1. 10-2r t
2. number dependence of the strength of the wake component in a low Reynolds number turbulent boundary layer over a grooved surface (XI), shown by filled symbols, with smooth flat plate relationship due to Coles, shown by the solid line and, smooth flat plate measurements due to P u r t e l l a (1981) shown by open symbols. the wall but in virtually most of the boundary layer.
3. The fallowing observation may be made in Fig. 20. The frequencies fl and f2 of the resonant cavity flow in model 111 (Figs. 1-3) are also the frequencies of the upper and lower branches respectively of the neutral stability curve at the same measurement station. The suggestion is intriguing that several significant frequencies of B turbulent boundary layer can be accommodated in the framework of stability. It may be that a sub-harmonic breakdown is being observed. Or, in view of the linear nature of Stokes layer, it may be that ageneral periodic motion has been built up by summing several harmonic components (Batchelor 1967; p. 193). At this stage, however, it is not clear whether this coincidence is merely a happenstance or it has any physical significance.
4. The Stokes layer is a feature common to both a laminar and a turbulent boundary layer. It is interesting to note that the existence of the Stokes layer in the present turbulent flow means that a viscous wave (Schlichting 1979) exists near the wall. In the classical instahility work of Tollmien, the role of viscosity is considered i n the critical layer as well as i n the Stokes layer. A viscosity-induced phase change of n takes place across each layer (see Yorkovin (1986) for a discussion of the role of Stokes layer i n f l a t plate boundary layer instability processes), In addition to this pilase change, another attractive characteri s t i c of Stokes layer is tliat the diffusion of "orticity in a turbulent boundary layer away from the wall can be envisaged in its framework. In contemporarystudies on turhulence production processes i n turbrilent boundary layers, the Stokes layer is ignored by many. lbwever, this does not seem justified. Experiments a1.w need t o be conducted t o see if the Stokes layer i n a turbulent flow can be efficiently di.sturbed t o produce results of engineering importance (Raodyopndhyay 1986a).