ME 363 - Fluid Mechanics
Homework #5
Due October 10, 2007
Fall Semester 2007
1] When an elongated cylinder is placed in a uniform flow, a broad, low-velocity wake develops downstream,
idealized as a V-shape. Pressures P1 and P2 are approximately equal. If the flow is 2-dimensional and
incompressible, with a width b into the page, derive a formula for the drag force F on the cylinder. Rewrite your
result in the form of a dimensionless drag coefficient defined as CD = drag force ÷ ρU2bL.
2] The model car weighs 17 N and is to be accelerated from rest by a 1-cm-diameter water jet moving at 75 m/s.
Neglecting air drag and wheel friction, estimate the velocity of the car after it has moved forward 1 m.
3] The boat is jet-propelled by a pump that develops a volume flow rate Q and ejects water out the stern at velocity
Vj. If the boat drag force is F = k*V2, where k is a constant, develop a formula for the steady forward speed V of
the boat.
Problem #1
Problem #2
Problem #3
Problem 1 solution (slightly different version, note the dimension L in this problem is 2L in your problem, the C D
doesn’t change but the FD is double)
Problem 2 solution:
for more details on the math, see the following page.
Problem 3 solution: