In a longitudinally ventilated tunnel, the backlayering effect induced by a fire depends primarily on the averaged wind speed near the fire. The effectiveness on detouring the smoke to exhaust shaft also depends on the averaged wind speed at the junction where smoke is detoured. In this study, a flow model set up using the commercial package ANSYS CFX is incorporated with the conjugate gradient method and proved to be able to determine the wind speed that can precisely control the smoke flow.