# Tagged Questions

A subset of [tag:fluid-dynamics] concerning primarily forces such as lift and drag generated on bodies as they move through gasses or as gasses move through the body (typically air).

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### How a fan moves air? [duplicate]

How does a fan moves air towards you (I mean in 1 direction). Also propeller and fan have different shapes, does it mean they work different?
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### What is going on in front of and behind a fan?

Why is it that when you drop paper behind a fan, it drops, and is not blown/sucked into the fan, whereas if you drop paper in front of a fan, it is blown away?
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### Why Are Normal Shock Waves Unstable in a Converging Channel?

While learning about shock waves in an introductory Gas Dynamics course, it was revealed that normal shocks are unstable if formed in a converging channel. Even if the local conditions ostensibly ...
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### Number of blades in a fan

How does the number of blades in a fan affect the flux of air? I read that 3 blades are the best option but some companies uses more blades because there's a misconception among people that more ...
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### Static,total and velocity pressure at a point

I have a past exam question that I am stuck on. $200$m$^3/s$ of air is entering a shaft of $5m$ diameter, the density is $1.18$kg/m$^3$, and I have shock loss of factor $X=.5$ I want static, ...
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### Is the flow in a ducted fan really incompressible?

I have a long duct with a fan in the middle of it. The fan causes a steady airflow in the pipe which is at low speeds (incompressible) the mean flow in the pipe is about 10 m/s. However the fan blades ...
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### Calculate flow rate of air through a pressurized hole

I was wondering about this: If there is a pressurized container, like a tank of compressed air at some pressure that is greater than the ambient air pressure, and this tank of air has a hole in it, ...
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### What does centre of lift depend on?

I've read in many places that centre of lift is about quarter chord of the wing and that post-stall lift (the part developed on lower surface) has centre midchord. The later makes sense; the pressure ...
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### Integrating pressure over a surface

Consider the 2D airfoil below. In engineering (and maybe physics) you will often see something like the following as an expression for the pressure force acting on a surface (in this case a curve ...
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Though the mathematical concepts underlying quadratic air drag are quite straightforward (a single variable differential, just like the linear drag equation), my text book (and online text books) ...
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### How to calculate the drag coefficient using terminal velocity?

I was wondering if it were possible to calculate the drag coefficient by allowing an object to reach terminal velocity. Can you rearrange the terminal velocity formula to give the drag coefficient?
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### How much more efficient is a road bike than a mountain bike? [closed]

What would be a good estimate on the difference of efficiency between a road bike and a mountain bike? A number of links cite all the usual reasons: thinner tires, better aerodynamics... But I'm ...
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### Why does a piece of paper travels up when inserted into a kite's thread?

When I was young, I would insert a piece of paper into a kite's thread. The paper would travel upward till it reaches the head of that kite. What makes it to go up against the gravity even if the kite'...
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### Could the phenomenon of vortex bursting be exploited to reduce wake turbulence?

One of the classic stories in the annals of aerospace engineering is the development (and subsequent redesign) of the F-18 and its Leading Edge Extensions (LEX) due to fatigue problems, problems that ...
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### Why do co-rotating vortices coalesce, but not counter-rotating ones?

In studying the aerodynamics of modern aircraft equipped with high-lift devices, I have discovered that quite a number of distinct trailing vortices are present in the immediate wake of an airplane in ...
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### How should I throttle my rocket to reach highest altitude? [closed]

"Real world" problem. Suppose we want to launch a rocket equipped with an engine which can be throttled as we prefer. Suppose also that the amount of fuel burnt per time is directly proportional ...
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### Why are the backs of airplanes curved?

I get the front part, but why is the back curved too? I do not see a problem with the back being flat.
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### What does a wing do that an engine can't?

This isn't a question of how a wing works -- vortex flow, Bernoulli's principle, all of that jazz. Instead, it's a question of why we need a wing at all. A wing produces lift, but why is that ...
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### How laminar or turbulent is air?

Consider an outdoors scenario, with good weather and no sensible air currents at the floor level. How turbulent or laminar is the air surrounding this environment?
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### How much wind does it take to tip a sign over? [closed]

Or said another way - how much counterweight does the base of a sign need to keep it from tipping over given a specific max wind? Assume the sign does not let wind through Assume the base of the ...
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### Maximum helicopter height [duplicate]

Helicopters or dual/quadcopters stop rising after reaching some height from the ground. What causes that? And what one should do if he want to prevent this, if he want the helicopter to keep rising up?...
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### Are Mach Diamonds radially symmetrical?

Videos like this show a form that suggests a radial symmetry in the gas jet. The same is reinforced by schematic illustrations. But other images, like this, look like there is a twisting structure ...
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### Experiment - Measuring final velocity of a parachute

I'm working on a project that involves a scale parachute. It is vital for the project to get a good measurement of the speed it has when it lands. We still haven't built the parachute, but we need a ...
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### Stagnant Air Deep Inside Vintage Mines

This is really a question more about mining ventilation than true "physics." I occasionally explore a southwest desert mine, but am aware of the hazards - typically passing over half a dozen vintage ...
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### Why air above airplane's wing moves faster? [duplicate]

One explanation I read: Because of the wing's geometry, the "upper" side of the wing is longer, so the air has to travel faster: My wondering: Who said (and what was his/her explanation) that ...
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### Uses of the Reynolds number

I have seen a lot of places talking about the Reynolds number and how it is calculated, but I have never seen an equation that actually made use of this number to calculate lift, drag, or other ...
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### What's the advantage of NASA's flying saucer over traditional aerodynamic models?

NASA has recently tested a saucer-shaped spaceship. What's the advantage of this new design over traditional aerodynamic designs? The test launch was performed from within atmosphere which would ...
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### Does the lift created by a wing change when the slats are deployed?

This is a conceptual question as much as an empirical one, but the question is: Does the lift of a wing change when the slats (or any other leading edge device) are deployed? I am stipulating that the ...
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### Cause for Power Transmission Tower “Breathing”

OK, this question is not your usual one: Last night while hiking solo from the mountains back to my car at the mountain/desert interface (Lone Pine, CA), I had a rather bizarre -- and downright spooky ...
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### Effect of Earth's magnetic field on aeroplanes?

As an aeroplane accelerates through the Earth's B-field, it experiences a changing flux $d\phi \over dt$ and a potential difference is induced along its wings. Given the wings are made of metal and ...
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### What is the exact cause of flow separation in a viscous fluid?

I recently got into a lengthy debate about the exact nature of boundary layer separation. In common parlance, we have a tendency to talk about certain geometries as being too "sharp" for a viscous ...
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### Why doesn't this model plane fly? [closed]

I have been designing a model plane for Design Technology for the past month or so, and today I laser cut my final design and assembled, it then tested it. Upon testing the plane does not get any lift,...
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### Why aren't airplanes like golf balls?

Ok this is a silly question but here it goes Although it is good to have a laminar flow of the air around the object for low drag but the laminar flow is prone the phenomena called separation (sounds ...
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### Why do tropical cyclones not tear themselves apart?

A tropical cyclone is the generic term for a hurricane, typhoon, or tropical storm. Tropical cyclones derive their energy from evaporation of water at the ocean surface which ultimately recondenses ...
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### Does it make sense to open one window all the way when the other window is much smaller?

I can't wrap my head around this idea because I don't know much about air flows. Say we have this imaginary apartment with two windows, one of which is two times smaller than the other: Will the ...
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### Why Don't Birds Stall?

I often hear about planes stalling when they lose lift due to low airspeed/too high angle of attack. Why don't birds stall? Does it have to do with the structure of their wings and their flexibility, ...
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### Why isn't jumping from a high altitude fatal?

After seeing this answer claiming that displacing matter "In a very short time", "no matter whether the matter is solid, liquid, or gas" (even though he concludes that falling from a high altitude is ...
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### Why is jumping into water from high altitude fatal?

If I jump from an airplane straight positioned upright into the ocean, why is it the same as jumping straight on the ground? Water is a liquid as opposed to the ground, so I would expect that by ...
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### Counter rotating propellers: better in same column or staggered?

In an octo-copter design, is it better to put four pairs of counter-rotating props over each other so that each pair works in a column together? Or will you get more power or efficiency from ...
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### Does the wing-propeller plane or jet-plane struggle up at the high altitudes because of low pressure?

We all know that wing-propeller planes rely on air to create a thrust. They suck the air in and push them back hard that it's opposite reaction pushes the plane (Newton's law). Does that mean wing-...
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### Toy helicopters at very high altitude

Because air is thin up there, toy helicopters usually have a maximum operational altitude. If the toy brought up by a real aircraft, higher than its intended maximum altitude, would it be able to ...
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### Stratospheric ship

Let's assume that we build a giant steel hull in a shape of cube with open top (2km long edge) and lift it to the top of stratosphere and then pump air out of it. Would it float on the outer layer of ...
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### Aerodynamics and thermodynamic [closed]

Why can we refer an object as being aerodynamic but we can't refer an object as being thermodynamic, and if an object is thermodynamic what does it even means?
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### How do birds generate thrust?

I have been watching this video carefully and I want to know how the wings of birds generate thrust. This is because the wings are more or less flapping up and down --- generating the lift. But I do ...
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### What would be the minimal size of an aerial vehicle capable of sustained suspersonic flight? [closed]

I wonder, what's the smallest possible size for a UAV capable of sustained supersonic flight at the current technology level? Let's say 10 minutes of flight at 1.1 M.
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### Modal analysis with aerodynamic damping

I'm using modal decomposition to predict the steady state response of a beam structure to harmonic loading. The structure itself is very lightly damped, but we know from experiments that the ...
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### Mathematically impossible for a vortex line to have loose ends?

Could someone show the math behind it? Source : "A vortex is a bunch of air circulating around itself. The axis around which the air is rotating is called a vortex line. It is mathematically ...
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### Why does air circulate on an airfoil — The Kutta Condition [duplicate]

Why does the air circulate on a flowing airfoil, thus giving rise to increased velocity (circulation + relative airspeed) above the wing and hence decreased pressure.