The motion of fluids (gases, liquids and granular material).

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Excess pressure for a surface

Calculate the excess pressure for a surface located to plans a) $e_{1}$ b) $e_{2}$ for the flow specified by : $v_{1} =-c(x_{2} + x _{1})$ , $v_{2} = c(x_{2} - x_{1})$ , $v_{3} = 0$ where $c = 0,5 \...
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112 views

If I was to poke two holes in a water filled balloon would it lose water twice as fast a a balloon with one hole?

if I was to assume that the only force behind water coming out of the holes is the pressure i.e., the height of the water remaining which is constantly changing (declining) as water leaves the balloon,...
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3answers
63 views

Flame shape and size(length) depending on gravity

How would the shape and size of a flame(from a simple candle for example) depends on the gravity? Suppose all the relevant information is known, like candle dimensions and its chemical composition, ...
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1answer
59 views

A fan is connected to a pipe, what happens to the airflow after the pressure drop?

Overview I am trying to find out what happens to the air flow within a pipe if a fan cannot generate the required pressure to overcome the friction within the pipe. After much research I am aware ...
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1answer
167 views

Equation of a water pump with a nozzle

If I have a small water pump with a flow f (L/s) and maximal pressure p (bar) and attach its output to a small tube having on the other end a nozzle with an area A (m^2). What is the equation that ...
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4answers
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Riddle: can you swim faster upstream than downstream (with respect to the water)?

A friend of mine posed a riddle to me: A man swims upstream in a river, which is flowing at an unknown rate. He is wearing swimming goggles. At a certain point he loses his goggles. 10 minutes later ...
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2answers
274 views

Bernoulli's equation for flow between cylinders

Let's consider the situation bellow noting that the water cylinders aren't equal and there will be water flowing to the right (As indicated by the arrow) Taking Bernoulli's equation at the tip of ...
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1answer
56 views

Is the flow of a viscous fluid in free space under no pressure gradient always laminar?

Consider a (Newtonian) incompressible viscous fluid in three spatial dimensions, whose velocity field $\mathbb{v}=\mathbb{v}(x,y,z,t)$ moves according to the Navier-Stokes equations $$\tag{1}\label{...
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76 views

What is the velocity in the Navier-Stokes equation?

I have been looking at the Navier-Stokes equation, and can't seem to find anywhere a clear description of what velocity it represents. From what I have read it could be any of the following: The '...
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1answer
46 views

Why does laminar flow turn turbulent(*) in external fluid flow and why doesn't a fully developed laminar flow turn turbulent in flow through pipes?

(*:with increase in distance from the leading edge.) I would appreciate if you give detailed explanation for the first question and then apply the same logic to answer the second.
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Modelling a flow due to an oscillating hemisphere?

Say I have a long cylinder filled with air, closed at one end and open at the other. Now say I place a compressible hemisphere at the closed end and make it oscillate (compression and expansion of the ...
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2answers
811 views

Dynamic water pressure from flow and diameter

I have to test some fire hydrants and need to get the dynamic pressure. For that I know I can use a Pitot tube but, I don't have one. After Googling a lot, I found two formulas allowing to compute the ...
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6answers
75 views

Can I recirculate water from an open reservoir to the bottom of a bigger, closed one, without a pump?

A fountain head pumps water out of the main tank into a 'pond' reservoir. Can the water recirculate back into the main tank without the help of another pump? I'm sorry if this a dumb question. I'm ...
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38 views

How to make an equal flow rate in 4 glass tubes with different inner diameters?

I would like to ask a question regarding a flow rate project I've been working on that required pneumatic software to solve. Unfortunately, I don't have the software, and I don't know how to control ...
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1answer
245 views

Does the termination point(level) influence water flow from a pipe

I get water to my home from a nearby Tank A at a certain height above ground level. I have a 1" pipe through which I get this water to my home.. I leave this water into my well by connecting a 1" tube ...
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24 views

Air pressure equalisation

I'm programming a physics simulation and have the air pressure impact the movement of rigid bodies in the system. I know how to (approximately, both for simplicity and performance) specify bodies' ...
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1answer
39 views

Cooling power of CO2 being released from a gas bottle

I need help in calculating how much a constant flow of CO2, being released into a small pipeline from a pressurised gas bottle, cools down the pipeline (in watts), to figure out if I'd need to heat up ...
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Steam boiler temperature and pressure

Take a pressure vessel containing water heated to the boiling point. Total steam evolution is 7 kg/hr (2 g/sec). The boiler is vented to standard atmosphere via 10 meters of 10 mm ID tubing with no ...
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3answers
49 views

Tank draining derivation: is it correct to use Bernoulli and continuity equations?

I'm confused about a classical exercise on tank draining, which asks to find the time requested for empty the tank. I'll put here the derivation I found on textbook and on the web, which I'm not ok ...
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1answer
70 views

Hamiltonian flow?

I was wondering what the Hamiltonian flow actually is? Here is my idea, I just wanted to know if I am correct about this. So let $(x(t),p(t))' = X_{H}(x(t),p(t))$ are the Hamilton's equations and $...
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32 views

How is zonal flow defined and computed?

The transition to turbulence in pipe flow was recently observed to be in the same universality class as directed percolation. This was done by reinterpreting the turbulence and laminar flow in terms ...
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2answers
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Can air be considered incompressible as long flow velocities are less than 100 m/s (Ma = 0.3)?

Multiple sources state that steady air flow (in open systems) can be considered incompressible at velocities less than 100 m/s (Ma = 0.3). Example: Deborah A. Kaminski, Michael K. Jensen, "...
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Power generation with a long, horizontal pipe, using atmospheric pressure differences

Straight to the point. If I float a pipe starting from a point in the ocean that has the atmospheric pressure of 101 kPa (Call it Point A) all the way to another geographic position on the ocean with ...
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1answer
39 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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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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447 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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2answers
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Is flow energy the same as work?

Studying thermodynamics I face this following text from Cengel's book. Throttling valves are usually small devices, and the flow through them may be assumed to be adiabatic (q = 0) since there is ...
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1answer
70 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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1answer
37 views

Why does increasing heat conductivity decrease Nusselt number?

Normally we expect that when heat conductivity(k) increases heat transfer is also increases. But for fluids increasing k decreases Nusselt number. My senses tell that using high conductive fluid cause ...
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3answers
57 views

Do you need to increase pressure to pump a fluid from a large pipe through a small pipe?

The application of this question is towards clogged arteries and blood flow. I am wondering why people with clogged arteries have high blood pressure if a smaller artery (assuming it is smooth and ...
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0answers
24 views

Are the blades of a propeller necessarily in the shape of wings or would flat blades work?

I think I have seen that airplane propellers are airfoils -- but I am not sure that flat blades would not work and if they do, how much more efficient are airfoils?
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1answer
61 views

How long time before all the water in a lake has been replaced? [closed]

Studying hydrology, and in that context we've seen some information about lakes and the times it takes for the water to renew completely in them. This got me interested in finding out how this is ...
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1answer
17 views

A mathematical question on an Ultrasonic flow meter (Time of flight)

I'm trying gain a better understanding of expressions used to derive a formula to calculate the flow velocity Here are the following expressions: $$t_d= \frac{L}{(v+ucosĪø)}$$ $$t_u= \frac{L}{(v-...
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1answer
60 views

When does the shock occurred?

I'm between this 2 conditions that when does the shock occurred over an airfoil. When the speed of point of an airfoil reach to sonic velocity of environment. When the speed of point of an airfoil ...
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2answers
129 views

Must flow be supersonic for disturbances not to affect upstream?

I'm studying oil production and found a fact that puzzled me. It states that fluid flow downstream of the wellhead must be supercritical in order not to disturb the flow upstream of it. From PetroWiki:...
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1answer
26 views

Liquid Flow In A Vertical Tube

A cylindrical vertical tube has uniform cross section $A_1$, and length $l$. It is open at both ends. Water enters from the top with a constant velocity $v_1$, and allowed to flow out from the bottom. ...
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Why is a controlled shear test compared to the natural flow?

Looking for the areas where contolled shear stress (CSS, where force is fixed while rate of deformation is measured) and controlled stress rate (CSR, where conversely the force is measured while ...
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2answers
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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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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 ...
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Energy of electrons in wire

The Drude Model helped me quite a lot to visualize how current could flow in a circuit. However, there is still a point that I cannot grasp in the explanation given by some people when they talk about ...
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4answers
2k views

Propellers and the Speed of sound

I was reading this website that described a novel wind turbine technology and it has this quote: In the mid-20th century, open propeller-driven planes with piston engines reached performance ...
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Calculating optimal heat dissipation/air-flow out-of/into an enclosure - hole size vs number of holes

I am not sure if this is the right place to ask, but I figured it would be a physics question. Essentially I have an enclosure that I will be using to house the components to build a computer. I am ...
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1answer
28 views

Can a piston-like displacement be miscible?

I have a question regarding miscibility and piston-displacement of fluids. My question is as follows: I am given that a fluid is displaced miscible. At the same time, I am also given that the ...
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1answer
41 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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2answers
79 views

When I open a window to air out the room, how does the smell disperse?

Let's say I'm in a room with some kind of noxious stink, possibly of flatulent nature. The quickest way to right the world that comes to mind is to open a window. When I open a window, how do the ...
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42 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 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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Temperature of throttled flow

In a pipe with a throat, closed off to the left, I'm pouring in $\dot{n}$ atoms per second from the left. These atoms combine and do a chemical reaction, which releases energy. To the right, past the ...
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Isentropic flow through nozzle

I've a question concerning the use of the Bernoulli principle. Let's say I have a pipe filled with air at atmospheric pressure, with a constricting section ala the figure below. The volume before the ...
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3answers
1k views

Is Torricelli's law “wrong” for big holes? - Tank draining problem

Consider a tank full of water with a constant cross-sectional area A1 placed vertically on the ground. Now someone drills a hole of an area A2 in the bottom of the tank, and the liquid starts escaping ...