For years Computational Fluid Dynamics (CFD) has been widely used across many branches of engineering including the Aerospace, Automotive, Biomedical Chemical, Marine, Oil, Gas, Petrochemical and Power Generation industries.

One dimensional (1D) CFD allows engineers to understand how flow rates and pressures may change within a network flow system of interconnected components. In contrast, three dimensional (3D) CFD allows design engineer to understand how detailed flows interact with all manner of complex geometry. Of course, 1D calculations are much faster, only a few minutes, when 3D CFD may take hours or days.

Co-simulation of a 1D with a 3D model will potentially offer the best of both worlds. It enables the sharing of boundary data between 1D and 3D models in a single or multi-domain system to facilitate the simulation of the overall fluid system coupled with more detailed CFD simulation of 3D flows within a critical part of the network.

 

The Concept

 

The initial typical concept is quite simple, to allow 3D CFD simulation to collaborate with 1D CFD or systems simulation. The 3D option is local and detailed but slow, the system consideration is fast but general and do not offer detailed information on typical components.

We had to consider two kinds of industrial need: the 1D user who is looking for a local detail or the 3D user who would like the more real as possible boundary conditions.

3D analysis is being used where the nature of the flow and the understanding required makes 3D analysis the appropriate tool; 1D simulation is applied to examine the fluid flow conditions of the remaining system, which can be captured by a 1D calculation with in-built sub-models for specific components as required. Boundary conditions and results are then passed around the complete system, allowing for a more complete and faster analysis. A link is in charge to handle communication of variables (and results) between the models. Most of the software vendors have to enable users to bi-directionally link their 3D CFD models (often via a simple to use and intuitive user interface) to a 1D fluid flow system network. This 1D network then analyses for pressures, flows and temperatures across the whole system, reporting boundary conditions (Steady Sate or Transient) directly back into the CFD model.

The 1D system can also allow thermal interactions or mechanical positions from other systems and incorporate those effects when determining the component CFD boundary conditions.

To ensure proper functioning of the coupling, the interface must not be located in a region in which flow separation or recirculation is expected and must be a planar face orthogonal to the expected main flow direction.

This process will also allow you to investigate the influence of a complex three-dimensional component on 1D fluid flow and system performance, to provide detailed information about 3D flow fields in the domain of interest; considering the influence of all relevant sub-systems modelled one-dimensionally and to obtain an immediate response of the sub-systems on design changes.

 

The situation ten years ago

 

Various editors gave detailed information on the different links available and showed some examples., mainly based on engine thermal management.

The results obtained were challenging and stimulating and showed that changes in 1D model would directly affect 3D model.

This first approach on industrial case presumed that realistic simulations were possible and gave scope for further development and optimization, but some people remembered that validation is important.

The way some companies has developed new links corresponded to the ideal solution for users. They did not need intermediate tools between 1D and 3D codes and did not have to worry about the coupling itself.

The goal was to provide a consistent interface for coupling multiple 1D codes with 3D, assuming that 1D problem is set up by an expert user of 1D code.

 

The MpCCI option

 

Around 2010, MpCCI (Mesh-based parallel Code Coupling Interface) has been developed and distributed by Fraunhofer-Institute for Algorithms and Scientific Computing (Fraunhofer SCAI) to support Abaqus, ANSYS, Fine/HEXA, Fine/TURBO, Flowmaster, Fluent, Flux, ICEPAK, MSC.Marc, Permas, STAR-CD and RadTherm amongst others.

MpCCI overcomes the challenges inherent in co-simulation – complex hardware requirements and challenging software engineering requirements – by using adapters (developed by each software vendors) to establish a direct connection between the MpCCI Coupling Server and the 1D or 3D CFD code.

This new interface could facilitate interactions between different software tools as it remains partially vendor independent but was probably less convenient than the former direct links. Indeed, the initial links here were created for specific 1D and 3D codes, so they are fully dedicated. The graphical user interface was user-friendly and often directly included in the 1D user interface. The only problem was that each 1D or 3D software was live; the version was regularly changing, and the link was generally dependent on both 1D and 3D code version.

Using MpCCI avoided the main part of this dependence but as it was a general tool, the user interface was less easy to use than the direct ones.


Figure 1: A coupling example presented at Gaydon with MpCCI.

 

The implementation was easy to set up but we could presume that for highly complex systems some convergence problems could occur.

 

Another option, the external workflow process of modeFRONTIER

 

ModeFRONTIER allows to manage all the logical steps of your engineering process with a single automated workflow.

Complex engineering problems often include the use of a myriad of in-house and third-party CAD, CAE and general use software, including 1D and 3D CFD, resulting in a disconnected, difficult to manage process.

Integration and process automation tools, like modeFrontier, for example, will streamline and automate the engineering process within an integrated workflow of various software to increase the overall efficiency, save time and reduce operational costs.



Figure 2: How to integrate and drive multiple CAE tools.

 

They will streamline the execution of complex simulation processes and effectively manage system-level optimization.

Complex engineering problems include the necessity to take into account multiple disciplines, consider high number of interrelated variables, and run multiple third-party simulation software.

The automated, workflow-based environment for multidisciplinary optimization enables the efficiently execution of complex integration chains, the breaking of large engineering problems into modules, and the scheduling of nested optimizations from within the main workflow to improve the overall efficiency of the design process.

The technology offers a variety of direct integration nodes to couple with the most popular engineering solvers.

Direct nodes seamlessly connect with the simulation models, extract relevant parameters and allow the selection of the process input and output in few clicks, through a simple guided process. Constraints and objectives just have to be added to run the process and exploit the benefits of process automation.

 

Flownex is offering direct 1D-3D co-simulation

 

Flownex made the choice to collaborate with ANSYS and decided to integrate their 1D system tool to the workbench environment.

Flownex is linked to both ANSYS CFD and FEA codes for more localized results where required.

In particular Flownex can be coupled with ANSYS Fluent to provide an interactive communication between a CFD 1D system and CFD 3D analyses. This allows the inclusion of the effect 3D complex geometries in terms of pressure losses, multiphase flows and non-homogenous heat transfer.

Flownex can also be coupled with the FEA code ANSYS Mechanical: thermal and pressure stress analyses can be performed starting from the temperature and pressure calculated by Flownex. Co-simulations between 1D network and FEA 3D code can be used to model conjugate heat transfer with 3D geometries: this approach keep the accuracy of temperature distribution in the solid domain and reduce the computational effort on the fluid side.



Figure 3: 1D and 3D flows are solved together, and information is exchanged at the boundaries. The entire simulation is controlled by Flownex interface.

 

One application of the coupling with ANSYS Fluent can be illustrated in the simulation of the HVAC system in a server room. In this case Flownex is used to simulate the fan, heat exchanger and ducting to the server room while ANSYS Fluent is used to simulate the heating of the air by the electronics and the air flow in the room.

The interface points are chosen at the vents that supply air to the room, where the temperature and flow results from Flownex are transferred to Fluent and the backpressure results from ANSYS Fluent are returned back to Flownex. In this simulation, control elements are added to automatically adjust the fan speed and the vent openings in order to study the transient scenario of the system.




Figure 4: Flownex and ANSYS Fluent link: HVAC system in a server room.

 

Flownex SE also adds value to structural simulations and can be used to transfer 1D flow results to a FEA simulation package such as ANSYS Mechanical.

The ANSYS Mechanical coupling finds application in many industries. An example of the ANSYS Mechanical coupling in the power generation industry is illustrated below.



Figure 5: An example of typical boiler tube failure locations.

 

It shows a model of a boiler where the flows are calculated using Flownex and the thermal stresses are calculated using ANSYS Mechanical. Using this analysis technique, start-up conditions can be simulated in a transient environment and boiler design can be modified to ensure the stresses are all within allowable limits before construction begins.



Figure 6: Zoom view of some of the custom compound components illustrating the network that was modelled in Flownex.

 

Flownex can also calculate pressure forces on elbows and pipes: these can be used as an input to mechanical design for the calculation of pressure stress analyses and frequency analyses based on pressure signal (using Fast Fourier Transform).



Figure 7: Flownex and ANSYS Mechanical link: Thermal stresses of a boiler.

 

Conclusion

 

Coupling 3D and 1D codes, the overall computational effort is reduced while the 3D fundamental modelling aspects are maintained at the same time.

It took nearly 20 years to have an efficient environment for such computation. At the beginning, the numerous software editors slowed down the creation of individual links, the second step often involved a third-party tool, but the recent merging of some software vendors shorten the collaboration.

Following this approach, several 1D tools are now linked with 3D tools in a unique simulation environment.

Now the coupling is completely automatic and be can used for sensitivity analyses to improve the system design.

Simulations can be optimised for both accuracy and minimization of run time, providing engineers with a robust set of simulation tools that meet industrial demands.

 

References

 

Soumoy, Vincent (2019). Coupling 1D and 3D CFD: Myth or Reality.: NAFEMS WOLRD CONGRESS, Quebec City (Canada).

 

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