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

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Are Normal Stress Differences in Newtonian Fluids Equal?

I was reading about normal stress differences in the context of Non-Newtonian fluids. There was a remark that the normal stress differences in Non-Newtonian fluids are not equal whereas the normal ...
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138 views

Why is there no upper-bound on power-efficiency of Hydro-turbines?

We have Betz's law, an upper bound on power efficiency, for wind-turbines. But there is no theoretical upper limit on hydro-turbines, Why this is so? Is it mainly due to the incompressibility of ...
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148 views

Physical interpretation of the change of diffusion term in navier stokes equations

In the Navier-Stokes Equations, there is one term accounting for convective flow and one term for diffusive flow. At high flow rates, the diffusive term becomes much smaller compared to convective ...
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138 views

Angle of rotation of an ellipsoid in a linear shear flow field

I am modeling the motion of an ellipsoid in a linear shear flow field. The ellipsoid is rotating about its shortest semi-principal axis which I have designated the $z$-axis in the body-fixed frame, ...
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45 views

Similarities between laminar-turbulence transition and others like BCS-BEC crossover, quark-hadron transition etc

From my limited readings on fluid dynamics, my understanding is that as the system changes from near-laminar flows to full turbulence, the dimensionless Reynolds number changes from $ R << 1$ to ...
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330 views

Compressible fluid flow through a branched pipe junctions

Backdrop: Designing a dust extraction system (LEV) with a branched junction. What principles allow for the calculation of volume flow rate and pressure at the inlets if the volume flow rate and ...
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57 views

What causes the 'tears' in yoghurt drinks?

Fluid dynamical instabilities are present in many different everyday things. The famous tears in wine for example are a classical example of a Marangoni effect, where surface tension gradients due to ...
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42 views

When rotating a liquid-filled jar what determines how much force goes to spinning the specimen floating inside?

Another way of stating it: When you have leaves in a (cylindrical) tea cup, if you rotate the cup (eg clockwise), the leaves largely stay in place relative to the table, but spin clockwise a little. ...
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52 views

Could you blow up a barrel (closed container) using a straw and water?

Imagine you fill up a barrel with water (or any other liquid) dig a small hole at the top and place a sufficiently large straw in it, so that there is no pressure loss or anything. Then you start to ...
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50 views

Upward bouyant forces?

When an object is submerged in a fluid (e.g., water), there is a pressure on the object due to weight of water by p=mgh, but why is there an upward buoyant force?
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34 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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Snowmaking in the tropics - an estimate of water evaporation

If I set up a snowmaker in the tropics and sprayed water with it how much water would I evaporate? How would I calculate?
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295 views

Is it possible to extract nitrogen from the air via a centrifuge

I was thinking about how they have cans of compressed nitrogen for filling paint cans, and also that they sometimes will blow nitrogen in to food containers before sealing them, and that got me ...
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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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2k views

Is it possible for wind to break the sound barrier?

I understand that in nature wind would never get high enough, but I am just curious as to whether physics would allow this to occur or not.
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65 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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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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61 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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121 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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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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439 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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If I bury a cylinder (top end sealed) in sand, how much force do I need to pull it out?

Let's say the cylinder has a diameter equal to its height of $25\;\mathrm{cm}$ the cylinder is sealed at the top and filled with sand the cylinder is also buried in the sand at sea at a depth of ...
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85 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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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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Why does a helium filled ballon move forward in a car when the car is accelerating?

I noticed that when I had a helium filled, latex ballon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the ballon went from the ...
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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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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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237 views

How fast would a truck have to go to pull a pedestrian onto the road? [closed]

Let's suppose a pedestrian P is walking or standing next to a highway. Suppose a truck T will drive by the said pedestrian at speed V leaving distance L between the two. Assuming L is a reasonably ...
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157 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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81 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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39 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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61 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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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 ...
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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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189 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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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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239 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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193 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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74 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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63 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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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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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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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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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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158 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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What cools a drink?

When you stick ice in a drink, AFAICT (the last physics I took was in high school) two things cool the drink:The ice, being cooler than the drink, gets heat transferred to it from the drink (Newton's ...
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123 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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Water under high pressure

If you were to sink a container to the bottom of a deep ocean and seal it there, then bring it up to the surface, would it retain its pressure? The answer for a gas is obviously yes, but what about ...
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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 ...