Oblique non-buoyant turbulent jets in a moving ambient flow

Abstract

Extensive experimental investigations have been carried out to research the behavior of non-buoyant and buoyant jets released into a stagnant or moving ambient flow. The experimental data can be used to verify the theoretical mathematical model based on the physical conservation laws, which can be employed in the design of engineering structures built for wastewater disposal through an ocean fall system. However, there is still limited experimental data for non-buoyant jets released into a moving ambient flow that are discharged at oblique initial angles to the ambient fluid, hence the confidence in the predictions of the mathematical model under these specific circumstances is limited. As the core objective of the current study, an experimental investigation is conducted into the mean behavior of the non-buoyant jet with oblique initial discharge angles to the ambient flow, and the relationship of the mean behavior with the initial discharge angle. The jet flow behavior performs differently in the different flow regions, as the jet flow is either weakly or strongly advected by the ambient flow. In addition, it is necessary to investigate the transition location between the weakly and strongly advected regions. A light attenuation technique is applied for the investigation of non-buoyant jets into a moving ambient flow, mainly because it enables the behavior of the jet flow with two-dimensional trajectory to be obtained with relatively simplicity. The technique only provides the integrated views of the jet flow. The integrated concentration data field is interpreted by assuming the mean cross-sectional concentration distributions, which is either the single-Gaussian in the weakly advected flow region or double-Gaussian (two merged stretched Gaussians) in the strongly advected flow region. Experiments are carried out with initial discharge angles from 10° to 90° and the experimental results show that the cross-sectional concentration structure in the strongly advected region gradually changes from the weak jet to momentum puff, characterized by increasing parameters involved in the double Gaussians assumptions. The spread in the weakly and strong advected regions varies linearly with the vertical distance away from the source for all initial discharge angles. The experimental data is used to obtain new spread coefficients as a function of the discharge angle, determine the shape parameters involved in the double Gaussian assumption, and find out the location of the transition region, described by the start and end point of transition region ( STR and ETR ). A previous numerical model, the Momentum Model, is updated to assist in the prediction of the flow behavior of non-buoyant jet with oblique initial discharge angles to the ambient fluid. Unlike the previous Momentum Model, the determination of the theoretical point of transition from the weakly to the strongly advected flow region, denoted as PTR , is based on the optimal experimental spread data fitting results rather than the relative magnitudes of the initial excess momentum flux and the ambient momentum flux. The experimental results indicate that the value of PTR in terms of the non-dimensional vertical distance, increases up to 45° and then decreases with increasing the initial discharge angle. Another significant difference between the newly updated and the original Momentum Model, is the modification used to deal with the transition angle. The current study applies the general double-Gaussian assumption and new experimentally determined spread function to simulate the jet flow behavior in the strongly advected region so that the complexity of the transition angle problem in the original Momentum Model can be avoided. Finally, predictions from the updated Momentum Model (trajectory and dilution results) are compared with data from the current and previous experimental investigations. It shows that the new predictions are consistent with the experimental data for a wide variety of the non-buoyant jet configurations with different initial discharge angle to the ambient flow. However, the updated Momentum Model still lacks reasonable predictions for the transition behavior, which is mainly because the cross-sectional concentration structure and spread behavior in the transition region are not modeled separately in the Momentum Model.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology