Hydraulic Jump Test
OBJECTIVES
To demonstrate how energy and force momentum concepts are applied in open channel flows.
To investigate the relationship between depths upstream and downstream of a hydraulic jump and to compare this with the theoretical relationship
Introduction A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface. The rapidly flowing liquid is abruptly slowed and increases in height, converting some of the flow's initial kinetic energy into an increase in potential energy, with some energy irreversibly lost through turbulence to heat. In an open channel flow, this manifests as the fast flow rapidly slowing and piling up on top of itself similar to how a shockwave forms. The phenomenon is dependent upon the initial fluid speed. If the initial speed of the fluid is below the critical speed, then no jump is possible. For initial flow speeds which are not significantly above the critical speed, the transition appears as an undulating wave. As the initial flow speed increases further, the transition becomes more abrupt, until at high enough speeds, the transition front will break and curl back upon itself. When this happens, the jump can be accompanied by violent turbulence, eddying, air entrainment, and surface undulations, or waves.
Theory
Hydraulic jumps are very efficient in dissipating the energy of the flow to make it more controllable & les erosive. In engineering practice, the hydraulic jump frequently appears downstream from overflow structures (spillways), or under flow structures (slvice gates), where velocities are height. A hydraulic jump is formed when liquid at high velocity discharges into a zone of lower velocity only if the 3 independent velocities (y1, y2, fr1) of the hydraulic jump equation conform to the following equation:
Apparatus
Glass walled flume with sluice gates & a spillway arrangement Point gauges Manometer & scales Pump
Procedure
The pump was started to supply water to the flume. The tail gate was closed to allow water to accumulate and to develop hydraulic jump The position of the hydraulic jump was adjusted by adjusting the amount of closure of slvice gate IThe depth of the bed of flume was measured by using a point gauge. In the next step , The water surface level was measured before it had crossed the spillway. The height of spillway & the depth of water over the spillway were measured. Using the point gauges the water surface level downstream of the jump was determined Then the y1 & y2 heights were measured
Discussion
In open channels, the transition from supercritical to subcritical flows is called a hydraulic jump. For low upstream Froude numbers, free-surface undulations develop downstream of the jump and the hydraulic jump is called an undular jump. New experiments on undular hydraulic jumps were performed in a rectangular channel in which the upstream flows were fully developed turbulent shear flows. In this paper, the main flow patterns are described. Visual and photographic observations indicate five types of undular jumps. One of the main flow characteristics is the presence of lateral shock waves for Froude numbers larger than 1.2. The results show that the disappearance of undular jump occurs for Froude numbers ranging from 1.5 to 2.9 and that the wave length and amplitude of the free-surface undulations are functions of the upstream Froude number and the aspect ratio yc/W. Hydraulic jump characteristics were measured over several artificially roughened test beds in a horizontal rectangular flume with smooth side walls. A smooth test bed, two strip roughness test beds and three densely packed gravel test beds provided a relative roughness range from 0.0–0.9. The testing program involved some 200 hydraulic jump observations which included flow rate, upstream depth, tailwater depth and jump length. Observations showed that boundary roughness reduces both the sequent depth and the length of a hydraulic jump, and that the observed reductions were related to both Froude number and the degree of roughness. The observed hydraulic jump characteristics were consistent with theory, and a proposed approximation for a theoretical hydraulic jump equation was found to compare favorably with the observed characteristics.
Advantages
Dissipates the energy of water over a spillway Prevents scouring on the downstream side of the dam structure Traps air in the water Reverses the flow of water Maintains a high water level on the downstream side
Disadvantages
Downstream turbulence can cause damage and degradation of channel banks May cause erosion on hydraulic surfaces Undesirable condition for fish passage
To control hydraulic jump and enhance hydraulic jump efficiency, sills such as sharp-crested weirs, broad-crested weirs or end sills at the bottoms of waterways are frequently used. The force acting upon such a sill in the hydraulic jump rapidly decreases to the minimum as the end point of roller at downstream of the hydraulic jump moves upward at a point where it almost overflows a sill. This impact of sills can make the length of the scour risk zone shorter than a normal hydraulic jump phenomenon. Then, as the hydraulic jump becomes shorter and moves further upstream, the force upon a sill gradually increases to reach a certain level. Since such a rapidly varying flow is characterized as having an uneven velocity distribution, the changes in force upon a sill seem to be because of the changes in velocity distribution occurring between the starting points of the hydraulic jump to its end point. Consequently, in a cross-section with unequal velocities, the momentum surges greatly. Theoretically, hydraulic jump controlled by sills can be interpreted using the momentum theory. However, in the absence of a precise theory of velocity distribution, an accurate quantification can hardly be made by just relying on theoretic interpretation. An experiment for simulating the behavior of a sluice gate and an associated hydraulic jump experiment have been carried out in different conditions to enable an investigation of the characteristics of hydraulic jump, and to analyze energy dissipation, the physical experiment collects data that are afterwards used to define relations, which are used as guidelines for design operations.
Conclusion
A hydraulic jump is defined as a rise in the level of water. This occur when a supercritical flow (Fr > 1) encounters a submerged object such as a dam or weir throwing the water upward and changing the flow from a supercritical flow to a subcritical flow (Fr < 1), which causes a “jump”. An advantage of hydraulic jumps is the ability to dissipate energy in dams, channels, and similar structures. A disadvantage of hydraulic jumps is the downstream turbulence which can cause erosion and degradation of channels
Reference
https://www.researchgate.net/publication/236154641_Experimental_Study_of_Hydraulic_Jump_Charact eristics_in_Sloping_Prismatic_Channels http://www.brighthubengineering.com/hydraulics-civil-engineering/55054-open-channel-flow-basicshydraulic-jump-calculations/ Douglas, J.F.; Gasiorek, J.M.; Swaffield, J.A. (2001). Fluid Mechanics (4th ed.). Essex: Prentice Hall. ISBN 0582-41476-8. Faber, T.E. (1995). Fluid Dynamics for Physicists. Cambridge: Cambridge University Press. ISBN 0-52142969-2.
