Questions tagged [fluid-dynamics]

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

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induced drag at supersonic speed [duplicate]

for a delta wing with supersonic leading edge at small angle of attack there is no stagnation point on leading edge so There is no connection between the top and bottom of the wing. Therefore, the air ...
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Why does adding heat to the subsonic flow increase the speed of the flow?

In Rayleigh flow, adding heat to the subsonic flow increases the speed and adding heat to the supersonic flow decreases the speed. But what is its physical cause?
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Why are the top of the F/A-18 strakes curved?

As seen in this picture, the top of the strake has a slight curve to it. I'm mainly talking about the curve seen if you were to look at it straight on. To be more clear, not the curve of the strake as ...
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Estimate the drag caused by wind on a free falling body

I'm trying to solve the following problem for my masters thesis: Estimate the drag caused by wind on a free falling body from H altitude. H is in range of 300-1000 meters. What would be the best way ...
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Why is the wave drag of a straight wing less than that of a swept back wing?

According to NASA's claims, when the swept back wing is inside the shock cone and the shock is not attached to the leading edge of the wing(subsonic leading edge), the wave drag of the swept wing is ...
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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 ...
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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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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 ...
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Do speed and density exhibit a linear relationship?

Does speed and density scale linearly? Say you’re going 500mph, and speed up to 600mph. The density of the air will decrease. Now if you’re going 1000mph and speed up 100mph, does the density decrease ...
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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 ...
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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 ...
199 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 ...
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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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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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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 ...
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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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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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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 ...
4k 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, ...
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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?
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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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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?
253 views

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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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?
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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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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....
336 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.)
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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 ...
405 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 ...
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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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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 ...
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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 ...
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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?
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