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Questions tagged [fluid-dynamics]

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

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Pressure Difference in a Rotating Fluid Coloumn

This question has intriqued me because two methods are generally employed to derive the shape of the surface(paraboloid) . Both methods give equal numerical answer but differ in which of them P or Q (...
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1answer
22 views

Is this air/water thrust/drag comparison correct?

On another list, someone asked regarding a blimp animal (hot-air balloon type object) propelling itself by using basically a bellows - and comparison to a nautilus (which bellows water to move around)....
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1answer
33 views

Why did this table tennis ball rise to the sky?

The nozzle makes the water flow through the upper part of the table tennis ball. If the angle and velocity of the water jet are well controlled, the table tennis ball will rise to the sky. The string ...
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3answers
75 views

Why can water be sprayed farther when the pipe outlet is narrowed?

Someone told me: This is because the flow rate remains unchanged, so the flow rate of small diameter pipelines is faster. I don't think this explanation is right: the diameter of the pipe outlet is ...
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21 views

Increased Stability of Vortex Rings by Pushing it

I've noticed in playing around with smoke rings that they are substantially more stable in their axial motion if pushed from behind by a hand, as can be seen in various Youtube videos. I was wondering ...
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33 views

Can Bernoulli Equation be applied across two fluids?

There is a beaker with two liquids placed above another. The beaker has a hole at bottom. So can I use Bernoulli equation across the interface of liquids.
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57 views

Why are golf balls the only (famous) object with dimples?

To my knowledge there isn't anything that uses dimples to reduce drag. It's weird to hear why dimples are useful and help the ball go farther while at the same time finding no other examples. I ...
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2answers
42 views

Increase Water flow through sealed tank

How can I increase the movement of water through a tank that is being supplied in the front and being drawn to the back of the tank with a pump? I am trying to dissipate heat and need more water ...
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2answers
86 views

Complex Potential describing inviscid fluid flow

I'm a Mathematics student, working through a homework sheet for a Fluid Mechanics module. The question is given: Consider the flow described by the complex potential $$w=4z+\frac{8}{z}.$$ ...
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47 views

Buoyant force when object is between two liquids

Suppose we have a ball floating in liquid A. Then we pour a liquid B on top of the ball (Assuming that the liquid is poured such that the ball doesn't oscillate) . The ball rises up. Why? According ...
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52 views

Time derivative of the continuity equation - accelerated flow

Given a fluid with accelerated motion $\vec a= d \vec v / d t$ (in one direction). The question is to write the continuity equation using the acceleration value. The continuity equation reads $$\frac{...
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2answers
63 views

How does flow work arises in enegy balance using Reynolds Transport theorem?

I will use the below mentioned form of Reynolds Transport theorem(usually derived in Fluid Mechanics context) to give a relation between a Control mass system(no mass in or out) and Control Volume(...
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31 views

How can i calculate the upwards force applied by a water jet on a suspended sphere [on hold]

First of all, i would like to possibly clear a few things up: yes, i do know the weight of the ball, but i would like to be able to calculate the upwards force of the water jet on the ball. (hence, i ...
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21 views

Rayleigh-Benard convection criterion

So, every text starts with a uniform heating of the bottom plate of a horizontal fluid for Rayleigh-Benard (RB) convection. Is "uniform heating" and "horizontal plate" an essential starting point? If ...
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1answer
33 views

How to solve this question based on bernoulli's principle? [on hold]

To Find : P1 (Initial Pressure) Can I solve this using bernoulli's equation ? I think that the given data is insufficient, I would also need one of the velocities to find P1. Is there any way to ...
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1answer
72 views

Where do the air bubbles inside the pool come from?

Here's a pic of my legs in the pool water. There are tiny air bubbles on them. I thought this is the result of air coming out of my leg, but I learnt that this is not the case. Also, there seem to be ...
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2answers
60 views

Bernoulli equation: how should one imagine $\frac{p}{\rho}$?

The Bernoulli equation states the following $$ \frac{1}{2}u^2 + \frac{p}{\rho}+ gz = \text{constant along a streamline} $$ where: $u$ is the fluid flow speed at a point on a streamline, $g$ is ...
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Did Usain Bolt really sustain 11 horsepower of exerted power during the 100 m sprint?

I've just come across a WP article which says: "The Jamaican sprinter Usain Bolt produced a maximum of 3.5 hp (2.6 kW) 0.89 seconds into his 9.58 second 100-metre (109.4 yd) dash" Out of curiosity, I ...
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How should energy loss in a hydraulic jump be calculated?

The energy loss in a hydraulic jump is still calculated with the old equation of Bresse from year 1860; (I.e., equation 7 in this paper from 2017) $$ \frac{\Delta E}{E_1} = \frac{(\sqrt{1+8Fr^2}-3)^3}...
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what's the difference between viscous flow and diffusion

For a one-componet gas flow in a channel. There are a pressure i.e density difference between two ends of the channel. There will be a mass flux. My question is how to describe this flux? it's a ...
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1answer
29 views

Derive properties of fluids using Monte Carlo method on brownian motion

Given a particle inside a fluid, it's known that its movement will be unpredictable due to the random collisions with the particles of the fluid. However, the distance from the origin of motion will ...
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2answers
47 views

Outflow velocity from a funnel over time for water without the simplifications made in Torricelli's law

I am trying to calculate the outflow velocity $v_2(t)$ from the orifice of a funnel for water. I am aware of Torricelli's law derived from the Bernoulli equation assuming $v_1(t)\approx 0$ resulting ...
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0answers
39 views

Why does Stirred (rotating) coffee alters cup’s resonance frequency? [duplicate]

I place a spoon in my cup of (dolce gusto lungo) coffee and tap the bottom of the cup with my spoon. It emits a certain (fixed) frequency. However, if I stir the coffee (but not so fast that the ...
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0answers
27 views

Variable mass system in fluid mechanics [closed]

A tank, initially at rest , is filled with water to a height H=4m. A small orifice is made at the bottom of the wall.Find the velocity attained by the tank when it becomes completely empty. (Assume ...
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Finding force exerted on a turbine blade by water flow

I am trying to find the maximum theoretical rotational power of a turbine from the following quantities: $\omega$, $r$, $\gamma$, $I$, and $t$. This is in an experimental setting, not an assignment,...
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1answer
19 views

Derivation of the expression of a propeller pitch distance

The pitch distance of a propeller is given by: $$p = 2\pi r \tan{\beta}$$ where $\beta$ is the pitch angle and $r$ is the radius. I cannot find how this expression was derived. Could you please ...
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23 views

Measure static pressure with measuring tubes of different diameter

Assume you have a horizontally placed cylindrical pipe with constant diameter. There is a flow of water, through this pipe. You want to measure the static pressure inside the tube, which should be the ...
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0answers
38 views

How to adjust power requirements for a propeller working against a pressure head [closed]

I'm in the process of building a hovercraft. In order to lift it, I'll need a motor directing the flow of air into a skirt. The specs for the motor I'm looking at say that it can turn a 15x5 dual-...
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0answers
31 views

Liquid behaviour under high pressure

If we put a liquid assume water in a cylinder (non isolated) and compress it up to 1000MPa. How can i calculate the temperature change?
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0answers
37 views

Cannot simplify expression with rotor and nabla with index notation [closed]

I need to handle simple operation which needs some skill in tensor algebra. I have to take $\mathrm{rot}$ from $ (\vec u \cdot\nabla)\vec u $. I am not very good at tensors operations, but I know ...
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0answers
7 views

Insert sensors in an aerostat with helium?

Hy is possible for you insert sensors (to calculate pressure and temperature) inside an aerostat with helium ? I ask this because... I have an helium balloon anchored to the ground by a rope with a ...
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0answers
30 views

Relation between electric charge and electric current distributions via velocity

Assume we have a charge $\rho(\vec{r},t)$ and current $\vec{\jmath}(\vec{r},t)$ distributions given in some region of space. From Maxwell's equations we know that the only relation between these two ...
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1answer
19 views

Potential source/sink flow. (Shifted in polar coordinates)

I was wondering how does the equation of velocity for potential source flow shifted from the origin to (R,Alpha)(this are the center coordinates) look like? Can anyone write a formula for this? I was ...
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73 views

Why does P = $\rho$gh seem to contradict equal accelerations of fluids in a closed U tube?

This is based on a problem. The closed U tube is as shown : I want to figure out time period of small oscillations if the liquid interface is slightly disturbed. My take : considering an ...
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1answer
44 views

How does atmospheric pressure affect dew point?

EDIT: I just want to understand why, intuitively, higher atmospheric pressure results in a higher dew point? (Rather than how to calculate dew point - I realise a clever person than I can probably get ...
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2answers
45 views

Hydrostatic Paradox - weighing issue

In the picture below, both tubes have the same pressure at the bottom. I understand that if they were weighed individually, they would show a different result since the water on the second tube is ...
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1answer
28 views

How to calculate the specific weight of air?

First of all sorry for my bad English ... I have to calculate the specific weight of air knowing that in the air there is actually a small percentage of water vapor. Therefore the specific weight of ...
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3answers
32 views

Feeling low pressure fluid with high velocity

In Bernoulli theorem physics we say that pressure is low when velocity is high but we feel pressure when fluid hits us why?
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2answers
50 views

Using fast flowing rapid to pump water

I am here in Zambia. I have a fast flowing river at a safari camp and I want to use this to pump water to a header tank some 30ft above the river. I am looking to create a tapered box with one end ...
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Does the “O” in this google doodle for Olga Ladyzhenskaya have anything to do with her work, or is it completely fanciful? [closed]

Google Doodle celebrates mathematician Olga Ladyzhenskaya She was famous for fluid dynamics and partial differential equations, both of which are beyond my pay grade. And she worked on the Navier-...
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Is that reasonable to use kinetic energy and potential energy of a fluid system to define the laminar, transition and turbulent region?

This is my assumption ( I don't know whether it exists or not ): I suppose a fluid system always contains two types of energy, i.e., kinetic energy and potential energy. The kinetic energy of a ...
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1answer
45 views

Energy balance of this machine is positive. What is wrong? [duplicate]

Machine on this picture is inspired by video: https://www.youtube.com/watch?v=rxFXsoqbfrk There must be error in the computation of energy balance or there is another negative energy. Can you help ...
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0answers
11 views

Scaling law or order estimation for turbulent viscosity in a rectangular cavity

Is there any recognized scaling law for the eddy viscosity inside a rectangular cavity? Indeed, that should mean some sort of homogenized value, since the quantity is spatially dependent. I have made ...
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0answers
19 views

Does the pressure caused by the force exerted on an ideal fluid in a Bernoulli tube decrease over an increasing distance? [closed]

Does the pressure caused by the force exerted on an ideal fluid in a Bernoulli tube decrease over an increasing distance? For example, someone pushes the water with a force from the left in this setup....
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0answers
18 views

Does Leidenfrost effect reduce weight

To a hot pan >100C some water is poured into due to lidenfrost effect it doesn't touch the pan so does the weight of the pan add to the pan in some way or like the vapour exerts force on the pan
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Will forcing a gas through an aluminum foam cool or heat it up

Metal foams are known to be porous and interconnected when in open cell form, will forcing a gas stream through the network heat it up due to restrictions and friction or cool it down due to large ...
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1answer
33 views

Why will fluid particles always move along streamlines in a steady flow?

It is stated at numerous places that in a steady flow, all particles move along the streamlines, i.e. streamlines and pathlines coincide. For example - WIkipedia states- In steady flow (when the ...
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Breaking the Strain Rate into normal and tangential components

I was looking through a set of notes on combustion and I found this definition of the strain rate at a point on a flame surface: $K_s = -\mathbf n \cdot \mathbf E \cdot \mathbf n $ where $ \mathbf n$...
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1answer
41 views

Drinking Straw physics [duplicate]

I was just wondering, when you put a straw in water and block the top, water doesn't get out from the bottom when you lift it up. What about alcohol like Vodka ( ethanol )? I am trying to make a ...
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2answers
55 views

How pressure and velocity is related? And its effect in cavitation

As per my knowledge I think when velocity increases and discharge is constant,the fluid(liquid) molecules get away from each other in the region where velocity is high and that's why they get ...