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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What happens when 2 air streams, one going faster but the same direction, are beside each other?

Inspired by this video, what happens when 2 air streams beside each other going the same direction, but one going faster, interact? Do they make turbulence and or vortices as said in the video? I’m ...
0 votes
1 answer
64 views

What does air becoming compressible change?

You always hear that around Mach 0.3, air technically becomes compressible. What actually changes when this happens, in subsonic and supersonic flow? Does it affect the formation of shocks?
1 vote
3 answers
162 views

Do slats and leading edge devices increase lift at a constant AoA and airspeed?

I've seen mixed sources on this subject. Some say leading edge devices allow the wing to obtain a higher AoA, and others say it flat out increases lift. For me, the latter makes most sense, because ...
0 votes
0 answers
37 views

Are oblique shocks formed at the leading edge-top surface of a wing?

Do oblique shocks form on the leading upper surface of a wing? I know bow shocks are formed there, but is that the same thing as I’m talking about? Oblique shocks would have to form there because the ...
1 vote
1 answer
294 views

When an oblique shock forms, what happens to the normal shock that helped it form?

What happens to the normal shock that helped the oblique shock form, in the first shock of a lambda shock? Oblique shock waves form because : "An oblique shock wave is a shock wave that, unlike a ...
3 votes
1 answer
216 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 ...
1 vote
2 answers
165 views

Do boundary layers become thicker or thinner as you speed up?

What happens to the thickness of boundary layers as you speed up? I recently watched a video about the SR-71, in which it said “the inlet spike develops a significant boundary layer of air” when ...
1 vote
1 answer
86 views

Why are pressures equal across the slip line of a lambda shock, but density is not?

Why are pressures equal across the slip line of a lambda shock, but density is not? In this answer, it says this : The surface Σ is a slip line between zones 3 and 4. Velocities will be parallel ...
1 vote
1 answer
40 views

How do boundary layers and shocks interact?

How do boundary layers and shocks interact? What difference is made with thinner and thicker BL's? I've recently been researching this topic, but at least for me there are no good search results on ...
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1 answer
105 views

Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? [duplicate]

Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? When going Mach 0.82, the shocks on the upper (and lower) surface don't go the whole chord length of the wing. (...
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1 answer
119 views

Why are bow shocks angled slightly downward at the leading edges of wings?

Why are bow shocks angled slightly downward on airfoils? In this picture if you look closely at the very bottom airfoil diagram, you'll see the "subsonic airflow" part is more below the ...
1 vote
2 answers
146 views

Why do boundary layers become more turbulent as they flow over a surface?

Why do boundary layers become more turbulent as they flow over a surface? This question originated from this one. What makes the boundary become more turbulent as it flows over a surface/wing? My ...
0 votes
2 answers
104 views

Why is a thicker boundary layer more turbulent?

Why is a thicker boundary layer more turbulent? I’m mainly referring to the effect of the varying speed over the boundary layer, and why it makes it more turbulent. I know it has something to do with ...
0 votes
2 answers
69 views

Wind tunnel flow over surface vs. non-wind tunnel (atmosphere) flow question

If you take an infinitely thin sheet of metal and stick it out of the window of an airplane at exactly 0 AOA, will the surfaces of it become low pressure if you speed up? In a wind tunnel, there is ...
0 votes
2 answers
141 views

Is air that is accelerating more turbulent?

Does air that is accelerating have more turbulence? It makes sense why air decelerating would make turbulent flow (e.x, the air during pressure recovery over the rear of an airfoil). Would speeding it ...
2 votes
0 answers
83 views

Why does the density decrease equal the speed gain at Mach 1?

In this answer, it says : "At small Mach numbers, changes in speed cause negligible changes in density, but as Mach approaches unity, both are of similar magnitude. With Ma>>1 , changes in ...
-1 votes
1 answer
275 views

Why does density decrease most above Mach 0.3, and not as much below Mach 0.3?

Why does density decrease more above Mach 0.3, and not as much below Mach 0.3? From what I've heard, it isn't a linear relationship but why is that? Asked differently, what makes it a non linear ...
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1 answer
166 views

Is my understanding of density/speed correct?

So I asked this question, and I wanted to know if my guess below was correct. I got an answer in the comments, but the reason for asking this question is because I was still a bit confused if my ...
0 votes
0 answers
38 views

What is the speed and temperature profile of the boundary layer at the stagnation point?

At the statgnation point where the air velocity is zero, what is the shape of the velocity and temperature profile of the boundary layer? Is the velocity profile of the boundary layer in the form of a ...
0 votes
0 answers
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Does lift decrease in hypersonic speed?

In John Bertin's Aerodynamics book, it is said that when the thickness of the boundary layer is low, the displacement thickness of the boundary layer is also low, and therefore the curvature of the ...
1 vote
1 answer
66 views

What is monotonous and non-monotonous flow?

I'm not an engineering graduate so I'm learning as I go along. I've just read a sentence that states: The transition shapes are identical, while the variety of growth rates of intermittency are non-...
1 vote
1 answer
109 views

How do I know which outer grid shape is best for CFD?

I’m looking into running a CFD simulation on an aerofoil in transonic flow (M=0.8) but am unsure about what external grid shape to choose for my computational domain? I read into 3 main types: C-grid ...
2 votes
1 answer
76 views

If I know both Re number and Mach, how do I incorporate both into my simulation?

I’m trying to simulate an aerofoil based on experimental data with flow in the following conditions: Mach=0.84 Reynolds number (Re) = 3*10^6 I wanted to keep my chord length at 1unit but then what ...
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 ...
1 vote
2 answers
279 views

why lift coefficient decreases at supersonic flow?

In Anderson's performance book, he wrote that the higher the speed, the greater the pressure difference between two points, and as a result, the lift coefficient is greater. But when we reach the ...
1 vote
2 answers
129 views

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 ...
2 votes
1 answer
192 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 ...
0 votes
1 answer
196 views

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 ?
3 votes
3 answers
825 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 ...
1 vote
0 answers
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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). ...
-1 votes
4 answers
3k views

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 ...
2 votes
3 answers
87 views

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 ...
1 vote
0 answers
84 views

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 ...
4 votes
1 answer
27k 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 ...
0 votes
0 answers
53 views

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 ...
2 votes
3 answers
331 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?
1 vote
2 answers
256 views

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 ...
1 vote
3 answers
625 views

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 ...
1 vote
1 answer
79 views

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?
23 votes
2 answers
57k views

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 ...
0 votes
0 answers
85 views

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)...
4 votes
4 answers
422 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 ...
0 votes
0 answers
42 views

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 ...
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 ...
64 votes
11 answers
50k views

How do wings generate lift?

Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
8 votes
1 answer
12k views

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 ...
11 votes
8 answers
3k views

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, ...
4 votes
1 answer
142 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 ...
3 votes
3 answers
868 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 ...
2 votes
1 answer
194 views

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; ...