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30 votes
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Why does this shockwave not touch the body that creates it?

After passing through the shock wave, the air still has to evade the body. If the shock-wave touched the body, the air would have to escape through infinitely thin space at infinitely high pressure ...
Jan Hudec's user avatar
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24 votes
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Will computational fluid dynamics (CFD) ever be 100% correct?

This is a broad question, hence a broad answer 😃 As high eng CFD now stands, it already is correct enough for, say, 95% of use cases. I am not aware of much utility in modelling complex turbulent ...
Jpe61's user avatar
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23 votes
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What is the reason for changing the speed reference (IAS or Mach number) with altitude?

Aircraft are limited by both air speed (VMO, affects loads on the structures) and Mach (MMO, formation of shock waves resulting in buffet). At low altitudes, the speed of sound is high so an aircraft ...
fooot's user avatar
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16 votes
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Why do aircraft need Mach number?

Mind you, not all aircraft need one, especially slower piston ones. As you get closer to higher speeds, where compressiblity becomes significant, and you are approaching the speed of sound, the air ...
Radu094's user avatar
  • 7,980
15 votes

How do wings generate lift?

HOW AN AIRPLANE GENERATES LIFT There are usually two popular fields of thought (excluding the debunked equal time theory) behind why an airplane flies; some think it is caused by an application of ...
Murey Tasroc's user avatar
  • 1,172
15 votes

Why do we consider that air comes into jet engine?

Both the air and the engine are moving very, very fast. Our galaxy and everything in it is moving at a blistering speed of several hundred miles per second. Of course, we don't feel this speed. ...
Chris's user avatar
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14 votes
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Why do we use dimensionless expressions in flight mechanics and aerodynamics?

Here is a dimensionful equation: $$L = \frac{\rho}{2}\cdot v^2\cdot \frac{2\cdot\pi\cdot b^2}{1+\sqrt{1+\left(\frac{b^2}{2\cdot S}\right)^2}}\cdot\alpha$$ Note that all ingredients are physical, ...
Peter Kämpf's user avatar
12 votes

Why don't we place the engine in the front of the wings?

In addition to the Antonov 72/74's, Boeing worked on the YC-14 concept as a replacement for the C-130's. Also, many seaplanes have to place the engines as far up as possible, to keep the engines and ...
Efe Ballı's user avatar
  • 1,189
11 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

100% correct, no. That's because CFD (and a great many other computer applications) work on numerical approximations. There's either no exact solution* known, or the exact solution isn't computable. ...
jamesqf's user avatar
  • 2,577
11 votes

Why do we consider that air comes into jet engine?

Because it doesn't affect the math, and because it helps with keeping relative change in perspective if we use the engine as a frame of reference instead of the airmass. Also because you are incorrect ...
Michael Hall's user avatar
10 votes

Why does this shockwave not touch the body that creates it?

In case of an infinestimally thin body in supersonic flow, the disturbances created propogate as Mach waves. However, as the the thickness of the body increases, the flow has to physically turn away ...
aeroalias's user avatar
  • 100k
10 votes

Is turbulence a random process?

The equations that describe fluid flow, the Navier Stokes equations, are generally considered chaotic. In particular turbulence is defined as " fluid motion characterized by chaotic changes in ...
DJClayworth's user avatar
  • 4,501
9 votes
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Why don't we place the engine in the front of the wings?

They can be, as on the An-72/74, which has the engines mounted above and forward of the wings in part to increase lift, utilizing the Coandă effect. One downside is that this makes access for ...
zymhan's user avatar
  • 2,238
9 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

First, the Millennium Challenge asks about the existence and uniqueness of a solution to the Navier-Stokes (NS) equations given any compatible initial/boundary conditions. This question is of ...
JZYL's user avatar
  • 11.1k
8 votes

How do wings generate lift?

The simplest answer that I know that is that is still accurate is that for any object to move through the air, some force must push the air in front of it out of the way (gravity, engines, momentum ...
Paul Smith's user avatar
  • 1,095
8 votes

Does the Reynolds number on a smaller model need to match the one of the original aircraft?

In wind tunnel testing you need to match several similarity parameters; the most well known are the Reynolds number and the Mach number. Since the Reynolds number depends on flow speed, viscosity and ...
Peter Kämpf's user avatar
8 votes

How can flow in the compressor of a jet engine go back to the inlet?

Note that the flow speed past the diffusor (which sits between the compressor and the combustion chamber) is very low in order to allow air and fuel to mix, the fuel to evaporate and the mixture to ...
Peter Kämpf's user avatar
7 votes

How do aircraft engine manufacturers achieve a higher bypass ratio while still meeting the thrust requirements for a given aircraft?

Early turbojets were so inefficient that adding a fan was considered but not implemented because that would had made the engines even more sluggish and narrowed the operating limits even more. If you ...
Peter Kämpf's user avatar
7 votes
Accepted

How do diffusers turn velocity into pressure?

Warning N°1: I'll avoid energy conservation, but cannot set aside mass conservation obviously. Warning N°2: Both speed and pressure settle in a way that abides the laws of physics. It's not like one ...
MrBrushy's user avatar
  • 778
7 votes

Why does this shockwave not touch the body that creates it?

Perhaps, at the root of your question, there is a misunderstanding of the nature of a shock wave. A shock wave is a surface, an ensemble of points, where the properties of the gas change ...
MariusMatutiae's user avatar
6 votes

How do wings generate lift?

Wings generate lift pushing air downwards. As a kid I used to stick my hand out of the open car window and tilt it - there is an upward force. A flat plate does this. So aircraft wings could be flat ...
Koyovis's user avatar
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6 votes
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What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?

According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off. ...
bwduncan's user avatar
  • 556
6 votes

How do diffusers turn velocity into pressure?

While we wait for a probably better answer, here's my tentative. You state that you understand the concept of constant mass flow. Let's for a moment write the mass flow properly: $$\dot{m} = \rho A ...
Federico's user avatar
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6 votes
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What does the NCrit parameter indicate in a CFD analysis?

The whole setting group is called transition settings, and refers to the boundary layer transition point (from laminar to turbulent). The forced transition trip locations are exactly that, expressed ...
AEhere supports Monica's user avatar
6 votes
Accepted

What is boundary layer tripping?

Tripping the boundary layer refers to the action of artificially transitioning a laminar boundary layer into a turbulent one. It can happen intentionally (via turbulator) or unintentionally (via ...
JZYL's user avatar
  • 11.1k
5 votes

Why does the diffuser section generate thrust in a jet engine?

But why is the diffuser also providing forward thrust? The diffuser slows down the flow to ease fuel-air mixing and combustion a bit later. If you only focus on entry and exit speeds, there would be ...
Peter Kämpf's user avatar
5 votes

In turbojet engines, does the fan play a key role in the air intake to the inlet?

In turbojet there is no fan. Very simply, turbofan means turbojet + fan in front connected to the LP shaft.
Afe's user avatar
  • 1,501
5 votes
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Is it possible to have the laminar flow above $Re=10^7$?

The airfoil always starts with a laminar boundary layer; at some point on the airfoil surface, the boundary layer may transition to turbulent. In a uniform free-stream, two modes of transitions ...
JZYL's user avatar
  • 11.1k
5 votes

Is it possible to have a separated boundary layer without having reversal flow?

I think I have a partial answer that I have found in a paper [1]: The following is a quote from the introduction: It is too narrow a view to use vanishing surface shearing stress or flow reversal as ...
Random Ape from Africa's user avatar

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