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

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What amount of force could be created by a person “swimming” in air?

Had a question on a physics test, a person is at rest on a frictionless surface and can not throw anything, can they move? I said yes, by "swimming" against the air and got docked because "it wouldn'...
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40 views

When does $ S\cdot v=\mathrm{cost}$ hold?

In fluids dynamics what are the limitations for the use of the volume flow rate conservation law? $$ S\cdot v=\mathrm{cost}$$ Does it still hold when there are external forces acting on the system, e....
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1answer
56 views

What is the direction of buoyancy in the bulk of liquid on earth?

I have a trouble when considering the direction of buoyancy in the bulk of liquid subjected to vertical temperature gradient. The liquid is heated from below or above that induces a natural convection ...
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93 views

Why do I spill lesser water if it's hotter?

If I pour water in a glass to make a cup of tea, I noticed that if the water that comes out of the kettle is very hot, almost no water is spilled. If the water is cold though, much more water is ...
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What about negative Pressure?

Here is something I see : Let's say the ideal fluid(water here) of density $\rho$ is drawn from a source by a motor and thrown upwards with a velocity $v$. Now we take the power of motor be ...
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456 views

An equivalent for the Bernoulli equation for viscous liquids

I was wondering whether there is any equation for viscous liquids (probably derived from energy and mass conservation principles) relating the pressure, rate of volume flow, area of cross section and ...
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1answer
31 views

Inertial Waves - Why neglecting the advecting term?

I'm trying to derive the dispersion relation for Inertial waves. In Cartesian coordinates: Inviscid and incompressible fluid is rotating uniformly with Angular Velocity: $\Omega = (0, 0, \Omega)$ ...
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45 views

The convective term in Reynolds Transport theorem

Reynolds Transport theorem states that $$\frac{D B_{sys}}{Dt}=\frac{\partial B_{CV}}{\partial t}-\dot{B_{in}}+\dot{B_{out}}$$ where $B_{in,t+\Delta t}=b_1m_{1,t+\Delta t}=b_1\rho_{1}\forall_{in,t+\...
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How does smoke move in the air and how can I direct it so it will go to a place I want it to go?

Let's assume a close room with 1-2 people who only one of them smoking cigarette. What is the equation describe the smoking spreading? is it diffusion? what are the parameters is so? Is there a way/...
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173 views

Nomenclature clarification concerning solitons

My experience with solitons is restricted to the classical setting, namely solutions to the quartic interaction $\phi^4$, the Sine-Gordon equation, and Korteweg–de Vries equations. I was explicit to ...
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1answer
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Best way to move teaspoon to dissolve sugar in a cup of tea

I know that there is asked questions about something like cooling down cope of coffee..etc, but this one is little different. My friend was watching TV program of English eating etiquette, and they ...
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Strain Flow in Fluid Mechanics

I am working on hydrodynamic instabilities and was looking for non axisymmetric vortex models that are not Stuart or Taylor green vortices. What I came across was multipolar vortices where a uniform ...
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When beer tapping, why does only one of the beers foam up?

In the paper Physics of beer tapping the author go through an explanation of the phenomenon of beer tapping, i.e. the foaming up of beer in glass bottle after it has been hit at the top of the bottle ...
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1answer
17 views

Lengthening in-flight evaporation time of water droplet in air

How one can increase the in-flight evaporation time of sprayed water droplets in air without changing the thermodynamic conditions of the ambient air ( i.e. RH, T ). For instance mixing water with ...
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1answer
50 views

What happens if a cylinder filled with propane gas is connected to an empty one of the same size

Lets say I have 2 100lb propane cylinders. One of the cylinders is filled to its maximum capacity while the other is empty. If I connect them to each other via a high pressure hose in an upright ...
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Can a droplet of water bounce back when it hits a water surface?

Is it possible that a droplet of water can bounce from the surface of a volume of water?
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43 views

Flow rate calculation of small nozzles in a linear system

I am currently working on a system for feeding multiple very small reactors, of which a prototype is described in the figure below (for scale, the distance between each nozzle is 9mm). The liquid ...
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1answer
73 views

Why is turbulence caused?

In high Reynolds numbers we have turbulent flow. This is because the inertial forces are much greater than the viscous forces. I understand inertial forces to be actually the fictional forces due to ...
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2answers
69 views

Difference between inviscid and viscous flow

In my lecture notes, I have a load of examples and I want to sort out which egs are viscous flow and which are inviscid flow. It is not always said if the flow is viscous or inviscid. Please can ...
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333 views

Computing the force generated by a sail on a boat

This is a follow up question to the question I asked yesterday: Problem understanding basic sail mechanics I am focusing purely on the Newtonian aspect of motion and I am computing this as follows:...
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1answer
34 views

Viscous fluid boundary condition

Consider an incompressible viscous fluid of kinematic viscosity $ν$ , dynamic viscosity $µ$ and density $ρ$ . A viscous boundary layer is located over a solid surface at $y = 0$ and $x > 0$. The flow ...
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1answer
85 views

Bernoullis in Parallel pipes

Im having trouble with a scenario where a flow 𝑄𝑆𝑡𝑎𝑟𝑡 splits into two parallel pipes 𝑄𝐴 and 𝑄𝐵 then rejoins 𝑄𝐸𝑛𝑑 before exiting the control volume what makes this scenario difficult is ...
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867 views

Will this type of engine produce thrust?

I was wondering that if we replace the combustion chambers in a jet engine with heaters heating the compressed air up to temperature near to plasma then what will be the case, will it produce any kind ...
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1answer
52 views

Newton and Stokes regime in settling velocity

Can you tell me please the differences between Newtonian and Stokes regime in settling velocities of particles in moving fluids? I know that Newtonian applies to higher Reynolds numbers. Does that ...
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6answers
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Why is a beam reach the fastest point of sail on modern sailboats?

I've heard that a beam reach (perpendicular to the wind) is the fastest point of sail on modern sailboats, but I haven't heard a satisfying explanation of the physics behind the claim. Triangular ...
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39 views

Finding boundary condition of stationary solid body

A fluid flows past a stationary solid body of arbitrary shape. Write down the boundary condition on the fluid velocity $\textbf u$ for an inviscid fluid and for a viscous fluid, at the solid surface. We ...
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Pressure drop through a flat vs. pleated filter

Consider two identical pipes where two identical air flows take place. The pipes are "obstructed" by two filters made of the same fabric, one flat, the other pleated as shown in the following picture. ...
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Time Scales Of Processes In Molecular Dynamics

Suppose I run a molecular dynamics simulation of a fluid sandwiched between solid walls which are periodic in the lateral directions and finite in the direction of the fluid film thickness. Now, I ...
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67 views

Why is it easier to remove stains with water?

And I don't mean stain removers, but literally tap water. See here's the thing, you can't remove a stain without some sort of friction, and water reduces friction. Yet if I say, spill some coffee and ...
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Water Jet scenario [duplicate]

So I have a question I came across that goes along the lines of Water is discharged from a 20 mm diameter nozzle to the atmosphere. The water jet leaves the nozzle with a velocity of 12 m/s and is ...
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76 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 t^2}+2\...
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0answers
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Physical Meaning of Over- / Underdetermined Acoustic Eigenvalue Problem

I am performing an eigenmode study on a system of acoustic ducts. The system consists of two large cylindrical volumes connected by several smaller cylindrical volumes (modeling a combustion chamber ...
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95 views

Rotational and irrotational flow

Can a rotational flow take place if there is no viscosity? If yes then will it always be rotational or will it become irrotational after some time?
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How do ceiling fans cool or heat?

Why does a ceiling fan blowing air downward cool in the summertime: whereas a ceiling fan sucking air upwards heats in the wintertime: ? Why isn't it the other way around?
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Three-dimensional centrifugal instability and the Rayleigh critertion

Lord Rayliegh postulated that for a two-dimensional symmetric swirling flow, a three-dimensional instability may occur if the angular momentum magnitude decreases in the radial direction with respect ...
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2answers
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How relevant are Reynolds number and flow regime in very small tubing at very low flow rates?

I know that in bigger pipes, Reynolds number and the dominant flow regime can be important in further decisions. Now, I have an application with tubing of about 0.5 mm in diameter and a flow rate of ...
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1answer
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Is a venturi pump feasible at very low pressure?

I need assistance (or experience feedback) in estimating if a Venturi pump would work at low pressure. A boiler under vacuum (10% atmosphere) will produce steam (at a given speed depending on the ...
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Contact Angle In Capillary

One Professor recently told me that in an insufficient length capillary, the contact angle doesn't change, its an inherent property which does not change. If the edges of capillary are extremely sharp ...
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1answer
58 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 ...
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1answer
44 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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2answers
56 views

How can pressure and velocity fluctuations in acoustic plane waves be in phase and still hold to the B.C. that velocity must be zero at a solid wall?

In linear plane-wave acoustics (no mean flow, small perturbations, etc.), it is often derived that the phase of a traveling pressure fluctuation wave and a fluctuation velocity wave are the same. For ...
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1answer
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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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1answer
23 views

How does gas friction emerge?

The fluid parcel model states that a gas can be modelled as the sum of infinitesimal small parcels. According to this model, the friction of a gas with a solid surface (e.g. the wall of a pipe) ...
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2answers
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Why does a container with fluid make different sounds at different fluid levels?

Have you ever noticed that when you are filling a container with fluid. As it approaches the top, it makes a different sound? You can tell by listening when your about to reach the top. Why is this?
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Water Moving Through Coffee Grinds

Like many of my peers, I rely on coffee as if it were prescribed to me. It might even be classified as an addiction if it weren't for the cultural norms surrounding a cup of coffee or tea in the ...
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0answers
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Why would oscillation of a gas column inhibit rather than promote mixing?

Many years ago I helped to support an experiment conducted in Japan which investigated the effects of high frequency oscillation ventilation (HFOV) on the mixing and distribution of gas into the lungs....
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1k views
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1answer
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Effect of wall friction on stagnation pressure and temperature

We have incompressible air flowing through a cylindrical pipe, from section A to section B. The process is considered to be stationary. My theory book says that, because of the frictional losses on ...
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503 views

Non-linear waves and shock formation

In the cases of non-linear acoustics, why is shock formation unlikely when the dispersion is strong when compared to the non-linearity of the wave?
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Can drag force and lift force be in the same direction in the following case?

In the attached picture, can I say that there is a lift force in the Y-direction, and a drag force too in the same Y-direction? $F_L$ proportional to $V_fx^2$ ? $F_D$ proportional to $V_py^2$ ? Is ...