What is a Hydraulic Jump?

part1, Controlling the flow of water is one of the fundamental objectives of modern infrastructure,. from flooding rivers to, irrigation canals, stormwater drainage facilities to aqueducts, and even the spillways. of dams. So, engineers need to be, able to predict how water will behave in. order to design structures that manage or control it. This is the definition of engineering:, to take theoretical knowledge of science and physics. (in this case fluid dynamics), and apply that information to, make decisions about the real. world.. Fluid dynamics might sound, as complicated as rocket science, but unlike rockets, you probably already. have some intuitions about how water flows., The study of how water with a free. surface behaves, that is not confined within a pipe, is, known as open channel hydraulics. And, fluids don’t always behave the. way you’d expect. A, flow profile can naturally transition from subcritical to supercritical (that is. from slow to fast), for example if, a channel changes to a steeper slope or. a cliff.. The classic demonstration of a hydraulic jump, can be seen at the bottom of your. sink. Open the faucet and watch how the flow behaves., You can see the fast. moving water right as the flow hits the sink and the abrupt transition, of the. hydraulic flow. You can also see that the flow is. controlled, on the upstream side, so nothing I do downstream affects the depth of this flow. This. video, is sponsored by Nord VPN. Feel safe online: anywhere, anytime. Hey, I’m Grady, and this is Practical Engineering. One of the most important parameters in fluid dynamics is velocity, or, how quickly. the water flows. If the flow velocity is exactly, equal to. the wave speed, we call the flow critical. Fast. moving flow is called supercritical. Slow, tranquil flow conditions are called subcritical. Sometimes velocity is a good thing, like when you‘re trying to move. a, lot of water quickly, for example in a flood. In this case, the wave. speed is faster than, the speed of the flow. But it’s more likely that these two. velocities, are different. Either way, the velocity of a flow is important, because it determines the speed at which the flow can travel, and how fast it can travel in a given. direction.. This field is especially useful in civil engineering where structures can‘t usually be, tested at. scale. For example, the spillway. of a dam, is a very large structure, and it can be very difficult to test the performance of a spillway at a. small scale. We can, not only test it, but we can also not test it at a very small scale, like a small. mountain. to see how well it works,, and then rebuild it if the performance is not up to standards.. But, when flow transitions the other direction - when a fast-moving supercritical flow transitions. to a more tranquil, subcritical condition - something much more interesting happens: the hydraulic jump. If you look at the video above, you can see how fast the water is moving in a subcritical. flow. Because of this,, the depth is controlled by downstream conditions. You’re also able to see that waves can travel. against the flow direction. You. can see that anything I do upstream is not changing the, depth of that flow. Many types of flow measurement devices rely on, forcing a flow to transition. from sub- or supercritical because there will be a unique relationship between flow, rate and. depth for a given geometry. Maybe we’ll talk more about flow measurement in a future. video., But, the most interesting phenomena in open-channel flow occurs when the flow transitions, from supercritical to subcritical in a channel. This transition, happens due to the pressure of the water, in the channel. We also care about the velocity, of waves or how quickly the pressure disturbances in a fluid can travel. Sometimes the pressure is a bad. thing. Sometimes it. is a. good thing. More on that later.. A, hydraulic jump can happen when it. flows into a slope or cliff, or it can flow into an incoming flume. Also hydraulic. transitions, into an area where the slope is steeper, or where the cliff is higher. This type of transition is called a tidal. jump and it happens when the slope of the cliff. changes.. It can happen due to pressure in the rocks and the cliff. Plus it. naturally built into the slope. Supercritical flow is, when I open a boring