. How do the automatic soda dispensers at fast food joints know when the cup is full? — MB, San Diego, CA
Those dispensers measure the volume of liquid they dispense and shut off when they've delivered enough liquid to fill the cup. They don't monitor where that liquid is going, so if you put the wrong sized cup below them or press the button twice, you're in trouble.
. How does the automatic cutoff valve on a gasoline pump work? How is it able to shut off the gas before the nozzle has become immersed in the liquid? I don't see how the pump could be so sensitive to back pressure in the gasoline. — NG, Bloomsburg, PA
As you suspect, the pump isn't able to detect the change in gasoline pressure that occurs when the fill level reaches the nozzle. Instead, the nozzle uses several hidden components to shut itself off when the tank is full. There is a small hole near the end of the nozzle that becomes blocked by the liquid gasoline as soon as the fill level reaches that hole. Blocking this hole with gasoline is what shuts off the valve. There is actually a thin tube inside the main gasoline delivery hose that operates this valve system. That tube runs from the hole in the nozzle to a vacuum pump inside the gasoline-pumping unit. While the pump is dispensing gasoline into a partially filled tank, air flows easily into the nozzle's hole and the pressure inside the thin tube remains close to atmospheric pressure. But when the level of gasoline rises high enough, it essentially blocks the hole and the pressure inside the thin tube drops. This pressure drop is what triggers the valve and stops the gasoline flow. Look for the hole near the end of the metal nozzle next time you fill your car with gasoline. In most cases, it's easy to see.
. What is a vortex? — M
A vortex is a region of fluid that's circulating in one direction around a line passing through that region. If you imagine yourself looking along that line, you would see the fluid flowing either clockwise or counter-clockwise around the line itself. Tornadoes and whirlpools are both vortices since they involve fluids circulating in one direction around a central line.
. What is the relationship between turbulence, laminar flow, and Reynolds number? — DD, SC
The Reynolds number is a measure of the way in which a moving fluid encounters an obstacle. It's equal to the fluid's density, the size of the obstacle, and the fluid's speed, and inversely proportional to the fluid's viscosity (viscosity is the measure of a fluid's "thickness"—for example, honey has a much larger viscosity than water does). A small Reynolds number refers to a flow in which the fluid has a low density so that it responds easily to forces, encounters a small obstacle, moves slowly, or has a large viscosity to keep it organized. In such a situation, the fluid is able to get around the obstacle smoothly in what is known as "laminar flow." You can describe such laminar flow as dominated by the fluid's viscosity—it's tendency to move smoothly together as a cohesive material.
A large Reynolds number refers to a flow in which the fluid has a large density so that it doesn't respond easily to forces, encounters a large obstacle, moves rapidly, or has too small a viscosity to keep it organized. In such a situation, the fluid can't get around the obstacle without breaking up into turbulent swirls and eddies. You can describe such turbulent flow as dominated by the fluid's inertia—the tendency of each portion of fluid to follow a path determined by its own momentum.
The transition from laminar to turbulent flow occurs at a particular range of Reynolds number (usually around 2500). Below this range, the flow is normally laminar; above it, the flow is normally turbulent.
. Why is viscosity important in motor oil for today's high revving engines?
If the oil in your car is has too little viscosity, it will easily flow out of the gaps between surfaces and will not lubricate them well. Those surfaces will experience sliding friction and wear. If the oil has too much viscosity, it will waste the engine's energy by opposing motion and turning work into thermal energy. Modern motor oils have carefully adjusted viscosities that balance the two problems. Since temperature affects viscosity (e.g., hot molasses has less viscosity than cold molasses), motor oils add chemicals that stabilize their viscosities over wide temperature ranges.
. Why does a hose squirt further when you cover the hole with your thumb?
The water entering the hose has a certain amount of energy per liter. That energy can be in one of three forms: pressure potential energy, gravitational potential energy, or kinetic energy. If you let it flow freely through the hose, most of that energy will become kinetic energy and the water will move quickly through the hose. But it will encounter frictional effects as it slides past the walls of the hose (its viscosity participates here) and it will convert much of its kinetic energy into thermal energy by the time it leaves the hose. However, if you pinch off the flow with your thumb, the water won't be able to convert its energy into kinetic form as it enters the hose. Most of the energy will remain as pressure potential energy. The water will move slowly through the hose and it will experience relatively little energy loss to frictional effects. Most of the energy will remain by the time the water reaches your thumb. Then, as the water flows past your thumb to the outside air, its pressure will drop suddenly and its energy will become kinetic energy. The water will spray out at very high speed.
. When you were showing us water faucets during class, each faucet had a corner immediately preceding the opening through which the water came out. Does that corner help slow the pressure of the water?
Most faucets do have a turn just before the water comes out and that turn is there to slow the water down. Unless the faucet is opened for maximum flow, the pressure of the water emerging from the valve part of the faucet is pretty close to atmospheric pressure, so there isn't any need to control that pressure. But the water emerging from the valve may be traveling very fast and it could easily spray across the room if there were nothing in its way. To prevent such sprays, most faucets are bent so that water spraying out of the valve will hit the bend and become turbulent. The turbulence will help it to convert its kinetic energy into thermal energy so that it will emerge from the faucet at low speed and atmospheric pressure. (Great question!—I'd never thought of this before).
. What is the difference between "thickness" and viscosity? Is viscosity just a fancy word for thickness?
Viscosity is a measurable quantity—a liquid has a specific viscosity as measured in units of poise or pascal-seconds. Thickness refers to the same characteristic as viscosity, but isn't a specific quantity. It's certainly correct to say that a thick liquid is a liquid with a large viscosity.
. What are vortex rings?
These rings (also called smoke rings) are moving portions of fluid that are moving relative to the surrounding fluid. They form a remarkably stable structure. The inner edge of the ring heads forward, while the outer edge head backward and the ring pulls itself through the air. Fluid dynamicists study these sorts of objects.
. How does Jell-O work? How come it congeals when it is cooled?
Jell-O is composed of long, stick-like molecules. When you dissolve it in hot water, those molecules separate, but as the liquid cools, they begin to stick together like a giant heap of straws. The water flows slowly through these straws because of frictional effects. The result is a stiff material that is given its structure by the straw heap. If you leave the Jell-O long enough, the water will seep out and make puddles on the plate.