Numerical Analysis on the Effectiveness of Gurney Flaps as Power Augmentation Devices for Airfoils Subject to a Continuous Variation of the Angle of Attack by Use of Full and Surrogate Models

Abstract

The disclosing of new diffusion frontiers for wind energy, like deep-water offshore applications or installations in urban environments, is putting new focus on Darrieus vertical-axis wind turbines (VAWTs). To partially fill the efficiency gap of these turbines, aerodynamic developments are still needed. This work in particular focuses on the development of a mathematical model that allows predicting the possible performance improvements enabled in a VAWT by application of the Gurney flaps (GFs) as a function of the blade thickness, the rotor solidity and geometry of the Gurney flap itself. The performance of airfoil with GFs was evaluated by means of detailed simulations making use of computational fluid dynamics (CFD). The accuracy of the CFD model was assessed against the results of a dedicated experimental study. In the simulations, a dedicated method to simulate cycles of variation of the angle of attack similar to those taking place in a cycloidal motion (rather than purely sinusoidal ones) was also developed. Based on the results from CFD, a multidimensional interpolation based on the radial basis functions was conducted in order to find the GF design solution that provides the highest efficiency for a given turbine in terms of airfoil and solidity. The results showed that, for the selected study cases based on symmetric airfoils, the GF positioned facing outwards from the turbine, which provides the upwind part of the revolution, can lead to power increments ranging from approximately 30% for the lower-solidity turbine up to 90% for the higher-solidity turbine. It was also shown that the introduction of a GF should be coupled with a re-optimization of the airfoil thickness to maximize the performance.

Some design guidelines to adapt a Darrieus vertical axis turbine for use in hydrokinetic applications

“…According to the technical literature [11,12], it was found that the height of the GF should be kept below the boundary layer thickness in order to obtain a beneficial lift-to-drag ratio. In previous works carried out by some of the authors for VAWTs applications [13,14], it was found that the best performance can be achieved when a GF with a height of 2%c was applied on the inner side of the airfoil.…”

“…The numerical investigation was performed by means of two dimensional CFD simulations with the commercial software ANSYS ® FLUENT ® . The computational framework was thoroughly assessed and validated with experimental data for various test cases by some of the authors in previous studies [13][14][15][16][17][18]. According to previous results, the unsteady Reynolds-Averaged Navier-Stokes (URANS) approach was adopted with the Menter's κ-ω SST (shear stress transport) model for turbulence closure.…”

“…TSR higher than the efficiency peak, while the number of nodes was increased to 1500 for low TSRs. In case of hydrofoil equipped with Gurney Flap, a much finer grid was necessary, according to [13][14][15], therefore a nodes' count on the blade surface was 2000. The resulting size of the different meshes is reported in Table 1, together with the azimuthal timestep adopted for the time advancing of the solution.…”

An attempt to measure cycloidal rotor fan in a rectangular duct

“…Mostly it is used as a propulsion of small unmanned aerial vehicles in aviation, in maritime its variant in the form of a cycloidal propeller is used for maneuvering the ship [2][3][4][5][6][7][8][9][10]. Thanks to its advantages, such as, the possibility of a quick change of the flow direction [9][10][11], higher flow efficiency than in the case of machines without cycloidal regulation and the possibility of using a Gurney flap (for fans) [12]. It can also be used for HVAC.…”

Special Issue “Numerical Simulation of Wind Turbines”

“…Developments are needed in each of them, from more accurate but computationally affordable simulation techniques [11,12], to innovative control logics [13] or effective methods for noise prediction [14]. Focusing more specifically on aerodynamics, it is apparent that the use of high-fidelity computational techniques is key to properly model the new generation of long and flexible blades, where aero-elastically-tailored designs are needed, and different flow control devices are often used to alleviate the loads [15,16]. As discussed, all these new methods and tools, however, should be developed synergically by the different groups of specialists, in view of that integration that represents the only way of properly addressing the reliable modeling of such a complex system like a wind turbine.…”