Water at 24 °C is flowing by gravity through a filter bed
of small particles having an equivalent diameter of 0.0060 m. The void fraction of the bed
is measured as 0.42. The packed bed has a depth of 1.50 m. The liquid level of water
above the bed is held constant at 0.40 m. What is the water velocity v′ based on the
empty cross section of the bed?