Simulating Unsteady Valve Motion Using Overset Mesh in STAR-CCM+

Simulating Unsteady Valve Motion Using Overset Mesh in STAR-CCM+

Introduction

To simulate the motion of an unsteady valve in STAR-CCM+, the overset mesh plus DFBI (Dynamic Fluid Body Interaction) technique can be employed. This article provides a concise guide on setting up an overset mesh simulation, focusing on the key steps and considerations for achieving accurate results. 

Using Overset Mesh to manipulate the valve involves creating two distinct regions: an Overset Region housing the moving valve disc and a Background Region. When dealing with models featuring tiny gaps, Overset Mesh becomes crucial due to the requirement of at least four layers of complete cells between the background and overset boundaries. This might result in a significant increase in cell count. It's essential for Overset and Background Regions to maintain similar-sized cells near the Overset Boundary, leading to potential increases in both cell count and runtime, especially for very small gap problems. However, this approach eliminates the need for remeshing or moving all vertices, ultimately saving simulation time.

Overset Mesh Basics

The overset mesh approach involves creating two distinct regions - an overset region containing the moving valve disc and a background region. For models with small gaps, a minimum of four layers of complete cells is essential between the overset and background boundaries, potentially increasing the cell count and runtime. It is crucial to ensure similar-sized cells near the overset boundary to optimize simulation efficiency. Please reference the user guide for the general workflow for using an overset mesh.

Tutorial: Overset Mesh Small Gap Modeling - Lobe Blower

The tutorial illustrates the workflow for an overset mesh simulation modeling air flow through a lobe blower. Rigid body rotation is applied to overset mesh regions surrounding each lobe, with prism layer shrinkage handling small gaps. This mechanism redistributes prism layer cells in narrow spaces, maintaining mesh quality.

Creating Overset Mesh Interfaces with Prism Layer Shrinkage

During simulation, variable values are exchanged through interpolation between meshes. Overset interfaces are crucial for associating background and overset regions, as well as between multiple overset regions. A minimum of two to three active cell layers is required for overset mesh modeling to resolve gaps between wall boundaries.

Defining Overset Topology

If the overset boundary is not closed, the overset hole cutting algorithm requires guidance to determine active parts. Setting up overset topology instructs the algorithm to identify the overset boundary during the simulation.

Setting up Overset Regions

In a typical overset simulation, one or more overset regions overlap the background. Alternatively, multiple overlapping overset regions without a background are possible. For a single overset region overlapping a background, designate the outer boundary as the overset mesh type. In cases with multiple overlapping regions without a background, outer boundaries remain as wall types, requiring the activation of dynamic overset behavior.

Mesh Preview Run

Before running a full transient physics simulation, a mesh preview run is recommended. This simulates mesh motion without physics models, allowing users to analyze and verify the defined motion, troubleshoot potential errors, and optimize settings. The unsteady solver with a large time-step is employed for this purpose.

By following these guidelines, users can effectively simulate unsteady valve motion using overset mesh in STAR-CCM+.

Additional helpful resources

  1. Multiple Walls in Contact - As the walls of the objects within the overset region converge with the background region's wall, they come into contact, resulting in the obstruction of flow between the object walls. Additionally, multiple objects within overset regions may approach each other, gradually closing the gap between them.

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