Dynamic Fluids: Definition, Properties, Formulas, and Differences

Dynamic Fluids, Static Fluids, And The Difference –  Static and dynamic fluids are an important aspect of life. Fluid is a substance that can flow. Liquids and gases are types of fluids. Because both of these substances have properties that can flow. Unlike rocks and various other hard objects, all solids are not included in fluids because solids cannot flow.

Liquids can be in the form of milk, cooking oil, and water. All liquids are included in the type of fluid. Especially with its nature that can flow from one place to another.

In addition to liquids, there are gases which also include fluids. Gas substances also have the ability to flow from one place to another.

Blowing wind is one example of air that can move from one place to another. Based on its movement, fluids are divided into static fluids and dynamic fluids.

Dynamic Fluids

Fluids are needed in everyday life. Every day, humans use fluids for drinking, washing clothes, breathing air and many other activities. This fluid can be divided into two parts, namely static fluid and dynamic fluid.

Dynamic Fluid Properties

In order to make it easier to study this dynamic fluid phenomenon, scientists have agreed to make assumptions about an ideal fluid. The ideal fluid properties include:

  • Is steady flow (flow velocity at a point is constant with time). If the velocity v at a point is constant, then the fluid flow can be said to be steady. An example of steady flow is a steady stream of water (low flow rate).
  • It is an incompressible flow, meaning that the flowing fluid does not change in volume or density when pressed. If the flowing fluid does not change in volume or density when pressed, then the fluid flow can be said to be incompressible.
  • It is a non-viscous flow. The fluid will not experience friction between one fluid layer and another fluid layer. In fact, fluid fluids also do not experience friction with the channel walls as a result of viscosity symptoms.
  • The flow has a current line and is not turbulent, meaning that each fluid particle will pass through the same path point and go in the same direction. Even though there is no absolutely ideal fluid, the fluid closest to the ideal fluid properties is water. So that research on fluids often uses water.

Fluid Flow Type

There are several types of fluid flow. The path traversed by a moving fluid is called a flow line. Here are some types of fluid flow, namely as follows:

  • Straight or laminar flow is smooth fluid flow. The layers next to each other glide smoothly and seamlessly on top of each other. In this flow the fluid particles will move along a smooth trajectory. In addition, these tracks do not cross one another. Laminar flow can be found in water flowing through a hose or pipe.
  • Turbulent flow is flow that is accompanied by the presence of erratic circles and resembles a vortex. Turbulent flow is often found in ditches and rivers.

General Characteristics of Dynamic Fluids

The general characteristics of fluid dynamics are as follows:

  • The fluids are considered incompatible
  • Even though there is motion of the material (it has no viscosity), the fluid is considered to move without friction.
  • Fluid flow is stationary flow, that is, the speed and direction of motion of fluid particles passing through a certain point are always fixed
  • It is independent of (steady) time, meaning it is a constant velocity at a certain point, and forms
  • Leliner flow or (layered)

Magnitudes in Dynamic Fluids

Debit friend (Q)

The total volume of fluid flowing in unit time, or

Where :

Q = flow discharge (m3/s)

A = cross-sectional area (m2)

V = fluid flow rate (m/s)

Fluid flow is often expressed in terms of flow rate

Where :

Q = flow discharge (m3/s)

V = volume (m3)

t = time interval (s)

Dynamic Fluid Laws

There are 2 laws that apply in fluid dynamics, namely the Law of Continuity and also Bernoulli’s Law.

1, The Law of Continuity

For example, in the event of watering flowers, water flowing from a hose with the end pressed down will flow faster. Why is that? This is due to the phenomenon of the Law of Continuity in flowing fluids.

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The Law of Continuity states that the discharge of water flowing at each point along the flow of the hose is constant or the same.

Mathematically it can be written as follows:

Debit or Q is the amount of volume of fluid flowing per unit time or written mathematically with

Volume can be calculated by multiplying the cross-sectional area of ​​the hose by the length of the hose or V = A cdot L.

So that the discharge equation becomes the length of the hose that is passed by the water can be calculated by means of the speed of the water multiplied by the time, or in other words the speed is the length of the interval divided by the time, v = frac{L}{t}.

So that the discharge equation becomes then the law of continuity can also be written as

2. Bernoulli’s law

Bernoulli’s law was discovered by Daniel Bernoulli, a scientist from Germany. From this discovery, in 1738, Bernoulli managed to publish a book entitled Hydrodynamica.

Bernoulli’s law is a law based on the law of the conservation of energy experienced by fluid flow. This law states that the amount of pressure (p), kinetic energy per unit volume, and potential energy per unit volume has the same value at every point along a flow line.

This law can be applied to any type of fluid flow as long as it satisfies the following conditions.

  • The fluid is incompressible.
  • The fluid has no viscosity.
  • The fluid flow is steady.
  • The fluid flow is laminar (fixed and does not form a vortex).
  • No energy is lost as a result of friction between the fluid and the wall and turbulence.
  • There is no transfer of heat energy.

In this case Bernoulli’s Law applies which states that each sum of pressure (p), kinetic energy per unit volume (½rv2) and potential energy per unit volume (rgh) has the same value at every point along the current line.

Bernoulli’s law when expressed in the equation becomes:

Information :

P1 = pressure in pipe 1 (N/m2);

P2 = pressure in pipe 2 (N/m2);

ρ1 = density of pipe 1 (kg/m3);

ρ2 = density of pipe 2 (kg/m3);

v1 = velocity of fluid in pipe 1 (m/s);

v2 = velocity of fluid in pipe 2 (m/s);

h1 = cross-sectional height of pipe 1 from the reference point (m);

h2 = cross-sectional height of pipe 2 from the reference point (m); And

g = acceleration due to gravity (m/s2).

Toricelli’s theorem (efflux rate)

The rate of water spraying from the hole is the same as water falling freely from a height. The rate at which water shoots out of this hole is called the efflux rate, while the phenomenon is called the Toricelli theorem.

If Bernoulli’s equation is applied at point 1 (surface of the container) and point 2 (surface of the hole). Because the diameter of the faucet/hole at the bottom of the container is much smaller when compared to the diameter of the container, the velocity of the liquid on the surface of the container is considered zero (v1 = 0). The surface of the container and the surface of the hole/tap is open so that the pressure equals atmospheric pressure (P1 = P2). Thus, the Bernoulli equation for this case is:

Application of Bernoulli’s Law

The principle of Bernoulli’s Law is also applied to the following objects that are often encountered in daily activities.

1. Parfum

When pressing the perfume sprayer downwards, the liquid at the bottom will move at a low speed, resulting in a higher pressure on the liquid below.

This can encourage liquid to move up through the perfume hose with a small size. When it reaches the top of the hose, the air on the suction cup will come out along with a burst of perfume.

2. Venturimeter pipe

A venturimeter pipe is a tool used to measure the flow rate of liquids. This ventirometer pipe is designed in the form of a pipe with a narrowing diameter. Based on the presence or absence of a pressure gauge, these devices can be divided into two, namely a venturimeter without a manometer and a venturimeter with a manometer.

A manometer is a device used to measure the air pressure in a closed space. When Sinaumed’s  wants to know the shape of the venturimeter pipe, look at the following picture.

The venturimeter shown in the previous figure does not have a manometer. Therefore, to determine the flow velocity of liquid entering sections 1 and 2, it can be formulated as follows.

Information :

A1 = cross-sectional area of ​​pipe 1 (m2);

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A2 = pipe cross-sectional area 2 (m2);

v1 = velocity at pipe cross section 1 (m/s);

v2 = velocity at pipe cross section 2 (m/s);

h = difference in the height of the liquid in the small pipe above the venturimeter (m); And

g = acceleration due to gravity (m/s2).

3. Pitot tube

A pitot tube is a device used to measure the speed of gas in a pipe. Look at the following picture.

Mathematically, the gas flow rate in the pipe is formulated as follows.


v = gas flow velocity (m/s);

Static Fluids

Static fluid or hydrostatics can be defined as a branch of physics that deals with pressure, water balance, and other fluids. Fluids at rest create problems that are far from trivial to solve when compared to dynamic fluids.

Definition of Static Fluids

Before delving into the discussion of the static fluid formula, it’s better if we understand its meaning first.

Static fluid is a fluid that is not moving (still) or a fluid in a state of motion but there is no difference in the speed of the fluid particles. It can also be said that the fluid particles move at the same speed and do not cause shear forces. For example, water in a glass that is not subjected to a force will remain still or in a river that flows at a constant speed.

Some Static Fluid formulas are as follows:

1. Density

In physics, density or a measure of the density of a homogeneous object is called density, which is mass per unit volume. The higher the density of an object, the greater the mass of each volume. It has the function of determining the substance. Each substance has a different density. Mathematically, density can be written as follows.

ρ = m/V


m = mass (kg or g),

V = volume (m3 or cm3)

ρ = density (kg/m3 or g/cm3).

If ρ of the water is greater than ρ of the object, then the object will float. If both ρ are the same, then the object will float in the water. However, if the object’s ρ is greater than the water’s ρ then the object will sink.

2. Hydrostatic pressure

Hydrostatic pressure at any depth will not be affected by the weight of the water, the surface area of ​​the water, or the shape of the water vessel. Hydrostatic pressure will only be affected by the area of ​​the object receiving it or the measuring depth.

Hydrostatic pressure pushes in various directions, is the force exerted on a quantifiable or measurable area based on the depth of the object.

The equation for this is:

Ph= ρ.g.h


ρ = density of water (for fresh water, ρ = 1,000 kg/m3)

g = the magnitude of the acceleration due to gravity (the acceleration of gravity on the surface of the earth is g=9.8 or 10 m/s2)

h = depth point measured from the water surface (m)

The unit used is N/m2 or Pascal (Pa). There is also pressure which is referred to as absolute pressure. Absolute pressure is the total pressure experienced by objects that are in the water, the formula:

P = Ph + Patm

Patm is atmospheric pressure.(1.013 x 105 Pa)

Pascal’s law

Pressure is defined as the force exerted divided by the area of ​​the object receiving the force. In formula form, it will be written as:

P = FA


F = the magnitude of the force (Newtons)

A = cross-sectional area (m2)

Pascal’s law states that the pressure exerted on a fluid in a confined space will be transmitted equally in all directions and can be summed up as:

P in = P out

So that:

FEnterAMin= FExitAExit

F1 = (d1d2). F2


d1 = surface diameter 1

d2 = surface diameter 2

Difference between Static and Dynamic Fluids

The difference between static fluid and dynamic fluid is as follows:

1. Understanding

Static fluid is a fluid that is in a stationary phase or a fluid that is not moving. Meanwhile, a dynamic fluid is a fluid that is in a state of motion.

2. Speed

Static fluid has no difference in velocity between fluid particles. It can be said that some fluid particles have a uniform velocity movement. So that the fluid has no shear force.

While the dynamic fluid has a constant speed with respect to time. In addition, dynamic fluids do not experience volume changes and are not viscous. In addition, dynamic fluids also do not experience rotation or turbulence.

Completion in fluid dynamic calculations, generally requires the calculation of various properties. Those things are speed, density, pressure, and temperature which function as space and time.

That’s all the discussion about dynamic fluids, static fluids, and also the differences. Thank you for visiting and hopefully useful!