy will be
attenuated after a long distance.
When Fr 1 > 1.7, the hydraulic jump becomes a typical hydraulic jump
with surface water roll, which is called a complete hydraulic jump. In addition, the complete hydraulic jump can be subdivided according to the Froude
number Fr 1 of the section before the jump. However, this classification is
only different in the strength and weakness of the surface phenomenon of
the hydraulic jump turbulence, and there is no essential difference, as shown
in Fig. 3.47a.
When 1.7 ≤ Fr 1 < 2.5, it is called weak water jump. There are many small
rolls on the water surface, the energy dissipation effect is not good, the energy
dissipation efficiency is less than 20%, but the cross-section after the jump
is relatively stable. Energy dissipation efficiency refers to the percentage of
energy consumed by the hydraulic jump in the total mechanical energy of
the section before jump, as shown in Fig. 3.47b.