Early On-Set Prediction Of Vortex-Ring State Of Quadrotors

The OpenFOAM CFD package was used to initially investigate the aerodynamic effects of vortex-ring state of a quadrotor, then to study various quadrotor flight maneuvers to minimize the thrust loses of vortex-ring state and followed by possible detection methods for a drone entering vortex-ring state. Vortex-ring state is characteristic of a decrease in the effective angle of attack of incoming airflow due to a rotor descending into its downwash. This causes significant loses in the thrust of the rotor, which typically leads to severe flight upsets for rotorcraft. A quadrotor was studied at varying descent velocities to investigate wake roll-up at the rotor tips and the subsequent effects on rotor thrust and power. The quadrotor was then subjected to non-vertical descent angles to investigate thrust loss mitigation approaches due to vortex-ring state. A method of detecting the on-set of vortex-ring state is proposed using various differential pressure measurements on the quadrotor. It has been shown that by monitoring the pressure difference between the top of the quadrotor body and the bottom of one of the quadrotor legs, a pressure drop can be seen just prior to the on-set of vortex-ring state. This pressure drop was shown to occur during descending flight regimes and may prove to be an effective pre-vortex-ring state warning system.

Early On-Set Prediction Of Vortex-Ring State Of Quadrotors

The OpenFOAM CFD package was used to initially investigate the aerodynamic effects of vortex-ring state of a quadrotor, then to study various quadrotor flight maneuvers to minimize the thrust loses of vortex-ring state and followed by possible detection methods for a drone entering vortex-ring state. Vortex-ring state is characteristic of a decrease in the effective angle of attack of incoming airflow due to a rotor descending into its downwash. This causes significant loses in the thrust of the rotor, which typically leads to severe flight upsets for rotorcraft. A quadrotor was studied at varying descent velocities to investigate wake roll-up at the rotor tips and the subsequent effects on rotor thrust and power. The quadrotor was then subjected to non-vertical descent angles to investigate thrust loss mitigation approaches due to vortex-ring state. A method of detecting the on-set of vortex-ring state is proposed using various differential pressure measurements on the quadrotor. It has been shown that by monitoring the pressure difference between the top of the quadrotor body and the bottom of one of the quadrotor legs, a pressure drop can be seen just prior to the on-set of vortex-ring state. This pressure drop was shown to occur during descending flight regimes and may prove to be an effective pre-vortex-ring state warning system.

Robust Wake Rollup Modelling Using Dyes

This project report gives details on a modification of VAPTOR, a program that can predict the aerodynamic performance of aircrafts using a potential flow method with a relaxed wake model. In VAPTOR the wake is modelled using distributed vorticity elements (DVEs). DVEs can induce velocities at certain points used to relax the wake. A DVE has inbuilt singularity protections i.e. prevents the calculated velocity to approach infinity, but when two adjacent DVEs have a very low relative angle, these protections lead to an error in the calculation of the velocity at its shared midpoint during the relaxation process. In most cases these errors are negligible until a rotor is analysed during hover or vortex ring state. In these special cases the wake rollup is more intense leading to relatively small angles. The subsequent errors caused by the singularity protections cannot be ignored since they cause the solutions to be erratic and not smooth. It also causes the wake DVEs to deform disproportionally which is a visual indication of the errors. The modification uses a method that involves splitting the DVE in order to eliminate the errors when calculating the velocity at the junction of two adjacent DVEs. The splitting is temporary and only applied during the calculation of the velocity at the junction. The algorithm for the splitting of the DVE and its implementation into MATLAB is provided in this report. The implementation is tested by ensuring that all conditions are kept the same except when splitting is enabled or disabled. A number of test runs were conducted, and an index called the Smoothness Index was created in order to quantify the improvements of the DVE splitting method. The results shown are promising as the solution with splitting enabled is twice as smooth as when the splitting is disabled. There is also a noticeable improvement during visual comparison of the wake diagrams when splitting is enabled and disabled. The results combined with the fact that the extra computation required to execute the DVE splitting method is negligible, the author recommends it be enabled in all cases. Having said that, the end user has full control whether he or she would like to use it or not. They can also change the parameters of splitting to suit their needs.

Robust Wake Rollup Modelling Using Dyes

This project report gives details on a modification of VAPTOR, a program that can predict the aerodynamic performance of aircrafts using a potential flow method with a relaxed wake model. In VAPTOR the wake is modelled using distributed vorticity elements (DVEs). DVEs can induce velocities at certain points used to relax the wake. A DVE has inbuilt singularity protections i.e. prevents the calculated velocity to approach infinity, but when two adjacent DVEs have a very low relative angle, these protections lead to an error in the calculation of the velocity at its shared midpoint during the relaxation process. In most cases these errors are negligible until a rotor is analysed during hover or vortex ring state. In these special cases the wake rollup is more intense leading to relatively small angles. The subsequent errors caused by the singularity protections cannot be ignored since they cause the solutions to be erratic and not smooth. It also causes the wake DVEs to deform disproportionally which is a visual indication of the errors. The modification uses a method that involves splitting the DVE in order to eliminate the errors when calculating the velocity at the junction of two adjacent DVEs. The splitting is temporary and only applied during the calculation of the velocity at the junction. The algorithm for the splitting of the DVE and its implementation into MATLAB is provided in this report. The implementation is tested by ensuring that all conditions are kept the same except when splitting is enabled or disabled. A number of test runs were conducted, and an index called the Smoothness Index was created in order to quantify the improvements of the DVE splitting method. The results shown are promising as the solution with splitting enabled is twice as smooth as when the splitting is disabled. There is also a noticeable improvement during visual comparison of the wake diagrams when splitting is enabled and disabled. The results combined with the fact that the extra computation required to execute the DVE splitting method is negligible, the author recommends it be enabled in all cases. Having said that, the end user has full control whether he or she would like to use it or not. They can also change the parameters of splitting to suit their needs.

A design methodology for rotors of small multirotor vehicles

A model is presented for the aerodynamic performance prediction of fixed-pitch rotors for small unmanned aerial vehicles. The method uses a blade element momentum theory based approach that is formulated specifically for small rotors operating in hover and edgewise flight. In order to validate the model, a rotor test stand is used to measure the performance of a commercially available rotor for several inflow angles and advance ratios. The predictions agree with measurements for operating conditions excluding conditions with suspected vortex ring state. The model is incorporated into a numerical optimization scheme to demonstrate its potential as a design tool. Designs are presented that minimize the power loading for single- and multi-point operating conditions. The optimized designs have hyperbolic twist distributions, higher solidities, and operate at lower tip-speeds than existing designs. A potential flow based model is also presented to predict the wake interactions between multiple rotors in configuration.

A design methodology for rotors of small multirotor vehicles

A model is presented for the aerodynamic performance prediction of fixed-pitch rotors for small unmanned aerial vehicles. The method uses a blade element momentum theory based approach that is formulated specifically for small rotors operating in hover and edgewise flight. In order to validate the model, a rotor test stand is used to measure the performance of a commercially available rotor for several inflow angles and advance ratios. The predictions agree with measurements for operating conditions excluding conditions with suspected vortex ring state. The model is incorporated into a numerical optimization scheme to demonstrate its potential as a design tool. Designs are presented that minimize the power loading for single- and multi-point operating conditions. The optimized designs have hyperbolic twist distributions, higher solidities, and operate at lower tip-speeds than existing designs. A potential flow based model is also presented to predict the wake interactions between multiple rotors in configuration.

Dynamic inflow model for a Floating Horizontal Axis Wind Turbine in surge motion

Floating Offshore Wind Turbines may experience large surge motions which, when faster than the local wind speed, cause rotor-wake interaction. Previous research hypothesised that this phenomena can result in a turbulent wake state or even a vortex ring state, invalidating the Actuator Disc Momentum Theory and the use of the Blade Element Momentum Theory. We challenge this hypothesis and demonstrate that the Actuator Disc Momentum Theory is valid and accurate in predicting the induction at the actuator in surge, even for large and fast motions. To achieve this, we derive a dynamic inflow model which mimics the vorticity-velocity system and the effect of the motion. The predictions of the model are compared against results from other authors and from a semi-free wake vortex-ring model. The results show that the surge motion and rotor-wake interaction do not cause a turbulent wake state or vortex ring state, and that the application of Actuator Disc Momentum Theory and Blade Element Momentum Theory is valid and accurate, when correctly applied in an inertial reference frame. The results show excellent agreement in all cases. The proposed dynamic inflow model includes an adaptation for highly loaded flow and it is accurate and simple enough to be easily implemented in most Blade Element Momentum models.