Hydrodynamics of paper making: Streamwise vortices generated upstream of a two-dimensional jet nozzle

The hydrodynamic stability of initially laminar flow in a 2D jet was investigated. The motivation for this study was understanding the effects of jet geometry and turbulence in the papermaking process. Utilizing the digital particle image velocimetry (DPIV) technique, eight experimental cases were conducted in a 2D jet.{09}The jet exit had a vertical blockage as much as 50% of the jet height. Two grid size screens, 0.73 x 0.73cm2 and 1.11 x 1.11 cm2, were used as turbulence generators. The Reynolds number range, based on the momentum thickness and the free stream velocity $1-2ms$, varied from 960 to 5500. Streamwise vortices occurred due to shear induced by the presence of the blockage. The size of the vortices was 0.5–1 cm, which is nearly proportional to the streak spacing found in a sheet of paper. With the presence of a turbulence generator, streamwise vortices still exist. The 0.73 x 0.73cm2 size turbulence generator had a greater effect on reducing the strength of the streamwise vortices than the 1.11 x 1.11cm2 size turbulence generator. The size of a mean spanwise vortex, located at the corner formed by the upper wall and the blockage, was found to be one half the length of the blockage. The study suggests that the streamwise vortices do not result from the streaks in the turbulent boundary layer, nor from the Gortler vortices. The spanwise and streamwise vortices are likely the product of inflexional instability.