In flow past a body or wall, early transition to turbulence can be induced by placing a trip wire on the wall across the flow, as in Fig. P6.5. If the trip wire in Fig. P6.5 is placed where the local velocity is U, it will trigger turbulence if Ud/ν=850, where d is the wire diameter [3, p. 388]. If the sphere diameter is 20 cm and transition is observed at ReD=90,000, what is the diameter of the trip wire in mm?