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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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How do wings generate lift?

Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
Jae Carr's user avatar
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25 votes
3 answers
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Why does this shockwave not touch the body that creates it?

In the shock wave of the next image (in a blunt body) Why the wavefront is in front of the object? It does not assume that if the object travels faster than the speed of sound wave front should be ...
XF-91's user avatar
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24 votes
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What is the reason for changing the speed reference (IAS or Mach number) with altitude?

Inspired by that question: How is the airspeed-Mach number transition handled in modern airliners? When pressure and density decrease IAS also decreases. When temperature and pressure/density ...
mins's user avatar
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13 votes
3 answers
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What are the limitations of the aerodynamics modelling used in flight simulators?

I've asked around, and it seems all flight training programs use a combination of flight simulators and aircraft flights to train pilots. This works well for most pilots who fly airliners which don't ...
user11377's user avatar
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4 answers
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Why are flight simulators bad at predicting drag and lift values at high angles of attack? (the nonlinear flow regime)

X-Plane is built on something called blade element theory. From my understanding, it says the aircraft performance can be found if the performance of 2D cross sections are calculated and integrated in ...
user11377's user avatar
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11 votes
8 answers
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Will computational fluid dynamics (CFD) ever be 100% correct?

CFD results never match real world numbers especially in turbulent 3D complex flow... What stops CFD being perfect, and will it ever be 100% correct? If the Navier-Stokes Millennium problem is solved, ...
22flower's user avatar
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11 votes
4 answers
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Why do we use dimensionless expressions in flight mechanics and aerodynamics?

Why do we use non-dimensional expressions in flight mechanics and aerodynamics? We could as well directly calculate forces and moments; would that not be more relevant to the specific problem?
SAMER BABIKIR's user avatar
8 votes
1 answer
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What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?

For a typical turbofan jet engine (two examples given in the title), what is the exhaust velocity and mass flow rate of air at sea level and cruising altitude (~ FL350)? Also, does the specific ...
flextempers's user avatar
8 votes
3 answers
9k views

What is the working principle of a Gurney Flap?

Recently I came across a airfoil improvement called a 'Gurney flap', see image. I don't really understand the working principle of a Gurney. Wikipedia states: The Gurney flap increases lift by ...
ROIMaison's user avatar
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7 votes
3 answers
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Why do aircraft need Mach number? [duplicate]

Why don't aircraft operates with IAS only? Is there any other usage of Mach number except monitoring of shock waves formation? How Mach number is calculated?
wiaim's user avatar
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7 votes
1 answer
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Why does strong shock wave-boundary-layer interaction produce a bubble reattachment?

The picture represents a lambda type shock wave, separating the boundary layer at its foot. Separation causes a recirculation bubble to appear. I understand why separation has happened, but what is ...
Darjan's user avatar
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7 votes
2 answers
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How do you calculate the lift coefficient of an airfoil at zero angle of attack?

Thin airfoil theory gives $C = C_o + 2\pi\alpha$, where $C_o$ is the lift coefficient at $\alpha = 0$. However, I couldn't find any equation to calculate what $C_o$ is which must be some function of ...
Aviator S's user avatar
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6 votes
2 answers
3k views

How do diffusers turn velocity into pressure?

There are many resources that give explanations for the behavior of nozzles and diffusers. But at the crux point they all say, "Velocity goes down and pressure goes up." As if that doesn't warrant any ...
BoddTaxter's user avatar
6 votes
2 answers
5k views

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

I am studying the thrust distribution of jet engines. But I'm now a bit confused. In Rolls-Royce's "The Jet Engine" book, http://aeromodelbasic.blogspot.com/2012/05/thrust-distribution-distribution-...
Jono's user avatar
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2 answers
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Is the air in a jet engine supercritical?

Nitrogen has a critical point of 126.2 K and 3.4 MPa (34 bar). Air contains 75 % of nitrogen For example, nitrogen has a critical point of 126.2 K (−147 °C) and 3.4 MPa (34 bar). source : https://...
ralf htp's user avatar
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6 votes
3 answers
5k views

What is the relation between drag and weight?

Given an aircraft type and fixing its wing configuration (flaps, slats) and fixing its speed and altitude and assuming it flies horizontally, what is the relationship between the aerodynamic drag and ...
polaris12246's user avatar
5 votes
2 answers
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Does the Reynolds number on a smaller model need to match the one of the original aircraft?

1.Airbus 380-800 : The overall length of it , is 72.72 m and its cruise speed is 903 km/h. Based on the mean chord of wing , its Reynolds number in cruise phase is around 75 millions. Now , a 1/18 ...
alireza abbassi's user avatar
5 votes
3 answers
1k views

Is turbulence a random process?

Is turbulence in air/fluid a random or a deterministic process, and why is it so hard to solve? For sure if it is random it can't be solved... Or maybe it just appears to be random because of our lack ...
user avatar
5 votes
2 answers
604 views

How is the lift generated by a number of out-of-plane staggered wings calculated?

I want to calculate the lift generated by a number of out-of-plane staggered wings but I don't think a valid approach is to simply use superposition and add the sum of the lift of all the wing's in ...
Jonny's user avatar
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5 votes
2 answers
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Is there a downwash equation?

I need to find an equation for the following problem: Suppose a full-size glider passes 10 feet over my head at high speed. No doubt I will feel the downward air pressure (downwash) caused by the ...
user32016's user avatar
5 votes
1 answer
221 views

What happens when the surface ends at the laminar zone of the boundary layer?

When a body travels through any fluid, due to viscosity of the fluid a boundary layer is formed around the surface of the body which separates the velocity of the fluid inside the boundary layer(near ...
user17838's user avatar
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1 answer
815 views

Are flame holders made of a special material other than that used in combustion chamber?

Are the flame holders made of materials which have higher friction coefficients than the combustion chamber in order to create turbulence?
kcihtrak's user avatar
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4 votes
2 answers
1k views

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

I understand that a higher bypass ratio leads to greater efficiency as less air moves through the core, and therefore less fuel is burned. However, how is the thrust requirement for the same aircraft ...
flextempers's user avatar
4 votes
4 answers
471 views

Will two counter rotating vortices cancel each other?

Suppose that an aircraft flies with its wing through a tip vortex of another aircraft which flew in the opposite direction. Suppose that the shed wing tip vortex of both aircraft are exactly the same ...
lWindy's user avatar
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4 votes
1 answer
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What is boundary layer tripping?

I came across the term "Boundary layer tripping" but I can't really underastand it. After searching on internet I cannot find any video or an image explaining that concept. Could you please explain ...
izri_zimba's user avatar
4 votes
1 answer
28k views

What does the NCrit parameter indicate in a CFD analysis?

When analyzing an airfoil in the XFLR5 software I am asked to input some data regarding the free stream. For this, a dialogue box pops up that has the following options: One field contains the ...
surya's user avatar
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4 votes
1 answer
145 views

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

To fix the ideas, let's consider an airfoil instead of a generic shape. As far as I understand, the separation of the boundary layer takes place after there is a region of reversal flow on the suction ...
Random Ape from Africa's user avatar
4 votes
1 answer
2k views

What is the difference between flutter and vortex induced vibration?

During flutter, the airfoil oscillates with a large amplitude, which generates vortices at the wake. In vortex induced vibration (VIV) the oscillations of the airfoil are caused by the shedding of ...
Upid's user avatar
  • 53
4 votes
1 answer
483 views

How is the flow of air through a NACA submerged inlet determined?

This question pertains to the design and/or selection of NACA submerged inlets. I am trying to understand what kind of airflow would enter a NACA submerged inlet/scoop and am looking for a good ...
Gerry's user avatar
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3 votes
3 answers
977 views

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

I learnt in class today that fast-moving air over the wing of the plane, which causes a difference in pressure on the airfoil due to something called Bernoulli's principle. If this is so why cant the ...
Linkin's user avatar
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3 votes
3 answers
836 views

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

In aerodynamic lessons and books about jet engines, it is always said that there is a risk that air flow in the combustion chamber or compressor can go back to reverse direction, towards the inlet if ...
Jawel7's user avatar
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3 votes
1 answer
412 views

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 ...
Wyatt's user avatar
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3 votes
3 answers
4k views

How do a helicopter's cyclic and collective transfer commands to the swash plate?

I understand the function of the cyclic and collective, and I am specifically wondering if anyone has a good image/video showing the mechanism of how the flight control "mixer" works! I can't seem to ...
Gerry's user avatar
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3 votes
2 answers
444 views

Could compressor stalls be caused by air in supercritical conditions?

It is likely that in compressors of jet engines appear conditions that cause the intaken air to become supercritical, cf Is the air in a jet engine supercritical? . Is it possible that ...
ralf htp's user avatar
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3 votes
2 answers
2k views

How to carry out a CFD simulation for determining the stability derivatives of an aeroplane?

I want to create a 3D model of an aircraft to determine the stability derivatives by using a computational fluid dynamics (CFD) simulation. I tried to google for tutorials that explain how it is ...
MrYouMath's user avatar
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3 votes
1 answer
2k views

Does the airflow from a propeller over a wing contribute to lift? [duplicate]

How I understand it, the perfect propeller would move air rearward at the exact same velocity as the free stream, i.e. the propeller air will merge with the free stream and have no real influence on ...
Jonny's user avatar
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3 votes
2 answers
562 views

How does the adiabatic cooling of wingtip vortices work?

This question on condensation and wingtip vortices, What is the mechanism by which condensation forms above wings?, attributes the reduction in temperature found in wingtip vortices to Adiabatic ...
Quentin Chester's user avatar
3 votes
3 answers
950 views

Does shrouding a propeller minimize induced drag by equalizing the downwash velocity along its blades?

EDIT: It's not a duplicate of Are ducted fans more efficient? That question and the answers doesn't address the reason for the higher theoretical efficiency, it is more about efficiency in practice (...
Aviator S's user avatar
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3 votes
1 answer
141 views

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 ...
Wyatt's user avatar
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3 votes
2 answers
3k views

How is Reynolds Number derived and used?

Reynolds number is the ratio between inertial forces and viscous forces according to wikipedia. However, I still do not understand its derivation process and how to, for example, calculate the ...
Harry Yang's user avatar
3 votes
1 answer
105 views

Why $\frac{\Delta p}{\overline q}$ is depending by weight, flap deflection and other parameters? Which are other parameters?

As you can see in this link (Roskam, Airplane Aerodynamics and Performance, Chapter 2.5.3 Airspeed corrections) ... ... in the first and second line of page 30 Roskam tells that, in hypothesis of ...
d.pensopositivo's user avatar
2 votes
1 answer
305 views

How do viscous drag and form drag individually change as a function of angle of attack?

Typically, when dealing with airfoils, the drag coefficient $C_d$ is given as a function of angle of attack. If we limit the discussion to 2D subsonic incompressible flows, then the drag is the sum of ...
IamNotaMathematician's user avatar
2 votes
1 answer
483 views

What aerodynamics forces are acting on a jet fighter during slow flight demonstration?

I have long been contemplating the forces involved in certain maneuvers in an aeroplane and some of those maneuvers begin to break down my understanding of the forces that specifically oppose gravity. ...
Ryan Griffith's user avatar
2 votes
1 answer
121 views

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 ...
Wyatt's user avatar
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2 votes
1 answer
207 views

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 ...
Wyatt's user avatar
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2 votes
1 answer
197 views

Can a propeller with 90° pitching and symmetrical airfoils generate thrust?

Imagine we have a plane that is already moving at a speed $v_{plane}$. At a certain time $t=0$, a motor starts moving a propeller whose blades consist on symmetrical airfoils with $90^\circ$ of ...
user3141592's user avatar
2 votes
3 answers
416 views

How does increasing camber increase lift coefficient?

How does increasing the camber of an airfoil (like the NACA 0018) increase its coefficient of lift? You're just curving the airfoil; I don't see how that increases lift for a given angle of attack?
MichaelK's user avatar
2 votes
1 answer
316 views

Does the flow in an afterburner have to be subsonic?

This question is prompted by the problem on page 350 of the fluid mechanics book by Landau and Lifshitz. The problem states, "A small amount of heat is supplied over a short segment of a tube in ...
Stephen Blake's user avatar
2 votes
1 answer
1k views

For an aircraft, what are the differences between nonlinear and linear equations of motion?

So, as far as I understand, linearized equations of motion are just nonlinear equations of motion converted to linear EoM's using approximations and assumptions to simplify/remove elements from the ...
synchh's user avatar
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2 votes
1 answer
62 views

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 ...
Wyatt's user avatar
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