D p = ∮ p n ⋅ i d S , Y = ∮ p n ⋅ j d S . displaystyle get started aligned D_ p &=oint pmathbf n cdot mathbf i ;mathrm d S,Y&=oint pmathbf n cdot mathbf j ;mathrm d S.end aligned By using the streamwise vector i parallel to the freestream rather than k inside the integral, we acquire an expression with the strain drag Dp (which includes