Turbulence Intensity Profiles Research Articles

Inner and Outer-Layer Similarity of the Turbulence Intensity Profile over a Realistic Urban Geometry

Inner and Outer-Layer Similarity of the Turbulence Intensity Profile over a Realistic Urban Geometry

New analytical wake models based on artificial intelligence and rivalling the benchmark full-rotor CFD predictions under both uniform and ABL inflows

New analytical wake models are derived from the soft computing technique, called Genetic Programming (GP) to predict wake velocities and turbulence intensity. The design of the wind farm’s appropriate layout is essential for minimizing cost and maximizing the wind farm power generation. This needs a precise wake velocity model to simulate the wake effect of the wind farm within a limited time duration. Furthermore, prediction of turbulence in the wake due to ambient flow and rotor-generated is extremely crucial owing to its contribution to fatigue loads and structural failures of the downstream wind turbines. This article discusses the classical to the recent analytical wake velocity and turbulence intensity models derived based on hard computing techniques in detail and their limitations. The significant constraints are the consideration of uniform inflow without integrating Atmospheric Boundary Layer (ABL) impacts for the forecast of wake velocity and estimation of single value of turbulence intensity while it radially varies at distinct downstream distances of the wind turbine. Eventually, these constraints are tackled and new analytical models for wake velocity and turbulence intensity profiles are formulated for both uniform and ABL inflows. The existing and proposed models are compared with the previous NREL Phase VI wind turbine CFD study for uniform and ABL inflows and it was observed that the proposed models are precise.

Effect of the wing trailing-edge flaps and spoilers position on the jet-vortex wake behind an aircraft during takeoff and landing run

The large-scale vortex structure of flow in the near wake behind an aircraft during its run on a runway is investigated numerically. The geometrical aircraft configuration was taken close to a mid-range commercial aircraft like Boeing 737-300. It included all essential elements: a body (fuselage), wings with winglets, horizontal and vertical stabilizers, engine nacelles, nacelle pylons, inboard flap track fairings, leading-edge and trailing-edge flaps, and spoilers. The position of flaps and spoilers corresponded to the takeoff and landing run conditions. Computational simulation was based on solving the Reynolds averaged Navier–Stokes equations closed with the Menter Shear Stress Transport turbulence model. Patterns of streamlines, fields of the axial vorticity and the turbulent intensity, vertical and horizontal velocity profiles in the wake are compared and discussed for both run regimes. The flow model was preliminary tested for validity by comparison of the calculated velocity profiles behind a reduced-scale aircraft model with those obtained in special wind tunnel experiments.

Turbulent flow and noise sources on a circular cylinder in the critical regime

The flow around a circular cylinder with long spanwise length has been investigated in the critical regime using a compressible wall-resolved Large Eddy Simulation (LES) for the first time. The flow at such a critical Reynolds number combines complex features: large favorable and adverse pressure gradient, separation and turbulence transition and flow reattachment. The results of the present simulation agree well with previous experimental and incompressible LES data, for the distribution of the mean wall-pressure coefficient that dominates the drag coefficient, and the skin-friction coefficient that illustrates the flow separation and reattachment behaviors. A weak reattachment is observed from the quasi-zero skin-friction in the reattachment region. A detailed study of the boundary-layer and shear-layer development around the cylinder with profiles of mean velocity and turbulence intensities confirms the transition, separation and reattachment behaviors shown by the skin-friction coefficient. The maximum tangential velocity and its location above the wall have also proven to be adequate measures of the edge velocity and associated boundary layer thickness. The Kelvin-Helmholtz instabilities have been observed in the shear layer and the ratio of the frequency of these instabilities and the fundamental vortex shedding frequency matches well with the existing scaling based on experimental data. The far-field noise obtained by both direct computation and acoustic analogy shows a dominant vortex shedding tone, but with additional broadband sources in the cylinder wake. These sound sources are evaluated from maps of filtered pressure signals and cross-correlation analysis of the pressure fluctuations around the cylinder and in the far-field.

A joint velocity-intermittency analysis reveals similarity in the vertical structure of atmospheric and hydrospheric canopy turbulence

Turbulent flow through and over vegetation continues to draw significant research attention given its relevance to a plethora of applications in earth and environmental science. Canopy flows are characterized by three-dimensional coherent vortical motions not directly accessible from single-point measurements, which pose a challenge to formalizing links between vegetation structure and turbulent motion. A joint velocity-intermittency technique is applied to velocity data collected within and above aquatic vegetation in a hydraulic flume and above a forested canopy. The approach reveals behavior that provides greater insight into canopy flow dynamics than may be inferred from the vertical profiles of mean velocity, turbulence intensity and Reynolds stresses, which are the quantities usually studied. There is a remarkable similarity in the structure of such flows between the forest canopy and the flume study despite large differences in morphology and stem rigidity. In particular, these results determine an outer flow type arising above 1.5 canopy heights, while turbulent in-rushing events are most significant at the zero-plane displacement. The approach also implies ways in which improved models for canopy turbulence may be developed.

Round-robin tests of porous disc models

Nine research teams organized a round-robin measurement campaign of the wake of two porous discs in a homogeneous and “low-turbulent’ flow. Mean streamwise velocity and turbulence intensity profiles at four diameters downstream of the discs were measured and compared through such metrics as the maximum of velocity deficit, the maximum of turbulence intensity, the wake width and the thrust coefficient. The dependence of these metrics on the inflow conditions (freestream turbulence intensity and Reynolds number based on the disc diameter) is discussed.

Velocity profiles of an electrohydrodynamic flow generator: CFD and experiment

The velocity profiles of the electrohydrodynamic flow generator were investigated experimentally and numerically. The generator of a needle-to-cylinder electrode configuration with varying interelectrode distance and supply voltage was studied. Experimental results were obtained with constant temperature anemometry technique and the numerical simulations were performed with Multiphysics Object-Oriented Simulation Environment (MOOSE) framework. The current-voltage characteristic of the device and the relationship between flow velocity and electric current were found out to qualitatively match the results from previous studies. Velocity profiles obtained experimentally and numerically showed varying degree of agreement throughout studied configurations of interelectrode distance and supply voltage. Generally, better agreement was found in cases with lower interelectrode distance, lower turbulence strength and better accuracy of the solution of the electric part of the problem. Some similarities between studied flow and the flow specific for confined jet arrangement were observed, although recorded turbulence intensity values were much higher. Turbulence intensity profile for the lowest interelectrode distance indicated the existence of the shear layer between jet core and the boundary layer. For other values of the interelectrode distance the shear layer and boundary layer could no longer be distinguished. Nondimensional velocity profiles for all investigated configurations showed high degree of similarity in the jet core region, however the similarity was lost in the remaining part of the flow.

Assessment of an improved Random Flow Generation method to predict unsteady wind pressures on an isolated building using Large-Eddy Simulation

A wide variety of improved or modified Random Flow Generation (RFG) methods have recently been proposed for simulating the Atmospheric Boundary Layer (ABL) in a Large-Eddy Simulation (LES) framework. A distinct advantage of RFG methods over precursor-successor methods is the significant savings that can be made in terms of setup and computational costs. A review of the literature indicates that RFG methods are mostly evaluated in terms of velocity statistics and spectra. However, many applications in computational wind engineering ultimately aim to simulate the wind loads acting on a building. It would therefore be useful to evaluate the real capabilities of RFG methods for predicting unsteady wind pressures acting on various shapes of building. In this paper, one of the available RFG methods based on a summation of Fourier harmonic functions is assessed for various shapes of isolated building (a full-scale cube, a wind-tunnel cube, a high-rise building and a low-rise building) in terms of pressure statistics and spectra. Comparisons between measurements and simulations show that the RFG method tested here could be particularly relevant for industrial applications, provided that the inlet turbulence intensity profile is adapted.

Development of a Computational System to Improve Wind Farm Layout, Part II: Wind Turbine Wakes Interaction

The second part of this work describes a wind turbine Computational Fluid Dynamics (CFD) simulation capable of modeling wake effects. The work is intended to establish a computational framework from which to investigate wind farm layout. Following the first part of this work that described the near wake flow field, the physical domain of the validated model in the near wake was adapted and extended to include the far wake. Additionally, the numerical approach implemented allowed to efficiently model the effects of the wake interaction between rows in a wind farm with reduced computational costs. The influence of some wind farm design parameters on the wake development was assessed: Tip Speed Ratio (TSR), free-stream velocity, and pitch angle. The results showed that the velocity and turbulence intensity profiles in the far wake are dependent on the TSR. The wake profile did not present significant sensitivity to the pitch angle for values kept close to the designed condition. The capability of the proposed CFD model showed to be consistent when compared with field data and kinematical models results, presenting similar ranges of wake deficit. In conclusion, the computational models proposed in this work can be used to improve wind farm layout considering wake effects.

Gust characteristics of near-ground typhoon winds

Based on 14 sets of records observed by 4 meteorological stations during 10 typhoons, a comprehensive investigation of gust characteristics, which are usually quantitatively expressed as the turbulence intensity and gust factor, for the near-ground typhoon winds was conducted. The direction- and mean-wind-speed-dependent features of these two gust parameters are analyzed at different heights. It is found that, in general, mean wind speed has little effect on gust parameters for off-sea winds coupled with a small number of over-land winds over 15 km of upstream fetch. Statistical characteristics, including the first four-order moments together with the probability distribution, are examined and turbulence intensity and gust factor profiles are compared with those given in various codes and standards for open-sea and flat-area conditions. The estimation of peak factor based on observations and a Gaussian distribution-based model is also presented, and they show generally good accordance for the level of mean values. The relationship between turbulence intensity and gust factor is also studied and compared with several pioneer results. Two typical upstream conditions, off-sea and over-land winds are separately analyzed, and they show an opposite tendency, i.e., the off-sea gust factor at 10 m is 8% higher than the over-land value while other heights show little difference. We attempt a rapid estimation of gust factor using the model coupled with the MODIS land-use roughness length data and compare the results with observations. Finally, some uncertainties that would induce scatter of gust characteristics and imperfections of the theoretical model are identified and discussed.

The amplification of large-scale motion in a supersonic concave turbulent boundary layer and its impact on the mean and statistical properties

Direct numerical simulation is conducted to uncover the response of a supersonic turbulent boundary layer to streamwise concave curvature and the related physical mechanisms at a Mach number of 2.95. Streamwise variations of mean flow properties, turbulence statistics and turbulent structures are analysed. A method to define the boundary layer thickness based on the principal strain rate is proposed, which is applicable for boundary layers subjected to wall-normal pressure and velocity gradients. While the wall friction grows with the wall turning, the friction velocity decreases. A logarithmic region with constant slope exists in the concave boundary layer. However, with smaller slope, it is located lower than that of the flat boundary layer. Streamwise varying trends of the velocity and the principal strain rate within different wall-normal regions are different. The turbulence level is promoted by the concave curvature. Due to the increased turbulence generation in the outer layer, secondary bumps are noted in the profiles of streamwise and spanwise turbulence intensity. Peak positions in profiles of wall-normal turbulence intensity and Reynolds shear stress are pushed outward because of the same reason. Attributed to the Görtler instability, the streamwise extended vortices within the hairpin packets are intensified and more vortices are generated. Through accumulations of these vortices with a similar sense of rotation, large-scale streamwise roll cells are formed. Originated from the very large-scale motions and by promoting the ejection, sweep and spanwise events, the formation of large-scale streamwise roll cells is the physical cause of the alterations of the mean properties and turbulence statistics. The roll cells further give rise to the vortex generation. The large number of hairpin vortices formed in the near-wall region lead to the improved wall-normal correlation of turbulence in the concave boundary layer.

Measurement of Transitional Surface Roughness Effects on Flat-Plate Boundary Layer Transition

An experimental study has been conducted to investigate the effects of transitionally rough surface on the flat-plate boundary layer transition. Transitional boundary layers with three different flat plates (ks+ = 0.07 ∼ 0.19, 2.71 ∼ 7.05, and 13.65 ∼ 41.09) have been measured with a single-sensor hot-wire probe. All of the measurements have been conducted under zero pressure gradient (ZPG) at the fixed Reynolds number (ReL) and freestream turbulence intensity (Tu) of 3.05 × 106 and 0.2%. Transitionally, rough surface does not affect the sigmoidal distribution of turbulence intermittency model; but induces earlier transition onset and shortens the transition length. For all surfaces, streamwise turbulence intensity profiles with similar values of turbulence intermittency are similar for the transition length less than 60%. Therefore, mean velocity profiles with the similar values of turbulence intermittency are similar regardless of surface conditions. However, downstream of 60% of the transition length, mean velocity defect increases as the surface roughness increases. Enhanced diffusion of turbulent kinetic energy from the near wall (y/δ < 0.1) to the outer part (y/δ ≈ 0.4) of the boundary layer due to the surface roughness is responsible for the increased momentum deficit.

Spreadsheet solution for the computation of the mean wind speed and turbulence intensity profiles according to the Romanian standard

The presented paper focuses on the computation of the mean wind speed and turbulence intensity profiles for all the cities from Romania. The calculation of both, the mean wind speed profile and the turbulence intensity profile, had as mathematical support the equations presented in the Romanian design standard for wind action CR 1-1-4/2012. The main objective of this paper was to provide a tool for the computation of the two wind action features. This method was based on creating a spreadsheet in Excel with which, in just a few seconds, a user could correctly obtain the two wind characteristics. This Excel dashboard can be used as a teaching material for students as well as input data for structural design engineers in the process of modelling and observing the behaviour of a building excited by wind action on a particular city in Romania.

Experimental study of the flow across an elliptic cylinder at subcritical Reynolds number

In this work, an incompressible flow across an elliptic cylinder with an axial ratio of 0.5 was experimentally studied in a low-turbulence horizontal wind tunnel at two Reynolds numbers: $ 1.5\times 10^4$ and $ 3.0\times 10^4$ . Hot-wire anemometry measurements were carried out in the cylinder wake to obtain turbulence intensity, average stream-wise velocity, higher-order central moments of velocity signals (i.e. skewness and flatness), and dimensionless vortex shedding frequency for the two Reynolds numbers. The obtained results showed that, the turbulence intensity profiles in the wake become broader as one moves toward downstream and peak turbulence intensities occur at points of high velocity gradient along the velocity profile. With increasing the Reynolds number, lower turbulence intensity and velocity defect parameters were observed within the near-wake zone. It was further observed that, upon the increase in Reynolds number at constant axial ratio, the wake became narrower with its Strouhal number (associated with the Karman vortex) remaining constant. In addition, the drag coefficient decreased with increasing the Reynolds number. When it came to the drag coefficient, the results of the present research were in good agreement with other experimental works.

Turbulence-resolving simulations of wind turbine wakes

Turbulence-resolving simulations of wind turbine wakes are presented using a high-order flow solver combined with both a standard and a novel dynamic implicit spectral vanishing viscosity (iSVV and dynamic iSVV) model to account for subgrid-scale (SGS) stresses. The numerical solutions are compared against wind tunnel measurements, which include mean velocity and turbulent intensity profiles, as well as integral rotor quantities such as power and thrust coefficients. For the standard (also termed static) case the magnitude of the spectral vanishing viscosity is selected via a heuristic analysis of the wake statistics, while in the case of the dynamic model the magnitude is adjusted both in space and time at each time step. The study focuses on examining the ability of the two approaches, standard (static) and dynamic, to accurately capture the wake features, both qualitatively and quantitatively. The results suggest that the static method can become over-dissipative when the magnitude of the spectral viscosity is increased, while the dynamic approach which adjusts the magnitude of dissipation locally is shown to be more appropriate for a non-homogeneous flow such that of a wind turbine wake.

Application of GPU-Based Large Eddy Simulation in Urban Dispersion Studies

While large eddy simulation has several advantages in microscale air pollutant dispersion modelling, the parametric investigation of geometries is not yet feasible because of its relatively high computational cost. By assuming an analogy between heat and mass transport processes, we utilize a Graphics Processing Unit based software—originally developed for mechanical engineering applications—to model urban dispersion. The software allows for the modification of the geometry as well as the visualization of the transient flow and concentration fields during the simulation, thus supporting the analysis and comparison of different design concepts. By placing passive turbulence generators near the inlet, a numerical wind tunnel was created, capable of producing the characteristic velocity and turbulence intensity profiles of the urban boundary layer. The model results show a satisfactory agreement with wind tunnel experiments examining single street canyons. The effect of low boundary walls placed in the middle of the road and adjacent to the walkways was investigated in a wide parameter range, along with the impact made by the roof slope angle. The presented approach can be beneficially used in the early phase of simulation driven urban design, by screening the concepts to be experimentally tested or simulated with high accuracy models.

Evolution of Hydrodynamic Characteristics with Scour Hole Developing around a Pile Group

This study concerns the evolution of flow field and hydrodynamic characteristics within the developing scour hole around a four-pile group with 2 × 2 arrangement. The instantaneous velocities in scour holes at four typical stages during the scouring process were measured by an acoustic Doppler velocimeter (ADV). The evolution and spatial distribution of the time-averaged flow field, turbulence, and the corresponding hydrodynamic characteristics within scour holes were compared. The time-averaged flow field shows that the reverse flow, downward flow, and horseshoe vortex are formed in the upstream of the pile group. During the scouring process, the mean components of flow characteristics (i.e., mean velocity, vorticity, and bed shear stress) around the pile group decrease while the fluctuating components (i.e., turbulence intensity) intensify simultaneously. Similarity of turbulence intensity profiles was found within different scour holes. The horseshoe vortex at upstream of each pile merges and the shear layer in the gap region extends when the dimension of the scour hole increases to that of equilibrium scour status, indicating that the four piles behave more like a single bluff body. With the development of scour holes, the large-scale horseshoe vortex system becomes more stable and the dissipation of small-scale eddies becomes more significant.

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.

Mapping of the turbulent round jet developing region using a constant temperature anemometer (CTA)

The fully developed round turbulent jet has been extensively studied, whereas the developing region is much less understood. The high shear and turbulence intensities in the most interesting parts of the developing region make them inaccessible to common measurement techniques such as Constant Temperature Anemometry (CTA) due to the high demands on the measurement techniques for accuracy of the measurements. Turbulence measurements are therefore planned using our in-house laser Doppler anemometer (LDA) system based on its capability to provide accurate measurements and with its inherent ability to properly distinguish velocity components. A rigorous measurement with the intended LDA system however demands impractical processing time, so knowing the critical points at which measurement are to be taken will save valuable time. This information is herein acquired significantly faster and more practically, however less accurately, with single-wire CTA. A high-resolution measurement was done using a computer-controlled single-wire CTA with the wire probe mounted perpendicular to the incoming flow from the jet orifice. The measurements covered several points in the radial (r-direction) along x/D=10, x/D=15, x/D=20 and x/D=30 downstream (where D is the jet exit diameter), with spatial resolutions ranging from 1 to 3 mm between the points, depending on how far the measurement was from the jet centerline. A proper alignment was also conducted prior to measurement so that the same points can be reached again for LDA measurement on the same jet afterwards. The radial profiles of mean velocity and turbulence intensity at each downstream position are presented to show the statistics of the air flow inside and outside the jet. As expected from theory, the mean profiles display a nearly Gaussian shape, spread out and tapered with the downstream direction. The highest velocities are located at the centerline.

Nondimensional analysis of wind noise and atmospheric surface-layer properties

Wind noise is a prominent limitation to the signal to noise ratio of acoustic sensors. Realistic expectations of signal detectability can be generated by predicting the noise floor prior to a sensor deployment; however, a relationship must be established between the wind noise measured by a sensor and atmospheric surface-layer properties. Under conditions of horizontal homogeneity and quasi-steadiness, Monin-Obukhov similarity theory relates friction velocity, temperature scale, and roughness length to the near-surface profiles of mean wind speed and turbulent intensity, which in turn are known to govern wind noise. It is expected that the ratio of one-third octave band root-mean-square sound pressure to the turbulent flux of momentum, Strouhal number, and dimensionless elevation have a nondimensional relationship that collapses wind noise data as a function of Monin-Obukhov parameters. In order to establish such a relationship, we analyze a dataset of wind noise recorded in Spring 2018 within the Army Research Laboratory’s Meteorological Sensing Array on the Jornada Experimental Range, New Mexico. This dataset consists of continuous recordings of ambient noise at several sites on audio microphones up to 20 m above ground level, co-located with a suite of high-fidelity meteorological instruments, including sonic anemometers.