Due to their potential to beat the Betz-Joukowsky limit for power extraction, diffuser-augmented ... more Due to their potential to beat the Betz-Joukowsky limit for power extraction, diffuser-augmented wind-turbines have experienced a great research interest, especially in the last two decades. This paper presents a thorough critical-analysis and review of the most important theoretical models conceived for the performance analysis and design of this wind-concentrator system. The models are classified and compared between each other, and their main analogies and differences are highlighted and explained. New bridging relations between several models are also laid down. All methods are verified and validated using new and/or existing numerical and experimental data. For the first time, the impact of the simplifying assumptions, typically used in these models, is evaluated and discussed on a quantitative basis. Attention is also paid to the optimization procedures aimed at evaluating the maximum power-coefficient attainable by a diffuser-augmented wind-turbine. It is revealed that none of these procedures is valid for a given duct geometry, whereas they still offer some usefulness from a design point of view. Finally, the review points out the main limitations, shortcomings and open-issues associated with theoretical models, paving the way for future research lines and improvements of this kind of models.
Diffuser-augmented wind-turbines are drawing increasing attention since they can beat the Betz-li... more Diffuser-augmented wind-turbines are drawing increasing attention since they can beat the Betz-limit referred to the rotor-area. However, their diffusion is still prevented by some issues including: 1) the attainable power has not yet been shown to be larger than that of an open-turbine with the same frontal-area, 2) the classical analysis methods rely on the one-dimensional-flow and no-tip-gap assumptions whose impact has never been quantified. The paper addresses these two items investigating the potential of ideal diffuser-augmented wind-turbines using a newly-developed Axial-Momentum-Theory approach, and an extended version of a free-wake ring-vortex actuator-disk model. In comparison with similar methods, the novelty of the first approach is that it accounts for the two-dimensional effects and the tip-gap presence. Since this approach cannot evaluate the performance of a turbine for a given duct-geometry, a ring-vortex method is also developed. This is the first low-computational-cost method relying on the exact solution of the inviscid-flow through a uniformly-loaded ducted-turbine with a finite-size tip-gap. It strongly couples the flow induced by the duct and the wake which are modelled as the superposition of ring-vortices. The combined use of axial-momentum and ring-vortex methods leads to the following results. Firstly, it is clearly shown that an ideal diffuser-augmented turbine can extract more power than a Betz disk with the same frontal-area. To strengthen this statement, a new duct geometry with a remarkable value of the exit-area power-coefficient equal to 0.6098 is presented. This value is significantly higher than that of a base-line NACA5415 duct profile, i.e. 0.4800. Secondly, the impact of the one-dimensional-flow and no-tip-gap assumptions is evaluated. It is also shown that the tip-gap has negligible effects. Moreover, the one-dimensional-flow hypothesis has a low impact for high values of the rotor load, while the errors grow up decreasing the rotor thrust.
International Journal of Turbomachinery Propulsion and Power, 2017
The paper investigates the impact of the standard approximations embodied in the well-known Momen... more The paper investigates the impact of the standard approximations embodied in the well-known Momentum Theory on its performance prediction capabilities. To this aim the results of the momentum theory, which is still widely used in all Blade Element/Momentum codes for the analysis and/or design of wind turbines, are compared with those obtained with an actuator disk model based on CFD techniques. In this method, the axisymmetric and steady Euler equations are solved with a classical finite volume approach, while the turbine effects are modelled through a set of axial and tangential body forces distributed over a disk shaped region representing the rotor swept surface. Since this method does not rely on the momentum theory simplifying assumptions, it can be suitably employed to verify the momentum theory validity. The analysis is carried out using the well documented experimental data of the NREL Phase VI wind turbine.
This paper completes the work presented in the companion paper [Bontempo et al., Appl. Ocean Res.... more This paper completes the work presented in the companion paper [Bontempo et al., Appl. Ocean Res., 58 (2016) 322 - 330] by presenting the investigation of the flow around a propeller ducted with a so-called accelerating duct. To this aim, both the axial momentum theory and a nonlinear actuator disk method are used. The straightforward application of the first approach reveals that if the duct and rotor thrusts are concordant, then a beneficial effect on the propulsive efficiency can be readily obtained by enclosing a propeller in an accelerating duct. When the more advanced nonliner actuator disk method is applied to verify the outcomes of the axial momentum theory additional information on the performance of the device are obtained. Moreover, the nonlinear actuator disk method is also employed to investigate, through experimental design techniques, the effect of the key geometrical parameters of the duct onto the efficiency and robustness of this kind of propulsive system. In particular, it has been found that a propulsive efficiency gain can be achieved through a duct thickness, camber and chord increase, and through an incidence decrease.
Diffuser-augmented wind turbines are known for the potential improvement in power extraction in c... more Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.
The popular Axial Momentum Theory relies on the steady, incompressible, axisymmetric and inviscid... more The popular Axial Momentum Theory relies on the steady, incompressible, axisymmetric and inviscid flow through a so-called actuator disk. The most important result of this theory is the famous Betz-Joukowsky limit stating that the maximum power coefficient achievable by an open disk is limited to 16/27. Generally, this value is obtained assuming a priori that the disk is radially uniformly-loaded and the flow is axially one-dimensional. This, however, does not prove that the uniform type is the optimal load, or else that it returns the maximum value of the extracted power. For this reason, this paper preliminary shows that 16/27 is the exact value of the maximum power coefficient of an uniformly loaded disk, even if the flow is not assumed as one-dimensional. Then, it proves, using a calculus of variation approach, that the radially uniform load is optimal. The proof refers to an approximate classical local form of the axial momentum equation. Finally, the paper points out that, since the proof of the Betz-Joukowsky limit relies on this simplifying assumption, the exact evaluation of the optimal radial distribution of the disk load, leading to the maximum value of the power coefficient, is still an open question.
International Journal of Wind Engineering and Industrial Aerodynamics, 2023
The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmente... more The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmented wind turbines (DAWTs), namely the use of an annular wing to enclose an existing open-rotor. To this aim, the paper presents a numerical analysis of the NREL-Phase-VI rotor enclosed into a shroud whose cross-section consists of the Selig-S1223 airfoil. Particular attention is devoted to the analysis of the blade pressure fields, velocity triangles, blade forces, tip-vortex and wake development. The data show that the duct induces a gain in the rotor inlet axial velocity and, therefore, in the local flow-angle. Consequently, the blade forces, the extracted work, and the risk of flow separation considerably rise. Thanks to the simultaneous increase in the ingested mass flow rate and extracted work, the DAWT experiences a higher power coefficient (C_{P,exit}) which, however, would be further improved if a coupled design-procedure was used. Indeed, in the present case, the maximum C_{P,exit} is obtained for the wind-speed value corresponding to the duct optimal flow behaviour. However, in this condition, the rotor operates at off-design with an extensive flow-separation on the blade suction-side. Finally, while the inefficiencies magnitude is specific of the analysed case, the conceptual relevance of the achievements remains valid in general.
The paper presents an extension to ducted rotors of the nonlinear actuator disk theory of Conway ... more The paper presents an extension to ducted rotors of the nonlinear actuator disk theory of Conway (J. Fluid Mech., vol. 365, 1998, pp. 235–267) and it is exact for incompressible, axisymmetric and inviscid flows. The solution for the velocities and the Stokes stream function results from the superposition of ring vortices properly arranged along the duct surface and the wake region. Using a general analytical procedure the flow fields are given as a combination of one-dimensional integrals of expressions involving complete as well as incomplete elliptic integrals. The solution being exact, the proper shape of the slipstream whether converging or diverging is naturally accounted for, even for heavy loads. A semi-analytical method has been developed that enables the flow induced by an actuator disk housed in a contoured duct to be solved duly accounting for the nonlinear mutual interaction between the duct and the rotor. Non-uniform load distributions, rotor wake rotation and ducts of ...
International Journal of Turbomachinery, Propulsion and Power
The paper presents a state-of-the-art review of turbine trailing edge flows, both from an experim... more The paper presents a state-of-the-art review of turbine trailing edge flows, both from an experimental and numerical point of view. With the help of old and recent high-resolution time resolved data, the main advances in the understanding of the essential features of the unsteady wake flow are collected and homogenized. Attention is paid to the energy separation phenomenon occurring in turbine wakes, as well as to the effects of the aerodynamic parameters chiefly influencing the features of the vortex shedding. Achievements in terms of unsteady numerical simulations of turbine wake flow characterized by vigorous vortex shedding are also reviewed. Whenever possible the outcome of a detailed code-to-code and code-to-experiments validation process is presented and discussed, on account of the adopted numerical method and turbulence closure.
The paper offers an analytical formulation of the two errors embodied in the momentum theory. The... more The paper offers an analytical formulation of the two errors embodied in the momentum theory. The first one originates from to the use of the differential form of the axial momentum equation and the second one from the linearisation of the tangential velocity terms. Both errors are evaluated comparing the axial velocity at the disk as predicted by the momentum theories with that one obtained thorough a semi-analytical actuator disk method based on the exact solution of the flow. Several cases characterised by different values of the thrust and advance coefficient are analysed, and the range of validity of the momentum theories is discussed in depth.
Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy
This paper presents the validation of a generalised semi-analytical actuator disk model as applie... more This paper presents the validation of a generalised semi-analytical actuator disk model as applied to the study of the flow around ducted propellers. The method, which returns the exact solution as a superposition of ring vortex, duly accounts for the rotation of the wake, the convergence of the slipstream, and the nonlinear mutual interaction between the duct and the propeller. Furthermore, it can deal with an arbitrary radial distribution of the load and ducts of general shape. In order to validate the previously mentioned actuator disk model, results obtained through it are compared with those provided by the so-called “CFD actuator disk method”. The latter is a widely diffused tool for the analysis of the flow around open and ducted propellers which models the rotor by means of radial profiles of blade forces distributed over a disk surface. In this paper, evidence has been given of the excellent agreement between the results of the two methods. Thanks to its extremely reduced c...
Due to their potential to beat the Betz-Joukowsky limit for power extraction, diffuser-augmented ... more Due to their potential to beat the Betz-Joukowsky limit for power extraction, diffuser-augmented wind-turbines have experienced a great research interest, especially in the last two decades. This paper presents a thorough critical-analysis and review of the most important theoretical models conceived for the performance analysis and design of this wind-concentrator system. The models are classified and compared between each other, and their main analogies and differences are highlighted and explained. New bridging relations between several models are also laid down. All methods are verified and validated using new and/or existing numerical and experimental data. For the first time, the impact of the simplifying assumptions, typically used in these models, is evaluated and discussed on a quantitative basis. Attention is also paid to the optimization procedures aimed at evaluating the maximum power-coefficient attainable by a diffuser-augmented wind-turbine. It is revealed that none of these procedures is valid for a given duct geometry, whereas they still offer some usefulness from a design point of view. Finally, the review points out the main limitations, shortcomings and open-issues associated with theoretical models, paving the way for future research lines and improvements of this kind of models.
Diffuser-augmented wind-turbines are drawing increasing attention since they can beat the Betz-li... more Diffuser-augmented wind-turbines are drawing increasing attention since they can beat the Betz-limit referred to the rotor-area. However, their diffusion is still prevented by some issues including: 1) the attainable power has not yet been shown to be larger than that of an open-turbine with the same frontal-area, 2) the classical analysis methods rely on the one-dimensional-flow and no-tip-gap assumptions whose impact has never been quantified. The paper addresses these two items investigating the potential of ideal diffuser-augmented wind-turbines using a newly-developed Axial-Momentum-Theory approach, and an extended version of a free-wake ring-vortex actuator-disk model. In comparison with similar methods, the novelty of the first approach is that it accounts for the two-dimensional effects and the tip-gap presence. Since this approach cannot evaluate the performance of a turbine for a given duct-geometry, a ring-vortex method is also developed. This is the first low-computational-cost method relying on the exact solution of the inviscid-flow through a uniformly-loaded ducted-turbine with a finite-size tip-gap. It strongly couples the flow induced by the duct and the wake which are modelled as the superposition of ring-vortices. The combined use of axial-momentum and ring-vortex methods leads to the following results. Firstly, it is clearly shown that an ideal diffuser-augmented turbine can extract more power than a Betz disk with the same frontal-area. To strengthen this statement, a new duct geometry with a remarkable value of the exit-area power-coefficient equal to 0.6098 is presented. This value is significantly higher than that of a base-line NACA5415 duct profile, i.e. 0.4800. Secondly, the impact of the one-dimensional-flow and no-tip-gap assumptions is evaluated. It is also shown that the tip-gap has negligible effects. Moreover, the one-dimensional-flow hypothesis has a low impact for high values of the rotor load, while the errors grow up decreasing the rotor thrust.
International Journal of Turbomachinery Propulsion and Power, 2017
The paper investigates the impact of the standard approximations embodied in the well-known Momen... more The paper investigates the impact of the standard approximations embodied in the well-known Momentum Theory on its performance prediction capabilities. To this aim the results of the momentum theory, which is still widely used in all Blade Element/Momentum codes for the analysis and/or design of wind turbines, are compared with those obtained with an actuator disk model based on CFD techniques. In this method, the axisymmetric and steady Euler equations are solved with a classical finite volume approach, while the turbine effects are modelled through a set of axial and tangential body forces distributed over a disk shaped region representing the rotor swept surface. Since this method does not rely on the momentum theory simplifying assumptions, it can be suitably employed to verify the momentum theory validity. The analysis is carried out using the well documented experimental data of the NREL Phase VI wind turbine.
This paper completes the work presented in the companion paper [Bontempo et al., Appl. Ocean Res.... more This paper completes the work presented in the companion paper [Bontempo et al., Appl. Ocean Res., 58 (2016) 322 - 330] by presenting the investigation of the flow around a propeller ducted with a so-called accelerating duct. To this aim, both the axial momentum theory and a nonlinear actuator disk method are used. The straightforward application of the first approach reveals that if the duct and rotor thrusts are concordant, then a beneficial effect on the propulsive efficiency can be readily obtained by enclosing a propeller in an accelerating duct. When the more advanced nonliner actuator disk method is applied to verify the outcomes of the axial momentum theory additional information on the performance of the device are obtained. Moreover, the nonlinear actuator disk method is also employed to investigate, through experimental design techniques, the effect of the key geometrical parameters of the duct onto the efficiency and robustness of this kind of propulsive system. In particular, it has been found that a propulsive efficiency gain can be achieved through a duct thickness, camber and chord increase, and through an incidence decrease.
Diffuser-augmented wind turbines are known for the potential improvement in power extraction in c... more Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.
The popular Axial Momentum Theory relies on the steady, incompressible, axisymmetric and inviscid... more The popular Axial Momentum Theory relies on the steady, incompressible, axisymmetric and inviscid flow through a so-called actuator disk. The most important result of this theory is the famous Betz-Joukowsky limit stating that the maximum power coefficient achievable by an open disk is limited to 16/27. Generally, this value is obtained assuming a priori that the disk is radially uniformly-loaded and the flow is axially one-dimensional. This, however, does not prove that the uniform type is the optimal load, or else that it returns the maximum value of the extracted power. For this reason, this paper preliminary shows that 16/27 is the exact value of the maximum power coefficient of an uniformly loaded disk, even if the flow is not assumed as one-dimensional. Then, it proves, using a calculus of variation approach, that the radially uniform load is optimal. The proof refers to an approximate classical local form of the axial momentum equation. Finally, the paper points out that, since the proof of the Betz-Joukowsky limit relies on this simplifying assumption, the exact evaluation of the optimal radial distribution of the disk load, leading to the maximum value of the power coefficient, is still an open question.
International Journal of Wind Engineering and Industrial Aerodynamics, 2023
The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmente... more The study investigates the soundness of a popular uncoupled design strategy for diffuser-augmented wind turbines (DAWTs), namely the use of an annular wing to enclose an existing open-rotor. To this aim, the paper presents a numerical analysis of the NREL-Phase-VI rotor enclosed into a shroud whose cross-section consists of the Selig-S1223 airfoil. Particular attention is devoted to the analysis of the blade pressure fields, velocity triangles, blade forces, tip-vortex and wake development. The data show that the duct induces a gain in the rotor inlet axial velocity and, therefore, in the local flow-angle. Consequently, the blade forces, the extracted work, and the risk of flow separation considerably rise. Thanks to the simultaneous increase in the ingested mass flow rate and extracted work, the DAWT experiences a higher power coefficient (C_{P,exit}) which, however, would be further improved if a coupled design-procedure was used. Indeed, in the present case, the maximum C_{P,exit} is obtained for the wind-speed value corresponding to the duct optimal flow behaviour. However, in this condition, the rotor operates at off-design with an extensive flow-separation on the blade suction-side. Finally, while the inefficiencies magnitude is specific of the analysed case, the conceptual relevance of the achievements remains valid in general.
The paper presents an extension to ducted rotors of the nonlinear actuator disk theory of Conway ... more The paper presents an extension to ducted rotors of the nonlinear actuator disk theory of Conway (J. Fluid Mech., vol. 365, 1998, pp. 235–267) and it is exact for incompressible, axisymmetric and inviscid flows. The solution for the velocities and the Stokes stream function results from the superposition of ring vortices properly arranged along the duct surface and the wake region. Using a general analytical procedure the flow fields are given as a combination of one-dimensional integrals of expressions involving complete as well as incomplete elliptic integrals. The solution being exact, the proper shape of the slipstream whether converging or diverging is naturally accounted for, even for heavy loads. A semi-analytical method has been developed that enables the flow induced by an actuator disk housed in a contoured duct to be solved duly accounting for the nonlinear mutual interaction between the duct and the rotor. Non-uniform load distributions, rotor wake rotation and ducts of ...
International Journal of Turbomachinery, Propulsion and Power
The paper presents a state-of-the-art review of turbine trailing edge flows, both from an experim... more The paper presents a state-of-the-art review of turbine trailing edge flows, both from an experimental and numerical point of view. With the help of old and recent high-resolution time resolved data, the main advances in the understanding of the essential features of the unsteady wake flow are collected and homogenized. Attention is paid to the energy separation phenomenon occurring in turbine wakes, as well as to the effects of the aerodynamic parameters chiefly influencing the features of the vortex shedding. Achievements in terms of unsteady numerical simulations of turbine wake flow characterized by vigorous vortex shedding are also reviewed. Whenever possible the outcome of a detailed code-to-code and code-to-experiments validation process is presented and discussed, on account of the adopted numerical method and turbulence closure.
The paper offers an analytical formulation of the two errors embodied in the momentum theory. The... more The paper offers an analytical formulation of the two errors embodied in the momentum theory. The first one originates from to the use of the differential form of the axial momentum equation and the second one from the linearisation of the tangential velocity terms. Both errors are evaluated comparing the axial velocity at the disk as predicted by the momentum theories with that one obtained thorough a semi-analytical actuator disk method based on the exact solution of the flow. Several cases characterised by different values of the thrust and advance coefficient are analysed, and the range of validity of the momentum theories is discussed in depth.
Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy
This paper presents the validation of a generalised semi-analytical actuator disk model as applie... more This paper presents the validation of a generalised semi-analytical actuator disk model as applied to the study of the flow around ducted propellers. The method, which returns the exact solution as a superposition of ring vortex, duly accounts for the rotation of the wake, the convergence of the slipstream, and the nonlinear mutual interaction between the duct and the propeller. Furthermore, it can deal with an arbitrary radial distribution of the load and ducts of general shape. In order to validate the previously mentioned actuator disk model, results obtained through it are compared with those provided by the so-called “CFD actuator disk method”. The latter is a widely diffused tool for the analysis of the flow around open and ducted propellers which models the rotor by means of radial profiles of blade forces distributed over a disk surface. In this paper, evidence has been given of the excellent agreement between the results of the two methods. Thanks to its extremely reduced c...
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Papers by Marcello Manna
To this aim the results of the momentum theory, which is still widely used in all Blade Element/Momentum codes for the analysis and/or design of wind turbines, are compared with those obtained with an actuator disk model based on CFD techniques.
In this method, the axisymmetric and steady Euler equations are solved with a classical finite volume approach, while the turbine effects are modelled through a set of axial and tangential body forces distributed over a disk shaped region representing the rotor swept surface.
Since this method does not rely on the momentum theory simplifying assumptions, it can be suitably employed to verify the momentum theory validity.
The analysis is carried out using the well documented experimental data of the NREL Phase VI wind turbine.
To this aim, both the axial momentum theory and a nonlinear actuator disk method are used.
The straightforward application of the first approach reveals that if the duct and rotor thrusts are concordant, then a beneficial effect on the propulsive efficiency can be readily obtained by enclosing a propeller in an accelerating duct.
When the more advanced nonliner actuator disk method is applied to verify the outcomes of the axial momentum theory additional information on the performance of the device are obtained.
Moreover, the nonlinear actuator disk method is also employed to investigate, through experimental design techniques, the effect of the key geometrical parameters of the duct onto the efficiency and robustness of this kind of propulsive system.
In particular, it has been found that a propulsive efficiency gain can be achieved through a duct thickness, camber and chord increase, and through an incidence decrease.
To this aim the results of the momentum theory, which is still widely used in all Blade Element/Momentum codes for the analysis and/or design of wind turbines, are compared with those obtained with an actuator disk model based on CFD techniques.
In this method, the axisymmetric and steady Euler equations are solved with a classical finite volume approach, while the turbine effects are modelled through a set of axial and tangential body forces distributed over a disk shaped region representing the rotor swept surface.
Since this method does not rely on the momentum theory simplifying assumptions, it can be suitably employed to verify the momentum theory validity.
The analysis is carried out using the well documented experimental data of the NREL Phase VI wind turbine.
To this aim, both the axial momentum theory and a nonlinear actuator disk method are used.
The straightforward application of the first approach reveals that if the duct and rotor thrusts are concordant, then a beneficial effect on the propulsive efficiency can be readily obtained by enclosing a propeller in an accelerating duct.
When the more advanced nonliner actuator disk method is applied to verify the outcomes of the axial momentum theory additional information on the performance of the device are obtained.
Moreover, the nonlinear actuator disk method is also employed to investigate, through experimental design techniques, the effect of the key geometrical parameters of the duct onto the efficiency and robustness of this kind of propulsive system.
In particular, it has been found that a propulsive efficiency gain can be achieved through a duct thickness, camber and chord increase, and through an incidence decrease.