Water at \(20{ }^{\circ} \mathrm{C}\) flows through a concentric annulus of inner diameter \(D_{1}=2.0 \mathrm{~cm}\) and outer diameter \(D_{2}=4.0 \mathrm{~cm}\). The surface roughness is \(0.002 \mathrm{~cm}\). Calculate the frictional pressure loss per unit length for a flow rate of \(0.01 \mathrm{~m}^{3} / \mathrm{s}\).