Experimental and numerical study on the influence of wall roughness on the ventilation resistance coefficient in a tunnel under construction

• A test platform for the influence of tunnel wall roughness on ventilation is developed. • The influence of roughness conditions on the resistance coefficient is revealed. • The Nicholas empirical formula of friction coefficient is modified. • A simple modified formula of tunnel resistance coefficient is proposed. Ventilation is essential for providing adequate amounts of fresh air, reducing environmental temperature and humidity to acceptable levels, and controlling the spread of harmful gases and dust, thus ensuring a safe and comfortable tunnel space. However, very little attention has been given to the variation in the resistance coefficient with wall roughness, which is of great significance for achieving an optimum and economically feasible ventilation design for a tunnel under construction. Hence, the main aim of this article is to study the influence of the height, shape, and spacing of rough elements on the resistance coefficient along the tunnel wall. Model testing and numerical simulation were applied, and the reliability of the numerical simulation method was verified by model testing. The results show that the resistance coefficient along the tunnel decreases with the increase in wind speed. When the wind speed is greater than 1.5 m/s, the resistance coefficient changes very little. The higher the rough height, the greater the resistance coefficient and the more significant the influence of the spacing on the resistance coefficient is. The influence of rectangular elements on the resistance coefficient is the largest; the influence of conical elements is the second and the influence of hemispherical elements is the smallest. The larger the spacing of the elements, the smaller the resistance coefficient. The influence of shape on the resistance coefficient is greater than that of the rough height. Attempts were also made to propose a correction coefficient α based on the average rough height Δ for the existing empirical formula; thus, we obtained a correction formula for calculating the resistance coefficient. The rationality of the correction formula was verified by a field test.