Three-dimensional Turbulent Structure Research Articles

Development of a multiple laser-sheet imaging technique for the analysis of three-dimensional turbulent explosion flame structures

The present work describes the development of a multiple laser-sheet imaging technique for the study of turbulent, premixed flame surface structures at high Karlovitz stretch factor values. Experiments were conducted using CH4 air mixtures at 365 K and 0.5 MPa and up to a root mean square turbulence velocity of 1.5 m/s. A high-speed Nd:YAG laser capable of pulsing up to 60 kHz in conjunction with a high-speed camera and a rotating mirror was used to reconstruct time-resolved three-dimensional turbulent flames. This has, for the first time, enabled the direct measurement of wrinkled flame surface areas, along with reaction progress variable and flame brush thickness. These are important parameters for the characterization of turbulent burning rates and provide more insight into the dynamic nature of the flames and their structures. In addition, the current data aid toward direct comparison with results from combustion simulation studies.

Multi-Scale Flow Estimation Within Stereoscopic PIV Processing Based On Deep Learning

Classical methods of particle-image velocimetry (PIV) based on state-of-the-art cross-correlation algorithms constitute a spatial filtering operation that leads to a loss of spatial resolution of the estimated velocity fields. This is associated with numerous limitations, e.g., when it comes to aligning high-fidelity numerical simulations, since the detection of small turbulent scales is often precluded. An optical flow neural network adapted for PIV processing, called RAFT-PIV, can overcome the drawback of reduced spatial resolution while maintaining the reliability and robustness of the classical approach. The present work aims to extend the application of the aforementioned deep learning based method by integrating it within a stereoscopic PIV processing workflow. Experimental data from an optically accessible single-cylinder combustion engine the flow field of which is characterized by complex, three-dimensional flow structures and a broad range turbulent scales are processed using RAFT-PIV. The 2D-3C vector fields obtained from stereoscopic reconstruction are compared against a classical cross-correlation based PIV evaluation, providing insight into instantaneous quantities and turbulent structures. A qualitative comparison of the instantaneous in-plane and out-of-plane velocity components reveals that the reconstructed flow obtained from the deep learning based estimation includes a wide range of resolved turbulent length scales. It outperforms the classical method. Furthermore, the findings of the present study demonstrate that the pixel accuracy has an effect on both physical quantities and turbulence characteristics. In particular, an increase in turbulent kinetic energy is observed across the engine cycles of the investigated engine flow states. Moreover, a higher fraction of three-dimensional isotropic turbulent structures is derived from RAFT-PIV compared to the results from the cross-correlation based technique.

Flow dynamics and turbulent coherent structures around sediment reduction plates of a sewer system

In the management of urban drainage networks, great interest has been generated in the removal of sediments from sewer systems. The unsteady three-dimensional (3D) flow and turbulent coherent structures surrounding sediment reduction plates in a sewer system are investigated by means of the detached-eddy simulation (DES). Particular emphasis is given to detailing the instantaneous velocity and vorticity fields within the grooves, along with an examination of the three-dimensional, long-term, average flow structure at a Reynolds number of approximately 105. Velocity vectors demonstrate continuous flapping of the flow on the groove wall, periodically interacting with ejections of positive and negative vorticity originating from the grooves. The interaction between the three-dimensional groove flow and the shear flow leads to the downstream transport of patches of positive and negative vorticity, which significantly influence sediment transport. The high-velocity shear flows and strong vortices generated in undulating topography, as identified by the Q-criteria, are the key factors contributing to the efficient sediment reduction capabilities of the sediment reduction plates. The sediment reduction plates with partially enclosed structures exhibit low sedimentation rates in grooves on the plate, a broader acceleration region, and a lesser impact on the flow capacity. The results improve the understanding of the hydrodynamics and turbulent coherent structures surrounding the sediment reduction plates while elucidating the driving factors behind the enhancement of sediment scouring and suspension capacities. These results indicate that the redesign of the plates as partially enclosed structures contributes to further improving their sediment reduction performance.

Planar wall plumes bounded by vertical and inclined surfaces

Planar wall plumes are gravity-driven flows where a fluid of lower (or higher) density than the ambient rises (or lowers) along a vertical or inclined wall. This study investigates planar wall plumes at five different wall slopes, ranging from a vertical wall (θ=90°) to a shallow inclination of θ=3°, using highly resolved direct numerical simulations. The three-dimensional turbulent structure of these supercritical flows is investigated in detail along with the streamwise evolution of the depth-averaged quantities. Simulations were performed in very large domains in order to focus attention on the behavior of the plumes in the near self-similar state, far downstream of the inlet. In the self-similar state, key quantities such as the entrainment rate, the basal drag coefficient, the Richardson number (or equivalently the Froude number), and the shape factors reach constant values, which dependent only on the slope. The present simulations, along with earlier results for subcritical currents at shallower slopes, provide a complete description of this dependency.

A Combined Delayed Detached Eddy Simulation and Linearized Navier–Stokes Equation Study on the Generation and Reduction of Aerodynamic Noises Inside Steam Turbine Control Valve With Acoustic Liner

Abstract This study was aimed at numerically investigating the source, generation mechanism, and strategy for reducing aerodynamic noises inside a steam turbine control valve. A delayed detached eddy simulation was performed to extract the three-dimensional unsteady turbulent flow structures formed within the serpentine flow passage of the turbine valve. Acoustic analogies, spatial Fourier transform, and spectral proper orthogonal decomposition on the delayed detached eddy simulation-simulated flow data were complementarily combined to clarify the generation mechanism of tonal and broadband aerodynamic noises. The results showed that broadband noises were produced by wall-attached jet flow and turbulent mixing flow between the annular wall jets and central reverse flow. High-intensity tonal noises were generated by the excitation of multi-order natural acoustic modes of the bell-shaped valve spindle. The intensive acoustic pressure pulsations concentrated inside the bell jar and propagated along the diffuser to the downstream turbine chamber. A novel ring acoustic liner was designed using the acoustic impedance model to reduce the valve noises without sacrificing the flow performance. The noise reduction effectiveness was evaluated by solving the linearized Navier–Stokes equations in the frequency domain.

Three-dimensional flow and turbulence structure around Sea Cucumber Apostichopus japonicus

The sea cucumber Apostichopus japonicus (A. japonicus) is a benthic invertebrate with a quasi-cylindrical body and several rows of papillae (conical prominence) on its body wall. As nutrient recyclers, A. japonicus plays a fundamental role in marine ecosystems and is one of the most important commercial aquaculture species in eastern Asia. Despite its ecological and economical importance, little is known concerning the flow characteristics surrounding A. japonicus on the organism scale. Research on the detailed flow structure at such a scale may serve as a significant step toward understanding the transport of sediment and nutrients in the vicinity of A. japonicus. This study sets up the D3Q27 multiple-relaxation-time lattice Boltzmann method to obtain the three-dimensional flow structure around an individual A. japonicus placed at the bottom of a flat-bed channel. The numerical model is validated by a water flume experiment. The flow patterns exhibit distinctive features at three different angles of attack (0°, 45°, 90°). The increasing inflow velocity (0.1m/s, 0.25m/s, 0.4m/s) brings no fundamental change to the structure of flow fields but induces more small-scale vortices and larger velocity fluctuations. This study investigates the complex turbulent flow fields around the A. japonicus and provides new insight for future studies on the interaction between water flow and A. japonicus.

Flow Dynamics and Pollutant Transport at an Artificial Right-Angled Open-Channel Junction with a Deformed Bed

Artificial open-channel junctions have sharp corners and a smaller width-to-depth ratio. Despite numerous studies on channel junctions, knowledge on turbulent structures and their role in mixing at artificial junctions is lacking. To fill this research gap, the present study uses a large-eddy simulation (LES) model to investigate the three-dimensional (3D) turbulent structures and their role in pollutant transport at a right-angled laboratory open-channel junction with a deformed bed. The deformed-bed junction represents a quasi-equilibrium condition and consists of a scour zone and deposition bar. The numerical model is validated against experimental data of the velocity field and turbulent kinetic energy. Comparisons of the flow field between the flat-bed condition and quasi-equilibrium deformed-bed condition showed a reduced flow separation zone and less developed recirculating gyre in the latter case because of the strong secondary currents. The coherence of the turbulent structures was drastically disrupted at the deformed-bed junction because the Kelvin-Helmholtz (KH) instability results in more randomly oriented residuals. In contrast, the breakdown of the shear layer at a flat-bed junction displayed the trail of the arch-shaped vortices. The role of turbulent structures in pollutant transport is elucidated by using a neutrally buoyant conservative tracer. Large turbulent structures associated with the KH instability help the tracer evolve faster at the deformed-bed junction.

Reconstruction of three-dimensional turbulent flow structures using surface measurements for free-surface flows based on a convolutional neural network

A model based on a convolutional neural network (CNN) is designed to reconstruct the three-dimensional turbulent flows beneath a free surface using surface measurements, including the surface elevation and surface velocity. Trained on datasets obtained from the direct numerical simulation of turbulent open-channel flows with a deformable free surface, the proposed model can accurately reconstruct the near-surface flow field and capture the characteristic large-scale flow structures away from the surface. The reconstruction performance of the model, measured by metrics such as the normalised mean squared reconstruction errors and scale-specific errors, is considerably better than that of the traditional linear stochastic estimation (LSE) method. We further analyse the saliency maps of the CNN model and the kernels of the LSE model and obtain insights into how the two models utilise surface features to reconstruct subsurface flows. The importance of different surface variables is analysed based on the saliency map of the CNN, which reveals knowledge about the surface–subsurface relations. The CNN is also shown to have a good generalisation capability with respect to the Froude number if a model trained for a flow with a high Froude number is applied to predict flows with lower Froude numbers. The results presented in this work indicate that the CNN is effective regarding the detection of subsurface flow structures and by interpreting the surface–subsurface relations underlying the reconstruction model, the CNN can be a promising tool for assisting with the physical understanding of free-surface turbulence.

Three-Dimensional Numerical Simulation of a Flat Plate Perpendicularly Submitted to Current With a Blockage Ratio of 0.214: URANS and Detached Eddy Simulation

Abstract The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier–Stokes (URANS) based on the realizable k–epsilon (RKE) turbulence model, URANS based on the k–omega shear stress transport (SST) turbulence model, and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117,000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate predictions of all. Moreover, by comparing the instantaneous vorticity contours and three-dimensional (3D) turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF)-based simulation of the free surface, it is demonstrated that the free surface has no significant effect on the mean drag coefficient and Strouhal number.

Numerical analysis of the flow in a cylindrical container injected air from a slit nozzle

This paper describes the flow characteristics of air injected from a slit nozzle in a cylindrical container. The numerical simulation was performed by the large-eddy simulation (LES) using ANSYS Fluent 19.0 based on the finite-volume method. The non-dimensional nozzle offset distance was set to yn/D = 0.4, where D is the diameter of the cylindrical container. The Reynolds number of the plane jet flow was settled at 2,000. The velocity profiles by the numerical simulation agreed with the experimental results. The jet reached near the bottom of container, and the jet flowed out from the left side of container. Although the flow in the cylindrical container showed a three-dimensional turbulent flow structure, the mean velocity field was generated symmetrically with respect to the central cross-section of the container perpendicular to the slit nozzle. The large recirculating flow around the z-axis occurred near the central cross-section of the container by the jet. In addition, the weak recirculated flows around the y-axis were observed near the upper side wall in the container.

Wind tunnel measurement of three-dimensional turbulent flow structures around a building group: Impact of high-rise buildings on pedestrian wind environment

The wind velocity and surface pressure of the model of a group of urban blocks made of low-rise buildings with a high-rise building in the center were measured using a wind tunnel. Not only the time-averaged velocity field, but also the turbulent statistics, i.e., the turbulent kinetic energy, normal component of the Reynolds stresses, gust factor (GF), and probability density function (PDF), and power spectrum density (PSD) of instantaneous velocity components were investigated. Based on the results, the impact of high-rise buildings on the general flow structure and the surrounding pedestrian wind environment was discussed. The presence of high-rise buildings drastically changes the pedestrian wind environment around buildings in terms of time-averaged and instantaneous flow properties due to the complex interaction between the flows around the building and in and above the surrounding street canyons. High-rise buildings create a large GF region where the downwash flow and the street flow collide. A clear correlation between the GF and the time-averaged horizontal velocity magnitude was observed regardless of the height of the central building. The PDF of instantaneous velocity components around the high-rise building was characterized by a large skew not only near the building corners of the high-rise building but also on the surrounding streets. The high-rise building produces large PSDs of the velocity fluctuation components, particularly near the upwind corner at a high frequency and on the surrounding street at an intermediate frequency. Furthermore, the results of the time-averaged and instantaneous flow properties provide a useful database for computational fluid dynamics (CFD) validation and model development studies.

A new idea to predict reshocked Richtmyer–Meshkov mixing: constrained large-eddy simulation

Abstract

Atmospheric turbulence profiling with multi-aperture scintillation of a Shack–Hartmann sensor

ABSTRACT Adaptive optics (AO) systems that use tomographic estimation of the three-dimensional structure of atmospheric turbulence require the vertical atmospheric turbulence profile, which describes turbulence strength as a function of altitude as prior information. We propose a novel method to reconstruct the profile by applying a multi-aperture scintillation sensor (MASS) method to scintillation data obtained by a Shack–Hartmann wavefront sensor (SH-WFS). Compared with a traditional MASS, which uses atmospheric scintillation within four concentric annular apertures, the new method utilizes scintillation in several hundreds of spatial patterns, which are created by combinations of SH-WFS subapertures. Accuracy of the turbulence profile reconstruction is evaluated with Bayesian inference, and it is confirmed that the turbulence profile with more than 10 layers can be reconstructed because of the large number of constraints. We demonstrate the new method with a SH-WFS attached to the 51-cm telescope at Tohoku University and we confirm that the general characteristics of the atmospheric turbulence profile are reproduced.

Transition in atmospheric boundary layer turbulence structure from neutral to convective, and large-scale rolls

Abstract

Experimental study of a turbulent structure in coplanar channels

The experimental results of a three-dimensional turbulent flow structure inside the cells of flat coplanar channels with mutually intersecting edges on opposite walls are presented. In the experiments, a two-component laser Doppler speed meter is used, which allows measuring the averaged and pulsating velocity components. It is shown that a flow structure varies depending on a place of measurements inside a coplanar channels cell.

Numerical Simulation of Coherent Structures in the Turbulent Boundary Layer under Different Stability Conditions

Coherent structures in the turbulent boundary layer were investigated under different stability conditions. Qualitative analyses of the flow field, spatial correlation coefficient field and pre-multiplied wind velocity spectrum showed that the dominant turbulent eddy structure changed from small-scale motions to large- and very-large-scale motions and then to thermal plumes as the stability changed from strong stable to neutral and then to strong unstable. A quantitative analysis of the size characteristics of the three-dimensional turbulent eddy structure based on the spatial correlation coefficient field showed that under near-neutral stability, the streamwise, wall-normal and spanwise extents remained constant at approximately 0.3 δ , 0.1 δ and 0.2 δ ( δ , boundary layer height), respectively, while for other conditions, the extent in each direction varied in a log-linear manner with stability; only the spanwise extent under stable conditions was also independent of stability. The peak wavenumber of the pre-multiplied wind velocity spectrum moves towards small values from stable conditions to neutral condition and then to unstable conditions; thus, for the wind velocity spectrum, another form is needed that takes account the effects of the stability condition.

Mean flow and turbulence structure over exposed roots on a forested floodplain: Insights from a controlled laboratory experiment.

The time-averaged and instantaneous flow velocity structures of flood waters are not well understood for irregular surfaces such as are created by the presence of roots and fallen branches on forested floodplains. Natural flow structures commonly depart systematically from those described for idealised roughness elements, and an important knowledge gap exists surrounding the effects of natural flow structures on vertical exchanges of fluid and momentum. An improved understanding of the flow structure is required to model flows over forested floodplains more accurately, and to distinguish their dynamics from non-vegetated floodplains or indeed floodplains with other vegetation types, such as reed or grass. Here we present a quantification of the three-dimensional structure of mean flow velocity and turbulence as measured under controlled conditions in an experimental flume using a physical reproduction of a patch of forested floodplain. The results conform in part to existing models of local flow structure over simple roughness elements in aspects such as flow separation downstream of protruding roots, flow reattachment, and the lowering of the velocity maximum further downstream. However, the irregular shape of the surface of the floodplain with exposed roots causes the three-dimensional flow structure to deviate from that anticipated based on previous studies of flows over idealised two-dimensional roughness elements. The results emphasise varied effects of inheritance of flow structures that are generated upstream—the local response of the flow to similar obstacles depends on their spatial organisation and larger-scale context. Key differences from idealised models include the absence of a fully-developed flow at any location in the test section, and various interactions of flow structures such as a reduction of flow separation due to cross-stream circulation and the diversion of the flow over and around the irregular shapes of the roots.

Turbulence length scales in a low-roughness near-neutral atmospheric surface layer

This paper investigated the integral length scales of turbulence in a low-roughness atmospheric surface layer (ASL), characterised by very smooth terrain in the Utah desert during near-neutral conditions, and evaluated the Engineering Sciences Data Unit (ESDU) 85020 and 86010 predictions for the turbulence length scales in a low-roughness ASL. The correlation integral method was used to estimate the integral length scales of the velocity components with longitudinal, lateral and vertical separations from sonic measurements on a vertical tower and spanwise array in the Surface Layer Turbulence and Environmental Science Test (SLTEST) field experiment. It was found that the longitudinal integral length scales calculated using near-neutral SLTEST data followed a logarithmic relationship with height proportional to the mean velocity profile with approximately constant integral time scale, however the sizes of the longitudinal components of the energy-containing eddies in the low-roughness flat terrain were 2–3 times smaller than those previously measured during field experiments in open country terrains. The calculated length scales with longitudinal separations over the very smooth terrain characteristics of the salt flats at Dugway were not consistent with those predicted by ESDU 85020. In contrast, the scaling of the lateral and vertical components of the three-dimensional turbulence structure with respect to the longitudinal component in the low-roughness ASL were consistent with similarity theory predictions in ESDU 86010 that the scaling ratios are independent of terrain roughness. Furthermore, this confirms the large dependence of the longitudinal turbulence length scales on the upstream terrain roughness and highlights the large variation of turbulence length scales observed at different low-roughness sites in the literature.

Numerical Investigation of Powered Jet Effects by RANS/LES Hybrid Methods

For the high-precision simulation of engine jet effects, an improved delayed detached eddy simulation (IDDES) method based on the two-equation shear stress transport (SST) model is developed, and the fifth-order finite-volume weighted essentially non-oscillatory (WENO) scheme is employed to enhance accuracy of spatial discretization, and then numerical investigation of powered jet effects by RANS/LES hybrid methods is carried out. The effects of the grid distributions and the accuracy of the spatial schemes are discussed during the RANS/LES validation analysis on the fully expanded jet flow and Acoustic Reference Nozzle (ARN) jet flow. The results show that, by enlarging the grid density and improving the accuracy of the spatial schemes, the velocity distributions in the jet flow can be better predicted, the non-physical steady flow after the jet nozzle can be shortened, the instantaneous flow structures are clearer and the turbulent intensities are more accurate. Then IDDES simulation of turbofan engine jet flow is carried out. The mixing characteristics of the external fan jet flow and internal core jet flow as well as the ambient flow are obtained, and the three-dimensional turbulent structures are also given.

Three-dimensional Eulerian-Lagrangian simulation of particle settling in inclined water columns

We present three-dimensional simulations of sedimentation of suspended solids in inclined vessels using a Eulerian-Lagrangian model developed specifically for the study of solid-liquid mixtures. The simulation results reveal the three-dimensional vortical flow structures that follow the occurrence of flow instability and induce strong mixing between the particle-laden and clear-fluid layers. We show that the tubes exhibit significant spanwise variation due to the presence of wall boundary layers and are subsequently transformed into three-dimensional turbulent structures. The vortices and turbulence result in significant resuspension from the particle-laden layer and weaken the convective effect that enhances sedimentation. We show that for the present flow condition and particle size, particle inertia plays an important role in particle resuspension. Analysis of the release of potential energy is presented to examine the sedimentation efficiency under different flow conditions. We show that the time history for a typical case can be divided into development, convection, and deposition stages. The analysis for cases with different particle sizes shows that the faster settling associated with larger particles stabilizes the flow by reducing the interfacial shear while settling. From simulation results using various angles of inclination, we can find the optimal angle of inclination to achieve the greatest efficiency in settling enhancement. Finally, critical modeling aspects, including the importance of different forcing terms and two- or three-dimensional considerations, are discussed.