Erosion and Air Entrainment by Stationary and Translating Circular Jets
Creators: Amin, Mahmud R
Publisher: name: University of Alberta Library
Year: 2023

<p>Circular turbulent jets are essential in many engineering applications and are thoroughly studied. Most of these studies involve stationary jets. Contemporary studies lack the understanding of translating jets partly because the experimental setup is complicated, and computational models cannot simulate air entrainment and scour by translating jets reliably. This thesis presents some phenomena related to translating turbulent circular jets: scouring sand beds, air entrainment in a quiescent waterbody, and impingement on a flat wall. These phenomena of translating jets were compared against stationary jets. However, there is a research gap in the scouring of sand beds by short impinging circular jets. Therefore, an experimental study of stationary short-impinging jet scouring was also conducted.<br>
The scouring of sand beds by stationary jets is crucial because the failure of hydraulic structures is often related to the downstream scouring of these structures. Many studies have been conducted previously on the scour of sand beds by circular jet of long impinging height of more than 8.3 times the jet diameter. The potential core of such a jet is completely diffused before it impinges on the bed. However, shallow waterbodies are often subjected to scouring by short jets of impinging height 5.5 times the jet diameter. Short impinging height is essential because it ensures a uniform jet velocity and strong impingement. Laboratory experiments were conducted with short axisymmetric jets and sands with mean diameters of 0.54 and 1.1 mm. Semi-empirical prediction equations for the ultimate scour hole and temporal development of the unsteady scour hole was developed.<br>
Practical applications like the fluidization of sand beds for dredging and clam collection, sewer cleaning, and movement of water vessels in shallow waterbodies involve sand beds scouring with translating jets. Therefore, experiments were conducted in a towing tank with a submerged sand bed and circular turbulent jets of different translating velocities to assess the erosion of sand beds under varied hydraulic conditions. A half-model configuration was used with a high-speed camera mounted at the side of the tank to capture sand movement using the Particle Image Velocimetry (PIV) technique. PIV analysis revealed the dynamics of the sand movement and properties of the dynamic scour by translating jets.<br>
Air entrainment by jets is useful in many industrial applications. Many studies are available on air entrainment by stationary jets. However, the mechanism of air-entrainment by a translating jet could be different. An experimental study was conducted with translating turbulent circular jets impinging in a quiescent water pool to study the air entrainment mechanism and bubble characteristics. Shadowgraph images taken with a high-speed camera were used for analysis. It was found that the maximum penetration depth of the bubbles is a function of the jet translating speed and the Capillary number of the air-water interface. The spatial distribution of the bubbles in the bubble swarm cross-section shows Gaussian distribution. However, the terminal velocity of the bubbles shows no noticeable effect of the jet translation.<br><br>
The impingement of jets is a significant engineering problem. Such problems include jet issuing from hydraulic outlets, the vertical takeoff of space-rovers, and spraying devices. A translating jet impingement adds more complexities to the stationary jet impingement problem. A submerged translating jet is typically subjected to the relative crossflow and the relative drag of the impingement surface. Therefore, laboratory experiments and numerical simulations were conducted to understand the complex interaction between the translating jet and the impingement surface. The experiments were performed inside a towing tank with a 19 mm diameter translating jet. The PIV technique was used to capture the velocity field. Computational Fluid Dynamics (CFD) models were developed for translating jet velocities ranging between 0.03 and 0.09 m/s in OpenFOAM software. The CFD results were compared against the PIV experiments. In addition to the velocity field, the CFD models provided the distribution of the wall pressure and shear stress in the impingement region and the effect of different translating velocities.<br>
In summary, this thesis presents experimental and computational studies related to stationary and translating circular turbulent jets. Practical applications of this work include but are not limited to water injection dredging, submarine cable trenching, shellfish collection, sewer cleaning, and scouring by moving water vessels and space rovers.</p>