Abstarct: Jets appear in various forms in the nature. Inverse jets have been appeared as turbulent flows in the places where the river enters into the delta. The purpose of this research is the numerical investigation of the penetration length of an Inverted jets penetrated into a square- shaped and closed channel, since the numerical investigation of jet has been performed broadly using several software. In 2002, a test have been performed in which a rectangular channel with a known length with steady flow into which a pipe is inserted against the flow so that the outgoing flow is entered into the fluid environment as a jet flow, The purpose of this test was to show that the penetration rate of the circular jet is varying with different speeds and different diameters of the nozzles. That finally leads to get the permeability coefficient. So we can compare the numerical model and laboratorial model. The test model is a rectangular channel (30cm × 30cm) with a length of 120cm in which a pipe with a length of 18cm inserted. As the steady speed of the fluid in the channel (water) is constant, the jets enter into the channel with 5 different diameters (1.5, 3, 6, 9 and 12 mm) each of which has different speed. Then they are numerically modelled with OpenFOAM software and the standard model of k-ε has been used to solve the turbulence equations. The results show a permeability coefficient as 2.65.