Function Of Speed Ratio Research Articles

A theoretical and experimental investigation of tri-helical gravure roll coating

Tri-helical, gravure roll coating, operated in reverse mode, is investigated via a complementary experimental and theoretical approach. Flow visualisation reveals the underlying flow structure within the roll-to-web transfer region and highlights when loss of stability, that is a propensity to streaking, occurs. The latter is found to be influenced in turn by the depth of the tri-helical grooves and the capillary number. Experiments show that as the web-to-roll speed ratio is increased, so too is fluid pick-out from the grooves, although the coated film thickness may decrease. Extensive data are provided for coated film thickness, pick-out and meniscus location as a function of speed ratio, for different operating parameters. A key feature of the present investigation is the formulation of a novel complementary mathematical model for the process. The base flow within the grooves is modelled as a Poisson equation in conjunction with simplified approximations for the attendant free-surface boundary conditions. The resultant equation set is written in terms of finite Fourier sine transforms and unique solutions obtained by varying the flux until a consistent pressure distribution throughout the coating bead is obtained. Results from the model are found to be in generally good agreement with their experimental counter-parts, and are seen to predict and capture all the major features of the process over the operating range explored.

Laminar-Turbulent Transition in Free Shear Layers

A STUDY has been performed to derive a threshold criterion for the existence of turbulence in a shear layer, and to see if such a threshold is useful in predicting transition onset in such a flow. This approach has already given satisfactory results in wake flows. As in the latter case, the present work is based on the existence of a minimum turbulence Reynolds number, and derives numerical results by utilizing established magnitudes of turbulence properties in the turbulent shear layer. Transition Reynolds numbers are predicted as a function of speed ratio, total temperature ratio, and Mach number jumps across the layer. New experiments are described in which free shear-layer transition was measured and, within an observed unit Reynolds-number effect, was found to parallel the predictions quite well.