Conducting any measurements within an actively used tunnel can be extremely expensive and time-consuming not to mention unfeasible in cases where the tunnel has not yet been constructed. As a result, tunnel ventilation assessments generally require computer-based fire models for smoke control design and design of ventilation systems that operate during normal tunnel operations.
Computational fluid dynamics (CFD) models are becoming more widely used in these highly complex evaluations to avoid problems with cost, traffic closures and other significant disruptions associated with real-time measurements. A problem exists, however, in the lack of standardized approaches to CFD modeling in tunnel ventilation assessments. Establishing guidelines for CFD modeling could not only benefit both end-users and designers and but also improve the quality of tunnel ventilation design.
CFD Modeling in Fire and Life Safety Assessments
Both one and three-dimensional computer modeling are useful in fire and life safety assessments and can be applied to a variety of facilities in multiple disciplines - such as security, accessibility, emergency management, pedestrian modeling, risk analysis, and threat and vulnerability assessment. The advantage of CFD three-dimensional modeling, however, is that it gives users access to a suite of tools that allow for customization of design strategies to address a wide range of challenging scenarios.
One use for CFD modeling is in developing computer models of fire scenarios to support fire hazard analyses. Engineers perform several representative scenarios using a CFD model to determine impacts from substantial fire barrier, automatic sprinkler and ventilation deficiencies. After evaluating the modeling output, engineers provide recommendations supporting life safety, property protection and rescue mission for egressing occupants.
Computer modeling of smoke movement and control in rail and road tunnels is also common in the industry. This modeling has the potential to refine system requirements to reduce cost and complexity. Typically, both fire dynamics CFD and timed egress models are used to evaluate solutions.
Standardizing CFD Modeling in Tunnel Ventilation Design
To accurately size and design the ventilation systems for emergency or fire operations, the aerodynamic phenomena and tunnel and ventilation duct network must be accurately modeled and simulated using one-dimensional and CFD software. The choice of these tools and where they are applied is a question for the ventilation experts. One of the primary considerations is the complexity and geometry of the spaces to be analyzed.
It is often the case that combining the two is the best option with boundary conditions for the CFD modeling obtained from the one-dimensional software. Boundary conditions refer to applying parameters obtained from one-dimensional model at the “limits” of shortened models created in CFD software.
Many experts predict that the use of CFD software will continue to grow within the industry. Therefore, for the benefit of CFD users and quality assurance, a standardized approach to CFD modeling must be established for tunnel ventilation assessments as well as other types of analyses within the fire and life safety discipline. Key areas to focus on are:
- using CFD software that has been validated and accepted in the industry with proven use on global infrastructure projects;
- establishing universal approaches to setting up CFD models so that there is no ambiguity;
- obtaining boundary conditions for CFD modeling from accurate, one-dimensional models of the complete tunnel network;
- establishing methodology for running simulations including sharing of computational resources, if required;
- post-processing and recording of data obtained from analysis;
- checking and reviewing CFD results; and
- presenting and publishing approach standardization.
Through our work on tunnel ventilation systems and CFD, it has become clear the industry is beginning to move toward standardization. Developing a clear process and partnering with industry leaders like Jensen Hughes will help create efficiencies and benefit end-users in the long run.