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

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Stop Motion 3D model of airflow over an airfoil

I am looking for a 3D stop motion image of air represented as molecules over a wing. Does anyone know where I can find an image that I can build a model from?
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2answers
185 views

Analogy between fluid dynamics and electromagnetism

I've been reading a set of notes (Chapter 13 of Caltech Ph 136 notes from 2004, by Blandford and Thorne) that draw an analogy between fluid dynamics and electromagnetism, identifying the magnetic ...
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1answer
73 views

How to calculate differential pressure

A ventilator fan mounted on the wall of an air tight room(except for an air intake with a given cross sectional area) is blowing air out of the room. If air displacement rate of the fan is given(CFM ...
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64 views

Kolmogorov/Energy spectrum for turbulent boundary layer

Previously, I have calculated energy spectrum for 3D isotropic turbulent flow data which is equally spaced in all three directions and then to compute the energy spectrum, one performs Fourier ...
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8 views

Poseuille number for flat channel?

For a flat channel with fully developed flow, the Poseuille number when using the hydraulic diameter is $$Po=C_fRe_{D_{H}}=24$$ But when using 2b as the length scale, the Poseuille number is ...
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1answer
73 views

Pressure in Fluids,in particular horizontal pressure

I am not sure about the pressure existing in fluids.If the vertical pressure acts due to gravity and the atmospheric pressure since it also changes due to height also acts due to gravity then why is ...
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3answers
128 views

How come water in a cup won't shoot out of a straw placed inside it?

I've been learning about pressure and fluid dynamics, and I've stumbled onto a problem. Say you have a straw in a cup. The surface area of the water in the cup is much greater than that of the straw. ...
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3answers
83 views

Helium and its unique behaviour

It is said that helium is lighter than air. What does that exactly mean ? If it is lighter than air,then there should be absolutely no trace of helium in earth's atmosphere(it should have flown into ...
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2answers
453 views

Why can, or can not, a perfectly incompressible fluid exist?

Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure. I wondered whether that is strictly true, or an approximation? Later I ...
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2answers
96 views

Can a solid object float on a frictionless liquid?

Assuming the solid is less dense than the liquid, will a solid object float on a frictionless liquid? I can imagine that due to the pressure gradient the object will float, but I can also imagine ...
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2answers
69 views

Microscopic source of pressure in an incompressible fluid

The pressure of a fluid can be explained microscopically in terms of molecules bouncing off the walls of a container. The molecules have a certain mass and speed, so when they bounce they transfer a ...
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4answers
251 views

Conserved quantities and total derivatives?

I am having a bit of a crisis in understanding of the physical meanings of total derivatives. When a quantity $\rho$ (be it a vector or a scalar) is said to be conserved, then (mathematically) ...
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1answer
168 views

Why aren't airplanes like golf balls?

Ok this is a silly question but here it goes Although it is good to have a laminar flow of the air around the object for low drag but the laminar flow is prone the phenomena called separation (sounds ...
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41 views

Has this boundary condition been used in fluid flow?

I would like to know whether anyone has seen a boundary condition used in a fluid flow problem, of the following type. Suppose viscous incompressible fluid is to the left of a plane $x_1=a$, so the ...
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37 views

Relationship between the fall time and projected area

I am investigating the relationship between the fall time of a paper tray and its projected area. In this investigation I have been using the following variables: Controlled Variable: The air ...
3
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1answer
64 views

What is meant by incompressible flow?

What is meant by incompressible flow? The density of the fluid is a constant, $\rho = constant$ The density of a fluid has a spatial dependence but remains constant in time, $\rho = ...
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3answers
132 views

Probability distribution in phase space and Liouville's theorem?

We can define a probability distribution over phase space (say 1D) $\rho(x,p)$ such that, for example, $$\langle x\rangle = \int x \rho(x,p) dxdp$$ etc. It can be shown here that such a distribution ...
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1answer
136 views

Sum of forces with liquid in rotation

It's not homework (I'm teacher). I would like to compute sum of forces on this study : The shape is symmetrical like that I'm sure the center of gravity is in the center of the shape. I compute ...
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1answer
42 views

Steady flow in fluid dynamics?

Steady flow is the condition in which the flow velocity profile does not vary with time. Mathematically this is translated to $\frac{\partial \mathbf{v}}{\partial t} = 0$, for example in the ...
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3answers
139 views

Can one pump water without providing an external source of energy? [closed]

I have a farm situated right beside a canal through which we use to get water for our crops. I am not financially sound to buy an electric motor. The water is 15ft below the ground level. So can ...
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1answer
89 views

Integrating the divergence of a velocity field (Fluid Mechanics)

I'm looking more for verification of my answer as I'm not 100% sure how valid my integration is here: If I have a 2-D incompressible fluid velocity field given by ...
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2answers
211 views

When pressure is exerted on parallel hydraulic pistons, do they start extending at the same time?

If there are two hydraulic cylinders connected in parallel, each with a different load (shown in the picture below), will they start extending at the same time? I'm having a disagreement with my ...
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2answers
2k views

Why does a nuclear explosion have directionality?

It just occurred to me that almost all images I've seen of the (in)famous mushroom cloud show a vertical column rising perpendicular to the ground and a horizontal planar ring parallel to the ground. ...
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34 views

What is the physical origin of acoustic modes in a duct?

Lets deal with wave propagation in a cylindrical duct. We ask the question: "what is the general form of a pressure wave which can propagate through the duct?" In answering this, we assume that the ...
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1answer
85 views

Curvilinear coordinate system around body of revolution

In Boundary-Layer Theory by Schlichting he gives the boundary-layer equations for a body of revolution according to the paper by Boltze$^1$. Unfortunately, this paper is in German. He apparently uses ...
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1answer
249 views

What is the difference between mechanical and thermodynamic pressure?

To start with I know thermodynamics deals with processes at equilibrium. Hence the thermodynamic pressure should most likely be the pressure of a fluid at equilibrium. I'm not sure if a fluid flow ...
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1answer
267 views

Does water turn solid under deep ocean because of high pressure?

I know that we can make water solid with high pressure, so I think water will be solid in the deep ocean? If that is true, the depth of the ocean would be limited because water will become ice? ...
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1answer
75 views

Center of mass of a cylinder from which liquid is flowing out

A cylindrical container is of mass M, radius R and height H. The cylinder is filled with a liquid of density D. The base of the cylinder has an outlet tap of radius r through which the liquid can flow ...
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105 views

What is the body force in the Navier Stokes Equations?

The incompressible Navier Stokes equations are: $\rho(\frac{\partial v_i}{\partial t} + v_j\frac{\partial v_i}{\partial x_j}) = -\frac{\partial p}{\partial x_i} + \mu\frac{\partial^2 u_i}{\partial ...
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1answer
65 views

Why does (potential) fluid flow bend around a solid surface in the flow?

Potential flow obeys Laplace's equation with certain boundary conditions (i.e. no fluid penetrates the solid body in flow, and far away from the body, the flow is uniform with a given velocity and ...
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1answer
60 views

woven appearance of water-streams by the side of the street

It's pretty rainy in Boston now, and there are streams of water by the sides of the street. Why do they have a 'woven' appearance? What's the formal name for this kind of fluid flow? With lots of ...
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1answer
107 views

Looking for the name of a physical phenomenon in fluids' mechanics

I have not even an idea about how I would search for that on Google, that is why I'm trying my chance here. As electrical engineer I have no clue about fluid mechanics. We all now that when water is ...
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2answers
95 views

How come a whistling kettle starts whistling only when water boils, and not long before - due to hot air escaping under pressure?

A whistling kettle will start to whistle when the water boils and turns into a jet of steam which then exits the small aperture in the spout. But why doesn't this happen much earlier - when the air ...
3
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2answers
161 views

How does throttling effect the upstream flow?

Let's assume we have a simple duct with a fan (compressor) situated somewhere along it. The fan runs at the same speed always. At the end of the duct, downstream of the fan, there is a throttle valve. ...
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160 views

If the MH370 black box did sink to 15000 ft, how long would it have taken?

I have been following MH370 on the news just as everyone and latest reports seem to indicate that the black-box could be found. A recent info-graphic http://t.co/lyBBE9C2hF shows the insurmountable ...
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1answer
58 views

Constant mass flux in fluid dynamics?

OK so I am having a conceptual crisis about Hydrodynamics. 1) Since mass should be conserved, is $\frac{dm}{dt}=0$ ? 2) But I know that the formula $\rho_1u_1A_1 = \rho_2u_2A_2$ exists... and ...
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1answer
54 views

Incompressiblity and speed of sound

I have read that usually if the speed of a fluid is much lower than the speed of sound (in that medium) then it can be treated as an incompressible fluid. Where does this condition come from? Is it ...
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2answers
135 views

Viscosity tensor in Navier-Stokes equation?

In my Hydrodynamics notes the viscosity term in the Navier-Stokes equation is of the form: $$ \nabla\cdot(\underline{\underline{h}}\cdot\nabla)\mathbf{u} $$ where $\underline{\underline{h}}$ is the ...
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112 views

Stress-energy-momentum tensor

In Wald's General Relativity, he writes on pg 61 For an observer with 4-velocity $v^a$, the component $T_{ab}v^a v^b$ is interpreted as the energy density, i.e. the mass-energy per unit volume, as ...
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92 views

Finding power with Drag Force equation

The mass of the car is 1500 kg. The shape of the body is such that its aerodynamic drag coefficient is $C_D=0.330$ and the frontal area is $2.50 m^2$. Assuming that the drag force is proportional to ...
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1answer
39 views

Evolution of minimization of surface tension

What are governing equations (or/and variational principles) for evolution of a simply connected body of water in vacuum? Initial state - for time $t=0$ we have a bounded simply connected set ...
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1answer
47 views

Modeling Water Surface Impact

I have a project that is to model the wave equation in a pond due to a pebble being thrown in. The project is quite simple as all we really have to model is the wave equation with an initial ...
3
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1answer
72 views

How can I see this equation describes advection?

My notes say that this is the equation for the "advection of a quantity A at speed v": $$ \frac{\partial \mathbf{A}}{\partial t} = \nabla\times(\mathbf{v}\times\mathbf{A}) .$$ Is this true? How can ...
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1answer
64 views

Pressure at the front and back of a fan

I have a fan which is inside a duct. When the fan is off, the pressure at the front and back of the blade remains the same. Now when it is running, the velocity of the air below will be more because ...
2
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1answer
75 views

Time taken in changing surface tension [closed]

I had read some where that the water in glasses has a high surface tension, but in lota (It is sort of a spherical container) the surface tension is lower. How can I measure the water surface ...
2
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1answer
94 views

Dyson air blade principle as a propulsion system for an autonomous underwater vehicle?

Any thoughts on applying this principle to an AUV (autonomous underwater vehicle) ? if the power source is a battery, would its efficiency be beneficial to that of a propulsion system?
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1answer
127 views

Vortex Air Intake

Why is a vortex air intake in an engine cause the fuel to burn more efficiently within a vehicle. Why is this more efficient.
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494 views

How much of the forces when entering water is related to surface tension?

When an object enters water with high velocity, (like in Why is jumping into water from high altitude fatal?), most of it's kinetic energy will be converted, eg to accelerate water, deform the object ...
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84 views

Eckert number in incompressible flow

Most of the time, temperature evolution for incompressible flow is computed assuming the Eckert number is negligible, that is the viscous dissipation and work of pressure variation is negligible ...
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43 views

Minimum amount of fluid to experience turbulence?

Turbulence is a challenge to model and simulate: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the ...