**Short Notes on Vortex Motion **

Contents

**Vortex Motion Definition**

- A
*whirling or*mass of fluid is called vortex flow.**rotating** - The flow of the fluid in a curved path.

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**Free Vortex Flow**

In this flow fluid mass rotates due to the conservation of angular momentum. The velocity profile is inversely proportional to the radius.

*v × r = constant*

The point at the center of rotation is called a singular point, where velocity approaches to infinite. An example of free vortex motion is whirling mass of liquid in a washbasin, whirlpool in the river, etc.

**Points to remember:**

- No
torque or energy is required.*external* - In free vortex flow,
can be applied.*Bernoulli’s equation*

**Forced Vortex Flow**

When a fluid is rotated about a vertical axis at a constant speed, such every particle of it has the same angular velocity, the motion is known as the forced vortex.

*v = r × ω*

*h = ω²r²/2g*

Where ‘h’ is a height of paraboloid, and r is the radius of the cylinder.

The volume of paraboloid = *1/2 π ×r² ^{ }h*

= 1/2 of the volume of circumscribing cylinder

**Points to Remember**

- The surface profile of a forced vortex flow is parabolic.
- Forced vortex requires a constant supply of external energy or torque.

An example of forced vortex flow is a rotating cylinder and flow inside the centrifugal pump.

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**Variation of Pressure**

*dp = (ρv²/r) × dr – ρg dz*

*z is in the upward direction*

**Equation of Free Vortex Flow**

*We know for free vortex v × r = constant = k*

and *dp = (ρv²/r) × dr – ρg dz*

dp = [{(ρ × k²)/r×r² }× dr] + ρg dz (putting *v* = k/r)

*On Integration for point 1 to point 2*

∫dp = ∫[{(ρ×k²)/r³ }× dr] – ρg ∫dz

P_{2} – P_{1} = ρ/2 (v_{1}² – v_{2}²) – ρg(z_{2} – z_{1}) (Again putting value of k )

P_{1} + ½ ρv_{1}² + ρgz_{1} = P_{2} + ½ ρv_{2}² + ρgz_{2} *(Bernoulli’s Equation is applicable in Free Vortex)*

**Equation of Forced Vortex Flow**

*We know for free vortex v/r = ω = constant*

dp = [(ρ×ω²r²)/r] × dr + ρg dz

*On Integration for point 1 to point 2*

∫dp = ρ×ω² ∫r × dr + ρg ∫dz

P_{2} – P_{1} = ρω²/2 (r_{2}² – r_{1}²) – ρg(z_{2} – z_{1})

*On Putting v = ω × r*

P_{2} – P_{1} = ρ/2 (v_{2}² – v_{1}²) – ρg(z_{2} – z_{1})

P_{1} – ½ ρv_{1}² + ρgh_{1} = P_{2} – ½ ρv_{2}² + ρgz_{2} *(Bernoulli’s Equation Not Applicable)*

**Free Vortex and Forced Vortex Flow Differences**

Let us look at some of the differences between Free Vortex and Forced Vortex.

Free Vortex Flow |
Forced Vortex Flow |

No external torque i.e. torque required to rotate is zero. | External torque is required to rotate the fluid mass. |

We know T = d (mvr)/dt and since T = 0,
So, mvr = constant or v × r = constant (mass is constant) |
ω = constantv/r = constant |

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