Assignment:
a) Calculate the power required to overcome the skin friction drag on the keel of a sailing dingy moving through water at 2m/s. The keel projects 0.8m into the water and it is 0.4m wide. Assume that the keel is a smooth flat rectangular plate and that transition from laminar to turbulent flow occurs instantaneously at Re = 4*10^5.
For water: v = 10^-6 m^2 / s p = 1000kg/m^3
b) In reality, the keel in (a) is not a thin flat plate but a slim ellipse with C_D = 0.1 for turbulent flow and a frontal area of 0.04m^2. Assuming that the entire boundary layer is turbulent, determine the contributions of each component of drag (skin friction drag and pressure drag) to the total drag force and explain your result. You may take the skin friction area to be the same as in (a).
The velocity profile for turbulent flow in a pipe is well described by the relationship:
u/u_max = (y/R)^(1/n)
n varies according to the relationship:
Re n
4*10^3 6
2.3*10^4 6.6
1.1*10^5 7
1.1*10^6 8.8
2*10^6 10
3.2*10^6 10
Demonstrate that this profile cannot apply all the way to the wall. Do not describe the actual flow in the near-wall region; merely consider the mathematics of the above equation.