Flow patterns and draining films created by horizontal and inclined coherent water jets impinging on vertical walls

Abstract

The flow patterns created by coherent water jets created by solid stream nozzles impinging on vertical polymethylmethacrylate (Perspex) and glass surfaces were studied for nozzles with diameters 2–4mm at angles up to ±45° from the horizontal. The flow rates studied ranged from 7.1 to 133gs−1 (26–480Lh−1; jet velocities 2.6–10.6ms−1). The width and height of the film jump marking the limit of the radial flow zone were compared with models based on that developed by Wilson et al. (2011), modified to include the effect of gravity and the angle of inclination for non-horizontal jets (incorporating the flow distribution model reported by Kate et al. (2007. Journal of Fluid Mechanics 573, 247–263)). The location of the film jump and the flow pattern around the impingement point were sensitive to the nature of the substrate at low flow rates, but insensitive to substrate nature at higher flow rates. The models predicted the film jump location with reasonable accuracy, and the width of the wetted region at the mid-plane was found to follow a simple relationship to the film jump width there. A first-order model for the width of the rope of liquid draining around the film jump gave a lower bound estimate of this dimension. The falling film generated below the impingement point exhibited three forms of behaviour: a wide film, termed gravity flow; a narrowing film, termed rivulet flow, and a wide film which split into two with the formation of a dry patch. The transition to form a dry patch was found to obey the minimum wetting rate criterion reported by Hartley and Murgatroyd (1964), once loss of liquid due to splashback was accounted for. Dry patch formation within the falling film was only observed with upwardly impinging jets, and the tendency to form dry patches was predicted with some success by a simple two-stream model.