Experimental verification of Type-II-eigenmode destabilization in the boundary layer over a compliant rotating disk

Destabilization of the Type-II eigenmode in boundary layers over compliant rotating disks was predicted theoretically by Cooper and Carpenter [J. Fluid Mech. 350, 231 (1997)]. Their results showed that for relatively low levels of compliance the Type-II eigenmode was destabilized, to be stabilized and ultimately eliminated for higher levels of compliance. The goal of the present study was to obtain the first experimental verification of the prediction that the Type-II mode can be destabilized at low levels of compliance. To this end a new type of rotating-disk apparatus was designed and a new type of material was used to produce suitable compliant walls for the experiments. Background noise in the new facility is substantially reduced in comparison with that in facilities used in related previous studies. This enabled the detection of substantially cleaner hot-film signals. Although the mean base flow remained unchanged, noise characteristics have been improved and turbulence intensities are significantly reduced. The measurements reveal not only the comparatively strong signals from the Type-I (cross-flow vortices) instability mode but also clear evidence of the Type-II eigenmode. In agreement with the theory of Cooper and Carpenter the data analysis shows that relatively low levels of wall compliance destabilize the Type-II mode.