Investigation of applying physics informed neural networks on the test case involving flow past C... more Investigation of applying physics informed neural networks on the test case involving flow past Converging-Diverging (CD) Nozzle has been investigated. Both Artificial Neural Network (ANN) and Physics Informed Neural Network (PINN) are used to do the training and prediction. Results show that Artificial Neural Network (ANN) by itself is already able to give relatively good prediction. With the addition of PINN, the error reduces even more, although by only a relatively small amount. This is perhaps due to the already good prediction. The effects of batch size, training iteration and number of epochs on the prediction accuracy have already been tested. It is found that increasing batch size improves the prediction. On the other hand, increasing the training iteration may give poorer prediction due to overfitting. Lastly, in general, increasing epochs reduces the error. More investigations should be done in the future to further reduce the error while at the same time using less train...
IECON 2016 - 42nd Annual Conference of the IEEE Industrial Electronics Society, 2016
In this paper, a comparative study of speed control efficiencies based on two dynamical models is... more In this paper, a comparative study of speed control efficiencies based on two dynamical models is presented. The common structure of dynamical models is developed using first principles of physics. The complex thrust mechanism is studied using two different techniques. First, we apply a novel data-driven approach to build up the non-linear mapping between tail angular motion and thrust generated, which is essential to specify the input gradient. Further, additional experiments are designed to acquire data necessary for modelling the input delay factor causing a time-delayed input to drive the system output. Second, analytically developed Lighthill's slender body theory is also employed to study thrust mechanism for comparative purposes. Though the theory has been used in robotic fish motion studies, it was originally formulated as an analytical tool for studying biological fish motion. Comparing the two theories help understand the predominant features in a robotic fish speed control. Lastly, because of the highly non-linear system dynamics, robust discrete-time Sliding Mode Controllers (SMC) are developed with the two dynamical models as basis respectively. Experimental verifications confirm that SMC based on data-driven model performs superior to the SMC based on the Lighthill's theory, by efficiently controlling the robotic fish's speed to track the time-varying reference speeds.
Submitted for the DFD07 Meeting of The American Physical Society Analysis of Non-symmetrical Flap... more Submitted for the DFD07 Meeting of The American Physical Society Analysis of Non-symmetrical Flapping Airfoils WEE BENG TAY, KAH BIN LIM, National University of Singapore — Simulations have been done to assess the performance of different types of non-symmetrical airfoils on lift, thrust and propulsive efficiency under different flapping configurations at a Reynolds number of 10,000. The variables studied include the Stroudal number, reduced frequency, pitch angle and phase angle difference. In order to analyze the variables more efficiently, the Design of Experiments using the response surface methodology is applied. The simulation results show that besides the flapping configuration, airfoil shape also has a profound effect on the efficiency, thrust and lift production. The 4 factors have different levels of significance on the responses, indicating the shape of the airfoil plays a part as well. Thrust production depends more heavily on these parameters, rather than the shape of t...
Wing-wing interaction (WWI), such as the clap and fling motion (CFM), occurs when two wings are f... more Wing-wing interaction (WWI), such as the clap and fling motion (CFM), occurs when two wings are flapping in close proximity of one another. Due to the interactions between the wings and their vortices, improvement in the wing's performance can be obtained. We intend to design a hovering 4-wing flapping MAV (FMAV) which makes use of WWI to improve its lift performance at Re = 5,000 (based on chord length), through the use of numerical simulations. In this study, the objectives include 2 (TL_FMAV2) / 4-wing (TL_FMAV4) WWI comparison and the effect of different types of flexibility on lift and drag. Results show that TL_FMAV4 produces more than twice the amount of lift compared to TL_FMAV2, but at the expense of higher drag and power requirement. Investigations on wing flexibility found that having a rigid spanwise and flexible chordwise wing produces the highest lift, minimum drag and power requirement. These results will be beneficial in the understanding of the underlying aerodynamics of WWI.
The purpose of this research is to understand the aerodynamics of Flapping wing Micro Aerial Vehi... more The purpose of this research is to understand the aerodynamics of Flapping wing Micro Aerial Vehicles (FMAVs) flying in V-formation and how V-formation can improve the endurance of FMAVs. We tested FMAVs with two types of flapping kinematics: (1) idealized sinusoidal flapping/pitching motion, (2) actual FMAV wing deformation capture. Using the design of experiment methodology, together with an immersed boundary method numerical solver, we investigate the effects of phase angle and x, y, z separation between the wings on the FMAVs’ performance. Results show that it is possible to obtain an overall thrust and lift improvement of up to 25% and 14% respectively, or a reduction of both 19%, compared to the single wing configuration. Lastly, for certain cases, we extend another row, leading to a total of five wings. Results show that in general, the additional thrust or lift experienced by the second row of wings is also experienced by the third row of wings. Hence, this may apply to subsequent rows too.
The objective of this research is to investigate the effect of serrations on quadcopter propeller... more The objective of this research is to investigate the effect of serrations on quadcopter propeller blades on noise reduction through numerical simulations. Different types of the 5 inch 5030 propellers, such as the standard, modified and serrated, are tested. The modified propeller has a portion of its blade’s trailing edge cut off to achieve the same surface area as that of the serrated blades to ensure a fairer comparison. Three-dimensional simulations propellers have been performed using an immersed boundary method (IBM) Navier–Stokes finite volume solver to obtain the velocity flow fields and pressure. An acoustic model, based on the well-known Ffowcs Williams-Hawkings (FW-H) formulation, is then used to predict the far field noise caused by the rotating blades of the propeller. Results show that due to the reduction in surface area of the propeller’s blades, there is a drop in the thrust produced by modified and serrated propellers, compared to the standard one. However, compari...
A three-dimensional numerical simulation of a four-wing (two wings on each side, one on top of an... more A three-dimensional numerical simulation of a four-wing (two wings on each side, one on top of another) flapping micro-aerial vehicle (FMAV), known as the Delfly micro, is performed using an immersed boundary method Navier-Stokes finite volume solver at Reynolds numbers of 5500 (forward flight condition). The objective of the present investigation is to gain an insight to the aerodynamics of flapping wing biplane configuration, by making an analysis on a geometry that is simplified, yet captures the major aspects of the wing behavior. The fractional step method is used to solve the Navier-Stokes equations. Results show that in comparison to the Delfly II flapping kinematics (a similar FMAV configuration but smaller flapping stroke angles), the Delfly-Micro flapping kinematics provides more thrust while maintaining the same efficiency. The Delfly-Micro biplane configuration generates more lift than expected when the inclination angle increases, due to the formation of a uniform leading edge vortex. Estimates of the lift produced in the forward flight conditions confirm that in the current design, the MAV is able to sustain forward flight. The potential effect of wing flexibility on the aerodynamic performance in the biplane configuration context is investigated through prescribed flexibility in the simulations. Increasing the wing' spanwise flexibility increases thrust but increasing chordwise flexibility causes thrust to first increase and then decrease. Moreover, combining both spanwise and chordwise flexibility outperforms cases with only either spanwise or chordwise flexibility.
The immersed boundary method (IBM) is now used extensively in many simulations involving moving b... more The immersed boundary method (IBM) is now used extensively in many simulations involving moving bodies, such as flapping wings. The unique feature of this method is that the entire simulation is performed on a Cartesian grid and the bodies in the simulation do not conform to the grid. Hence, there is no complicated grid movements needed and grid quality issues are avoided. Despite the clear advantages of the IBM, it has its problems as well. One of the most important disadvantages with regard to flapping micro aerial vehicle (FMAV) simulations is the increase in grid size with increasing Reynolds number (Re). Unlike conforming grids which has the flexibility to adjust the cells very close to the wall, the Cartesian grid used in the IBM is more rigid as it is not aligned with the wall of the body. Hence it will require more grids to capture the boundary at the same accuracy as the conforming grid. FMAVs generally operate in the Re range of 5,000 to 10,000, hence, a large number of grid cells will be required, especially in 3D, to accurately simulate and capture the boundary layer. Moreover, there are two classifications of IBM -the continuous and discrete forcing approach, depending if the forcing is applied to the equation before or after discretization. Due to the smoothing of the forcing function inherent in the continuous approach, it has been argued that this approach is not suitable for relatively high Reynolds number flow. At that regime, the discrete forcing approach is thought to be more suitable because it is able to provide a sharp representation of the immersed boundary of the bodies. The objective of this paper is to validate the suitability of both the continuous and discrete forcing approach for flows at Re = 5,000 to 10,000, by comparison with numerical and experimental results of plunging airfoils in literature. Results obtained with conforming grid flow solvers are included for comparison. Results show that both the continuous and discrete forcing IBM solvers are able to give reasonably accurate results in terms of thrust coefficient, although the discrete forcing IBM solver is able to give smoother results at a lower grid resolution. This indicates that it is able to capture the thin boundary layer better. The vorticity contours plots of both IBM solvers are visually similar compared to the referenced results. There is disagreement in the lift coefficient, but it applies to all the numerical solvers. This could be due to the transition from laminar to turbulent flow. Lastly, the discrete forcing IBM solver is used to simulate a 3D NACA0012 wing undergoing plunging motion at a Re of 10,000. The results compare relatively well, given the lower resolution due to limited computational resources. This shows that the IBM solvers are capable of doing 3D flapping wing simulations.
ABSTRACT A modified ALE method combining a body conforming mesh and a topology changing algorithm... more ABSTRACT A modified ALE method combining a body conforming mesh and a topology changing algorithm is developed to simulate unsteady symmetric fluid problems with bodies in close proximity. By using this method, the advantages of the original ALE method are used, while the disadvantages are reduced or eliminated. A mesh topology changing algorithm is used to create a computational domain boundary, while still having a smoothly deforming mesh in the remaining part of the mesh. It is shown that at higher Reynolds numbers the newly developed method performs better in terms of spurious oscillations than an immersed boundary method (IBM).
Numerical simulations have been performed to examine the interference effects between an upstream... more Numerical simulations have been performed to examine the interference effects between an upstream flapping airfoil and a downstream stationary airfoil in a tandem configuration at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the distance of the tail and its angle of attack on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that efficiency and average thrust can be increased up to 10% and 25% respectively when a stationary airfoil is placed downstream. The simulations reveal how the vortex-shedding pattern of the airfoils are affected by the interaction between them. As the angle of attack of this airfoil increases from 0 to 45o, high lift is generated at the expense of rapidly decreasing efficiency and thrust. The results are not very sensitive to the shape of the airfoil; similar results are obt...
ABSTRACT This paper investigates the effect of active chordwise flexing on the lift, thrust and p... more ABSTRACT This paper investigates the effect of active chordwise flexing on the lift, thrust and propulsive efficiency of three types of airfoils. The factors studied are the flexing center location, standard two-sided flexing as well as a type of single-sided flexing. The airfoils are simulated to flap with four configurations, and the effects of flexing under these configurations are investigated. Results show that flexing is not necessarily beneficial for the performance of the airfoils. However, with the correct parameters, efficiency is as high as 0.76 by placing the flexing centre at the trailing edge. The average thrust coefficient is more than twice as high, from 1.63 to 3.57 with flapping and flexing under the right conditions. Moreover, the single-sided flexing also gives an average lift coefficient as high as 4.61 for the S1020 airfoil. The shape of the airfoil does alter the effect of flexing too. Deviating the flexing phase angle away from 90° does not give a significant improvement to the airfoil’s performance. These results greatly enhance the design of a better performing ornithopter wing.
The numerical simulation of an insect-sized 'X-wing' type biplane... more The numerical simulation of an insect-sized 'X-wing' type biplane flapping wing configuration is performed in 3D using an immersed boundary method solver at Reynolds numbers equal to 1000 (1 k) and 5 k, based on the wing's root chord length. This X-wing type flapping configuration draws its inspiration from Delfly, a bio-inspired ornithopter MAV which has two pairs of wings flapping in anti-phase in a biplane configuration. The objective of the present investigation is to assess the aerodynamic performance when the original Delfly flapping wing micro-aerial vehicle (FMAV) is reduced to the size of an insect. Results show that the X-wing configuration gives more than twice the average thrust compared with only flapping the upper pair of wings of the X-wing. However, the X-wing's average thrust is only 40% that of the upper wing flapping at twice the stroke angle. Despite this, the increased stability which results from the smaller lift and moment variation of the X-wing configuration makes it more suited for sharp image capture and recognition. These advantages make the X-wing configuration an attractive alternative design for insect-sized FMAVS compared to the single wing configuration. In the Reynolds number comparison, the vorticity iso-surface plot at a Reynolds number of 5 k revealed smaller, finer vortical structures compared to the simulation at 1 k, due to vortices' breakup. In comparison, the force output difference is much smaller between Re = 1 k and 5 k. Increasing the body inclination angle generates a uniform leading edge vortex instead of a conical one along the wingspan, giving higher lift. Understanding the force variation as the body inclination angle increases will allow FMAV designers to optimize the thrust and lift ratio for higher efficiency under different operational requirements. Lastly, increasing the spanwise flexibility of the wings increases the thrust slightly but decreases the efficiency. The thrust result is similar to one of the spanwise studies, but the efficiency result contradicts it, indicating that other flapping parameters are involved as well. Results from this study provide a deeper understanding of the underlying aerodynamics of the X-wing type, which will help to improve the performance of insect-sized FMAVs using this unique configuration.
A numerical investigation is performed on the interference effects in single or biplane flapping ... more A numerical investigation is performed on the interference effects in single or biplane flapping airfoil propulsion in the presence of a stationary downstream tail. At a Reynolds number of 1000, this corresponds to the regime of small micro aerial vehicles. The objective of this study is to provide insight into the complex wing-tail interaction effects occurring in flapping-wing propulsion configurations. The effect of the relative distance between the airfoils, as well as the positioning and incidence angle of the tail, is investigated. Adding a tail behind a single flapping airfoil increases the efficiency and average thrust by up to 10 and 25%, respectively. For the biplane flapping airfoils without tail, overall efficiency and average thrust per airfoil increase up to 17 and 126%, respectively, with respect to the single airfoil due to the formation of a strong momentum jet. The effect of adding a tail behind the biplane flapping airfoils depends on the tail's orientation and location. Increasing the incidence angle of the tail generates higher lift, although at the expense of decreased efficiency and thrust. Lastly, shifting the vertical position of the tail to have it coincide with the middle heaving position of the leading top airfoil gives the best overall performance.
Investigation of applying physics informed neural networks on the test case involving flow past C... more Investigation of applying physics informed neural networks on the test case involving flow past Converging-Diverging (CD) Nozzle has been investigated. Both Artificial Neural Network (ANN) and Physics Informed Neural Network (PINN) are used to do the training and prediction. Results show that Artificial Neural Network (ANN) by itself is already able to give relatively good prediction. With the addition of PINN, the error reduces even more, although by only a relatively small amount. This is perhaps due to the already good prediction. The effects of batch size, training iteration and number of epochs on the prediction accuracy have already been tested. It is found that increasing batch size improves the prediction. On the other hand, increasing the training iteration may give poorer prediction due to overfitting. Lastly, in general, increasing epochs reduces the error. More investigations should be done in the future to further reduce the error while at the same time using less train...
IECON 2016 - 42nd Annual Conference of the IEEE Industrial Electronics Society, 2016
In this paper, a comparative study of speed control efficiencies based on two dynamical models is... more In this paper, a comparative study of speed control efficiencies based on two dynamical models is presented. The common structure of dynamical models is developed using first principles of physics. The complex thrust mechanism is studied using two different techniques. First, we apply a novel data-driven approach to build up the non-linear mapping between tail angular motion and thrust generated, which is essential to specify the input gradient. Further, additional experiments are designed to acquire data necessary for modelling the input delay factor causing a time-delayed input to drive the system output. Second, analytically developed Lighthill's slender body theory is also employed to study thrust mechanism for comparative purposes. Though the theory has been used in robotic fish motion studies, it was originally formulated as an analytical tool for studying biological fish motion. Comparing the two theories help understand the predominant features in a robotic fish speed control. Lastly, because of the highly non-linear system dynamics, robust discrete-time Sliding Mode Controllers (SMC) are developed with the two dynamical models as basis respectively. Experimental verifications confirm that SMC based on data-driven model performs superior to the SMC based on the Lighthill's theory, by efficiently controlling the robotic fish's speed to track the time-varying reference speeds.
Submitted for the DFD07 Meeting of The American Physical Society Analysis of Non-symmetrical Flap... more Submitted for the DFD07 Meeting of The American Physical Society Analysis of Non-symmetrical Flapping Airfoils WEE BENG TAY, KAH BIN LIM, National University of Singapore — Simulations have been done to assess the performance of different types of non-symmetrical airfoils on lift, thrust and propulsive efficiency under different flapping configurations at a Reynolds number of 10,000. The variables studied include the Stroudal number, reduced frequency, pitch angle and phase angle difference. In order to analyze the variables more efficiently, the Design of Experiments using the response surface methodology is applied. The simulation results show that besides the flapping configuration, airfoil shape also has a profound effect on the efficiency, thrust and lift production. The 4 factors have different levels of significance on the responses, indicating the shape of the airfoil plays a part as well. Thrust production depends more heavily on these parameters, rather than the shape of t...
Wing-wing interaction (WWI), such as the clap and fling motion (CFM), occurs when two wings are f... more Wing-wing interaction (WWI), such as the clap and fling motion (CFM), occurs when two wings are flapping in close proximity of one another. Due to the interactions between the wings and their vortices, improvement in the wing's performance can be obtained. We intend to design a hovering 4-wing flapping MAV (FMAV) which makes use of WWI to improve its lift performance at Re = 5,000 (based on chord length), through the use of numerical simulations. In this study, the objectives include 2 (TL_FMAV2) / 4-wing (TL_FMAV4) WWI comparison and the effect of different types of flexibility on lift and drag. Results show that TL_FMAV4 produces more than twice the amount of lift compared to TL_FMAV2, but at the expense of higher drag and power requirement. Investigations on wing flexibility found that having a rigid spanwise and flexible chordwise wing produces the highest lift, minimum drag and power requirement. These results will be beneficial in the understanding of the underlying aerodynamics of WWI.
The purpose of this research is to understand the aerodynamics of Flapping wing Micro Aerial Vehi... more The purpose of this research is to understand the aerodynamics of Flapping wing Micro Aerial Vehicles (FMAVs) flying in V-formation and how V-formation can improve the endurance of FMAVs. We tested FMAVs with two types of flapping kinematics: (1) idealized sinusoidal flapping/pitching motion, (2) actual FMAV wing deformation capture. Using the design of experiment methodology, together with an immersed boundary method numerical solver, we investigate the effects of phase angle and x, y, z separation between the wings on the FMAVs’ performance. Results show that it is possible to obtain an overall thrust and lift improvement of up to 25% and 14% respectively, or a reduction of both 19%, compared to the single wing configuration. Lastly, for certain cases, we extend another row, leading to a total of five wings. Results show that in general, the additional thrust or lift experienced by the second row of wings is also experienced by the third row of wings. Hence, this may apply to subsequent rows too.
The objective of this research is to investigate the effect of serrations on quadcopter propeller... more The objective of this research is to investigate the effect of serrations on quadcopter propeller blades on noise reduction through numerical simulations. Different types of the 5 inch 5030 propellers, such as the standard, modified and serrated, are tested. The modified propeller has a portion of its blade’s trailing edge cut off to achieve the same surface area as that of the serrated blades to ensure a fairer comparison. Three-dimensional simulations propellers have been performed using an immersed boundary method (IBM) Navier–Stokes finite volume solver to obtain the velocity flow fields and pressure. An acoustic model, based on the well-known Ffowcs Williams-Hawkings (FW-H) formulation, is then used to predict the far field noise caused by the rotating blades of the propeller. Results show that due to the reduction in surface area of the propeller’s blades, there is a drop in the thrust produced by modified and serrated propellers, compared to the standard one. However, compari...
A three-dimensional numerical simulation of a four-wing (two wings on each side, one on top of an... more A three-dimensional numerical simulation of a four-wing (two wings on each side, one on top of another) flapping micro-aerial vehicle (FMAV), known as the Delfly micro, is performed using an immersed boundary method Navier-Stokes finite volume solver at Reynolds numbers of 5500 (forward flight condition). The objective of the present investigation is to gain an insight to the aerodynamics of flapping wing biplane configuration, by making an analysis on a geometry that is simplified, yet captures the major aspects of the wing behavior. The fractional step method is used to solve the Navier-Stokes equations. Results show that in comparison to the Delfly II flapping kinematics (a similar FMAV configuration but smaller flapping stroke angles), the Delfly-Micro flapping kinematics provides more thrust while maintaining the same efficiency. The Delfly-Micro biplane configuration generates more lift than expected when the inclination angle increases, due to the formation of a uniform leading edge vortex. Estimates of the lift produced in the forward flight conditions confirm that in the current design, the MAV is able to sustain forward flight. The potential effect of wing flexibility on the aerodynamic performance in the biplane configuration context is investigated through prescribed flexibility in the simulations. Increasing the wing' spanwise flexibility increases thrust but increasing chordwise flexibility causes thrust to first increase and then decrease. Moreover, combining both spanwise and chordwise flexibility outperforms cases with only either spanwise or chordwise flexibility.
The immersed boundary method (IBM) is now used extensively in many simulations involving moving b... more The immersed boundary method (IBM) is now used extensively in many simulations involving moving bodies, such as flapping wings. The unique feature of this method is that the entire simulation is performed on a Cartesian grid and the bodies in the simulation do not conform to the grid. Hence, there is no complicated grid movements needed and grid quality issues are avoided. Despite the clear advantages of the IBM, it has its problems as well. One of the most important disadvantages with regard to flapping micro aerial vehicle (FMAV) simulations is the increase in grid size with increasing Reynolds number (Re). Unlike conforming grids which has the flexibility to adjust the cells very close to the wall, the Cartesian grid used in the IBM is more rigid as it is not aligned with the wall of the body. Hence it will require more grids to capture the boundary at the same accuracy as the conforming grid. FMAVs generally operate in the Re range of 5,000 to 10,000, hence, a large number of grid cells will be required, especially in 3D, to accurately simulate and capture the boundary layer. Moreover, there are two classifications of IBM -the continuous and discrete forcing approach, depending if the forcing is applied to the equation before or after discretization. Due to the smoothing of the forcing function inherent in the continuous approach, it has been argued that this approach is not suitable for relatively high Reynolds number flow. At that regime, the discrete forcing approach is thought to be more suitable because it is able to provide a sharp representation of the immersed boundary of the bodies. The objective of this paper is to validate the suitability of both the continuous and discrete forcing approach for flows at Re = 5,000 to 10,000, by comparison with numerical and experimental results of plunging airfoils in literature. Results obtained with conforming grid flow solvers are included for comparison. Results show that both the continuous and discrete forcing IBM solvers are able to give reasonably accurate results in terms of thrust coefficient, although the discrete forcing IBM solver is able to give smoother results at a lower grid resolution. This indicates that it is able to capture the thin boundary layer better. The vorticity contours plots of both IBM solvers are visually similar compared to the referenced results. There is disagreement in the lift coefficient, but it applies to all the numerical solvers. This could be due to the transition from laminar to turbulent flow. Lastly, the discrete forcing IBM solver is used to simulate a 3D NACA0012 wing undergoing plunging motion at a Re of 10,000. The results compare relatively well, given the lower resolution due to limited computational resources. This shows that the IBM solvers are capable of doing 3D flapping wing simulations.
ABSTRACT A modified ALE method combining a body conforming mesh and a topology changing algorithm... more ABSTRACT A modified ALE method combining a body conforming mesh and a topology changing algorithm is developed to simulate unsteady symmetric fluid problems with bodies in close proximity. By using this method, the advantages of the original ALE method are used, while the disadvantages are reduced or eliminated. A mesh topology changing algorithm is used to create a computational domain boundary, while still having a smoothly deforming mesh in the remaining part of the mesh. It is shown that at higher Reynolds numbers the newly developed method performs better in terms of spurious oscillations than an immersed boundary method (IBM).
Numerical simulations have been performed to examine the interference effects between an upstream... more Numerical simulations have been performed to examine the interference effects between an upstream flapping airfoil and a downstream stationary airfoil in a tandem configuration at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the distance of the tail and its angle of attack on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that efficiency and average thrust can be increased up to 10% and 25% respectively when a stationary airfoil is placed downstream. The simulations reveal how the vortex-shedding pattern of the airfoils are affected by the interaction between them. As the angle of attack of this airfoil increases from 0 to 45o, high lift is generated at the expense of rapidly decreasing efficiency and thrust. The results are not very sensitive to the shape of the airfoil; similar results are obt...
ABSTRACT This paper investigates the effect of active chordwise flexing on the lift, thrust and p... more ABSTRACT This paper investigates the effect of active chordwise flexing on the lift, thrust and propulsive efficiency of three types of airfoils. The factors studied are the flexing center location, standard two-sided flexing as well as a type of single-sided flexing. The airfoils are simulated to flap with four configurations, and the effects of flexing under these configurations are investigated. Results show that flexing is not necessarily beneficial for the performance of the airfoils. However, with the correct parameters, efficiency is as high as 0.76 by placing the flexing centre at the trailing edge. The average thrust coefficient is more than twice as high, from 1.63 to 3.57 with flapping and flexing under the right conditions. Moreover, the single-sided flexing also gives an average lift coefficient as high as 4.61 for the S1020 airfoil. The shape of the airfoil does alter the effect of flexing too. Deviating the flexing phase angle away from 90° does not give a significant improvement to the airfoil’s performance. These results greatly enhance the design of a better performing ornithopter wing.
The numerical simulation of an insect-sized 'X-wing' type biplane... more The numerical simulation of an insect-sized 'X-wing' type biplane flapping wing configuration is performed in 3D using an immersed boundary method solver at Reynolds numbers equal to 1000 (1 k) and 5 k, based on the wing's root chord length. This X-wing type flapping configuration draws its inspiration from Delfly, a bio-inspired ornithopter MAV which has two pairs of wings flapping in anti-phase in a biplane configuration. The objective of the present investigation is to assess the aerodynamic performance when the original Delfly flapping wing micro-aerial vehicle (FMAV) is reduced to the size of an insect. Results show that the X-wing configuration gives more than twice the average thrust compared with only flapping the upper pair of wings of the X-wing. However, the X-wing's average thrust is only 40% that of the upper wing flapping at twice the stroke angle. Despite this, the increased stability which results from the smaller lift and moment variation of the X-wing configuration makes it more suited for sharp image capture and recognition. These advantages make the X-wing configuration an attractive alternative design for insect-sized FMAVS compared to the single wing configuration. In the Reynolds number comparison, the vorticity iso-surface plot at a Reynolds number of 5 k revealed smaller, finer vortical structures compared to the simulation at 1 k, due to vortices' breakup. In comparison, the force output difference is much smaller between Re = 1 k and 5 k. Increasing the body inclination angle generates a uniform leading edge vortex instead of a conical one along the wingspan, giving higher lift. Understanding the force variation as the body inclination angle increases will allow FMAV designers to optimize the thrust and lift ratio for higher efficiency under different operational requirements. Lastly, increasing the spanwise flexibility of the wings increases the thrust slightly but decreases the efficiency. The thrust result is similar to one of the spanwise studies, but the efficiency result contradicts it, indicating that other flapping parameters are involved as well. Results from this study provide a deeper understanding of the underlying aerodynamics of the X-wing type, which will help to improve the performance of insect-sized FMAVs using this unique configuration.
A numerical investigation is performed on the interference effects in single or biplane flapping ... more A numerical investigation is performed on the interference effects in single or biplane flapping airfoil propulsion in the presence of a stationary downstream tail. At a Reynolds number of 1000, this corresponds to the regime of small micro aerial vehicles. The objective of this study is to provide insight into the complex wing-tail interaction effects occurring in flapping-wing propulsion configurations. The effect of the relative distance between the airfoils, as well as the positioning and incidence angle of the tail, is investigated. Adding a tail behind a single flapping airfoil increases the efficiency and average thrust by up to 10 and 25%, respectively. For the biplane flapping airfoils without tail, overall efficiency and average thrust per airfoil increase up to 17 and 126%, respectively, with respect to the single airfoil due to the formation of a strong momentum jet. The effect of adding a tail behind the biplane flapping airfoils depends on the tail's orientation and location. Increasing the incidence angle of the tail generates higher lift, although at the expense of decreased efficiency and thrust. Lastly, shifting the vertical position of the tail to have it coincide with the middle heaving position of the leading top airfoil gives the best overall performance.
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