Large Coherent Structures Research Articles

Optical-acoustics source analysis of supersonic jet noise reduction using micro vortex generators

A new supersonic jet noise reduction technology has been developed using Micro Vortex Generators (MVGs) by the collaboration between the University of Cincinnati and the Naval Research Laboratory. MVGs are used on model scale nozzles that are representative of GE F404 engine nozzles. Noise reductions up to −10 dB have been observed in both laboratory measurements and LES simulations at conditions related to take off in the overexpanded regime. Analysis of the acoustic field and flow field using Schlieren visualization reveal the noise reduction mechanisms associated with MVGs. Direct visualization of the changes in shock cell spacing, Large Coherent Structures (LCS) formation, and their convective velocity are identified and those changes modify the downstream propagating hydrodynamic waves and the upstream propagating acoustics waves. Spectral Proper Orthogonal Decomposition (SPOD) is utilized to examine the flow sources at frequencies associated with the noise components observed in the acoustic spectra to explain the noise reduction mechanisms of MVGs.

Spanwise distribution of the loads on a hydrofoil working in the wake of an upstream propeller

The spanwise distribution of the loads acting on a hydrofoil working in the wake of a propeller is analyzed both as a function of the propeller load and the incidence of the hydrofoil. The analysis is based on data from Large Eddy Simulations, conducted on cylindrical grids consisting of O(109) points. Although very large coherent structures are shed by the propeller at outer and inner radii (tip and hub vortices), the average loads on the hydrofoil are characterized by peaks correlating with the radial coordinate of maximum load of the propeller blades, equivalent to about 70% of their outer radius. The azimuthal velocity of the propeller wake causes a displacement of the stagnation point in front of the hydrofoil, affecting substantially the pressure fields on its port and starboard sides and, as a result, the loads it experiences. This flow physics suggests that twisted geometries, able to decrease the local incidence angle, can produce substantial benefits on the stresses experienced by rudders operating in the wake of upstream propellers.

Coherent structure formation via spatial phase coupling in two-dimensional (potential) vorticity-conserved systems

We employ the phase synchronization concept, which is universally adopted to study collective dynamics in many-body systems, to investigate the formation mechanism of coherent structure in a two-dimensional (potential) vorticity conserved system. It is shown that the gradient of mean (potential) vorticity can mediate ``attractive'' coupling among local vortex excitations, so that the vortices get synchronized and a large coherent structure then forms. The phase synchronization concept develops a method for studying self-organization processes in fluid turbulence.

A three-dimensional dynamic mode decomposition analysis of wind farm flow aerodynamics

High-fidelity large-eddy simulations are suitable to obtain insight into the complex flow dynamics in extended wind farms. In order to better understand these flow dynamics, we use dynamic mode decomposition (DMD) to analyze and reconstruct the flow field in large-scale numerically simulated wind farms by large-eddy simulations (LES). Different wind farm layouts are considered, and we find that a combination of horizontal and vertical staggering leads to improved wind farm performance compared to traditional horizontal staggering. We analyze the wind farm flows using the amplitude selection (AP) and sparsity-promoting (SP method) DMD approach. We find that the AP method tends to select modes with a small length scale and a high frequency, while the SP method selects large coherent structures with low frequency. The latter are somewhat reminiscent of modes obtained using proper orthogonal decomposition (POD). We find that a relatively limited number of SP-DMD modes is sufficient to accurately reconstruct the flow field in the entire wind farm, whereas the AP-DMD method requires more modes to achieve an accurate reconstruction. Thus, the SP-DMD method has a smaller performance loss compared to the AP-DMD method in terms of the reconstruction of the flow field.

Large‐eddy simulation of very stable boundary layers. Part II: Length scales and anisotropy in stratified atmospheric turbulence

AbstractIn very stable boundary layers (SBLs), turbulence is expected to be weak, anisotropic, and non‐stationary. Moreover, viscous effects likely become relevant under very weak‐wind conditions. This study investigates the stratification regimes based on the relative strength of the forces that control the flow (i.e., buoyancy‐ or viscosity‐dominated flow) and the degree of turbulence anisotropy in large‐eddy simulations of the very SBL with weak winds. The simulations explored here correspond to an Ekman‐layer‐type boundary layer with geostrophic winds equal to and 2 ms, and surface cooling rates of 1 and 3 Khr, at high latitude. According to the buoyancy Reynolds number and the horizontal Froude number , these SBLs are in strongly stable conditions (i.e., and ). Moreover, the vertical profiles of indicate that the turbulent flows are in an energetic state near the surface but gradually transition to a viscosity‐affected stratified state in the upper part of the SBL as viscous effects become important. For the most stable simulation, the results show that even the small scales are somewhat affected by buoyancy. The invariant analysis of the Reynolds stress anisotropy tensor shows that the flow is in an anisotropic state and is governed by the streamwise component of the turbulent flux (one‐component turbulence). Additionally, based on the two‐point spatial correlation function, the shape of the large coherent turbulent structures is nearly isotropic in the vertical direction and anisotropic in the horizontal direction, and their size increases with height. The evaluation of the turbulent kinetic energy budget shows that the most stable simulation is in a non‐stationary state, which impacts the turbulence mixing efficiency measured by the flux Richardson number.

A Ship-Based Characterization of Coherent Boundary-Layer Structures Over the Lifecycle of a Marine Cold-Air Outbreak

Convective coherent structures shape the atmospheric boundary layer over the lifecycle of marine cold-air outbreaks (CAOs). Aircraft measurements have been used to characterize such structures in past CAOs. Yet, aircraft case studies are limited to snapshots of a few hours and do not capture how coherent structures, and the associated boundary-layer characteristics, change over the CAO time scale, which can be on the order of several days. We present a novel ship-based approach to determine the evolution of the coherent-structure characteristics, based on profiling lidar observations. Over the lifecycle of a multi-day CAO we show how these structures interact with boundary-layer characteristics, simultaneously obtained by a multi-sensor set-up. Observations are taken during the Iceland Greenland Seas Project’s wintertime cruise in February and March 2018. For the evaluated CAO event, we successfully identify cellular coherent structures of varying size in the order of 4 times 10^2 m to 10^4 m and velocity amplitudes of up to 0.5 m hbox {s}^{-1} in the vertical and 1 m hbox {s}^{-1} in the horizontal. The structures’ characteristics are sensitive to the near-surface stability and the Richardson number. We observe the largest coherent structures most frequently for conditions when turbulence generation is weakly buoyancy dominated. Structures of increasing size contribute efficiently to the overturning of the boundary layer and are linked to the growth of the convective boundary-layer depth. The new approach provides robust statistics for organized convection, which would be easy to extend by additional observations during convective events from vessels of opportunity operating in relevant areas.

Improved Quadrant Analysis for Large-Scale Events Detection in Turbulent Transport

Quadrant analysis has been widely used to investigate the turbulent characteristics in the atmospheric boundary layer (ABL). Although quadrant analysis can identify turbulent structures that contribute significantly to turbulent fluxes, the approach to the hyperbolic hole and its parameter, referred to as hole size, remains uncertain and varies among different studies. This study discusses an improved quadrant analysis with an objective definition of the hole size for the isolation of large coherent structures from small-scale background fluctuations. Eddy covariance data collected 50 m above the grass canopy were used to analyze and evaluate the proposed method. This improved quadrant analysis ensures that the detected large coherent eddies play a dominant role in transporting fluxes, occupying 10% of the total time, with mean flux contributions ranging from 62% to 95% for momentum and 35–104% for scalars. The separated background small-scale eddies are isotropically characterized by a comparable time duration and flux contributions in each quadrant. It is observed that under an unstable atmosphere, large-scale ejections are more active than sweeps, while under stable conditions, they are equally important. Furthermore, mechanical-driven transport under near-neutral conditions only enhances ejection and sweep motions of momentum. In contrast, the buoyancy-driven scenarios under unstable conditions enhance the large-scale activities of sensible heat alone.

Wave-packets in a reacting, imperfectly-expanded supersonic jet

The present work focuses on the large coherent structures of a turbulent, reacting and imperfectly-expanded supersonic jet. These have been extracted from a Large Eddy Simulation (LES) including chemical reactions by Spectral Proper Orthogonal Decomposition (SPOD). While SPOD is now a standard tool for non-reactive flows, very few studies have dealt with reactive jets. The article shows that the pressure and axial velocity fields exhibit coherent wave-packets with a well-defined wavelength and amplitude envelope. The shock/recompression cells have a strong effect on the amplitude of the wave-packet, but a weak effect on the wavelength. The temperature field exhibits a lower self-coherence and exhibits small scale structures upstream, generated within the mixing layer, and an irregular pattern of larger structures downstream in agreement with Prasad and Morris (2020) [65]. A high-energy coherent structure has been observed at a Strouhal number of St=0.4 (normalized by the jet diameter and the jet exit velocity), which appears to result from a mutual reinforcement between the main shear layer instability and the vortex shedding from the normal shock. An attempt is made to model the wave-packets using the parabolized stability equations (PSE) about the mean flow, ignoring the chemical reactions. The PSE solution fails to model the temperature fluctuations, but shows rather good agreement with the leading SPOD modes of pressure and axial velocity downstream. This reveals that the flame impacts the pressure and velocity wave-packets in a weak manner: the coherent structures in the pressure and velocity field are generated by hydrodynamic convective instability, without being significantly altered by the reactive nature of the flow.

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer (TBL) is actively controlled by the synthetic jet generated from a circular hole. According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry (TR-PIV) system, the average drag reduction rate of 6.2% in the downstream direction of the hole is obtained with control. The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream, while the reverse vortex decays rapidly. From the statistical average, it can be found that a low-speed streak is generated downstream. Induced by the two vortex legs, the fluid under them converges to the middle. The drag reduction effect produced by the synthetic jet is local, and it reaches a maximum value at x + = 400, where the drag reduction rate reaches about 12.2%. After the extraction of coherent structure from the spatial two-point correlation analysis, it can be seen that the synthetic jet suppresses the streamwise scale and wall–normal scale of the large scale coherent structure, and slightly weakens the spanwise motion to achieve the effect of drag reduction.

Experimental Study of Energy Contribution by Coherent Structures in a Turbulent Boundary Layer

Kang, Y.-D. and Koo, B., 2021. Experimental study of energy contribution by coherent structures in a turbulent boundary layer. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 579–583. Coconut Creek (Florida), ISSN 0749-0208. An experimental study was carried out to investigate a contribution of large coherent structures in turbulent flow. Both dynamometer and Clauser fitting method were adopted to calculate the frictional velocity near the wall having uncertainty. In order to quantify coherent structures, Particle Image Velocimetry (PIV) system was used to reveal their characteristics connecting through boundary layer. Variable Interval Time Averaging (VITA) technique scanned through flow fluctuations at the edge of boundary layer, whether there was a large deviation from the background flow. According to the presence of large scale structures, the flow field was divided into weak and strong cases to validate their existence. Changing a free-stream velocity from 0.5 m/s to 1.0 m/s increased the Reynolds stress more than 3 times. Proper Orthogonal Decomposition (POD) analysis was also applied to compare the energy budgets between them.

Time-resolved sphere and fluid motions in turbulent boundary layers

This paper extends the study by Tee et al. (2020) to investigate the effect of large coherent structures on motion of spheres with specific gravities of 1.006 (P1) and 1.152 (P3) at Reτ = 670 and 1300 (d+ = 56 and 116). The sphere and fluid motions are tracked simultaneously via 3D particle tracking and stereoscopic particle image velocimetry over the streamwise-spanwise plane, respectively. With sufficient mean shear, sphere P1 lifts off of the wall upon release before descending back towards the wall at both Reτ. It typically accelerates strongly over a streamwise distance of less than one boundary layer thickness before approaching an approximate terminal velocity. By contrast, the denser sphere P3 does not lift off upon release but mainly slides along the wall. At lower Reτ where wall friction is stronger, this sphere translates with unsteady velocity, significantly lagging the local fluid. The streamwise velocities of both spheres correlate strongly with the fast- and slow-moving zones that approach and move over them. In most runs, both spheres lag the local coherent structures and travel with either fast- or slow-moving zones throughout the observed trajectories. Vortex shedding, which is most prevalent for sphere P3 at Reτ = 670, is also important. The sphere spanwise motion is prompted by wall friction, spanwise fluid motion, and/or meandering of the coherent structures, and spheres do not appear to migrate preferentially into slow-moving zones.

Friction drag reduction based on a proportional-derivative control scheme

Dielectric barrier discharge plasma actuators (DBD-PAs) are deployed experimentally for the first time in a feed-forward proportional-derivative (PD) control system, where the fluctuating wall-pressure Pw is demonstrated to be an effective feed-forward signal, to manipulate a turbulent boundary layer for drag reduction. A floating-element force balance with an area of 50 mm (streamwise length) × 200 mm (spanwise length) is deployed to capture the spatially averaged drag variation behind the DBD-PAs. The DBD-PAs generate streamwise vortices, whose occurrence synchronizes with the output signal of the controller with a predominant frequency of 40 Hz under the optimally tuned PD control. The control system proves to be effective, achieving a spatially averaged drag reduction by 16%, and efficient, cutting down its energy consumption by 30% at a negligibly small expense of drag reduction compared with the open-loop control. It has been found that the optimally tuned PD control aptly increases the voltage applied to the DBD-PAs upon detecting large Pw fluctuations or coherent structures, accounting for the savings in input power, Pinput. The experimental data have been carefully analyzed, which cast light upon the underlying physical mechanism behind the drag reduction. The reason behind the efficient control is also clearly elaborated.

The impact of the seabed morphology on turbulence generation in a strong tidal stream

Highly energetic turbulent flow structures are observed in strong tidal flows. If they are suspected to result from interactions between the flow and the seabed morphology, the physical processes involved in their generation, as well as their impact on the structure of the flow, are not yet fully understood. Here, the lattice-Boltzmann method is used to simulate a strong tidal flow in large-eddy simulation. The effect of the seabed morphology on the hydrodynamic characteristics of the flow is studied. A high spatial variability of the turbulent kinetic energy (TKE) production is observed. The flow average velocity is significantly reduced in areas of high TKE production. These areas are not necessarily associated with the largest seabed landforms. However, some seabed landforms of specific shapes are identified as turbulence generators. The areas of high turbulence are associated with trails of vortices successively released from the seabed and following similar trajectories. The generation of a large coherent flow structure is observed at the intersection of two vortex trails, suggesting that such a structure, that could be identified as the large boils commonly observed at the surface of strong tidal power flows, could result from the aggregation of smaller vortices.

Influence of gap-ratio on flow dynamics and heat transfer for a square cylinder approaching a moving wall in turbulent regime

Flow past a square cylinder near a moving wall is investigated for 0.1 ≤ G/D (gap-ratio) ≤ 1.0 at a constant Reynolds number of 22,000 using large eddy simulations. Influence of the gap-ratio on various flow features has been studied. It is observed that for G/D ≤ 0.3, a regular Kármán vortex shedding from the cylinder gets suppressed which causes a large change in the flow dynamics. Unlike the case of a stationary wall, no ground vortex is formed in the present case. For G/D = 1.0, turbulence is strong in the near wake region and for low G/D values (≤ 0.3) it gets weaker and shifts downstream. With a decrease in G/D, an early reattachment of the lower shear-layer with the bottom surface of the cylinder occurs while the upper shear-layer is deflected and K-H instability occurs away from the top surface. The so-called ‘extreme events’ which contain high frequency fluctuations are observed for G/D = 1.0 and these disappear for low G/D values. Large coherent structures are observed for high G/D (≥ 0.5) while for low G/D uniformly distributed small structures are formed. Evolution of enstrophy has been examined for G/D = 0.1 and the role of the strain-rate tensor in the enstrophy production has been investigated. The lift coefficients are negative for all G/D values while drag coefficients decrease with a decrement in G/D and are nearly constant for G/D ≤ 0.3. Due to the formation of large coherent structures, power spectra for high G/D follows -5/3 law over a large range of frequency. Unlike the case of a laminar flow, Nusselt number on the cylinder surface monotonically decreases with a decrease in G/D.

Large eddy simulation of film cooling effectiveness on a turbine vane pressure side with a saw-tooth plasma actuator

The effects of a saw-tooth plasma actuator (STPA) on the pressure side film cooling in a turbine vane have been studied by using large eddy simulation. The phenomenological plasma model was adopted to solve the electrohydrodynamic (EHD) force vector generated by the STPA, which was placed downstream of the cooling hole. The time-averaged flow fields without and with the STPA are comparatively analyzed, and results show that the jet flow reattaches to the pressure surface earlier owing to the EHD force vector. Furthermore, the momentum injection effects of the STPA reduce the jet lift-off caused by the counter rotating vortex pair (CRVP) and help the wall vortex pair entraining more coolant to the pressure surface. Then the spatial-temporal evolutions of coherent structures are presented in conjunction with the variations of the centerline temperature, which show that the elongated CRVPs are the dominated flow patterns of the film cooling, and they reduce in the structure size and extend farther downstream distance due to the STPA aerodynamic actuation. Consequently, the intermittent coherent structures are shed from the tail end of the CRVPs later. Moreover, the coherent structures are reduced in the structure size and intermittency because of the STPA aerodynamic actuation, and the centerline temperature is varied with their developments while approaching downstream. Finally, the iso-surfaces of the transient temperature qualitatively and quantitatively indicate that the STPA aerodynamic actuation enlarges the wall-coverage of the coolant. Overall, the STPA aerodynamic actuation greatly improves the film cooling effectiveness by regulating the evolutions of the large scale coherent structures, especially in the range of 40% to 60% blade axial chord.

The outer disc in shambles: Blind detection of Monoceros and the ACS with Gaia’s astrometric sample

Context. The Gaia astrometric sample allows us to study the outermost Galactic disc, the halo, and their interface. It is precisely at the very edge of the disc where the effects of external perturbations are expected to be the most noticeable. Aims. Our goal is to detect the kinematic substructure present in the halo and at the edge of the Milky Way (MW) disc and provide observational constraints on their phase-space distribution. Methods. We download, one HEALpix at a time, the proper motion histogram of distant stars, to which we apply a wavelet transformation to reveal the significant overdensities. We then analyse the large coherent structures that appear in the sky. Results. We reveal a sharp yet complex anticentre dominated by Monoceros (MNC) and the Anticentre Stream (ACS) in the north – which we find have intensities comparable to the Magellanic Clouds and the Sagittarius stream – and by MNC South and TriAnd at negative latitudes. Our method allows us to perform a morphological analysis of MNC and the ACS, both of which span more than 100° in longitude, and to provide a high purity sample of giants with which we track MNC down to latitudes as low as ∼5°. Their colour-magnitude diagram is consistent with extended structures at a distance of ∼10−11 kpc that originated in the disc, with a very low ratio of RR Lyrae over M giants, and with kinematics compatible with the rotation curve at those distances or slightly slower. Conclusions. We present a precise characterisation of MNC and the ACS, two previously known structures that our method reveals naturally, allowing us to detect them without limiting ourselves to a particular stellar type and, for the first time, using only kinematics. Our results will allow future studies to model their chemo-dynamics and evolution, thus constraining some of the most influential processes that shaped the MW.

Single-shot observation of breathers from noise-induced modulation instability using heterodyne temporal imaging.

We report phase and amplitude measurements of large coherent structures originating from the noise-induced modulation instability in optical fibers. By using a specifically designed time-lens system (SEAHORSE) in which aberrations are compensated, the complex field is recorded in single-shot over long durations of 200 ps with sub-picosecond resolution. Signatures of Akhmediev breather-like patterns are identified in the ultrafast temporal dynamics in very good agreement with numerical predictions based on the nonlinear Schrödinger equation.

Experimental study on the three-dimensional wake structure of the DrivAer standard model

An automatic particle imaging velocimetry (Auto-PIV) measuring technique was developed based on the external triggering and automatic control technology. Measurements of the DrivAer model wake were performed using Auto-PIV in the spanwise and vertical direction. Three-dimensional time-averaged velocity distribution was reconstructed and the spatial distribution of the large coherent structures was described. The results indicate that regions with high velocity fluctuations mainly locate at the shear layer of the upper and lower edge of the recirculation bubble. Further proper orthogonal decomposition results show that the velocity fluctuations account for a very little proportion of kinetic energy compared with the time-averaged velocity, supporting that the DrivAer model has steady aerodynamic characteristics. The smooth outlines of the DrivAer notchback model can delay the flow separation on the top and the C-pillar of the model, which inhibits the formation of the back recirculation bubble at the rear window and reduces the size of the recirculation bubble behind the baggage compartment. These promote the pressure recovery at the rear of the DrivAer notchback model, and thus reduces the pressure drag. At the rear of the model, the airflow on both sides converges towards the center due to the pressure difference, producing the streamwise vorticity by shearing the baggage compartment, then separates at the rear of the baggage compartment and generates a pair of secondary vortices. Overall, the aerodynamic behavior of the DrivAer model is robust.

A Python implementation in graphic processing unit of a lattice Boltzmann model for unstable three-dimensional flows in immersed permeable media

The implementation of a lattice Boltzmann model for three-dimensional permeable media with localized drag forces is presented. The model was previously introduced for two-dimensional geometries and follows the basics of the immersed boundary method. Permeable flows are much less stable than their counterparts in porous media and generally produce large coherent flow structures, such as vortex lines, rolls, and wakes. In addition, in permeable media, the small-scale geometry often needs to be represented to a high degree of detail in order to capture certain transport phenomena, such as micro-convection or pollination. Hence, both calculation speed and memory requirements are under strain. The present model was implemented in a graphic processing unit showing excellent performance in the calculation of stable and unstable flows in a rectangular channel partially obstructed by an array of parallel wires. In particular, the model is able to deal with small and medium spatial scales without losing the heterogeneous nature of permeable flows in the homogenization process. The algorithm to manage memory issues is described in detail, and the results of the test case for stable and unstable conditions show the capability of the method to simulate these types of flows.

Anomalous transport due to large coherent structures in a magnetized toroidal plasma

Anomalous transport due to large coherent structures, or coherent events, is studied experimentally in a magnetized toroidal discharge plasma. The present analysis emphasizes anomalous plasma losses as well as transport of electron thermal energy density across magnetic field lines. The experiment was carried out in the Blaamann device at the University of Tromsø. The diagnostics are based on data for potential, density and electron temperature variations as detected by Langmuir probes. Most of the data analysis uses conditional sampling. The flux of thermal energy density can be broken down into several components which are analyzed separately. Experimental means for obtaining the space-time variation of the complete polarization drifts are demonstrated, and the results compared with the fluctuating -velocities. For the present conditions these polarization drifts contribute only little to the plasma losses, but for other discharge parameters in heavier gases they can be important. Our results show that while the density, the potential and the electric field variations are dominated by large scales, the vorticity of the -flow as well as the ion polarization drifts are strongly intermittent.