Roll waves create unstable and turbulent behaviour in channels and chutes and also change the behaviour of water entering the energy dissipater structures. In the present study, the free surface of water in the chute is simulated by numerical models. The roll waves’ occurrence is investigated in the numerical model for the discharge up to 30 m3/s, the longitudinal slope varying from 17 to 20% and the width of chute varying from 3.4 to 4.0 m. The ratios of the hydraulic depth to the chute width and also the minimum velocity to the maximum velocity are calculated along the chute length for different discharges, widths and longitudinal slopes of the chute. Results indicated that for the chutes in which the ratio of hydraulic depth to the chute width less than 0.16 the ratio of minimum to maximum velocity is less than 0.75, the formation of roll waves is inevitable.

Investigation on the formation of roll waves in chutes

John S. Latsis Foundation
Tima Foundation
Minas Kulukundis Trust
Muscular Dystrophy Association Hellas
Department of Applied Mathematics and Theoretical Physics
Queens' college, Cambridge
Disability Resource Centre, Cambridge
Engineering and Physical Sciences Research Council (EPSRC)

Rotating thin films : the vortex fluidic device

Slug flow is a two-phase fluid flow pattern, characterized by a series of liquid slugs dispersed by relatively large air bubbles. Air bubbles produced and trapped during the slug flow phenomenon co...

Computational investigation on the effects of rib on the slug flow phenomenon; using OpenFOAM

In channels with steep slopes, the flow is supercritical and the formation of roll waves is expected. In order to evaluate the flow behaviors in steep open channels, the Azad dam spillway is simulated numerically and its flow characteristics are compared with the physical model tests. In this study, the hydraulic variables including hydraulic depth and water surface profile are studied along the steep open channels. The k- ɛ turbulence model is considered in this study. It is shown that in the conditions in which the angle of the wave forehead is less than 35 degrees, the formation of roll waves is inevitable.

Analyzing the turbulent flow on steep open channels

Pulsating waves (also known as roll waves) might occur on the free surface of extreme events like mud and debris flows, among others, usually intensifying the caused damage. This technical note aims to inform about the roll wave phenomenon developing in a free-surface laminar flow, and analyze its generation criteria, centered on the concepts of Froude number and disturbance frequency. The complete linear stability analysis of the new depth-averaged model was proven a useful theoretical tool in determining new generation criteria for roll waves developing in non-Newtonian fluids. The results showed that the roll wave generation depends on two criteria: the first is associated to the minimum Froude number, and the second is related to the cut-off frequency. In addition, we have confirmed that the new generation criteria can be verified via numerical simulation based on a second model with full equations (Fluent software). Globally, the emergence of roll waves is favored by the non-Newtonian properties of the flowing fluid and the fact that the cut-off frequency decreases along with the minimum Froude number. Lastly, both generation criteria were tested in order to examine a case study involving the occurrence of roll waves in a watershed.

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Roll waves and their generation criteria

Uniform flow will become unstable when the channel slope is very steep. When this happens, the free surface will form a series of roll waves. This paper, using the Fluent software and applying the volume of fluid model to simulate free surface flow and the k – e turbulence model, analyzes conditions for roll wave formation in free surface flow with the aim of predicting their occurrence in chutes. This study describes how the type of cross-section of the chutes influences the formation of roll waves. Hydraulic characteristics of the flow in the physical model of the flood control structure for Azad Dam spillway, which consists of a side spillway, chute, and flip, are three-dimensionally simulated. Numerical predictions were compared against the experimental data and results show that there is a good agreement between numerical and experimental results. Results indicate that an increase in side slope tends towards flow stability and the slower formation of rolling waves. Also, the rolling waves are expected to be generated for sections of the chute when the difference between the maximum dynamic pressure and the minimum dynamic pressure in those sections is above 9,000 Pa.

Behrouz Aghebatie

Papers

Numerical analysis of roll waves in chutes

Investigation on the formation of roll waves in chutes

Computational investigation on the effects of rib on the slug flow phenomenon; using OpenFOAM

Analyzing the turbulent flow on steep open channels

Computational modeling on slug flow phenomenon in inverted siphon and improving its performance