A model of a hydraulic turbine is tested at a head of 1/4th of that under which the full scale turbine works. The diameter of the model is half of that of the full scale turbine. If N is the RPM of the full scale turbine, then RPM of the model will be
- A. N/4
- B. N/2
- C. N
- D. 2N
Answer: C. N
Explanation: Unit speed $N / \sqrt{H}$ should be constant? No, $DN/\sqrt{H}$ is constant for similarity. $D_m N_m / \sqrt{H_m} = D_p N_p / \sqrt{H_p}$. $(D_p/2) N_m / \sqrt{H_p/4} = D_p N_p / \sqrt{H_p}$. $0.5 N_m / 0.5 = N_p$. $N_m = N_p = N$.