Computational study on aerodynamic characteristics of a flying wing MAV

In this paper an effective method is developed to study aerodynamic characteristics of a flying wing micro air vehicle (MAV). The method is based on momentum source method (MSM), low Mach number preconditioning and lower-upper symmetric Gauss-Seidel (LU-SGS) implicit dual time-stepping algorithm on hybrid dynamic meshes. The S-A turbulence model is also applied to capture flow separation. Momentum source items are utilized to replace the propeller in the numerical simulation by simplifying the unsteady flow into a steady one. Compared with wind tunnel experimental results, the computed results indicate that the method developed is capable of dealing with steady and unsteady aerodynamic characteristics of flying wing MAV

Vortex generator design and application on the flow control of top-mounted subsonic intake at high angle of attack

A detailed orthogonal design of a vane-type vortex generator (VG) was performed by varying angles of attack and geometrics including delta, cropped-delta, rectangle and trapezium configurations. Influences of different shapes, angles of attack and geometric parameters of VG on the wake vortex were analysed. Results show that the vortex strength and drag of the triangle VG are minimal while the vortex strength and drag of the rectangular is maximal. The rectangular configuration has the highest wake vortex core orientation, while subsequently followed by cropped-delta, trapezoid and triangle layouts. Due to the fact that mounted height and angle of attack of VG have a significant influence on the wake vortex, a coherent computational campaign was conducted on a Unmanned Aerial Vehicle (UAV) equipped with a VG in front of the top-mounted subsonic intake. The aerodynamic interference from VG are numerically examined when the UAV operated at high angles of attack. Research revealed the development of forebody vortex and the effectiveness of flow control by vortex generator on the performance of the intake. Comparing to cases without VG, the distortion index |DC60| of the intake decreased 7.1 % and 5.9 % respectively at the angle of slip β=2° and 4°, while the total pressure recovery remains almost the same

Vortex generator design and application on the flow control of top-mounted subsonic intake at high angle of attack

A detailed orthogonal design of a vane-type vortex generator (VG) was performed by varying angles of attack and geometrics including delta, cropped-delta, rectangle and trapezium configurations. Influences of different shapes, angles of attack and geometric parameters of VG on the wake vortex were analysed. Results show that the vortex strength and drag of the triangle VG are minimal while the vortex strength and drag of the rectangular is maximal. The rectangular configuration has the highest wake vortex core orientation, while subsequently followed by cropped-delta, trapezoid and triangle layouts. Due to the fact that mounted height and angle of attack of VG have a significant influence on the wake vortex, a coherent computational campaign was conducted on a Unmanned Aerial Vehicle (UAV) equipped with a VG in front of the top-mounted subsonic intake. The aerodynamic interference from VG are numerically examined when the UAV operated at high angles of attack. Research revealed the development of forebody vortex and the effectiveness of flow control by vortex generator on the performance of the intake. Comparing to cases without VG, the distortion index |DC60| of the intake decreased 7.1 % and 5.9 % respectively at the angle of slip β=2° and 4°, while the total pressure recovery remains almost the same

Parametric study on the water impacting of a free-falling symmetric wedge based on the extended von Karman's momentum theory

The present study is concerned with the peak acceleration azmax occurring during the water impact of a symmetric wedge. This aspect can be important for design considerations of safe marine vehicles. The water-entry problem is firstly studied numerically using the finite-volume discretization of the incompressible Navier-Stokes equations and the volume-of-fluid method to capture the air-water interface. The choice of the mesh size and time-step is validated by comparison with experimental data of a free fall water-entry of a wedge. The key original contribution of the article concerns the derivation of a relationship for azmax (as well as the correlated parameters when azmax occurs), the initial velocity, the deadrise angle and the mass of the wedge based on the transformation of von Karman momentum theory which is extended with the inclusion of the pile-up effect. The pile-up coefficient, which has been proven dependent on the deadrise angle in the case of water-entry with a constant velocity, is then investigated for the free fall motion and the dependence law derived from Dobrovol'skaya is still valid for varying deadrise angle. Reasonable good theoretical estimates of the kinematic parameters are provided for a relatively wide range of initial velocity, deadrise angle and mass using the extended von Karman momentum theory which is the combination of the original von Karman method and Dobrovol'skaya's solution and this theoretical approach can be extended to predict the kinematic parameters during the whole impacting phase.Comment: arXiv admin note: text overlap with arXiv:2207.1041

Effects of wave parameters on load reduction performance for amphibious aircraft with V-hydrofoil

An investigation of the influence of the hydrofoil on load reduction performance during an amphibious aircraft landing on still and wavy water is conducted by solving the Unsteady Reynolds-Averaged Navier-Stokes equations coupled with the standard $k-\omega$ turbulence model in this paper. During the simulations, the numerical wave tank is realized by using the velocity-inlet boundary wave maker coupled with damping wave elimination technique on the outlet, while the volume of fluid model is employed to track the water-air interface. Subsequently, the effects of geometric parameters of hydrofoil have been first discussed on still water, which indicates the primary factor influencing the load reduction is the static load coefficient of hydrofoil. Furthermore, the effects of descent velocity, wave length and wave height on load reduction are comprehensively investigated. The results show that the vertical load reduces more than 55$\%$ at the early stage of landing on the still water through assembling the hydrofoil for different descent velocity cases. Meanwhile, for the amphibious aircraft with high forward velocity, the bottom of the fuselage will come into close contact with the first wave when landing on crest position, and then the forebody will impact the next wave surface with extreme force. In this circumstance, the load reduction rate decreases to around 30$\%$, which will entail a further decline with the increase of wave length or wave height

Numerical Prediction of Unsteady Aerodynamics of a Ducted Fan Unmanned Aerial Vehicle in Hovering

Recently, ducted fan unmanned aerial vehicles (UAVs) have attracted considerable attention due to their potential for application in both civil and military missions. Compared with free propellers, the presence of duct can in principle decrease the flow contraction after propeller, and gives the potential to fly efficiently with high security, compact structure, and low noise. In the present study, a ducted fan UAV is designed using the open source code OpenProp. The computational fluid dynamics (CFD) simulation model using sliding mesh technique is established and validated as a reliable tool for highly vortical flows by propeller thrust experiment. The effect of the duct, revolution speed, and distance between propellers on the aerodynamic characteristics of the ducted fan UAV is evaluated in detail. Results show that the unducted coaxial upper and lower propellers generate 3.8%, 4.3% more thrust than the unducted single propellers, respectively, and the unducted upper and lower propellers generate 55.9%, 34.9% more thrust than ducted propellers, respectively. The ducted fan UAV generates 5.7% more thrust and consumes 39.1% less power than the unducted coaxial propellers. The thrust of the ducted fan UAV increases first and then follows with a decreased tendency as the distance between propellers increases

Numerical Prediction of Unsteady Aerodynamics of a Ducted Fan Unmanned Aerial Vehicle in Hovering

Recently, ducted fan unmanned aerial vehicles (UAVs) have attracted considerable attention due to their potential for application in both civil and military missions. Compared with free propellers, the presence of duct can in principle decrease the flow contraction after propeller, and gives the potential to fly efficiently with high security, compact structure, and low noise. In the present study, a ducted fan UAV is designed using the open source code OpenProp. The computational fluid dynamics (CFD) simulation model using sliding mesh technique is established and validated as a reliable tool for highly vortical flows by propeller thrust experiment. The effect of the duct, revolution speed, and distance between propellers on the aerodynamic characteristics of the ducted fan UAV is evaluated in detail. Results show that the unducted coaxial upper and lower propellers generate 3.8%, 4.3% more thrust than the unducted single propellers, respectively, and the unducted upper and lower propellers generate 55.9%, 34.9% more thrust than ducted propellers, respectively. The ducted fan UAV generates 5.7% more thrust and consumes 39.1% less power than the unducted coaxial propellers. The thrust of the ducted fan UAV increases first and then follows with a decreased tendency as the distance between propellers increases

Experimental Investigation of Aerodynamics of Flapping-Wing Micro-Air-Vehicle by Force and Flow-Field Measurements

This study explores the aerodynamic characteristics of a flapping-wing micro aerial vehicle (MAV) in hovering configuration by means of force and flowfield measurements. The effects of flapping frequency and wing geometry on force generation were examined using a miniature six-component force sensor. Additional high-speed imaging allowed identification of the notable different deformation characteristics of the flexible wings under vacuum condition in comparison to their behavior in air, illustrating the relevance of aeroelastic effects. Flow visualization around the flapping wing by means of planar particle image velocimetry (PIV) measurements revealed the formation, development, and shedding of the vortical structures by the wings during flapping motion, with particular emphasis on the clap-and-fling phase. Further stereoscopic PIV measurements performed in the wake showed a momentum surplus wake induced by the clap-and-fling, indicative of thrust generation. The vortical structures in the wake formed during instroke and outstroke were characterized using a three-dimensional wake reconstruction from the planar measurements

Experimental Investigation on the Aerodynamics of a Bio-inspired Flexible Flapping Wing Micro Air Vehicle

An experimental investigation on a 10 cm bio-inspired flexible Flapping-Wing Micro Air Vehicle (FWMAV) was conducted in both hovering and forward-flight conditions with the objective to characterize its aerodynamic performance. The measurements in hovering conditions were performed with the particular objective to explore the effect of different wing configurations (i.e. different aspect ratios and wing flexibilities), whereas forward flight tests in a wind tunnel were carried out to assess the aerodynamic performance of the FWMAV as a function of flow speed, flapping frequency and body angle. The cyclic variation of forces (lift and thrust) generated as a result of the wing flapping was captured by means of a high-resolution force sensor, in combination with high-speed imaging to track the wing motion. Results of measurements in hover show that the flapping frequency, aspect ratio and wing flexibility have a crucial impact on the efficiency and the force generation during the flapping cycle. An estimated flight envelop for the MAV's operation is defined from the data obtained in the wind tunnel measurements. Furthermore, additional tests on several brushless DC motors provide a feasible option in future engine selection and design