Fluid Dynamics

The study of the motion of fluids (liquids and gases) and the forces causing this motion, Fluid Dynamics has several subdisciplines, including hydrodynamics and aerodynamics. To ask and answer questions about Fluid dynamics and its subdisciplines, come on in to the Fluid Dynamics category.

1,958 Questions
Fluid Dynamics

What is shock waves in fluid mechanics?

The speed of sound is constant in solids, liquids and gases. If something happens in such a medium the mechanical energy of it can propagate only at the speed of sound. If something is traveling through the medium faster than the speed of sound then it is pumping energy into a wave that contains more energy than what the simple passage of the object contains. This is the sonic boom hear by passing jet aircraft.

Fluid Dynamics

How does toilet flush work?

Check out the Youtube video.

Fluid Dynamics
Newtons Laws of Motion

How can the unit of upthrust can be derived?

Upthrust is the upward or buoyant force acting on an object in a liquid. It is a force, and so has the units of force. This is pounds in the imperial system, and Newtons in the SI.

Fluid Dynamics

Does toothpaste have high viscosity?


Fluid Dynamics

Who is the father of fluid mechanics?

pascal & Archimedes

Fluid Dynamics
Bombs and Explosive Materials

What makes the whistling noise on bombs as they fall?

If you've ever seen a bomb you will notice that there are tail-fins. There are two purposes to these fins.

  1. To make the bomb fall where it is intended to go. Hitting a target with a bomb is a hard thing to do and a lot of technology goes into making a bomb the right shape to 'fall straight.'
  2. These tail-fins are also modified to make the screaming sound as they fall; to add a dimension of terror to a bombing raid.
Fluid Dynamics

What is necessary to create a convection current?

One Sentence Answer:

Natural convection currents will be created when the temperature gradient in a fluid is in a different direction than the force of gravity and that includes liquids, gasses and even plasmas.

Conditions for Convection:

Normally when discussing convection the type of convection is assumed to be "natural convection" otherwise known as "thermal convection" caused by heating a fluid. There is also "forced convection" where fluid movement is caused by an artificial mechanism like a fan. Convection ovens, for instance, are actually forced convection where a fan moves heated air. Thus, it is obvious that forced convection can occur in any fluid.

That said, it remains true that natural convection can occur in any fluid.

Natural convection requires three conditions.

1. A Nonuniform temperature in the fluid.

2. A change in density of the fluid with change in temperature.

3. A gravitational field to create buoyancy.

Discussion "natural convection":

We can explain the conditions in order.

0. We could add condition "0" to the list and that is the condition that one has a fluid and not a solid. A solid will maintain its shape under the influence of a force, i.e. a solid will not flow. Convection does not occur in solids. (Some people will call a "glass" a liquid, but that is not appropriate here. The glassy state is still an amorphous solid state. Contrary to some old wives' tales, glass does not flow.)

1. A Nonuniform temperature in the fluid is required because we are discussing thermal convection caused by temperature differences. We could also discuss convection caused by chemical or physical differences in a fluid. For instance, it is well known that if a body of salt water experiences evaporation, the density of the surface regions increases and as a result, the surface water will sink. That is a form of convection that does not require a temperature difference and is not what we call "forced convection." There are other examples where a fluid becomes inhomogenous for reasons other than a temperature gradient, e.g. phase separation, and density changes result in fluid flow.

2. A change in density of the fluid with change in temperature is the driver of convection. If the temperature is uniform and the fluid is therefore uniform in density, there are no buoyant forces and no force to cause motion in the fluid. Of course, one can imagine a fluid which for some reason does not change in density with temperature. Most fluids that decrease in density as temperature increases, gasses for instance. Rarely, a fluid will increase in density as temperature increases, water for instance. (If you increase the temperature of water from 0 C to 4 C, the density increases by 0.013 %.) There is no known material that does not change density with temperature.

3. A gravitational field to create buoyancy is required so that the force pulling on the dense region of the fluid is unbalanced from the force on the less dense regions and the unbalanced force causes motion. This turns out to be an important problem for people living in zero gravity environments. Fluid currents induced by convection are important for heating liquids. In fact, this is also important for combustion. Trying to burn a candle in a zero gravity environment is difficult. (This is not to say that convection due to forces other than gravity is impossible. One can imagine other forces that would penetrate a fluid, e.g. electrical, magnetic, centrifugal. Whatever the force, a kind of convection would occur if there is an imbalance in the force acting on difference regions of the fluid.)

Finally, we should mention plasmas, which exhibit convection the same as any other fluid. The presence of electric and magnetic fields complicate the fluid dynamics, but convection can still take place. The Sun is the obvious example, but only the outer third of the Sun exhibits convection. The dynamics of the Sun's convection zone involves more than temperature, particularly the turbulent outermost region where forces of magnetic fields create dramatic effects. Note that the absence of convection in the deep interior does not violate our basic assertion that convection occurs in all fluids because the gravitational field inside the Sun in very nonuniform, dampening vertical density fluctuations. But, if you want to get picky, then you could claim that the interior of the Sun is one place that fluid convection does not take place!

Earth Sciences
Fluid Dynamics

How do convention currents stop?

Simple Answer:

When the driving force for convection is removed, the system will return to thermal equilibrium.


If convection exists in a fluid because a heat source is maintaining a temperature gradient and then that heat source is removed, the fluid will return to a stationary state with uniform temperature in a sequence of three overlapping stages.

1. With no driving force, the system initially continues the current flow and transport of heat, but the flow will slow and cease due to internal fluid viscosity

2. When system inertia of initial flow dissipates, new convection forces develop due to remaining thermal gradients and push fluid towards a configuration where density profile are perpendicular to gravitational forces.

3. Existing temperature gradients in the fluid will diminish through thermal conduction as the system proceeds toward a final uniform temperature.

Swimming Pools
Pool Care and Cleaning
Fluid Dynamics

Should the surface water in your pool be moving a lot?

Yes, I believe it means your pump and filter are running correctly. You may want to adjust your jet so that the water discharge is just below the surface and circulating around the pool properly. My pool company told me to place the return jet at an angle facing away from the skimmer. It should be angled so that very little water movement can be noticed from the surface. The pool companies often set the showroom return jets in a way that is angled too high. They do this just for "show." If the return is set right, surface materials end up in the skimmer and bottom materials end up in one general spot in the center which is very easily vacuumed.

Fluid Dynamics

What do you call the curvature of a water surface in a glass tube?

The meniscus.

Related Information:

A meniscus can be concave or convex.

For more information, see related links, below.

Fluid Dynamics

Heat transfer by convection requires what?

Simple Answer:

Heat transfer by convection requires a fluid, heat source and gravity. Natural heat transfer by convection requires there to be a heat source causing nonuniform temperature in the fluid so that buoyancy of the warmer fluid causes it to rise.


When we say heat is transferred by convection, we mean the thermal energy (heat) is actually carried by the material to distinguish from heat conduction through a material.

Convection requires the presence of matter that can move, e.g. liquid or gases exhibit convective heat flow but solids do not.

Normally, convection is driven by buoyancy, so one also needs to have the liquid (or gas) change density with temperature.

Aside: Convection is why we say "heat rises." And, an interesting side observation is that when water cools near the freezing point, it actually gets lighter as the temperature drops from 4 C to 0 C, hence causing the cooler water in that range to be less dense and more buoyant.

Aside: There is also something called forced convection where fluid is moved by artificial means such as air pushed by a fan. Most home heating occurs with "forced convection" but years ago natural convection resulting from buoyancy of hat air was the cause for air circulation.

Industrial Engineering
Electrical Engineering
Fluid Dynamics

What is an eddy current?

In magnetism:

An eddy current is induced into a metal when magnetic lines of force move across it. A South pole causes circulating current in clockwise direction while a North pole causes current in counter-clockwise direction. These eddy currents thus buck the applied forces. Eddy currents are undesirable when induced into transformer cores causing power loss. Lamination of core material reduces current flow in the core. Current induced into the secondary winding of a transformer is a used to step-up or step-down voltages so that they can be of a correct size for end-use applications.


When time-varying magnetic field is applied to electrical machines like transformers, a time-varying emf is induced in the transformer cores. A short circuit occurs at the molecular level in the core. Due to less resistance, a large current begins to flow in the core. This causes heating in the core. Actually the path of the current is circular resembling the circular waves in a pool of water (eddy). Hence these currents are called eddy currents.

In fluids:

In water flow, an eddy is a current that flows opposite the normal flow. If on a river, an eddy is a current that will flow upstream in a side channel filling it, even if the flow is in an opposite direction of the original flow. It is equivalent to a stream's water level rising because the river it feeds has more water in it than the stream, thus making the water flow upstream. It can also be an area that seems not to have a current at all.


Just like there exists a magnetic path due to current (charge) flow in a conductor (direction given by right hand rule), the thing works other way as well...

When there is a flux path crossing a current conducting material, there exists current paths around the flux line on the conductor plane centered to the point where flux line meets the plane. These currents are eddy currents.

Commonly available in magnetic circuits. Laminations are done to minimize the ability to flow eddy currents.

Fluid Dynamics

Where can you get heat n mass transfer notes?

Fluid Dynamics
The Difference Between

What is difference between penstock and pressure shaft?

Pressure Shaft is designed to resist Internal as well as External Pressure, but the Penstock is designed to resist Internal Pressure only.

- Saravanan.S

Fluid Dynamics

How deep will a vacuum pump water from?

i believe the question should be stated as "How high can a pump pull liquid when mounted above the liquid source". an old pump adage is that a pump doesn't suck. sounds dumb, but it refers to the necessity of having a positive pressure at the suction of the pump greater than the required net positive pressure req'd by the pump. NPSHa must be greater than NPSHr. in any system open to atmosphere the surface of the fluid will have 14.7psi (at sea level) X 2.31 ft/psi, or roughly 34' of head, or NPSHa, available. the manufacturers performance curves will show the NPSHr of the pump at any given flow for a given impeller trim. by subtracting this NPSHr from the calculated surface pressure you can arrive at a general maximum lift that the pump can run at. there will also be line friction losses that will reduce this height, and typically we subtract another 2-3' for a fudge factor as you would not want to run on the ragged edge. so, a pump with an NPSHr of 8' would be able to lift cold water approx 22' before cavitating. getting it primed is another issue, and having said all this, there is a type of centrifugal that can successfully trick this seemingly rigid restriction on lift. the typical home commercial jet pump can lift from many times this limited depth by taking a portion of the high pressure discharge and sending it down a separate pipe and into the suction pipe. this effectively increases the suction pressure and allows this type of pump to lift from quite a depth. a really neat way around having to install a down-hole pump submersible. of course the type of pump, what you are pumping, temperature, vapor pressure, specific gravity, and viscosity will all affect the height that a pump can lift a fluid.

If the question is ' at what height we can place the suction side of pump from the water level from where it is pulled up', then if we are not considering the NPSH (which is not practical of course) , then i think that the maximum height of the suction side will be the height which will balance the pressure which is on the water level below from where it is pumped. If the pressure there is atmospheric pressure (at the water level below suction side)then maximum height of water rising is 10.3 m around(will vary according to the fluid being pumped). Above it water will not rise whatever vacuum you create though pump.(Its just like in barometer where mercury doesn't rise above 760mm, although there is vacuum above it in the tube. This is because at base of inverted tube of barometer, pressure is balanced). In practical of course the height is much less of course otherwise cavitation will take place when pressure falls below Vapor pressure of the Liquid being pumped.

Fluid Dynamics

Why can a gas be squeezed into a smaller container?

This is possible because gases have the physical property of elasticity, due to the fact that there are relatively large spaces between the molecules of gases.

Fluid Dynamics

For a particle in circular motion the resultant acceleration is?

Since no values are given, the answer must be a general one. A particle in circular motion undergoes centripetalacceleration. Inertial motion is straight line motion. Any change in motion (including direction) requires positive or negative acceleration. In order to move along a circular (or any curved) path, a particle's direction of motion is in a constant state of diversion from straight line inertial motion, so it moves under a contant state of acceleration.

Math and Arithmetic
Fluid Dynamics

A pebble has a mass of 35 grams and a volume of 14 cubic centimeters What is its density?


Fluid Dynamics

How do convection currents transfer thermal energy in a fluid?

Short Answer:

A convection current transfers thermal energy by physically carrying it from one place to another.


This answer will considering only "natural convection" currents that arise due to the difference in density of a fluid caused by heating. Forced convection results from some artificial process like a fan moving warm air.

When a region of a fluid is less dense that its surrounding, it feel a buoyant force. Conversely for a more dense region. A fluid current results with less dense material going up in the gravitational field and more dense fluid going down. Normally, heating a fluid in one region decreases its density (though the opposite works as well) and so the hotter fluid moves up. That motion is natural (thermal) convection. Motion of the hot fluid carries thermal energy, hence the answer to the question. "A convection current transfers thermal energy by physically carrying it from one place to another."

Fluid Dynamics

Explain what determines the viscosity of a liquid?

the shape and size of the molecules that the fluid consists of, if they are big, wavy molecules then the liquid will be viscous because the molecules struggle to move over each other.

Home Improvement
Fluid Dynamics

Why is my Interior home water pressure low?

Old house, the pipes fill up with sediment. A 1/2 inch pipe may have a quarter of an inch or less. On some new houses there may be a pressure regulator on the line coming in. Somewhere between where it comes into the house and the first faucet or the water heater, whatever comes first. May just be rust and sediment at the faucet. Take the screen off the faucet and see if it is plugged.

Fluid Dynamics

Why are falling drops of water spherical?

The shape of falling water drops is caused by the polar bonding of the water molecules. The aggregate effect of this bonding is referred to as surface tension which can be regarded as an air/water boundary.This surface tension tends to minimize the surface area of the drop. The shape with the smallest surface area is a sphere.

Related information:

A major factor distorting the shape of falling water drops, is aerodynamic drag as the drop moves through the air. That's why the drops only approximate a spherical shape. It is sometimes convenient to think about this bonding configuration as the water having a skin. However, while it is sometimes convenient and useful, it is important to remember that it is only marginally accurate as a description of reality.

Fluid Dynamics

Why the divergent part of venturimeter is longer than convergent part?

Actually when the flow emerges from the throat area of venturi to enter into the diverging section, their is a negative pressure gradient i.e, in layman terms fluid is trying to flow from low pressure region to high pressure region according to Bernoulli equation. In this adverse pressure gradient, there is boundary layer separation, in simple terms, the fluid leaves the surface of the wall. Due to this there can be energy loss or the fluid can't recover the pressure fully leading to head loss. So if divergent section is long that means more gradual diverging section, due to which the adverse pressure gradient is less so less chance of boundary separation and hence less loss. Also large diverging section will ensure proper development of flow, i.e. fluid sticking to the wall back after separation.

Fluid Dynamics

How do you explain air resistance?

In fluid dynamics, drag (sometimes called air resistance or fluid resistance) refers to forces that oppose the relative motion of an object through a fluid (a liquid or gas).

Drag forces act in a direction opposite to the oncoming flow velocity.[1] Unlike other resistive forces such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.[2]

For a solid object moving through a fluid, the drag is the component of the net aerodynamic orhydrodynamic force acting opposite to the direction of the movement. The component perpendicular to this direction is considered lift. Therefore drag opposes the motion of the object, and in a powered vehicle it is overcome by thrust.

In astrodynamics, and depending on the situation, atmospheric drag can be regarded as an inefficiency requiring expense of additional energy during launch of the space object or as a bonus simplifying return from orbit.

Mechanical Engineering
Fluid Dynamics

How does one define negative absolute pressure?

As vacuum pressure.


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