# 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 ...

8

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, ...

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

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 ...

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

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 ...

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

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

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

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

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

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

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

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 ...

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

It is possible that the fan motor is slowing down as it heats up. If you can supply data such as motor temperature and RPM, that will verify my answer.

1

The fans work by entrainment of surrounding air by the jet blowing out of a slit in the ring. Obviously it is optimized for entraining air from behind the ring, but it should also pull in air from the sides if the back is blocked. Most fluid dynamics textbooks will discuss the problem of a planar jet coming out of a wall, this is essentially the same ...

1

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 ...

1

There isn't a simple formula for fan noise, but the physics can be worked out from the fundamental equations of fluid mechanics and acoustics. It isn't a simple problem however. The noise created by fans is complex and from several fundamental sources, and its amplitude depends on frequency. Here is an example of a fan noise frequency spectrum for a cooling ...

1

The air stream from the fan is created by separate blades that break up the flow and create cells of low pressure and high pressure in the stream. When the air moves past the paper, it is split into two streams, which push and pull the paper from side to side. You are right that the paper moves away from high pressure and into low pressure. This creates a ...

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 ...

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 ...

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