Frequently Asked Questions

‍

Just an internet connection. You will be able to access the platform and run your optimizations via your internet browser. The software platform runs on secure HPC resources that we provide. We do not expect our clients to have the computing power available in-house. For postprocessing, a discrete GPU is recommended. This will make the 3D visualization of your results easier. You can both store the results on the platform and download them to your own storage. Be aware of the fact that you need to have enough storage space if you would like to download the results.

‍

Yes, the designs are manufacturable. Manufacturing constraints can be included in the case setup. The software supports various manufacturing techniques, and constraints depend on the technique. It is important to note that the manufacturing constraints are defined mathematically in our software, and as engineers we recommend to critically appraise the resulting design, since minor errors may occur. You can set constraints for various manufacturing techniques like CNC milling, sheet metal forming, 3D printing, die casting, stamping, injection molding and much more.

‍

No, the software is not restricted to just electronics or even thermal problems. It can design components for any application that requires thermal management or flow distribution. Anything that produces heat or that involves flow is within the software’s scope. In summary, all thermal and flow problems can be solved. Industries that are using our software include but are not limited to automotive, laser technology, aerospace, home appliances, medical imaging, etc.

‍

The designs are created using a topology optimization approach. In topology optimization, contrary to shape optimization, there is no need for an initial design input. This allows for a wider search on the design space, thus increasing the probability of finding better performing and more innovative designs.

‍

In CFD analysis, the accuracy of the simulation is closely related to the resolution of the mesh. Broadly speaking, smaller mesh elements (higher resolution) can capture smaller physical phenomena, thus increasing the accuracy of the simulation. On the other hand, simulations with a higher resolution will require more computational resources (cores, memory, time).

In Diabatix Coldstream, once a base resolution is chosen, the mesh will be automatically generated, taking into account not only physical phenomena, but also geometric inputs to provide the best possible ratio of quality to computational cost.

‍

It is easy to extend the logic of the previous FAQ to the design creation. A better mesh generates better data that is used to generate more accurate designs.

It should also be noted that the resolution of the mesh will determine the minimum allowable feature size, meaning that the smallest quantity of material that can be added or removed depends on the size of the cells in the design region.

In order to meet the user input, Diabatix Coldstream will automatically define the resolution of the design space based on the minimum feature criteria entered.

‍

The duration of the design run depends on the inputs of the case. If the design region is very big and one selects the smallest feature size, Coldstream will automatically generate a mesh with a very high resolution. Coldstream does this to make sure the user input is followed. It is very important to have a good feature size to design region ratio. You can contact the support engineers to give you more guidance on which feature size to select for your design case.

This ratio in combination with the amount of credits selected, can result in a very demanding CFD problem with the resources available. This translates to a longer running time. In conclusion, assigning more credits to a design case will decrease the computational time.

‍

Diabatix Coldstream is based on an incompressible flow solver, therefore, at the moment supersonic flows are not allowed on the platform. Liquids are generally accepted as being incompressible, but this is not the case for gases. A good rule of thumb to take into account when setting a forced or mixed convection case is that the Mach Number should be < 0.3 ( ~110 m/s for air at standard sea level conditions). If Mach > 0.3, the compressibility effects cannot be ignored and the results of the simulation will not be accurate.

‍

Diabatix Coldstream can do external aerodynamic optimizations, however this is not its main forte. Our Diabatix sales and support engineers can advise you on how to set up each specific case.

A possible approach can be, if the goal is to reduce the drag forces, to create a “windTunnel” and setting the objective to powerLossMinimization between the inlet and outlet. Since Diabatix Coldstream uses a topology optimization approach, the design region has to be smartly chosen in order to allow not only addition, but also removal of material from the object being optimized.

‍

Turbulence is the chaotic movement of the fluid. This chaotic movement happens on different length scales throughout the entire fluid domain, in turbulence we speak about "eddies" when talking about this chaotic movement. There are thus eddies of different length scales. Turbulence models, as the name suggests, try to model the turbulent behavior of fluids in motion while maintaining the computational cost at an acceptable level. An oversimplified explanation is that turbulence models mimic the very small details of the flow that cannot be captured by the mesh.

If the computational resources allow, a DNS (Direct Numerical Simulation) approach will resolve all the details of the flow, and there is no need for a turbulence model. These simulations are normally restricted to simple academic problems and are not feasible for industrial simulations.

For slightly more complex simulations, normally restricted to one region (e.g. aircraft components, flow in a pipe, ...), the turbulence can be modeled using LES (Large Eddy Simulation) models, which as the name suggests resolve the largest eddies while modeling the smallest features (on both length and time scales). These models can be seen as a filter, and to achieve good results require very specific meshes that can become prohibitively expensive to simulate. Therefore, for industrial applications, they are reserved for very sensitive flow problems, where the turbulent behavior of the flow plays a major role (e.g. aeroacoustics).

Besides the LES models, there are the RANS (Reynolds-Averaged Navier-Stokes) models which are the industry standard. These models, as the name indicates, take an average of the full spectrum of fluctuations. As the turbulent behavior is fully modeled, the mesh can be coarser than for the LES simulations,. This allows to focus the computational power on problem scale and complexity (e.g. multiphysics, full industrial machinery, combustion, ...)

Both LES and RANS models are extensively covered by the literature and validated against experimental tests with very good results.

Diabatix Coldstream has several different RANS models available:

• KOmegaSST
• KOmega
• kEpsilon
• RNGKepsilon
• Laminar

‍

The turbulence model selection depends on the problem at hand. As a rule of thumb:

• kEpsilon - One of the most commonly used models, it tends to better predict the flow away from the walls and tends to be quite robust. It is mainly used for external aerodynamics flows and fully turbulent flows.
• kOmega - Tends to perform better close to walls and for low Reynolds flows than the kEpsilon model, and can predict separation.
• RNGkEpsilon - a variation of the kEpsilon model with a mathematical approach that attempts to take into account different length scales to model the eddy viscosity, instead of a single one.
• kOmegaSST - This model is a variation of the kOmega model and can be seen as an hybrid between the kOmega and the kEpsilon model, taking into account the best of both, uses kOmega near the walls and switches to kEpsilon away from the walls. It is the default turbulence model used in Diabatix Coldstream.
• Laminar - for very low Reynolds flows or purely laminar flows. A low Reynolds number means that viscous forces are much more important than inertial forces. The high viscosity can dampen out the chaotic movement, so there is no turbulent behavior in the fluid.

‍

Diabatix Coldstream uses a topology optimization approach, meaning it can transform one material into another, thus creating a new design. If a design is already present there are a number of options:

1. First option: you strictly want to remove material from the existing design. The design region will be the original design, meaning that material can be removed (transformed into another material).
2. Second option: you strictly want to add material to the existing design. The design region will be the region attached to the original design, thus allowing for material growth.
3. The third and final option: a combination of the two previous options. The design region must partly overlap with the existing design and partly be made up of the domain attached to the existing design. In theory, Coldstream will be able to remove material and add material to the existing design (depeding on the targets chosen, it is possible that only one of the two manipulations is performed).

‍

The number of objectives is not limited to 1, so multiple objectives are possible. In order to control the relative importance of the objectives, the weighting factor can be set to a value in the range [0,1]. The values of the different weighting factors do not have to add up to 1, this relative weighting is done automatically by Diabatix Coldstream.

‍

No, once the simulation is stopped it cannot be restarted from the point where it was killed. A new optimization should be submitted. If you press the 'Stop calculating'-button, all data is irretrievably deleted.

‍

The fanInlet and fanOutlet are boundary conditions that model the behavior of a fan, meaning that they alter the momentum of the flow based on a performance curve.

Diabatix Coldstream provides the user with a large library of performance curves for fans currently on the market, however the user is free to create their own fan. In order to do this, a set of at least 2 pairs of points representing volumetric flow (m^3/s) and total pressure loss (Pa) should be provided. Please note that the units should be in S.I., and that the total pressure should be used and not the static pressure.