A CFD Study Of Wind Turbine Aerodynamics
A CFD Study of Wind Turbine AerodynamicsChris Kaminsky*, Austin Filush*, Paul Kasprzak* and Wael Mokhtar**Department of Mechanical EngineeringGrand Valley State UniversityGrand Rapids, Michigan 49504Email: kaminscw@mail.gvsu.edu, filusaus@gmail.com, kasprzap@mail.gvsu.edu andmokhtarw@gvsu.eduAbstractWith an ever increasing energy crisis occurring in the world it will be important to investigatealternative methods of generating power in ways different than, fossil fuels. In fact, one of thebiggest sources of energy is all around us all of the time, the wind. It can be harnessed not onlyby big corporations but by individuals using Vertical Axis Wind Turbines (VAWT). VAWT’soffer similar efficiencies as compared with the horizontal axis wind turbines (HAWT) and in facthave several distinct advantages. One advantage is that unlike their HAWT counterparts, theycan be placed independently of wind direction. This makes them perfect for locations where thewind direction can change daily.To analyze the effectiveness of a VAWT, methods of computational fluid dynamics (CFD) wereused to simulate various airflows and directions. The analysis began with a literature analysisinto the subject, in order to determine the types of airfoils that were most effective. After theresearch was done, the system was modeled in SolidWorks and imported into Star CCM toperform a CFD analysis. The first part of the CFD analysis analyzed the 2D flow over thechosen airfoil(s). Next, the analysis looked at the flow over a 3D representation of the airfoil(s).The 2D and 3D simulations used different angles of attack and speeds (15 & 30 mph) todetermine when separation occurred at the various speeds. Finally, a full VAWT assembly wascreated and analyzed at various wind directions at the same wind speeds. The full assemblyincluded 3 airfoils that were attached into a 5ft high, 3 ft diameter structure. Each step of theanalysis included importing the CAD file into Star CCM as an IGES file, selecting physicalmodels, generating a numerical mesh, and applying boundary conditions. All three portions ofresearch studied scalar and vector properties, such as pressure and velocity. This paper describesthe research of a VAWT using the NAC A0012-34 airfoil.* Mechanical Engineering Student** PhD, Assistant Professor of Mechanical Engineering, ASEE member.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education
IntroductionThe energy crisis the world is going to be in, in the next 100 years when the Earth’s supply offossil is no more, isn’t foreign matter to anyone. Since the start of the industrial revolutionhumans have been using fossil fuels to power their machines that make more machines. Societythinks that the “gravy train” that is fossil fuels will be around for a long time but analysis on thecurrent supplies and the ever growing population of the planet suggest that we may run out verysoon. The Klass model assumes a continuous compound rate and it is a computationalapproximation of when we will run out of the various fossil fuels. Depletion times for oil, coaland natural gas are approximately 35, 107, and 37 years.1 These figures do not give much timefor alternative methods to be found or perfected. The use of VAWT on residential andcommercial properties could help to extend these deadlines out further or even eliminate them ifthe right technologies come about.Literature ReviewWind turbines use the kinetic energy of the wind and convert it to mechanical energy. This isthen used to produce electricity, grinding of grain or pumping of water (windmills, wind pumps).There are two types of wind turbines, horizontal and vertical. Vertical axis wind turbines(VAWT) have the rotor shaft vertically. These types of wind turbines are advantageous becausethey do not need to be pointed into the wind in order to function. Also, the generator andgearbox are able to be placed near the ground, thus allowing for easy maintenance. On the otherhand, these types of turbines typically have a lot lower rotational speed, meaning higher torquesinvolved. Combine this effect with the very dynamic loading that is generated on the blades anda more expensive drive train is needed. This dynamic loading can however be reduced by usingmore than 2 blades.2 The dynamic loading that is generated on VAWTs requires higher materialexpenditures for a given construction as compared to HAWTs but when compared to the overallcosts of a small VAWT installation, the increased cost is negligible.3 Also, HAWTs arepresently cheaper because they have been produced for a long time and in larger numbers. Asresearch into VAWTs increases, the cost for such wind turbines will decrease as the processbecomes more efficient.4Symmetrical blades were chosen for this project because they are easier to manufacture. A studyby Liu Shuqin of Shandong University of Chin focused on the power generation based onchanging the type of blades that were used and this was used to give an insight into choosing theblade shapes for our VAWT design. The experimental setup is given below in Table 1. Theresults showed that self-pitch streamline symmetrical blades generated a higher power outputthan the others, as shown in Figure 1.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education2
Table 1: Experimental Setup5ParametersPitch TypeNumber of BladesBlade ShapeSize of BladeWeight of EachBladeDiameter of ArmRotationGeneratorWind Turbine ASelf-pitch5Streamlinesymmetrical blades120 cm (L) x 20 cm(W) x 2.5 cm (T)1.65 kgWind Turbine BFixed-pitch5Streamlinesymmetrical blades120 cm (L) x 20 cm(W) x 2.5 cm (T)1.65 kgWind Turbine CSelf-pitch5Arc blades120 cm (L) x 20 cm(W) x 1.75 cm (T)1.15 kg2m2m2m300 W permanentmagnet generator300 W permanentmagnet generator300 W permanentmagnet generatorFigure 1: Power generation of various types of blades5Another reason it was decided to choose symmetric airfoils was that although these types ofairfoils are not the most efficient at producing lift, they do produce a lift-stall effect that allowsthe system to obtain equilibrium. The lift that is created from the wind passing over the airfoil iseventually lost and the blade goes into its stall condition, but the other blades pick up more liftand keep the cycle going. This in turn regulates the speed of the entire VAWT.2An aspect ratio of 13.6 was chosen as a result of literature review. A study of an application ofdesign of a small capactity ( 10kW) fixed-pitch straight bladed VAWT, similar to application ofthis design study, utilized this aspect ratio. The study claimed 13.6 to be an efficient aspect ratiofor the application.6 Utilizing the maximum dimensions of the VAWT, the chord length wascalculated to be 4.41 inches using this aspect ratio.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education3
Present WorkThe presented study utilized CFD simulations to analyze the flow field around a vertical axiswind turbine (VAWT) that could be used for residential use. The design parameters for thechosen VAWT consisted of having 3 blades, a height of 5 ft and a diameter of 3 ft. Using theUIUC Airfoil Coordinates Database the NACA 001234 airfoil was chosen. A graphicalrepresentation of this airfoil as well as its associated properties can be seen below in Figure 2.The airfoils and the full assembly were then analyzed at various angles of attack and for differentwind speeds, as listed in Table 2. The objective of the analysis was to be optimized the attackangle in order to obtain the greatest possible lift force. The drafting software that was used tomodel the VAWT and the far field geometries that surround it and its components wasSolidWorks. Below, Figures 3 and 4 show images of the CAD models that were used for the2D/3D airfoil configurations and for the 3D full assembly respectively.Table 2: Simulations StudiedCaseYaw AngleWind Speed (mph)o ooo2D Airfoil0 , 5 , 10 , 1515, 30o ooo3D Airfoil0 , 5 , 10 , 1515, 30oooo3D Full Assembly0 , 30 , 60 , 9015, 30Figure 2: NACA 0012347Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education4
Figure 3: VAWT 2D & 3D Airfoil ConfigurationsFigure 4: VAWT Full Assembly ConfigurationComputational MethodThe analysis of the VAWT was done using STAR CCM , developed by CD Adapco as standardcommercial software. The analysis used a computational finite volume method to analyze the2D, 3D airfoil only and 3D full assembly cases with using a segregated flow solver. Modeling ofthe turbulence in each of the cases was done using the 2 equation SST k-ω model. This selectionfor the turbulence model was made because the k-ω model offers great analysis in both fullydeveloped flow and along the boundary layer regions.Three models were used in generating the mesh regions around the VAWT that were used in theanalysis of the 3 cases. These were the prism layer and surface remesher, and the trimmer.Approximately 600,000 cells were generated for the 2D analysis cases, around 900,000 cellswere generated in the various 3D airfoil analysis cases and for the full complete VAWT analysisabout 1.3 million cells were used in the simulations. Using STAR CCM’s volumetric controls,regions of importance, such as separation regions, were able to be analyzed more accurately.Figures 5a-5c shows the boundary conditions, surface mesh and volume mesh that were used inthe analysis of the 2D airfoil cases. Figure 6a-6c shows the boundary conditions, surface meshProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education5
and volume mesh that were used in the analysis of the 3D airfoil cases. Figures 7a-7c shows theboundary conditions, surface mesh and volume mesh that were used in the analysis of the 3DVAWT full assembly cases.Figure 5a: 2D Airfoil- Fluid Domain Boundary ConditionsFigure 5b: 2D Airfoil- Surface MeshFigure 5c: 2D Airfoil- Volume Mesh and Plane SectionProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education6
Figure 6a: 3D Airfoil- Fluid Domain Boundary ConditionsFigure 6b: 3D Airfoil- Surface MeshFigure 6c: 3D Airfoil- Volume Mesh and Plane SectionProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education7
Figure 7a: 3D VAWT Full Assembly- Fluid Domain Boundary ConditionsFigure 7b: 3D VAWT Full Assembly- Surface MeshFigure 7c: 3D VAWT Full Assembly- Volume Mesh and Plane SectionProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education8
CFD Results And DiscussionsThe first part of the analysis involved studying the effects that varying attack angles and windspeeds had on the flow regions. As expected, increasing the attack angle of the airfoil createdlarger regions of separation causing what is known as the stall effect. Increasing the wind speedcaused the separation regions to be more exaggerated with a greater amount of turbuluencepresent. Knowing the stall angle is important to the design of vertical axis wind turbines becauseof the lift-stall effect that was mentioned earlier in the literature review section. Having aVAWT system with very large separation regions would prove to be very inefficient because ofthe large amounts of drag that would exist.Figures 8 and 9 below show pressure distribution plots for 2 of the cases, more results can befound in Appendix A. Figure 8 shows the results from the 0 degree attack angle and it shows analmost equal pressure distribution on both the top and bottom of the wing. This makes sense dueto the symmetry of the chosen airfoil. Increasing the attack angle to 15 degrees, as shown inFigure 9, shows a dramatic change in the pressure distribution plot. It can be seen that there is avery large low pressure region on top of the wing and the stagnation point has moved to thebottom of the airfoil.Stagnation pointEqual pressure distributionFigure 8: 2D Airfoil Pressure Distribution for 0 degrees at 15 mphProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education9
Low pressure regionStagnation pointHigh pressureFigure 9: 2D Airfoil Pressure Distribution for 15 degrees at 15 mphFigure 10 and 11 below show the results of the velocity distribution for the same 2 cases asshown above, more results can be found in Appendix B. Figure 10 shows the 0 degree attackangle and similar to the pressure distribution, it shows approximately equal velocities both on topand under the airfoil. This is also due to the symmetry of the chosen airfoil. Just as before,increasing the attack angle, as in Figure 11, shows a very large separation region on top of thewing. In this region vortexes and reversal of flow exist in a turbulent manner.Equal velocity distributionFigure 10: 2D Airfoil Velocity Distribution for 0 degrees at 15 mphProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education10
Turbulent/separation regionHigher velocityLower velocityFigure 11: 2D Airfoil Velocity Distribution for 15 degrees at 15 mphThe same analysis that was done above on the 2D airfoil was done to a 3D airfoil model and itproduced similar results. Figures 12 and 13 show the pressure distributions on the airfoil alongwith velocity streamlines and Figures 14 and 15 show the velocity distribution around the airfoil.The streamlines become more turbulent as the angle of attack is increased, this is shown inFigures 12 and 13.Figure 12: 3D Airfoil Pressure Distribution for 0 degrees at 15 mphProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education11
Figure 13: 3D Airfoil Pressure Distribution for 15 degrees at 15 mphFigure 14: 3D Airfoil Velocity Distribution for 0 degrees at 15 mphProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education12
Figure 15: 3D Airfoil Velocity Distribution for 15 degrees at 15 mphFurther analysis of the VAWT involved studying the net lift force generated at the various anglesof attack and wind speeds. The results show that at higher speeds the critical angle of attackhappens faster than at lower speeds. Looking at the results from the 2D airfoil analysis of the liftforce versus attack angle (Figure 16), the results show that the stall condition occurred at fasterwind speed at approximately 8 degrees. The results for the lower wind speed are incomplete andthe stall condition could not be seen in the scope of the speeds that were tested for this analysis.The faster wind speed condition produced the most net lift force, which was expected. The netlift force results for the 3D airfoil analysis (Figure 17) show similar results to that of the 2Dscenario but this graph does not appear to show clear stall conditions. This could be because themodeling of the 2D airfoil is a simplified analysis and the 3D airfoil.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education13
0.3Net Lift Force (lbf)0.250.20.1515 MPH0.130 MPH0.050-0.05051015Angle of Attack (degrees)20Figure 16: 2D Airfoil- Lift Force vs. Attack AngleNet Lift Force (lbf)2.521.515 mph130 mph0.5005101520Angle of Attack (degrees)Figure 16: 2D Airfoil- Lift Force vs. Attack AngleThe full assembly 3D results were intended to be run for 4 different angles of attack, 0, 30, 60and 90 degrees and 2 wind velocities 15mph and 30mph for a total of 8 runs. This would cover asampling of the angles that turbine will see during operation, and since the pattern repeats after120 degrees, there is no need to redo those results. Several different mesh models were run untilone yielded results that seemed logical. At first from looking at the velocity distribution, Figure17, the results seemed to make sense.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education14
Figure 17: 3D Full Assembly- velocity distributionAfter analyzing the results further however, several anomalies arose. For one, the residual plot,shown below in Figure 18, had large spikes in turbulent kinetic energy and specific dissipationrate as shown. These errors were far above acceptable results and were cause for concern.Figure 18: Residuals plot for 3D full assemblyProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education15
Also, further post processing tools used did not yield logical results. In the pressure plot below,Figure 19, there were several spots of abnormally large or abnormally negative results. A closeup view is available in Figure 20. These numerical errors resulted in illogical results.Figure 19: Pressure distribution with streamlinesAbnormal pressureincrease on insideface of airfoilFigure 20: Close-up on pressure distributionUpon further investigation into these results, it was found that the inside of the trailing edge hadsome large irregularities from the surface mesh, Figure 21. This only happened on a single one ofthe airfoils each time however. To investigate if this was indeed the cause of the error, severalother angles of air were run and it was found that each time, there was a large error on the airfoilwhere the air hit it nearly perpendicular to the inside trailing edge.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education16
Figure 21: Surface mesh irregularitiesThis theory was further confirmed by looking at the velocity distribution at the boundary layer(Figure 22) of the affected airfoil. To eliminate the possibility that it was an issue with theboundary layer itself, several models were run with anywhere between 10 and 20 prism layers. Ithad no effect; the results were the same every time.Figure 22: Boundary layer analysis of velocity distributionSeveral attempts were made to fix the issues with the surface mesh such as adding feature curvesnear the trailing edge and increasing the number of cells all the way to 6 million cells. Thesefixes were not enough to smooth out the problem areas. A solution could not be found in the timelimit of the study so no further results were able to converge.ConclusionsThe use of the STAR CCM software to analyze the air flow around a vertical axis wind turbineproved to be a very effective tool. A full analysis could be performed before any construction ofsuch a project even began. The modeling of the airfoil in both the 2D and 3D cases allows theuser to study the aerodynamics of various geometries at different physical settings to get a truefeel for how the specific airfoil might behave in real world applications. Wind speeds of 15 and30 mph were studied, as well as varying attack angles from 0 to 15 degrees, the results of thisresearch on the NACA 001234 airfoil showed it could be a very viable choice for a residentialProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education17
VAWT. The 2D analysis gave a stall angle of about 8 degrees, however, the 3D analysis, itbeing more accurate, did not provide us with a stall angle. This suggests that further analysiswould provide more information on the critical angle of attack. CFD software is a very strongtool but it can be difficult to master. The results for the 3D full assembly analysis of vertical axiswind turbine were incomplete. Even though the results yielded were less than desirable, analysisof why the simulations failed proved to be very helpful in the mastery of CFD software use.
Wind turbines use the kinetic energy of the wind and convert it to mechanical energy. This is then used to produce electricity, grinding of grain or pumping of water (windmills, wind pumps). There are two types of wind turbines, horizontal and vertical. Vertical axis wind turbines (VAWT) have the rotor shaft vertically.
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The CFD software used i s Fluent 5.5. Comparison between the predicted and simulated airflow rate is suggested as a validation method of the implemented CFD code, while the common practice is to compare CFD outputs to wind tunnel or full-scale . Both implemented CFD and Network models are briefly explained below. This followed by the .
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CFD (Computational Fluid Dynamic) Model Comparison between Two Independent Models 1.ANSYS CFD was utilized by CCNY (CCNY City College of New York) 2.COMSOL CFD was employed by QRDC (QRDC is an R&D Company in Chaska, MN) 3.A virtual wind tunnel was constructed to examine the performance of an INVELOX system 4.The results are in agreement
A.2 Initial Interactive CFD Analysis Figure 2: Initial CFD. Our forward trained network provides a spatial CFD analysis prediction within a few seconds and is visualised in our CAD software. A.3 Thresholded and Modified CFD Analysis Figure 3: Threshold. The CFD is thresholded to localise on
performing CFD for the past 16 years and is familiar with most commercial CFD packages. Sean is the lead author for the tutorial and is responsible for the following sections: General Procedures for CFD Analyses Modeling Turbulence Example 3 - CFD Analysis
CFD Analysis Process 1. Formulate the Flow Problem 2. Model the Geometry 3. Model the Flow (Computational) Domain 4. Generate the Grid 5. Specify the Boundary Conditions 6. Specify the Initial Conditions 7. Set up the CFD Simulation 8. Conduct the CFD Simulation 9. Examine and Process the CFD Results 10. F
