How do the automatic soda dispensers at fast food joints know when the cup is fu…

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 s…

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?

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?

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?

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.

Does super cooled helium act in a viscous or non-viscous manner?

Does super cooled helium act in a viscous or non-viscous manner?

Below 2.17 K, liquid helium behaves very differently than normal fluids. It behaves as though it were made of two intermingled fluids: one that is normal in every way and the other that is completely without viscosity. Depending on what sort of experiment you do, you will see one or the other fluid. If you swirl the liquid helium with a stick, you will see the viscous fluid component swirling and splashing. If you pour the liquid helium through a filter made of tightly packed dust, you will see the non-viscous component rushing through. No normal fluid can travel through packed dust, because its viscosity slows its travel until it doesn’t move at all. But the viscosity-free component of liquid helium can flow easily through any holes, no matter how small. It can flow through holes that even helium gas has trouble passing.

How does Jell-O work? How come it congeals when it is cooled?

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.

What are vortex rings?

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.

What is the difference between “thickness” and viscosity? Is viscosity just a …

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.