Robert Ettema

Detached Eddy Simulation Investigation of Turbulence at a Circular Pier with Scour Hole

This paper uses results from detached eddy simulation to reveal the dynamics of large-scale coherent eddies in the flow around a circular pier with an equilibrium scour hole. This is important for the sediment transport because the local scour process is controlled to a large extent by the large-scale coherent structures present in the near-bed region. The present paper investigates the dynamics of these coherent structures, their interactions and their role in entraining sediment in the later stages of the scour process when the horseshoe vortex system is stabilized by the presence of a large scour hole. The pier Reynolds number was 2.06× 105 , outside the range of well-resolved large-eddy simulation (LES). Additionally, scale effects are investigated based on comparison with LES results obtained at a much lower Reynolds number of 16,000 in a previous investigation. The paper provides a detailed study of the dynamics of the main necklace vortices of the horseshoe vortex system, including an investigation...

Particle-image velocimetry for whole-field measurement of ice velocities

Particle-image velocimetry (PIV) is introduced herein as a useful experimental and field technique for determining and mapping whole fields of ice and water velocities. The technique, which is becoming extensively used in other applications of fluid mechanics, lends itself very well for this purpose. The primary requirements to use the technique are a suitable series of images of moving ice, or model ice in a hydraulic model, a computer equipped with a frame-grabber, and the pertinent software. The images may comprise direct video images, a series of photographs, or radar-generated records of moving ice. The present paper briefly describes the principles underlying PIV and shows how PIV can be adapted for use in hydraulic-model studies of ice-movement problems. The technique is demonstrated for a hydraulic model study of ice movement through a confluence of two rivers. The techniques utility suggests that it also could be a useful adjunct for remote sensing of ice movement on rivers, in coastal regions, or on oceans.

On internal friction and cohesion in unconsolidated ice rubble

Abstract Unconsolidated ice rubble is a mass of ice fragments, often floating in water, that has not solidified by freezing of pore water or by freeze bonding between individual ice-rubble fragments. It is argued herein that unconsolidated ice rubble undergoing continuous shear deformation is essentially cohesionless; i.e., in terms of the Mohr-Coulomb failure criterion, it should not exhibit a cohesive intercept at zero confining pressure and at large shear deformation. Contacting pieces of submerged ice rubble, however, have a propensity to freeze bond with one another, even at very low confining pressures and in nonfrigid air, such that unconsolidated ice rubble may tend to self-solidify. It is this propensity that significantly contributes to the comparatively large values of internal-friction angle that have been reported for ice rubble, and which may cause relationships between shear strength and confining pressure to be nonunique.

Flow and sediment transport measurements in a simulated ice-covered channel

Laboratory flume experiments conducted to illuminate flow field and sediment transport characteristics in ice-covered sand-bed channels are discussed. The overall purpose of the experiments was to examine how level ice covers affect flow distribution, flow turbulence characteristics, and sediment transport rates. The experiments were conducted with a nonrefrigerated flume fitted with a sand bed and plywood panels used to simulate cover-floating ice covers. A discriminator laser-doppler velocimeter was used to measure water and sediment particle velocities simultaneously. The addition of a cover and an increase in its roughness increases flow depth and decreases bulk flow velocity, thereby reducing sediment transport rates compared to those in open water flow. The Reynolds shear stress, turbulence intensities, and distribution of sediment size over the depth also change.

Simple-shear box experiments with floating ice rubble

Abstract A simple-shear box was used to study the shear strength characteristics of floating layers of vertically unconstrained ice rubble comprised of parallelpiped ice blocks. A comparative set of experiments was also performed using floating layers of parallelpiped plastic blocks in order to determine the origin of cohesion in ice rubble. Experiments were also performed using mushy ice. However, the shear-box proved not to be useful for determining the shear testing of mushy ice. The shear strength of a layer of ice rubble was found to depend on normal stress, which in turn was found to depend on rubble thickness, layer porosity, and shear rate. The dependence on shear rate of normal stress and, as a consequence, of shear strength of a layer of floating ice rubble is attributed to the development of freeze-bonds between the ice blocks comprising the rubble layer. It is argued that, at slower shear rates, more and stronger freeze-bonds develop than at higher shear rates, thus enabling the layer to withstand larger normal stresses and, consequently, shear strengths that increase with decreasing shear rates. If the influence of freeze-bonding on normal stress is taken into account, and if a Mohr-Coulomb failure criterion is used to characterize shear strength, it is found that a floating layer of ice rubble undergoing continuous-shear deforms as a cohesionless material; or at least as a material with unique cohesive properties.

Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction

This report reviews the present state of knowledge regarding bridge-abutment scour and the veracity of the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, which is to be considered in updating the scour estimation methods that are recommended by AASHTO, and used generally by engineering practitioners. Though considerable further progress has been made since 1990, the findings indicate that several important aspects of abutment scour processes remain inadequately understood and therefore, are not included in current methods for scour depth estimation. The state-of-the-art for abutment scour estimation is considerably less advanced than for pier scour. Moreover, there is a need for design practice to consider how abutment design should best take scour into account, as scour typically results in the geotechnical failure of an abutment’s earthfill embankment, possibly before a maximum potential scour depth is attained hydraulically. Abutment scour herein is taken to be scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. This definition excludes other flow and channel-erosion processes such as lateral geomorphic shifting of the bridge approach channel but includes contraction and abutment scour as part of the same physical processes that should be treated together rather than separately in their estimation. The review shows that, since 1990, advances have been made in understanding abutment-scour processes, and in (1) estimating scour depth at abutments with erodible compacted earthfill embankments, and at those with solid-body (caisson-like) foundations; (2) identifying the occurrence of at least three distinct abutment scour conditions depending on abutment location and construction; and (3) utilizing the capacity of numerical modeling to reveal the flow field at abutments in ways that laboratory work heretofore has been unable to provide. The review identifies and evaluates leading scour formulas and suggests a framework for developing a unified abutment scour formula that depends on satisfying several targeted future research needs.

Observations from an aufeis windtunnel

Abstract Presented are observations from a special type of windtunnel, termed an “aufeis windtunnel”, used to investigate the initial spreading of aufeis forming on sloped planar surfaces subject to wind. The observations, which extend earlier work conducted on aufeis formation in a flat-sloped flume, are largely qualitative, though they include data on trends in aufeis morphology. The aufeis windtunnel facilitated investigation of the influences on initial aufeis formation of gravity and wind drag. Tilting the windtunnel and varying wind speed, respectively, produced the influences. Both gravity and wind drag may drive the spreading of shallow flows of freezing water introduced at the upstream end of the windtunnel test section. The combination of the two driving mechanisms results in a variety of aufeis morphologies (height and spacing of surface features) on the windtunnels aluminum base. For such surfaces, the initial morphologies appear essentially wavelike (or terraced), and their spacing and height vary with slope and wind speed. On the other hand, aufeis initially forming on a smooth surface of ice develops a finer scale surface morphology. The differences in morphology reflect differences in the location and spacing of initial sites of ice-crystal growth that leads to aufeis formation. Whereas ice crystals may emerge and grow from any site on an ice surface, ice-crystal growth began at surface irregularities (scratches or burrs) on the aluminum base of the windtunnel.

The Horseshoe Vortex System Around a Circular Bridge Pier on Equilibrium Scoured Bed

Large eddy simulation (LES) is used to investigate the horseshoe vortex (HV) system around a circular bridge pier located on scoured bed corresponding to equilibrium conditions. The deformed bed bathymetry is obtained from experiments. Simulation is performed for two different inflow boundary conditions, one in which the upstream flow is fully turbulent including the turbulent fluctuations obtained from a precalculated LES simulation in a periodic channel (Re=18,000) and one in which the mean streamwise velocity profile corresponding to the first simulation is used (Re=5000, no turbulent fluctuations). The former case corresponds exactly to the experimental conditions, the latter serves to highlight the effect of incoming turbulence on the HV system. The dynamics of the instantaneous coherent structures associated with the HV system around the cylinder and the spectral content of the flow in this region are analyzed. Special attention is given to the interaction between the legs of the horseshoe vortices and the flow behind the cylinder and to the distribution of the bed shear stress. The statistics of the flow are calculated. It is found that the bed shear stress fluctuations around the local mean values can be very high, especially in the scour and near wake regions. The mean flow fields show that for the given scour bathymetry and flow conditions, the mean HV system contains one primary eddy situated fairly close to the cylinder. Inside this eddy the mean pressure fluctuations and resolved kinetic energy levels are very high. Maximum mean bed shear stress levels are observed beneath this eddy as well as close to the small corner vortex at the base of the cylinder.

A review of scour conditions and scour-estimation difficulties for bridge abutments

This paper is a review of the scour conditions, and scour-estimation difficulties, associated with scour at abutments of bridges over rivers and streams. A recent survey of abutment-scour problems in the U.S., Korea, and abroad, shows that several scour conditions can occur, and that existing relationships for scour estimation, besides being inaccurate do not match the scour conditions. Moreover abutment scour is found to be primarily a concern for bridges over smaller rivers and streams than for larger rivers, because inadequate design and monitoring attention has been give to abutment scour at the many small bridges. This paper also discusses the difficulties confronting accurate estimation of abutment-scour depth. The difficulties, which include similitude aspects of laboratory experiments on scour at bridge abutments, complicate the development of reliable scour-estimation relationships. In a practical sense, the difficulties imply that design relationshipsfor scour-depth estimation inevitably have to be of approximate accuracy given the great variability of channel conditions at most abutments.

Laboratory experiments on frazil ice growth in supercooled water

Abstract Presented herein are the results of a laboratory investigation of the influence of turbulence and water temperature on the nature and rate of frazil ice growth in a turbulent body of supercooled water. The results indicate that the rate and the quantity of frazil ice growth increase with both increasing turbulence intensity and with decreasing water temperature at the instant of seeding, when a small fragment of ice is placed in the supercooled water. The turbulence characteristics of a flow affect the rate of frazil-ice growth by governing the temperature to which the flow can be supercooled; by influencing heat transfer from the frazil ice to surrounding water; and by promoting secondary nucleation, crystal, platelet and floc fracture, thereby increasing the number of nucleation sites available for further frazil ice growth. Larger frazil ice platelets, beginning as single crystals then becoming laminar fusions of crystals, were observed to form in water supercooled to lower temperatures. However, platelet size generally decreased with increasing turbulence intensity, as platelets with a major diameter larger than a certain value tend to break when buffeted by turbulence eddies. The investigation of frazil ice growth involved the use of a simplified analytical model, in which the rate of frazil ice growth is related to temperature rise of a turbulent volume of water due to the release of latent heat of fusion of liquid water to ice. Experiments conducted in a turbulence jar with a heated, vertically oscillating grid served both to guide and to calibrate the analytical model as well as to afford insights into frazil ice growth.