Matrix of downstream nodes using LightGraphs

Does anyone have an idea as to how you would create a vector of ‘downstream nodes’ for a given node in a network, using LightGraphs? For example, see the network attached, all downstream nodes of 17 would be [19, 20, 21, 22, 23, 24]. The full topology of the network is as follows:

(l, i, j)
(1, 1, 2)
(2, 2, 3)
(3, 3, 4)
(4, 4, 5)
(5, 4, 6)
(6, 6, 7)
(7, 6, 8)
(8, 8, 9)
(9, 8, 10)
(10, 10, 11)
(11, 11, 12)
(12, 12, 13)
(13, 11, 14)
(14, 10, 15)
(15, 15, 16)
(16, 16, 17)
(17, 17, 18)
(18, 17, 19)
(19, 19, 20)
(20, 19, 21)
(21, 19, 22)
(22, 22, 23)
(23, 22, 24)
(24, 16, 25)
(25, 25, 26)
(26, 15, 27)
(27, 27, 28)

Where l is the branch number, i is the ‘from node’ and j is the ‘to node’ of that branch.