Session A23: Free-Surface Flows: Natural Flows

Interfacial gas exchange in vegetated channels under free surface waves

Fluxes of dissolved oxygen (DO) at the air-water (AW) interface are crucial for availability of oxygen to aquatic biota. Turbulence generated by aquatic vegetation canopies is known to influence the exchange of dissolved gases with the atmosphere. In this study we performed experiments on a wave flume to determine the gas transfer rates under free surface waves of varied amplitude and frequencies through surrogate vegetation. We used arrays of rigid cylinders to mimic submerged and emergent vegetation canopies and used optical DO sensors to monitor the interfacial reaeration rates within and downstream of the canopy. We used Acoustic Doppler Velocimetry (ADV) to characterize the flow inside and outside the canopy and performed 2D planar Particle Image Velocimetry (PIV) to reveal the detailed dynamics inside the canopy. Data revealed enhanced gas transfer within the vegetation array once stem-scale turbulent wakes develop. Frequency of waves was observed to be a dominant factor enhancing gas transfer rate across all wave amplitudes. The results allow us to identify the critical parameters governing the interfacial gas transfer dynamics, and to develop theoretical predictors for surface gas transfer rates on vegetated oscillatory flows.   

Modeling of transport processes of invasive carp eggs in channels and rivers

We developed a stochastic egg drift model, SDrift, to study the transport processes of invasive carp eggs in rivers. SDrift is a Lagrangian particle tracking model that incorporates egg characteristics in water as a function of post-fertilization time and temperature. Turbulent velocities are simulated using the solution for the governing stochastic differential equation. We examined the longitudinal dispersion of simulated eggs in a variety of channels with an increasing complexity of river morphology, including a straight open channel, an idealized curved channel, and an 8-km reach in the Lower Missouri River. The hydrodynamics in the straight channel were constructed using classic flow and turbulence profiles in open channels. The hydrodynamics in the idealized curved channel and the river reach were determined using three-dimensional computational fluid dynamics simulations. In the river reach case, the simulations were calibrated with measured water surface elevation and validated with measured velocity distributions in 22 cross sections at two river discharges. The results show that the model can adapt to a wide range of mixing conditions, and reveal the effect of localized turbulence on egg retention and detention, which may have strong influences on the survival of fish.   

Effect of curvature on the reconfiguration of flexible blades immersed in incompressible flows

Marine plants anchored on the ocean floor are exposed to constantly changing ocean currents. In response to the moving flow, their flexible leaves sway rhythmically and so change posture with considerable displacement. This flow-induced deformation arising from nature evolution facilitates aquatic plants to withstand the hydrodynamic loading, also by virtue of the curvature enhanced stiffness behind which there are still open questions. In order to cover this scientific gap, we provide a systematic computational investigation of the reconfiguration of a clamped cylindrical patch by exploring the design space spanned by curvature radius and Cauchy number. The investigation was carried out with an extensively-validated software which employs a partitioned approach to solve Fluid-Structure Interaction (FSI) problems. The fluid field is evolved by means of a finite-difference flow solver with direct immersed boundary forcing, whereas the discretization of the structural domain relies on a NURBS-based Isogeometric method, which has proven to be very efficient in capturing large strain gradients with minimal degrees of freedom. Preliminary results support the capability of curvature-enhanced stiffness to reframe the deformation dynamics of elastic blades.   

Identification of coherent structures in the flow field around a cylindrical pier using Finite Time Lyapunov Exponents

Local scouring around a bridge pier is one of the most common reasons for bridge failure. The flow around a bridge pier is three-dimensional. It contains different flow features, such as down flow in front of the pier, vortex formation at the upstream base of the pier, scour hole at the bottom of the pier and vortex shedding in the pier downstream. The interaction among all these features makes the 3D flow around a bridge pier very complex. The coherent structures around the pier play a vital role in forming the scour hole and transporting sediment from scour hole to the pier downstream. Hence, the accurate prediction of these coherent structures and their interactions is necessary to understand the scouring mechanism better. But, the current methods fail to accurately extract the coherent structures from the velocity field.

Lagrangian coherent structures (LCS) are the ridges of Finite Time Lyapunov Exponents (FTLE). LCS is used to understand the overall flow geometry and material transport. These structures separate the regions with different dynamical behaviour in the flow and extract the geometry which is often hidden in the velocity field. Therefore, these structures often provide an excellent tool for understanding the behaviour of time-dependent systems, especially for understanding particle transport. Since the LCS govern the particle transport, it may even influence the sediment moment and hence the formation of the scour hole.

The present study carried out a 3D Reynolds Averaged Navier Stokes (RANS) simulation with a k-ω SST turbulent model to simulate the velocity field around the pier with different stages of scour hole (initial flatbed, intermediate scour hole and equilibrium scour hole). The Lagrangian Coherent Structures (LCS) are extracted from the velocity field as the ridges of the FTLE field. The obtained LCS accurately represented the three-dimensional coherent structures formed around the pier and the interactions among them.   

Atypical turbulent junction flow characteristics upstream of model boulders mounted atop a rough, porous bed

Boulders are common flow obstacles in steep, gravel-bed streams that affect local sediment bed morphology via their influence on local hydrodynamics. While turbulent flow characteristics in the junction region, which occurs where a boundary layer meets a wall-mounted obstacle, are well described for many engineered flow obstacles, uncertainty remains about flows upstream of boulders largely due to the uncertain effects of the rough, permeable gravel bed and variable submergence. This laboratory flume study provides insights to turbulent flow characteristics upstream of cylindrical model boulders via new volumetric particle image velocimetry (PIV) tests conducted at fully and partially submerged conditions. While near-bed, mean-flow separation is a prominent feature of most junction flow studies, a striking and atypical finding here is that flow separation was substantially delayed and was not observed in measurements (made within 0.1 diameters upstream). In addition, no clearly discernable mean horseshoe vortices were observed via mean streamline and vorticity plots, which are typically found in junction flows. These findings suggest the presence of atypical junction flow characteristics at boulders, which should be considered in river engineering and stream restoration efforts.