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# What Are Core Hours?

This article explains the concept of core hours and its importance for the user.

Core hour is a unit that SimScale uses to know how much resources were used for simulation. This depends on how many cores were used and how long it took to finish the simulation. So, 1 core hour is consumed when a simulation was run on 1 core and it took 1 hour to finish it. You can calculate this with the formula given below:

$$Core\,Hours = Time\ required\ to\ run\ simulation \times Number\,of\,cores\ used$$

Hence, for two simulations with the same number of cores assigned the one that runs longer naturally ends up consuming more core hours.

There are several factors that influence the number of core hours that you use for simulation. You can read these factors below:

### Initial Conditions

They define the starting values for each solution field. Initial conditions that are realistic and closer to the expected values can accelerate your simulation, which in turn reduces the core hours that you use. Values from experiments or from other simulations can be used as your initial conditions.

### Mesh

Finer mesh means increased cell count and thus increase in the computational expenses. Although the accuracy of results increases with mesh density it also uses additional cores.

The ‘Automatic’ number of processors will choose the most economical number of cores for your simulation. However, you can manage the resources you use under Simulation control and assign a suitable number of cores. For example, if you need to run your simulation faster, you can assign up to 96 cores (only available for users with the professional plan).

It is also important to remember that the number of cores also relates to the amount of memory available for the calculation. So, be sure to select a suitable number of cores so you will not run out of memory.

### Result Control

Result control allows the user to get additional results out of the simulation at a specified write frequency. However, the more results you want to output the more data needs to be saved consuming more core hours. Therefore, be sure to only use the necessary result outputs.

### Numerical Settings

The numerical settings can also influence the number of core hours that you use:

• Relaxation factors: These control the stability and speed of convergence of a simulation. Higher relaxation factors will accelerate the convergence of your simulation, which in turn reduces the core hours burned. However, this will introduce higher fluctuations which can cause inaccuracy in the results or lead to divergence.
• Solver tolerances: Tolerances control the residuals of your simulation. The lower the residuals, the better the results, however, to achieve low residuals more computational resources are required, resulting in more core hours used.
• Numerical schemes: They determine how the equations relevant to your simulation will be solved. First-order schemes are less accurate but will use fewer core hours, resulting in faster convergence as compared to second-order schemes.

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