.. _python_API_unsteady:

.. currentmodule:: flow360

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Unsteady Simulations
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Unsteady (time-accurate) simulations capture the dynamic behavior of flows that change significantly over time. Flow360 provides powerful capabilities for resolving transient flow phenomena with high fidelity and computational efficiency.

.. grid:: 1

    .. grid-item-card:: 🌀 2D Cylinder Vortex Shedding
        :link: 2D_cylinder
        :link-type: doc
        
        Learn how to set up and analyze vortex shedding behind a 2D cylinder, a classic benchmark for unsteady flow phenomena.

When to Use Unsteady Simulations
===============================

Unsteady simulations are necessary when:

* The flow field changes significantly with time
* You need to capture periodic or transient phenomena (vortex shedding, buffeting)
* The problem involves large separated regions or wake interactions
* You're analyzing acoustic phenomena or aerodynamic noise
* You're investigating dynamic stability or fluid-structure interaction
* The Reynolds number exceeds critical values for your geometry

Key Unsteady Simulation Concepts
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* Setting appropriate time step sizes based on flow physics
* Understanding physical vs. dual time stepping approaches
* Determining sufficient total simulation time for statistical convergence
* Selecting output frequency for capturing transient phenomena
* Setting subiterations per physical time step

Time Stepping Best Practices
==========================

* **Time step size:** Small enough to resolve the highest frequency of interest
* **Statistical convergence:** Run for multiple periods of the slowest phenomenon
* **Physical time steps:** Typically 50-100 steps per period of interest
* **Subiterations:** Sufficient to reduce residuals by 1-2 orders of magnitude per time step


.. toctree::
   :hidden:

   2D_cylinder