Large-scale Structures In Turbulent Flows Research Articles

Comparative assessment of URANS, SAS and DES turbulence modeling in the predictions of massively separated ship airwake characteristics

An early assessment of the ship airwake characteristics is one of the most challenging tasks associated with the designing of vessels. The design of warship superstructures has traditionally followed the basic polyhedron shape (box type structures) to achieve the desired stealth capability. However, presence of such a box shape bluff superstructure generates massively separated airwake over the ship helodeck region. This airwake results into complex flow phenomena which carry strong velocity gradients in space and time, along with widely varying turbulence length scales. Under such conditions, the launch and recovery of a shipboard helicopter operations are very hazardous. Thus, an accurate assessment of the resultant ship airwake flow phenomena at early design stages is desirable.We present a comparative time-accurate assessment study in order to gain a better understanding of the capability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS), the Scale-Adaptive Simulation (SAS) and the Detached Eddy Simulation (DES) turbulence models in predicting turbulent ship airwake characteristics. Detailed comparisons are conducted with respect to the in-house experimental data. Results show that the DES and SAS produce nearly similar trends of the mean flow properties when compared to the experimental results. However, comparisons of velocity spectra indicate that SAS can resolve the dominant large-scale turbulent flow structures with less computational burden. Further, this study also attempts to compare the variation of mean flow quantities with steady RANS approach in order to quantify the percentage variation between the predictions of the steady and unsteady turbulence modelling approach.

Turbulence modifications induced by the bed mobility in intense sediment-laden flows

An experimental dataset of high-resolution velocity and concentration measurements is obtained under intense sediment transport regimes to provide new insights into the modification of turbulence induced by the presence of a mobile sediment bed. The physical interpretation of the zero-plane level in the law of the wall is linked to the bed-level variability induced by large-scale turbulent flow structures. The comparison between intrinsic and superficial Reynolds shear stresses shows that the observed strong bed-level variability results in an increased covariance between wall-normal ($w^{\prime }$) and streamwise ($u^{\prime }$) velocity fluctuations. This appears as an additional Reynolds shear stress in the near-wall region. It is also observed that the mobile sediment bed induces an increase of turbulence kinetic energy (TKE) across the boundary layer. However, the increased contribution of interaction events ($u^{\prime }w^{\prime }>0$, i.e. quadrants I and III in the ($u^{\prime },w^{\prime }$) plane) induces a decrease of the turbulent momentum diffusion and an increase of the turbulent concentration diffusion in the suspension region. This result provides an explanation for the modification of the von Kármán parameter and the turbulent Schmidt number observed in the literature for intense sediment transport.

Natural snowfall reveals large-scale flow structures in the wake of a 2.5-MW wind turbine

To improve power production and structural reliability of wind turbines, there is a pressing need to understand how turbines interact with the atmospheric boundary layer. However, experimental techniques capable of quantifying or even qualitatively visualizing the large-scale turbulent flow structures around full-scale turbines do not exist today. Here we use snowflakes from a winter snowstorm as flow tracers to obtain velocity fields downwind of a 2.5-MW wind turbine in a sampling area of ~36 × 36 m(2). The spatial and temporal resolutions of the measurements are sufficiently high to quantify the evolution of blade-generated coherent motions, such as the tip and trailing sheet vortices, identify their instability mechanisms and correlate them with turbine operation, control and performance. Our experiment provides an unprecedented in situ characterization of flow structures around utility-scale turbines, and yields significant insights into the Reynolds number similarity issues presented in wind energy applications.

Flow and turbulent structures around simplified car models

External car aerodynamics study has great importance in overall car efficiency and ride stability, being a key element in successful automotive design. The flow over car geometries shows three dimensional and unsteady turbulent characteristics. Additionally, vortex shedding, flow reattachment and recirculation bubbles are also found around the bluff body. These phenomena greatly influence the lift and drag coefficients, which are fundamental for ride stability and energy efficiency, respectively. The aim of the present study is focused on the assessment of different LES models (e.g. VMS or SIGMA models), as well as to show their capabilities of capturing the large scale turbulent flow structures in car-like bodies using relative coarse grids. In order to achieve these objectives, the flow around two model car geometries, the Ahmed and the Asmo cars, is simulated. These generic bluff bodies reproduce the basic fluid dynamics features of real cars. First, the flow over both geometries is studied and compared against experimental results to validate the numerical results. Then, different LES models are used to study the flow in detail and compare the structures found in both geometries.

Flow Structure around Bridge Piers of Varying Geometrical Complexity

Piers with back-to-back stems or columns and piers for which part of the foundation becomes exposed as a result of the development of scour over large periods of time or because of severe flood events are fairly common at bridge waterways. The present paper uses eddy-resolving numerical simulations to study flow and turbulence structure at piers of complex shape and/or with multiple components. In particular, the study considers cases with one and two back-to-back pier columns for which the section of the main column is neither circular nor rectangular. In addition to a design case for which the foundation of each pier column is submerged, the study analyzes a case when scour exposes part of the foundation of the main column. The results show that the shape and size of the pier column have a significant effect on the spatial and temporal distributions of the bed friction velocity induced by the horseshoe vortex system. The large-scale shedding behind the main column greatly influences flow structure and increases bed friction velocity around the downstream column for piers with two back-to-back columns that are aligned with the incoming flow direction. The present study shows that the presence of large-scale unsteady coherent structures in the vicinity of the bed around piers of complex shapes results in very complex distributions of the bed friction velocity and in large-scale temporal oscillations of the bed friction velocity. The results of eddy-resolving simulations strongly suggest the need to account for the effect of these large-scale oscillations around the mean value when bed friction velocity distributions are used to estimate the flux of entrained sediment in movable bed simulations that do not resolve the large-scale turbulent flow structures.

Structural dissimilarity of large-scale structures in turbulent flows over wavy walls

Turbulent flows over a wavy wall are investigated in channels with a wavy bottom and a flat top with different channel heights. Flow structures are determined from proper orthogonal decompositions of velocity fields measured with particle image velocimetry. Three different channel heights are considered, which are characterized by blockage ratios β (half channel height to wave amplitude ratio) 3.3, 6.7, and 10. Measurements are evaluated at comparable Reynolds numbers (Re) around 10 000. Structural similarity of large-scale structures, which is valid at β = 6.7 and 10, no longer holds at β = 3.3. Furthermore, characteristic regions of flows over wavy walls exhibit different locations in the case of the smallest channel height.

Laboratory measurements of large-scale near-bed turbulent flow structures under spilling regular waves

The instantaneous turbulent velocity field created by the breaking of spilling regular waves on a plane slope was measured in a plane running parallel to the slope using particle image velocimetry. The measurement plane was located at a height of about 1 mm above the bed. The measurement area encompassed the region where the large eddies generated at incipient wave breaking impinged on the bottom inside the surf zone. A total of 30 trials were conducted under identical experimental conditions. In each trial, six consecutive wave cycles were recorded. The measured velocity fields were separated into a mean flow and a turbulence component by ensemble averaging. The instantaneous turbulent velocity fields were analyzed to determine the occurrence frequency, location, geometry and evolution of the large eddies, and their contributions to instantaneous shear stresses, turbulent kinetic energy and turbulence energy fluxes. The motion of single glass spheres along the bed was also investigated. The two-phase flow measurements showed that the velocity and displacement of large solid particles on a smooth bed were significantly affected by the magnitude and direction of turbulence velocities. Overall, this study has examined the kinematic and dynamic properties of large eddies impinging on the bed and the interaction of these large-scale turbulent flow structures with the mean flow. The study has also highlighted the important role of large eddies in sediment transport.

Experimental Study of Fine Sand Particle Settling in Turbulent Open Channel Flows over Rough Porous Beds

Results are presented from laboratory studies investigating the behavior of fine sand particles within turbulent open channel flow conditions flowing over rough, porous beds. A particle tracking technique was employed to record and analyze sand particle motion within the flow, while mean and fluctuating flow velocities were measured by an acoustic Doppler velocimeter probe. Measured particle settling rates show a strong influence from flow turbulence, being generally enhanced in the near-bed and intermediate flow regions and retarded in the outer flow region, compared to their fall velocity in still water conditions. Experiments also reveal the relative degree of settling enhancement to increase with decreasing particle size. Correlation between particle and small-scale fluid motions is demonstrated through a quadrant analysis technique, with higher-order events for the two phases found to be dominated by ejections and sweeps associated with the bursting process. Particle interactions with large-scale turbulent flow structures, revealed through flow visualization with a moving frame of reference, are found to result in particle accumulation in peripheral trajectories on the downflow side of local eddy structures. Analytical and theoretical considerations suggest that both these turbulence scales provide preferential transportation mechanisms that will account for the enhanced sand particle settling rates observed.

Comparison of Computational Fluid Dynamics Approaches for Simulating Weapons Bay Flow

Comparison of Computational Fluid Dynamics Approaches for Simulating Weapons Bay Flow

Improved conditional averaging technique for plasma fluctuation diagnostics

Conditional averaging is one of the standard methods to detect large-scale structures in turbulent flows. By means of synthetic and experimental data it is shown that the usual trigger conditions are not sufficient to obtain reliable structure amplitudes. Our analysis shows that the amplitude reduction is caused by trigger jitter and a finite probability for trigger errors to occur. Using an improved conditional averaging scheme based upon a cross-correlation analysis, the possibility to remove these errors and to obtain correct amplitudes is shown.

Size, shape and dynamics of large-scale turbulent flow structures in a gravel-bed river

In this paper, we present a detailed investigation of the size, scale and dynamics of macro-turbulent flow structures in gravel-bed rivers. We used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel-bed river. The array was deployed successively in various configurations in order to quantify the vertical, lateral and longitudinal extent of the flow structures and to estimate their advecting velocities. To depict the spatial and temporal properties of the flow structures, we used space–time velocity matrices, space–time correlation analysis and coherent-structure detection schemes. The results show that the large-scale turbulent flow structures in a gravel-bed river occupy the entire depth of the flow and that they are elongated and narrow. The length of the structures is 3 to 5 times the flow depth while the width is between 0.5 and 1 times flow depth. In spite of the high roughness of the bed, these values are similar to those reported in the literature for laboratory experiments on large-scale turbulent flow structures. The dynamics of the large-scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near-bed region. Our evidence suggests that large-scale flow incursions trigger ejections in the near-bed region that can develop into megabursts that can reach the water surface.

Dynamics of large-scale structures in turbulent flow over a wavy wall

We describe the dynamics of large-scale structures in a developed turbulent flow between a train of waves and a flat wall. A water channel facility, for which the wavelength, A, of the bottom wall equals the channel height and the wave amplitude is ten times smaller, is used. The channel is sufficiently wide so that structures of spanwise scale O{1.5Λ} meander laterally. The paper dicusses the temporal behaviour and the meandering motion at a Reynolds number of 4500, defined with the half channel height and the bulk velocity. Digital particle image velocimetry is performed in a horizontal plane with a field of view of 2.6A x 2.7A. Ten ensembles of 90 consecutive image pairs are acquired at a rate of 15 Hz, a temporal resolution sufficient to assess how the largest flow scales evolve in time. The streamwise velocity u(x, z, t) is filtered using the dominant eigenfunctions that are obtained by a proper orthogonal decomposition analysis. The very large temporal scales of the meandering motion of the O{1.5Λ} structures could be followed over measurement times of up to 6 s, during which they are convected downstream by distance of 65 wavelengths

Speckle tomography of turbulent flows with density fluctuations

The speckle tomography technique is used for reconstructing both large-scale structures in turbulent flows and the microstructure of turbulence. The technique is based on multi-projectional line-of-sight speckle photography measurements with a subsequent computer-assisted tomographic reconstruction of the interior structure of the flowfield. The large-scale structure is reconstructed using the Radon integral equation, and the microstructure is analysed using a statistical approach and a novel Erbeck–Merzkirch integral transform. Digital speckle photography and speckle tomography methods are described. Numerical simulation of the optical technique is performed using digital ray tracing through a turbulent flowfield. The methods are illustrated by the 3D "averaged" temperature fields in turbulent convective flows obtained earlier and by the recent reconstruction of 3D correlation functions of density variations in turbulent flows. Local values of turbulence (Kolmogorov) microscale are evaluated using these correlation functions and the Erbeck–Merzkirch integral transform The precision of the reconstruction and the spatial resolution achieved are analysed.

Shock associated noise of supersonic jets from convergent-divergent nozzles

Results of experimental and theoretical studies of the characteristics of shock associated noise from imperfectly expanded supersonic jets over an extensive range of underexpanded and overexpanded operating conditions are described. This kind of broadband noise is believed to be generated by the weak but coherent interaction between the downstream propagating large scale turbulent flow structures in the mixing layer of the jet and the nearly periodic shock cell system. Theoretical reasoning based on this mechanism leads to the scaling formula that the intensity of shock associated noise varies as ( M j 2 − M d 2) 2 where M j and M d are the fully expanded jet operating Mach number and nozzle design Mach number, respectively. This formula holds for underexpanded as well as overexpanded jet Mach numbers. In addition, a peak frequency formula is also derived from the same model. The noise intensity, directivity and spectra of supersonic jets from a convergent-divergent nozzle of design Mach number 1·67 were measured in an anechoic facility over the Mach number range of 1·1 to 2·0. The effect of jet temperature was investigated by operating the jet at three temperature conditions. These sets of data provide sufficient information for fully assessing the relative importance and characteristics of shock associated noise of supersonic jets from convergent-divergent nozzles. Comparisons between theoretical results and measurements show very favorable agreement.