Doctor of Philosophy
Mechanical and Materials Engineering
Khayat, Roger E
The circular liquid jet impingement with subsequent hydraulic jump formation is studied theoretically using boundary-layer and thin-film approaches. Three different scenarios are tackled, namely, an accelerated Newtonian jet impinging on a stationary disk, a steady Newtonian jet impinging on a rotating disk, and a steady viscoelastic liquid jet impinging on a stationary disk. Accordingly, the effects of jet acceleration, gravity, centrifugal forces, and fluid elasticity on the flow behaviour and the jump are examined. The results are validated against numerical simulation and existing measurements. The findings show that the thickness of the boundary layer developing near impingement diminishes with jet acceleration but remains unaffected by centrifugal forces, aligning with previous findings in rotating boundary-layer flows. On the other hand, the film thickness increases with jet acceleration and disk rotation but decreases with fluid elasticity. For a jet velocity that increases linearly with time, the jump radius and conjugate depths increase over time, but their evolution is predominantly linear only at later times. The flow responds to the jet acceleration quasi-steadily near impingement but exhibits a long-term transient behaviour near the jump after the acceleration is halted. In the case of a rotating disk, two flow regimes are identified. The first regime corresponds to weak disk rotation characterized by the formation of a jump, and the second regime corresponds to dominant disk rotation characterized by the formation of a hump. In the jump regime, the flow transitions from predominantly azimuthal near the disk to predominantly radial towards the free surface, while in the hump regime, the flow maintains an azimuthal character around the hump. The vortex associated with the jump is found to diminish with increasing rotation, indicating a potential occurrence of a type 0 jump on a rotating disk. For the viscoelastic case, the radial flow across the jump is faster compared with the Newtonian case. In addition, the fluid elasticity yields an increase in the jump height and a decrease in its radius, resulting in the jump occurring in closer proximity to the impingement point.
Summary for Lay Audience
An impinging jet flow can be described as a stream of liquid hitting a solid surface. This kind of flow is widely used in many cooling and cleaning applications. A feature of this flow is that the liquid spreads out as a thin layer when it hits the surface and then suddenly gets deeper, which is known as a hydraulic jump. If the hydraulic jump appears as a ring, like when water from the faucet hits the kitchen sink, it is called a circular hydraulic jump. In general, the liquid moves quickly and is not very deep before the jump, while it slows down and gets deeper afterwards. The latter has a negative impact on the performance of practical applications. For this reason, researchers want to find out how these jumps occur in different situations.
This thesis theoretically investigates the behaviour of impinging flow and the circular hydraulic jumps in three scenarios. First, the speed of the liquid was continuously increased. In this case, the change in the circular size of the jump was tracked, showing that the jump gets bigger with time. Interestingly, when the liquid speed stopped increasing, the jump continued to expand with time. In the second situation, the amount of liquid coming out remained the same over time, but the surface it hits rotated. It was discovered that when the rotation is small, the jump is formed. But, when the rotation is large, the jump turns into a small bump which later disappears with increased rotation. In the last scenario, the surface did not rotate, and the liquid coming out remained the same. However, the liquid was changed; instead of water, a thicker liquid like paint was studied. Compared to water, it was found that the jump was smaller and occurred closer to where the liquid hits the surface. To explain all the scenarios, mathematical equations describing the physics of the problem were used. Subsequently, the results were compared to existing computer simulations and experiments, which matched well. Overall, this work has helped in filling a knowledge gap and will encourage more research in this field.
Baayoun, Abdelkader, "Interplay of Forces in Impinging Jet Flow and Circular Hydraulic Jump" (2023). Electronic Thesis and Dissertation Repository. 9656.