Talia Tokyay

Lock-exchange gravity currents with a high volume of release propagating over a periodic array of obstacles

Large eddy simulations are employed to investigate the structure and evolution of a bottom-propagating compositional gravity current in a rectangular horizontal plane channel containing a series of identical large-scale obstacles (dunes and square ribs) at the channel bottom. Simulation results show that below a certain value of the additional drag force per unit streamwise length induced by the bottom obstacles (low drag cases), the gravity current propagating over an array of obstacles transitions to a regime where the average front velocity is close to constant. Past its initial stages, the total kinetic energy, E k , increases in time proportional to t 1/3 , where t is the time since release. This behaviour is similar to the slumping phase observed for currents propagating over a flat bed, with the exception that in the latter case the temporal increase of E k during the later stages of the slumping phase is much faster ( E k ~ t ). Simulation results also show that above certain value of the drag force per unit streamwise length induced by the obstacles (high drag cases), the slumping phase can be very short. In this case, similar to currents propagating in a porous medium, the current transitions to a drag-dominated regime in which the front velocity decays proportionally to t β , with β = −0.28, once the discharge of lock fluid at the position of the lock gate becomes close to constant in time.

Gravity current flow past a circular cylinder: forces, wall shear stresses and implications for scour

The flow of compositional gravity currents past circular cylinders mounted above a wall is investigated numerically. Two- and three-dimensional Navier–Stokes simulations are employed to quantify the force load on the cylinder, along with the friction velocity at the bottom wall near the cylinder, for Reynolds numbers in the range of 2000–45 000. While two-dimensional simulations accurately capture the impact stage, they are seen to overpredict the force and friction velocity fluctuations throughout the transient stage. Comparisons between gravity current and constant-density flows past circular cylinders show that the impact and transient stages are unique to gravity current flows. During the quasi-steady stage, on the other hand, the wake structures and the values of the drag, the peak-to-peak lift, the vortex shedding frequency and the friction velocity below the cylinder are comparable. The friction velocity below the cylinder depends chiefly on the Reynolds number formed with the front velocity and the gap width. The maximum friction velocity at impact is about 60% larger than during the quasi-steady stage or in a constant-density flow. This raises the possibility of aggressive erosion behaviour at impact, which may occur in a spanwise localized fashion because of the larger friction velocity near the lobes.

Lock‐exchange gravity currents with a low volume of release propagating over an array of obstacles

The study discusses, based on high-resolution 3-D large eddy simulation results, the evolution of lock-exchange Boussinesq gravity currents with a low volume of release (LVR) propagating over an array of identical two-dimensional (2-D) obstacles (large roughness features in the form of dunes and ribs corresponding to eroded substrates) in a rectangular horizontal channel. The study analyzes the effect of the shape and height of the roughness elements and scale effects between Reynolds numbers at which most laboratory experiments of lock-exchange currents are conducted and Reynolds numbers closer to field-scale currents in geophysical applications on the temporal variation of the front velocity, mixing, and flow structure within the gravity current. The temporal evolution of the flow instabilities (e.g., Kelvin-Helmholtz billows, forward and backward propagating interfacial waves forming as a result of the interaction of the front of the current with the obstacles) and bed friction velocity distributions are analyzed during the different stages of the evolution of the current. The focus is on the later stages of the propagation of LVR currents, after the transition to the self-similar drag-dominated regime has started. The differences between the evolution and structure of gravity currents with a low and a high volume of release are highlighted.

The effects of a submerged non-erodible triangular obstacle on bottom propagating gravity currents

The flow induced by a compositional (e.g., temperature or salinity driven) gravity current propagating over a fixed non-erodible triangular bottom-mounted obstacle is investigated based on 3-D large eddy simulations. The paper discusses how the flow physics (e.g., type and characteristics of the reflected bore, dynamics of flow instabilities affecting mixing, and turbulence structure) and main flow variables (e.g., the proportion of the flow advected over the obstacle, the height of the reflected flow and speed of the reflected bore, the height of the lower layer and front speed of the current downstream of the obstacle, and the drag force) change as a function of the incoming gravity current type (lock-exchange vs constant-flux), relative obstacle height, and Reynolds number. A particular focus is on the flow structure during the two possible quasi-steady regimes that can occur in such flows. The predictive capabilities of shallow flow theory models to estimate the main flow parameters during these two r...

A Numerical Study of Wind Loads on Large Highway Sign Structures

Accurate estimation of wind forces on large highway sign structures is important due to possible structural failure of these sign structures under strong winds. The use of large panels for the traffic signs is increasingly more common as we attempt to better manage the highway traffic flow and automate the highway systems. In order to be able to predict behavior of these structures, accurate knowledge of the forces on these structures must be known. An important aspect of wind loads on highway structures that is generally not appreciated is the effect of interactions among the panel structures on these forces. The main objective of the proposed study is to use Computational Fluid Dynamics (CFD) tools to determine the wind loads and pressure distributions by accurate numerical simulations of the air flow characteristics around large highway sign structures under severe wind speeds conditions. The pressure distributions are needed in case a detailed structural dynamics analysis will be performed. In particular, the present study investigates the effect induced by the presence of back-to-back signs.

Large-eddy simulations of gravity current flows past submerged cylinders

As the offshore oil and gas industry moves towards deeper ocean environments, submarine structures such as oil and gas pipelines become increasingly exposed to less understood hazards, among them gravity and turbidity currents. Our incomplete understanding of the interaction between gravity currents and submarine structures has motivated several recent experimental [1, 2] and numerical [3, 4, 5, 6, 7] investigations. Whereas previous studies focus on the force exerted on submerged cylinders and on the two-dimensional dynamics of the interaction, the current investigation places emphasis on the magnitude of the wall shear stresses near the cylinder calculated from threedimensional simulations. This shear stress is related to the process of scour near submarine structures [8].