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 does flow separation cause an increase in pressure drag?
Why does flow separation cause an increase in pressure drag?
In this question, I am relying on this answer for some information, so it might be essential to read that answer to understand what I am ...
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Can a sonic boom occur in supersonic Mach number?
If the wing's leading edge has a sufficient swept angle , then it is called a subsonic leading edge.
Therefore, if the air speed is supersonic, the air speed on the wing will still be subsonic.
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Why does flow separation cause a loss of lift, considering the low pressure is the cause of lift above the wing?
When the flow separates, the main stream of air is no longer passing by the upper surface of the wing. Usually this means that the air pressure will decrease above the wing, because the freestream ...
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Will the air be low pressure above an airfoil going less than Mach 0.3?
If flow only becomes compressible (>5%) above Mach 0.3, and the only way for air to accelerate is by having a pressure gradient¹, will the air be low pressure on top of a wing going less than Mach ...
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Why is there no Betz law in reverse?
Betz's law says that only 16/27 of the power in a wind stream can be converted into electrical power by a wind turbine. However, it appears that 100% of the power supplied to a fan motor can be ...
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Why does the airflow turn less when you extend further from the body?
In this answer, it says as follows:
Because "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 ...
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How does the weak / strong shock solution work?
In this chart, you can see that there are 2 possible angles for a shock to be at when both the deflection angle and Mach number are the same.
Circled in red is the line of an object going Mach 2, and ...
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How will a normal shock forming on a wing create drag?
When you're going close to supersonic speeds, the air over the wing is actually getting accelerated to supersonic. When the pressure recovery area comes, this accelerated air turns into a shock ...
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Is it possible for a shockwave to reflect off of the ground, making the observer hear 2 shocks?
Is it possible for a shockwave from a supersonic aircraft to reflect off of the ground, forcing the observer to hear 2 shockwaves?
In this picture, the shockwave from this aircraft is shown to reach ...
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Why do tip vortices seem to 'bend' inwards at the tip of a plane wing?
Why do tip vortices seem to 'bend' inwards at the tip of a plane wing?
Here if you look closely, the tip vortices seem to bend or travel inwards very slightly after the aircraft has passed. In this ...
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Will a plane 'shake' when it stalls?
I haven't been able to confirm this, but does a plane shake when it stalls? If so, why?
I was able to find some info on this, but from a non-confirmed source.
As the plane shakes and rattles, it's a ...
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Does the pressure recovery become not as extreme as you speed up?
Does the pressure recovery area on a wing become not as intense as you speed up? If so, why?
The pressure recovery is the effect that decelerates the air after it has been accelerated by the airfoil. ...
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Why will the bending of air over an airfoil make a force? [duplicate]
As seen here, the air will bend downwards when it travels over the upper surface. Nothing new here.
The reason it does this is because if the air was not to bend, it would create a vacuum.
The ...
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Will the 3D effect change shockwave angle/strength?
Similar to how the 3D relieving effect will change the shockwave angle (see above), will this same effect change the shock angle and strength further away from the body? As a shock goes through ...
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Why is there an area of different density right at the nose of a plane going supersonic?
A normal subsonic enough, but still compressible flow will compress when it gets to the planes nose. But in supersonic flow, there won’t be this compression. There will just be a shockwave which turns ...
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Why will a positive stagger increase biplane efficiency, while a negative stagger will decrease efficiency?
On a biplane, why will a positive stagger (top wing more forward relative to the bottom one) be more efficient than a negative stagger? In both cases the wing’s pressure fields are more separate, so ...
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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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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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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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