Maybe you even developed a new preference. Preferences are personal choices and can differ from person to person. Laws of physics, by contrast, determine how the water flows. You might think the box was empty when you poured all the water out, but it was not. Something else filled the box—something you cannot see: air. If liquid is poured out, another fluid such as air will take its place or the container will collapse.
Unless you take very special measures, the empty space created by pouring out water does not stay empty for long! In your case the container probably held its form and air rushed in. Sometimes air can flow in and water can pour out simultaneously. The two flow alongside each other, resulting in fluent or laminar flows.
You probably saw this when you poured while gripping side panel C. Holding the box by this side allows you to tilt the box quite far before the water level reaches the spout. This makes it easier to create a nice stream of water without filling the spout opening entirely, leaving space for air to flow in while water pours out.
When you poured while holding side panel A, a much smaller tilt of the box made the water pour out but it almost instantly covered the spout completely, leaving no room for air to flow in. As water pours out and there is nothing to replace it, a pressure imbalance builds up.
If this imbalance is great enough, the water flow can stop for a moment to suck in air, which results in water glugging out. Each glug is accompanied by a break for air to flow. This is an example of a chaotic or turbulent water flow, which is much harder to control. Cleanup Dry off your workspace, clean the glass and recycle your box if you can. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.
See Subscription Options. Discover World-Changing Science. Materials Empty rectangular soup or milk box such as shelf-stable soy milk with a spout on top Water Marker Glass or cup to pour into Paper towels or rag to dry up spills A workspace that can tolerate water spills Paper and pen optional Preparation The box has six sides: one on the top with the spout, one on the bottom and four side panels that connect the top and the bottom. Write the letter A on the side panel farthest from the spout.
Go around the box, lettering the next side panel B, then C and, finally, D. Procedure Fill the box with water. The water level should be about one half inch from the top. In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids liquids and gases. It has several subdisciplines, including aerodynamics the study of air and other gases in motion and hydrodynamics the study of liquids in motion. Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.
Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. Laminar flow is desirable in many situations, such as in drainage systems or airplane wings, because it is more efficient and less energy is lost. Turbulent flow can be useful for causing different fluids to mix together or for equalizing temperature.
According to McDonough, most flows of interest are turbulent; however, such flows can be very difficult to predict in detail, and distinguishing between these two types of flow is largely intuitive. An important factor in fluid flow is the fluid's Reynolds number Re , which is named after 19th century scientist Osborne Reynolds, although it was first described in by physicist George Gabriel Stokes.
McDonough gives the definition of Re as, "the ratio of inertial to viscous forces. Note that Re is not only a property of the fluid; it also includes the conditions of its flow such as its speed and the size and shape of the conduit or any obstructions. At low Re , the flow tends to be smooth, or laminar, while at high Re , the flow tends to be turbulent, forming eddies and vortices.
Re can be used to predict how a gas or liquid will flow around an obstacle in a stream, such as water around a bridge piling or wind over an aircraft wing. The number can also be used to predict the speed at which flow transitions from laminar to turbulent.
The study of liquid flow is called hydrodynamics. While liquids include all sorts of substances, such as oil and chemical solutions, by far the most common liquid is water, and most applications for hydrodynamics involve managing the flow of this liquid. That includes flood control, operation of city water and sewer systems, and management of navigable waterways.
Hydrodynamics deals primarily with the flow of water in pipes or open channels. Geology professor John Southard's lecture notes from an online course, " Introduction to Fluid Motions " Massachusetts Institute of Technology, , outline the main difference between pipe flow and open-channel flow: "flows in closed conduits or channels, like pipes or air ducts, are entirely in contact with rigid boundaries," while "open-channel flows, on the other hand, are those whose boundaries are not entirely a solid and rigid material.
Due to the differences in those boundaries, different forces affect the two types of flows. For instance, most city water systems use water towers to maintain constant pressure in the system. This difference in elevation is called the hydrodynamic head. Liquid in a pipe can also be made to flow faster or with greater pressure using mechanical pumps.
The flow of gas has many similarities to the flow of liquid, but it also has some important differences. First, gas is compressible, whereas liquids are generally considered to be incompressible.
Balachandran describes compressible fluid, stating, "If the density of the fluid changes appreciably throughout the flow field, the flow may be treated as a compressible flow.
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