Copyright (2009) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Horton, NA & Pokrajac, D 2009, 'Onset of turbulence in a regular porous medium: An experimental study', Physics of Fluids, vol 21, no. 4. and may be found at http://pof.aip.org/resource/1/phfle6/v21/i4/p045104_s1