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Questions tagged [fluid-dynamics]

The quantitative study of how fluids (gases and liquids) move.

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Angle of attack and lift: is there a model?

The relation between the angle of attack $\alpha$ of an airplane's wing and the lift $F$ generated by it is quite complex. If I treat air particles as tennis balls hitting the wing at the front, then ...
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Normal Stress differece

What is the purpose of normal stress difference (t11-t22=N1) in uniaxial elongation and simple shear flow? what this different tell us about ?
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Viscous Force On A Rotating Cylinder

In this question asked in Irodov, it is taken that the friction force acting on a unit area of a cylindrical surface of radius $r$ is given by $σ = ηr(∂ω/∂r)$. A fluid with viscosity $η$ fills the ...
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Movement of bubbles in a cup of tea

Why is it that when I try to scoop bubbles out from the surface of a cup of tea, they always slip off the spoon even if some of the tea does not?
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How do vortex filaments move?

In fluid dynamics (Navier-Stokes, incompressible case), a vortex filament, as I understand it, is a curve of points $x\in\mathbb R^3$ such that the velocity field $v\colon \mathbb R^3\times\mathbb R\...
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Traceless stress tensor

What does it mean, when the viscous (or viscoelastic) stress tensor is traceless $\tau_{rr}+\tau_{\theta \theta}+\tau_{\phi \phi}=0$? Why if the viscoelastic model is linear it is traceless and if ...
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Adiabatic and isothermal astrophysical flows

Astrophysical flows around black holes (like accretion and winds) can be adiabatic as well as isothermal. In adiabatic flows, the flow is non-dissipative except at the shock location. However, in case ...
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Is this working by communicating vessels?

https://www.youtube.com/watch?v=JzuamK0AyC4 I think the first 2 bottles are working by communicating vessels because i think that the third bottle is with atmospheric pressure and since the second ...
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1answer
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Diameter to prevent water flow into a closed-end tube?

Imagine I drill three holes of different diameter into a large block of plastic but the holes don't go all the way through. (They form three close-ended tubes, see the image.) I then submerge the ...
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Clarification on Bernoulli's Principle

I need a clarification about Bernoulli's principle. The standard way in which this principle is taught is related to a picture like this one where usually the two sections have different areas and ...
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velocity gradient in cylindrical coordinate [on hold]

In cartesian coordinate,the velocity gradient $\nabla \mathbf{v}$ is defined as: \begin{equation} \begin{pmatrix} \partial_x v_x && \partial_y v_x && \partial_z v_x\\ \partial_x v_y &...
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What kind of forces develop on a solid shape suspended in bubbly fluid? Is there a name for the study of this sort of interaction?

Let's say I have a solid shape - a sphere, a cylinder, I don't know if it matters - and I place that solid in a container of water. The forces on the shape consist of gravity and buoyancy (due to ...
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How can the velocity field $V_z$ be obtained with this equation? [on hold]

I need to obtain the formula for the velocity field between 0
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Can Bernoulli's theorem be applied to laminar flow?

Since in streamline flow the velocity of a particle at any point is same, Bernoulli's theorem can be applied to streamline flow, but could it be applied to laminar flow?
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Is this wire-melting phenomenon likely to be a manifestation of the Plateau–Rayleigh instability, and have I done my math right?

In LECTURE 5: Fluid jets from MIT's 1.63J/2.26J Advanced Fluid Dynamics equation 23 is $$\omega^2 =\frac{\sigma}{\rho R_0^3} k R_0 \frac{I_1(k R_0)}{I_0(k R_0)}\left(1 - k^2 R_0^2 \right)$$ and is ...
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Sound of water filling up a bottle

So there are many questions addressing but I am yet to find a correct response. Most answers indicate that the bottle should be modeled as a Helmholtz resonator or as a closed organ pipe with ...
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Non-viscous incompressible Fluid between two coaxial cylinder

Consider a non-viscous incompressible fluid lies between two coaxial cylinders. The domain occupied by the fluid is defined as $0<z<\xi$, $A<r<B$. The coaxial cylinders slowly rotate ...
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Volumetric Flow with Air Compressor [on hold]

If I have a 5 gallon, 125 psi air compressor and I connect that to a pvc pipe with holes in it, how would I go about determining the amount of air flow through each hole in that pipe. It might be ...
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Does a thickness of a bubble become constant over time?

Assuming no gravity, imagine you have a bubble floating in the air, does its thickness have a tendency to become constant? State a bit differently, if we start with a bubble with variable thickness, ...
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Is velocity of a fluid the gradient of something physically significant?

For incompressible flow, $$\nabla\cdot \mathbf v=0.$$ That means $\mathbf v$ got to be the gradient of some scalar field. How can I find the scalar field? Is it physically important?
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What does it really mean by “faster flow, lower pressure”?

My primary school physics textbook trys to brainwash me with the idea that a region in a fluid with fast velocity (often) has lower pressure than a region that flows slowly. Examples of this include ...
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Would this experiment help solve turbulence in dynamical fluids? [closed]

(Inspiration from Tomamak fusion reactors + computational construction of fractals + machine learning.) Fluid turbulence is complex and is apparently random, but it’s not. It’s chaotic. Hypothesis:...
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First law of therodynamics appiled across a normal shock

This is a snippet from the book Compressible Flow by Anderson. Here, he is trying to evaluate the change in entropy across the shock using the relation, $s_2 - s_1 = c_p \ ln \frac{T_2}{T_1} - R \ ln ...
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In a spinning tube, how low can the vacuum in the center be?

If you put a gas in a tube, and spin the tube [edit: along the axis of the cylinder], you will get a centrifuge. What's the maximum pressure differential between edge and centre? Asked another way: ...
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I was wondering if the mass of an object can be measured by the effect it has on a body of water if it was dropped from a certain height? [duplicate]

The question: Height of Water 'Splashing' asks about the height of the splash when an object is dropped into water, but there are other parameters that can depend on the mass and velocity of ...
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+500

Physical intuition behind torque converter

A torque converter (also here) is a device used in some cars. It uses several "fans" coupled through a liquid (transmission fluid) in order to perform the function of a clutch, but more importantly it ...
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Siphon/Pump Hybrid For Getting Water From a River [closed]

Forgive me if this answer is already up on here, but I couldn't find anything particular to what I am trying to do. I need to lift water approximately 50 feet vertically and 200 feet horizontally from ...
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Air Flow through non-uniform size Duct with Holes

We are a manufacturing plant and this is one query which we could not find the answer too. So, came here asking for help. Problem: In the attached image, you can see the air/steam inlet from the top ...
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1answer
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Water beyond 10.3 metres [closed]

Using a centrifugal pump, you can only pump upto 10 metres. Thats fine. But, Can't i pump water beyond 10.3 metres by placing the suction head exactly at 10 metres( or below that) and extending the ...
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1answer
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When water flows over a curved surface, will it reduce the pressure on the curved surface? Is this because of the viscosity of water?

I think it's because water moves on a curved surface with centripetal force. Because the gravity of water provides centripetal force, the pressure of water on the curved surface is reduced. When the ...
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single pool skimmer or multiple better

Im designing a swimming pool, long rectangular shape with skimmers. Im having some debate as to whether one single skimmer at the centre of the long side, or multiple skimmers positioned along the ...
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Convert Normal water to cool instantly [closed]

Is it possible to convert the hot/normal water to cold steam instantly(may be within 1/2hr) without using refrigerators etc. If yes, how far can the cold steam travel. I am not sure with all the fluid ...
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Fuel efficiency of supersonic planes

I stumbled upon an interesting plot; in particular, the dependence of wave drag on the Mach number: It is curious to see that the drag coefficient drops so abruptly in the supersonic regime, but I am ...
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Liquid sand: is it really a liquid?

This question follows the lead of a previous question (What kind of fluid is sand?), where basic properties of sand were discussed. A comment to that same question linked to a videoclip where sand ...
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1answer
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Calculating radiative heat transfer between air and walls, what to use for the emissivity of air?

For example, say I have heated some air, I push it through a tunnel with cold black walls. Will the air radiate heat to the walls or will the walls radiate heat to the air? I understand that the ...
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Imperfect or perfect fluid in Einstein Field Equation

I'm trying to solve the Einstein Field Equations in an unconventional way (at least not usual from what is done in most basic textbooks). So basically, I specified a metric tensor (specifying a ...
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Is the pressure gradient perpendicular to the flow direction or along the flow direction?

I know that the pressure gradient is the pressure difference per unit length, but I'm not sure what direction it is.
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What's the difference and relationship between perfect fluid and superfluid?

Sorry for this naive question, I just cannot find good references talking about the difference and relationship between perfect fluid and superfluid. Could someone explain this to me or point me to ...
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Block completely immersed inside the fluid

Consider a block with density $\sigma$ kept in liquid of density $\rho$, where $ \rho > \sigma$. Now what is the cause of force by which it moves upwards? Is it buoyant force? If yes, then how? As ...
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What kind of fluid is sand?

Questions: To what extent is it possible to treat (dry) sand in presence of gravity as a fluid? How does sand differ from other more standard fluids like liquid substances? Since the definition of "...
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Will a cantilever tube vibrate if fluid enters from its free end and flows into the fixed end?

For dynamics of a cantilever pipe with fluid flowing from fixed end towards free end, there are values of frequency for a threshold of flow velocity that gives unstable, stable and neutral systems, ...
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Physical reason behind the origin of shock waves in astrophysics [duplicate]

Shock waves arises in astrophysics in accretion flows and in winds. But we know that shock waves usually occurs in supersonic flows when the flow encounters any obstacle or when the properties of the ...
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Viscous flow through a concave tube

Consider gravity-driven incompressible flow through a concave tube with known radius $R(z)$, where pressure is atmospheric $P_a$ on both ends: How does the viscous pressure distribution $P(z)$ ...
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Is a hole better for ceating a toroidal vortex than a pipe?

Does a hole work better for creating a toroidal vortex, where the fluid starts to turn as it exits, than a pipe, where the flow inside is more laminar. Some high power vortex guns have a cone shaped ...
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1answer
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Do viscous and inviscid flow have opposite effects on pressure?

Consider flow through the converging and diverging portions of a tube with varying radius: In inviscid flow, Bernoulli's equation states that velocity at the throat will be the highest, so this area ...
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1answer
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Bubbles in a Pipeline 2

This question has kind of been asked before here, but aren't the gas bubbles in the pipeline also a moving fluid and so as they get constricted shouldn't their pressure decrease too. If so how are ...
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Two spheres falling through a pipe

Imagine a pipe of radius $a$ perpendicular to the ground. A liquid flows in the pipe. Now you throw (instantaneously) two balls with radius $b$ (where $b << a$) through the pipe: one through the ...
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Does liquid have normal velocity when it moves on Inclined plane?

After the ball moves horizontally to Inclined plane, because the direction of motion is not parallel to Inclined plane, the ball will have normal velocity (perpendicular to Inclined plane); after the ...
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Frictional losses in flexible tubing with multiple branches opening up to a large tank

I am working on designing an experiment where I will be creating a wall-jet flow where the wall is a rotating wing. After some thought I realized I need to estimate the pressure losses due to the pipe ...
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Motion of cyclone in land less area

Lets say a tropical cyclone is formed, and there is no land for it to fall or dissipates, what will happen to that cyclone, will it move in circles due to coriolis force being more and more as it ...