HBI Haerter AG is an internationally leading engineering consultant specialized in the design, planning and execution of projects in the field of tunnel ventilation, prediction of emissions, safety issues and the aerodynamics and thermodynamics of road and rail tunnels. HBI offers the entire range of services related to tunnel ventilation, from feasibility studies to measurements, detailed studies and project execution all the way to refurbishment and optimization during operation.
More and more roads are being built below ground, not only in mountainous areas, but likewise in densely populated areas. Tunnels and underground train stations rely on technology to provide appropriate environmental conditions for the people using them. Ventilation systems are needed to supply fresh air and remove exhaust gases. In emergencies such as accidents or fire, people need to be able to get out of danger zones. This not only requires emergency guiding systems but also a design that facilitates an air flow leaving a clear zone under the smoke layer in which to breathe and find escape routes.
“The ventilation design of a tunnel system or station building is extremely complex,” says Erwin Schnell, project manager, HBI Haerter. “It cannot be verified and improved using a prototype but needs to be right the first time, so it requires simulation using computational fluid dynamics (CFD).”
This is not just a job but a mission to the experienced specialist engineer who commenced numerical simulation as early as 1989 in the fields of aerospace and automotive engineering. Presenting the results of a transient exhaust system simulation in a VR-CAVE he took advantage of virtual immersive engineering first in 1998 and as of early 2013 he significantly pushes augmented reality as well. Specializing in fire and smoke simulations, Schnell is one of 40 experts forming HBI Haerter. With more than 800 successfully completed reference projects, this group of engineers operating from two locations in Switzerland and two more in Germany and Australia is an internationally leading consultant in tunnel ventilation design.
To calculate airflows induced by traffic or ventilation and smoke dispersion in tunnel systems and train stations, HBI Haerter simulation engineers are using Simcenter™ STAR-CCM+™ software. This package is part of the SimcenterTM portfolio, a comprehensive suite of simulation software and test solutions from product lifecycle management (PLM) expert Siemens Digital Industries Software.
Although not limited to fluid behavior, the software is centered on CFD to solve and analyze problems that involve fluid flows by successive approximation using numerical analysis and data structures. It is based on the finite volume method for representing and evaluating the Navier–Stokes equations. This is also termed computational continuum mechanics, hence the CCM in Simcenter STAR-CCM+.
“The entire flow field is represented by an adequate number of simply connected volume volumetric elements, which are generated within a process called spatial discretizaion or simply meshing,” says Schnell. “Velocities and scalar quantities such as temperature or smoke concentration are calculated within each individual element.”
When he started tackling simulations using computer-aided engineering (CAE) software nearly 30 years ago, Schnell chose Simcenter™ STAR-CD™ software, the legacy predecessor of Simcenter STAR-CCM+.
“More than any other software on the market at the time, it was discernibly made by engineers for engineers,” says Schnell, “and so is its modern-day successor that I have been using since it came to market in 2005.”
Conceived from a blank sheet of paper and built using an object-oriented approach, the software was designed to simultaneously solve fluid flow and heat transfer problems.
“In my point of view, Simcenter STAR-CCM+ is much more than just a piece of software,” says Schnell. “It is a process that can easily be adapted tailored to the entire variety of user-specific needs.”
Among the variations between customer projects is the quality and format of geometry data created using different computer-aided design (CAD) software tools. Simcenter STAR-CCM+ supports various formats to import 3D solids from all relevant CAD software systems.
Featuring a built-in, fully parametric 3D feature-based CAD modeler, it also allows for creating and modifying geometries directly within the software.
“This provides us with opportunities to successively optimize geometries by setting different parameters,” says Schnell. “As the software is capable of iterating through parameter lists, it makes investigating the effects of design variations an extremely efficient task.”
This is further enhanced by the built-in Design Manager capability that enables users to set up and automatically evaluate families of designs within Simcenter STAR-CCM+, including process management and performance assessment. The software not only allows easy access to all operations, it can also use parameters to automatically recreate meshes following geometry modifications.
“Using Simcenter STAR-CCM+, we can be sure to always get reproducible results that are reduced to the best possible minimum of meshing influences,” says Schnell.
Recent versions of Simcenter STAR-CCM+ can also be directly linked with Teamcenter® software to import and export product lifecycle management extensible markup language (PLM XML) files created in Teamcenter. These are master files containing all the names, attributes and locations of the actual CAD data. The underlying CAD data can come in practically any of the popular formats (including the JT™ data format, Parasolid® software, or NX™ software, to name but a few). This improves associativity with the CAD data, so simulation engineers are sure to always get the right model. Engineers of the various disciplines can collaborate with little or no extra effort using the workflow tools offered by Teamcenter.
On the other hand, Simcenter STAR-CCM+ provides simulation engineers with tools helping them concentrate on functional modeling without getting into too much geometric detail.
“In a recent project, all I got was a rather primitive sketch,” says Schnell. “Using only generic geometrical forms such as cubes and cylinders, I was able to confidently present valid simulation results within four hours.”
This capability positioned the customer to make a well-founded design decision on short notice, a requirement that keeps gaining in importance. It also means that it is easy to learn how to use this software, so new employees can quickly become productive.
It is similarly simple to include various aspects in multiphysics simulations to get a comprehensive match of the real-world performance of a product or scenario. In Simcenter STAR-CCM+, all of these capabilities are accessed from a single integrated environment that hosts the geometry, mesh, boundary conditions, physics models and simulation results. It also offers an easily accessible application programming interface (API) to enable coupling with other simulation tools for co-simulation.
“In emergency situation modeling for underground train station design, we integrate a heat source, ventilation systems, the aerodynamics of moving trains and the effects of people on the platforms with the funneling geometry of the architecture,” says Schnell. “With an overset mesh that allows for creating hierarchic simulation scenarios, Simcenter STAR-CCM+ enables us to combine independent aspects into truly comprehensive multiphysics simulations. But its not just fire and smoke investigations that are carried out by HBI´s CFD experts. The whole variety of capable features accessible within STAR-CCM+ are applied e.g. VoF, DEM, FSI, Mulitphase Flow etc.”
The richer the digital twin, however, the more number-crunching power the simulation requires. HBI Haerter operates compute clusters at the Zurich headquarter to provide processing power inhouse. In case of capacity overloads additional compute resources are accessible on short call at HLRS in Stuttgart. As Simcenter STAR-CCM+ employs client-server architecture, it enables users to solve problems from a lightweight computer, while the computationally expensive math is done on a remote machine. A power session license unique to this Siemens Digital Industries Software product allows simulation engineers to run simulations on as many processors as needed for a fixed annual rate.
“Using massive parallel processing, we achieve very short turnaround times,” says Schnell. “Compared to all other CFD simulation software tools I am familiar with, Simcenter STAR-CCM+ features very efficient decompositioning so we can effortlessly adjust the number of processors to meet the requirements of the simulation task at hand.”
Part of the ever-increasing level of realism offered in Simcenter STAR-CCM+ is the use of virtual reality (VR) headsets as human-machine interfaces. The VR headsets allow users to dive into the simulation, for example, to verify the possibility of safe escape in case of fire.
“Users can release massless particles to visualize the flow field as well as the relevant smoke and temperature layers at platform level,” says Schnell. “In complete safety, they can walk to the exits nearby and determine whether escape routes are smoke free.”
One of the benefits is that in a VR simulation, fire experts are able to analyze vulnerabilities in places that would not be accessible in real life or in classic documentation. In early 2018, Schnell began negotiations with insurance companies and government authorities. They may soon approve construction based on numerical verification, supplied in the form of tours through smoke-contaminated buildings in virtual reality.