Influence of sampling frequency and time on pressure fluctuation characteristics of axial-flow pump
In order to precisely analyze the pressure fluctuation in axial-flow pumps,the unsteady flow in such a pump was calculated based on the time-averaged N-S equations,the standard k-ε turbulence model and SIMPLEC algorithm with a structured mesh under a specific tip gap,then the pressure signals monitored were sampled with different sampling frequencies and times to show their effects on the pressure fluctuation characteristic.The results show that the point at f =0 stands for the average static pressure physically if the pressure was used as the Y-axis of FFT in a frequent domain diagram.Besides,the sampling frequency and time exhibit a significant effect on the pressure fluctuation feature and a too small sampling frequency is responsible for the aliasing phenomenon in time-and frequency-domain.It was derived that a spectral leakage at the basic frequency doesn't occur when the sampling frequency is an integer multiple of the product of the time for impeller rotating one revolution and the number of blades.Furthermore,a general equation for determining sampling time was established for the pressure fluctuation signals obtained in an axial-flow pump by numerical computation.According to that equation,sampling time is dependent on pump rotational speed.Obviously,such a finding forms the base for processing of pressure fluctuation signal obtained in CFD simulations in an axial-flow pump.
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