Questions tagged [fluid-dynamics]

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

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To what extent can atoms be *specularly* reflected (in the context of high velocities in a rarified atmosphere)?

This is a follow up to earlier comments below this answer regarding orbits at low altitude. The suggestion is to try to obtain energy from the Earth's rotation to maintain an orbit with a very low ...
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Would a rotating sphere placed within a pipe with air flowing through it experience a lifting force due to the Magnus Effect?

I am wondering if a rotating sphere placed with a pipe/duct with air flowing through the pipe will cause the rotating sphere to experience a lifting force due to the Magnus Effect. Moreover, if the ...
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Force calculation to close the valve and also the minimum force require to keep it close [closed]

I was thinking of calculating the force as Drag force $$N = \frac{C_d\cdot A\cdot \rho\cdot v^2}{2}$$ where $C_d$ - drag coefficient of circular disc, $A$ - Area od Disc (diameter - 20mm), $\rho$- ...
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How can we create suction, without the use of a vacuum?

How is it possible to create suction without the use of a vacuum? For example: As I understand, positive displacement pumps transport fluid by creating a vacuum at their inlet to draw in a distinct ...
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What determines handedness of wind circulation around pressure extrema in the atmosphere?

I'm reading a textbook (Atmospheric Science - An Introductory Survey 2nd ed, John M. Wallace • Peter V. Hobbs) which states (p14): The winds observed in the Earth’s atmosphere closely parallel the ...
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Poking the water droplet developed at the spout of a leaky tap

I was observing a leaky tap today and I noticed that when I poked the approximately hemispherical surface developed before the water dropped, the water quickly came off as a stream. If I don't disturb ...
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How does object move in bottom of swimming pool? [closed]

Suppose there is an object $O$ (swimming goggles) that has fallen to the bottom of a swimming pool. I have the swimming pool circulation pump turned on. Initially, the object is at some position $P_1$....
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Why does the second piston accelerates slower than first? Why it happens so? [duplicate]

In the image above we can have $A2>A1$. We know it from Newton second law that $F=m×a$. It means that if we apply large force on any object then, it will accelerate with large value too. My ...
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Local thermodynamic equilibrium and fluid continuum

Two hypotheses commonly used as a basis for all Fluid Mechanics are: Fluid continuum (i.e. we don't consider the space between particles) Local thermodynamic equilibrium: useful for fluids without ...
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1answer
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In which direction will air flow? [closed]

Here is a problem: I am having trouble with deciding between choices B and D. The sub-hemispherical bubble will have lesser volume than the hemispherical one for a given radius of the tube, and ...
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Why is the stress component in a tensor related to deformation along two dimensions

While trying to derive the constitutive laws of Newtonian fluids, I came across this equation: $$\tau_{ij} = \mu\left(\frac{\partial v_i}{\partial x_j}+\frac{\partial v_j}{\partial x_i}\right)$$ I ...
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What is the mechanism between whirlpools repelling and attracting each other?

I read somewhere that whirlpools in water will attract each other if they are spinning in opposite direction but will repell eachother if they are spinning in same direction. How does this exactly ...
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Why doesn’t a laminar flow launched upward get infinitely wider? [closed]

Law of continuity states that A*V is constant.. Then as laminar flow gets slower as it shoots upward, shouldn’t it get infinitely wider? As V approaches 0, A should increase infinitely. What real ...
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How can a Kestrel hover in the wind?

Kestrels are birds of prey commonly found in Europe, Asia, Africa, and the North America. They belong to the falcon family but have a unique ability to hover in the air. You can find a whole bunch of ...
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How to estimate size of hole damaged in the tank? [closed]

the vessel crew reported that they suspected bottom hull of the vessel is leaked as they found out a Void Tank have a sea water ingressed and now the tank full with Sea Water. Luckily the tank ...
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Viscous forces acting on a thin square plate moving along the top surface of a water tank

Consider a thin square plate floating on a viscous liquid in a large tank. The height ݄ of the liquid in the tank is much less than the width of the tank. The floating plate is pulled horizontally ...
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Oil pooling along grates in heating pan

Recently I went to cook something on my cast iron pan. I turned the burner on, drizzled in some oil (I think it was olive oil) and started chopping some vegetables. When I turned back a few moments ...
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Why does radius of curvature increase only after height is maximized in capillary rise?

In an experiment to measure the surface tension of water using capillary rise method, a student uses a capillary whose length is shorter than the height of capillary rise (obtained theoretically for ...
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Confusion about Newton's third law [closed]

The question: A toy rocket consists of a container of water and compressed air.Water is pushed vertically downwards through a nozzle by the compressed air. The rocket moves vertically upwards. The ...
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Why doesn't oil produce sound when poured?

I've noticed that unlike other liquids, when pouring olive oil for example, I don't hear any sound at all from it. Usually you can hear an audible sound as a cup gets filled with water, as the sound ...
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1answer
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For competitive swimmers, why does drag increase when pushing off of the wall near the surface rather than deep in the water?

I found this experiment which showed that there is the least amount of drag between 1.25 and 1.50m below the surface of the water, while there is increased drag at just 0.25m below the surface. The ...
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Bernoulli Equation Normal to the Streamline

Good Evening, I'm thinking about the expression of the Bernoulli Principle in the tangential and normal directions of a StreamLine. My question is: If I solve the velocity field in a fluid dynamics ...
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Calculate venturi effect for equilibrating pressure drop

Let me first say I am a novice in fluid dynamics, I am not sure about terminology but I will try to explain myself as clearly as possible. I need some help for understanding the following case: in a ...
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1answer
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In freestyle swimming does flutter kick recovery causes sinking?

As per my understanding in flutter kick, we kick downwards and the water pushes the swimmer up. However, in flutter kick the swimmer is also actually kicking the water upwards when recovering from the ...
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Deriving velocity of sound in a relativistic perfect fluid from weinberg

The following derivation is from Weinberg's gravitation and cosmology. $o=\frac{\partial N^\alpha}{\partial x^\alpha}=\frac{\partial ({nU^\alpha})}{\partial x^\alpha}=\frac{\partial}{\partial t}({n(1-...
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Confusion between Steady flow and Space-Time invariant density field

Let's take the example of a steady flow with velocity field as $u=\beta r (\tilde{r}) $. Now, we can find a density field $\rho (r,t)$ that satisfies the mass conservation equation of $\partial\rho/\...
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Newtonian mechanics vs. Pressure in Flight [duplicate]

Which one is better to describe lift of a plane wing: Newton's third law of motion: (The wing applies a force on air and the air applies an equal and opposite force on wing - which causes lift.) OR ...
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Consideration about Bernoulli Theorem

I was considerating the Bernoulli Statement about the fact that in a Stationary and Inviscid Flow an increase in the speed of a fluid particle correspond to a decrease in its static pressure. I'm ...
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If you raise the temperature setting on a boiler, does the boiler hold hot water longer?

My family has two showers and whenever someone is using the other shower, my brother raises the temperature setting on the boiler. My brother is trying to argue with me that raising the boiler's ...
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Efflux velocity vessel filled with air and fluid

I want to calculate the effelux velocity for this open vessel with area $A_1$ with a small outlet with area $A_2$ as a function of time. The atmospheric pressure is $p_0$. The vessel has height $h = ...
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Velocity of liquid in tank [closed]

How do i find the average velocity of efflux. I tried integrating v=√(2gx) from x to 0 then dividing by x. I have to find out the time it would take water to get empty till the hole in the tank. I was ...
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Use a single function to express the relationship between spalding mass transfer (B) number of water droplet and relative humidity (RH)?

I'm totally inexperienced in fluid physics but I need to put spalding mass transfer number of water droplet and relative humidity into one function in my work. Through some research, I knew that $B=\...
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Pressure term in Bernoulli's principle

I have encountered two different explanations of why the pressure term appears in the equation for Bernoulli's principle. The first one is: (1) "The fluid has to speed up as it enters the ...
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Will the water level keep rising up after attaining terminal velocity?

Suppose I have an infinitely long container of water and I dropped a metal ball into it. Initially, the water level will rise up due to displacement of volume. But will it keep rising when the whole ...
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Query regarding a published description of a mechanical hydraulic system analogous to $V = IR$ in an electrical system

The screenshot is taken from the wikipedia article for a resistor - https://en.wikipedia.org/wiki/Resistor I understand that a larger head results in greater hydrostatic pressure, therefore a greater ...
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Peculiar flow of water from an aerated faucet

Let me acknowledge that my friend found out about this effect first, who later showed it to me. When you slowly open an aerated tap, bubbles that form near the opening of the tap start to merge to ...
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From where do the bubbles come from in an aerated tap? [duplicate]

It is common knowledge that when the opening of a tap has many holes, the water flows out as a stream of bubbles. But if you open the tap slowly, the water flow is laminar without any bubbles at first....
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Blob of falling water

If a "blob" of about 100-200ml of water is dropped from higher than about a first floor (using English nomenclature: ground, first floor, second, etc), it'll stay together as a blob until it'...
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Optimal shape for air inlet with least disturbance

I aim to measure the atmospheric $\text{CO}_2$ concentration (also many greenhouse gases) of a city. The measurement will be made on a tower, at the height of $130$ m. I have a KNF diaphragm pump and $...
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Newton's second law and moving through a fluid

It is harder to move an object through a dense fluid like water than compared to with a less dense fluid like air. Is an explanation for this possible with Newton's 2nd law? There are people that say ...
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Unsteady flow inside a pipe with heat exchange and pressure drop

I am studying the behaviour of a hot air gas inside a circular-area (constant area $A$) pipe. The gas exchanges heat through the wall and experiences pressure drop while flowing inside the pipe. I ...
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Name of the Rayleigh–Taylor demonstrator toy

There is a common physics "toy" consisting of two liquids sealed into the space between two sheets of plexiglass. When turned upside down, it demonstrates the Rayleigh–Taylor instability. I ...
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Vortex methods and Helmholtz theorem

I'm pretty new to vortex methods in fluid dynamics, I found in this paper (page 12, line2) that Helmholtz theorem states: that the dynamics of the vorticity field can be described by vortex elements, ...
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How to determine stability of shocks in hydrodynamics?

Shock waves are very common in relativistic as well as non-relativistic hydrodynamics. The stability of the shocks are usually studies using numerical simulations. However, I am interested to know ...
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Similarity between Magnus effect and Lorentz force

The Magnus effect causes a rotating cylinder, that moves perpendicularly to the axis of rotation, to experience a transversal force (i.e. perpendicular to the rotation axis and the primary direction ...
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1answer
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Shouldn't the water released from the big bottle stops when the water in the small bottle reaches the tube?

I am trying to make an automatic watering system and I have a simple question Shouldn't the water released from the big bottle stops when the water in the small bottle reaches the tube? I think the ...
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More confusion about the faster-than-wind Blackbird

There's been plenty of discussion on here about the Blackbird as featured in this video. I get that it harvests energy from the relative velocity of air to ground. But I still can't wrap my head ...
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Material Derivative on a Surface Element

I'm recently working on material derivatives, defined as follows. $$\frac{D }{Dt}=\frac{\partial }{\partial t}+v\cdot \nabla $$ It is easy to show that a material(or convective) derivative of an ...
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How pressure and flow rate change if we aplly force (pressure) on horizontal section when on left side is pump and right side is open to air?

Consider this picture with pump on left hand side with pressure P1 (or some very big container with water, so big that its volume is not changing) and right hand side is open to air (pressure PA). ...
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The lift of an airplane is similar to that of a balloon. It uses the density difference of air, right?

The lift of an airplane is similar to that of a balloon. It uses the density difference of air, right? The air density on the top surface of the balloon is low and the air pressure is small. The air ...

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