Final Report Summary - GUM (Active GUrney on Main Rotor blades)
Executive Summary:
In the first part of the project the main activity was the development of the new post-processing technique to calculate pressure field from PIV velocity data. This new method was firstly defined mathematically and then validated by comparison with a set of appositely produced experimental data. These data were obtained by means of a special test-rig that allow to measure both pressures and velocity around an oscillating airfoil.
The results of this assessment activity have been accurately reported and an extract can be find on the project website. The obtained results were quite satisfying and constituted a remarkable improvement with respect to the state of the art of these methodologies.
Secondly, the project activity was focused on the rotor tests. A new PIV test rig was specifically designed and set up for these tests. The rotor tests were divided in two different campaigns. In the first one, a set of passive blades were designed, produced and tested on the whirl tower provided by GRC1 in the large wind tunnel of Politecnico di Milano. These passive blades were tested to evaluate the effectiveness of different size passive Gurney flaps and to check and adjust the PIV setup waiting for the active blade set. The second campaign concerned the tests on the active blades equipped with active Gurney flaps system. This specific campaign enabled to gather a large amount of information about the effects of different Gurney flap actuation laws on blade performance.
Project Context and Objectives:
The main objective of the GUM project was to measure the performance of a helicopter main-rotor model with an active Gurney flap (AGF) integrated in each blade. This goal should be assessed by means of a comprehensive wind tunnel test campaign carried out in the large Wind Tunnel of Politecnico di Milano.
In addition to the performance characterisation, the test campaign included the use of Particle Image Velocimetry (PIV) aimed to achieve a detailed insight into the physical effects of the AGFs. Indeed, PIV was used to measure the velocity field and to obtained a velocity data base to be used for the evaluation of the pressure distribution around the blade. The derivation of a pressure field from PIV data is a very attractive technique widely investigated in the recent literature. In the frame of this project a novel approach to calculate pressure field from velocity data was developed to be also applicable to rotational flows. Thus, PIV measurements and the apposite post-processing technique enabled the evaluation of the sectional aerodynamic coefficients acting on the blade section with the Gurney flap. This activity integrated the information about the resultant forces and moments measured on the whole rotor.
Project Results:
The innovative and accurate procedure to compute the pressure field from PIV velocity measurements was based on the uncoupling of the velocity and pressure fields in the incompressible Navier–Stokes equations which relies on exact integral conditions for the pressure. By means of the Glowinski and Pironneau method, a set of linear PDE problems for the unknown pressure field has been derived and its numerical solution has been implemented by the finite element method.
The performance of the proposed method has also been compared with that provided by other methods. The comparison shows a remarkable smoothing of the velocity error, which allows the proposed method to be significantly more accurate than the other techniques proposed so far.
Thus, the proposed method has been applied to phase averaged PIV measurements of a challenging aeronautical problem as the oscillating airfoil. The
comparison between the computed pressure distribution and available pressure measurements on the airfoil surface showed pretty encouraging results. The reconstructed
pressure field presents the expected structures of the examined cases (e.g. separated flow and reattaching flow). A direct comparison between the results
obtained with the present method and those computed by the Poisson method for the pressure-field reconstruction showed an overall better performance of the proposed
method. The validation of the proposed methodology represents an outstanding output of the present project compared to the state-of-the-art.
The tests with passive Gurney flap allowed to obtain a measure of the effect of different Gurney size. In fact, also small Gurney with a relatively limitated radial extension (from 60% to 70% of the blade) and located at the 90% of the chord showed apparent increases in blade lift. Thus, they demonstrated to be useful for blade dynamic control.
The blade section aerodynamic coefficients were calculated from PIV data using the control volume approach. The considered condition is characterized by not negligible compressibility effects (local Mach number equal to 0.35). Furthermore, the flow on the integration contour is outside the boundary layer (except for a very thin segment in the wake) and can be therefore considered irrotational. Thus, due to these considerations the Bernoulli equation for compressible flow was used for pressure calculation on the integration contour. Viscous stresses along the contour of the control surface were neglected as their contribution to the calculation of the aerodynamic loads were negligible, while turbulent stresses obtained from the measured velocity fluctuations were considered in the calculation.
The velocity field around the blade section was reconstructed from the measurements carried out on the upper and lower surface of the airfoil.
The validation of the method was carried out by comparison of the vertical aerodynamic load calculated for the clean blade section configuration with the results of the blade element momentum theory in hovering. In particular, a classical blade element momentum theory (BEMT) approach was used to calculate the distribution of the aerodynamic loads along the clean blade span for the selected hovering test condition. With this aim, the performance of the clean blades was computed considering the measured rotor thrust as trim requirement.
The discrepancy between the vertical force coefficient computed from PIV data using the control volume approach and the BEMT analysis results is in the order of few percents. The good level of accuracy obtained for the calculation of the vertical aerodynamic force validates the applicability of the method for the calculation of the blade section performance from PIV data. The method was therefore applied to the PIV measurements carried out around the blade section equipped with the Gurney flaps to evaluate the effects of the flaps with different height by comparison with the clean condition for the same commanded collective angle.
An apparent increase of the vertical force coefficient was computed for the blade section configuration equipped with the different Gurney flaps with respect to the clean geometry for this hovering test condition. In particular, as it can be expected, the computed vertical force coefficient is higher increasing the Gurney flaps height with a maximum increase of 37% of the clean blade section value obtained for the flap with height of 2.5 mm. The good level of confidence of this computed trend was confirmed by the comparison of the rotor thrust coefficient measured in hovering for the different blade configurations.
The blade equipped with active Gurney flaps, provided by GRC1, were extensively tested although some limitations in the test conditions were necessary. In particular, the RPM have to be reduced in forward flight with dynamic flap extension to avoid incorrect flap motion. Nevertheless, the results can be considered very interesting and will be useful to address the full-scale tests on a real helicopter that will be the last step of the investigation on active rotor blades in the frame of GRC1 program. Indeed, the tests demonstrated that a significant increase in rotor efficiency can be achieved by means of active Gurney flap.
The PIV measurements at 65% of the retreating blade have been post-processed with the new developed technique in order to obtain the pressure distribution in the field around the blade section. In order to obtain the aerodynamic force coefficient an inviscid approach based on compressible Bernoulli theorem was preferred as to this aim only the external velocities were necessary.
Potential Impact:
The expected results were a set of experimental data to the effects that can be achieved by the use of Gurney flaps on the helicopter rotors, particularly in terms of energetic efficiency. The collected data will be very useful to plan the full-scale tests that will be carried out with a real helicopter in the frame of GRC1 program.
This study can lead to more efficient helicopters, less engine power requirement, less fuel burn, less emission, less pollution. Furthermore, the new PIV data post-processing technique will be a very useful tool for the scientific community in the field of fluid-dynamics research. All these achivements will become also part of the cultural background of Politecnico di Milano that is teaching to a wide number of students coming from Europe and the rest of the world.
List of Websites:
https://home.aero.polimi.it/gumproject/
In the first part of the project the main activity was the development of the new post-processing technique to calculate pressure field from PIV velocity data. This new method was firstly defined mathematically and then validated by comparison with a set of appositely produced experimental data. These data were obtained by means of a special test-rig that allow to measure both pressures and velocity around an oscillating airfoil.
The results of this assessment activity have been accurately reported and an extract can be find on the project website. The obtained results were quite satisfying and constituted a remarkable improvement with respect to the state of the art of these methodologies.
Secondly, the project activity was focused on the rotor tests. A new PIV test rig was specifically designed and set up for these tests. The rotor tests were divided in two different campaigns. In the first one, a set of passive blades were designed, produced and tested on the whirl tower provided by GRC1 in the large wind tunnel of Politecnico di Milano. These passive blades were tested to evaluate the effectiveness of different size passive Gurney flaps and to check and adjust the PIV setup waiting for the active blade set. The second campaign concerned the tests on the active blades equipped with active Gurney flaps system. This specific campaign enabled to gather a large amount of information about the effects of different Gurney flap actuation laws on blade performance.
Project Context and Objectives:
The main objective of the GUM project was to measure the performance of a helicopter main-rotor model with an active Gurney flap (AGF) integrated in each blade. This goal should be assessed by means of a comprehensive wind tunnel test campaign carried out in the large Wind Tunnel of Politecnico di Milano.
In addition to the performance characterisation, the test campaign included the use of Particle Image Velocimetry (PIV) aimed to achieve a detailed insight into the physical effects of the AGFs. Indeed, PIV was used to measure the velocity field and to obtained a velocity data base to be used for the evaluation of the pressure distribution around the blade. The derivation of a pressure field from PIV data is a very attractive technique widely investigated in the recent literature. In the frame of this project a novel approach to calculate pressure field from velocity data was developed to be also applicable to rotational flows. Thus, PIV measurements and the apposite post-processing technique enabled the evaluation of the sectional aerodynamic coefficients acting on the blade section with the Gurney flap. This activity integrated the information about the resultant forces and moments measured on the whole rotor.
Project Results:
The innovative and accurate procedure to compute the pressure field from PIV velocity measurements was based on the uncoupling of the velocity and pressure fields in the incompressible Navier–Stokes equations which relies on exact integral conditions for the pressure. By means of the Glowinski and Pironneau method, a set of linear PDE problems for the unknown pressure field has been derived and its numerical solution has been implemented by the finite element method.
The performance of the proposed method has also been compared with that provided by other methods. The comparison shows a remarkable smoothing of the velocity error, which allows the proposed method to be significantly more accurate than the other techniques proposed so far.
Thus, the proposed method has been applied to phase averaged PIV measurements of a challenging aeronautical problem as the oscillating airfoil. The
comparison between the computed pressure distribution and available pressure measurements on the airfoil surface showed pretty encouraging results. The reconstructed
pressure field presents the expected structures of the examined cases (e.g. separated flow and reattaching flow). A direct comparison between the results
obtained with the present method and those computed by the Poisson method for the pressure-field reconstruction showed an overall better performance of the proposed
method. The validation of the proposed methodology represents an outstanding output of the present project compared to the state-of-the-art.
The tests with passive Gurney flap allowed to obtain a measure of the effect of different Gurney size. In fact, also small Gurney with a relatively limitated radial extension (from 60% to 70% of the blade) and located at the 90% of the chord showed apparent increases in blade lift. Thus, they demonstrated to be useful for blade dynamic control.
The blade section aerodynamic coefficients were calculated from PIV data using the control volume approach. The considered condition is characterized by not negligible compressibility effects (local Mach number equal to 0.35). Furthermore, the flow on the integration contour is outside the boundary layer (except for a very thin segment in the wake) and can be therefore considered irrotational. Thus, due to these considerations the Bernoulli equation for compressible flow was used for pressure calculation on the integration contour. Viscous stresses along the contour of the control surface were neglected as their contribution to the calculation of the aerodynamic loads were negligible, while turbulent stresses obtained from the measured velocity fluctuations were considered in the calculation.
The velocity field around the blade section was reconstructed from the measurements carried out on the upper and lower surface of the airfoil.
The validation of the method was carried out by comparison of the vertical aerodynamic load calculated for the clean blade section configuration with the results of the blade element momentum theory in hovering. In particular, a classical blade element momentum theory (BEMT) approach was used to calculate the distribution of the aerodynamic loads along the clean blade span for the selected hovering test condition. With this aim, the performance of the clean blades was computed considering the measured rotor thrust as trim requirement.
The discrepancy between the vertical force coefficient computed from PIV data using the control volume approach and the BEMT analysis results is in the order of few percents. The good level of accuracy obtained for the calculation of the vertical aerodynamic force validates the applicability of the method for the calculation of the blade section performance from PIV data. The method was therefore applied to the PIV measurements carried out around the blade section equipped with the Gurney flaps to evaluate the effects of the flaps with different height by comparison with the clean condition for the same commanded collective angle.
An apparent increase of the vertical force coefficient was computed for the blade section configuration equipped with the different Gurney flaps with respect to the clean geometry for this hovering test condition. In particular, as it can be expected, the computed vertical force coefficient is higher increasing the Gurney flaps height with a maximum increase of 37% of the clean blade section value obtained for the flap with height of 2.5 mm. The good level of confidence of this computed trend was confirmed by the comparison of the rotor thrust coefficient measured in hovering for the different blade configurations.
The blade equipped with active Gurney flaps, provided by GRC1, were extensively tested although some limitations in the test conditions were necessary. In particular, the RPM have to be reduced in forward flight with dynamic flap extension to avoid incorrect flap motion. Nevertheless, the results can be considered very interesting and will be useful to address the full-scale tests on a real helicopter that will be the last step of the investigation on active rotor blades in the frame of GRC1 program. Indeed, the tests demonstrated that a significant increase in rotor efficiency can be achieved by means of active Gurney flap.
The PIV measurements at 65% of the retreating blade have been post-processed with the new developed technique in order to obtain the pressure distribution in the field around the blade section. In order to obtain the aerodynamic force coefficient an inviscid approach based on compressible Bernoulli theorem was preferred as to this aim only the external velocities were necessary.
Potential Impact:
The expected results were a set of experimental data to the effects that can be achieved by the use of Gurney flaps on the helicopter rotors, particularly in terms of energetic efficiency. The collected data will be very useful to plan the full-scale tests that will be carried out with a real helicopter in the frame of GRC1 program.
This study can lead to more efficient helicopters, less engine power requirement, less fuel burn, less emission, less pollution. Furthermore, the new PIV data post-processing technique will be a very useful tool for the scientific community in the field of fluid-dynamics research. All these achivements will become also part of the cultural background of Politecnico di Milano that is teaching to a wide number of students coming from Europe and the rest of the world.
List of Websites:
https://home.aero.polimi.it/gumproject/