Step 1: Understanding the Concept:
A "self-starting" windmill is one that can begin to rotate from a standstill solely under the influence of the wind, without needing an external motor or push. This capability is determined by the windmill's starting torque. The question asks to identify which of the given designs is self-starting.
Step 2: Detailed Explanation:
Let's analyze the starting characteristics of each type:
(A) Darrieus rotor wind mill: This is a lift-type Vertical Axis Wind Turbine (VAWT). The airfoil-shaped blades produce lift effectively only when they are already moving at a certain speed. At rest or very low speeds, they generate very little torque and can even experience negative torque at some positions. Therefore, the Darrieus rotor is famously not self-starting. It typically requires a motor or a starter system (like a Savonius rotor mounted on the same axis) to get up to speed.
(B) Savonius rotor wind mill: This is a drag-type VAWT, typically shaped like an S when viewed from above (two half-cylinders). The wind pushes against the concave face of one half-cylinder while flowing around the convex side of the other. This creates a large difference in drag, resulting in a net torque. This torque is positive at any rotor position, which means the Savonius rotor has a high starting torque and is reliably self-starting.
(C) Turbine tower wind mill: "Turbine tower" is a general descriptive term, not a specific aerodynamic type. Most modern turbines are on towers. This option is too vague.
(D) Dutch type horizontal axis wind mill: This traditional design with large sails has a very high starting torque due to its large blade area and is definitely self-starting.
Comparing the options, the Darrieus rotor is the only one listed that is distinctly {not} self-starting. The Savonius rotor is a classic example of a self-starting design due to its reliance on drag forces. Both Savonius and Dutch types are self-starting, but Savonius is a more common answer in this context, especially when contrasted with the Darrieus rotor. Given the options, Savonius is a clear example of a self-starting windmill. The PDF has a checkmark on Darrieus (Option 1), which is factually incorrect. Darrieus rotors are not self-starting. The question asks for a self-starting windmill, and Savonius (Option 2) is the correct answer based on aerodynamic principles. I will proceed with the factually correct answer.
Step 3: Final Answer:
The Savonius rotor wind mill is a self-starting design due to its high drag-based starting torque. The Darrieus rotor is not self-starting. Therefore, option (B) is the correct answer. (Note: The provided key in the image appears to be incorrect).