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Questions tagged [fluid-mechanics]

Fluid mechanics is the branch of physics which involves the study of fluids (liquids, gases, and plasmas) and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; and fluid dynamics, the study of the effect of forces on fluid motion.

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Why will the max lift point not be at the center of a swept but constant chord wing?

(In this question when I say "swept wing" it means the blue wing seen at the bottom. So no fuselage or anything) If you look at this graph from this answer, you'll see the max lift point of ...
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What happens in order to make a shock when the flow first turns?

Imagine you have a really zoomed in view of a 2d ramp, which is at 20 degrees. Zoom in on the exact point the ramp turns from horizontal to 20 degrees. When the first supersonic air molecules reach ...
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Why will the pressure distribution of a swept wing promote stall?

Here it explains how the bound vortex will change the lift distribution of a wing. What is meant by the lift distribution changing is that there is less lift at the tip, meaning the pressure ...
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Why are shockwaves allowed different angles at different distances from a body?

It's been (well) established that a shockwave can only be at the angle of the Mach cone. (I now understand this part). However, in this picture: you can see that the shock is at 2 angles. I also ...
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Why will a slower moving parcel of air above a wing create drag in non-inviscid flow?

This answer explains why a thicker boundary layer will cause a type of form drag. It uses inviscid flow to explain this. It is a great answer, but I can't seem to see how this applies in a real world ...
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How does a thickening boundary layer create form drag?

Imagine a normal wing with flow over it, and to simplify things it's at 0 AoA. Drag is being made mainly because of form drag and skin friction drag. I'm not sure on this, but I don't think there ...
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Why will the flow turning angle be 0 (or close to) inside of the Mach cone?

(Disclaimer : This probably seems like my 56th question on the same topic, but this will be my last relating to Mach cone angle and shock angles, as I think I understand most of the topic) So the ...
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Can a shock in certain circumstances be at a lesser angle than the Mach cone?

(Excuse the drawing quality) What will happen if a shockwave has an angle less than that of the Mach cone? In this picture, the black line represents the Mach cone. The orange line represents the ...
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Does the flow turning amount change as you get further away from an object?

Will the flow turning amount change as you get further away from an object? This answer explains why shockwaves will extend past the body that made them (good starting point). This question is asking ...
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Why does the angle of a shock change when the flow leaves the influence area of an object?

In this (great) answer, it says: "far enough" from the Concorde's nose, the path followed by the airflow is not affected by its presence and it's just a straight line. Close to the nose we ...
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What is the main reason raked wingtips are more efficient than winglets? [duplicate]

What is the main reason that these: Are more efficient than these?: My main guess is that they improve L/D compared to winglets, as they provide more upwards lift making them more efficient. It ...
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Why does the shock angle always seem to equal the "sound cone" angle?

(I'm going to use the term "sound cone" for the area you can hear an aircraft when it's at Mach ≥ 1, illustrated by this picture. Not considering the shockwave.) So as explained in this ...
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Can a sound wave travel through a shockwave?

Say you have a scenario where there is a shockwave, and you sent a normal sound wave towards it. Could the sound wave travel through that shockwave? If so/not, why? Would it depend on the strength of ...
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induced drag at supersonic speed [duplicate]

for a delta wing with supersonic leading edge at small angle of attack there is no stagnation point on leading edge so There is no connection between the top and bottom of the wing. Therefore, the air ...
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Why does adding heat to the subsonic flow increase the speed of the flow?

In Rayleigh flow, adding heat to the subsonic flow increases the speed and adding heat to the supersonic flow decreases the speed. But what is its physical cause?
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Does separation happen easier with less dense air?

Does less dense air affect when separation happens? So if you have a cylinder traveling in a straight path like the one above, there is separation happening at a given speed. If you were to change ...
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Why doesn’t the air expanding behind a shockwave reverse the effects of wave drag?

Wave drag is (from my understanding) the difference in pressure between the front and rear of an object due to shockwaves, making an opposing force. This needs higher pressure in front of said object ...
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Why are the top of the F/A-18 strakes curved?

As seen in this picture, the top of the strake has a slight curve to it. I'm mainly talking about the curve seen if you were to look at it straight on. To be more clear, not the curve of the strake as ...
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What is the main cause of pressure recovery on airfoil?

What is the main cause of pressure recovery over a wing? I’ve narrowed it down to be one of or a combination of 2 things: The higher pressure air from the bottom of the wing equalizing with the upper ...
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Does the back pressure on an airfoil stay the same with different speed?

(Inspired by this answer) The back pressure is the force on the rear side of an airfoil (in the pressure recovery zone). When the air streams reach the pressure recovery area on an airfoil, they are ...
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Why do shocks travel further down the wing as the Mach number increases?

Inspired by this question, why do shocks move further back on a wing as the Mach number increases? Normal shocks on the surfaces of wings form when the air molecules of the pressure recovery area on ...
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Is a stream of air / jet exhaust low pressure?

Inspired by this answer, is a moving stream of air (Jet exhaust, rocket plume, etc) low pressure? I’ve always thought that a higher pressure area (say the combustion chamber of a rocket engine) will ...
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Why doesn't the Coanda effect also pull air from inside the jet / streamline?

As explained nicely in this answer, the Coanda effect will make a streamline of air draw from the surrounding air outside of the streamline. This picture shows it nicely : So my question is what ...
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Why can't you hear an aircraft traveling at Mach 0.95 until it passes?

So say an aircraft is traveling at Mach 0.95 towards you. It's almost going faster than the sound waves it's making. Now say there's a speaker or something on the nose of the aircraft facing forward. ...
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4 answers
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Which is more efficient, thickening an airfoil or adding a bottom-surface curve?

So if you were to take a standard airfoil and increase the thickness, you’d increase the lift (Bernoulli’s principle), but it would also increase the drag. Now imagine you have the same airfoil, but ...
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What happens to a shockwave when it goes through different temp/density air?

Specifically I am asking what happens to a shock as it travels through different density air. This answer's comments has some good information about it. As said in the linked answer, a shock will bend ...
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Wouldn't the induced downwash "pull" the wing of a plane down?

I asked a similar question on this recently, (here), and the answer said this: Downwash is not a force. It is a small increment in the velocity vector. Downwash can change the local angle of attack --...
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Elevator / horizontal tail effectiveness at high speeds?

Take for example a jet fighter, when at low speeds vs high speeds, airspeed will differ a lot. As the turning moment is due to the lift force from the tail and lift increases quadratically with ...
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How is the pressure constant across the boundary layer but density isn't?

In $P=ρRT$, ρ is density, R is the gas constant, and T is the temperature in Kelvin. In the boundary layer, the temperature and density varies, so why doesn't the pressure? If you have a slip line in ...
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What way does induced downwash make a wing roll?

Say you have a wing, no sweep or anything to simplify things. One side of it, let's just say the left, magically doesn't have a tip vortex, therefore no induced downwash from the tip vortex. Would the ...
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1 vote
1 answer
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What causes the vapor cone when an aircraft is traveling at M ≤ 1? [duplicate]

What causes the vapor cone of an aircraft going M ≤ 1? I know that the vapor is from the air expanding, but why is it in the cone shape? What shock system forms it into that shape?
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Why don't shockwaves expand rearwards when they expand out?

Why don't shockwaves expand rearward when they expand out? In this for some reason really big picture, you can see the shocks expanding out past the actual plane. Why don't they also expand rearward? ...
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How does a shear layer make vortices?

How does a shear layer make vortices? If you have 2 streams of air that are moving at different speeds or opposite directions right beside each other, how would that make a vortex? What causes the ...
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What causes this vortex pattern on the slip line of a shockwave?

What are these little vortex patterns from on the slip line of these shocks? At first I thought it was vortex shedding, but it looks a tiny bit different to me (than vortex shedding). You can see the ...
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What causes the oscillation in vortex shedding?

When studying this video, I was wondering what actually causes the oscillation? Any source online that I found said that it was from oscillating low pressure zones, but what causes that?
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Will supersonic flow return to subsonic through shocks over a flat surface?

Will supersonic flow return to subsonic through shocks over a flat surface? Imagine supersonic flow over a plate. Will it always try to return to subsonic through a series of normal shocks? (Pretend ...
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What happens when 2 air streams, one going faster but the same direction, are beside each other?

Inspired by this video, what happens when 2 air streams beside each other going the same direction, but one going faster, interact? Do they make turbulence and or vortices as said in the video? I’m ...
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What does air becoming compressible change? [duplicate]

You always hear that around Mach 0.3, air technically becomes compressible. What actually changes when this happens, in subsonic and supersonic flow? Does it affect the formation of shocks?
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Do slats and leading edge devices increase lift at a constant AoA and airspeed?

I've seen mixed sources on this subject. Some say leading edge devices allow the wing to obtain a higher AoA, and others say it flat out increases lift. For me, the latter makes most sense, because ...
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Are oblique shocks formed at the leading edge-top surface of a wing?

Do oblique shocks form on the leading upper surface of a wing? I know bow shocks are formed there, but is that the same thing as I’m talking about? Oblique shocks would have to form there because the ...
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When an oblique shock forms, what happens to the normal shock that helped it form?

What happens to the normal shock that helped the oblique shock form, in the first shock of a lambda shock? Oblique shock waves form because : "An oblique shock wave is a shock wave that, unlike a ...
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3 votes
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What determines the angle at which a shock will form?

What determines the angle that a shock will form? If you look at a lambda shock, you’ll see it isn’t straight up, but with an angle (both the front and rear part of the lambda shock). The second ...
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Do boundary layers become thicker or thinner as you speed up?

What happens to the thickness of boundary layers as you speed up? I recently watched a video about the SR-71, in which it said “the inlet spike develops a significant boundary layer of air” when ...
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Why are pressures equal across the slip line of a lambda shock, but density is not?

Why are pressures equal across the slip line of a lambda shock, but density is not? In this answer, it says this : The surface Σ is a slip line between zones 3 and 4. Velocities will be parallel ...
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How do boundary layers and shocks interact?

How do boundary layers and shocks interact? What difference is made with thinner and thicker BL's? I've recently been researching this topic, but at least for me there are no good search results on ...
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Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? [duplicate]

Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? When going Mach 0.82, the shocks on the upper (and lower) surface don't go the whole chord length of the wing. (...
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Why are bow shocks angled slightly downward at the leading edges of wings?

Why are bow shocks angled slightly downward on airfoils? In this picture if you look closely at the very bottom airfoil diagram, you'll see the "subsonic airflow" part is more below the ...
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Why do boundary layers become more turbulent as they flow over a surface?

Why do boundary layers become more turbulent as they flow over a surface? This question originated from this one. What makes the boundary become more turbulent as it flows over a surface/wing? My ...
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Why is a thicker boundary layer more turbulent?

Why is a thicker boundary layer more turbulent? I’m mainly referring to the effect of the varying speed over the boundary layer, and why it makes it more turbulent. I know it has something to do with ...
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Wind tunnel flow over surface vs. non-wind tunnel (atmosphere) flow question

If you take an infinitely thin sheet of metal and stick it out of the window of an airplane at exactly 0 AOA, will the surfaces of it become low pressure if you speed up? In a wind tunnel, there is ...
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