Various methods of flow control for enhanced aerodynamic performance have been developed and applied to enhance and control the behavior of aerodynamic components. The use of Coandă effect for the enhancement of circulation and lift has gained renewed interest, in particular with the progress of CFD. The present work addresses the influence, effectiveness, and configuration of Coandăjet fitted aerodynamic surface for improving lift and
In line with the efforts to enhance the use of green energy technology for energy extraction, conversion, and propulsion, the fundamental principles and mechanisms that play key roles in these technologies have been the focus in many current research efforts as well as the present research work, since these have the eventual potential of national development significance. Since one of the first attempts to generate electricity by using the wind in the United States by Charles Brush in 1888 [
Wind energy conversion research and development efforts have their origin on fluid physics, thermodynamics, and material sciences. Learning from various innovations introduced in aircraft technology, wind energy technology can take advantage of flow circulation modification and enhancement.
Flow control for enhanced aerodynamic performance has taken advantage of various techniques, such as the use of jets (continuous, synthetic, pulsed, etc.), compliant surface, vortex cell, and the like [
In this conjunction, and with the progress of CFD, the use of Coandă effect to enhance lift has attracted renewed interest [
In the design of a novel Wind turbine using with the latest aerospace engineering technology, including aerodynamics, new lightweight materials, and efficient energy extraction technology, highly efficient aerodynamic surface came as a key issue. Recent efforts in the last decade have also indicated that Coandă effect has been given new and considerable considerations for circulation control technique [
The progress of high speed computers exemplified by the availability of new generations of notebooks has made possible the use of firstprinciplesbased computational approaches for the aerodynamic modeling of Wind turbine blades, to name an example. As pointed out by Xu and Sankar [
In the design of circulation control technique, one may note the progress of stallcontrolled and pitchcontrolled turbine rotor technology for dealing with power production at low and high wind conditions [
However, over the past decades, commonly used airfoil families for horizontal axis Wind turbines such as the NACA 44, NACA 23, NACA 63, and NASA LS
Annual energy improvements from the NREL airfoil families are projected to be 23% to 35% for stallregulated turbines, 8% to 20% for variablepitch turbines, and 8% to 10% for variablerpm turbines. Optimizing airfoil’s performance characteristics for the appropriate Reynolds number and thickness provides additional performance enhancement in the range of 3% to 5% [
Circulation control wing (CCW) technology is known to be beneficial in increasing the bound circulation and hence the sectional lift coefficient of airfoil. This technology has been extensively investigated both experimentally and numerically [
This causes the boundary layer and the jet sheet to remain attached along the curved surface due to the Coandă effect (i.e., a balance of the radial pressure gradient and centrifugal forces) causing the jet to turn without separation. The rear stagnation point location moves toward the lower airfoil surface, producing additional increase in circulation around the entire airfoil. The outer irrotational flow is also turned substantially, leading to high value of lift coefficient comparable to that achievable from conventional high lift systems, as illustrated in Figure
(a) Circulation control wing/upper surface blowing STOL aircraft configuration, from [
Numerical simulation carried out by Tongchitpakdee et al. [
With such background, the present work is aimed at the search for favorable Coandăjet lift enhanced configuration for wind turbine designs, focusing on twodimensional subsonic flow, by performing meticulous analysis and numerical simulation using commercially available CFD code. Results obtained from recent researches will be utilized for directing the search as well as for validation. For example, best lift enhancing configuration [
To arrive at desired design configurations, one is lead to come up with desired logical cause and effect laws as well as to find ways to carry out optimization schemes. It is with such objectives in mind that using numerical analyses, parametric studies can be carried out and may offer some clues on relevant parameters which may be utilized in a multivariable optimization (and to a larger scale, multidisciplinary optimization). The present work will investigate the influence, effectiveness, and configuration of Coandăjet fitted aerodynamic surface, in particular S809 airfoil, for improving its lift augmentation and
Twodimensional incompressible Reynoldsaveraged NavierStokes (RANS) equation will be utilized to perform the analysis and to obtain numerical solutions on a computational grid surrounding a reference airfoil. The governing equation is given by (steady state and ignoring body forces)
Computational fluid dynamics codes will be used to analyze the flow around twodimensional airfoil. Various CFD codes are available and are alternatively utilized. Careful review and analysis of the application of the first principle in the numerical computation are carried out prior to the utilization of commercially available CFD codes for the present study. For validation of the computational procedure, resort is made to appropriate inviscid case, and online aerodynamic codes are utilized. For a twodimensional geometry the mesh generator partitions the subdomains into triangular or quadrilateral mesh elements [
The basis of the computational approach in the present work is the twodimensional incompressible Reynoldsaveraged NavierStokes (RANS) equation (
Typical
Turbulence modeling in CFD is very essential in the choice of grid fineness and obtaining the correct simulation of particular flow field. For this purpose, one has to choose a particular model out of a host of available turbulence models developed to date. One of these turbulence models that is considered to be appropriate and user friendly is the
The
The dimensionless distance in the boundary layer, sublayer scaled, representing the viscous sublayer length scale, plays significant role in capturing relevant physical turbulence phenomena near the airfoil surface commensurate with the grids utilized in the numerical computation. In this regards, the flow field in the vicinity of the airfoil surface is usually characterized by the law of the wall, which attempts to identify intricate relationships between various turbulence model scaling in various sublayers. For example, the wall functions approach (wall functions were applied at the first node from the wall) utilized by
Based on preliminary attempts to choose the appropriate turbulence model which yields desirable results, in the present work,
For practical implementation purposes, it is worthwhile to introduce an auxiliary parameter
Following iterative procedure elaborated by Kuzmin et al. [
Various studies [
For wall functions, the computational domain starts a distance
By applying the wall function at the nodes of the first meshing layer of the computational grid at a distance
The distance
Therefore, care is exercised in the choice of grids in the vicinity of the airfoil, to obtain a certain acceptable error tolerance (which may also be attributable to numerical error and uncertainties). The plausibility of the numerical results will be judged by comparison to other established results in the literature (numerical or experimental) for specific cases. In the present study, it was found that the turbulence intensity of 5% and length scale of 0.01 m yield results that agree with benchmarking data. It is noted that Howell et al. [
In the present work, the grids were constructed following free mesh parameter grid generation using an algebraic grid generator and varied from extremely coarse up to extremely fine grids. However, for the S809 airfoil case, near the surface of the airfoil, boundary layer based meshing is carried out throughout. Grid sensitivity study on the lift force per unit span for clean airfoil at zero angle of attack is plotted in Figure
Grid sensitivity study for clean S809 airfoil at zero degree angle of attack; (a) lift force per unit span; (b) drag force per unit span.
Figure
The range of free mesh parameters.
Parameter  Range 

Maximum element size  0.156–0.42 
Minimum element size  0.018–0.12 
Maximum element growth rate  1.08–1.13 
Resolution of curvature  0.25–0.3 
Resolution of narrow regions  1 
In order to control the
Computational grid meshing layer properties.
Parameter  Range 

Number of mesh layers in the boundary layers 

Boundary layer mesh expansion factor 

Thickness of first mesh layer  0.00005–0.0002 
Thickness adjustment factor 

Along with the requirement of the
The grid generator is sufficiently general so that one can easily vary the jet slot location and size. Grid spacing and clustering can have significant effects on Wind turbine load and performance predictions [
Computational grid around a typical airfoil, shown here for S809 Coandă configured airfoil.
In general, the initial conditions are set to be equal to the properties of the freestream flow condition [
The outer boundary is placed far away from the blade surface, at a minimum of 6 chords (6 C). Nonreflecting boundary conditions are applied at the outer boundaries of the computational domain. The jet is set to be tangential to the blade surface at the Coandăjet nozzle location. The jet velocity profile is specified to be uniform at the jet exit. On the airfoil surface, except at the jet exit, noslip boundary conditions are applied. To gain benefits, the jet velocity is here designed to be larger than the potential flow velocity at the vicinity of the outer edge of the boundary layer. In addition, the thickness of the Coandăjet is designed to be less than the local boundary layer thickness.
Outer boundary conditions are chosen to minimize blockage effect, that is, significant discrepancies with free flight situations, whether theoretically or numerically simulated or experimental. Wall effects blockage changes streamline curvature interaction boundary layers and these should be minimized. The best measure is by carrying out computational experiments with reasonable outer boundaries of the computational grid and validating with both outer potential flow results and numerical parametric studies. This rationale has shown that, for the case considered, an outer grid boundary distance to the computational domain centerline of six chords produces results with high accuracy, as can be justified in the results presented in Figure
Prior to its utilization, various baseline cases have been tried out, with favorable results. As a case in point, for benchmarking purposes the code has been applied to calculate the liftslope characteristics of S809 airfoil (clean configuration, without Coandă) and GTRI CCW Dual Radius airfoil (with Coandă) and compared to the results obtained by Somers [
Comparison for validation of S809 airfoil of CFD computational results using COMSOL
Comparison of liftcurve slope for GTRI Dual Radius CCW airfoil with LE blowing on CFD computation using COMSOL
Computational results exhibited in Figure
Similar numerical simulation was carried out for GTRI Dual Radius CCW airfoil with leading edge blowing and compared to the experimental data (in Figure
The results exhibited in both examples above serve to indicate that the computational procedure and choice of turbulence model seem to be satisfactory for the present computational study and could lend support to further use of the approach in the numerical parametric study.
All computations in the present study were performed on a laptop computer with a 2.10 GHz Intel Core Duo processor, 4 GB of RAM, and 32bit operating system. Typical computation time for the computation of the flow characteristics around a two dimensional airfoil is in the order of 4 hours with around 300,000 degrees of freedom by using stationary segregated solver in
For the purpose of assessing the influence and the effectiveness of the Coandă enhanced
TE construction of the Coandă configured airfoil (the jet flow is tangential to the rounded circular sector).
Next, we would like to investigate the influence of specifically designed airfoil geometry for Wind turbine application, and for this purpose a typical S809, in clean and Coandăjet equipped configurations. S809 airfoil represents one of a new series of airfoils which are specifically designed for HAWT applications [
The twodimensional numerical simulation study for the S809 airfoil is carried out in logical and progressive steps. First, the numerical computation is performed on the clean S809 airfoil, then on the Coandă configured S809 airfoil without the jet (i.e., after appropriate modification due to TE roundingoff and backstep geometry), and then finally on Coandă configured S809 airfoil in its operational configuration.
To address threedimensional wind turbine configurations, particularly for the optimum design of a horizontal axis Wind turbine (HAWT), logical adaptation should be made, taking into account the fact that different airfoil profiles may be employed at various radial sections. Certain assumptions have to be made in order to project twodimensional simulation results to the threedimensional case, which may be necessary to evaluate the equivalent Betz limit.
The flow field in the vicinity of the TE for both configurations is shown in Figures
Velocity fields of S809 airfoil (a) with and (b) without Coandăjet.
The TE radius plays an important role in the Coandă configured design airfoil, since it may positively or negatively influence the downstream flow behavior. Tongchitpakdee et al. [
In contrast to the needs of TE roundingoff radius, performance degradation associated with it always stands as an issue due to the drag penalty when the jet is in the off mode. To overcome such drawback, the TE radii should be specifically and carefully designed. For that purpose, simulations at several TE radius (from 10 mm to 50 mm) have been performed, at a fixed Coandăjet momentum coefficient
(a) The effect of TE radius on
Moving the location of the Coandăjet forward implies the increase of the TE radius. Consequently, the camber of the airfoil will also be changed. This in turn will produce changes in the angle of attack of the airfoil compared to the baseline case. Therefore, although the present study looks into the influence of modifying TE radius, while holding other airfoil geometrical parameters constant, the zero angle of attack condition is at best possible only as first approximation. The robustness of the computational procedure adopted in the present computational setup may be able to take into account all these changes. Such a case is indeed revealed in Figures
Variation of the Coandăjet thickness from 0.5 mm to 3.0 mm at a fixed
(a) The effect of jet thickness on the
The performance of Coandă configured airfoils is dependent on the jet momentum conditions, which are important driving parameters. Figures
The effect of jet momentum on the
Figure
The effect of Coandăjet location on the
It should be noted that for the purposes of the present work, a uniform jet velocity profile has been adopted; this could be readily modified for more realistic modeling or design requirements. Numerical results indicate that there exists an optimum Coandăjet configuration, which has been the subject of parametric study as exhibited in Figures
A significant design parameter for boundary condition, which has been utilized to characterize Coandăjet applications by many investigators [
This expression shows that for a given constant
To justify the results of the present study, and to give us a physical explanation of the effect of Coandăjet, one may attempt to carry out simple calculation using first principle and KuttaJoukowski law for potential flow and compare the lift of the Coandă configured airfoil with the clean one obtained using CFD code. One has
For the threedimensional configuration, there is a physical relationship between the Wind turbine shaft torque (which is a direct measure of the extracted shaft power) with
It should be noted that the results obtained in the present work is limited to twodimensional wind turbine blade analysis, which at best can be interpreted to be valid at 0.7 blade span distance from the wind turbine rotor hub axis. The ambient air freestream wind speed
With due considerations of the threedimensional case, from the numerical results gained thus far, it can be deduced that circulation control, which in this particular case obtained by utilizing TE Coandăjet, can considerably increase the torque generated through the
Consequently, Coandăjet has the potential to increase the energy output delivered by a Coandă configured Wind turbine. To this end, one may recall that the maximum power that can be delivered by a Wind turbine cannot exceed the Betz limit, which is given by
Hence, with the introduction of Coandăjet, using considerations reflected by (
With regard to the design implementation of the present results and concepts, the maximum theoretical power that can be extracted from the freestream (ambient air) in the real threedimensional situation is given by (
CFD numerical experiments have been carried out to elaborate work reported earlier [
CFD numerical computations for the flow field around twodimensional airfoil S809 have been carried out with the objective to study the extent to which the introduction of Coandăjet enhances the aerodynamic performance of the airfoil, here represented by the
The introduction of Coandăjet on both airfoils carried out in the present work results in enhanced
Roundingoff of the TE along with the introduction of the Coandăjet seems to be effective in increasing
With due considerations of prevailing threedimensional effects, the twodimensional numerical study can be used to direct further utilization of the CFD computational procedure for Wind turbine blade studies and their design optimization. Numerical results presented have been confined to zero angle of attack case, which has been considered to be very strategic in exhibiting the merit of Coandăjet as lift enhancer. The numerical studies could be extended to increasing the angle of attack to obtain more comprehensive information, for which the choice of turbulence model will be more crucial.
The study also shows that the maximum total energy output of Coandă configured airfoil may exceed that predicted by Betz limit. With all the results obtained thus far, it is felt that the present work is by no means exhaustive. Other issues may still be explored, such as how could the ambient air energy input that can be drawn by the Coandăjet configured Wind turbine either from the nacelle or elsewhere be utilized to energize the Coandăjet and, for that matter, to lower the cutin speed of HAWT or the starting speed of VAWT.
Computational fluid dynamic
Circulation control wing
Trailing edge radius (mm)
Airfoil chord length (m)
Lift force (N)
Drag force (N)
Coandăjet thickness (mm)
Lift over drag ratio
Trailing edge
Short takeoff landing
Dimensionless wall distance for a wallbounded flow
Friction velocity
Distance to the nearest wall
Kinematic viscosity
Wall shear stress
Density
Turbulent viscosity, as defined by (
Horizontal axis wind turbine
Mach number
Turbulent model constant, as defined by (
Momentum coefficient
Lift augmentation
Lift augmentation due to Coandăjet
Megawatt
Megawatt hour.
The authors assure that there is no conflict of interests whatsoever with any commercial products, including CFD codes, that have been utilized in the present work. Any commercial software codes that are used in the present work, such as Fluent and COMSOL, are used for the purpose of arriving at research results and have been acquired through purchase, without any vested interest whatsoever from the authors.
The present research was initiated under Universiti Putra Malaysia (UPM) Research University Grant Scheme (RUGS) no. 0502100928RU, CC91933, and the Ministry of Higher Education Exploratory Research Grant Scheme Project Code no. 5527088. The first and corresponding author would also like to thank Universitas AlAzhar Indonesia for the opportunity to carry out the present research at Universiti Putra Malaysia.