# Tag Info

41

The eyes are measuring the number of photons of each color that are hitting a given point of the retina – that are coming from some direction. This is a function of time, $f(t)$, for each point. However, when this function is changing too quickly, the eye can't see the changes. Effectively, the eye may also see the average of $f(t)$ in each period of time ...

25

There is a YouTube video that visualizes the air flow around a propeller for various configurations. I caught a screen shot of a moment that more or less shows what is going on: As you can see, this happens at 2:07 into the clip - this happens to be for a dual rotor configuration (two counter rotating blades) but the principle is the same. Behind the ...

21

The fan motor provides a torque $\tau$ which has to accelerate $\alpha$ the fan blades whose moment of inertia is $I$: $$\tau=I\alpha$$ Given how long it takes for the fan blades to stop the frictional torques must be fairly low and so the torque applied by the motor to keep them going must also be low. With the relatively small torque rating, even if the ...

10

You don't have a fluorescent light but an LED light that time-multiplexes different colors to achieve white. The fan blades act as a stroboscope and make the switching frequency of these LEDs visible. Or... I am wrong! So back to the drawing board: I did the experiment. Most fluorescent lights in my house do not show this effect, at all, but one does, ...

10

You might be thinking in comparison to a desk or handheld electric fan. As mentioned by @Farcher, $\tau = I\alpha$. $I$, the moment of inertia of a spinning body around a particular axis of rotation, is calculated as follows: $$I = \iiint\rho(x,y,z)||r||^2\ dV$$ Or with uniform density, $$I = \rho\iiint||r||^2\ dV$$ From this formula, you can see that ...

10

A much simpler way of thinking about this is to consider energy. When the fan is spinning it has quite a lot of kinetic energy (try to stop it by putting your finger in the way to confirm this (don't actually do this!)). That kinetic energy goes as the square of the rotation rate, in fact. So as the fan starts, the motor needs to add energy to it. It ...

8

It's a bit complicated (Wikipedia). Induction motors work in sync with the AC frequency but have no torque at 0 RPM so they need some arrangement to get them started.

6

There seems to be a lot of B.S. in the ads for this product, making it seem much more complicated than it really is. It seems that the fan contains an ordinary fan in its base, and squirts out high speed jets of air from around the big ring. These jets of air push on the ambient air. This slows down the individual jets but pushes along a greater volume of ...

6

Here's a standard fan with some (hard to see) arrows indicating air flow. The fan works by pulling air in and then making it move faster. The air flow behind the fan is slow moving and wide (you can see the arrows behind the fan coming from above and below the fan blades) whereas the air flow in front of the fan is fast moving and narrow (which follows ...

5

In summertime, the ceiling fan blows air downwards and cools down your body using the wind chill effect. In wintertime, if you have an active heating system at home, it will heat up the air in the room. Hotter air moves up and accumulate near the ceiling, colder air being down. A ceiling fan in reverse direction moves cold air up pushes hot air downwards to ...

4

If the bearings were to be considered frictionless, then the maximum speed of the fan will not decrease, though it will take the fan longer to reach the maximum speed. Because as the moment of inertia of the impeller increases its angular acc. will decrease (for the same torque applied), therefore it will take the fan longer to reach its maximum speed. The ...

3

This will depend on exactly what kind of motor you have. If your fan is a brushed DC motor, then the fan speed will be slightly lower, since the new impeller is heavier than the old, so there will be slightly greater bearing friction. The added friction will serve as a power loss, and the motor will have to run slightly slower. If the motor is a brushless ...

3

Think about the air around the fan at any given time: The amount of air flowing into the fan must equate to the air flowing out of the fan. The amount of air that passes through an area in a given time is related to the velocity of the air i.e. the faster the air is moving, the more air that can flow through a fixed area/hole/slot. The fan blades apply a ...

2

Intuitively, it seems entirely possible. The "shape" of the air current blown out by both fans looks like a hollow cylinder, since the air is blown at high velocity near the ends of the blades, and lower velocity towards the center. If you put two of these fans facing each other, the two "cylinders" of air current will collide with each other and create a ...

2

For your first question, think of the air kind of like sand. If you just slide you hand across the surface, some grains beneath the surface will move due to friction between the sand grains transferring the energy of your hand to other sand grains. The same thing happens in the air inside the ring, as air shoots out of the slit surrounding the ring it ...

2

Unlike John Rennie, I think that the problem is not in the efficiency of this system but in the fact, that it will not generate considerable lift. So even if marketing materials are completely true and Dyson Air Multiplier is more energy efficient than conventional fans this efficiency only applies to moving air (which is its intended use) but not to the ...

2

This is really a comment, but it got a bit too long for the comment box. The term efficiency has a very specific meaning in Physics. In the case of a fan we can measure the power the fan consumes simply by measuring what current it draws from the main. We can in principle measure the power the fan produces by measuring the velocity of the air stream ...

2

My previous answer was based on physical principles. This answer is based on biological principles. I noticed in the question @JoeBlack stated "I feel air" meaning you are using your own biology to sense the air flow. Air flow can be felt by pressure of turbulence on the skin. It can also be felt by perceived temperature. Usually a person turns on a fan ...

2

Simply put, it's because a fan imparts momentum on the air (i.e. accelerates it), so in front of the fan you get a roughly conical jet of high speed air. At the back side of the fan there is a low-pressure region which makes the surrounding air move towards the fan (following the pressure gradient) from a large solid angle, as you already stated. This is ...

2

I don't agree with this part: the momentum of the entering water is exactly cancelled by the momentum carried out by exiting water The pump is adding momentum to both the water that enters and the water that exits...in the same direction; thus the equal and opposite momentum imparted on the apparatus. As the water goes through the tube, it is gaining ...

2

The answer is that your view or sight is different from the bare images made by the imaging optics of your eye on your retina. A "view" also includes signal processing from the brain that tracks what you fix your gaze on. Light can pass between the blades and form an image of the retina for at least some time. It's true that there are also blades blocking ...

2

Because human eyes and brains are slow, they cannot resolve the motion of the blades, but only see the average of the moving blades and the image in the background (this is actually primarily really due to the slow reaction time of the cones, which is slow, as is demonstrated by the fact that a 24 frames per second video does not appear as single images but ...

2

Your goal is to maximize the temperature of the room. I assume this is in steady state. If your heater runs indefinitely, the steady state temperature will be the one where the heat loss through the walls / ceiling / floor / windows equals the heat input. You will achieve this by pointing the heater away from the surface with the greatest heat loss. For ...

2

When the fan starts spinning, each blade starts from rest. Newton's first law of motion states that unless a force is applied to it, the [velocity][1] of a body does not change. That property is inertia. To speed up, the blades must accelerate. Newton's second law of motion states that the force necessary for an acceleration of a body is proportional to and ...

2

Either way will cool you in Summer: Either way, keeps the air circulating in the room, and the moving air increases the rate at which perspiration can evaporate from your sweaty body. Either way will warm you in Winter. Either way keeps the air circulating in the room, thereby preventing thermal stratification (i.e., it prevents all the heat from collecting ...

1

Background The issue of higher speeds having smaller penetration results from the ram pressure, which scales as: $$P = \frac{1}{2} \rho \ v^{2}$$ where $\rho$ is the mass density and $v$ is the flow speed. Even if you have thin fins in a radiator, they can start to "look like" sails to the incident flow if the flow moves fast enough causing the drag to ...

1

The most likely explanation is that at lower speeds you get a more laminar flow. To quote the wiki article: At low velocities, the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids.[2] In ...

1

One way to think of this is to imagine a high-speed camera which takes 1000 frames per second (which is 40x as many as a regular film camera). Then imagine putting the frames into groups of 40, and averaging each group, so that you now have a film with a normal framerate, where each frame is the average of 40 high speed frames. The resulting film would be ...

1

Away from the cylinder, water is stationary. It is being accelerated as it approaches the cylinder: once inside, it is conceivable that its momentum doesn't change (incompressible fluid, ignoring vorticity). Water leaving the other end of the cylinder is carrying away momentum. So looking at the entire body of water in which the cylinder is submerged, there ...

1

The wind flow does not need to be variable. Things can still wobble because of turbulence. If you could visualize the airflow around the object, you could see that it is not a smooth flow, but a flow with vortices. These vortices form near the object but then detach from it. The force acting on the object is different while a vortex forms vs. when it ...

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