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AERODYNAMIC DESIGN AND TESTING OF BLADE PROFILES FOR VERTICAL AXIS WIND TURBINES

Objective

THE OBJECTIVE OF THIS PROJECT IS THE INVESTIGATION OF THE INFLUENCE OF UNSTEADY AND ROTATIONAL FLOW PHENOMENA ON THE PERFORMANCE OF VAWT. THE INVESTIGATION WAS CARRIED OUT WITH THE HELP OF A NUMERICAL MODEL USING SURFACE SINGULARITY DISTRIBUTIONS. TYPICAL AIRFOIL SECTIONS WHICH HAVE BEEN USED OR HAVE BEEN PROPOSED FOR VAWT APPLICATIONS WERE EVALUATED.
INFORMATION WAS PRODUCED ON TWO ESSENTIAL CRITERIA FOR THE SELECTION OF AIRFOIL SECTIONS : THE FORM OF THE ADVERSE PRESSURE GRADIENT (ESSENTIAL FOR THE SEPARATION) AND THE VARIATION OF THE INSTANTANEOUS FORCES (THE NORMAL BEING ESSENTIAL FOR THE STRUCTURAL DESIGN OF THE VAWT). BASED ON THIS EVALUATION A FAMILY OF AIRFOIL SECTIONS HAS BEEN DEFINED. THEIR USE COULD IMPROVE THE PERFORMANCE CHARACTERISTICS OF THE VAWT. IN ORDER TO TEST BLADES WITH THOSE AIRFOIL SECTIONS A VAWT MODEL HAS BEEN DESIGNED AND CONSTRUCTED. THE TEST WILL TAKE PLACE IN THE 3.5 M X 2.5 M SECTION OF THE NTUA WIND TUNNEL.

THE EVALUATION OF TYPICAL AIRFOIL SECTIONS WHICH EITHER HAVE BEEN USED OR HAVE BEEN PROPOSED FOR VAWT LEAD TO SOME CONCLUSIONS REGARDING THE CHARACTERISTICS WHICH CAN AIRFOIL SUITABLE FOR VAWT SHOULD HAVE. THESE CHARACTERISTICS ARE DEFINED BY ANALYSING TWO ESSENTIAL CRITERIA FOR THE SELECTION OF AIRFOIL SECTIONS FOR VAWT WHICH HAVE NOT BEING ANALYSED IN THE PAST: THE VARIATION OF THE INSTANTANEOUS FORCES (THE TANGENTIAL BEING ESSENTIAL FOR THE ENERGY PRODUCTION AND THE NORMAL FOR THE STRUCTURAL DESIGN OF THE VAWT) AND THE FORM OF THE INSTANTANEOUS PRESSURE DISTRIBUTION ON THE AIRFOIL (ESSENTIAL FOR THE SEPARATION). THE ANALYSIS HAS BEEN CARRIED OUT WITH THE AID OF A NUMERICAL MODEL USING SURFACE SINGULARITY DISTRIBUTIONS ON THE AIRFOIL AND VORTEX PARTICLES WITH FINITE CORES ON THE WAKE.
IN THIS MODEL BOTH ROTATIONAL AND UNSTEADY PHENOMENA ARE TAKEN INTO ACCOUNT. THE METHOD, THOUGH, IS LIMITED TO NON-VISCOUS FLOW CALCULATIONS.

THE EVALUATION SHOWED THAT FLOW CURVATURE AND UNSTEADY EFFECTS CREATE AN ASSYMETRIC LOADING BETWEEN THE UPSTREAM AND DOWNSTREAM PART OF THE ROTOR WHICH HAS IMPLICATIONS IN THE FATIGUE LIFE AND THE PERFORMANCE (PREMATURE STALLING) OF THE WIND TURBINE. IN ORDER TO COMPENSATE THESE EFFECTS AN AIRFOIL HAS BEEN DESIGNED WHICH HAS THE THICKNESS DISTRIBUTION OF A NACA 0015 AIRFOIL AND A MEAN LINE WHICH IS AN ARC OF THE CIRCLE OF ROTATION. THE AIRFOIL IS SHOWN TO HAVE A GOOD AERODYNAMIC BEHAVIOUR WHICH IS AMELIORATED IN SOME RESPECTS IF A FIXED BLADE PITCH IS USED.

BASED ON THE ABOVE RESULTS OTHER AIRFOILS WITH DIFFERENT THICKNESS DISTRIBUTIONS HAVE ALSO BEEN DESIGNED AND THE RELATIVE ADVANTAGES AND DISADVANTAGES OF EACH ONE HAVE BEEN DEFINED. THREE OF THEM HAVE FINALLY BEEN SELECTED. THEIR PERFORMANCE FOR DIFFERENT TIP SPEED RATIOS AND DIFFERENT CHORD TO RADIUS RATIO IS COMPARED WITH THAT OF THE CLASSICAL NACA 0015 AIRFOIL. BLADES WITH THESE AIRFOIL SECTIONS ARE NOW UNDER CONSTRUCTION IN ORDER TO BE TESTED EXPERIMENTALLY IN THE WIND TUNNEL.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

NATIONAL TECHNICAL UNIVERSITY OF ATHENS
Address
Iroon Polytechnioy 5
15780 Athens
Greece