Experimental Study on the deposition behavior of turbidity currents

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Large eddy simulation of turbidity currents in a narrow channel with different obstacle configurations

Turbidity currents are frequently observed in natural and man-made environments, with the potential of adversely impacting the performance and functionality of hydraulic structures through sedimentation and reduction in storage capacity and an increased erosion. Construction of obstacles upstream of hydraulic structures is a common method of tackling adverse effects of turbidity currents. This paper numerically investigates the impacts of obstacle’s height and geometrical shape on the settling of sediments and hydrodynamics of turbidity currents in a narrow channel. A robust numerical model based on LES method was developed and successfully validated against physical modelling measurements. This study modelled the effects of discretization of particles size distribution on sediment deposition and propagation in the channel. Two obstacles geometry including rectangle and triangle were studied with varying heights of 0.06, 0.10 and 0.15 m. The results show that increasing the obstacle height will reduce the magnitude of dense current velocity and sediment transport in narrow channels. It was also observed that the rectangular obstacles have more pronounced effects on obstructing the flow of turbidity current, leading to an increase in the sediment deposition and mitigating the impacts of turbidity currents

Large eddy simulation of turbidity currents in a narrow channel with different obstacle configurations

© 2020, The Author(s). Turbidity currents are frequently observed in natural and man-made environments, with the potential of adversely impacting the performance and functionality of hydraulic structures through sedimentation and reduction in storage capacity and an increased erosion. Construction of obstacles upstream of hydraulic structures is a common method of tackling adverse effects of turbidity currents. This paper numerically investigates the impacts of obstacle’s height and geometrical shape on the settling of sediments and hydrodynamics of turbidity currents in a narrow channel. A robust numerical model based on LES method was developed and successfully validated against physical modelling measurements. This study modelled the effects of discretization of particles size distribution on sediment deposition and propagation in the channel. Two obstacles geometry including rectangle and triangle were studied with varying heights of 0.06, 0.10 and 0.15 m. The results show that increasing the obstacle height will reduce the magnitude of dense current velocity and sediment transport in narrow channels. It was also observed that the rectangular obstacles have more pronounced effects on obstructing the flow of turbidity current, leading to an increase in the sediment deposition and mitigating the impacts of turbidity currents

Air flow effect on the behavior of lock-exchange gravity current

The main goal of this study is investigating the effect of air flow above the free surface on the behavior of gravity current. Lock-release gravity current has been simulated in a channel, by using VOF method, for modeling free surface at the interface of gas and liquid phases. Eulerian approach is used to consider the presence of particles in the flow. The results of simulation with free surface assumption are in a well agreement with the previous experimental results. It is observed that the flows containing particles with larger diameter experience higher deposition rate, due to their higher terminal velocities which are 0.000129m/s, 0.000359m/s and 0.000808m/s for the particles with 12μm, 20μm and 30μm diameters respectively. Increasing the size of particles diameter leads to decrease in the driving force, the front position of flow containing particles with 30μm diameter is 11% less than that of flow containing particles with 12μm diameter, thereby the flow velocity decays quickly. The results show that the presence of particles leads to a reduction in the value of entrainment rate. It is concluded that the velocity of air-phase affects the shape of flow and instabilities. By considering three different values of 0.1m/s, 0.12m/s and 0.18m/s for the air-phase velocity, it is observed that the amount of run-out length, in the case where the air velocity is 0.18m/s, is nearly 3% more than that in other cases at the end of channel, moreover it leads to an increase in the value of entrainment rate

Stress analysis of a second stage gas turbine blade under asymmetric thermal gradient

In this study the main causes of the failure of a GE-F9 second stage turbine blade were investigated. The stress distribution of the blade which has 6 cooling vents in three modes (with full cooling, closure of half of the cooling channels, and without cooling) was studied. A three dimensional model of the blade was built and the fluid flow on the blade was studied using the FVM method. The stress distribution due to centrifugal forces applied to the blade, temperature gradients and aerodynamic forces on the blade surface was calculated by the finite element model. The results show that the highest temperature gradient and as a result the highest stress value occurs for the semi-cooling state at the areas near the blade root and this status is true for the full cooling mode for the regions far from the root. However, the field observations showed that the failure occurred for the blade with the semi-cooling state (due to closure of some of the channels) at areas far from the root. It is discussed that the main factor of the failure is not the stress values being maximum because in the state of full cooling mode (the state with the maximum stress values) the temperature of the blade is the lowest state and as a result the material properties of the blade show a better resistance to phenomena like hot corrosion and creep

Presence of Holmboe waves in particle-laden intrusive gravity current

Abstract The present study evaluates the prevalence of Holmboe waves in an intrusive gravity current (IGC) containing particles, employing large Eddy simulation (LES). Holmboe waves, a type of stratified shear layer-generated wave, are characterised by a relatively thin density interface compared to the thickness of the shear layer. The study demonstrates the occurrence of secondary rotation, wave stretching over time, and fluid ejection at the interface between the IGC and a lower gravity current (LGC). Results indicate that, aside from J and R, the density difference between the IGC and the LGC has an impact on Holmboe instability. However, a reduction in the density difference does not manifest consistently in the frequency, growth rate, and phase speed, though it does cause an increase in the wavelength. It is important to note that small particles do not affect the Holmboe instability of the IGC, while larger particles cause the current to become unstable and vary the characteristics of Holmboe instability. Moreover, an increase in the particle diameter size results in an increment in the wavelength, growth rate, and phase speed; but is accompanied by a decrease in frequency. Additionally, the enlargement of the bed slope angle makes the IGC more unstable, encouraging the growth of Kelvin–Helmholtz waves; however, this causes Holmboe waves to disappear on inclined beds. Finally, a range for the instabilities of both Kelvin–Helmholtz and Holmboe is provided