How to refine a 2D cylinder blockMesh

For the audience who do not know what a 'blockMesh' is, it is a Mesh tool which comes along with the open-source software called 'openFoam'. Many university students and researchers have begun using openFoam for their flow simulation problems. There is another mesh tool called the 'snappyHexMesh', which is normally employed to mesh complex geometry for the flow simulations.
In this post, although, I will restrict the information only to a 2D cylinder mesh - that is, a 2D mesh housing a cylinder. I will largely talk about how to play around with parameters in the blockMeshDict in order to obtain a refined mesh. I will also briefly talk about the turbulence models that can be used to run a high Reynolds number simulation on a refined grid.

If you are wondering where to start or where to access a case file, you can check the following website: http://www.wolfdynamics.com/. They have an extended compilation of case files and tutorials working up to almost 1.5 GB. If you are a beginner and are looking for a place to download and work on openFoam files, the website will do you justice. If you are an expert in openFoam and still require some insight into complex meshing and simulations, this again is a perfect place to look into. Having said that, the case folder to download from the website is named as "vortex shedding". This folder, in turn, is sub-divided into 12 cases, each of which is distinct in terms of solvers used and the refinement assigned. 

A coarse 2D mesh looks somethings like the following:

The above is a 2 dimensional mesh, and of course if you wish to make a 3D mesh, all you have to do is extend the co-ordinates in the Z-direction. This mesh as such is made up of several blocks, each of which are represented by a set of co-ordinates assigned inside the blockMeshDict. The case file is again available to download from the above mentioned website. The mesh can be used to perform simulations to study the force Coefficients, whose medium is Air. If you are contemplating on running a cylinder simulation with water as the medium, it is better to start refining the mesh and extending the domain in the X-direction.
The most important refinement parameters given inside the blockMeshDict for a coarse mesh can be represented below as:

If you are planning to refine the case, do so by playing around with the above parameters. Once done, you may be able to achieve a refined mesh, whose grading parameters can be the following:


And now the mesh domain as viewed in the paraFoam looks like the image attached below.


The above image may well be misleading because the thick lines in the domain aren't lines itself, but are very closely packed cells. The above mesh has a Y+ value of 60, and if you are planning to run the kOmegaSST turbulence model, you still might require further refinements to bring the Y+ value down to anything less than 1. But instead of refining the above mesh any further, I had used a scaling command to reduce the cell domain, which automatically brought down my Y+ value. The command is : transformPoints –scale “(0.015 0.015 0.015)”
The kOmegaSST model proved unsuccessful because the Clrms value for a Reynolds number of 1*10^6 was high. The error may be due to the fact that one did not use a 3D mesh for the kOmegaSST turbulence model without a wall-function. And so the scaling factor was scraped and a k-Epsilon was used along with a wall-function, in which the y+>30. Now, the simulation produced good estimates for the force Coefficients when compared with the experimental results. For more information, please check out the following video tutorial and contact me at misaacmark24@gmail.com for the case files. 


In the consecutive posts let us explore the mesh tool 'snappyHexMesh'. Thank you!

Comments

Post a Comment

Popular posts from this blog

snappyHexMesh tutorial for a simple Geometry

Image
In this section I will address how to use the snappyHexMesh tool to mesh a Cylinder. We can either create an stl file for the cylinder - place it inside the  triSurface/constant  folder and call the same into the background mesh, or a simple cylinder can be directly created by using the 'searchable cylinder' parameter within the snappyHexMeshDict.  cylinder     { type searchableCylinder;   point1 (0 0.0 -0.5);  point2    (0 0.0 0.5); radius 0.01585;     } A refined mesh can be viewed below. The radius of the cylinder is of the order e-02 which gives us some insight into what the cell size for the back ground mesh can be. The blockMesh for the background mesh will be addressed later. Next in line in the snappyHexMeshDict is the Refinement box parameter. This box defines how refinement around the cylinder and importantly how the wake refinement can be achieved. The refinement box separates the finer refinement from the background mesh grid.  refin
Read more

snappyHexMesh Tutorial For a Complex Geometry and External Aerodynamics

Image
The geometry chosen for external fluid dynamics simulation is that of an offshore structure. In that, a deep-draft semi-submersible structure  is scaled scaled down to a value  of 0.01 (scaled down by a factor of 100). The semi-submersible structures are employed at locations where the oil extraction from the sea bed is considerably deep. These are movable structures which are half submerged during application. A working deck comes right on top of the structure where the cranes and oil rigs are fitted. The semi-submersible consists of four hull appendages which connect the working deck with a pontoon structure. The pontoon structure gives buoyancy to the semi-submersible in deep waters but does not come directly in contact with the ocean bed.  Figure No.1: DDS The effects of flow induced vibration on the above four columns during high flow currents is indeed the case of interest. Once a good mesh is achieved, the platform is set for the study on the effects of Vortex Induced
Read more

AIRCRAFT OpenFOAM SIMULATION (BOEING 747)

Image
Before I begin to furnish the results, a quick look at the computational resources that were utilized for the simulation. I possess a minimal capability laptop manufactured by Hewlett-Packard, which runs on 2 core(s) and a 4 GB RAM. The objective of the simulation is to set up a 3D mesh for an Aircraft flow simulation with a resolved boundary layer of Y+ < 1, and to study the Residuals . The kOmegaSST turbulence model was adopted. The STL file downloaded online is a scaled Boeing 747 model, with the elevator tabs deflected downwards. Figure No.1: Rear View The cell size in the blockMesh is 0.05, owing to the fact that a scaled 747 model was selected, otherwise a cell size of 0.5 can be executed for an un-scaled model. Regardless, any of the following cell size can be applied (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0). Four boundary layers were generated with an expansion ratio of 1.2,  adding to the total cell count of over 3 million cells. A steady-state solver
Read more