Q1: When no air is left below the nappe and water stream adheres to the down stream face of the weir, it is known as

ANS:C - clinging nappe

A "clinging nappe" refers to a specific condition observed in the flow of water over a weir or spillway. In this condition, the water stream adheres closely to the downstream face of the weir or spillway without separating or breaking away. When water flows over a weir, it forms what is known as a nappe, which is the thin sheet of water that flows over the crest of the weir. In the case of a clinging nappe, the water adheres closely to the downstream face of the weir, maintaining contact with the surface. The clinging nappe condition typically occurs under certain flow conditions, such as when the velocity of the water is high enough to prevent the formation of air cavities or air entrainment beneath the water surface. It may also occur in weirs with specific geometries or when the flow is well-controlled. The clinging nappe condition is desirable in some hydraulic structures because it ensures efficient flow over the weir without air entrainment or excessive turbulence. However, it may also indicate certain design considerations or flow conditions that need to be addressed to ensure the proper functioning of the hydraulic structure. Overall, the clinging nappe condition is characterized by the close adherence of the water stream to the downstream face of the weir, providing a smooth and efficient flow of water over the structure.