NEAR-FIELD MIXING OF PLANE BUOYANT-JET DISCHARGE ON THE SURFACE OF OPEN CHANNEL FLOW

摘要

Results of an experimental study on near-field mixing characteristics of a plane buoyant-jet discharge on the surface of an open channel flow are presented and discussed. The experiments were conducted in a 0.4 m wide, 10 m long flume, with a fresh water discharge. Salt-water was added, vertically and uniformly distributed in the channel width, to the fresh water flow, through a head box located on the water surface, at a section where the boundary layer was fully developed. Three components of flow velocity were measured in the near-field of the discharge by means of an ADV with a sampling frequency of 25 Hz. Mean local values of the flow density in the same region were measured by taking water samples with an hypodermic needle. The experimental conditions were parameterized in terms of length scales associated to the jet/plume and jet/cross-flow transitions. Different regimes were identified. A cross-flow dominant regime generates a flow structure with a low velocity and high density region near the free surface, which thickens linearly to cover the flow depth in an extension of about two flow depths downstream from the discharge, and with dilutions lower than 50% in the same region. A jet dominant regime generates a well-mixed region that extends vertically from the discharge throughout the flow depth, with dilutions close to 70%. Maximum flow velocities occur near the bottom. A plume dominant regime generates a situation that is intermediate to those already described, with an almost stagnant salt wedge at the bottom that extends upstream from the discharge, and a relatively well-mixed region downstream with dilutions over 80%.