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Questions tagged [flow]

The motion of fluids (gases, liquids and granular material).

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15 views

Is it possible to equate the radius of a hole using poiseuille's law?

I want to calculate the radius of a hole from the leak rate. I have come across Poiseuille's law which includes the radius of pipe/hole. Is it possible to equate the radius of the hole from the volume ...
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Is Coefficient of viscosity frame dependent? Why/Why not?

Is Coefficient of viscosity frame dependent? Why/Why not?
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1answer
54 views

What are good textbooks or documents to learn numerical simulation about open channel flow like river flow?

I'm trying to learn about numerical simulation about river flow. The problem is I am a CFD beginner (just studied about hydraulics). So I don't know history or important topics about numerical ...
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2answers
74 views

Why is the stress tensor simplifying like this

\begin{eqnarray} \nabla \cdot \boldsymbol \tau &=& 2 \mu \nabla \cdot \boldsymbol \varepsilon\\ &=& \mu \nabla \cdot \left( \nabla\mathbf{u} + (\nabla\mathbf{u}) ^\mathrm{T} \right)\\ &...
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11 views

Maximum velocity of isentropic compression

In the isentropic compression tables, if P/Po = 0.9999, then A/A* = 57.8738. This is from the isentropic compression table found here: http://www.cchem.berkeley.edu/cbe150a/isentropic_flow.pdf Am I ...
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Finding the flow rate of a gaseous oxidizer flowing through a solid fuel (and they're combusting!)

I'm wondering, is there a way to calculate the volumetric flow rate or mass flow rate of oxygen flowing through a solid fuel (ex. paraffin wax) and combusting with it? After the combustion, the flow ...
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5answers
3k views

Water Bottle Rocket Thrust - two calculation methods not matching

I have been launching water bottle rockets with my kids and we are in the process of creating a simulation of the launch using numerical methods. I am a mechanical engineer, but fluid dynamics is not ...
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1answer
34 views

Air velocity from a 5mm pipe

At first, I will try to describe my problem. I have a compressed air at 0.4 MPa (4 bars) that goes trough a control valve and a flow meter into two 5 mm pipes. Flow meter shows the value of 200 ...
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1answer
25 views

How can the characteristic length of flows also be a measure of a fluid element?

This is how Wikipedia derives Reynolds number: $${\displaystyle {\begin{aligned}\mathrm {Re} &={\frac {\text{inertial forces}}{\text{viscous forces}}}={\frac {ma}{\tau A}}={\frac {\rho V\cdot {\...
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2answers
38 views

How are velocities defined in the usual form of the continuity equation?

Often in Physics classes we solve problems involving fluid flows in pipes. Usually they are solved by a combination of Bernoulli's law and continuity law, which states that the same volume of fluid ...
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1answer
30 views

Using height to calculate Pressure (independent variable) and Flow rate (dependent variable)?

The investigation is experimentally verifying Poiseuille's equation. The independent variable is the changing pressure and the dependent variable is flow rate. My experiment plan is having a simple ...
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3answers
84 views

Bernoulli's law for turbulent flow

Is Bernoulli's law valid for turbulent flow for a non-viscous, non-compressible, non-rotating fluid with isoentropic flow? An ideal fluid has coeffieicent of viscosity=0,which implies that it's ...
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33 views

What is the theoretical flow rate of water through a tube leaving a container?

The question comes from this image: There is a container with $h$ amount of water above a hole of diameter $r$. This hole leads to a tube that the water exits through. Using the pressure on both ...
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1answer
31 views

Hagen-Poiseuille equation

The Hagen-Poiseuille equation states that $$\Delta p = \frac{8\mu LQ}{\pi R^4}$$ where $\mu$ is the dynamic viscosity of the fluid. I'm just wondering, can the dynamic viscosity be replaced by ...
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44 views

Derivation of Stokes Formula using stream function

I need help with derivation of Stokes Formula. I have found derivation on Wikipedia (https://en.wikipedia.org/wiki/Stokes%27_law), but I don’t know how can I get from equation (1) to equation (2). How ...
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1answer
44 views

Why is the pressure difference the same for B and C?

So this is the question from brilliant.org: Which pipe (or pipes) carries water with the highest speed? The answer given was A. I picked C, the explanation wasn't clear to me. It says: The ...
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1answer
57 views

What happens during cavitation?

Cavitation is vapor bubbles and when they implode creates a shock wave, before imploding the bubble can reach temperatures of 10000k. I want to know what conditions must be met to make cavitation, ...
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1answer
24 views

What is the reason for the presence of a developing region in a boundary layer?

Why does a fluid first flow as shown in the entrance region before flowing as shown in the developed region ? In other words, why wouldn't it flow in developed region from the beginning ?
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Continuously cooling air at the entry to a vertical tube to get a denser air flow at exit

If you have a well insulated vertically aligned pipe open at the top and bottom to ambient air, and then continuously cool the air at the top entry cross-sectional diameter to create a free-falling ...
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2answers
251 views

Why do proofs of Bernoulli's equation assume that forces on opposite ends point in different directions?

I've read 4 different books and yet nobody explains why forces $F_1$ ($=p_1A_1$) and $F_2$ ($=p_2A_2$) point in different directions. Shouldn't $F_2$ point in the same direction as $v_2$? Since we'...
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1answer
82 views

Why does the “air” close or open doors?

Assume we have an empty room with two doors on the opposite sides of it. If we open one of the doors, the door on the opposite side automatically closes "harder", and if we close one of the doors the ...
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37 views

Why does the wind close doors?

Assume you have two rooms, both with holes in the wall (more commonly known as open windows), which connect each room to the exterior. The two rooms are connected, but the connection can be closed by ...
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1answer
39 views

Is there a Coandă effect outside the atmosphere?

For example, when air is blown through a spoon in the atmosphere, the air flow bends toward the spoon. People say it's the Coandă effect. So, outside the atmosphere, when we let air blow through the ...
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2answers
37 views

How to avoid water tension in a hose?

I am doing a project in which I have to flow water between two containers, which are connected at the bottom by a 1/4" hose. Remarkably, the surface water tension is such that water does not flow ...
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1answer
43 views

(SOLVED) Calculate the velocity of a gas inputted in pressure difference

I'm trying to make a generally simple calculation but I'm not sure if I'm approaching this correctly. So as I'm making assumptions solving this problem, please comment or correct me wherever I'm wrong ...
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34 views

How can you calculate the total time taken for water to completely flow out of a container given the amount of water initially in the container?

I'm doing an experiment on changing the amount of water in the container and seeing how the time is taken changes with it. The container has a little hole at the bottom where the water flows out from. ...
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3answers
115 views

Can the flow be irrotational if the viscous forces act on fluid?

I tried to answer the question only using the definitions and the Navier-Stokes equation: $$\rho \frac{Dv}{Dt} = -\nabla P +\rho g -\mu[\nabla \times(\nabla \times v)] $$ In my opinion if the ...
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1answer
43 views

Flow in parallel vertical pipes

Consider a tank open to atmospheric pressure, with a large cross-section area A1 and height H, that contains an incompressible fluid. Two vertical pipes with length H and cross-section area A2 are ...
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2answers
36 views

Does density vary with time in a steady flow?

Steady flow is defined as the type of flow where density, pressure and velocity don't change with time, as far as my understanding goes. However, in the video below, they say that the density in a ...
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1answer
29 views

Physical meaning of the convection term in the momentum equation of acoustic wave

In deriving the acoustic wave equation, the momentum equation is used. $$\frac{\partial \mathbf{u}}{\partial t}+ (\mathbf{u}\nabla)\mathbf{u}=-\frac{1}{\rho} \nabla p$$ Intuitively, the convection ...
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3answers
179 views

Conversion between Power and Flow rate of Fan taking into consideration the geometry of the fan

I need to find the relation between two properties of a fan, that is between power and flow-rate. I expect such a formula to depend on the pressure and on the geometry of the fan. I have found in ...
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1answer
72 views

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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1answer
65 views

Specific total enthalpy VS Specific enthalpy

What is the difference between specific enthalpy and specific total enthalpy in the context of fluid flow?
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35 views

Does laminar flow of a mixture tend to result in mass separation?

In a laminar flow regime, a fluid separates into streamlines with a parabolic flow profile. Do particles within such a flow tend to become aligned by mass (e.g. lighter molecules travelling together ...
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28 views

Most effecient negative shape to create laminar flow

I was looking at laminar flow filters to aid in my mycology work I'm doing. Most filters use an aluminum extrusion with fins. I figured the options out there are pretty expensive for what you get and ...
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50 views

Wich is the correct flow rate $Q$ solved with iterations?

I have a little question about this problem I computed the right answer this way First I applied Bernoulli between the top and the exit $$\frac{P_{t}}{\rho g}+z_{t}+\frac{v_{t}^2}{2g}-H_{1}-H_2-H_{\...
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17 views

Flow velocity ambiguity in transition region

I have calculated the following short table concerning the stationary flow of water in a 1 meter long pipe with a diameter of 16 mm and a completely smooth inner surface. The flow is driven by the ...
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0answers
15 views

$Gm/r$ in momentum equation of compressible flow for cylindrical geometry

In the case of compressible flow, due to the gravitational field, the source term added in the momentum equation (which is gravitational potential) is $Gm/r$ in the case of cylindrical geometry and $...
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1answer
28 views

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? ...
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3answers
4k views

What is the shape of the upper boundary of water hitting a screen?

When I saw that thing I did not understand how that shape is formed? To be ideal, take a vertical smooth plane. Now aim at the wall with a thin water tube. Then the outer layer forms a "parabolic ...
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2answers
63 views

What is irrotational flow? How to judge?

For example, when the wing moves horizontally, the direction of fluid flow changes first to upward at the leading edge of the wing and then to downward at the trailing edge. Does it rotate? If the ...
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0answers
31 views

calculate CFM of a ceiling fan

I'm trying to calculate, or even understand what data do i need to calculate the CFM of a ceiling fan. in many countries this is a given data in the product details but not where I'm from. I have the ...
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17 views

Acoustics air flow vs the human body

My question to you all is... Is it possible to move air acoustically. ie to move air over a 2.1m distance to be felt by the human body with a cooling effect but not heard by the human ear ? I'm just ...
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0answers
65 views

Problem estimating the flow rate in natural draft chimney using Bernoulli equation including heat and friction losses

I am trying to build a simple model for fluid flow and heat transfer in a natural draft chimney. I've found some literature about it (most of them giving the same result as the equations on the ...
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1answer
49 views

Why does blood pressure drop when blood vessels rupture?

Some people say that because the flow is large, the pressure is low. Is that right? Why does the pressure drop when the flow rate is high?
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1answer
24 views

How to measure flow rate using Toricelli's law?

For my experiment, I want to investigate the relation between height and velocity using Torricelli's law. I would like to know how I would measure the flow rate from volume in a spouting cylinder.
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62 views

Viscous losses in loop helical flow

Imagine flow of the following kind. The flow line for each flow particle is a helix with its axis bent so that the beginning attaches to the end. How can I determine the viscous losses in this flow ...
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3answers
107 views

Do drones fly because of low pressure above them or high pressure below them?

I was working on some drone battery life solutions and was stuck with the question whether drones fly because of low pressure created above them by the rotor or due to high pressure created below them....
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47 views

A doubt on this paper regarding Ricci flow on space-time

https://arxiv.org/abs/1812.06239 In this paper,the authors use ricci flow to construct Lifshitz spaces. But it is known that ricci flow is limited by Riemannian manifold, which has a positive metric....
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Macroscopic hydrodynamics, is there a fluid “vacuum” force?

Background I was looking at the following simulated fish movement article and how the physics could be improved (ie, made more realistic) with out a huge impact on performance. It looks like they ...