An incompressible fluid flows between a pair of infinite plates separated by a distance $L$. The top plate is moving with a constant velocity $U$, whereas the bottom plate is stationary, as shown in the figure. The difference of the stream functions $\left(\psi_{T}-\psi_{B}\right)$ at the two plates for a laminar and fully-developed flow is equal to
- $\dfrac{U L}{2}$
- $U L$
- $2 U L$
- $4 U L$
