Hydrostatic Equilibrium: 5 Essential and Exciting Principles in Engineering

Hydrostatic equilibrium is a fundamental concept in fluid mechanics, astrophysics, and planetary science. It describes the balance between the gravitational force and the pressure gradient force in a fluid. This balance is crucial for understanding the behavior of fluids in various contexts. These contexts range from the Earth’s atmosphere to the structure of stars.

The principle of hydrostatic equilibrium helps determine the pressure exerted by the fluid. This pressure acts on the wall of a column or container.

Consider a column filled with a stationary mass of static fluid. Take any cross-section of the column parallel to the earth’s surface. The pressure is the same on this cross-section. However, if we take another cross-section of the column, the pressure differs from the first one. So, the pressure is changing from height to height.

Hydrostatic Equilibrium Equation Derivation

Assume the following dimension of the column:

1. The cross-sectional area of the column is S.
2. Pressure is p at height z from the bottom of the column.
3. The density of the fluid is ρ.

Now, we will analyze a small portion of the column with height dz and area S.

The three forces acting on this small portion of the column are as follows:

1. Pressure p is acting in the upward direction = pS
2. The force due to pressure \(p+dp\) acts in a downward direction = (p+dp)S
3. The force due to gravity acting downward = ρ S dz g

Force due to gravity is equal to mass times acceleration = mg

Mass (m) = Volume × Density = V ρ

Volume = Area × Height = S dz

Therefore, m = S dz g

And force due to gravity = ρ S dz g

The resultant force on a small portion must be zero. Forces acting upward are taken as positive. Forces acting downward are taken as negative.

Divide equation (1) by S.

The equation above is the desired basic equation that can be used for obtaining the pressure at any height.

Incompressible Fluid

For incompressible fluids, density is independent of pressure.

Integrating the Equation above, we get

The equation above shows that the pressure reaches its maximum level at the base. This is true for the column or container of the fluid. The pressure decreases as we move up the column.

Consider that the pressure at the bottom of the column is p₁, where z = 0, and the pressure at any height z above the base is p₂ such that p₁ > p₂, then

Integrating the above equation, we get

Where, p₁ and p₂ are expressed in N/m². ρ is in kg/m³. z is in m. g is in m/s² in the SI Unit.

The above equation helps in obtaining the pressure difference in a fluid. This can be done by measuring the height of the vertical column of the fluid between any two points.

Compressible Fluids

For compressible fluids, density changes with pressure.

For an ideal gas, the density is given by the following relation:

Here,
p represents absolute pressure.
M indicates the molecular weight of the gas.
R stands for the universal gas constant.
T signifies the absolute temperature.

Putting the value of ρ from the ideal gas law equation into the equation of Hydrostatic equilibrium,

Rearranging Equation,

Integrating Equation, we get

Integrating the above equation between two heights z₁ and z₂ where the pressures acting are p₁ and p₂, we get

The equation above is known as the Barometric Equation. It gives us the idea of pressure distribution within an ideal gas for isothermal conditions.

Hydrostatic Equilibrium in a Centrifugal Field

In a rotating centrifuge, a layer of liquid moves outward due to the axis of rotation. Centrifugal force holds it against the bowl’s wall.

The surface of the liquid is shaped like the paraboloid of the revolution. Still, in industrial centrifuges, the rotational speed is so high. The centrifugal force greatly exceeds the force of gravity. As a result, the liquid surface is virtually cylindrical and coaxial with the axis of rotation.

This situation is shown in Figure 2: Hydrostatic Equilibrium in a Centrifugal Field.

Here,
r₁ = Radial distance from the axis of rotation to the free liquid surface.
r₂ = The radius of the centrifugal bowl.

The entire mass of the liquid indicated in the figure rotates as a rigid body. No layer of liquid slides over another.

Under these conditions, the pressure distribution in the liquid is determined by the principle of fluid statics.

The pressure drop over any ring of rotating liquid is calculated as follows:

Considering the ring of liquid shown in the figure above and the volume element of thickness ρ at radius r.

Where,
dF = Centrifugal Force.
dm = Mass of liquid in an element.
ω = angular velocity (rad/s)

If ρ is the density of liquid and “b” is the breadth of the ring,

Eliminating dm gives,

The change in pressure over the element is the force exerted by the element of the liquid divided by the area of the ring

The pressure drop over the entire ring is

Assuming the density is constant and integrating gives

Conclusion

The concept of hydrostatic equilibrium explains how fluids behave within columns or containers. It also explores how pressure changes with height. In a stationary fluid column, pressure remains constant along horizontal cross-sections parallel to the earth’s surface. However, it varies from one cross-section to another as you ascend.

For incompressible fluids, the pressure decreases with increasing height. This is expressed as p + zρg = constant. Here, 'p' is pressure, 'z' is height, ‘ρ‘ is density, and 'g' is the acceleration due to gravity.

In contrast, compressible fluids like ideal gases exhibit a pressure-height relationship defined by the Barometric Equation. This equation is expressed as ln(p) + g(M/RT)dz = constant. Here, M represents molecular weight, R is the universal gas constant, and T is the absolute temperature.

The concept extends to hydrostatic equilibrium in rotating centrifuges, where a layer of liquid is pushed outward by centrifugal force. The pressure distribution follows fluid statics principles. The pressure drop over a rotating liquid ring can be calculated using dp = ω²ρrdr.

These insights into hydrostatic equilibrium give a comprehensive understanding of fluid behavior in various contexts. They also explain pressure variations in fluids. Their application in rotating systems is also covered.

FAQ’s

Read Also:

Fluid Statics and Its Application

Manometers

Reference:

McCabe, W. L., Smith, J. C., & Harriott, P. (2005). Unit Operations of Chemical Engineering (7th ed.). McGraw-Hill Education.