# Questions tagged [fluid-dynamics]

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

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### Fluid Dynamics: Stagnation, Static and Dynamics Pressures

I would like to ask how energy changes forms in a flow. I understand the concept of stagnation pressure, static and dynamic. I was wondering how energy changes form from static to dynamic. What ...
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### Wedge flow boundary layer

Why does the velocity of free stream outside the boundary layer in wedge flow changes in direction of flow ($x$)? Why thickness of boundary layer on the wedge is less than thickness of boundary layer ...
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### Circular motion at $Re\ll 1$

Let's assume a bead of mass $m$ is moving on a circular trajectory in a fluid, and that the typical velocity, the radius of the bead $a$ and the viscoisity of the fluid $\eta$ are such that we are in ...
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### Potential flow pressure on a smooth surface

For an incompressible potential flow around a smooth rigid body, is it true that the pressure on the surface of the body is proportional to $a\cos^2\theta+b$ where $\theta$ is the angle the inward ...
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### Can a gust of wind have a distinct blade-like edge?

I was staring at a puddle as a gust of wind hit it, producing the following wave pattern ... To my mind, the only way this could occur is if the gust of wind had a distinct leading edge, much like if ...
1k views

### The physics of bottle flipping

Recently it has become a trend to flip a bottle which is partially filled with water and land it on it's bottom. This video shows the phenomena :https://www.youtube.com/watch?v=2E7rS1q4NbU What i ...
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### Lifting line trailing vortex/vortex sheet inside flow field

With refference to the lifting line model, do we consider trailing votex or vortex sheet to be inside the flow field ? because the vortex core is rotational and we violate one of the flow field ...
182 views

### can you light a butane lighter while cruising at high altitude?

According to this Wikipedia article, cabin pressure is normally maintained at 75kPa. The article suggests that at this pressure it would be difficult or impossible to maintain an open flame due to low ...
140 views

### Pressure required for gas to flow through an orifice

Imagine I have an upside-down cone, which is filled with a gas - let's say it is air. At the bottom of this cone is an extremely small hole (a few microns in diameter). How do I go about calculating ...
100 views

### 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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### How to survive a direct hit of a Tsunami?

It's probably a question too general for this forum, but I'm not sure. The other day, I was in the beach, near the sea, and a big wave came towards me. So, instead of running from the wave, I decided ...
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### Grad student needing help with a fluid mechanics issue in project

As the subject states I have hit a wall in a fluid mechanics associated problem in designing an experiment for my grad project. Long story short, I'm an engineering undergrad who got a broad ...
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### Energy recovery from refinery cooling tower

Is there any possibility of recovering exhaust energy from a cooling tower by putting a wind turbine above it? Would this affect the performance of the cooling tower?
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### In a completely closed system with no access to Air, will stirring (from a motor) decrease cooling time?

Given: A closed, square bottle of water with a Thermo-electric Cooler cooling the water inside a bottle by chilling a piece of metal that touches the water. No leaks, no access to air, all cooling ...
204 views

### de Laval nozzle

My question is for de Laval nozzle Given the inlet pressure $p_i$, ambient pressure $p_e$, and the geometry of the Laval nozzle ( local Area/Throat area : A/A*) relationship, what is the mach number ...
44 views

### Calculating The Potential Dimensions of an Effective Pipette

Is there a formula for calculating the potential variable dimensions of a non-bulb pipette (using a finger to block air ingress) to ensure fluid is retained within the pipette. For example, as a ...
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I am mathematician and I am interested to know more about fluid mechanics. I need an explanation of the physical meaning of the derivative of velocity with respect to position.
581 views

### Effect of mass on terminal velocity

How does increasing the mass of heavy and light objects affect their terminal velocity in viscous and inertial drag?
108 views

### Dumping term in equation

I want to know the physical interpretation of the term $$\int\limits_0^\infty {g(s){\psi _{xx}}(x,t - s)ds}$$ in some equation. This term is called infinite memory and the function $g$ is called ...
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### Why is the tone coming from my cup, caused by the strirring of my coffee, decreasing when I put creamer into it?

I noticed that when I put creamer in my coffee while stirring it, the tone of the ceramic cup (caused by the spoon touching the cup) is decreasing. I've searched the internet, but couldn't find any ...
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### Is it possible that water flow stops in a very long pipe due to friction forces?

I've been thinking about a hypothetical problem concerning fluid flow across two distant countries. Assume that country A wants to share water from a river with country B. Country A builds a dam to ...
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### Does the temperature of the body of an aircraft increases its efficiency?

I am actually working on a prototype of a Plane, i was wondering if the temperature of the body of the plane can make the Plane more efficient, if the structure is at 25Ā°C needs less energy to ...
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### Conservative form of Navier-stokes equation in cylindrical coordinates

Is it possible to write the conservative form of Navier-stokes equation in cylindrical coordinates? Almost all texts I have referred (Frank M. White, Kundu & Cohen,G.Batchelor) have it in non-...
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### Vortice flow comparison

Suppose the flow of a 1 square meter circle surface vortex. If instead of 1 vortex I stick say 100 smaller vortices in the same area. Will the total flow of 100 parallel vortices be larger that the ...
827 views

### Calculation of Drag coefficient for infinite cylinder

I would like to calculate the drag coefficient for an infinite cylinder for various Reynolds numbers. I'm using $C_D=\large{\frac F{\frac 12Av^2\rho}}$ My question is how do I calculate force ? ...
456 views

### Buoyancy and natural convection in a stratified water tank

I'm working on a simple calculation-model for a hot-water-storage. At the moment I'm stuck on implementing natural convection. To calculate the temperature I've implemented a simple 1D-model in ...
5k views

### Physical reason of adding salt when we're cooking rice (osmosis)

Why do we add salt when we're cooking rice? I know one reason is related to the boiling point of water but someone said it is also related to "Osmosis". What is the relation between adding salt and ...
458 views

### acceleration of water when it travels through garden hose

How to derive average acceleration of water as it travels through nozzle of garden hose. Suggest some assumptions for better understanding
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### Conceptualising the switch from discrete molecules to the continuum (fluid dynamics)

I understand that $\rho(x,t)$ represents the density of the small volume surrounding the point $x$. However what have I done with the discrete molecules in the process of averaging?
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### Can I explain lift in this way?

** As shown above, Suppose the wing moves horizontally.** The red arrow is the direction of air movement at the top of the wing. This direction is not perpendicular to the normal line at the top of ...
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### what is the relationship between the velocity of a wave source and the amplitude of a shock wave?

I'm having trouble understanding the kind of interference leading to shock wave formation and how it relates to the velocity of the wave source.I came across this image: It illustrates the formation ...
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In Bernoulli's equation, should the pressure variable be gauge pressure, atmospheric pressure, or absolute pressure?
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### How to explain imaginary kinematic viscosity of a vacuum?

According to the connection between the SchrĆ¶dinger equation and the Navier-Stokes vacuum has the imaginary kinematic viscosity $\frac{ih}{2m}$. How to explain it? For the formation of the viscosity ...
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### If I blow into a tube, how long does it take for air to exit the other end?

I have a tube open on both ends. I blow into one end. How long does it take before I can detect air coming out the other end and why?
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### Is there a drag in a moving fluid due to pressure (reaction) from the walls of the tube?

Consider a tube of linearly varying cross section... Now consider a section of the tube...There are pressure on the ends of this section due to the fluid... Now consider the walls, there is a ...
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### Flow in bend pipe

What happens to a flow in a bent pipe of uniform cross sectional area? Will there be any change in flow velocity at the outlet? If yes, why does it happen? What happens at the bent area of the pipe? ...
259 views

### Capillary rise and excess pressure

In a capillary rise energy must be provided to lift a column of liquid in a capillary above the original surface. Can you explain briefly and in molecular terms where this energy comes from?
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### Jet engine and thrust

Let's consider two identical jet engines operating at the same point. The first is attached on a plane's wing which is moving and the second is attached on the ground and remains still in order to be ...
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### Air Resistance Effects on a Potato, a Dirt Clod, and a Rock

Let me preface this by saying that I know little to nothing about physics, so I understand any personal attacks on my intelligence and gladly accept them so long as I learn something. Having said ...
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### How can i calculate the speed of a fan in a commercial jet engine?

How can i calculate the speed of a fan in a commercial jet engine if i already know the Ļ(Air density), Cp (Maximum output), A (Rotor swept area), V(wind speed) and Drag coefficient? and what other ...
2k views

### Why does surface of a liquid behave as a stretched membrane?

Surface tension is the special property of a liquid due to which the liquid tends to minimize its surface area. This property is due to a net inward imbalanced force due to the fact that no liquid ...
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### Outflow from a tank

Consider a tank with an outflow a the bottom: with the unsteady Bernoulli equation:$$\int_S \frac{\partial v}{\partial t} \ dS+\frac{v^2}{2}+\frac{p}{\rho}+gz=0$$where $s$ is the path between $S_1$ ...