Bahmanpouri, Farhad (2019) Experimental study of air entrainment in a hydraulic jump on pebbled rough bed. [Tesi di dottorato]

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Tipologia del documento: Tesi di dottorato
Lingua: English
Titolo: Experimental study of air entrainment in a hydraulic jump on pebbled rough bed
Autori:
AutoreEmail
Bahmanpouri, Farhadfarhad.bahmanpouri@unina.it
Data: 10 Luglio 2019
Numero di pagine: 251
Istituzione: Università degli Studi di Napoli Federico II
Dipartimento: Ingegneria Civile, Edile e Ambientale
Dottorato: Ingegneria dei sistemi civili
Ciclo di dottorato: 31
Coordinatore del Corso di dottorato:
nomeemail
Papola, Andreapapola@unina.it
Tutor:
nomeemail
Carlo, Gualtieri[non definito]
Hubert, Chanson[non definito]
Data: 10 Luglio 2019
Numero di pagine: 251
Parole chiave: Hydraulic jump, Pebbled roughness, Two-Phase flow, Air entertainment, Turbulence
Settori scientifico-disciplinari del MIUR: Area 08 - Ingegneria civile e Architettura > ICAR/01 - Idraulica
Depositato il: 13 Giu 2019 15:42
Ultima modifica: 16 Giu 2020 09:47
URI: http://www.fedoa.unina.it/id/eprint/12732

Abstract

Hydraulic jumps occur when upstream supercritical flow suddenly transitions into downstream subcritical flows. Extremely turbulent flow of the hydraulic jump associated with the development of large-scale turbulence, surface waves and spray, air entrainment and energy dissipation. Hydraulic jumps are commonly taking place in natural waterways and hydraulic structures. Enhancement of energy dissipation, fluid mixing or flow re-aeration are the usage of hydraulic jumps. Research of hydraulic jumps yet is a challenge and to date study of turbulence and two-phase air-flow properties are mostly limited to hydraulic jumps in smooth rectangular channels. Past studies on uniformly distributed roughness focused on the identification of the conjugate depth relationship and mean velocities flow and few of them measured even the turbulence and air-water parameters. No past study was conducted to investigate the effect of non-uniform bed roughness on the air-water flows in hydraulic jumps. The present study systematically investigated the effects pebbled rough bed upon the basic flow patterns and air-water flow properties. The free-surface and air-water flow measurements were respectively recorded with the pointer gauge and intrusive phase-detection probes. Basic parameters of hydraulic jump including flow patterns and free-surface dynamics were carried out on smooth and rough bed configurations with two and one gate opening at upstream, respectively. The study of basic flow patterns covered a wide range of Froude numbers from 1.31 to 4.94 and Reynolds numbers from 4.2×104 to 2.3×105. The hydraulic jumps on the rough bed showed some characteristic flow patterns including a preaeration of the flow upstream of the hydraulic jump, an upwards shift of the jump roller resulting in a reduction of jump length, generation of larger eddies advecting downstream. An investigation of the fluctuations in impingement perimeter showed consistent statistical properties of longitudinally oscillating impingement positions across the central flow region on both bed types. Fluctuation of longitudinal water surface profile, exhibited the same trend on both bed types but a slightly higher standard deviation for smooth bed which was due to higher oscillation of water surface. Basic properties of air –water flow including void fraction, bubble count rate, interfacial velocity and turbulent properties including turbulence intensity, correlation time scales, advection and length scale were investigated on both rough and smooth bed configurations with the same gate opening. Experiments of two-phase flow were conducted for a range of discharges 0.06 ≤ Qw ≤ 0.1 m3/s, corresponding to upstream Froude numbers of 1.7 ≤ Fr1 ≤ 2.84 and to Reynolds numbers of 1.4 × 105 ≤ Re1 ≤ 2.2 × 105. The overall distributions of air-water flow properties were similar on both rough and smooth bed configurations. Comparative analysis highlighted some distinctive effects of the bed roughness including an upwards shift of the hydraulic jump and an increase in bubble count rate, maximum and mean void fractions, and turbulent intensity in the region close to the jump toe. The bubble-turbulence interplay induced the occurrence of bubble clustering, which was analysed on both rough and smooth bed configurations. The characteristic air-water time scale including particle chord time and length as well as properties including inter-particle arrival time, cluster count rate, cluster size and cluster proportion were investigated. Interparticle arrival time (IAT) analysis was also studied on both rough and smooth bed configurations. The result showed that near the jump toe, number of clustered bubbles was higher on rough bed which linked to big air entities and a higher rate of bubble concentration. The present thesis provides a description of the turbulent two-phase flow in hydraulic jump on pebbled rough bed. It is expected that this study would improve our knowledge of such a complex hydraulic process and address solid justification for future theoretical and numerical studies that have a long way ahead.

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