Instabilities of Decaying Flow in a Rectangular Channel

摘要: Instabilities near the wall of a rectangular water channel in decaying freestream flow is investigated experimentally. A sliding piston imparts trapezoidal velocity variation (constant acceleration from rest, constant velocity, and deceleration to rest) onto channel. With onset deceleration, space invariant unsteady jet like profiles develop opposite direction as evident particle image velocimetry (PIV) measurements. Strong shear with reverse are observed phase after stops. Velocity at this stage found be highly inflectional. Dye visualization shows that outer layer becomes unstable resulting formation array vortices similar Kelvin–Helmholtz instability. Secondary sign evolve inner close due adverse pressure gradient induced by primary vortices. Vortex not below critical value Reynolds number. Due nature base flow, i.e., decreasing number, instability do break down turbulent flow. The wavelength ($$\lambda $$) scales average boundary-layer thickness $${\bar{\delta }}$$, being taken start appearance vortex. ratio $$\lambda /{{\bar{\delta }}}$$ $$\sim 2.6$$. vortex time convective scale 16/ (\overline{ \Delta u/ \delta })$$, where $$\Delta u = u_{max} - u_{min} $$ u_{max}, $$, $$\delta maximum minimum respectively any instant. Visualization transverse plane reveals vortical structures appear later times stream-wise indicating three dimensional