Laminar Wind Research Articles

Diffuser Augmented Wind Turbine (DAWT) Technologies: A Review

Diffuser Augmented Wind Turbines (DAWT) are an optimised class of wind turbines that use the theory of a Diffuser to accelerate and direct air flow onto a wind turbine rotor to drive it for higher rotations-per-minute and power output. This power output is typically measured and rated in terms of the power augmentation. Diffuser theory was pioneered in the 1970’s and has re-emerged in recent years with a range of new technological approaches to achieving laminar wind profiles, low exit pressures, improved pressure recovery, reduced blade tip losses, improved torque generation and adaptability to wind directional and speed changes. Research has been pivotal in the advancement of design and performance of DAWT’s with CFD remaining a critical analysis tool. Power augmentations have been realistically achieved within the range of 2-3 for small-medium scale turbines. In this review, ground-based Diffuser technologies have been presented primarily according to rotor type. Large-scale on-shore and off-shore concepts have been presented along with airborne technologies. Building-integrated DAWT’s are then presented with a description of some of the influential economic and technical factors that currently affect and will continue to affect the development of the DAWT industry. DAWT’s prove to have a realisable significant potential even when considered in applications across the wind turbine sector. The current DAWT industry is mostly research-based with very little commercialisation as the majority of technologies presented here are in their early developmental stages. It is expected that subsequent research and innovation in this field will be able to advance this issue to allow DAWT’s a credible chance as a key player in the wind technology sector.

Evolution of Electronic Acoustic Targets: Research of Supersonic Mathematical Models

Приведена история совершенствования математических моделей сверхзвуковых акустических мишеней для стрелкового оружия на кафедре «Вычислительная техника» ИжГТУ имени М. Т. Калашникова и в Институте механики УрО РАН за 25 лет. Электронные акустические мишени широко используются для испытания и диагностики стрелкового и артиллерийского оружия на производствах и военных приемках и для повышения их точности. Исследованы модели мишеней с различным размещением акустических датчиков, мишени, инвариантные к позиции стрельбы, и мишени, работающие на открытом воздухе при ветреной обстановке. В каждой модели проанализированы обоснованные упрощения физических процессов. Показаны основные факторы, влияющие на точность определения точки попадания. Приведены математические и конструкторские методы по снижению погрешностей работы акустической мишени, такие как несимметричное расположение акустических датчиков на рамной конструкции для невырождения системы уравнений, использование взвешенных моментов времени при обработке сигналов с датчиков для инвариантности ориентации датчиков, учет умеренного ламинарного ветра при испытаниях на открытом воздухе путем обоснованного усложнения математической модели и увеличения количества датчиков, учет угла курса и угла падения траектории объекта в пространстве мишени для определения точки вылета путем преобразования координат в математической модели, использование вторичной регрессионной математической модели для уменьшения неучтенных погрешностей. Сделан вывод о перспективности дальнейших исследований и усовершенствований математических моделей сверхзвуковых электронных акустических мишеней с целью повышение их точности и снижения себестоимости.

Experimental Investigation on Flutter Similitude of Thin-Flat Plates

This paper shows the experimental results of the flutter speed of thin-flat plates with free leading edge in axial flow as a function of plates’ geometry, fluid densities, and viscosities, as well as natural frequencies of the plates. The experiment was developed based on similitude theory using dimensional analysis and Buckingham Pi Theorem. Dimensional analysis generates four dimensionless numbers. Experiment was conducted by placing the thin-flat plates in a laminar flow wind tunnel in order to obtain the relationship among those dimensionless numbers. The flutter speed was measured by varying the flow velocity until the instability occurred. The dimensional analysis gives a map of the flutter Reynolds number as a function of a new type of dimensionless number that is hereby called flutter fluid structure interaction number, thickness-to-length, and aspect ratios as the correcting factors. This map is a very useful tool for predicting the flutter speed of thin-flat plates in general. This investigation found that the flutter Reynolds number is very high at the region of high flutter fluid structure and thickness-to-length ratios numbers; however, it is very sensitive to the change of those two dimensionless numbers. The sensitivity is higher at lower aspect ratio.

Assessment of the fatigue failure of an All Aluminium Alloy Cable (AAAC) for a 230kV transmission line in the Center-West of Brazil

Abstract The aim of this work was to conduct an evaluation on the causes for the premature fatigue failure of an AAAC 900 MCM conductor used in a 230 kV transmission line located at the Center-West of Brazil. Each phase of the line had a double cable configuration separated by vertical rigid spacer clamps. To carry out the analysis, vibration recorders were installed on the spacer clamps at two different positions in the span and fatigue tests on the cable were conducted. The Miner's rule was applied and the remaining life of the cable was estimated to be greater than 33 years. The fact that the actual life of the cable is much smaller than the estimated one was associated to the conjoint action of three factors: (i) the use of rigid spacer clamps between the two conductors; (ii) the choice of an AAAC conductor (whose fatigue resistance proved to be significantly less than that of an ACSR) and (iii) the existence of a high frequency laminar wind regime. To avoid new failures the rigid spacer clamps were uninstalled. This action drastically reduced vibration in the line so that life estimates increased for more than 100 years.

Use of two indicators for the socio-environmental risk analysis of Northern Mexico under three climate change scenarios.

The aims of this study were to (1) find critical areas susceptible to the degradation of natural resources according to local erosion rates and aridity levels, which were used as environmental quality indicators, and (2) identify areas of risk associated with the presence of natural hazards according to three climate change scenarios defined for Mexico. The focus was the municipality of Lerdo, Durango (25.166° to 25.783° N and 103.333° to 103.983° W), which has dry temperate and very dry climates (BSohw and BWhw). From the Global Circulation Models, downscaling techniques for the dynamic modeling of environmental processes using climate data, historical information, and three regionalized climate change scenarios were applied to determine the impacts from laminar wind erosion rates (LWER) and aridity indices (AI). From the historic period to scenario A2 (ScA2, 2010–2039), regarding greenhouse gas emissions, the LWER was predicted to reach 147.2 t ha−1 year−1, representing a 0.5 m thickness over nearly 30 years and a change in the AI from 9.3 to 8.7. This trend represents an increase in drought for 70.8 % of the study area and could affect 90 % of the agricultural activities and approximately 80 % of the population living in the southeastern Lerdense territory.

Prediction of the Effect of Vortex Generators on Airfoil Performance

Vortex Generators (VGs) are widely used by the wind turbine industry, to control the flow over blade sections. The present work describes a computational fluid dynamic procedure that can handle a geometrical resolved VG on an airfoil section. After describing the method, it is applied to two different airfoils at a Reynolds number of 3 million, the FFA- W3-301 and FFA-W3-360, respectively. The computations are compared with wind tunnel measurements from the Stuttgart Laminar Wind Tunnel with respect to lift and drag variation as function of angle of attack. Even though the method does not exactly capture the measured performance, it can be used to compare different VG setups qualitatively with respect to chord- wise position, inter and intra-spacing and inclination of the VGs already in the design phase.

Modeling the potential impact of climate change in northern Mexico using two environmental indicators

Modeling the deterioration of natural resources, especially water and soil that results from the global effects of climate change has become a powerful tool in the search for mitigation and adaptation measures. The objectives of this research were: (1) to model the potential impact of climate change for the period 2010-2039, and (2) to offer advice about future risks based on local radiative forcing or critical areas and taking into account two indicators of environmental quality, the aridity index (AI) and laminar wind erosion (LWE). Evaluation techniques for natural resources, similar to those applied by the Instituto Nacional de Ecología y Cambio Climático (National Institute of Ecology and Climate Change) were used for studies of ecological land use. The inputs include climate information (current and future), soil cover and edaphic properties related to the municipality of Gómez Palacio, Durango, Mexico (25.886° N, 103.476° W). According to calculations estimated from the anomalies for the mean annual rainfall and mean annual temperature, in a future climate change scenario, an average impact of approximately 63% would be caused by LWE, and the AI would change from its historical value of 9.3 to 8.7. It is estimated that the average impact on the AI in the future will be 0.53 ± 0.2.

WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. II. RADIAL DEPENDENCE AND GLOBAL PICTURE

Non-ideal magnetohydrodynamical effects play a crucial role in determining the mechanism and efficiency of angular momentum transport as well as the level of turbulence in protoplanetary disks (PPDs), which are key to understanding PPD evolution and planet formation. It was shown in our previous work that at 1 AU, the magnetorotational instability (MRI) is completely suppressed when both Ohmic resistivity and ambipolar diffusion (AD) are taken into account, resulting in a laminar flow with accretion driven by magnetocentrifugal wind. In this work, we study the radial dependence of the laminar wind solution using local shearing-box simulations. Scaling relation on the angular momentum transport for the laminar wind is obtained, and we find that the wind-driven accretion rate can be approximated as M_dot~0.91x10^(-8)R_AU^(1.21)(B_z/10mG)^(0.93)M_Sun/yr, where B_z is the strength of the large-scale vertical magnetic field threading the disk. The result is independent of disk surface density. Four criteria are outlined for the existence of the laminar wind solution: 1). Ohmic resistivity dominated midplane region; 2). AD dominated disk upper layer; 3). Presence of (not too weak) net vertical magnetic flux. 4). Sufficiently well ionized gas beyond disk surface. All these criteria are likely to be met in the inner region of the disk from ~0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this radius, angular momentum transport is likely to proceed due to a combination of the MRI and disk wind, and eventually dominated by the MRI (in the presence of strong AD) in the outer disk. Our simulation results provide key ingredients for a new paradigm on the accretion processes in PPDs.

Aeroacoustic measurements on a NACA 0012 applying the Coherent Particle Velocity method

Aeroacoustic measurements on two NACA 0012 airfoil sections with different chord length and sharp trailing edge were conducted at the Laminar Wind Tunnel (LWT) of the University of Stuttgart. The LWT is a closed test section wind tunnel with a very low turbulence level and an acoustically optimized diffusor section allowing for high-quality aerodynamic as well as aeroacoustic measurements. Trailing edge noise measurements were performed using the Coherent Particle Velocity (CPV) method, which is based on a cross-spectral analysis of two hot-wire sensor signals placed on the suction and the pressure side of the airfoil trailing edge, respectively. At high angles of attack, the cross-spectral analysis of the two sensor signals used for the measurement of the trailing edge noise can be prone to a disturbing influence of hydrodynamic fluctuations. Hence, continuous shifts in the phasing of the cross-correlation are observed mainly for low sensor distances to the trailing edge. The quantitative evaluation of the trailing edge noise predominately in the low frequency range is, therefore, considerably disturbed. A new approach is proposed, which allows for the correction of the cross-correlation function based on the averaged single wire auto-spectrum. The results are compared to measurements with increased sensor distance and show good agreement. In the following, trailing edge noise measurements were performed on a NACA 0012 airfoil in a wide range of angles of attack (α = 0°–8°) and free-stream velocities ( $$u_{\infty} = 30{-}70\,{\hbox{m/s}}$$ ). The tripped flow cases exhibit a very good consistency for the scaling of the 1/3 octave spectra based on outer variables. Moreover, a common intersection point of the sound pressure level was observed for trailing edge noise spectra measured at constant free-stream velocity and different angles of attack. In cases without boundary layer tripping, the presence of an acoustic feedback loop was observed and linked to the presence of a laminar separation bubble on the pressure side in the vicinity of the trailing edge. Finally, a comparison of the aeroacoustic measurements based on the CPV method showed reasonably good agreement with published data obtained with both a microphone array and the Coherent Output Power method in open-test section facilities.

WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. I. SUPPRESSION OF THE MAGNETOROTATIONAL INSTABILITY AND LAUNCHING OF THE MAGNETOCENTRIFUGAL WIND

We perform local, vertically stratified shearing-box MHD simulations of protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account the effects of both Ohmic resistivity and ambipolar diffusion (AD). The magnetic diffusion coefficients are evaluated self-consistently from a look-up table based on equilibrium chemistry. We first show that the inclusion of AD dramatically changes the conventional picture of layered accretion. Without net vertical magnetic field, the system evolves into a toroidal field dominated configuration with extremely weak turbulence in the far-UV ionization layer that is far too inefficient to drive rapid accretion. In the presence of a weak net vertical field (plasma beta~10^5 at midplane), we find that the MRI is completely suppressed, resulting in a fully laminar flow throughout the vertical extent of the disk. A strong magnetocentrifugal wind is launched that efficiently carries away disk angular momentum and easily accounts for the observed accretion rate in PPDs. Moreover, under a physical disk wind geometry, all the accretion flow proceeds through a strong current layer with thickness of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4 times the sound speed. Both Ohmic resistivity and AD are essential for the suppression of the MRI and wind launching. The efficiency of wind transport increases with increasing net vertical magnetic flux and the penetration depth of the FUV ionization. Our laminar wind solution has important implications on planet formation and global evolution of PPDs.

Landing Surface Color Preferences of Spathius agrili (Hymenoptera: Braconidae), a Parasitoid of Emerald Ash Borer, Agrilus planipennis (Coleoptera: Buprestidae)

The color preferences for landing surfaces were examined for Spathius agrili (Hymenoptera: Braconidae), a parasitic wasp introduced for biocontrol of emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae). Lures with the 3-component pheromone blend of male S. agrili were used to activate upwind flight by virgin female S. agrili in a laminar flow wind tunnel. Paper discs with halves of two different colors (combination pairs of black, white, red, yellow, green, or purple), with the pheromone lure in the center, were tested to quantify preferences for landing on one color over another. Females landed preferentially on green, yellow, and white surfaces, and landed the least frequently on red, black, and purple surfaces. Changes in color preferences due to adjacent colors were observed and discussed.

Tuned vibration control in aeroelasticity of slender wood bridges

Tuned vibration control in aeroelasticity of slender wood bridges is treated in present paper. The approach suggested takes into account multiple functions in aeroelastic analysis and flutter of slender wood bridges subjected to laminar and turbulent wind flow. Tuned vibration control approach is presented with application on actual bridge. Some results obtained are discussed.

Semi-Empirical Modeling of Turbulent Anisotropy for Airfoil Self-Noise Predictions

In the present paper an approach to derive anisotropic turbulence properties from standard Reynolds-averaged Navier-Stokes simulations is proposed. The theory includes models for anisotropic velocity spectra and scaling functions that can be used to derive other turbulence parameters required for the turbulent boundary-layer trailing-edge interaction noise prediction. This type of noise is supposed to be affected by the inhomogeneous and anisotropic flow character near the trailing edge of an airfoil or wing. For this purpose, two-point turbulent velocity correlation measurements in the near wake of two airfoils (NACA 0012 and NACA 64 3 -418) at high Reynolds numbers (Re = 1.5 x 10 6 and 2.5 x 10 6 ) were conducted in the Laminar Wind Tunnel of the University of Stuttgart. A detailed postprocessing along with assessment of the experimental data has been performed for the modeling of different isotropic and anisotropic velocity spectra and related length scales. For this modeling, an anisotropy amplitude scaling method and Kolmogorov local-isotropy hypothesis have been applied. Furthermore, two different semi-empirical functions have been correlated to the resultant anisotropy scaling factors to model the effects of turbulence anisotropy in numerically predicted Reynolds-averaged Navier-Stokes data. The proposed spectrum modeling allows an accurate evaluation of the turbulence mean dissipation rate. The new models were validated and applied to a turbulent boundary-layer trailing-edge interaction noise prediction method (Rnoise) to analyze the impact of anisotropy on the total predicted noise spectra.

Validations and improvements of airfoil trailing‐edge noise prediction models using detailed experimental data

ABSTRACTThis paper describes an extensive assessment and a step by step validation of different turbulent boundary‐layer trailing‐edge noise prediction schemes developed within the European Union funded wind energy project UpWind. To validate prediction models, measurements of turbulent boundary‐layer properties such as two‐point turbulent velocity correlations, the spectra of the associated wall pressure fluctuations and the emitted trailing‐edge far‐field noise were performed in the laminar wind tunnel of the Institute of Aerodynamics and Gas Dynamics, University of Stuttgart. The measurements were carried out for a NACA 643‐418 airfoil, at Re = 2.5 ×106, angle of attack of −6° to 6°. Numerical results of different prediction schemes are extensively validated and discussed elaborately. The investigations on the TNO‐Blake noise prediction model show that the numerical wall pressure fluctuation and far‐field radiated noise models capture well the measured peak amplitude level as well as the peak position if the turbulence noise source parameters are estimated properly including turbulence anisotropy effects. Large eddy simulation based computational aeroacoustic computations show good agreements with measurements in the frequency region higher than 1 kHz, whereas they over‐predict the sound pressure level in the low‐frequency region. Copyright © 2011 John Wiley & Sons, Ltd.

Comprehensive evaluation and assessment of trailing edge noise prediction based on dedicated measurements

An extensive assessment and step by step validation of turbulent boundary-layer trailing-edge interaction (TBL-TE) prediction was conducted based on the boundary layer properties calculated by three different aerodynamics methods, XFOIL, Wilcox EDDYBL and the RANS solver FLOWer. For this purpose detailed measurements of turbulent boundary-layer properties like two-point turbulent velocity correlations, the spectrum of the associated wall pressure fluctuations (WPFs) and the emitted trailing-edge noise have been performed in the Laminar Wind Tunnel (LWT). The measurements were performed for the NACA 0012 airfoil. Most of the investigated cases show that the numerical WPF and far-field radiated noise models capture the measured peak amplitude level as well as the peak position remarkably well, if the turbulence noise source parameters are estimated properly including turbulence anisotropy effects.

Unsteady Separation Control on Wind Turbine Blades using Fluidic Oscillators

Fluidic oscillators are actuators that are essentially constituted of a flow vane with no moving parts. They are very effective in generating an oscillating velocity field, and because of their robustness and potential to meet most application requirements they have been thoroughly investigated in previous years. In this work fluidic oscillators have been embedded in an airfoil representative of the outboard sections of wind turbine blades, and subsequently tested at full-scale Reynolds numbers 2.0·10 6 ≤ Re ≤ 4.8·10 6 in the laminar wind tunnel at the University of Stuttgart. The effects of the unsteady actuation on the lift and drag strongly depend upon Re, the level of actuation, and the state of the airfoil surface. However, strong improvements have been obtained throughout the whole testing envelope, with relative lift increase spanning from a minimum of 10 to over 60% and substantial stall margin extension. In addition, employing fluidic oscillators strongly reduces the suction surface boundary-layer thickness and the unsteadiness of the mean flow velocity.

Comparative in-flight and wind tunnel investigation of the development of natural and controlled disturbances in the laminar boundary layer of an airfoil

This paper describes in-flight and wind tunnel research into laminar-turbulent transition. Measurements were carried out with a laminar wing glove for a glider (Twin II Grob G103), which could also be used in the large laminar wind tunnel at the Institute for Aerodynamics and Gasdynamics in Stuttgart. The central aspect of the investigation was the survey of the temporal–spatial development and propagation of natural as well as controlled generated waves. For the experiments performed, varied sensor arrays were used which allowed the two-dimensional acquisition of flow information on the glove (surface hot-wire and piezo foil sensors). Thus the amplification and the spatial distribution of the disturbances could be measured and compared in flight as well as in the wind tunnel, beginning with the very early linear amplification stage to the early non-linear stage of transition. For the investigation of controlled transition, multiple spanwise adjacent harmonic point sources were used which were operated independently.

Experimental measurements of two-dimensional planar propagating edge flames

The study of edge flames has received increased attention in recent years. This work reports the results of a recent study into two-dimensional, planar, propagating edge flames that are remote from solid surfaces (called here, “free-layer” flames, as opposed to layered flames along floors or ceilings). They represent an ideal case of a flame propagating down a flammable plume, or through a flammable layer in microgravity. The results were generated using a new apparatus in which a thin stream of gaseous fuel is injected into a low-speed laminar wind tunnel thereby forming a flammable layer along the centerline. An airfoil-shaped fuel dispenser downstream of the duct inlet issues ethane from a slot in the trailing edge. The air and ethane mix due to mass diffusion while flowing up towards the duct exit, forming a flammable layer with a steep lateral fuel concentration gradient and smaller axial fuel concentration gradient. We characterized the flow and fuel concentration fields in the duct using hot wire anemometer scans, flow visualization using smoke traces, and non-reacting, numerical modeling using COSMOSFloWorks. In the experiment, a hot wire near the exit ignites the ethane–air layer, with the flame propagating downwards towards the fuel source. Reported here are tests with the air inlet velocity of 25 cm/s and ethane flows of 967–1299 sccm, which gave conditions ranging from lean to rich along the centerline. In these conditions the flame spreads at a constant rate faster than the laminar burning rate for a premixed ethane–air mixture. The flame spread rate increases with increasing transverse fuel gradient (obtained by increasing the fuel flow rate), but appears to reach a maximum. The flow field shows little effect due to the flame approach near the igniter, but shows significant effect, including flow reversal, well ahead of the flame as it approaches the airfoil fuel source.

Future needs and laminar flow technology

Abstract After highlighting the need for laminar flow technology, we give an overview of the large-scale laminar flow wind tunnel and flight tests performed within European programmes and consider the prospects of the latest developments.

Flight tunnel responses of Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae) to olfactory and visual stimuli.

Cohorts of mass-reared, naive Diachasmimorpha longicaudata, parasitoids of tephritid fruit flies, were released in a laminar airflow wind tunnel to study their responses to visual and olfactory stimuli associated with their host habitat. Parasitoids were five times more likely to land on yellow plastic spheres emitting the odor of ripe, guava fruit (Psidium guajava L.) than to spheres emitting clean air. The rate of landing was not modified by the presence of green artificial leaves adjacent to the spheres in the tunnel or by the inclusion of green leaf volatiles emanating with the guava odors. However, hovering activity was twice as frequent around spheres adjacent to artificial leaves than around isolated spheres. Oviposition activity on spheres emitting guava odor was not affected by the presence of artificial leaves nor by green leaf volatiles. This suggests that inexperienced D. longicaudata may be instinctively attracted to foliage and to fruit odor but that landing (arrestment) and oviposition are influenced more by odor than by the appearance of fruit or foliage. D. longicaudata are not instinctively attracted to larvae of Bactrocera dorsalis in the absence of host-habitat stimuli. More wasp activity occurred around oviposition units containing larvae if the odor of ripe/overripe guava was present. Successful wasp reproduction occurred only in units with guava odor.