The figure shows the three-zone grid in a simple diverging duct. Zone 1 is the upstream 25% of the duct, zone 2 is the next 25% of the duct, and zone 3 is the downstream 50% of the duct. Thus, the downstream boundary of zone 1 is coupled to the upstream boundary of zone 2, and the downstream boundary of zone 2 is coupled to the upstream boundary of zone 3.