Is the critical Reynolds number a universal constant?

Due to reports accumulated over several years about the observed differences of the critical Reynolds number for different pure gases, and even for liquids, we raise the question of the universality of the critical Reynolds number for the onset of turbulence. We summarize the results, and briefly discuss the physical idea which prompted these experiments in the first place. These experiments raise the fundamental question on whether the Navier-Stokes equation should be the sole and unique starting point of turbulence research.