ANS:B - velocity gradient is zero at the solid surface.

When a fluid flows over a solid surface, the statement that best describes the situation is: velocity is not zero at the solid surface. Here’s why:

• Velocity Profile: In fluid dynamics, when a fluid flows over a solid surface (such as flow in a pipe or over a plate), the velocity of the fluid at the solid surface is not zero. The fluid adheres to the surface due to viscosity and exhibits a phenomenon known as the "no-slip condition," where the fluid velocity at the surface matches the velocity of the surface itself.
• No-Slip Condition: According to the no-slip condition, the fluid directly at the solid surface moves with the same velocity as the surface, assuming the surface is stationary or moving at a different velocity than the fluid. This means the velocity of the fluid is not zero at the solid surface; rather, it matches the surface velocity due to frictional effects.
• Velocity Gradient: There is a velocity gradient within the fluid layer adjacent to the solid surface. This gradient starts from zero at the surface (due to the no-slip condition) and increases further into the fluid.
• Resistance and Layers: The resistance between the solid surface and the fluid is actually higher compared to the resistance between fluid layers themselves, especially near the surface where viscosity plays a significant role.