This study presents an experimental investigation on a diffuser augmented wind turbine (DAWT). A screen mesh is used to simulate the energy extraction mechanisms of a wind turbine in experiment. Different screen porosities corresponding to different turbine loading coefficients are tested. Measurements of the axial force and of the velocity distribution in radial direction are reported. The general purpose is to highlight the dependency between the diffuser and the screen, and to compare the radial velocity distributions in the diffuser between unloaded and loaded conditions. It is shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition. Moreover, the thrust on the diffuser largely depends on the screen loading. For the present configuration, the thrust on the screen with high loading coefficient contributes for more than 70% of the total thrust on the DAWT. Smoke visualizations and radial velocity profiles reveal that the high loading screen induces flow separation on the outer surface of the diffuser, justifying the results of the thrust measurements. It is also inferred that the flow separation leads to loss of thrust and has a great effect on the total pressure drag. It should be emphasized that the experimental results indicate that the flow field around the diffuser is strongly affected by the choice of screen porosity, that is, turbine loading. And that, the thrust coefficient of the diffuser does not show a linear dependence on the thrust coefficient of the screen. The axial momentum theory, therefore, is not a solid predictor for DAWT performance with high loaded screens. @en

The present study aims at providing experimental performance analysis of ducted wind turbines (DWTs) through momentum theory. To simplify the DWT model, a screen emulating the rotor is adopted by applying the actuator disc theory. Two duct models and five different screens are employed for different configurations to investigate the aerodynamic performance of the DWT in this experimental study. Duct 1 (C T , d u c t = 0.91 ) is characterized by an aerofoil-shaped cross-section and duct 2 (C T , d u c t = 1.17 ) has a cambered shape. Specifically, the five screens vary in porosity from low (C T , s c r e e n = 0.46) to high thrust coefficient (C T , s c r e e n = 1.12). Results show that the total thrust coefficient (C T , D W T ) varies with screen thrusts and ducts geometry. The aerofoil-shaped duct 1, caused by the non-linear behaviour of the aerodynamic force on an aerofoil-shaped body at increasing angle of attack, is more sensitive to the screen loading compared to duct 2. Thrust on duct 1 is highly dependent on screen thrusts owning to the existence of inward oriented lift, while thrust on cambered plate duct 2 stays constant. The high thrust and solid blockage this duct creates, accelerates the flow through the lesser loaded centre where the screen is located. The data also show that the growth of duct thrust has a positive effect on the overall performance of DWT. Both high screen loading and inappropriate aerofoil shape duct may lead to flow separation which causes the momentum loss for DWT. Moreover, the optimal value of C T , s c r e e n = 0.89 for a bare wind turbine doesn’t seem apply to the tested DWTs. The optimal value of C T , D W T depends both on duct geometry and the screen loading. The velocity measurements further indicate that flow separation occurs at the duct 1 external (pressure) surface, and shows the presence of stalled flow around centre line inside the duct. Finally, a comparison of the pressure distribution of duct with different screen loadings reveals that adding a screen inside duct 1 leads to loss in lift and hence to mass reduction. @en

A new high-lift aerofoil modification for the duct has been developed and will be experimentally tested in a small wind tunnel. Aerofoils for such wind tunnel ducts typically operate in the low Reynolds number range from 2 × 105 to 6 × 105. The effect of a duct and of rotor on power and pressure drop were considered separately in previous studies. This paper focuses on the optimization of aerofoil geometry for a Reynolds number of 3 × 105 taking into account of the presence of a screen, having a pressure drop similar to a real rotor. In particular, the current work concentrates on obtaining high lift, instead of high lift-to-drag ratio. Since high lift is the only desirable feature when modifying an aerofoil for ducts, the factors most related to enhanced high-lift low Reynolds numbers aerofoil performance are investigated. Previous experimental data of a three-dimension aerofoil-shaped duct model are used. Combining these data, and applying the Liebeck type high-lift design philosophy, which is to make use of an optimal pressure recovery with aft loading, variations in thickness, camber, and the shape of leading and trailing edges are analysed through the fully inversed method. The XFOIL 6.99 code was adopted as the analyse tool in this study. With the specified velocity distribution, it is found that an increase of both camber and thickness of the duct leads to an increase in lift coefficient with the presence of the pressure drop. In particular, the thickness increment for the aft part of the aerofoil generates higher lift coefficient. The installation of screen divides the duct into two parts, the duct fore part starts from the leading edge until the screen plane, while the duct aft part includes the screen plane to trailing edge. It is observed from previous experimental data that, with the screen presence, the front stagnation point moves towards the inner part of the duct. Consequently, the pressure coefficient reduces in the front part of the suction side, although the pressure differences, between the upper surface and the lower surface, of the duct fore part enlarges. Decreasing the leading edge radius, in essence, accelerates the airflow around it so that a negative area was created. Building on these results, the modified aerofoil model is fabricated and will be tested in a wind tunnel experiment. The test two-dimension model, with the assumption of symmetrical flow, is composed of an aerofoil and a uniform porous screen to simulate half part of the rotor from centreline. The aerofoil has a chord length of 20 mm and the screen has a length of 130 mm in vertical direction. To find the highest lift coefficient of this 2-dimension model, measurements will be conducted with the varying angle of attack and wind speed. Moreover, to investigate the effect of screen loading onto the configuration, there will be two different screens tested. Since the experiment will be carried out in April 2017 the comparison with the XFOIL 6.99 predictions cannot be provided at present, but will be shown during the symposium. @en

This paper reports an experimental investigation on the effect of the duct geometry on the aerodynamic performance of an aerofoil shaped ducted wind turbine (DWT). The tested two-dimensional model is composed of an aerofoil equipped with pressure taps and a uniform porous screen. The experimental setup is based on the assumption that the duct flow is axisymmetric and the rotor can be simulated as an actuator disc. Firstly, different tip clearances between the screen and the aerofoil are tested to point out the influence of this parameter on the DWT performance in terms of aerofoil pressure distribution, aerofoil lift and flow field features at the duct exit area. Then, the combined effect of tip clearance, of the angle of attack and of the screen position along the aerofoil chord is evaluated through a Design of Experiments (DoE) based approach. The analysis shows that, among the analysed range of design factor variation, increasing angle of attack and the tip clearance leads to a beneficial effect on the lift and back-pressure coefficients, while they show a poor dependence upon the screen axial position. Finally, the configuration characterized by the maximum value of all three main factors (15 degree of angle of attack, 5% of tip clearance and 30% backward to the nozzle plane), has the best values of lift coefficient and back-pressure coefficient.@en

In this paper, a numerical investigation on the effect of gurney ap (GF) on the performance of a diffuser augmented wind turbine (DAWT) is presented. The flow-field around the DAWT is obtained by solving the Reynolds-averaged-Navier-Stokes (RANS) equations. The turbine is modelled as an uniformly loaded actuator disc (AD) that imposes a resistance to the passage of the flow. Comparison of the numerical results with experimental measurements in similar conditions shows that the numerical approach used satisfactorily reproduces the mean flowfield. GF heights equal to 2% and 4% of the diffuser chord length were investigated. Results show that separation induced by GF creates a low pressure region at the diffuser exit, that increases the mass ow through the diffuser and the power coeffcient of the DAWT. @en