Turbulence Integral Length Research Articles

Wire mesh fences for manipulation of turbulence energy spectrum

Manipulation of turbulence within an atmospheric boundary layer flow by application of woven wire mesh fences is investigated. Turbulence properties behind fences of different porosities and mesh opening widths were determined from velocity measurements in a wind tunnel. It is found that with the application of a fence with a porosity of 0.46, the streamwise turbulence intensity can be reduced from the inflow level of 12.5%–8.8% and the integral length scale can be reduced from 380 to 270 mm. The results show that behind the mesh fences turbulence kinetic energy decays as a power law function of the downstream distance for all wire mesh fences tested in the wind tunnel. The decay rate of turbulence kinetic energy is faster, and a larger reduction in the integral length scale is achieved for fences with porosities between 0.46 and 0.64 compared to higher porosities of between 0.73 and 0.75. Porosity of the woven wire meshes is found to be the key parameter which influences their turbulence reduction performance. In the end, application of the wire mesh fences for reduction of wind loads on solar panels and heliostats is discussed. Evaluation of wind loads based on the reduction of turbulence intensity and integral length scale shows that up to 48% and 53% reduction in peak drag and lift forces on a heliostat, respectively, can be achieved with application of mesh fences.

Influence of turbulence integral length scale on aerostatic coefficients of bridge sections

This study investigates the influence of turbulence on the aerostatic coefficients of typical bridges by obtaining force measurements via the simulation of turbulence in wind tunnels. The traditional simulation method for turbulence based on spires and grilles cannot be employed for the accurate simulation of the large integral scale of turbulence in an actual wind field. Thus, in this study, the turbulence integral length scale was effectively increased using an active control grid. The wind fields of different turbulence integral scales were generated by controlling the vibration frequency of the active control grid. The aerostatic coefficients of five typical bridge sections (single-box girder, twin-box girder, truss girder, edge girder, and edge-box girder) were measured under turbulent and uniform flow. The test results were compared and analyzed, which revealed that the drag coefficients increased in accordance with a decrease in the reduced turbulence integral length scale, and they were lower under turbulent flow than under uniform flow.

Turbulent flow characteristics over forward-facing obstacle

The local flow field and wake region over a forward-facing obstacle were experimentally investigated, when the water surface above the obstacle was dipped compared to flow further upstream and further downstream. The obstacle was modelled resembling the shape of an airfoil with a flat top having stoss-side slope and downstream lee-side slope designed for minimal flow separation. The submerged obstacle showed an evolutionary change in the flow field with a dipped water surface shape over the crest; and consequently three distinct flow regions were observed: upstream region, crest region and the downstream. The velocity data were recorded using a three-dimensional micro-acoustic Doppler velocimeter along the flume centreline for three flow Reynolds numbers and different localised Froude numbers. The paper highlights the flow and turbulence prime parameters: second-order statistics, turbulence kinetic energy, eddy viscosity, turbulence production, probability-density functions and the contributions of stress fraction into four quadrants to the total shear stress along the flow. The spectral analysis was performed to examine the energy distributions, vortex-shedding frequencies with corresponding Strouhal numbers. Variations of turbulence integral length scale along the flow passing over the obstacle were investigated to examine the eddy turnover scales. The turbulent features were fairly inspiring since it led to a pronounced acceleration on the crest location like a jet with a potential core region, and elongated deflection in the downside slope. The results provide a basic turbulence in natural environment for various phases of ebb and tide over a submerged smooth obstacle or mountain hills in river/ocean.

Observational study of wind characteristics, wind speed and turbulence profiles during Super Typhoon Mangkhut

On the basis of the wind records from 356-m-high Shenzhen Meteorological Gradient Tower (SZMGT) which is the tallest of its kind in Asia, this paper presents an observational study of wind characteristics during Super Typhoon Mangkhut and aims to enhance the understanding of the boundary layer wind structure of strong typhoons over land. First of all, the raw records are selected for subsequent analysis through quality control concerning flow distortion, thermal stability, stationarity, and spikes. Then wind speed profiles, as well as their associations with upstream terrain conditions, are presented and discussed. Moreover, parameters for describing atmospheric turbulence, involving gust and peak factors, turbulence intensities, turbulence integral length scales, and turbulence ratios, are investigated considering different surface roughness regimes. Furthermore, Gaussianity, power spectral density, and coherence of wind velocities are investigated. The outcome of this study is expected to facilitate the wind-resistant design of high-rise structures in typhoon-prone regions and provide references for typhoon boundary layer simulation.

Flow and heat transfer in a rotating cavity with de-swirl nozzles: An LES study

Large eddy simulation was applied for studying the flow and heat transfer characteristics in the rotating cavity with de-swirl nozzles. Simulations were performed at the rotational Reynolds number of 1.4 × 105 and nondimensional flow rate of 4040. Compared with vertical nozzle, de-swirl nozzle generates lower swirl ratio, pressure loss and Nusselt number. Distributions of radial velocity show that de-swirl nozzle generates lower velocity in the Ekman boundary layer, but higher velocity in the core region. In the corner region of shroud and jet, the ‘dead zone’ of coherent vortices exists for vertical nozzle. However, for de-swirl nozzle, the ‘dead zone’ disappears due to the impact of inclined injection, and accordingly, the scale of time-averaged counter-rotating vortex pair is reduced effectively. In the middle and lower sections of cavity, plenty of vortex stripes are formed both for vertical and de-swirl nozzles; however, in the Ekman layer, turbulent kinetic energy and Reynolds normal stress for vertical nozzle is much higher than de-swirl nozzle. The distributions of turbulent energy spectrum and energy integral length scale show that, for de-swirl nozzle, large-scale eddies take the dominant role in middle section, while small-scale eddies dominate the flow fields in upper and lower sections of cavity.

The effect of the integral length scale of turbulence on a wind turbine aerofoil

The effect of free stream turbulence on a DU96w180 wind turbine aerofoil is investigated through wind tunnel experiments. Wind turbine blades experience large scale, high intensity turbulent inflow during their service life. However, the effect of turbulence is normally neglected in the assessment of their aerodynamic performance. This is normally justified based on common assumptions on the effect of the integral length scale of turbulence, which supposedly only acts in the low-frequency range of the energy spectrum, hence affecting the angle of attack instead of the aerodynamic behaviour. In this study, an experimental setup implementing passive grids is developed to vary independently turbulence intensity and integral length scale in wind tunnel testing, with a range spanning I~5–15% and L~8−33cm respectively. Results show that turbulence effects are not negligible even at the largest integral length scales, provided that a critical value for the turbulence intensity is achieved. Turbulence is found to increase mean and fluctuating Lift and delay separation in stalled conditions.

Normal and typhoon wind loadings on a large cooling tower: A comparative study

Large cooling towers are sensitive to wind effects with their increasing heights and flexibilities. Unlike traditional approaches, which employed Code-defined normal winds to check the loading characteristics, this paper developed a framework for checking the typhoon-induced wind loading on a large cooling tower using Monte Carlo simulations and multi-fan wind tunnel tests. Some distinct characteristics of typhoon winds were compared with those of Code-defined normal winds. Furthermore, wind characteristics of incoming normal and typhoon winds in terms of vertical profile of mean wind speed, turbulence intensity, turbulence integral length scale and power spectrum density of fluctuating winds were well reproduced by a feedback control process of a multi-fan actively controlled wind tunnel. The surface wind pressure distributions of a large cooling tower under these conditions were then investigated by testing a 1:600 reduced scale model. Mean and fluctuating external wind pressures along the circumferential direction under various incoming winds were discussed and quantitatively formulated with eight-term trigonometric equations. Moreover, the cross correlations of wind pressures in the circumferential and meridian directions and correlations with structure base forces, i.e. integral drag and lift forces, were investigated. Non-Gaussian characteristics in terms of skewness and kurtosis of fluctuating wind pressures were also analyzed under two wind climates. Peak factors for modeling extreme wind pressures were examined and compared with those of various models. Finally, the extreme wind loads on a large cooling tower obtained from different wind pressure combinations were compared with peak-factor-theory-based results to identify an appropriate combination for structural design.

Large-eddy simulations of the inlet grid-generated turbulence

In this paper, a new technique of turbulence generation in large eddy simulation (LES) is studied and verified. In order to generate turbulence similar to the wind tunnel test, the proposed grid inlet technique places the grid on the Inlet boundary to achieve the following effects: changing the grid size controls the turbulence integral length scale and changing the distance from inlet controls the turbulence intensity. The purpose of this paper is to explore the domain requirements of grid-inlet technology by studying the turbulence characteristics of three different grid inlets. In particular, this paper further studies the effects of domain sizes on the lateral correlation of fluctuating wind by arranging the transverse positions of monitoring points irregularly and in equal proportion. Meanwhile, the isotropic hypothesis of gird-generated turbulence is verified by power spectrum. The results show that the turbulence intensity is unaffected by the domain sizes, the larger calculation domain corresponds to the gentler changing trend of the lateral correlation of the fluctuating wind and the flow fields under the three different domain sizes basically satisfy the isotropic hypothesis. The above results are helpful for the further application of the grid inlet technique.

Incorporation of turbulence in a nonlinear wake-oscillator model for the prediction of unsteady galloping response

The aeroelastic behavior of certain slender bluff bodies is known to be dominated in smooth flow by the interference of vortex-induced vibration and transverse galloping. In turbulent flow, wind tunnel experiments revealed a more complicated behavior, with some features that have not been fully understood yet. With the aim to explain the oscillation response observed for a sharp-edge rectangular cylinder with a side ratio of 1.5 and to make a step ahead in the simulation of the self-excited behavior of such bodies, a nonlinear wake-oscillator model that provided promising results in smooth flow is considered. The differential equations are randomized to account for the contribution of three-dimensional, partially-correlated, turbulent velocity fluctuations attacking the structure. Given the physical basis of the mathematical model, all the parameters but one are estimated based on wind tunnel tests on a stationary sectional model. In contrast, given the lack of wake measurements, one of the coupling coefficients in the equations is calibrated based on the cylinder response for a given value of the nondimensional mass-damping parameter (Scruton number). Then, the model is used to predict the vibrations recorded for very different values of the Scruton number, showing satisfactory accuracy. This fact highlights the key role played, even in turbulent flow, by vortex shedding and the importance of its nonlinear interaction with oncoming wind velocity fluctuations. The equations are also used to investigate the effect of turbulence intensity and integral length scale. In agreement with the experimental evidence, the response of the cylinder is found to be little sensitive to flow velocity fluctuations with small intensity (say, less than about 4%), but is remarkably affected by strong turbulence. The simulations also disclose a significant influence on the vibration amplitude of the integral length scale of turbulence. Finally, the proposed model might represent a useful engineering tool to estimate the unsteady galloping behavior of a slender structure immersed in a realistic large-scale turbulent flow, which seldom can be correctly reproduced in the wind tunnel.

Validation of a large eddy simulation methodology for accelerated nozzle flows

ABSTRACTPrediction of aeroengine exhaust plume near-field development requires knowledge of velocity and turbulence distributions at nozzle exit. The high Reynolds number nozzle inlet boundary layers of engineering practice are fully turbulent, but acceleration can induce re-laminarisation. Thus, to reproduce nozzle exit conditions accurately, large eddy simulation (LES) for plume prediction must be capable of capturing re-laminarisation and any subsequent boundary layer recovery. Validation is essential to establish a credible LES methodology, but previous studies have suffered from lack of nozzle inlet/exit measurements in the test cases selected. Validation data were here taken from an experiment on a convergent round nozzle with a parallel exit extension to allow boundary layer recovery. LES inlet condition generation applied a rescaling/recycling method (R2M), whose performance was validated against measurements of first and second moment statistics as well as the turbulence integral length scale. Simulations employed two sub-grid-scale (SGS) models; these produced similar predictions up to the end of the nozzle convergent section, but marked differences appeared for the nozzle exit turbulence field. The Smagorinsky model predicted much lower turbulence levels than measured, whereas the Piomelli and Geurts model revealed the presence of a small separation region at the convergence/parallel section corner, which led to higher exit turbulence and much better agreement with measured data.

Turbulent Properties of Triple Elliptic Free Jets With Various Nozzle Orientation

Abstract An experimental investigation of nozzle orientation effects on turbulent characteristics of elliptic triple free jets was carried out for three nozzle configurations. The first configuration had each nozzle oriented along the minor plane (3_Minor), the next had two nozzles oriented along the minor plane and one along the major plane (Min_Maj_Min) and the last configuration had one nozzle oriented along the minor plane and two along the major plane (Maj_Min_Maj). The experiments were conducted using modified contoured nozzles with a sharp linear contraction for a nozzle spacing ratio of 4.1d, a nozzle equivalent diameter of 9 mm, and Reynolds number of 10,000. Nozzle orientation effects on the mean velocity, turbulent intensity, and Reynolds shear stress were discussed. The velocity decay, jet spread, merging point (MP), combined point (CP), and potential core length were used to characterize the effects of nozzle orientation on the mixing performance. The 3_Minor configuration had shorter potential core length and closer MP location which are indicative of a faster mixing in the converging region. The early merging of 3_Minor led to higher levels of streamwise turbulent intensity. One-dimensional plots revealed that jets approached self-similarity at a faster rate in the major axis. The orientation of the middle jet was found to be a key factor in determining transverse diffusion of the Reynolds shear stress in the plane of observation. Two-point correlations were used to provide insight into the effects of nozzle orientation on the spatial coherence of the large-scale turbulence structure and integral length scale.

Assessment of Tabulated Premixed Conditional Moment Closure Model Using Direct Numerical Simulation

ABSTRACT The Conditional Moment Closure (CMC) is a proven methodology to model non-premixed flames, but an emerging technique for analyzing turbulent premixed flames. However, solving a full set of CMC equations is a time-consuming process. The Tabulated Premixed CMC (TPCMC) model is a modification of the premixed CMC model that seeks to reduce the time consumed in solving equations by making the premixed CMC model amenable to the tabulation of chemistry. TPCMC model considers the effect of small scale mixing of reactants and products using the scalar dissipation rate and effect of large scale turbulence on the flame using the beta probability density function. The validation of TPCMC model using Direct Numerical Simulation (DNS) data has been attempted in this paper. DNS of growth of an ignition kernel in a turbulent medium was performed for this purpose. Two sets of simulations, namely 3D DNS with three-step reduced chemistry and 2D DNS with multi-step chemistry, have been carried out in this study using PENCIL code. The Favre averaging of this DNS data was performed on the surfaces of constant radius to assess the accuracy of TPCMC model. The variations in turbulence intensity and integral length scale were considered to generate DNS database. TPCMC model accurately predicted the variation of fuel mass fraction and temperature for all the cases considered in this study and captured the qualitative trends in the CO mass fraction where a negligible deviation from the actual values was observed. The influence of initial parameters related to turbulence considered in this study was negligible on the accuracy of predictions obtained from the model.

Spanwise Cylinder Wake Hydrodynamics and Fish Behavior

AbstractFlows generated near hydro‐engineering structures are characterized by energetic three‐dimensional flow structures that are markedly different from naturally occurring fish habitats. The current study evaluated the interaction of Nile tilapia (Oreochromis niloticus) with spanwise rollers in the turbulent wake of a cylinder in both the wake bubble and the vortex shedding further downstream. The flow field hydrodynamics were measured using an acoustic Doppler velocimeter for Reynolds number (ReD) regimes ranging from 3,730 to 33,590, over a streamwise length of six diameters downstream of the cylinder, and revealed a pair of alternating vortices rotating about a spanwise axis, which were rendered asymmetric by the bed boundary proximity. Fish avoided areas where vorticity, turbulence intensity, turbulent kinetic energy, eddy size, and Reynolds shear stress were highest. Events of stability loss, referred to as spills, were significantly correlated to the turbulence integral length scale relative to fish standard length, with the peak number of spills occurring when the eddy length approached 45% to 50% of the fish length. Spill events significantly depended on ReD, Reynolds stress, and vorticity and varied according to fish length and weight. Among zones of similar Reynolds stress and vorticity magnitude, spills were most frequent when Reynolds shear stress was positive, downward acting and eddies rotated clockwise, which highlights the importance of direction and orientation of flow structures in determining the hydrodynamic forces that affect fish swimming stability. Recommendations are made for the inclusion of these metrics in the design and refinement of hydro‐engineering schemes.

Flame propagation characteristics and explosion behaviors of aluminum dust explosions in a horizontal pipeline

The effects of airflow velocities, particle sizes, and dust concentrations on the flame propagation characteristics and explosion behaviors of aluminum dust explosions in a horizontal pipeline were experimentally investigated. The results showed that the distributions of the airflow velocity and turbulence integral length both exhibited convex parabolic distributions. After ignition, the flame propagation velocities of 5μm and 30μm aluminum dust explosions both experienced acceleration processes. The positive feedback coupling among the combustion, expansion, and turbulence governed the acceleration mechanism. The flame propagation characteristics and explosion behaviors were significantly affected by the airflow velocity. As the airflow velocity increased from 6.4m/s to 13.3m/s, the maximum average flame propagation velocity increased from 60m/s to 140m/s. Moreover, the maximum explosion pressure increased from 36kPa to 83kPa, and the maximum explosion impulse increased from 78Pa.s to 576Pa.s.

Comparison of wind characteristics at different heights of deep-cut canyon based on field measurement

The typical U-shaped deep-cut canyon is widely distributed in the western mountainous areas of China, especially in Sichuan province and Yunnan province. The deep-cut canyon has the characteristics of the high drop in elevation, high-temperature difference, and complex wind environment. A 50 m high meteorological mast with a total of eight anemometers was erected in such topography, and a long-span suspension bridge will be constructed in the area where the meteorological mast is located. Based on the long-term monitor data, the wind characteristic parameters including average and fluctuating wind characteristics and coherence between different heights are investigated. The results are as follows. The dominant wind direction which depends on the topography is north–south. The attack angle of wind is mainly less than zero, and its probability distribution obeys the hypothetical Gaussian distribution. Both the increases in height of anemometer and in wind speed reduce the dispersion of the attack angle of wind. The gust factor has a similar change law of attack angle of wind. Turbulence intensities are affected by the height of the anemometer and the wind speed, and they are different from the recommended value of China Codes. In terms of turbulence integral length scale, the value increases with an increase in the height of the anemometer in the same component. The largest value occurs in the longitudinal direction and the smallest occurs in the vertical direction at the same level. The coherence between any two locations is relatively strong, and the longitudinal component is stronger than others. The measured wind power spectrum for longitudinal, lateral, and vertical wind in deep-cut canyon fits the von Kármán model better.

Regimes of premixed turbulent spontaneous ignition and deflagration under gas-turbine reheat combustion conditions

We address the problem of turbulent spontaneous ignition vs. deflagration propagating against a mean residence time gradient, particularly relevant to the stabilization of quasi-steady diesel flames and gas-turbine reheat combustion, by performing a large series of statistically one-dimensional premixed turbulent flame DNS. An autoignitve mixture of hydrogen-vitiated air representative of gas-turbine reheat combustion is considered. First, we demonstrate that turbulence can trigger the transition from spontaneous ignition to a deflagration. We then construct a spontaneous ignition vs. deflagration regime diagram based on inflow velocity, turbulent velocity fluctuations and integral length scale and identify the mechanism that governs the transition from one propagation mode to the other. Finally, we propose a theoretical model that captures accurately this transition and assess the validity of a proxy based on chemical explosive mode analysis to determine the local propagation mode in complex configurations.

Characteristics of turbulence in a face-centred cubic porous unit cell

Direct numerical simulations (DNS) are performed in a triply periodic unit cell of a face-centred cubic (FCC) lattice covering the unsteady inertial, to fully turbulent, flow regimes. The DNS data are used to quantify the flow topology, integral scales, turbulent kinetic energy (TKE) transport and anisotropy distribution in the tortuous geometry. Several unique flow features are observed within this low porosity configuration, where the mean flow undergoes strong acceleration and deceleration regions with presence of three-dimensional helical motions, weak wake-like structures behind spheres, stagnation and jet-impingement-like flows together with merging and spreading jets in the main pore space. The jet-impingement and weak wake-like flow structures give rise to regions with negative total TKE production. Unlike flows in complex shaped ducts, the turbulence intensity levels in the cross-stream directions are found to be larger than those in the streamwise direction. Furthermore, due to the compact nature and confined geometry of the FCC packing, the turbulent integral length scales are estimated to be less than 10 % of the bead diameter even for the lowest Reynolds number studied, indicating the absence of macroscale turbulence structures for this configuration. This finding suggests that even for the highly anisotropic flow within the pore, the upscaled flow statistics are captured well by the representative volumes defined by the unit cell.

Direct numerical simulation of droplet deformation in turbulent flows with different velocity profiles

The deformation and fragmentation processes of single droplet in turbulent velocity flows are compared with the continuous uniform velocity inlet profile flows. Direct numerical simulation (DNS) coupled with volume of fluids (VOF) method is adopted to solve the flow equations and track the liquid–gas interfaces. By varying the turbulence integral length scales, different inlet velocity profiles are generated. Meanwhile, the inlet velocity magnitude changes in a wide range to present the turbulence and Weber number effects on droplet deformation. The temporal and spatial evolution processes of the droplet are taken into consideration. After analyzing the velocity distribution, the motion of the turbulent flow fronts and the scales of the turbulent vortices, the deformation differences between the uniform and turbulent fields are clearly explained. Different from uniform velocity profile, droplet shows an unsteady and unsymmetrical deformation in the turbulence fields. In the later stage, the droplet breakup in the turbulent fields is more intensive and thorough. Due to the velocity fluctuations, the pressure around the droplet is abruptly distributed which causes the droplet to be turned, rotated and side-squeezed. The droplets tend to be turned up and down with the Half integral scale profiles, be turned and rotated with the Quarter while be squeezed sideways with the Tenth. This asymmetrical deformation tendency may stimulate or counteract the transport process. This will help improve the comprehension of the droplet breakup in internal combustion engines and other industrial activities.

On the Variation of Turbulence in a High-Velocity Tidal Channel

This study presents the variation in turbulence parameters derived from site measurements at a tidal energy test site. Measurements were made towards the southern end of the European Marine Energy Centre’s tidal energy test site at the Fall of Warness (Orkney, Scotland). Four bottom mounted divergent-beam Acoustic Doppler Current Profilers (ADCPs) were deployed at three locations over an area of 2 km by 1.4 km to assess the spatial and temporal variation in turbulence in the southern entrance to the channel. During the measurement campaign, average flood velocities of 2 ms−1 were recorded with maximum flow speeds of 3 ms−1 in the absence of significant wave activity. The velocity fluctuations and turbulence parameters show the presence of large turbulent structures at each location. The easternmost profiler located in the wake of a nearby headland during ebb tide, recorded flow shielding effects that reduced velocities to almost zero and produced large turbulence intensities. The depth-dependent analysis of turbulence parameters reveals large velocity variations with complex profiles that do not follow the standard smooth shear profile. Furthermore, turbulence parameters based on data collected from ADCPs deployed in a multi-carrier frame at the same location and time period, show significant differences. This shows a large sensitivity to the make and model of ADCPs with regards to turbulence. Turbulence integral length scales were calculated, and show eddies exceeding 30 m in size. Direct comparison of the length scales derived from the streamwise velocity component and along-beam velocities show very similar magnitudes and distributions with tidal phase.

Wind tunnel study of separated and reattaching flows by particle image velocimetry and pressure measurements

This article presents a comprehensive investigation on the separated and reattaching flows over a blunt flat plate with different leading-edge shapes by means of particle image velocimetry and surface pressure measurements. Wind tunnel tests are performed in both smooth and various turbulent flow conditions, and the separated and reattaching flows are examined as a function of Reynolds number ( Re), leading-edge shape, turbulence intensity, and turbulence integral length scale. It is shown through the particle image velocimetry and pressure measurements that the Reynolds number effect is significant regarding the mean vorticity field, but with little effect on the mean velocity field. For the effects of leading-edge shape, the distributions of pressure coefficients respond strongly to the change in leading-edge angle, and both the velocity (streamwise and vertical) and vorticity fields have a clear dependence on the leading-edge shape. For the effects of freestream turbulence, the mean pressure coefficient responds strongly to turbulence intensity, whereas the fluctuating and peak suction pressure coefficients are dependent on both turbulence intensity and integral length scale. The size of the separation bubble contracts aggressively with increasing turbulence intensity, but it remains approximately invariant in response to the change in turbulence scale in the tested range.