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The science
What an ejector does
A jet pump has no moving parts. Motive water at pressure pp accelerates through a nozzle into a suction chamber; the fast jet drags suction water along, the two streams mix in a throat and a diffuser turns the remaining velocity back into pressure at the outlet, pd. The flow ratio is M = Qs/Qp and the area ratio R = An/At (nozzle over throat).
Equations used
Nozzle and suction entry (loss coefficients Kn = 0.05, Ken = 0.3): pp - p1 = ½ ρ (1+Kn) vj2 and ps - p1 = ½ ρ (1+Ken) vs2 with vs = Qs/(At - An); together they fix the jet speed for a given M. Throat momentum balance: (p1 - p2) At = ρ [(Qp+Qs) v2 - Qp vj - Qs vs] + ½ ρ Kth v22 At with Kth = 0.02 L/d. Diffuser: pd = p2 + ½ ρ v22 (1 - Kd). The pressure ratio is N = (pd - ps)/(pp - pd) and the efficiency is η = M N; at M = 0 with no losses the exact result is N = (2R - R2)/(1-R)2.
The operating point and the cavitation limit
The delivery needs ρ g (H + k Q2); the page finds the flow ratio at which the jet's pd(M) equals that, by bisection. If M would exceed the flow at which the pressure leaving the nozzle plane, p1, reaches the vapour pressure of the water, the suction stream chokes: Qs,max = (At - An) √(2 (ps - pv) / (ρ (1+Ken))). The tanks drain and fill, so the lift grows and the delivery head rises while the sim runs.
Honest simplifications
One-dimensional incompressible flow with fixed loss coefficients and uniform velocity at the end of the throat (real mixing is incomplete in a short throat); no air entrainment and no compressible-vapour (steam ejector) behaviour; the 3D device is scaled up (1 mm of nozzle diameter = 0.03 m of scene) and the delivery height is compressed in the standpipe picture. Typical real jet pumps reach 25 to 35 % efficiency, which this model reproduces.