Transient secondary currents behind a compression wave in an irregular channel

Abstract

A compression wave is an unsteady rapidly-varied open channel flow motion, characterised by an increase in water depth. A detailed investigation was conducted in a prismatic asymmetrical channel, to better understand the physical processes observed in tidal/tsunami-bore affected estuaries with irregular topography. The prismatic channel was equipped with a 1V:5H transverse slope across the full channel width. The free-surface data presented three-dimensional unsteady flow features, and the velocity measurements showed a drastic impact of the surge on the flow field, with strong three-dimensional features.With the arrival of the surge front, the transverse velocity component became large in the shallow-water section, indicating some unsteady secondary motion and recirculation during and shortly after the passage of the surge. The Reynolds shear stress data were associated with large turbulent stresses and shear stress fluctuations. For d 1 /D < 1, both velocity and shear stress data showed a transient longitudinal x-z shear plane, about y/W  0.5-0.6, with conjugate transverse flow reversal for 0.5-0.6 < y/W < 1. In the transient shear plane, high Reynolds stress magnitudes and shear stress fluctuations were observed behind the surge front, up to one to two orders of magnitude larger than boundary shear stress levels observed in steady flows in compound channels.

Compression waves in semi-circular channel

“…Although the current measurements were conducted on the channel centreline, the arrival of the compression wave induced a three-dimensional transient response of the velocity field. The response of the flow field to the surge may be analysed as an impulse problem (Lighthill 1978, Kiri et al 2020. Theoretical considerations would predict large transverse velocities, on both sides, in the shallow-water sections, with an unsteady secondary motion and flow recirculation next to the sides (Fig.…”