Plane turbulent buoyant surface jets and jumps

The paper reports on measurements of the flow generated by a plane buoyant jet discharging vertically into shallow water. The study comprises visualization experiments, mean-velocity and turbulence measurements with a two-channel laser-Doppler anemometer and temperature measurements with thermistor probes. According to the previous investigation of Jirka & Harleman (1979) (JH) the flow may be either stable with the heated discharge water leaving the near field in a warm water layer adjacent to the surface, or unstable with flow recirculation and re-entrainment of heated water into the jet. The stable situation usually involves an internal hydraulic jump associated with a roller. Both stable and unstable situations were investigated, the limiting case of a non-buoyant jet representing the unstable one. In order that a roller representing an internal hydraulic jump developed in the relatively short test channel in the buoyant situations, a strong downstream control had to be imposed by inserting a slightly submerged weir. Most experiments were carried out at a depth-to-discharge-width ratio of 100, and in this case the strong upstream control caused the hydraulic jump to be flooded for both of the densimetric Froude numbers studied (F = 9.9 and 21). In each case, a thick upper layer of nearly uniform temperature developed, with a larger layer thickness for F = 21. Below this layer was a relatively thin interface with temperature gradients and below this a counterflow of cold ambient water. For both Froude numbers, the flow was stable in the sense of JH, but only marginally so in the higher-Froude-number case. The observed trends of the flow behaviour follow the stability analysis of JH, but the dilution of the heated water, which was determined from the temperature measurements, is different from that predicted by the JH mixing analysis. The dilution is much lower in the present case with the flooded jump than in the JH analysis and experiments without specific downstream control and with a much longer test channel and thus no flooded jump.

Section: T ("C)supporting

confidence: 91%

Section: General Flow Characteristicsmentioning

confidence: 99%

Experiments on vertical plane buoyant jets in shallow water

Andreopoulos¹,

Praturi²,

Rodi³

1986

“…Stefan & Hayakawa 1972); there have been numerous studies similar to Yih & Guha (1955) where inflows were discharged horizontally into stationary deep ambients either as buoyant surface jets (e.g. Wilkinson & Wood 1971;Stefan & Hayakawa 1972;Rajaratnam & Subramanyan 1985;Arita, Jirka & Tamai 1986), or as negatively buoyant bottom jets (e.g. Rajaratnam & Subramanyan 1986;Baddour 1987;Barahmand & Shamsai 2010).…”

Section: Introductionmentioning

confidence: 99%

Stationary internal hydraulic jumps

Lawrence

Armi

2022

This is a theoretical and laboratory study of stationary internal hydraulic jumps. These jumps are rapid transitions between internally supercritical flow, generated by placing a sill on the bed of a horizontal rectangular channel, and internally subcritical flow, generated by installing a downstream contraction. This contraction generates an approximately uniform flow downstream of the jump; thus mimicking barotropically driven two-layer flows, as found in tidally driven flows over underwater sills, and flows over mountain ranges driven by large-scale pressure gradients. Upstream of the jump a train of Kelvin–Helmholtz billows forms on the interface between the layers. Upper layer fluid is entrained into these billows, which are subsequently advected into the lower portion of the jump. These billows are broken down by the turbulence of the jump, and the entrained upper layer fluid is mixed with lower layer fluid. Downstream of the jump the upper layer remains homogeneous, the density step at the interface is weakened, the upper portion of the lower layer is approximately linearly stratified, and the lower portion of the lower layer is undisturbed. This altered density profile is the downstream conjugate state of the jump. When the contraction is narrowed the jump moves upstream and ‘drowns’ part of the train of billows, reducing the amount of entrainment. Thus, while the jump is responsible for mixing fluid from the upper layer into the lower layer, it is the position of the jump relative to the upstream train of billows that determines the amount of entrainment.

“…Investigations of turbulent jet flows near free surfaces have, until recently, been concerned primarily with flows of relevance to civil and hydraulic engineering, where the effects of buoyancy are often significant. investigated the scaling behaviour of the mean flow field of turbulent, non-buoyant surface jets, and Rajaratnam & Subramanyan (1985) investigated that of planar buoyant surface jets. Swean et al (1989) report measurements of mean velocities and turbulent fluctuations in a two-dimensional turbulent jet issuing a t a free surface.…”

Section: Introductionmentioning

confidence: 99%

Turbulence measurements in a round jet beneath a free surface

Anthony¹,

Willmarth

1992

The results of an experimental investigation of a turbulent jet flow issuing from a circular nozzle beneath and parallel to a free surface are presented. Measurements of the mean velocity vector and all components of the Reynolds stress tensor were made using a three-component, underwater laser-Doppler velocimeter (LDV). Visualizations of the flow field using both fluorescent dye and free-surface shadowgraphs were made in support of the measurements. The jet is observed to form a shallow surface current, the lateral extent of which is significantly greater than that of the primary jet flow that produces it. Flow visualization reveals the surface current to consist largely of fluid structures ejected from the jet. These structures remain coherent within the current, apparently as a consequence of reduced turbulent mixing just beneath the surface. LDV measurements of the turbulence within the surface current reveal that near the jet centreline, where the interaction between the jet and the free surface is most intense, the velocity fluctuations normal to the free surface are diminished in approaching the surface, while the fluctuations parallel to the surface are enhanced. Away from the jet centreline, toward the edges of the surface current, the vertical and cross-stream fluctuations become approximately equal in magnitude, whereas the streamwise fluctuations become diminished. This is attributed, in part, to orbital motions beneath surface waves generated by the interaction of large-scale jet structures with the surface. When oleyl alcohol, an insoluble surface-active agent, was added to the free surface, the surface current was not observed. Comparisons between a jet issuing beneath both clean and surfactant-contaminated free surfaces, and a jet issuing beneath a solid wall are made to identify the role of streamwise vorticity and of secondary vorticity generated at boundaries on the development of these flows.