# 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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### Boundary layer thickness calculation

I have tried to calculate all the losses associated with the serpentine intake to calculate the total pressure recovery. I have considered the frictional losses, pressure loss due to bend, approach ...
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### In wind tunnel testing "same Mach number" is not "same airspeed"?

Wikipedia: "Mach number: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical Mach number in a wind tunnel and around ...
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### 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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### 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 ...
401 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 ...
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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 ...
202 views

### 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 ...
146 views

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^... 1answer 1k views ### 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 ... 2answers 78 views ### 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 ... 1answer 255 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 ... 3answers 2k 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 ... 2answers 203 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).... 2answers 636 views ### 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 ... 3answers 352 views ### 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 ... 1answer 274 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 ... 2answers 13k 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 ... 1answer 731 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 ... 1answer 1k 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 ... 2answers 96 views ### 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? 1answer 145 views ### 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 ... 1answer 354 views ### 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 ... 1answer 586 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 ... 3answers 616 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 (... 0answers 73 views ### 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.... 0answers 115 views ### 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 ... 1answer 656 views ### 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? 2answers 6k views ### 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 ... 2answers 383 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 ... 2answers 521 views ### 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://... 1answer 234 views ### 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 ... 1answer 471 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 (... 2answers 4k 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-... 2answers 1k views ### 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 ... 3answers 4k views ### 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? 3answers 3k 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 ... 2answers 785 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 ... 1answer 344 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. ... 2answers 3k views ### 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 ... 1answer 524 views ### 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? 1answer 209 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 ... 0answers 336 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$coordinate of the airfoils (which are necessary to compute the chord) on the transformed plane (... 2answers 688 views ### How do aircraft engine manufacturers achieve a higher bypass ratio while still meeting the thrust requirements for a given aircraft? I understand that a higher bypass ratio leads to greater efficiency as less air moves through the core, and therefore less fuel is burned. However, how is the thrust requirement for the same aircraft ... 2answers 2k views ### How is Reynolds Number derived and used? Reynolds number is the ratio between inertial forces and viscous forces according to wikipedia. However, I still do not understand its derivation process and how to, for example, calculate the ... 1answer 97 views ### Why$\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 ...
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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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### 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 ...