To probe whether increasing friction with the walls will promote or degrade the instability, we decided to study the effect of varying the wall roughness of the tube on the exponential growth rate of h More specifically,Darias et al. (2011)and Sánchez et al. (2009)found that the exponential growth rate of
h increased almost lin-early with the amplitude of oscillation, for experiments performed at frequencies between 7.5 and 12 Hz. Our goal was to test this linear dependence, with tubes of different wall roughnesses. We will also test a model proposed bySánchez et al. (2009), to see if it can be applied to cases with large wall roughness.