Question

4. ( 10 pts) Chapter 9 External Flows The fixed keel of the sailboat is about 38 in. deep. The boat moves at a speed of 3 knots (i.e. 5.1 fts) in a lake with water temperature of 40°F. The keel can be approximated as a rectangular flat plate with an average length in flow direction /=(34+15)/2 = 24.5 in, and a depth of 38 inches. (Considering a laminar BL to turbulent BL transition happens at Rev = 2x70' in this situation.) 1) (3 pts) What is the Reynolds number Re, for flow over the plate? 2) (3 pts) What is the coefficient of drag for the plate, Cos? 3) (3 pts) What is the friction drag force on the keel? (notice there are two sides) 4) (1 pt) If the downstream keel flow BL transitions into turbulent BL. what is the effect of this on the overall skin friction drag? h

Answer 1

8.3 14 Giveni t=24.Sin=2.042 ft. T= 40€ (vs 1.664x10-5 It's).(3=1.939 Sluys I ft33. V-s.Iftis d. 38 in : 3.167 ft. (1) Reynold's no: (Ree) 5.1 X 2. 0 42 ve 6.258 Xias Ree o Ans:) V 1.664x1.5 since Ree - 2x10 to Laminar frow thoughout (2) Tof .. for caminar flow over flat platea 1.33 1.33 cos 5 1. 6812 X10-3 12 (Reel (6. 2588135 )l2 (3) Frictional dragi {A = 2x1 d .. (both sides) Fo = 1 / EQT AV? Fo = 'x16812813–81.939x12.934 = 2X 2.042 X 3.167 A:12.934 Ft?? x 5.12 Fo=0.5483 ebf D Ans )

increases (4) when flow transition takes place the drag force due to high turbulance & effect of surface roughness.