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

learn more… | top users | synonyms (1)

3
votes
1answer
2k 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, $\rho = \rho(\mathbf{r})$...
2
votes
3answers
364 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 ...
5
votes
1answer
282 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 ...
2
votes
2answers
1k 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 ...
4
votes
3answers
270 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 ...
1
vote
2answers
400 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 $\vec{U}(x,y,t)=u(x,t)\vec{x}+v(y,t)\...
1
vote
5answers
3k 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 ...
23
votes
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. ...
1
vote
1answer
163 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 ...
5
votes
3answers
462 views

Why do tropical cyclones not tear themselves apart?

A tropical cyclone is the generic term for a hurricane, typhoon, or tropical storm. Tropical cyclones derive their energy from evaporation of water at the ocean surface which ultimately recondenses ...
2
votes
1answer
246 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 ...
3
votes
1answer
3k 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 (...
7
votes
1answer
4k 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? ...
1
vote
1answer
484 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 ...
0
votes
3answers
1k 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 ...
2
votes
1answer
274 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 ...
4
votes
1answer
69 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 ...
-1
votes
1answer
140 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 ...
6
votes
2answers
607 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
votes
2answers
2k 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. ...
2
votes
2answers
197 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 ...
1
vote
1answer
144 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 doesn'...
2
votes
1answer
76 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 ...
2
votes
2answers
343 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 ...
3
votes
2answers
239 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 ...
1
vote
2answers
2k 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 $...
2
votes
1answer
55 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 $\Omega$...
1
vote
1answer
142 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
votes
1answer
106 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 ...
1
vote
1answer
167 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
votes
1answer
171 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
votes
1answer
255 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?
2
votes
1answer
680 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.
10
votes
2answers
2k 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 ...
1
vote
1answer
115 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 ...
0
votes
1answer
330 views

Measuring cross-section area of a flexible tube (5mm dia approx)

I want to measure the avg. inner cross-section area of a flexible tube of outer diameter 5mm. Since the cross-section isn't a circle exactly, use of vernier caliper to measure inner diameter fails. ...
2
votes
1answer
34 views

Consequences of tensor form due to various symmetries

I'm reading a paper on 2D hydrodynamics, specifically on the drag of a rod in a 2D fluid. It is a low-Reynolds number regime, therefore linear hydrodynamics and the velocity of the rod can be ...
2
votes
1answer
307 views

Darcy Flow in porous material - consider porosity in cross-section area?

According to Darcy's Law, the volumetric flow rate Q of a fluid occuring due to a pressure difference $\Delta p$ over a distance L and through a cross-sectional area A of a porous medium (volume V) ...
2
votes
2answers
126 views

Non-unique pressure for Navier-Stokes for incompressible fluid

The Navier-Stokes system for incompressible fluid says $$ \begin{aligned} \dot{\mathbf{u}} + (\mathbf{u}\cdot\nabla)\mathbf{u}-\nu\Delta\mathbf{u}+\nabla{p} &= 0,\\ \nabla\cdot\mathbf{u}&=0. \...
0
votes
0answers
53 views

Propeller Force & Momentum

An air stream is created by a propeller at velocity $\nu = 93.6\frac{m}{s}$ To calculate force applied by the propeller I used this formula: $$F=\frac{1}{2}\rho\nu^2A$$ where: $\rho = 1.29 kg/m^3$...
0
votes
1answer
149 views

Streamlines for steady flow problem

I need to calculate the streamlines for the steady flow given by the velocity field $$\mathbf{u}=(u,v,w) = \left( 0,\frac{y}{y^2 +z^2} , \frac{z}{y^2 +z^2}\right)$$ Now I've been able to notice that $...
2
votes
2answers
435 views

Which temperature to evaluate fluid properties in pipe?

I am always confused about which temperature to evaluate fluid properties at. Let's say I have a helical pipe and I know the inlet temperature, outlet temperature, and surface temperature and the ...
1
vote
1answer
71 views

Speed of liquid being blocked at end of pipe

How fast would water go if at the end of of a 1 inch diameter pipe was closed by a valve? The system is as follows: 5 meter high source of water that feeds a 1 in pipe. The pipe goes straight down ...
4
votes
1answer
257 views

Water falling, why does it spin?

I have noticed that water, when falling will rotate. Looking closely at a thin stream from a faucet and placing a flat object mid stream you will see the water is rotating. The further down the stream ...
5
votes
3answers
441 views

Home experiment question

I want to make a home experiment where I roughly explain the phenomenon of water remaining in a straw if you close one end of the straw. So I'm thinking that the weight of the water is pulling it ...
3
votes
2answers
255 views

What causes drag in a fluid?

What causes resistance of an object to motion within a fluid like water? Please explain to me the molecular dynamics of the situation.
5
votes
2answers
144 views

Does space have a “radiation pressure” caused by subatomic particles?

If space is not actually a void, then there's something shooting around in it, I'm guessing a lot like molecules in a gas. Could we apply a $(PV/T = k)$ -like gas law to it. Obviously we're not ...
1
vote
1answer
666 views

Calculating water flow as function of the height of water above the inlet pipe [closed]

The Source of Water is Pond where the water level do change randomly. A pipeline of 200mm pipe diameter run for around 2500 m. The Height difference between the level of the inlet pipe and the level ...
2
votes
4answers
511 views

In the formula for the mass of a drop of water on the end of a tube, is r internal or external radius?

In the formula for the mass $m$ of a drop of water forming on the end of a vertical capillary tube, $$m = \frac{2\pi r \gamma}{g},$$ does $r$ refer to the internal or external radius of the ...
0
votes
0answers
62 views

What's the shocks mach number in this specific problem?

The shock wave I'm interested in occurs for a special Riemann problem, known as 1D Noh's problem. It means basically that two streams of air (equal density) are colliding at $x = 0$ for $t = 0$; I ...