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 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 ...
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What causes lift in aerofoil: Bernouli's principle or Newton's third law? [duplicate]

Reading materials online, I observe some explanation based on Bernoulli's principle, quoting the difference of air pressure above/below and difference of speed of airflow. While some say's Bernoulli's ...
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Is it possible to have the laminar flow above $Re=10^7$?

For pipe flow I came across the information that the onset of turbulent flow occurs at approximately $Re=10^3$ to $Re=10^4$, while for boundary layers on the airfoil the onset occurs in between $Re=10^...
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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 ...
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What are the effects of separation at hypersonic rarefied flows?

Shock wave causes an adverse pressure gradient which causes an airfoil to stall. Also because gas density is much lower boundary layer becomes laminar and unable to prevent separation. Correct me if ...
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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 ...
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3answers
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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 ...
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141 views

Why is the circulation around a centrifugal impeller blade tip the opposite to a wing tip?

Why does the circulation around a centrifugal pump blade tip go from low pressure to high pressure as an effect of the slip caused by the non-uniform velocity distribution (the opposite of a wing tip)....
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Why does the Biot-Savart law give the induced velocity of a infinitesimal segment of a vortex filament at point P?

My question is about the proof of how the Biot-Savart law can be used for vortex filaments. This is what I have in my textbook: However, I'm not certain how it was derived - I understand that the ...
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Why is the stagnation point on the upper surface before the start of circulation?

In many textbooks, such as Anderson (Fundamentals of Aerodynamics), Bertin (Aerodynamics for Engineers), and Houghton (Aerodynamics for Engineering Students), the authors present the idea that there ...
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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 ...
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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 ...
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456 views

Boundary layer momentum thickness

Could someone please explain the concept of the boundary layer momentum thickness and its difference to boundary layer thickness calculation? Some explanation about the application of the boundary ...
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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 ...
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In turbojet engines, does the fan play a key role in the air intake to the inlet? [closed]

In turbojet engines, does the fan play a key role in the air intake to the inlet?
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1answer
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Is high speed buffet possible at supersonic speed?

Since high speed buffet is caused by boundary layer separation due to shock wave and at supersonic flight there is no shock wave directly onto the airfoil (it is attached to the trailing edge), is ...
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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 ...
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1answer
398 views

Why do we see a reduction in temperature in a wind tunnel by increasing the velocity, but in a real flight it's the opposite?

In a real flight, when the speed of an aircraft increases, we see an increase in temperature too. So that at high Mach numbers the surfaces of the aircraft will be very hot. Now I have encountered ...
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435 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 (...
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Is winglet height equivalent to duct height?

Winglets increase effective wingspan by causing vorticies to be generated further away from the wing root in proportion to the height of the winglet (if vertical), likewise with winglets on propellers....
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How much lift can be generated by the intake lip of a duct?

How to calculate the extra lift gained by the intake lip of a duct? Aside from improving efficiency by preventing tip vortices, if a duct has a bell-shaped intake, this lip around the edge creates ...
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1answer
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Why is the delta wing naturally stable in pitch?

Any thoughts on why the delta wings are naturally more stable in pitch, so they don't require any tail surface? What's the physics behind this?
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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 ...
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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 ...
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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://...
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Is there a symbols chart or legend for common aerospace equations?

I've read a lot of answers on this site that make references to various aerospace equations. So many of these are awesome responses, but I have trouble understanding the math because I don't know ...
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1answer
362 views

Why is the Aerodynamic Centre not affected by changes in Angle of Attack? [duplicate]

I understand that the Aerodynamic Centre (AC) is the point in the aircraft where the pitching moment never changes with a change in Angle of Attack (AoA). My understanding of the Centre of Pressure (...
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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-...
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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 ...
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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?
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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 ...
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2answers
447 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 ...
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1answer
302 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. ...
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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 ...
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How is the direction of the hydraulic fluid changed so that the cylinders and pistons can move backwards and forwards?

How is the direction of the hydraulic fluid changed, so that components and actuators (cylinders and pistons) can move backwards and forwards?
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1answer
115 views

What is the link OR difference between change in momentum & pressure difference?

I ask in regards to the the thrust formula, which includes change in momentum, as well as difference in pressure between inlet & outlet of the engine to calculate generated thrust; this implies ...
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0answers
202 views

Determination of a Joukowski airfoil chord (demonstration)

I'm currently studying Aerodynamics, and one thing that I noticed is that the maximum and minimum $x$ coordinate of the airfoils (which are necessary to compute the chord) on the transformed plane (...
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2answers
589 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 ...
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2answers
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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 ...
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1answer
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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 ...
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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 ...
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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?
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1answer
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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 ...
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548 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?
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605 views

What knowledge is needed to build a small aircraft?

It is my dream to make a basic flyable small scale aircraft (the size of a model aircraft). What is the basic knowledge needed to be able get started in the first place? In particular, I would like ...
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3answers
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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 ...
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1answer
179 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 ...
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Is there a real temperature change along a streamline when its velocity changes?

Stagnation enthalpies are given as $H_T=H+\frac{v^2}{2}\ $ Along an adiabatic inviscid steady flow streamline Stagnation Enthalpies are said to be constant. $H_{T1}=H_{T2}$ $H_1+\frac{{v_1}^2}{2}...
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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 ...
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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 ...