.. _knowledge_base_FAQ:

Frequently Asked Questions
=============================

Where are my Flow360 credentials stored locally?
------------------------------------------------

Your Flow360 API key is stored locally in a configuration file at:

.. code-block:: python

   ~/.flow360/config.toml


Does Flexcompute charge for data storage?
--------------------------------------------

For some data storage we do not charge, but for some data we do. See details:

+---------------+----------------------------------------------------------+
| resource      | storage price                                            |
+===============+==========================================================+
| surface mesh  | free                                                     |
+---------------+----------------------------------------------------------+
| volume mesh   | free                                                     |
+---------------+----------------------------------------------------------+
| case          | 0.00166 FlexUnit/GB/day (or 0.05 FlexUnit/GB/month)      |
+---------------+----------------------------------------------------------+

To reduce data storage costs we encourage users to\:

1. Delete case if results will no longer be needed
2. Download results to local drive and delete case from Flow360

SolverError: Error when calculating patch interpolation coefficients. What does this mean?
---------------------------------------------------------------------------------------------

When running a case with a rotational volume zone, if you get a solver error that looks like this:

.. code-block::

   Error when calculating patch interpolation coefficients. Patch has a distance mismatch of 0.7883948e-01
   with matching patch at location [1.7592032e-1, 1.0184957e-03, -4.7304816e-1].

This means that at a given time step, once the mesh has been rotated according to the prescribed motion, the maximum distance between the two interfaces is 0.7883948e-01. If this maximum distance between the two patches is greater than 20% of the maximum edge length contained in either patch, this error will be raised. It indicates that either the center of rotation/axis of rotation is likely incorrect or that the mesh is too coarse in this location.

.. _relativeVelocityExplain:

What does the "VelocityRelative" mean in the volumeOutput of the case configuration?
----------------------------------------------------------------------------------------

In a rotational domain, the absolute velocity, :math:`\overrightarrow{\boldsymbol{U}}_\text{absolute}`, of each fluid element could be treated as the summation of a relative velocity, :math:`\overrightarrow{\boldsymbol{U}}_\text{relative}`, and a velocity due to the rotating frame, :math:`\overrightarrow{\Omega}\times \overrightarrow{r}`. The "VelocityRelative" means the :math:`\overrightarrow{\boldsymbol{U}}_\text{relative}`:

.. math:: \overrightarrow{\boldsymbol{U}}_\text{absolute} = \overrightarrow{\boldsymbol{U}}_\text{relative} + \overrightarrow{\Omega}\times \overrightarrow{r}

It should be noted that the above relative velocity is zero on any no-slip walls within rotational blocks.