Graham I. Duck

Effect of a Flexible Wall on a Reattaching Turbulent Shear Layer

Digital particle image velocimetry was applied to investigate turbulent flow of air between a flexible wall and a rigid surface containing a backward-facing step (BFS). The inflow condition corresponded to a Coanda jet issuing from a nozzle that was located upstream of the BFS. The flexible wall was represented by a sheet of paper under tension that was positioned above the BFS. Two additional configurations, which involved the BFS without the flexible wall and the BFS in proximity to an inclined rigid upper wall, were considered in this study. In all three cases, the flow fields were characterized in terms of patterns of time-averaged velocity, out-of-plane vorticity, streamline topology and turbulence statistics. High-speed photography and unsteady pressure measurements were employed to characterize the flow-induced deformation of the flexible wall and the flow oscillations. The profile of the paper sheet could be approximated by linear segments, which, in conjunction with the rigid surface that contained the BFS, formed a diverging channel configuration. Confinement of the incoming flow by the flexible wall delayed flow reattachment to the rigid bottom surface downstream of the BFS. Patterns of turbulence statistics in the presence of the flexible wall shared qualitative similarity with the corresponding parameters of diverging channel flows as well as classical Couette-Poiseuille flows.

Two-sided laser device for online paper caliper measurement and control

Strong demand exists for a non-contacting paper caliper measurement which can be used as an input to a paper thickness control system. Caliper sensors requiring sheet contact suffer from errors related to dirt or coating build up and from high maintenance costs related to wear. These sensors can also damage the product by picking holes and marking sheets. Details of an on-line measurement device which employs two opposed laser displacement sensors and an inductive displacement sensor are presented. The sheet is held perpendicularly to the sensors with a Coanda air clamp. Dust and temperature control features which enable the sensor to operate reliably in an industrial environment are discussed. Results of production trials of this sensor are presented. Sub-micron profile agreement to lab and contacting caliper measurements has been demonstrated on light sheets. Results are presented of measurements on a wide range of paper grades from coated and uncoated light sheets to coated board.

Experimental Investigation of Flow Over a Backward-Facing Step in Proximity to a Flexible Wall

Turbulent flow between a flexible wall and a solid surface containing a backward-facing step (BFS) was investigated using digital particle image velocimetry (PIV) and high-speed photography. Motivation for this study stems from paper manufacturing industry, where high-speed, wall-bounded jets of air are employed in air clamp devices to precisely position moving sheets of paper at the specified locations with respect to other equipment. In the current investigation, stationary sheet of paper under tension was positioned in proximity to the BFS. The incoming air flow emerged from a planar nozzle that was located in the solid wall upstream of the BFS. Curvature of the nozzle wall resulted in a favorable pressure gradient condition. As a result, the flow upstream of the BFS was attached to the solid wall due to the Coanda effect. Flows corresponding to two values of the Reynolds number (3000 and 3600) based on the step height and the maximum flow velocity at the step location were characterized in terms of patterns of instantaneous and time-averaged velocity, out-of-plane vorticity, streamline topology, and turbulence statistics. In addition, paper sheet oscillation was characterized using high-speed photography. Frequencies of the natural vibration modes of the flexible wall (paper sheet) were well separated from the hydrodynamic frequencies corresponding to the oscillations of the shear layer downstream of the BFS, which resulted in the absence of resonance in the system and low characteristic amplitudes of the paper sheet oscillation. In the time-averaged sense, interaction between the separated flow downstream of the BFS and the flexible wall (paper sheet) resulted in deformation of the paper sheet and formation of diverging channel geometry between the sheet and the solid wall. As the inflow velocity increased, the paper sheet was pulled closer to the surface of the air clamp, which resulted in increased confinement of the incoming Coanda jet. The flow reattachment length calculated on the basis of time-averaged flow patterns increased with the increasing Reynolds number.