Wind Tunnel Blockage Research Articles

An Experimental Study of the Aerodynamics and Performance of a Vertical Axis Wind Turbine in a Confined and Unconfined Environment

This paper presents the results of a wide experimental study on an H-type vertical axis wind turbine (VAWT) carried out at the Politecnico di Milano. The experiments were carried out in a large-scale wind tunnel, where wind turbines for microgeneration can be tested in real-scale conditions. Integral torque and thrust measurements were performed, as well as detailed aerodynamic measurements to characterize the flow field generated by the turbine downstream of the rotor. The machine was tested in both a confined (closed chamber) and unconfined (open chamber) environment, to highlight the effect of wind tunnel blockage on the aerodynamics and performance of the VAWT under investigation. The experimental results, compared with the blockage correlations presently available, suggest that specific correction models should be developed for VAWTs. The experimental thrust and power curves of the turbine, derived from integral measurements, exhibit the expected trends with a peak power coefficient of about 0.28 at tip-speed ratio equal to 2.5. Flow measurements, performed in three conditions for tip speed ratio equal to 1.5, 2.5, and 3.5, show the fully three-dimensional character of the wake, especially in the tip region where a nonsymmetrical wake and tip vortex are found. The unsteady evolution of the velocity and turbulence fields further highlights the effect of aerodynamic loading on the wake unsteadiness, showing the time-dependent nature of the tip vortex and the onset of dynamic stall for tip speed ratio lower than 2.

Blockage-tolerant wind tunnel measurements for a NACA 0012 at high angles of attack

Good quality post-stall aerofoil force data at low Reynolds numbers is needed for the analysis of vertical-axis wind turbines (VAWTs). Discrepancies between existing studies are shown to affect modelled performance of VAWTs, with wind tunnel blockage identified as a possible cause.New data is presented for two different-sized NACA-0012 aerofoils, taken in blockage-tolerant and conventional solid-walled wind tunnels. The tolerant tunnel has transversely slotted walls and can be configured with either single or double slatted walls, with adjustable wall porosity. Conventional tunnel results require processing with blockage corrections that are less than ideal for application to stalled aerofoils. The tolerant tunnel does not need any corrections. Results are given for the best single and double slatted wall tunnels, chosen based on which tunnel wall porosity gives the closest force measurements for the two aerofoils.The single slatted wall data is similar to that from the corrected conventional tunnel. The double slatted tolerant tunnel has the best performance overall based on similarity of results for the aerofoils but post-stall force peaks are significantly lower than for the conventional tunnel.A relationship between post-stall lift and drag peak magnitude and blockage is hypothesised for conventional tunnel data that persists even after the application of corrections.

Wind tunnel experiments of a newly developed two-bladed Savonius-style wind turbine

Wind tunnel experiments have been conducted with a newly developed two-bladed Savonius-style wind turbine specifically meant for a small-scale energy conversion. This novel shape of the turbine blade is evolved from a series of experiments with different types of blades in the recent past. The developed two-bladed turbine is tested in an open type test section and its performance is assessed in terms of power and torque coefficients. Experiments have also been conducted with other standard blades such as semi-circular, semi-elliptic, Benesh and Bach types in order to have a direct comparison. In this study, all the reported experimental data are inclusive of wind tunnel blockage corrections. Further, the effects of Reynolds number on the dynamic and static characteristics are also discussed. The present investigation demonstrates a gain of 34.8% in maximum power coefficient with the newly developed two-bladed turbine.

Aerofoil behaviour at high angles of attack and at Reynolds numbers appropriate for small wind turbines

The aerodynamic characteristics of a NACA0018 aerofoil have been investigated experimentally for incidence angles ranging from [Formula: see text] to [Formula: see text] in closed-jet and open-jet wind tunnels with different blockage coefficients at Reynolds numbers from 60,000 to 140,000. The results provide a comprehensive data set for studying the performance of typical, small-scale Darrieus wind turbine blades which mainly operate at relatively low Reynolds number and experience extreme angles of attack, particularly during start-up. Measurements in both very high and very low blockage, open-jet wind tunnels capture a “second-stall” phenomenon at high angles of attack, but this behaviour is not observed in the closed-jet wind tunnel confirming the sensitivity of aerofoil performance at extreme incidence to wind tunnel configuration. Surface flow visualisation suggests that the “second-stall” occurs when the flow separation point near the leading edge of the aerofoil moves from the suction side to the pressure side which leads to a sudden change of wake structure. In the closed-jet wind tunnel, the tunnel walls constrain the wake and prevent the flow from switching from one regime to another. The measured data are also used to demonstrate that established wind tunnel blockage corrections break down under these extreme, post-stall angles of attack.

Investigation on Wind Tunnel Blockage Effects of High-Rise Building

As the significant difference in wind tunnel data between two scaled models of a practical engineering, the cause of this difference, which was regarded as blockage effect, was analyzed in this paper. Research shows that, for wind tunnel test with more surrounding buildings, blockage effect may result in serious data distortion, especially for lateral wind pressure, and existing correction method does not possess universality. Moreover, blockage ratio, as a global concept, is not appropriate to evaluate blockage effect of wind tunnel pressure data at different parts of building models. This study has reference value for blockage effect of wind tunnel pressure date in practical engineering.

Aerodynamic Measurements on a Vertical Axis Wind Turbine in a Large Scale Wind Tunnel

This paper presents the first results of a wide experimental investigation on the aerodynamics of a vertical axis wind turbine. Vertical axis wind turbines have recently received particular attention, as interesting alternative for small and micro generation applications. However, the complex fluid dynamic mechanisms occurring in these machines make the aerodynamic optimization of the rotors still an open issue and detailed experimental analyses are now highly recommended to convert improved flow field comprehensions into novel design techniques. The experiments were performed in the large-scale wind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for micro generation can be tested in full similarity conditions. Open and closed wind tunnel configurations are considered in such a way to quantify the influence of model blockage for several operational conditions. Integral torque and thrust measurements, as well as detailed aerodynamic measurements were carried out to characterize the 3D flow field downstream of the turbine. The local unsteady flow field and the streamwise turbulent component, both resolved in phase with the rotor position, were derived by hot wire measurements. The paper critically analyses the models and the correlations usually applied to correct the wind tunnel blockage effects. Results highlight that the presently available theoretical correction models do not provide accurate estimates of the blockage effect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in particular, seem to play a key role for the prediction of the turbine performance; large-scale unsteadiness is observed in that region and a simple flow model is used here to explain the different flow features with respect to horizontal axis wind turbines.

Passing Through the Wind Turbine Thrust Singularity

I T HAS long been known that the popular and useful controlvolume actuator-disk model for wind turbines encounters a troublesome behavior as the thrust coefficient for the rotor/disk approaches 1, which occurs when the velocity at the rotor/disk approaches half that of the freestream level and the axial velocity at the downstream outlet approaches zero. This is discussed in detail in nearly all texts and papers involving wind turbines (as, for example, in [1–8]). Beyond a thrust-coefficient value of 1, it is well understood that the current version of the model does not apply, because it predicts flow entering the control volume at the downstream outlet, thus negating its further use and leaving a gap in the theory for an important range of the controlling parameters. To date, this gap has been addressed largely with either empirical expressions and/or computational fluid dynamics (CFD) methods. Based on Glauert’s 1926 analysis of data taken for windmilling helicopter blades in open-jet wind tunnels (as discussed in [1]) and its adaption to wind turbines by Stoddard [5], it is now widely accepted that the classical model begins to break down somewhere near a rotor/disk velocity ratio of 0.6 (induction factor of 0.4). For larger induction factors, it is assumed that a turbulent-wake state ensues, with reverse and/or unsteady flow effects dominating the flow as the rotor/disk velocity further reduces to zero: i.e., the rotor/disk stalls. It is further widely accepted that Glauert’s empirical expression for the rotor/disk performance prevails in this turbulent-wake region, with several attempts at improving its utility, as summarized by Buhl [8]. Additionally, recent CFD studies by Sorensen et al. [6] andMikkelsen [7] appear to reinforce this empirical model. Recently, however, two studies [9,10] using control-volume actuator-disk modeling of wind-tunnel blockage influences on rotor/ disk performance uncovered an entirely new family of well-behaved steady-state solutions that exist in the turbulent-wake parameter space: i.e., for induction factors from 0.4 all the way to 1. Moreover, the results of [9] implied the existence of a new well-behaved solution family for induction factors above 0.5, even for the zeroblockage condition. The current work provides this new exact solution for induction factors from 0.5 to 1 based on a limit-analysis solution of the control-volume actuator-disk equations. This new solution provides the heretofore missing piece of the basic theory, filling the gap for induction factors of 0.5 to 1. Comparisons of the new analytical solutions with experiments and CFD studies of actuator disks provide strong evidence of its validity. The relevance of the results to CFD modeling, wind-tunnel testing, and/or field testing of wind turbines is discussed.

A wind-tunnel study of flow distortion at a meteorological sensor on top of the BT Tower, London, UK

High quality wind measurements in cities are needed for numerous applications including wind engineering. Such data-sets are rare and measurement platforms may not be optimal for meteorological observations. Two years' wind data were collected on the BT Tower, London, UK, showing an upward deflection on average for all wind directions. Wind tunnel simulations were performed to investigate flow distortion around two scale models of the Tower. Using a 1:160 scale model it was shown that the Tower causes a small deflection (ca. 0.5°) compared to the lattice on top on which the instruments were placed (ca. 0–4°). These deflections may have been underestimated due to wind tunnel blockage. Using a 1:40 model, the observed flow pattern was consistent with streamwise vortex pairs shed from the upstream lattice edge. Correction factors were derived for different wind directions and reduced deflection in the full-scale data-set by <3°. Instrumental tilt caused a sinusoidal variation in deflection of ca. 2°. The residual deflection (ca. 3°) was attributed to the Tower itself. Correction of the wind-speeds was small (average 1%) therefore it was deduced that flow distortion does not significantly affect the measured wind-speeds and the wind climate statistics are reliable.

Wind tunnel blockage corrections: Review and application to Savonius vertical-axis wind turbines

An investigation into wake and solid blockage effects of vertical axis wind turbines (VAWTs) in closed test-section wind tunnel testing is described. Static wall pressures have been used to derive velocity increments along wind tunnel test section which in turn are applied to provide evidence of wake interference characteristics of rotating bodies interacting within this spatially restricted domain. Vertical-axis wind turbines present a unique aerodynamic obstruction in wind tunnel testing, whose blockage effects have not yet extensively investigated. The flowfield surrounding these wind turbines is asymmetric, periodic, unsteady, separated and highly turbulent. Static pressure measurements are taken along a test-section sidewall to provide a pressure signature of the test models under varying rotor tip-speed ratios (freestream conditions and model RPMs). Wake characteristics and VAWT performance produced by the same vertical-axis wind turbine concept tested at different physical scales and in two different wind tunnels are investigated in an attempt to provide some guidance on the scaling of the combined effects on blockage. This investigation provides evidence of the effects of large wall interactions and wake propagation caused by these models at well below generally accepted standard blockage figures.

Performance measurement of a twisted three-bladed airfoil-shaped H-rotor

In the present study, performance of a twisted three-bladed airfoilshaped H-rotor was measured experimentally for different values of chord Reynolds number of flow and ratios of height-to-diameter (H/D). The rotor was 20 cm in height (H) and 5 cm in chord with about 26° twist at the trailing end. Tests were conducted in an open circuit subsonic wind tunnel for ten H/D ratios, namely 0.85, 1.0, 1.10, 1.33, 1.54, 1.72, 1.80, 1.92, 2.1 and 2.2. The wind tunnel blockage effect was also taken into consideration. The power coefficients (Cp) and torque coefficients (Ct) were evaluated for the selected values of H/D. It was observed that at any H/D, both Cp and Ct increased with the increase of tip speed ratio up to a certain limit, but then decreased even though tip speed ratio was increased. Moreover, there were optimum values of H/D at which both Cp and Ct were the highest. Further, lift and drag coefficients of the rotor were evaluated by using Prandtl lifting line concept and by computational fluid dynamics (CFD) approach. Moreover, the power loss due to the parasitic drag of the support arms was also calculated.

Wind-tunnel blockage effect on drag coefficient of circular cylinders

This paper explains how to correctly measure the drag coefficient of a circular cylinder in wind tunnels with large blockage ratios and for the sub-critical to the super-critical flow regimes. When dealing with large blockage ratios, the drag has to be corrected for wall constraints. Different formulations for correcting blockage effect are compared for each flow regime based on drag measurements of smooth circular cylinders performed in a wind tunnel for three different blockage ratios. None of the correction model known in the literature is valid for all the flow regimes. To optimize the correction and reduce the scatter of the results, different correction models should be combined depending on the flow regime. In the sub-critical regime, the best results are obtained using Allen and Vincenti's formula or Maskell's theory with <TEX>${\varepsilon}$</TEX>=0.96. In the super-critical regime, one should prefer using Glauert's formula with G=0.6 or the model of Modi and El-Sherbiny. The change in the formulations appears at the flow transition with a variation of the wake pattern when passing from sub-critical to super-critical flow regimes. This parameter being not considered in the known blockage corrections, these theories are not valid for all the flow regimes.

Experimental Investigations on the Effect of Overlap Ratio and Blade Edge Conditions on the Performance of Conventional Savonius Rotor

The torque of a conventional Savonius rotor is studied experimentally for overlap ratio from 0.10 to 0.70, blade edge condition (flat and round) and the change in Reynolds number. The results indicate that the coefficient of static torque improves marginally for round edged rotors for rotor angles up to 45°. The coefficient of static torque decreases as the overlap ratio is increased to 0.70 from 0.50 and is independent of the Reynolds numbers studied in the range between 120000 to 200000. Wind tunnel blockage ratio is studied. The power coefficient increases with the increase in the Reynolds number, whereas the coefficient of static torque is independent of the Reynolds number in the range studied. The coefficient of power, torque coefficient and coefficient static torque are independent of blockage ratios at a given Reynolds number.

Solidity and Blade Number Effects on a Fixed Pitch, 50 W Horizontal Axis Wind Turbine

Experimental studies were conducted on a modified Rutland 500 horizontal axis wind turbine to evaluate numerical implications of solidity and blade number on the aerodynamic performance. Wind tunnel data were acquired on the turbine with flat-plate, constant-chord blade sets and optimum-designed blade sets to compare with theoretical trends, which had indicated that increased solidity and blade number more than conventional 3-bladed designs, would yield larger power coefficients, CP. The data for the flat plate blades demonstrated power coefficient improvements as the range of solidities was increased from 7% to 27%, but did not indicate performance gains for increased blade numbers. It was also observed that larger pitch angles decreased the optimum tip speed ratio range significantly with a small (5% or less) change in maximum CP. The optimum-design 3-bladed rotors produced an increased experimental CP as solidity increased, with reduced tip speed ratio, at the optimum operating point. As blade number was increased at a constant solidity of 10% from 3 to 12 blades, aerodynamic efficiency and power sharply decreased, contrary to the numerical predictions and the flat plate experimental results. Low Reynolds numbers and wind tunnel blockage effects limit these conclusions and a full scale prototype rotor is being constructed to examine the results of the numerical and experimental studies using a side-by-side comparison with a commercially available wind turbine at the Clarkson University wind-turbine test site.

Wind tunnel blockage effects on aerodynamic behavior of bluff body

In wind tunnel experiments, the blockage effect is a very important factor which affects the test results significantly. A number of investigations into this problem, especially on the blockage correction of drag coefficient, have been carried out in the past. However, only a limited number of works have been reported on the wind tunnel blockage effect on wind-induced vibration although it is considered to be fairly important. This paper discusses the aerodynamic characteristics of the square model and square model with corner cut based on a series of the wind tunnel tests with various blockage ratios and angles of attack. From the test results, the aerodynamic behavior of square models with up to 10% blockage ratio are almost the same and square models with up to 10% blockage ratio can be tested as a group which behaves similarly.

The use of topographic scale models in predicting eolian dust erosion in hilly areas: field verification of a wind tunnel experiment

Eolian erosion of natural dust in an area of 53 ha, situated in the northern Negev Desert, is investigated in detail both in the wind tunnel (erosion simulations on a topographic scale model) and in the field. The wind tunnel results and the field results show good agreement, indicating that wind tunnel simulations of eolian dust erosion on scale models of complex topography may predict, at least to an acceptable level, the erosion-sensitive areas in a hilly landscape of nearly uniform surface roughness. The experiments also indicate that a restricted height distortion of scale models may be acceptable to determine the areas of highest erosion. In addition, wind tunnel blockage percentages up to 10% may be allowed to obtain acceptable erosion patterns on the model. When surface roughness is more or less uniform, eolian erosion of dry dusty sediments is almost entirely determined by macro-topography. If, however, surface roughness is highly variable over the field, the initiation of eolian erosion is also largely a function of local micro-topography.

The generalization and simplification of wind loads and implications for computational methods

The increasing power of computational methods in fluid dynamics raises the question of their future development in the context of wind engineering. To what problems can they best be applied? Will computational methods replace analytical and experimental methods such as wind tunnels? What are the new opportunities and limitations to their use? Here we will consider primarily wind loading but the remarks reply also to engineering design, for wind energy, air pollution and other problems. The chain like character of the wind loading process is pointed out. The determination either by computation or experiment of the pressure field on a body is one vital link. The contribution of computational methods to strengthening weaknesses in other links is described. Examples include the study of various storm types, the influence of complex terrain conditions and corrections for wind tunnel blockage. It is pointed out that generalizing the approach to be applicable to a wide variety of structures and simplifying the description so that important design sensitivities can be identified are both important. The paper suggests that orthonormal decomposition of the pressure field together with the use of influence functions and mode shapes are important tools. The computational implications are discussed.

Measurements of Fluctuating Pressures and Forces on a Circular Cylinder in the Reynolds Number Range 104 to 2·5 × 105

Experiments have been carried out on long, smooth, circular cylinders to establish accurate data on the fluctuating pressure distributions and r.m.s. lift and drag forces in sub-critical flow (104 <R< 2·5 × 105), on an elemental slice of the cylinder. Results of parametric studies of the effects of variation of Reynolds number, aspect ratio, turbulence of the incident flow and wind tunnel blockage are presented. Many other results have been published on this topic since 1950, but wide variations exist between them; the work reported here was undertaken to produce reliable data for its own sake and to provide the basis for a study of spanwise effects.

Wind tunnel blockage effects on drag coefficient and wind-induced vibration

Wind tunnel blockage effects on not only drag coefficient but also wind-induced vibration were experimentally investigated. It was found in this study that Maskell's theory is applicable to porous body and hexagonal section as well as flat plate and rectangular section, and that blockage factor can be expressed in a general form to enable the correction of blockage effect for arbitrary section. The magnitude of blockage effect on drag coefficient was significant even at 5% of blockage ratio, while that on wind-induced vibration was found to be not so remarkable. However, there exists more serious effect on the dynamic responses of slender models where flow reattachment likely occurs.

A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert)

ABSTRACTThe deposition of natural dust in an area of 53 ha, situated in the northern Negev desert, is investigated in detail both in the wind tunnel (dust storm simulations over a topographic scale model) and in the field. The wind tunnel results and the field results show a high degree of agreement, indicating that scale‐model simulation may be considered an important technique for future loess and desert research. More dust settles on windward slopes than on leeward slopes, which is in contradistinction with the widespread wind shadow concept. Air‐flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too.In the case of dust deposition on topographic scale models, a restricted height distortion of the model will not necessarily lead to serious problems. In addition, wind tunnel blockage percentages up to 13% may be allowed in order to obtain acceptable dust deposition patterns for the scale model.A mean gross dust deposition of about 200–250 g m−2 year−1 is calculated for the northern Negev desert for 1987. Thus, if the settled dust can be protected against erosion in the cultivated areas in the Negev, the dust content of the top soil will markedly increase with time. However, it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and, hence, to a higher dust content in the atmosphere. At any rate, a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields.From the air dust concentration data and the dust deposition data, a deposition velocity of 4.7cms−1 can be calculated for Avdat dust.

Freestream turbulence and wind tunnel blockage effects on streamwise surface pressures

Wind tunnel experiments were conducted to study the effects of blockage and freestream turbulence on streamwise surface pressures in a separation bubble. Mean and fluctuating pressures were measured near the leading edge of short axisymmetric cylinders in grid-generated turbulence and in smooth flow. Mean pressures were found to be strongly dependent on turbulence intensity but were little affected by turbulence scale. On the other hand, fluctuating pressures were dependent on both turbulence intensity and scale. The parameter σ u/ u (L x /D) 0.15 provided reasonable correlation of the fluctuating data for L x /D<2.0 and σ x/ u>0.06 . Blockage corrections were obtained for streamwise pressures measured near separation. A blockage ratio of 5% caused the mean pressure to increase in magnitude by ∼ 15% and the standard deviation and peak pressures to increase by ∼ 20%. The results of these experiments demonstrate the importance of minimizing wind tunnel blockage when measuring fluctuating pressures on building models.