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

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Why do water waves with longer wavelengths travel faster?

When wind blows over the sea, it excites a range of wavelengths. Why do longer wavelengths travel faster?
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38 views

effect of opening the barrier between a tube with one side pressurized and the other side a vacuum

What would happen when a tube with an air space pressurized to 500 atm by the water exerted at 5 km below the surface was opened into a vacuum chamber rising up 5 km to the surface? The bottom of the ...
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15 views

Scale analysis for the vertical component of the Boussinesq equation to obtain a complete hydrostatic relation

This is actually an exam question which i have been trying to solve for my studies. Consider the Boussinesq vertical equation: \begin{align} \frac{\partial \widetilde{w}}{\partial t} = - ...
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40 views

Plane landing into a tunnel

Typical action movies cliché is landing a plane in a tunnel or even a flight in a tunnel. Let's drop any doubts about supreme heroic piloting skills of the characters etc. and focus on aerodynamics. ...
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23 views

Difference between droplet and sphere?

So I was reading a section in a book(Advanced transport phenomena, Gary Leal), that was analyzing the equation of motion of a sphere, that is pushed by the action of an applied force F, toward a solid ...
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18 views

Which paramteres in the Weber number belong to the object and which to the fluid?

The Weber number (We) "can be thought of as a measure of the relative importance of the fluid's inertia compared to its surface tension" (Wikipedia): $$\mathrm{We} = \frac{\rho v^2 l}{\sigma}$$ So ...
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78 views

If a hole is drilled at the bottom of a vessel, why is the pressure of the liquid leaving the vessel equal to atmospheric pressure?

My question is similar to this one : Should the liquid come out of the tank if a hole is drilled in the vertical wall? , but I still don't understand why the pressure within a free jet of ...
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14 views

How to quantify the spreadness of beam of air molecules as they escape from a capillary to a vacuum?

Consider a tube from an oxygen cylinder attached into a vacuum chamber. When the cylinder is opened, the molecules escape out into vacuum chamber. What makes the molecules spread out when they escape ...
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16 views

In a flowing fluid, why the particles are gathering together under critic distance and separating above this distance?

In a flowing fluid has an inner pressure which gives it some abilities. So this fluid is flowing from narrow gates, the speed of flow is increasing while the pressure of fluid is decreasing. If we ...
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38 views

Excess corn starch in a non-newtonian fluid

I'm currently performing a high school experiment regarding non-newtonian fluid. And as a further exploration, I am aiming to figure out whether excess corn starch in the solution will cause a ball to ...
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27 views

Fluid dynamics: valve and pressure drop [closed]

I have a question regarding valves: Let's assume a district heating system where you want to control the mass flow in a sub-station with a valve. Is (in normal cases) the diameter of the valve ...
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657 views

Liquid wave faster than sound in the above gas: possibility implications

I recently heard that tsunamis (meaning "harbour wave") can travel over 800 kilometres per hour (500 mph), not so far from the speed of sound in the air. May it happen, in general, that a wave in a ...
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61 views

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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49 views

Working Out Shaft Torque in a Stirred Tank with a Cylindrical Cavern Formation (using a non Newtonian fluid)

Here is some background to the problem (in a stirred tank): "With yield stress non-Newtonian (viscoplastic) fluids, it is possible to generate an agitated volume around the impeller, defined as a ...
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11 views

Why Flow meters are some times showing -ve fluctuating Values in Pressurized Pipe lines [closed]

In our Underground Water reservior we are pumping water by Hydro-pneumatic pumps to maintain same pressure till last connection. in order to measure the flow we have installed precise electromagnetic ...
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14 views

Finding the inner rotational velocity of a vortex incrementally

This is for improving accuracy in a prototype hydro design spreadsheet. Hypothetically: In a 6.6m tank, water enters tangentially at 0.575m^3/s with a velocity of 0.7m/s. The height of the resulting ...
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1answer
45 views

Mean free path in QFT

I'm trying to understand the hydrodynamic approximation of a general QFT when the large $k$ and $\omega$ DOF have been integrated out i.e that at highly enough temperature every non-trivial QFT ...
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23 views

How to predict the group velocity of the waves in the liquid?

The question appeard in the book. The phase velocity of the waves on the liquid surface is given by $$V_{ph}=\sqrt{\frac{2*\pi S}{\lambda \rho}} $$ where S is the surface tension and $\rho$ is the ...
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27 views

Backing pressure for a turbo-molecular pump

It is known that turbo-molecular pumps are to be operated at already low levels of pressure. So the turbo is always backed by another pump (usually a scroll, rotary pump) that ensures this. One should ...
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1answer
41 views

water stream cut off abruptly; why there will occur sprinkling?

Suppose that water flows with constant velocity $V$ and constant pressure $p$ through a pipe with diameter $d$. Now the pipe is suddenly cut such that the water will splash out of the pipe into the ...
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27 views

Pressure on a piston [closed]

There are 2 syringes X and Y connected to each other horizontally. If pressure in atmosphere (P) is 1x105 Nm-2 and the pressure we should apply on X to have the velocity when liquid come out is ...
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15 views

Equalization /regarding Double walled tanks

In the environmental world - petroleum is stored in "double walled" tanks. They claim the secondary tank holds 100% or greater than the contents of the tank. for example - 100% plus 10% of the ...
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1answer
76 views

Patterns in falling viscous fluids

It is a common observation that honey(or any other viscous fluid),tends to overlap/coil/wind up as it reaches the rigid surface. There is this little bend near the pile-up. Why is this so? How does a ...
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59 views

velocity of water exiting a 20 m hole at the bottom of water column 5000 m in height

What would be the velocity of water exiting a tube that has a height of 5000 m from a hole with a diameter of 20 m at the bottom, if the height of the water remains constant at 5000 m? Would it be ...
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36 views

Modelling an Airplane Question

For an assignment of mine I am creating a very basic model of an airplane, where I choose to consider a fixed amount of parameters that affect flight and to disregard others which I find too difficult ...
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99 views

Is it possible that viscous flow has a higher lift to drag ratio than inviscid flow?

I have a code for airflow around an airfoil. When I compare viscous and non-viscous flow at the same Mach number for a cruise, viscous flow has a higher lift to drag ratio than non-viscous flow. Is it ...
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49 views

Ball touching the side of a tube — DOUBT

I have a small doubt in understanding a physics concept. If a steel ball is dropped in a thin tube of glycerol, it will eventually reach its maximum or terminal velocity. However, if the steel ball ...
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66 views

Derivation of the relativistic equation of energy conservation for a perfect fluid

I'm currently attempting to struggle through the first chapter of Sean M. Carrol's spacetime and geometry. I'm a bit stuck, most likely because of not understanding the mathematical operation. ...
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26 views

What is the short time limit of Maxwell viscoelastic fluids?

The Maxwell model for viscoelastic fluids writes: $$ \tau\stackrel{\triangledown}{\sigma}+\sigma=2\eta D(v) $$ where $D(v) = \frac{1}{2}(\nabla v +\nabla v^T)$, $v$ velocity and $\sigma$ stress tensor ...
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64 views

Dispersion Relation for Convective Wave Equation

The convective wave equation governs the propagation of sound in the presence of mean flow: $$ \frac{D^2p'}{Dt^2}-\bar{c}^2\nabla^2p'=0 $$ or, expanded out: $$ \frac{\partial^2p'}{\partial ...
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20 views

Ratio of Stagnation to Normal Flow Properties in Incompressible Flows

I know in compressible flows the ratio of stagnation to normal flow variables is a function of the Mach number and usually the specific heat ratio. Does this ratio mean anything for incompressible ...
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41 views

Flow Rate in a tube with zero roughness [closed]

Assume the following setup. Given the Diameter of the pipe, difference in elevation (z1 - z2), pipe length, what would be the theoretical way of determining the flow rate (Q) within tube? Assume ...
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How do I compute mass flow rate for a compressible fluid?

Consider the vacuum chamber in the figure. If a hole with cross section area $A_{cs}$ is created, how can I approximate the maximum mass flow rate, $\dot{m}$, for a compressible fluid (e.g. air)?
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35 views

Noncoalescingِ drops on vibrating liquid surface [duplicate]

if you set a dish filled with soapy water onto a loudspeaker or other vibrator and make vibration, When it oscillates, it is possible to hold small droplets on its surface for a long time. I mean, ...
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22 views

Noise created due to change in Air Pressure

Background : My blower in one of the experiments is generating noise due the a short airflow path to atmosphere. The reason is due to a sudden change in pressure (high to low) the high noise is ...
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73 views

How to calculate velocity of air through a pipe? [closed]

I was wondering if there is any equation that helps to calculate air velocity through a pipe? I cannot find anything although I did find that velocity of air (not in a pipe) can be found by: Is there ...
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1answer
50 views

Noncoalescing and bouncing droplets

if you set a dish filled with water onto a loudspeaker or other vibrator and make vibration, When it oscillates, it is possible to hold small droplets on its surface for a long time. I mean, when you ...
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2answers
58 views

Why jets entrain mass?

This very beautiful picture shows a jet of fuel in the combustion chamber of a diesel engine. Does anydody could explain me what is the physical reason that the jet entrains air? Why jets and plumes ...
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22 views

Can cyclic acceleration/deceleration be worse for fuel consumption?

Recently I have started to drive my car slower on the highway in order to save money on gas. However, I feel like I am spending at least as much fuel as before. One possible explanation that I thought ...
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1answer
35 views

relationship between electrical resistance and Hagen Poiseuille's law

For the electrical resistance of a conductor, we have $$R = \rho \frac{l}{A}$$ Noting the structural similarity between the Hagen-Poiseuille law and Ohm's law, we can define a similar quantity for ...
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23 views

Bernoulli principle and viscous loss

Imagine I have a fluid flowing at a rate Q in a closed circuit with no viscous losses (Bernoulli's principle applies + total energy is equal to the kinetic energy). Now, I add a resistor with ...
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Does continuity equation hold if the flow is accerelated?

I'm Studying the streamline flow, specifically the continuity equation Bernoulli's Principle. Consider the following system where a liquid flows through the pipe of a uniform area of cross section ...
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60 views

In enthalpy, is PV the pressure energy?

In the case of a fluid, does the $PV$ contribution to the enthalpy correspond to the pressure energy in fluid mechanics? (Related: What is Pressure Energy?)
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Are eductors working on the Bernoulli principle?

This is a picture of an eductor. The wikipedia article claims that it works on the Bernoulli principle. The Bernoulli principle is famous due to the misconception about how aeroplanes fly and ...
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1answer
33 views

Why do we need the kinematic viscosity?

Kinematic viscosity is the well known dynamic viscosity divided by density. What difference does it make to divide by density? What is so useful about this?
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Heat system: Dynamic - quasi-stationary - static

I am an engineer in the field of district heating and in the future I will work together with people from computer science and physics. I have a question regarding dynamic/quasi-stationary and ...
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45 views

Toricelli's law of efflux in moving container

Suppose there is a big tank of water with a small hole near the bottom. Now if this tank starts moving with say a velocity 'v', then will Toricelli's law hold? I tried directly applying Bernoulli's ...
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Pressure in a fluid

If a fluid is flowing along a vertical line with a constant velocity, will the pressure at every point be the same and irrespective of height?
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Question about propellers

A ship moves forwards because a rotating propeller sends water backwards. The diameter of the propeller is as large as possible because in order to be efficient it must push the the greatest feasible ...
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Symmetry Arguments: Flow Through Cylinder

Why can for symmetry reasons a steady, viscous, incompressible flow, obaying the N.S equation: $$\rho(v \nabla)v = -\nabla p + \eta \Delta v $$ That flows through a cylindrical(very long) pipe not ...