Questions tagged [fluid-dynamics]

Is a subdiscipline of fluid mechanics that describes the flow of liquids and gases

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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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-1 votes
4 answers
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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 ...
Jawel7's user avatar
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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 ...
Jawel7's user avatar
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What should be the ideal distance between wings in a multi-winged air plane?

What is multi-wing or multi-plane: For those who may not know, multi-wing air planes or multi-planes are aircraft that exceed the three sets of wings. The idea is while increasing the number of wings,...
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How the placement of the pitot and static ports decided?

Forgive the this topic placement as it might be similar to other older ones (Although I a bit sure that the main contents might be different than other older ones). In the upcoming days I will have a ...
k. theodosis's user avatar
-3 votes
2 answers
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What is the optimal jet engine size for maximum range? [closed]

I under stand the flying wing has been calculated to have an optimal length. Is their an optimal blade length within a turbine engine for planes as well?
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Can an ionized airflow be controlled magnetically?

I am thinking about the possibility of replacing the control surfaces of an aircraft with thrust vectoring. The most straightforward form of that would be mechanically adaptable nozzles which ...
Simon Henn's user avatar
1 vote
1 answer
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Is it expected to see transition from laminar to turbulent BL at low Reynolds numbers?

I have used XFOIL to do the following analysis: NACA0018 airfoil. Reynolds number $2\times 10^4$ (low Reynolds number). Critical Amplification factor, $N_\mathrm{crit} = 3.54$ (corresponding to a ...
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1 vote
3 answers
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Why can't supersonic flow work its way upstream?

I am reading this textbook today. And it described supersonic flow as follows: In a supersonic flow, because the local flow velocity is greater than the speed of sound, disturbances created at some ...
Yihong Zhu's user avatar
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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, ...
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How to find the velocity of the working fluid through the fan in a turbofan engine?

Is there any formula for the velocity of the working fluid (air) through /the fan of a turbofan engine? If not, how can I find the velocity?
ganesh1102's user avatar
4 votes
1 answer
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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 ...
Random Ape from Africa's user avatar
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What happens to the assumption ∂p/∂y≈0 in the region of separated boundary layer? is it still valid?

I am reading about Boundary Layer(BL) and seems that in the derivation of the governing equations there is an assumption that the pressure, $p$, in $y$ direction is constant, that is: $${\partial p \...
Random Ape from Africa's user avatar
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1 answer
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Why are there two local maximums on some CL vs AOA curves?

I know what a stall is but I don't understand why there are in some cases two local maximums on the $C_L$ vs AOA curve of some airfoils. What is the reason for the mini stall that occurs before the ...
user217340's user avatar
1 vote
2 answers
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Circulation around an airfoil at negative α

I was recently reading this very helpful string What is the physical meaning of circulation found in Kutta condition? and it explained very nicely potential flow around an airfoil. My question then is ...
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Modeling of the aerodynamic characteristics of the wing

As a part of the engineering thesis I work on the methods of modeling of aerodynamic characteristics of wing. In it, I rise an issue related to the impact of changes of aircraft attitude during ...
Karol Chojecki's user avatar
1 vote
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Does an increase in Mach number lead to a decrease in Total Pressure Recovery (TPR) for supersonic inlets?

I am curious if the total pressure recover for supersonic inlets, in general, decrease for an increase in Mach number.
Jose De La Pena's user avatar
1 vote
3 answers
672 views

How does the turbulent boundary layer thickness affect separation?

I read that for a turbulent boundary layer, a thicker (turbulent) boundary layer separates earlier than a thinner (turbulent) boundary layer? Why is that? Or is it the other way around?
RNG's user avatar
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1 answer
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What are the "beta lines" of a gas turbine engine component?

For a compressor in particular, what do the beta lines represent? I know for a compressor map you have the lines of constant spool speed, and the contours of isentropic efficiency. I also understand ...
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Does Betz's Law apply to jet engine turbines?

Betz's Law says the extractable power from wind is only 59% (16/27) of its original power. $$P_{max} = \frac{16}{27} \frac{1}{2} \rho A v^3$$ Rho is the gas density, A is the cross section area of gas ...
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Calculate resultant force from pressure distribution?

Can you explain with example how can I calculate magnitude and direction of resultant force from pressure distribution over airfoil?
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1 answer
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Does low pressure induces high velocity or high velocity induces low pressure?

Does low pressure induces high velocity or high velocity induces low pressure? I am wandering what is the cause and what is the result. I mean i.e. Bernoulli principle or situation in the core of a ...
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Does flow separation starts earlier at higher airflow speeds?

For same AoA,will flow separation starts earlier (point of separation more upstream) at higher airflow speeds (=higher air inertia) compare to lower airflow speeds? High speed airflow has more air ...
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1 answer
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Velocity induced by a straight vortex filament

Consider a straight vortex filament as shown below. At each point, there's a point vortex of strength $\it T$. Consider that a point $P$ is there on the outermost circle of flow induced by the vortex ...
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What happens when a "ground effect" plane approaches the sound barrier?

Does the shock wave inhibit or contribute to lift? What is the ideal speed to fly near or on the speed of sound while using the ground effect?
Justintimeforfun's user avatar
3 votes
3 answers
2k views

Is multi element wing good option for slow flight airplanes?

Can we use multi element wing for slow flight planes, like F1 multi element rear wing? If we know that pressure act prependicular to the surface why this two upper elements are "almost" ...
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2 votes
4 answers
696 views

Why high air speed at flat surface don't always produce low pressure?

Can you explain why flat surfaces don't produce low pressure in both experiments ,even air has high speed in both case? 1)experiment where flat surface DON'T produce low pressure: http://www.cabrillo....
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2 votes
1 answer
221 views

On an aerofoil or circular cylinder why does the airflow stay attached after passing the point of maximum thickness?

Why does it follow the curvature rather than separating? Its not due to the coanda effect as the accelerated flow near the surface of the aerofoil can't be considered a jet. It's not due to surface ...
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How accurate is CFD software compared to wind tunnel tests? [duplicate]

How accurate is computational fluid dynamics (CFD) software compared to wind tunnel tests? (I mean CFD computed with super-computers like the aircraft industry has.)
CFD 404's user avatar
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How does a diffuser (in ground effect) accelerate the air in front of it?

A diffuser reduces speed and increases pressure, so how does a diffuser accelerate air in front of it if we have an adverse pressure gradient situation? Wouldn't be better that we put the lower ...
IQ 240's user avatar
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3 votes
1 answer
388 views

Why does turbofan exhaust appear to reverse direction?

Yesterday I was at the airport planespotting. I observed the exhaust of a couple of turbofan engines, and found it curious how the fluid seemed to behave as it was leaving the engine and gaining ...
Erik's user avatar
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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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2 answers
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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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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
5 votes
2 answers
860 views

Why do shockwaves progressively move aft as speed increses?

As an airfoil moves from subsonic to supersonic, the shocks that develop over the surface move progressively toward the trailing edge. What is the physical explanation for this behavior? As a side ...
Nick Hill's user avatar
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2 votes
1 answer
219 views

What causes the vortex on the tip of this vertical stabilizer (CFD Simulation)

This is a computational fluid dynamics simulation of an airfoil that is oriented to look like a vertical stabilizer. After watching the video, it becomes clear that the tip of the stabilizer causes a ...
Pheric's user avatar
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4 votes
0 answers
675 views

How far can the sound of a sonic boom travel? [closed]

Are there conditions or distances from the plane to cloud to the observer in which a sonic boom from a plane is muted?
Muze's user avatar
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1 vote
2 answers
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What happens to the high pressure below an airfoil when the angle of attack increases?

I recently came across this question in a principles of flight exam: When the angle of attack of an airfoil increases the high pressure below the airfoil increases, the induced drag increases the ...
Skydemon's user avatar
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2 answers
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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 ...
Andrew Xie's user avatar
1 vote
3 answers
679 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 ...
Nick Hill's user avatar
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8 votes
3 answers
617 views

What are accurate flight simulation models called? Is there a standard model?

I'm risking a question that might be "opinion based", but I hope it will not be seen as that. This question is touched upon in the following questions: What are the differences between PC and ...
AlphaCentauri's user avatar
4 votes
1 answer
420 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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1 vote
2 answers
681 views

Why do wingtip vortices affect the upwash ahead of the aerofoil?

I understand that wingtip vortices increase the downwash aft of the aerofoil and the theory that they will alter the effective airflow angle, reducing effective angle-of-attack, with the total ...
Lee Johnston's user avatar
1 vote
1 answer
1k 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 ...
Guha.Gubin's user avatar
7 votes
2 answers
3k views

What is the difference between static and dynamic stall?

What are the static and dynamic stalls on the aircrafts wing? When would we encounter static and when dynamic one? What is the difference between them?
Darjan's user avatar
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1 vote
1 answer
2k views

Is the shear centre of a wing the same as its aerodynamic centre?

How can the shear centre of a wing be located and is it the same as the aerodynamic centre of a wing? Lastly, I know how the centre of pressure interacts with the aerodynamic centre but not the wing'...
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8 votes
2 answers
858 views

Transonic buffet - reason for a rapid movements of the shockwave along the wing chord line?

I know that transonic aerodynamic buffet is caused by the separated turbulent boundary layer striking the airframe (horizontal stabilizer, wings, fuselage) with considerable force causing a high ...
Darjan's user avatar
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0 votes
1 answer
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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?
Darjan's user avatar
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