Question 1Multiple choiceFluid Mechanics
A large open tank of water has a short, smooth, well-rounded nozzle in its side wall with an exit diameter of 50 mm. The centreline of the nozzle is 4.0 m below the free surface, which is held at a constant level. The jet discharges to the atmosphere. Neglect losses and the velocity of the free surface, and take . What is the volumetric flow rate from the nozzle?
- A.
12.3 L/s
- B.
17.4 L/s
- C.
69.6 L/s
- D.
154 L/s
Worked solution
Apply the Bernoulli equation along a streamline from the free surface (point 1) to the jet just outside the nozzle (point 2). Both points are at atmospheric pressure, the free-surface velocity is negligible, and the elevation difference is m:
This is Torricelli's result: the jet velocity is that of free fall through the head :
The nozzle area uses the diameter, so the radius is 25 mm:
Continuity gives the flow rate:
Q ≈ 17.4 L/s (option B)
The most tempting wrong answer is A (12.3 L/s), which uses and drops the factor of 2 that comes from the kinetic-energy term in Bernoulli. Option C (69.6 L/s) uses for the area instead of , the classic radius-versus-diameter slip. Option D (154 L/s) forgets the square root and uses .