جستجوی مقالات مرتبط با کلیدواژه « ملخ » در نشریات گروه « فنی و مهندسی »

In recent decades, the use of drones as part of urban and regional transport infrastructure, as well as military applications, has developed extensively. Contrarily, their high aerodynamic noise is one of the serious challenges to obtaining a certificate of airworthiness for these flying devices. The most important source of noise in multi-rotors, airplanes, and propeller drones with electric motors or even turboprop engines is the noise caused by propellers. In the present study, to learn more about the mechanisms affecting the propeller's noise at low Reynolds numbers and to investigate the sensitivity of the noise to the propeller rotational speed and the polar angle of its propagation, the far-field noise of a drone propeller has been evaluated experimentally. The results show that the propeller's noise includes tonal and broadband noise, so that at low frequencies, the largest propeller tonal noise occurs at the blade pass frequency, and its harmonics are still visible up to high frequencies. On the other hand, the broadband components of noise predominate in the medium and high-frequency ranges. The increase in propeller rotational speed has led to an increase in the amplitude of both the tonal and broadband noise spectrum, so that the values of the overall sound pressure level and the noise spectra of the propeller at the first blade pass frequency have increased with the increment of the propeller speed. Finally, the results showed that broadband noise has a higher sensitivity to the polar angle of sound propagation than tonal noise.

Unmanned aerial vehicles (UAVs) like drones (quadcopters, hexacopters, octocopters etc.) can be a source of a significant acoustic noise. High noise makes them less suitable for use in densely populated urban areas, particularly during take-off, landing and low level flight, due to the noise annoyance. This paper reviews methods and proposes some concepts for attenuation of UAV noise. Both passive and active solutions are considered. Solutions with piston engine silencer, Q-tip propeller, more propeller blades, absorptive and reflective barrier, ducted propeller, sound absorbing ducts, synchrophaser, multichannel active noise cancellation (ANC) system with secondary sources circularly arranged around the propeller and the system with magnetically excited propeller blades are mentioned.

This work aims to prediction of laminar/turbulent transition which plays an important role on aerodynamics of wing section. In this respect the flow around the NACA2415 airfoil simulated in a Computational Fluid Dynamics (CFD) solver in different regimes with and without propeller flowfield. For predicting the transition onset, two approaches were used: The first is based on time history of the skin-friction coefficient for determining the transition onset and the transition length on the airfoil. The second is to apply transition γ-〖Re〗_θ model for laminar/turbulent transition simulation. For investigation of transition effect, the simulation repeated by use of a classical turbulent model and both results was compared with experimental data. The comparison shows that taking into account the transition effects gives a good agreement with experiment. Relative error of calculated drag coefficients for the transition based simulation is lower than 10%, while fully turbulent simulation are 70% overestimated in some incidences. Slipstream of upstream propeller changes flow pattern and boundary layer characteristics over the wing. Indeed in presence of propeller, spanwise load distribution and laminar/turbulent transition onset were affected. In propeller flowfield, increasing of velocity normal component over wing surface causes transition delay. Movement of transition onset to trailing edge on the upper surface in propeller downwash is representative of such phenomenon. On the other hand, in upwash region, the transition onset moves upstream. With the increasing propeller rotational speed, this tendency augments and so the transition onset on the wing upper surface moves far downstream in propeller downwash.

Variable pitch propeller (VPP) are used in advanced helicopters, in order to achieve greater efficiency, better stability and achieve higher altitudes. This study is going to assess the behavior of VPP propeller with coupled non-linear displacement in three degrees of freedom. Accordingly, the behavior of this type of propeller with Changes of elastic axis distance, Length, mass, speed, polar radius of gyration, Stiffness in three degrees of freedom, and pitch have been investigated. In this paper, Gallerkin method is used to extract natural frequencies and the results were evaluated with the results reported by other researchers. The results show convergence and accuracy of the used method. In this study, it was found that parameters of mass, length and rotational speed of the propeller have effect on the natural frequencies, and all modes of vibration. However, other parameters except for the pitch angle effect on the odd or even number of frequency modes. It was also found that the pitch angle in the static mode does not effect on natural frequencies but in the case of rotation of propeller, affect on natural frequency of vibration modes as sine or cosine form.

Because of various applications of UAVs, research in this field has been developed increasingly in recent years. Propeller has considerable importance as a key factor in producing propulsion in such vehicles. Having information about a propeller’s performance variations in different operational conditions is very important in order to choose a suitable propeller for a predefined mission of the flying vehicle. For this aim, in this research a test stand was designed and fabricated to evaluate the static performance of electromotor driven propellers with application in UAVs. After collecting data by performing experimental tests, the results were compared to those obtained from the numerical and analytical techniques. In order to verify the results, a propeller was modeled and a computational method was applied based on k-ε, RNG turbulence model. The comparison of experimental, analytical and computational results shows an acceptable agreement between them. According to the results, the difference between analytical and empirical results is 0.4%, the difference between computational and empirical results is 0.3% and the difference between analytical and computational results is about 1.23%. Also in the range of the rotational speed of the propeller, the difference between computational and empirical results became less than 10% in most cases, implying the validity of the applied computational method and correctness of experimental test procedure.