H&S Simulation and
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3.5 Advanced Concepts
Users can model advanced flow characteristics (rotary zones or porous media…), which would otherwise require large meshes or sophisticated geometries leading to expensive simulations. This modelling can be done through the Advanced concepts feature (most relative to our project):
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Momentum Sources: Momentum sources can be used to simulate ventilators, fans, and other similar devices without having to model the exact geometry and motion of the device.
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Passive Scalar Sources: Passive scalar sources can be used to simulate the propagation of species like smoke from a burning car in a garage or the diffusion of dust or pollutants in a tunnel. This is only valid if these species get transported with the fluid without having any effect on the flow physics.
Figure 6: Advanced Concepts
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3.5.1 Numerics
Users can set the equation solver for the simulation, change the convergence settings or choose the time integration scheme for dynamic simulations. These settings mainly influence the stability and efficiency of the simulation run, but also the quality of the results may depend on these settings.
In most cases, the standard settings should be a good choice and should not be changed without reason.
However, all numerical settings are made available for users to have full control over the simulation. These settings are divided into three categories:
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Properties: All properties regarding the iterative solvers of velocity and pressure equations are set here. Relaxation factors, residual controls, and solver-specific tweaks are among these settings. However, depending on the solver (e.g. PIMPLE, PISO, …), these settings will be adjusted. For each field, a Help message is provided on the platform.
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Solver: In this part, linear solvers used in computing each variable could be chosen separately. Upon choosing a solver, a set of preconditions/smoothers and their tolerances become available. To assist with selecting the best solver, a Help message is provided for each field.
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Numerical Schemes (the default selections are the best choice): These schemes determine how each term in the governing equations should be discretized. Schemes categorized in the following groups:
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Time differentiation
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Gradient
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Divergence
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Laplacian
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Interpolation
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Surface-normal gradient
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3.5.2 sIMULATION cONTROL
Figure 6: Simulation Control
End TimE - Steady state
Steady state simulations are time-independent, that is, the equations solved do not include time derivatives. Hence, End time here represents the end of the simulation. No more iterations will be executed beyond this value.
Delta T - Steady state
For steady state simulations, Delta t represents the iteration step size. In other words, it reflects how aggressively you want to reach the end of simulation. It is also referred to as a pseudo-time step. Hence,
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑡𝑒𝑟𝑎𝑡𝑖𝑜𝑛𝑠 = 𝐸𝑛𝑑 𝑡𝑖𝑚𝑒 /𝐷𝑒𝑙𝑡𝑎 𝑡
(Although there can be different combinations of End time and Delta t for the same Number of iterations, it is advised to keep the step size small, usually 1)
Write Control and Write Interval
Under Write control, the user can select different methods to specify the frequency with which the simulation results will be written. The frequency is specified under Write interval.
The different methods are:
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Time step: Specify the number of time steps to be skipped between two successive writes of results. For example, A write interval value of 3 for a time step length Delta t of 2 seconds means that the results will be written every 3 x 2 = 6 seconds.
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Clock time: Clock time means the actual time or real time. Specify an interval in seconds of this real time which will act as a gap between two successive writes.
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Runtime: With Runtime, data can be written every specified interval seconds of simulated time.
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CPU time: It represents the amount of time for which the CPU is used to process instructions during the simulation run. Hence, the specified interval will be in seconds of this CPU time.
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Adjustable runtime: It is the same setting as Runtime although the time step Delta t may be adjusted to match with the write interval.
Number of Processors
One of the biggest advantages of using SimScale is that simulations are run in parallel. This means that different parts of the simulation domain are allotted to different cores and each part is run simultaneously. This speeds up the computation time.
mAXIMUM rUNTIME
Here, the maximum time limit can be set in real time after which the simulation will be terminated automatically irrespective of the value set under End time. This setting is important since it helps to control time expenses, especially during the initial iterations.
Potential Flow initialization
Potential flow initialization works by solving a pressure equation given the velocity initial and boundary conditions, which often provides better initial conditions for the simulation. Toggle-on this setting to accelerate convergence and improve stability during initial time steps if you are experiencing these problems.
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