Experiments have shown that the transition from laminar to turbulent conditions for flow normal to the axis of a long cylinder occurs at a critical Reynolds number of ReD,c = 2 X 105, where D is the cylinder diameter. Moreover, the transition from incompressible to com- pressible flow occurs at a critical Mach number ofMac = 0.3. For air at a pressure of p = 1 atm and tem- perature T = 27°C, determine the critical cylinder diameter Dc below which, if the flow is turbulent, com- pressibility effects are likely to be important.