Two-dimensional Wind Tunnel Test Research Articles

A Critical Analysis on Low-Order Simulation Models for Darrieus Vawts: How Much Do They Pertain to the Real Flow?

To improve the efficiency of Darrieus wind turbines, which still lacks from that of horizontal-axis rotors, computational fluid dynamics (CFD) techniques are now extensively applied, since they only provide a detailed and comprehensive flow representation. Their computational cost makes them, however, still prohibitive for routine application in the industrial context, which still makes large use of low-order simulation models like the blade element momentum (BEM) theory. These models have been shown to provide relatively accurate estimations of the overall turbine performance; conversely, the description of the flow field suffers from the strong approximations introduced in the modeling of the flow physics. In this study, the effectiveness of the simplified BEM approach was critically benchmarked against a comprehensive description of the flow field past the rotating blades coming from the combination of a two-dimensional (2D) unsteady CFD model and experimental wind tunnel tests; for both data sets, the overall performance and the wake characteristics on the midplane of a small-scale H-shaped Darrieus turbine were available. Upon examination of the flow field, the validity of the ubiquitous use of induction factors is discussed, together with the resulting velocity profiles upstream and downstream the rotor. Particular attention is paid on the actual flow conditions (i.e., incidence angle and relative speed) experienced by the airfoils in motion at different azimuthal angles, for which a new procedure for the postprocessing of CFD data is here proposed. Based on this model, the actual lift and drag coefficients produced by the airfoils in motion are analyzed and discussed, with particular focus on dynamic stall. The analysis highlights the main critical issues and flaws of the low-order BEM approach, but also sheds new light on the physical reasons why the overall performance prediction of these models is often acceptable for a first-design analysis.

The influence of wing twist on pressure distribution and flow topology

Empirical data serve as the foundation to computational modeling in the initial stages of the wind turbine design process. In case of aerodynamic simulations, the empirical input is comprised of lift and drag data obtained in quasi two-dimensional wind tunnel tests. In the simulations, the global flow over an entire blade is finally approximated as a spatial summation of the obtained 2-D data, which stands in strong contrast to the true operation of a wind turbine and consequently leads to a higher level of uncertainty. Especially, the near-root region of the blade experiences highly three-dimensional flow conditions, particularly in regions of the blade where the flow separates from the airfoil. This study aims to accentuate the difference between airfoil data obtained in quasi two-dimensional wind tunnel tests compared to airfoil data from a wing with an imposed three-dimensional spanwise pressure gradient. For this, a geometrically altered wing section with a spanwise twist is tested in a wind tunnel and compared to CFD computations.

A Modified Beddoes–Leishman Model for Unsteady Aerodynamic Blade Load Computations on Wind Turbine Blades

A modified version of the Beddoes–Leishman (B-L) dynamic stall model is presented. A novel approach was applied for deriving the effective flow separation points using two-dimensional (2D) static wind tunnel test data in conjunction with Kirchhoff's model. The results were then fitted in a least-squares sense using a new nonlinear model that gives a better fit for the effective flow separation point under a wide range of operating conditions with fewer curve fitting coefficients. Another model, based on random noise generation, was also integrated within the B-L model to simulate the effects of vortex shedding more realistically. The modified B-L model was validated using 2D experimental data for the S809 and NACA 4415 aerofoils under both steady and unsteady (oscillating) conditions. The model was later embedded in a free-wake vortex model to estimate the unsteady aerodynamic loads on the NREL Phase VI rotor blades consisting of S809 aerofoils when operating under yawed rotor conditions. The results in this study confirm the effectiveness of the proposed modifications to the B-L method under both 2D and three-dimensional (3D) (rotating) conditions.

Wind tunnel analysis of the aerodynamic loads on rolling stock over railway embankments: the effect of shelter windbreaks.

Wind-flow pattern over embankments involves an overexposure of the rolling stock travelling on them to wind loads. Windbreaks are a common solution for changing the flow characteristic in order to decrease unwanted effects induced by the presence of cross-wind. The shelter effectiveness of a set of windbreaks placed over a railway twin-track embankment is experimentally analysed. A set of two-dimensional wind tunnel tests are undertaken and results corresponding to pressure tap measurements over a section of a typical high-speed train are herein presented. The results indicate that even small-height windbreaks provide sheltering effects to the vehicles. Also, eaves located at the windbreak tips seem to improve their sheltering effect.

수상 태양광발전 시스템의 풍력계수 산정에 관한 실험적 연구

In recent years, green house effect related natural disasters occur throughout the world. Carbon dioxide, mainly comes from the fossil fuel burning, is suspected to be the cause of green house effect. To reduce the emission of carbon dioxide, we need to find alternative energy resources such as photovoltaic energy. In this paper, the basic characteristics of wind force coefficient on a PV panel installed on the floating type PV energy generation system are investigated though the two-dimensional wind tunnel tests. Test variables included the angle of PV panel, direction of wind, number of rows of PV panel and attached or not attached frame. Based on the results obtained through the wind tunnel tests, it was found that the wind force coefficient can be used as a preliminary data in the design of the structure.

The critical velocity for aggregate blow-off from a built-up roof

A new experimental technique is presented for calculating the critical conditions under which loose gravel on a roof becomes airborne. The critical condition for gravel blow-off from the top of a roof depends on the building geometry, particle properties, and the wind conditions. A series of two-dimensional wind tunnel tests were run to measure the critical condition for particle removal. The experimental results demonstrate that the critical condition for blow-off, parameterized in terms of a particle densimetric Froude number, is a function of the dimensionless particle size (d⁎) and the building geometry. Results for buildings without a parapet show that the critical particle densimetric Froude number has a power-law relationship with the dimensionless particle size as Frd2=8.1(d⁎)−0.44 for the range of parameters tested. For buildings with a parapet, the densimetric Froude number for the critical condition depends on both Reynolds number and parapet height to building height ratio. The experimental results indicate that buildings without a parapet are not always the most prone to blow-off, and that under certain conditions, a small parapet height can increase the risk of gravel removal.As the critical Froude number is dependent on the Reynolds number, raw data from small scale experiments cannot simply be scaled up by using a Froude number. Further, it is demonstrated that the current approach of using the Shields Diagram (or equivalent data) to scale results from small to large scale are also flawed as the motion initiation mechanism is different. Therefore, existing design guides should be re-visited and full-scale experiments should be conducted in order to fully analyze the risk of blow-off.

Scaling between Wind Tunnels–Results Accuracy in Two-Dimensional Testing

The establishment of exact two-dimensional flow conditions in wind tunnels is a very difficult problem. This has been evident for wind tunnels of all types and scales. In this paper, the principal factors that influence the accuracy of two-dimensional wind tunnel test results are analyzed. The influences of the Reynolds number, Mach number and wall interference with reference to solid and flow blockage (blockage of wake) as well as the influence of side-wall boundary layer control are analyzed. Interesting results are brought to light regarding the Reynolds number effects of the test model versus the Reynolds number effects of the facility in subsonic and transonic flow.

The influence of Reynolds and Mach numbers on two-dimensional wind-tunnel testing: An experience

AbstractIn this note, the experimental results of wind-tunnel measurements of lift coefficient and lift-curve slope for aerofoil NACA 0012 obtained from the VTI-Institute Zarkovo, are presented. The results were obtained from tests and integrations of surface static-pressure data over a model of the NACA 0012 aerofoil section. The data were obtained for a free-stream Mach number range of 0·25-0·8 and a chord Reynolds number range of 2-25MRe. The essential results of these measurements along with the results from other authors are presented and evaluated. The principal factors which influence the accuracy of two-dimensional wind-tunnel test results are analysed. The influences of Reynolds number, Mach number and wall interference with reference to solid and flow blockage as well as the influence of side-wall boundary-layer control are analysed. Interesting results brought to light the Reynolds number effects of the test model versus Reynolds number effects of the facility in subsonic and transonic flow as well as the effects of the side-wall boundary-layer control and wall interference.

Effects of Blade Tip Clearances on the Suction-corner Performances of a High-load Axial Compressor Cascade

Strong adverse flows tend to occur in the suction surface/wall corner of high-load blade cascades of axial compressors. In a two-dimensional cascade wind tunnel test, effects of tip clearances on the cascade pressure coefficient Cp and flow structures in the corner were surveyed. Addition of a tip clearance was found to alter the flow conditions in the suction corner of axial compressor blade cascade. Tip clearance of 0.4 mm for the test blade of chord length of 100 mm in a typical high-load stator blade cascade increased the pressure coefficient Cp and lessened the corner loss coefficient compared with the cases with no clearance or with wider clearance. The issuing jet through the tip clearance of 0.4 mm was found to have eliminated strong vortices in the corner zone. It suggests a very useful means of improving the axial compressor performances.

Evaluation of Airfoil Dynamic Stall Characteristics for Maneuverability

In severe maneuvers, out of necessity for a military aircraft or inadvertently for a civil aircraft, a helicopter airfoil will stall in a dynamic manner and provide lift beyond what would be calculated based on static airfoil tests. The augmented lift that occurs in dynamic stall is related to a vortex that is shed near the leading edge of the airfoil. However, directly related to the augmented lift that results from the dynamic stall vortex are significant penalties in pitching moment and drag. An understanding of the relationship between the augmented lift in dynamic stall and the associated moment and drag penalties is the purpose of this paper. This relationship is characterized using data obtained in two-dimensional wind tunnel tests and related to the problem of helicopter maneuverability.

Freestream Velocity Corrections for Two-Dimensional Testing with Splitter Plates

The velocity correction method for two-dimensional wind-tunnel testing is expanded for the case of testing airfoils between splitter plates. Despite the common nse of splitter plates, the peculiar effects of this testing arrangement on the velocity correction are not well documented. The use of splitter plates in the test section has several advantages, but there are unfortunate consequences (such as spillage, entrainment, and blockage to varying degrees) that require the freestream velocity to be measured directly between the splitter plates with the airfoil in the test section. In this case, the standard velocity correction for wall interference is not sufficient. An additional correction must be made to account for the change in velocity ahead of the airfoil model caused by the circulation about the airfoil. The results reveal that failing to correct for the circulation effect can yield significant errors in the aerodynamic coefficients. The proposed velocity measurement and correction method provides results that are in agreement with data obtained in the NASA Langley Research Center low-turbulence pressure tunnel facility. The method can be readily implemented in the data acquisition and reduction process.

Wind Tunnel Test of the S814 Thick Root Airfoil

The objective of this wind-tunnel test was to verify the predictions of the Eppler Airfoil Design and Analysis Code for a very thick airfoil having a high maximum lift coefficient designed to be largely insensitive to leading-edge roughness effects. The 24 percent thick S814 airfoil was designed with these characteristics to accommodate aerodynamic and structural considerations for the root region of a wind-turbine blade. In addition, the airfoil’s maximum lift-to-drag ratio was designed to occur at a high lift coefficient. To accomplish the objective, a two-dimensional wind tunnel test of the S814 thick root airfoil was conducted in January 1994 in the low-turbulence wind tunnel of the Delft University of Technology Low Speed Laboratory, The Netherlands. Data were obtained with transition free and transition fixed for Reynolds numbers of 0.7, 1.0, 1.5, 2.0, and 3.0 × 106. For the design Reynolds number of 1.5 × 106, the maximum lift coefficient with transition free is 1.32, which satisfies the design specification. However, this value is significantly lower than the predicted maximum lift coefficient of almost 1.6. With transition fixed at the leading edge, the maximum lift coefficient is 1.22. The small difference in maximum lift coefficient between the transition-free and transition-fixed conditions demonstrates the airfoil’s minimal sensitivity to roughness effects. The S814 root airfoil was designed to complement existing NREL low maximum-lift-coefficient tip-region airfoils for rotor blades 10 to 15 meters in length.

Unsteady aerodynamics associated with a horizontal-axis wind turbine

The wind turbine industry is currently facing many difficulties constructing efficient wind turbine machines caused by the inability to adequately predict structural loading and power output. Available evidence from wind turbines in a field environment suggests that formation of complex unsteady separated flowfields may be responsible for many aspects of wind turbine component failure. To examine this possibility in more detail, the Combined Experiment was developed. A full-scale wind turbine was constructed and operated in a field environment. The environment chosen was subject to wide variations in wind speed and direction and subsequently generated an extensive set of data for a variety of inflow test conditions. A single wind turbine blade was instrumented with pressure transducers and strain gauges with several data sets collected across a wide spectrum of typical and limiting wind turbine operating conditions. Surface pressure data taken at various spanwise locations along the blade demonstrated highly transient and spatially complex aerodynamic behavior for even the most basic operating conditions. Integrated normal force coefficient data showed enhanced lift values significantly beyond that predicted from steady-state two-dimensional wind-tunnel test data. Surface pressure data and integrated moment coefficient data suggested formation of coherent vortices consistent with the dynamic stall process observed in wind-tunnel tests for pitching wings. The unsteady, three-dimensional aerodynamic behavior for this wind turbine was then discussed and summarized.

Incompressible Navier-Stokes Computation of the NREL Airfoils Using a Symmetric Total Variational Diminishing Scheme

The purpose of this paper is to present a numerical study of flow fields for the NREL S805 and S809 airfoils using a spatially second-order symmetric total variational diminishing scheme. The steady two-dimensional flow is modeled as turbulent, viscous, and incompressible and is formulated in the pseudo-compressible form. The turbulent flow is closed by the Baldwin-Lomax algebraic turbulence model. Numerical solutions are obtained by the implicit approximate-factorization method. The accuracy of the numerical results is compared with the Delft two-dimensional wind tunnel test data. For comparison, the Eppler code results are also included. Numerical solutions of pressure and lift coefficients show good agreement with the experimental data, but not the drag coefficients. To properly simulate the post-stall flow field, a better turbulence model should be used.

Active control of super-tall structure vibrations under wind action by a boundary-layer control method

The authors have been developing active control methods for suppressing the aerodynamic vibrations of super-tall structures, like pylons of super-long bridges and super-tall buildings, which will be constructed in the next century. Boundary-layer control by rotating cylinders at the leading edge is treated as an active control method for suppressing aerodynamic vibrations. Previous wind tunnel tests conducted by using a two-dimensional square prism gave good results for suppression of the aerodynamic response due to vortex-excited vibration and galloping. On the basis of the two-dimensional wind tunnel test results, three dimensional wind tunnel tests were conducted. The experimental results had a relatively good agreement between two and three dimensional tests.

Effects of end plates and blockage of structural members on drag forces

The aerostatic and aerodynamic characteristics of structural members were investigated using two-dimensional wind tunnel tests. In these tests problems occur owing to the effects of end plates and blockage. End plates are used to produce two-dimensional flow around models. There is an optimum size for end plates, because the degree of the two-dimensional characteristics of the air flow depends on the size of the end plates. The required condition for the end plates was determined by measurement of the wind velocity distributions and base pressures around the testing body in relation to the formation of the wake. It is concluded that the required dimensions of the end plates can be determined on the basis of Kármáns theory of vortex streets. In practice, the diameter of the end plate should be larger than 8.5 times the model depth. On changing the blockage ratio with end plates of a constant size, the aerostatic forces change significantly. This is the result of the combined effects of end plates and blockage.

Combined four-wall interference assessment in two-dimensional airfoil tests

Two different procedures are examined for combining the correction method developed by Barnwell and Sewall for the effects of the sidewall boundary layer in two-dimensional wind-tunnel tests with the assessment and correction method due to Kemp for the effects of the upper and lower tunnel walls. One procedure utilizes the similarity transformation defined by Sewall to eliminate velocity perturbations induced by the sidewall boundary layer from consideration in the assessment and correction method as sources of error. The other procedure combines perturbations from all four walls before assessment and correction. In general, the two procedures yield different corrections to Mach number and angle of attack, either of which can be considered valid. Pressure distributions corrected for higher order interference effects are provided by either procedure.

A compatibility assessment method for adaptive-wall wind tunnels

A new method is described for assessing the compatibility between inner and outer flow regimes in adaptive-wall wind tunnels. The method is applicable to both two- and three-dimensional flows, and unlike other schemes, requires the measurement of only one velocity component. Moreover, a complete solution to the outer flowfield is not required with the new method. Computer simulations of two- and three-dimensional flows are presented along with data from a two-dimensional pilot wind tunnel test using the new method.