The demand of micro air vehicles (MAV) with multiple rotors is increasing in both military and civil applications because of their versatility on various missions. However, the use of MAVs for some missions still has limited success because of their noise pollution. One of the main noise sources is aeroacoustic sound produced by the low Reynolds number flows around the rotors. There have been many previous studies about small-scale rotor systems of MAVs during the past decades, but they mainly focused on investigations of the aerodynamics rather than the acoustics. Several studies considering the acoustics have started recently. However, only steady loading forces computed by using Blade Element Momentum Theory (BEMT) were considered in the previous studies, and the noise from unsteady flow phenomena were not taken into account. The main objective of the current study is to investigate the noise mechanisms and further to find ways to reduce the noise levels in unsteady low Reynolds number flows. A Non-linear Vortex Lattice Method (NVLM) is used to simulate the unsteady low Reynolds number flow and model the corresponding noise sources. The tonal components of far-field noise is predicted by using an acoustic analogy based on Ffowcs Williams-Hawkings (FW-H) equations. These numerical methods are applied to low Reynolds number propeller and rotor cases, and validated upon experimental data. Then, they are used to investigate the influence of the number of blades on both aerodynamic and aeroacoustic performance and provide further insight into prominent sources of noise.
An efficient multi-objective optimization method is proposed for node-based spacecraft radiator design. In contrast to a conventional manual approach to node-based radiator design with a thermal model, the proposed approach finds an optimum design solution for radiator node combinations through an optimization algorithm. The important parameters of radiator design are radiator size and topology, represented by discrete binary design variables allocated to each node division in the candidate radiator region. The optimization problem is formulated as a multiobjective problem with two or more objectives to minimize the number of radiator nodes and temperature margins of unit boxes, and a genetic algorithm suitable for multi-objective optimization is used. A small thermal model (verification thermal model) was developed to verify the proposed multi-objective optimization of the node-based spacecraft radiator design method, and test problems for this thermal model were defined. The numerical optimal solutions for test problems show good agreement with the analytic optimal solutions. Therefore, the applicability and feasibility of the multi-objective optimization of node-based spacecraft radiator design method to practical radiator design were confirmed.
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