Clustered, Stacked and Imbricated Large Coastal Rock Clasts on Ludao Island, Southeast Taiwan, and Their Application to Palaeotyphoon Intensity Assessment

This work investigated the characteristics of a boulder field on the exposed south east coast of Ludao Island (Green Island) in southern Taiwan. Although the region regularly experiences seasonal Pacific typhoons, fieldwork on Ludao was prompted following the double-strike of Typhoon Tembin in August 2012, which followed an unusual looping track and was one of the strongest storms to affect the island in recent decades. In Wen Cuen Bay, large limestone and volcanic clasts (103–105 kg) occur both as isolated individuals and also grouped into distinct clusters across the gently-sloping emerged reef platform of Holocene age. Some individuals reach megaclast proportions. Observations revealed limited evidence for the production of new coastal boulders by Typhoon Tembin. However, clustering, stacking and notable imbrication of old large clasts provide evidence for multiple high-energy palaeoevents. Stacking and imbrication are significant depositional features, implying that (partial) lifting by wave transport was responsible. Boulders deposited by Typhoon Tembin suggest that storm produced minimum flow velocities of 3.2–5.1 m/s. This range of minimum flow velocity (MFV) values is lower than the 4.3–13.8 m/s range inferred from the pre-Tembin boulders, which indicates that older storm washovers must have been stronger, judging from their ability to stack and imbricate large clasts. One explanation for high upper values of palaeoevent MFVs is that localized funnelling of water flow through narrow relict channels (inherited spur-and-groove morphology, oriented perpendicular to the modern reef edge) concentrates onshore flow energy into powerful confined jets. Support for this hypothesis is the positioning and train-of-direction of the main imbricated boulder cluster at the landward head of one such feature. Geomorphic controls amplifying wave-driven flow velocities across the emerged Holocene reef mean that a palaeotyphoon origin is sufficient for explaining large clast stacking and imbrication, without the need to invoke a tsunami hypothesis.

Assessment of Different Turbulence Models on Simulations of Confined Jets in a Crossflow at Supercritical Pressure

Numerical investigations of using two different turbulence models of K-ε and K-ω on mixing characteristics of two confined jets in a crossflow at supercritical pressure have been performed. The confined jets were at 180 degrees from each other injecting into a round tube. The jet to crossflow mass flows ratio, r, was 2.96. Reynolds Averaged Navier Stokes (RANS) equations were solved using Siemens PLM CCM+ software. Results indicate higher mixing rate with K-ω turbulence model. Higher vorticity and lower turbulent kinetic energy are observed with k-ω turbulence model. Increased mixing indicate reduced velocity and pressure gradients and cooler fluid toward the tube wall.

Shock transformation and hysteresis in underexpanded confined jets

This study investigates the shock transformation in an underexpanded jet in a confined duct when the jet total pressure is increased. Experimental study reveals that the Mach reflection (MR) in the fully underexpanded jet transforms to a regular reflection (RR) at a certain jet total pressure. It is observed that neither the incident shock angle nor the upstream Mach number varies during the MR–RR shock transformation. This is in contradiction to the classical MR–RR transformations in internal flow over wedges and in underexpanded open jets. This transformation is found to be a total pressure variation induced transformation, which is a new kind of shock transformation. The present study also reveals that the critical jet total pressures for MR–RR and RR–MR transformations are not the same when the primary pressure is increasing and decreasing, suggesting a hysteresis in the shock transformations.

Numerical Study of Turbulent Confined Jets Impinging on a Heated Substrate for Thin Film Deposition

This paper reports on the study of confined jets and jets interaction in terms of increasing chemical transport. The context of this study is the atmospheric pressure plasma-enhanced chemical vapor deposition, higher thin film growth rate being desired, while maintaining total flow rate as low as possible. Turbulence mixing and enhanced heat transfer are the physical mechanisms identified as being capable of increasing the growth rate at atmospheric pressure. A numerical study of jets impinging on a heated substrate was carried out using quasicompressible Reynolds-Averaged Navier–Stokes (RANS) equations. Abe–Kondoh–Nagano (AKN) low-Reynolds k-ε and standard k-ε models were tested using an unconfined impinging jet at Reynolds number Re = 23,750 for jet diameter to plate-spacing ratios of H/d = 2 and H/d = 6. Results were compared with experimental data from the literature. Based on numerical results and in accordance with existing findings, the AKN low-Reynolds k-ε was shown to be reasonably accurate and was thus chosen for the numerical study. The effects of flow rate, hole diameter and length, jet-to-jet spacing, confinement width, and jet number were investigated numerically for inline jets confined between two vertical planes for jet Reynolds numbers between 810 and 5060. The configurations with the greatest turbulent intensity were studied, with the addition of diluted species transport and consumption. A laminar flow setup with a slot jet (Re = 79.5) was compared to two injection designs consisting of a simple set of 12 impinging gas jets (Rej = 2530; H/d = 3) with and without the adjunction of a wire to break the jets (Rej = 1687; H/d = 2). The two turbulent injection methods improved growth rate by 15%, which mainly resulted from a larger gas heating by the surface due to turbulent heat exchange in the jet impact zone.

Simulation of Multiple Confined Impinging Jets on a Heated Plate Using the Lattice Boltzmann Method

A laminar two-dimensional thermal-flow generated by multiple confined jets impinging on an isothermal plate is investigated numerically using the lattice Boltzmann method. The impinging plate is kept at a constant high temperature while a cold air is flowing through the jets. The effect of different parameters (Reynolds number (Re), and the ratio between the jets height (H) and jets width (W))on the hydrodynamics and thermal characteristics of the flow field is discussed. The Reynolds number is ranging from 50 to 400, and the (H/W) ratio is varying between 1 and 3W.