11-1C Explain when an external flow is two-dimensional, three-dimensional, and axisymmetric. What type of flow is the flow of air over a car? 11-2C What is the difference between the upstream veloc- ity and the free-stream velocity? For what types of flow are these two velocities equal to each other? 11-3C What is the difference between streamlined and blunt bodies? Is a tennis ball a streamlined or blunt body? 11-4C Name some applications in which a large drag is desired 11-5C What is drag? What causes it? Why do we usually try to minimize it? 11-6C What is lift? What causes it? Does wall shear con tribute to the lift? 11-7C During flow over a given body, the drag force, the upstream velocity, and the fluid density are measured. Explain how you would determine the drag coefficient. What area would you use in the calculations? 11-8C During flow over a given slender body such as a wing, the lift force, the upstream velocity, and the fluid den- sity are measured. Explain how you would determine the lift coefficient. What area would you use in the calculations? 11-9C Define the frontal area of a body subjected to exter- nal flow. When is it appropriate to use the frontal area in drag and lift calculations? 11-10C Define the planform area of a body subjected to external flow. When is it appropriate to use the planform area in drag and lift calculations? 11-110 What is terminal velocity? How is it determined? 11-12C What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils? 11-13C What is the effect of surface roughness on the fric- tion drag coefficient in laminar and turbulent flows? 11-14C In general, how does the drag coefficient vary with the Reynolds number at (a) low and moderate Reynolds num- bers and (b) at high Reynolds numbers (Re > 10)