Large-scale characteristics of a stably stratified turbulent shear layer

Implicit large eddy simulation is performed to investigate large-scale characteristics of a temporally evolving, stably stratified turbulent shear layer arising from the Kelvin–Helmholtz instability. The shear layer at late time has two energy-containing length scales: the scale of the shear layer thickness, which characterizes large-scale motions (LSM) of the shear layer; and the larger streamwise scale of elongated large-scale structures (ELSS), which increases with time. The ELSS forms in the middle of the shear layer when the Richardson number is sufficiently large. The contribution of the ELSS to velocity and density variances becomes relatively important with time although the LSM dominate the momentum and density transport. The ELSS have a highly anisotropic Reynolds stress, to a degree similar to the near-wall region of turbulent boundary layers, while the Reynolds stress of the LSM is as anisotropic as in the outer region. Peaks in the spectral energy density associated with the ELSS emerge because of the slow decay of turbulence at very large scales. A forward interscale energy transfer from large to small scales occurs even at a small buoyancy Reynolds number. However, an inverse transfer also occurs for the energy of spanwise velocity. Negative production of streamwise velocity and density spectra, i.e. counter-gradient transport of momentum and density, is found at small scales. These behaviours are consistent with channel flows, indicating similar flow dynamics in the stratified shear layer and wall-bounded shear flows. The structure function exhibits a logarithmic law at large scales, implying a $k^{-1}$ scaling of energy spectra.

Detecting vortical structures in time-resolved volumetric flow fields

The detection of three-dimensional coherent vortical structures that get advected as well as deformed with time is a challenge. However, it is critical for the statistical analysis of these vortices, for example, the quasi-streamwise vortices (QSVs) in the near field of a turbulent shear layer, where cavitation inception typically occurs. These structures exhibit underlying correlations among different properties that can be derived from the velocity gradients. Exploiting these correlations, a pseudo-Lagrangian vortex detection method is proposed that uses k-means clustering based on vorticity magnitude and direction, values of λ2, strain rate structure, axial stretching, and location. The method facilitates the finding that QSVs have pressure minima that are lower than those in the surrounding flow, including the primary spanwise vortices. These minima typically appear after a period of axial stretching and before contraction events.

Energy estimation of resonant waves in channels with lateral cavities

<p>Macro-roughness elements, such as lateral cavities and embayments, are usually built in the banks of rivers for different purposes. They can be used to create harbors, or to promote morphological diversity that enhance habitat suitability in an attempt to restore the sediment cycle in channelized rivers. In presence of lateral cavities, shallow water flows may exhibit a rhythmic water surface oscillation, called seiche. The formation of the seiche is triggered by the partially bounded in-cavity water body which leads to the generation of a standing wave. Amplitude and periodicity of the seiche is jointly controlled by the dominant eigenmodes of the standing wave and by the turbulent shear layer structures created at the opening of the cavity. Seiches have been studied in the past decades putting the focus on their impact on river hydrodynamics and morphodynamics. However, the study of the seiches from an energy harvesting perspective is still unexplored. Seiche waves could represent a distributed hydropower source with a low environmental impact, being energy extraction possibly integrated with river restoration works. In this work, we use an in-house  numerical simulation model to reproduce the water surface oscillation in a channel with multiple lateral cavities and study their wave energy potential. The interaction of multiple cavities has an additional effect in the propagation and formation of multiple standing waves, ultimately leading to two-dimensional and multi-modal seiche waves. Therefore, a detailed analysis of the seiche amplitude and energy spatial distribution is presented.</p>

Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

Previous studies that investigated orographic precipitation have primarily focused on isolated mountain barriers. Here we investigate the influence of low-level blocking and shear-induced turbulence on the cloud microphysics and precipitation formation in a complex inner-Alpine valley. The analysis focuses on a mid-level cloud in a post-frontal environment, by combining observations from an extensive set of instruments including ground-based remote sensing instrumentation, in situ instrumentation on a tethered balloon system and ground-based precipitation measurements. During this event, the boundary layer was characterized by a blocked low-level flow and a turbulent shear layer, which separated the blocked layer near the surface from the stronger cross-barrier flow aloft. Cloud radar observations indicate changes in the microphysical cloud properties within the turbulent shear layer including enhanced linear depolarization ratio (i.e., change in particle shape) and increased radar reflectivity (i.e., enhanced ice growth). Based on the ice particle habits observed at the surface, we suggest that needle growth and aggregation occurred within the turbulent layer and that collisions of fragile ice crystals (e.g., dendrites, needles) might have contributed to secondary ice production. Additionally, in situ instrumentation on the tethered balloon system observed the presence of a low-level feeder cloud above a small-scale topographic feature, which dissipated when the low-level flow turned from a blocked to an unblocked state. Our observations indicate that the low-level blocking (due to the downstream mountain barrier) caused the low-level flow to ascend the leeward slope of the local topography in the valley, thus producing a low-level feeder cloud. Although the feeder cloud did not enhance precipitation in the present case, we propose that local flow effects such as low-level blocking can induce the formation of feeder clouds in mountain valleys and on the leeward slope of foothills upstream of the main mountain barrier, where they can act to enhance orographic precipitation through the seeder-feeder mechanism.

Turbulent shear-layer mixing: initial conditions, and direct-numerical and large-eddy simulations

Aspects of turbulent shear-layer mixing are investigated over a range of shear-layer Reynolds numbers, $Re_{\unicode[STIX]{x1D6FF}}=\unicode[STIX]{x0394}U\unicode[STIX]{x1D6FF}/\unicode[STIX]{x1D708}$, based on the shear-layer free-stream velocity difference, $\unicode[STIX]{x0394}U$, and mixing-zone thickness, $\unicode[STIX]{x1D6FF}$, to probe the role of initial conditions in mixing stages and the evolution of the scalar-field probability density function (p.d.f.) and variance. Scalar transport is calculated for unity Schmidt numbers, approximating gas-phase diffusion. The study is based on direct-numerical simulation (DNS) and large-eddy simulation (LES), comparing different subgrid-scale (SGS) models for incompressible, uniform-density, temporally evolving forced shear-layer flows. Moderate-Reynolds-number DNS results help assess and validate LES SGS models in terms of scalar-spectrum and mixing estimates, as well as other metrics, to $Re_{\unicode[STIX]{x1D6FF}}\lesssim 3.3\times 10^{4}$. High-Reynolds-number LES investigations to $Re_{\unicode[STIX]{x1D6FF}}\lesssim 5\times 10^{5}$ help identify flow parameters and conditions that influence the evolution of scalar variance and p.d.f., e.g. marching versus non-marching. Initial conditions that generate shear flows with different mixing behaviour elucidate flow characteristics in each flow regime and identify elements that induce p.d.f. transition and scalar-variance behaviour. P.d.f. transition is found to be largely insensitive to local flow parameters, such as $Re_{\unicode[STIX]{x1D6FF}}$, or a previously proposed vortex-pairing parameter based on downstream distance, or other equivalent criteria. The present study also allows a quantitative comparison of LES SGS models in moderate- and high-$Re_{\unicode[STIX]{x1D6FF}}$ forced shear-layer flows.