In this paper forced flow structure and mixed convection in a ventilated porous enclosure with a local contaminant source centrally positioned on the floor have been investigated numerically. The physical model for the momentum conservation equation makes use of the Darcy-Brinkman equation, which allows the no-slip boundary condition on a solid wall to be satisfied. The set of coupled equations is solved via the SIMPLE algorithm. Comparisons with previously published work are performed and found to be in excellent agreement. An extensive series of numerical simulations is conducted in the range of, 200 <= Re <= 10,000, 0.02 <= Re . Da <= 20 and, 0.01 <= f = (Gr(s) . Da(2)) / (Re . Da) <= 1000 where Re is the Reynolds number, Da is the Darcy number, Gr(s) is the solutal Gras of number and f is derived from the discretized V equation and represents the relative magnitude between the effective solutal buoyancy and the effective external force. Streamlines are produced to illustrate the forced flow structure transition from eddy-free motion to multi-eddy pattern and Re . Da >= 0.2 is the critical condition of eddy emergence. The analysis of overall Sherwood number and iso-concentrations indicate that the parameter f is appropriate to be regarded as the criterion of mixed convection in porous medium. (C) 2018 Published by Elsevier Ltd.