In this paper, the non-stationary feature of the pressure fluctuation is uncovered with the Wigner-Wille distribution (WVD). In order to faithfully extract the frequency information, we consider that the widely used concept -- major frequency f_m of powder spectrum density (PSD) should be replaced by the parameter -- local peak weighted average f_d, which is the generalized form f_m in non-stationary case. The quantitative result from our comparative study of the effect of static bed height on the f_m and f_d indicates f_d is in accordance with the theoretical prediction of Verloop and Heertjes (1) while f_m fails to. thus, it provides a solid evidence to the idea.
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