Aerodynamic interactions between rotors are important factors affecting the performance of in-plane multirotor Unmanned Air Vehicles (UAVs) or drones, which are the majority of small size UAVs (or mini-drones). Optimal design requires knowledge of the flow features. The low Reynolds number of many UAV rotors raises the question of how these features differ from those expected by traditional analytical methods for rotorcraft. Aerodynamics of a set of side-by-side rotors in hover over a range of rotor separation and Reynolds number is studied using high-speed Stereo Particle Image Velocimetry (SPIV) and performance measurements. The instantaneous and time-averaged SPIV data presented here indicate an increase in inter-rotor wake interactions with decrease in rotor spacing and Reynolds number. A dip in rotor efficiency at small rotor spacing at low Reynolds number is observed through thrust and torque measurements. The basic components of in-plane multirotor wake and velocity profiles are identified and discussed to help generalize the findings to a wide range of drones. However, the data provide confidence in traditional analysis tools, with small modifications.

Unmanned multi-rotor VTOL vehicles have recently gained importance in various applications such as videography, surveillance, search and rescue etc. suited to their small size and relatively cheap construction. Small scale UAVs struggle in providing satisfactory performance in terms of payload, range, and endurance because of higher viscosity-dominated losses, and due to yet to be understood rotor-rotor and rotor-airframe aerodynamic interactions. Viscosity dominated rotational flow field makes most potential flow methods, such as free wake model, invalid. A full N-S based approach for this problem is too expensive. Thus, a multi-rotor aerodynamic interaction study is necessary for understanding crucial phenomena, which will help in developing physics-based models which will be instrumental in multi-rotor UAV performance prediction and design optimization. In present work, a flow visualization and a high-speed stereo Particle Image Velocimetry (SPIV) study is done on two low Reynolds number multi-rotor arrangements with the aim of capturing vortex-vortex, blade-vortex and vortex-duct interactions. The first arrangement is a coaxial rotor in forward flight and another is an in-plane quad-rotor with and without duct. Instantaneous and average PIV data is being presented here with some observations and corresponding interpretations.

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In order to progress in the area of aeroacoustics, experimental measurements are necessary. Not only are they required for engineering applications in acoustics and noise engineering, but also they are necessary for developing models of acoustic phenomenon around us. One measurement of particular importance is acoustic impedance. Acoustic Impedance is the measure of opposition of acoustical flow due to the acoustic pressure. It indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency and can be a characteristic of the medium.The aim of the present paper is to give a synthetic overview of the literature on impedance measurements and to discuss the advantage and disadvantage of each measurement technique. In this work, we investigate the three main categories of impedance measurement techniques, namely reverberation chamber techniques, impedance tube techniques, and far-field techniques. Theoretical principles for each technique are provided along with a discussion on historical development and recent advancements for each technique.

Coaxial rotor uninhabited aerial vehicles (UAVs) are compact compared to single rotor UAVs of comparable capacity. At the low Reynolds numbers (Re) where they operate, the simplifying assumptions from high Re rotor aerodynamics are not valid. The low Re coaxial rotor flowfield is studied including aerodynamic interactions and their effect on performance. The evolution of the wake is captured using high-speed stereo particle image velocimetry (SPIV). Improvement of upper rotor performance due to viscous swirl recovery from the lower rotor is discovered and then verified by analyzing PIV data. Interesting vortex–vortex sheet interactions are observed under the coaxial rotor affecting wake structure spatially and temporally. A qualitative model explaining the observed wake interaction phenomena is presented. Comparison with the performance of high Re rotors shows higher profile and induced drag at low Re for the same thrust coefficient.