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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Why will all air slow down by the same amount in an adverse pressure gradient?
Boundary layer separation is caused by air slowing down to zero and reversing before reaching the end of the body. This is due to friction (viscous effects) + adverse pressure gradient.
The formation ...
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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 ...
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effect of wing chord length on lift coefficient
In the bertin's Aerodynamics book
It is written that reducing the wing chord increases the lift coefficient( decreases stall angle) and as a result, the wingtip stalls earlier than the wing root
how ?
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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 ...
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Calculating Lift-To-Drag ratio of a rotating propeller with zero forward velocity
I'm simplifying some blade element theory problems by assuming that the propellers are perpendicular to the ground and their motion is constrained to being purely vertical (without forward velocity).
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Why do we consider that air comes into jet engine?
While calculating air pressure and temperature during cruise of jet engines, we consider that air comes into the jet engine inlet with our flight velocity.
But why? In fact, air stops stagnant in ...
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Is contact between flow and inner walls of nozzle required for thrust generation?
I uploaded a picture below explaining over-expanded flow/nozzle condition with regards to thrust.
This aviation source tells that “extra nozzle walls don’t generate any additional thrust because flow ...
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How can over-expanded flow go out of nozzle when it has less pressure than ambient pressure?
We know that over expansion in flow means that nozzle exit pressure of flow is lower than ambient pressure at specific altitude. It gets narrower while exiting nozzle causing loss in efficiency.
My ...
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Do we use static pressure in thermodynamic cycles like brayton instead of total pressure?
I observed that pressure plotted in the graph above increases from 0 to 1.
0 is inlet entrance and 0' (not shown in the graph) somewhere between 0 and 1 is compressor entrance.
Now, we know that at ...
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Why are inlet and outlet (exhaust) pressure considered to be the same in a turbojet engine?
Why do we take outlet/exhaust pressure (station 6) as the same as in inlet (station 1) ?
In T-S diagram, we see that outlet temperature is much higher than inlet temperature since flow in outlet has ...
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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?
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Trailing edge flap disadvantages
What does ‘high local surface curvature’ mean with respect to trailing edge flaps?
I’ve been reading up on them, and that came up as one of their disadvantages?
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Why does pressure difference between top and bottom surface of wings increase as speed increases?
We observe that lift force increases as speed increases on aircraft wings. Theoretically, there must be increase in pressure difference between top and bottom surface of wings.
What I don't understand ...
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Streamwise Pressure Gradient on Curved Walls
Could someone help me to understand how can a wall generate differents streamwise pressure gradients just by its curvature? I can understand how it works in cross-flow (perpendicular to the streamline)...
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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 ...
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Is the same working principle of a vacuum ejector (fluid eductor, pressure ejector or "motive flow") viable to be used as a propulsion system? [duplicate]
Some time ago I've made a question involving Jetoptera bladeless propulsion, and got a comment saying they didn't know how that method of propulsion would make any difference. But I forgot to ask more ...
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location of aerodynamic center at hypersonic speed?
as we know , at low subsonic speed the aerodynamic center is located at 25% of chord line and for supersonic flow it lies at 50% of chord line. what about hypersonic speed? is it moves further aft?
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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, ...
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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 ...
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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 ...
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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 ...
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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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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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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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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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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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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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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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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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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 (...
