分野別セミナー

The rotating disk or von Karman boundary layer is the archetypical flow for the study of laminar-turbulent transition processes that occur naturally on the wings of aircraft and is often used to demonstrate laminar flow control
applications. This investigation concerns the novel approach of controlling linear disturbance development via a
time-periodic modulation of the otherwise steady disk rotation rate. The model is an example of a three-dimensional oscillatory boundary layer that forms by coupling a Stokes layer with the von Karman flow. Results of recent direct
numerical simulations are reviewed and illustrate the significant control benefits brought about by disk modulation. Both local and global forms of linear instability are stabilised by the application of time-periodic motion. In the
former instance a radial homogeneous flow approximation is modelled whereby the radial dependence of the base
flow is ignored. Under this assumption numerical simulations are performed at flow conditions corresponding to
convective and absolute instability. For each of these forms of linear instability time-periodic modulation establishes a significant reduction in the spatial and temporal growth rates associated with the disturbance leading to a delay in their onset. Greater control benefits are achieved as the modulation amplitude increases while the stabilising effect
is optimised for non-dimensional modulation frequencies near 10. For the genuine radially inhomogeneous base
flow comparable control benefits are again realised.