In this article, we will discuss the speed of efflux, its calculation, Torricelli’s law, its derivation, venturimeter, its working principle and its working equation.

**Torricelli’s Law**

As per the Torricelli’s law for Newtonian fluids, the speed of efflux of a fluid passing through a sharp edged hole at the bottom of a tank filled with the fluid to a depth of h is the same as the speed that a body would acquire in freely falling condition when falling from a height h.

**Derivation **

Consider a tank with a small hole in its side at a height y1 from the bottom, containing a liquid of density ρ. The air above the liquid is at pressure P and its surface is at height y2.

From the equation of continuity we can write

A_{1}v_{1 }= A_{2}v_{2}

Or v_{2} = A_{1}v_{1}/A_{2} ………………………….(1)

If the cross sectional area of the tank A_{2} is much larger than that of the hole (A2 >>A1), then we may take the fluid to be approximately at rest at the top, i.e. v_{2} = 0. Now applying the Bernoulli equation at points 1 and 2 and noting that at the hole P_{1} = P_{a} (the atmospheric pressure), we have

P_{a} + 1/2ρv_{1}^{2}+ ρgy_{1 }= P_{b} + 1/2ρv_{2}^{2} + ρgy_{2} ……………………………(2)

Here, let the difference in the height = h (as shown in the figure) = y_{1}-y_{2}

From equation 1 and 2, we have

**Venturimeter**

Venturimeter is a flow measurement instrument. Here, a converging section of a pipe is used to give an increase in the flow velocity and a corresponding pressure drop from which the flowrate of the fluid is deduced based on the Bernoulli’s equation.

Let us consider the figure shown above. Here we can see the block diagram of a Venturimeter. Here we can see small converging part, a throat and a diverging part. Here, we apply the Bernoulli’s equation between the inline section and the throat section. The pressure difference is measured using a manometer.

P_{1} + 1/2ρV_{1}^{2} = P_{2} + 1/2ρV_{2}^{2}

Where P_{1} is the pressure in the inline section and p_{2} is the pressure in the throat section, V_{1} is the velocity of the fluid passing through the inline section and v_{2} and the velocity of the fluid passing through the throat section and ρ is the density of the liquid.

Now, from the equation of continuity, we can say

Volumetric flow rate = V = ¼ πD^{2}u_{1} = ¼ πd^{2}u_{2}

Where, V is the volumetric flow rate of the liquid, D is the diameter of the pipe and d is the diameter of the throat.

Combining the two equations, we can write

Where, β is the ratio of diameters, d/D.

Here, we introduce a venture coefficient (C) considering the loss due to pipe friction and change in the total pressure,

Where, Δp is the pressure difference and C is the coefficient of Venturimeter.

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