Question

Question 3-Concept Questions (10 points) a) What is the reason for having surface dimples (roughness) on a golf ball? Explain in 2-3 sentences. b) Draw the variation of lift and drag coefficients for a subsonic airfoil (at high Re number) over a Mach number range of 0.1-1.1. Explain the reason for trends Specify the primary drag mechanism for the following shapes in airstream at different Mach c) numbers. a. Symmetrical airfoil at an angle of attack of 4 degrees at M-0.3. b. Sphere at M-0.2 at Re number of one million. Sphere at M <<0.1 at Re-0.3 c. Flat plate at an angle of attack at M-2.5 d. Airplane in a high lift configuration (such as landing) e. What is the reason for using turbofan engines in modern commercial jets as opposed to the turbojet engines? Explain in 2-3 sentences. d) Define aerodynamic center and the center of pressure for an airfoil. e) f) What is the difference between static stability versus dynamic stability? 8) Explain the difference between the critical Mach number and divergence Mach number? Which one is larger?

Answer 1

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Solution: a) When an object moves through air with a certain velocity, aerodynamic pressure (drag) acts on it which reduces its velocity. The purpose of dimples is to create a thin turbulent boundary layer of air along the surface of the golf ball. This causes a smooth flow of incoming air along the dimpled surface and thus reduces the size of wake. In short, the drag force on the dimpled golf ball decreases as compared to the non-dimpled ball.