Q $3($a$)$

Explain why the drag coefficient experienced by a cylinder in cross flow drops

suddenly when $R{e}_D>2{10}^5$ as illustrated in Fig. $3\mathrm{.}1$ by answering the following

questions:

Figure $3\mathrm{.}1$ Drag coefficient for cylinders and spheres in cross flow and their

dependence on the Reynolds number.

i$.$ Provide the definition of the drag coefficient and Reynolds number defining all

terms used. $[2$ Marks$]$

ii$.$ Plot the pressure distribution as a function of the angular position around the

cylinder starting at the forward stagnation point for the two cases $R{e}_D$

$2{100}^5$ and $Re{e}_D>2{10}^5.[2$ Marks$]$

iii. Assuming that drag is primarily due to the pressure on the cylinder surface,

conclude why the drag force drops when the Reynolds number changes $R{e}_D$

$2{10}^5$ to $R{e}_D>2{10}^5.[2$ Marks$]$