24
votes
Accepted
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 ...
- 30k
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. ...
- 17.3k
13
votes
Why are there two local maximums on some CL vs AOA curves?
The mini stall is the stall that we normally talk about when considering the flight of an aeroplane, and is therefore the main stall. Below the AoA where the first stall occurs, the wing profile is ...
- 62.2k
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. ...
- 2,587
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 ...
- 27.2k
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 ...
- 4,510
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 ...
- 11.1k
8
votes
Accepted
What is the difference between static and dynamic stall?
Static stall is what we typically think of, when we think of stall. Slowly increasing AOA and then loss of lift as the flow separates.
But pitch rate (rate of increase of AOA) has an impact too. At ...
- 1,973
8
votes
Accepted
What are the "beta lines" of a gas turbine engine component?
Beta lines are either linear or parabolic shaped lines that are conveniently placed and spaced between the surge line and the choke line of the compressor (or fan) characteristic (also known as ...
- 1,795
7
votes
Accepted
Why do shockwaves progressively move aft as speed increses?
The answer is deeply rooted in the theory of compressible fluid dynamics, so for a fully satisfactory take on the matter you might want to refer to a textbook on that topic (such as Thompson or ...
- 86
6
votes
Accepted
Why does turbofan exhaust appear to reverse direction?
What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers ...
- 4,877
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 ...
- 11.1k
6
votes
Do stall strips mean that wing washout is not required?
Rectangular wings can get away without washout because that planform tends to have favourable root to tip stall progression as a characteristic of the rectangular shape.
The stall strips aren't ...
- 134k
6
votes
Accepted
Why can't supersonic flow work its way upstream?
I don't really understand what upstream or downstream mean. Can someone explain to me please?
For an observer sitting in a flying airplane, it looks as if a stream of air is rushing towards and then ...
- 236k
5
votes
Accepted
Is the shear centre of a wing the same as its aerodynamic centre?
No. These are very different things.
The shear centre (SC) is a purely mechanical (structural) feature of the wing design. It is a point along the chord such that application of force - no matter ...
- 9,103
5
votes
Accepted
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 ...
- 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 ...
5
votes
Why do we consider that air comes into jet engine?
The answer to the last question is "Galilean relativity", the assumption that the same laws of physics hold in any inertial frame of reference. In the example, it is presumably easier to do ...
4
votes
Will computational fluid dynamics (CFD) ever be 100% correct?
No, it will never be 100% accurate. Air or water are quite well described by the Navier-Stokes equations assuming the fluid is a continuum. Usually it is also assumed that they are Newtonian fluid.
...
- 1,079
4
votes
Why do we consider that air comes into jet engine?
Our universe is so constructed that it doesn't matter if the air is moving and the model is standing still, or if the model is moving and the air is standing still. In either case, air is coming into ...
- 22.1k
4
votes
Why does density decrease most above Mach 0.3, and not as much below Mach 0.3?
When you combine the equations for isentropic flow
you can plot
"density compared to density at zero speed, M=0", i.e. $\rho / \rho_0$, as a function of M.
Anderson's Fundamentals of ...
- 12.6k
4
votes
Why are the top of the F/A-18 strakes curved?
The airflow at some point over the top of a wing that is close to a fuselage will not necessarily be in the exact direction of flight. You would then expect the strake orientation to be finessed to ...
- 22.1k
3
votes
On an aerofoil or circular cylinder why does the airflow stay attached after passing the point of maximum thickness?
This is due to the Coandă effect.
https://www.britannica.com/technology/fluidics#ref129652
In the 1930s Henri Coandă, a Romanian scientist, described what is now
known as the Coandă effect, a major ...
- 1,656
3
votes
Accepted
What causes the vortex on the tip of this vertical stabilizer (CFD Simulation)
The subtitle of the video states:
This video shows a high-fidelity CFD simulation of flow control applied to realistic wing profiles using PHASTA and ParaView Catalyst. Work done by Michel Rasquin ...
- 62.2k
3
votes
Accepted
What happens to the high pressure below an airfoil when the angle of attack increases?
Your answer is correct. The pressure distribution from the xfoil simulation on a NACA 0012 are shown below for AOA 0 and AOA 5. As you can see, the lower side pressure does increase. However, most of ...
- 11.1k
3
votes
Do stall strips mean that wing washout is not required?
6 degrees of washout means the angle of incidence is 6 degrees lower at the tip than the wing root, causing the wing root to stall first. Leading edge slats serve the same function, especially in ...
- 20.9k
3
votes
Bed sheet as wing skin?
Absolutely you can use a fabric for covering and airframe. I used to build large scale R/C aircraft much like they used to build real world aircraft. I used fine silk to cover the wings, elevator, ...
- 31
3
votes
Accepted
What aerodynamic phenomenae can decrease the pressure gradient in Boundary Layer Ingestion?
First, let me explain the context of your question: It is about a concept for a future airliner:
Boeing Blended-Wing-Body Model 450-1U, taken from NASA/CR-2006-214534.
In the artist impression ...
- 236k
3
votes
Accepted
Why is the stagnation point on the upper surface before the start of circulation?
The illustrations under the time-invariant, inviscid, irrotational and incompressible (potential) description is a bit misleading, in my opinion. In the real world, assuming the airfoil starts from ...
- 11.1k
3
votes
Is multi element wing good option for slow flight airplanes?
Excellent application inverted on the drag racer, which brings home the point perfectly. If you have enough excess thrust, devices like these can dramaticly increase Coefficient of Lift, but at the ...
- 20.9k
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