Computational procedures and results of an upwash jet arising from two opposing plane wall jets based on the Reynolds averaged Navier-Stokes equations are discussed in this thesis. The Navier-Stokes equations were solved using a staggered grid and a C-QUICK scheme. The SIMPLE-C algorithm is adopted to get the pressure field for the divergence-free velocity field. The present code was tested against various benchmark flows prior to the application to the upwash jet flow. For the calculation of the flow, a steady and an unsteady numerical approaches were followed. For the steady computation, we adopted various eddy viscosity models(the standard k-ε model, the RNG k-ε model and the Bardina's model) and the Reynolds stress transport model with various diffusion term closures. Results of the steady computation indicated that the jet half-width was very much underpredicted, and hence the velocity profiles of the upwash jet were in very poor agreement with the experimental data. It was concluded that the large jet half width of the upwash jet was not caused by the intense turbulence diffusion. This led to the supposition that the large jet half width of the upwash jet was due to the low frequency oscillation of the flow. The oscillatory behavior of the upwash jet was confirmed by the power spectrum of the velocity fluctuation in a similar flow configuration. To predict the oscillatory flow, we adopted an unsteady version of the standard k-ε eddy viscosity model. The outflow boundary condition was founded to be crucial to yield an oscillatory flow. The use of a convective outflow boundary condition resulted in a periodically oscillating jet flow. The jet half-width distribution obtained by taking the time average of the periodic velocity profiles was in significantly better agreement with the experimental data.