Water at $24C$ is flowing by gravity through a filter bed of small particles having an equivalent diameter of $0\mathrm{.}006m.$ The void fraction of the bed is measured as $0\mathrm{.}42.$ The packed bed has a depth of $1\mathrm{.}5m.$ The liquid level of water above the bed is held constant at $0\mathrm{.}4m.$ What is the water velocity $v^{\text{'}}$ based on the empty cross$-$section of the bed?