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Human thermal plume is quite important to the study of airflow organization in the indoor environment,especially in the micro-environment research such as personalized ventilation,infectious disease transmission through air,etc.In order to investigate the unsteady fluctuation of the thermal plume around human body,a series of transient numerical simulations are conducted in this study.Numerical simulation based on 9.7 million grids and 0.02 s time step is performed to obtain the detail quantitative data of flow field.The obvious fluctuation and separation are captured in the upper flow region of human body based on the high resolution grids.The maximum time-averaged velocity of the thermal plume is found to be 0.25 m/s while the maximum fluctuate velocity is about 0.07 m/s.The further analysis of frequency spectrum shows that the thermal plume around the body is mainly dominated by the low frequency fluctuation which is lower than 1 Hz and the principal frequency is around 0.1 Hz.In order to overcome the drawback of the high computation cost for application of the engineering simulation,a new numerical simulation method combining a modified k-ε turbulence model and coarse grids is presented.This modified k-ε model can reduce the calculation error of Reynolds stress in the flow region of natural convection through redefining the turbulence viscosity coefficient segmentally and avoid a high numerical viscosity appeared due to the central difference scheme.It can reasonably predict the general fluctuation velocity and the frequency distribution during simulation process in coarse grids and show a huge potential to be applied to the engineering application.