A wind tunnel is 6m long and has a cross section of 1m^2. Air flows through at 70m/s at standard sea-level conditions (density = 1.225 kg/m^3, mu = 1.7894E-05). At what angle should the walls diverge to maintain a constant velocity between x=1.5m and x=6m? This is needed to compensate for the boundary layer displacement thickness.