Pengaruh Variasi Diameter Nozzle Jet terhadap Proses Air Entrainment pada Vertical Plunging Water Jet Menggunakan Downcomer dengan Ketinggian Jet 25 cm di Atas Permukaan Air = The Effects of Nozzle Diameter on An Air Entrainment of Vertical Plunging Water Jet Using a Downcomer with a Height of 25 Cm on The Water Surface