If U∞ = free stream velocity, u = velocity at y, and δ = boundary layer thickness, then in a boundary layer flow, the momentum thickness θ is given by
1. \(\theta = \mathop \smallint \limits_0^\delta \frac{u}{{{U_\infty }}}\left( {1 - \frac{u}{{{U_\infty }}}} \right)dy\)
2. \(\theta = \mathop \smallint \limits_0^\delta \frac{u}{{{U_\infty }}}\left( {1 - \frac{{{u^2}}}{{U_\infty ^2}}} \right)dy\)
3. \(\theta = \mathop \smallint \limits_0^\delta \frac{{{u^2}}}{{U_\infty ^2}}\left( {1 - \frac{u}{{{U_\infty }}}} \right)dy\)
4. \(\theta = \mathop \smallint \limits_0^\delta \left( {1 - \frac{u}{{{U_\infty }}}} \right)dy\)