# Questions tagged [flow]

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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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### 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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### 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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1 vote
116 views

### 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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1 vote
67 views

### 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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207 views

### Why will air keep accelerating in Mach 1+ flow, when it would decelerate in subsonic flow?

So I was clarifying a few things in a comments section of an answer, and I had quite a few questions so I thought I'd ask my main one in a separate post. In the second comment on this answer, it says ...
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### Does speed affect the pressure and lift distribution at the wingtip?

So I was wondering if speed affects the pressure distribution at the wingtip. The second comment on this answer says (if I interpret it correctly) that speed doesn't change the pressure distribution ...
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1 vote
152 views

### Forces acting on ram air parachute (for pulling ships)

I recently stumpled across this question about the ram-air parachutes for use in pulling ships. In this answer it is stated that the parachute needs to travel perpendicularly to the ship's motion in ...
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### Why a kite flying at 1000 feet in "figure-of-eight loops" serves to "multiply the pulling effect of the airflow" on the ship to which it is attached?

In CNN's June 30, 2023 Giant kites could pull cargo ships across the ocean – and slash their carbon emissions “What differentiates it from other wind solutions,” says Bernatets1, “is that the wing is ...
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### Is contact between flow and inner walls of nozzle required for thrust generation?

I uploaded a picture below explaining over-expanded flow/nozzle condition with regards to thrust. This aviation source tells that “extra nozzle walls don’t generate any additional thrust because flow ...
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1 vote
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### How can over-expanded flow go out of nozzle when it has less pressure than ambient pressure?

We know that over expansion in flow means that nozzle exit pressure of flow is lower than ambient pressure at specific altitude. It gets narrower while exiting nozzle causing loss in efficiency. My ...
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### What is the most efficient way to reduce interference drag when surface imperfections are unavoidable?

Interference drag can start to appear when there’s imperfections or excrescences on an aircraft outer surface. For example the following image shows some of these typical imperfections: Source of ...
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### Do tufts show boundary layer flow or outer flow?

Outer flow and boundary flow are often different. Do tufts at the wing surface show boundary layer flow or outer flow? Picture source. Paint shows boundary layer flow. Picture source
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### why normal surface pressure over wedged cone is greater than freestream pressure

I came across this question in "Fundamentals of Aerodynamics" by John Anderson, although I knew how to solve the question, I didn't understand why we had the surface pressure on the wedge 1....
208 views

### What exactly happens to kinetic energy in the turbulent layer on an aerofoil?

Sources I've read contains contradicting information. As far as I know, when the laminar airflow breaks up, it becomes a thicker and draggy layer, causing a layer of what essentially is slowed air; ...
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### Why do planes employ vortex generators if the laminar flows exhibit a far lower skin friction drag than turbulent flows?

I am a student and still relatively new regarding fluid flows. Our professor asked this and I can't find a source to answer this question.
1 vote
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### Does this formula hold true for a source sheet covering a closed body of arbitrary shape?

Let me explain the picture above: We have a uniform flow and a source sheet cover the the surface of an arbitrary closed body. Let s be the distance measured along the source sheet, λ=λ(s) is the ...
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### How do I calculate the normal component of the velocity at the midpoint of a source straight panel?

I am reading "Fundamental of Aerodynamics " by J.D.Anderson Fifth edition. Now I am at the chapter 3. If you got the book, please go to the page 286, the part that says: "when j = i, the contribution ...
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1 vote
390 views

### What causes these losses in this propeller simulation?

While running a CFD simulation comparison of 2 vs 3 blade propeller versions (48" diameter, 23" pitch, 3570RPM), I've come up with: 2-blade: 347N of static thrust @ 110Nm Torque 3-blade: 220N of ...
282 views

### Why is the flow ahead of a shock wave chaotic?

Here is the image of a shock wave on a cone in a small supersonic wind tunnel, the white lines are the condensations formed in the test chamber due to flow expansion. Why are the condensations so ...
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