H14B-01 INVITED
Identification of vortices in complex flows
Dating back to Leonardo da Vinci's famous sketches of vortices in turbulent flows, fluid dynamicists for over five centuries have continued to visualize and interpret complex flows in terms of motion of vortices. Nevertheless, much debate surrounds the question of how to unambiguously define vortices in complex flows. This debate has resulted in the availability of many vortex identification criteria---mathematical statements of what constitutes a vortex. Here we review the popularly used local or point- wise vortex identification criteria. Based on local flow kinematics, we describe a unified framework to interpret the similarities and differences in the usage of these criteria. We discuss the limitations on the applicability of these criteria when there is a significant component of vortex interactions. Finally, we provide guidelines for applying these criteria to geophysical flows.
H14B-02
Turbulent Coherent Flow Structures In Gravel-Bed Rivers: An Approach Combining Flow Visualisation And Velocity Measurements
There is compelling evidence showing that flow turbulence in gravel-bed rivers is dominated by large-scale flow structures that are elongated and that occupy the entire depth of the flow. These structures scale with flow depth (Y) and their length is between 2 to 6Y. There are many questions concerning the origin, scales and dynamics of these coherent flow structures. Recent progress in answering these questions has been achieved through a combination of approaches that uses the field as a laboratory. Experimental designs relying on data derived from flow visualisation and from records of various lengths and resolutions of single and multiple point velocity measurements have been used to obtain pictures of the space-time dynamics of turbulent processes in rivers. In this paper, we present results from a range of experiments conducted in situ in gravel-bed rivers and highlight the relative contribution of various techniques and methods to our understanding of turbulence in gravel-bed rivers. Five main results are emphasised: 1. the frequency-magnitude scalings of the turbulent flow structures in the boundary layer which exhibit very robust relations over a range of conditions; 2. the relative role of mean flow velocity and depth on the dynamics of ejections; 3. the imbrication of intermittent sweeps and ejections into self- similar large scale flow pulsations; 4. the very local effects of a pebble cluster on the structure of the turbulent boundary layer and 5. the presence of a double shedding frequency of turbulent structures behind protuberant obstacles into the ambient flow. Even though there are limitations to the approach combining flow visualisation and velocity measurements (e.g. quantifying vorticity), the evidence provided by these methods encompasses a range of scales, bed roughness and forms in an general view of turbulent flows in rivers.
H14B-03 INVITED
Educing Coherent Structures and Their Impact From Planar Velocimetry Measurements in Laboratory Boundary Layers
Planar velocimetry techniques, like particle image velocimetry (PIV), provide detailed instantaneous velocity information over a plane of interest with a dynamic spatial range sufficient for resolving nearly all dynamically- significant length scales in moderate-Reynolds-number (Re) turbulent flows. While these techniques only provide a slice through a given flow, such data, in concert with the recent development of novel analysis methods, can provide significant insight into the structural building-blocks of wall-bounded turbulent flows. This talk will highlight various eduction schemes for identifying dominant coherent structures in instantaneous planar velocity fields as well as conditional averaging methods for assessing the ‘average' spatial characteristics of these structures. Representative results will be presented from laboratory PIV studies of smooth- and rough-wall zero- pressure-gradient turbulent boundary layers. While identifying the spatial characteristics of the underlying flow structure is important, understanding the role that such structures play in the evolution of wall turbulence is of equal or greater importance, particularly in the development of modeling and control strategies. To this end, the impact of coherent structures on the evolution of wall turbulence will be highlighted using conditional averaging analysis of the aforementioned PIV data sets.
H14B-04
Coherent turbulence structures in lateral separation eddies
An abrupt expansion in channel width generally produces lateral flow separation. A zone of recirculating flow is produced downstream of the point of separation that is termed a lateral separation eddy. Often the abrupt expansion is preceded upstream by a rapid constriction of the channel width. Lateral separation eddies are common in the Colorado River in Grand Canyon downstream of tributary debris fans. They are also produced by spur dikes, which are engineering structures designed to promote lateral sedimentation and eliminate lateral channel migration. Accurate prediction of the sediment transport field in a lateral separation eddy is difficult because of the presence of large and energetic turbulence structures. We have produced a three-dimensional numerical model of turbulence employing the large eddy simulation (LES) technique, in which large scale turbulence is directly calculated by integration of the spatially-filtered Navier-Stokes equations. Our model employs a moving boundary-fitted coordinate system to capture the irregular geometry of the channel and the moving free water surface. The model was used to simulate the flow and large-scale turbulence structures downstream of a single spur dike and in the Thirty Mile lateral separation eddy in Grand Canyon. The lambda-2 technique was used to extract and visualize the large-scale vortices. In both simulations, the upstream channel constriction produces strong secondary circulation resulting in a large vortex core in the main channel flow that is oriented downstream. Past the point of separation, vertical Kelvin-Helmholtz vortex cores are periodically produced along the free shear layer in both simulations. In the spur-dike simulation, the vertical Kelvin-Helmholtz vortex cores interact with the downstream-directed vortex core to produce regularly-shaped vortex cores which periodically give rise to strong, near-bed, cross-stream velocities directed toward the zone of flow reattachment. In the Grand Canyon simulation, the vertical cores and downstream core interact to produce highly irregularly- shaped vortex cores and episodic strong near-bed velocities directed into the lateral separation zone. Presumably, these periodic and episodic, near-bed, cross-stream velocities are important for the flux of sediment into a lateral separation eddy.
H14B-05
Organised Coherent Motion in Atmospheric Boundary Layer Flow in the Proximity to Tall Plant Canopies as Detected in Acoustic Doppler Profiler and Tower-based Observations
We investigated coherent structures above and in a tall plant canopy during a field campaign at a mountainous site in Germany (WALDATEM-2003). Data from a remote sensing acoustic Doppler system in concert with in-situ point measurements of turbulence in flow velocity and scalars deployed on towers yielded continuous observations from the forest ground to 200 m above the ground with a vertical resolution of 10 m at a sampling frequency of 0.4 and 20 Hz respectively. Coherent structures were extracted from time series utilizing wavelet transform techniques allowing for single structure analysis and averaged statistics of detected events. In addition to their spatiotemporal scales, we focused on the identification of generating mechanisms and surface parameters affecting coherent structures. Time scales were on the order of 20 to 36 s depending on the upstream topography and canopy morphology. Lateral transport dominated scalar coherent exchange. Vertical profiles of time scales in longitudinal and vertical velocities were mirror images showing an increase/ decrease, respectively, with height. Time scales in scalars were nearly height-constant. The ratio of the contribution of coherent structures to total vertical exchange was 0.2 for momentum and 0.25 to 0.4 for sensible heat. Analysis of power spectra confirmed an interaction between inactive eddies of atmospheric boundary layer scale and the horizontal flow in 4 % of all studied cases only, mainly under near-neutral stratification. Evaluation of the Mixing-Layer Analogy suggested that vertical shear caused by the immense canopy drag was the dominant generating mechanism. However, daytime coherent structures were found to be a superposition of shear generated events and convectional eddies. The latter led to an increase of vertical coherency in the flow around noon. At night, terrain induced linear gravity waves showed similar time scales as coherent structures emphasizing the need to differentiate between these two fundamentally different phenomena. Topography in combination with canopy morphology was found to affect coherent structures through modifying characteristic shear length scales. Based on how deep coherent structures penetrate the plant canopy, a classification of exchange regimes was proposed to qualitatively describe coupling and decoupling in tall canopies.
H14B-06
Infrared Remote Sensing of Coherent Structures in an Estuarine River
Coherent structures in rivers are generated by the interaction of the flow with bathymetric and shoreline features. These coherent structures produce surface signatures that can be detected and quantified using remote sensing instruments such as infrared (IR) cameras and microwave radars. Furthermore, the existing evidence suggests a number of relationships between coherent structures and flow characteristics that have the potential to allow flow parameters to be inferred from remote measurements. The Coherent Structures in Rivers and Estuaries Experiment, or COHSTREX is a five-year, multi-institutional collaboration to determine the extent to which the remotely-sensed signatures of coherent structures can be used to initialize and constrain predictive models for river and estuarine flows. Following a brief overview of COHSTREX, we report on the use of IR imagery to characterize and quantify the flow in the Snohomish River, in Everett, WA during the 2006 COHSTREX field campaign. Applications of IR techniques include using DPIV techniques to derive surface velocity and detecting coherent structures such as vortices and boils. Here we will focus on the finding that the thermal signature of boils generated by the flow over a submerged sill can be used to detect the presence of stratification due to an estuarine salt wedge. The boils were observed to have both warm and cold surface signatures depending on the phase of the tide. In the absence of stratification, the boil signature was warm relative to the surrounding undisturbed surface. A warm signature is consistent with disruption of the cool thermal boundary layer that is typically present at the surface of natural water bodies. When stratification was present, the boil signature was cool relative to its surroundings. Comparisons with in situ temperature and salinity measurements show that a cold signature is due to deep, colder water from the salt wedge being brought to the surface. We also found that near-surface diurnal heating due to solar radiation can significantly affect the thermal signatures of boils and other surface disruptions such as surface wakes. Our findings indicate that the thermal signature of coherent structures generated by flow over topography can provide information on the presence and degree of stratification. http://cohstrex.apl.washington.edu/
H14B-07 INVITED
The links between flow structure and suspended sediment over sand dunes revealed using phase coherence wavelet analysis
Alluvial sand dunes are characterized by their association with large-scale turbulence that may advect through the entire boundary layer thickness. Such turbulence has been held responsible for both controlling the flow-depth scaling of dunes and also the suspension of sediment above these bedforms. Here, we present field data from the Rio Paraná, Argentina, which reveals the nature of large-scale turbulence and associated sediment suspension over large alluvial sand dunes. The study uses three-dimensional velocity data obtained using a 600 kHz Teledyne RDI acoustic Doppler current profiler. Additionally, we calibrate the backscatter acoustic intensity from the instrument to allow simultaneous analysis of the temporal patterns of velocity and suspended sediment. Wavelet analysis is used to examine the nature and origin of the dune-related macroturbulence, which appears principally associated with eddy shedding and wake flapping in the dune leeside. Furthermore, phase coherence wavelet analysis shows that streamwise and vertical velocities are strongly and inversely correlated over both dune crest and trough, where flow decelerations are linked to fluid upwellings and vice versa. The inverse correlation between streamwise velocity and suspended sediment concentration (SSC), and the positive correlation between vertical velocity and SSC, indicates that fluid ejections sourced near the bed, and associated with the lee side flow, are suspending sand into the overlying flow field and are thus important mechanisms of sediment transport. http://homepages.see.leeds.ac.uk/~eardpa/index.shtml