Questions tagged [fluid-dynamics]

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

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Finding the force acting on a fluid: compressible Navier-Stokes equation

The compressible Navier-Stokes equation is given by $$ \rho[\frac{\partial\textbf{v}}{\partial t}+(\textbf{v}\cdot\nabla)\textbf{v}] = -\nabla p +\eta\Delta\textbf{v}+[\zeta+\frac{\eta}{3}]\nabla(\...
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Does pipe depth in water affect required suction for lift? Or just the height of the pipe above the water line?

Lets say you have 2 pipes submerged in a well where the water level is 20' down. The bottom of one pipe is 100' below the water level and the other is only 1' below. Would they require equal or ...
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Is it possible to solve for pressure loss in turbulent flow along a pipe purely analytically?

I understand it's possible to find pressure loss using formulas such as the Darcy Weisbach equation works with turbulence - but to my knowledge the friction factor is derived using empirical means. Is ...
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Why are aerodynamic / streamlined shapes always stumpy at the front?

I'm building an autonomous boat, to which I now add a keel below it with a weight at the bottom. I was wondering about the shape that weight should get. Most of the time aerodynamic shapes take some ...
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How to measure the doppler effect perpendicular to the movement in laser doppler anemometry

I'm trying to grasp the principle of Laser Doppler Anemometry. I like it as it is quite genius I'd say. However, what struggles me is the circumstance that the Doppler effect perpendicular to the ...
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How to find the heat flux for an ideal gas?

The conservation equations for the fluid mechanics of an ideal gas are \begin{align*}\dot{\rho}+\nabla \cdot (\rho u)=0 \;&\text{(mass conservation)} \\ \dot{(\rho u)}+\nabla \cdot (\rho u u) + \...
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Conditions for water to go down from high places

I've noticed that all of the above-ceiling water tanks are piped in a way that ensures to be open to the air at the tank, and then these water-carrying pipes go down to the kitchen or rest rooms,.....
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At what speed does air drag becomes important?

Imagine an object that wants to move at a speed $v$ trough a fluid. I was thinking that when i ride a bike, there is generally a speed at which air drag becomes a problem, but if i ride slow i does ...
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Question on fluid and basic mechanics

The question given is : Two vertical cylindrical vessels A and B of horizontal cross–sectional areas S and 2S are connected at their bottoms with a horizontal tube of cross sectional area 0.5S. An ...
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Which side of the surface does surface tension act on?

I just realized there's something extremely basic about surface tension that I don't understand. Surface tension is a property of the interface between two different materials, such as water and air, ...
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Why is pressure constant during a phase change of water in a piston with a constant weight weighing down on the water?

If I take a piston (such as the classic expanding gas piston) with a constant weight on the plate and liquid water beneath the plate. I understand by Pascal's principle that the pressure is constant (...
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Heat Modelling of an electronic assembly with Solidworks

I am trying to model an electronic assembly in Solidworks Flow Simulation. I am very new at this but I was able to figure out most of the components however, I am not sure how to go about adding the ...
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Where do incompressible fluids arise in nature?

The assumption that a fluid is incompressible seems to be an extremely common (and useful!) approximation to model fluids. I'm wondering what physical properties of a fluid make it approximately ...
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Ocean waves breaking slowly at the shore during periods of turbulence

During storms, I have often watched ocean waves breaking at the shore as if they were in slow motion. I've wondered if the slow curling of the waves during these times was somehow due to their ...
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Assumptions in a Moving Fluid

I saw this paper recently "On Integrals of the Hydrodynamic Equations That Correspond to Vortex Motions" by Hermann Von Helmholtz. It can be found here. In this research paper he discovered ...
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Pressure loss in concentric reducer with low Reynolds number and high viscosity fluid

This is my first question in stack Exchange,I'm making the overall calculus por a piston dossifier of viscous fluid like Honey and Carob Syrup, and I am currently struggling with the minor losses of ...
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What are some examples of moving a fluid without changing pressure?

Bernoulli's principle states that, in a steady, incompressible, inviscid flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. ...
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Why is it impossible to look equal Reynolds number conditions between prototype and model for the big wind turbine?

From the given Screenshot of the book the two equations shows equal Reynolds number both in prototype and model. I am not understanding that red marked line why it is saying so. Why is it impossible ...
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How do firefighting helicopters refill their water reservoirs while hovering?

I have been curious these days about how firefighting helicopters go about refilling their water reservoirs while hovering, and I was wondering, If the helicopter hovers $5 m$ above the refill ...
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What's the $P+P$ and $P-P$ on the opposite sides of the fluid particle. What is the meaning of this?

Does it mean adding really small pressures on one side of the particle and subtracting from the other side to cancel out? Also, what's with the delta sign and it's reverse? What's it called? (Sign on ...
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How can I theoretically determine the effect of acoustic perturbations on laminar-turbulent transition in a heat plume (i.e. rising smoke)?

I've done some preliminary reading on how Tollmien-Schlichting waves play a part in these perturbations, but is there a formula that connects sound frequency with the transition stage?
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Reason for motion of a fluid

I was making coffee for myself, when I observed that when I poured coffee from the frying pan (having curved edges), the fluid went directly into the cup without falling; but when I transferred coffee ...
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Why do we use relative velocity when describing flow rate of exhaust gases leaving a nozzle?

Reference Why do we take the relative velocity instead of the absolute analysis in analysis of flow rates? More particularly when we write down momentum, isn't the 'velocity' term which we multiply ...
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Simple analytical model for fluid flow in “Mushroom cloud”

In potential flow theory there are simple analytical models (formulas) for velocity-field of elementary features (like source, sink, dipole, vortex etc.) Is it possible to write simple analytical ...
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What is the effect of “The Ground Effect” on a load bearing structure?

If a train was made to utilize the ground effect, (or plane made to follow a rail) not to have any contact with the ground other than air, would the rail need to be so heavily built since the air is ...
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Pressure Variations in an Atomizer

This question has been asked two times Similar question 1 Similar question 2 Now my question will be different from all those questions, since I will be asking where exactly I am having trouble ...
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Why do we need to do work to maintain motion of a fluid?

On page 355 of principles of physics by Resnick walker and Halliday, the author writes: Work must also be done on the system (at the input end) to push the entering fluid into the tube and by the ...
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Bubble Detachment Dynamics

How does a gas bubble detach from a wall (say, for example, bubbles which form when water in a vessel is heated) it was sticking to? I have an idea... It is that when we increase the internal pressure ...
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Looking for a physics textbook on fluid mechanics [duplicate]

The title pretty much explains itself, could someone recommend me a textbook on fluid mechanics for someome who knows nothing about it? I've already learnt some vector calculus and I'd like to learn ...
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Fluid Mechanics - Bubbles

Why do gas bubbles initially stick to the walls of the bottle in carbonated drinks? And what happens when we open the cap? How do they separate from the walls?
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Experimental Data for Contact Angles

I am doing some experiments with contact angle of liquids on chrome stainless steel and was wondering if in the literature there is already experimental data for contact angles of different fluids on ...
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Is there a contradiction between the continuity equation and Poiseuilles Law?

The continuity equation states that flow rate should be conserved in different areas of a pipe: $$Q = v_1 A_1 = v_2 A_2 = v\pi r^2$$ We can see from this equation that velocity and pipe radius are ...
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Root Mean Square Displacement of Diffusion and Radial Diffusion Function

I read, that for normal diffusion the root mean square displacement $\sqrt{\langle x^2(t)\rangle}$ (for particles at the origin) can be interpreted as the mean distance the particles have with respect ...
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Source Term Inhomogenous Helmholtz Equation

So if we look at the inhomogenous Helmholtz equation \begin{equation} \nabla^2 u+k^2u=-f \end{equation} and include the sommerfield boundary condition \begin{equation} \lim_{r \rightarrow \infty } r^\...
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What is the Reynold's number of a plate moving through a fluid?

I read here that the equation of resistive drag $F = -bv$ can be used, assuming the particle is moving through a fluid at relatively slow speeds where there is no turbulence (i.e. low Reynolds number,...
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Why does fluid the backward curl as it gushes out from the nozzel in the forward direction?

Which phenomena is responsible for this kind of backward curled flow of the fluid (air)? Kindly if anyone knows about it,please explain.
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Zero velocity at a point in a streamline

Given a streamline, is it possible that the velocity at some point on it is non-zero but then at some other point(for example, stagnation point) it is zero? I mean does that make sense? My existing ...
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Vortex formation in an airfoil

How is a starting vortex formed when an airfoil (i.e cross-section of wings of a plane) starts to move?
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Proving $ \vec{V} \cdot (\vec{\nabla}\vec{V}) = (\vec{\nabla}\cdot\vec{V})\vec{V} $ using index notation

In my fluid mechanics course we encounter a lot of vector calculus problems, one of which I have been struggling with for a while now. We must prove that $$ \vec{V} \cdot \left(\vec{\nabla}\vec{V}\...
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Fluid flow out of a nozzle

When air or any fluid is forced out of a nozzle, why does the flow take a conical shape just outside the opening? And is there any way we can calculate the vertex angle of this cone?
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Is there some easy way to change the wettability of a sphere?

I am doing some experiments with spheres impacting a body of water and would like to change the wettability of the sphere. Is there some relatively easy way to do this? A procedure in one paper is ...
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Vortex study using video analysis?

I would like to know if they can recommend books or works where vortices are studied through video analysis (for example with TRACKER). I am interested in this because in an undergraduate experimental ...
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Can one model fluid flow only by using the convection–diffusion equation?

Modeling fluid flow in a coil seems very complicated, especially for turbulent flow. Having Navier-Stokes equations seems to be the right way to go, but I was wondering... can one use the convection-...
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What is a formal proof for Kutta-Joukowski Theorem in compressible flow?

Texts, such as Drela, Flight Vehicle Aerodynamics, indicate that the Kutta-Joukowski Theorem holds in compressible flow. Indeed, the KJT seems hold in linearized supersonic flow around a 2D flat plate....
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Flow rate and extrusion speed of a syringe

I want to calculate the flow rate of a liquid (very high viscosity, but we can assume laminar flow, for now, so I get an idea of how to go about this problem) I understand that I have to use ...
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Pressure at the opening of tube in Siphon

Consider a siphon tube... Now consider the opening of the tube at the lower end. Isn't the pressure at the mouth of the tube, at this lower end, something intermediate between $P_a$ and $P_a + \rho gh$...
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Does a ceiling fan block air flow from a window or a door?

A ceiling fan creates a vertical circular airflow. Does this airflow along a window or a door in the room reduce the airflow through them? I would guess air trying to flow into the room would face ...
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Flow rate in a curved pipe

Suppose I have in a constant gravitational field a shallow water tray that empties at height $h$ into a round pipe that opens underwater in another shallow tray at height $0$. My intuition suggests ...
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When we open the lid of an unused cold bottle of coke the gas inside comes out fast with a sound. Why?

For any movement we need a force. Which force does push the gas coming fast out the bottle of cold coke drink when we open the lid especially for the first time?
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Why do air compressors have separate inlets/outlets?

Our fridge recently broke down, and now I'd like to turn the leftover compressor into an air compressor. I've also got an old tank from a water pump. This one has only 1 opening. All the air ...

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