多孔介质porosity&void fraction 定义辨析

rock218

在帮助文档中介绍多孔介质的时候，有两个定义不能完全明白：a is the porosity of the media defined as the ration of the volume occupied by the fluid to the totail volume;  b is the void fraction defined as the volume of voids divided by the volume of the packed bed region.  我觉得a 也就是 porosity 翻译成孔隙度比较好，那void fraction 翻译成什么呢，它们的实质是不是一样的？