Siphon principle and applications

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Do you know about the siphon effect? The water in the container can flow out automatically through the elbow pipe that is above the water level in the container, as if something is \"sucking\" the water out of the container (see the image on the right). 　　Why does water flow automatically over high bends? It turns out that this is due to the pressure difference at work. When the pressures at the same liquid level on both sides of the bent pipe are different, the water (or other liquid) in the pipe will flow toward the side with the lower pressure. http://www.ephyst.com/wlsl/issue/37/acc/02.gif The diagram on the left shows the principle of how this pressure difference causes water to flow. 　　For a siphon effect to occur, the water outlet end of the elbow must be lower than the water inlet end. Let the horizontal plane at the inlet end be A, and the horizontal plane at the outlet end be B. The atmospheric pressure acting on both horizontal planes is P0. However, inside the right tube, there is a column of water with a length of h below plane A; therefore, the pressure at that same level as A in the right tube is P = P0 – ρgh. Since P0 > P, this causes the column of water to flow towards the right tube and eventually exit through point B. From this, a definition of the siphon phenomenon can be given: The phenomenon in which liquid flows on its own due to the pressure difference resulting from the height difference between the liquid levels at the two ends of a communicating vessel is called the siphon phenomenon. 　　A careful analysis of the schematic diagram of siphoning also shows that the elbow must first be filled with water, and no air allowed to enter inside the elbow; otherwise, the pressure of the air inside will alter the pressure difference between the two ends, preventing the siphoning effect from occurring. http://www.ephyst.com/wlsl/issue/37/acc/03.jpg Let’s start with an experiment: take a metal can, remove its lid, make a circular hole in the bottom, insert a rubber stopper with a hole in it, and then push a glass tube with both ends open and a diameter of 15–20 millimeters into that hole, as shown in the diagram. The height of the glass tube inside the cylinder is 5/6 of the cylinder’s total height. Take another large test tube with a diameter of 30–40 millimeters (slightly taller than the height of the glass tube inside the cylinder), and place it over the glass tube with its opening facing downward. Try to elevate the test tube (you can use three plastic blocks stuck to the bottom of the cylinder to raise it, without blocking the opening of the tube), and seal the space between the rubber stopper and the bottom of the cylinder, as well as between the rubber stopper and the glass tube, with melted paraffin. Now, start adding water to the canister. As long as the water level does not exceed line A, no water will flow out from the lower opening; but once a little more water is added and the level exceeds line B, almost all the water in the canister will flow out. In fact, the purpose of using a glass tube with an outer tube is to create a bent pipe, which is equivalent to the device shown in the image below on the left. When the water level in the container exceeds line B, water flows out from the lower opening; it continues to flow until the water level drops below line A, at which point the flow stops. http://www.ephyst.com/wlsl/issue/37/acc/05.jpg The siphon effect has many clever applications in production and daily life. For example, in public toilets, the toilets need to be flushed regularly with water; since these systems operate without human supervision, it’s necessary to prevent water from flowing continuously unchecked. In such cases, an automatic device based on the siphon principle can be used (as shown in the image on the right). 　　Adjust the water release valve so that water flows slowly into the container below. When the water level in the container rises above the top of the elbow, the elbow becomes filled with water, and water then flows out from the lower outlet to flush the sink. The water level in the container continues to drop, but as long as it remains above the upper opening of the elbow, water will continue to flow; once the upper opening is exposed above the water surface, the flow stops. This is the mechanism of siphoning. Adjusting the height of the upper port on the elbow pipe can change the water flow volume per flush ； By adjusting the flow rate of the water release valve, it is possible to change the time interval between two rinses. http://www.ephyst.com/wlsl/issue/37/acc/06.jpg Modern toilets also often make use of the siphon principle in order to generate greater suction and remove waste from the toilet bowl. 　　Have you noticed that the siphon principle used in toilets is exactly the same as that in the experiment setup above? 　　Finally, let’s take a look at an interactive Flash animation: the siphon fountain. Can you understand from this how the siphon principle is utilized?