An improved correlation for pressure drop in a tunnel under traffic jam using CFD Esmaeel Eftekharian a , Omid Abouali a,n , Goodarz Ahmadi b a School of Mechanical Engineering, Shiraz University, Mollasadra Street, Shiraz, Iran b Aeronautical & Mechanical Engineering Department, Clarkson University, Potsdam, NY, USA article info Article history: Received 4 June 2014 Received in revised form 9 December 2014 Accepted 20 April 2015 Available online 28 May 2015 Keywords: CFD Traffic jam Tunnel One directional Pressure drop abstract Undoubtedly, an accurate estimation of pressure drop due to vehicles can contribute to correctly pre- dicting the number of jet fans which ultimately leads to choosing the optimum ventilation system in a tunnel. The main objective of the present study is to improve the 1D (one-dimensional) model, which currently is unable to consider the effects of combination of cars in traffic jam on the total drag force. The CFD technique has been applied to model several tunnels in different lengths under severe traffic jam conditions; this way, the difference between pressure drop predicted by CFD simulation and 1D model can be investigated. The CFD simulation results were used to develop a new correlation for pressure drop due to cars inside the tunnel as a function of tunnel average air velocity and tunnel length. Additionally, the differences between the pressure drop due to the cars predicted by the CFD and the 1D model for different traffic conditions were investigated. It was shown that the existing 1D model underestimates or overestimates the pressure drop depending on the traffic condition and the drag-area value used in analysis. Finally, 1D model is modified by a simple equation for the pressure drop in a tunnel in congested traffic conditions using an equivalent friction factor resembling the stopped vehicles. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Providing a safe breathing air in tunnels is one of the most important issues in the design and operation of tunnels in urban environment. In this regard, providing a correct estimation of drag force related to the presence of vehicles inside the tunnel is of crucial importance. This information is important for prediction of the number of the ventilation jet fans required for a tunnel under traffic jam. Jang and Chen (2002) have proposed an approach based on the optimization techniques in order to determine the drag coefficient of vehicles under transient condition in a road tunnel. It was shown experimentally that the vehicle spacing affects the drag coefficient of vehicles. That is the presence of a vehicle in the upstream of another car affects the drag coefficient of both vehicles. It was also concluded that the combined drag coefficient of a number of isolated vehicles can be lower than that of combination of vehicles (Watkins and Vino, 2008). Drag coef- ficient of a single moving vehicle in a tunnel evaluated from the CFD simulation is in good agreement with that obtained from 1D model. The drag result for two moving vehicles, however, is totally different. It was shown that an increase in vehicle space or a decrease in vehicle velocity leads to an increase in drag coefficient (Wang et al., 2014). In curved tunnels, it was found that as the radius of curvature of the tunnel reduces, the drag coefficient of a moving vehicle as well as the traffic force induced in the tunnel rises (Wang et al., 2011). Investigation of the airflow field as well as pollutant dispersion inside the tunnel under traffic jam condition is another issue attracting attention (Bari and Naser, 2005 , , 2010). Bari and Naser (2010) investigated dispersion of CO, CO 2 and O 2 inside a tunnel with a burning vehicle when the cars under traffic jam are emit- ting pollutants and the jet fans are running. They showed that after about 8 min, the level of pollution exceeds the permissible level. They also analyzed the effectiveness of jet fan performance for removing the pollution emitted by the vehicles under the severe traffic jam condition, and also showed that the absence of jet fans will pose a threat to the health of passengers and drivers. In the same vein, Eftekharian et al. (2014) investigated the per- formance of banana and traditional jet fans in traffic jam in an urban tunnel and they demonstrated that in traffic congestion, both types of jet fans almost induce an identical volume flow rate into the tunnel; additionally, they carried out the impacts of adverse wind on the tunnel volume flow rate and it was shown that in this regard, the results predicted by CFD simulation have a remarkable difference with those of engineering design. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jweia Journal of Wind Engineering and Industrial Aerodynamics http://dx.doi.org/10.1016/j.jweia.2015.04.013 0167-6105/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ98 711 613 3034; fax: þ98 711 628 7508. E-mail address: abouali@shirazu.ac.ir (O. Abouali). J. Wind Eng. Ind. Aerodyn. 143 (2015) 34–41