Features of the flow velocity and pressure gradient of an undular bore on a horizontal bed

Experimental results are presented regarding the free-surface elevations, velocity fields, and horizontal pressure gradients of dambreak-generated undular bores propagating over a horizontal bottom. Ultrasonic wave gauges, high-speed particle image velocimetry, and a flow visualization method are used to investigate the flow fields. Features of the full-depth velocity field and pressure gradient (equal to minus the sum of the local and convective accelerations) in the free stream over the boundary layer are elucidated with respect to the phase of the free-surface elevation and are categorized into four temporal stages. For stage I with rising free-surface elevations, a successive increase in the magnitude of the pressure gradient (with a negative value) corresponds to a favorable pressure gradient in the free stream. Around the zero-up/down-crossing phase of each leading wave evolving in stage II, the pressure gradient has a negative/positive maximum, revealing the maximum favorable/adverse pressure gradient in the free stream. However, the pressure gradient is zero at each crest or trough phase, showing an instantaneous zero pressure gradient. Within stage III characterized by a constant free-surface elevation, the pressure gradient is almost zero. In stage IV with descending free-surface elevations, the pressure gradient first increases from nearly zero to a positive maximum (representing the maximum adverse pressure gradient), then keeps this value for a period of time, and eventually decreases to zero. Subsequently, flow reversal with an increase in thickness over the bottom and free-stream velocity equal to zero takes place.