How to extract raw data from OpenFOAM using sampleDict

We often need to take the data on a specific point. In OpenFOAM we can use sampleDict utility.

In the sampleDict file, we can add the following line for extracting data on a set of specific points.

curve // name of the file (can be any name)
type cloud; //type cloud for arbitrary ponits
axis xyz; // the points are scattred in xyz direction
points // list of points
(3.93 0.28 0.7897)
(5.93 0.28 1.2)
(7.93 0.28 2.2)

Sound from high-Reynolds number flow over bluff bodies

– This paper aims to investigate the aerodynamic sound generated from flow over bluff bodies at a high Reynolds number. By taking circular and square cylinders as two representative geometries for the cross-section of bluff bodies, this study aims to clarify the difference in flow formation and sound generation between the two types of bluff bodies. Furthermore, the possibility for a downstream flat plate to be used as sound cancellation passive mechanism is also discussed in this study.

– Sound source from the near field is numerically solved by using the Unsteady Reynolds-Averaged Navier Stokes equations. While for the sound at far-field, the compact sound theory of Curle’s analogy is used.

– Magnitude of the generated sound is dominant by the aerodynamic forcer fluctuations, i.e. lift and drag, where the lift fluctuation gives the strongest influence on the sound generation. The square cylinder emits 4.7 dB higher than the sound emitted from flow over the circular cylinder. This relates to the longer vortex formation length for the case of square cylinder that provides space for more vortex to dissipate. It is suggested that downstream flat plate is possible to be applied for a sound cancellation mechanism for the case of circular cylinder, but it would be more challenging for the case of square cylinder.

Practical implications
– This study include implications for the development of noise reduction study especially in high-speed vehicles such as the aircrafts and high-speed trains.

– This study identified that there is possible method for sound cancellation in flow over bluff body cases by using passive control method, even in flow at high Reynolds number.

email me (researchsukri[at] if you want the copy of the paper.

SnappyHexMesh: How to get a smooth mesh

This post explains briefly how can we get a smooth mesh when using the snappyHexMesh in OpenFOAM 2.0.

The main ingredient is the ‘surfaceFeatureExtract’ utility.

Here are the steps:

1- blockMesh: the BCs can be defined here, example: inlet and outlet patches.

2- extract the surface edge from the .stl file, example: tank.stl
command : “surfaceFeatureExtract -includedAngle 150 -writeObj constant/triSurface/tank.stl features”

this utiliy will create .eMesh file in /constant/triSurface/ : example tank.eMesh

3- in the snappyHexMeshDict, edge refinement is made by referring the eMesh file name, example: tank.eMesh.

4- run the snappyHexmesh utility.



This post is about how to calculate yPlus in OpenFOAM.

1) with wall function; simply type ‘yPlusRAS’
2) without wall function;
i- type wallShearStress
ii- calculate manually: y+ = sqrt(wallShearStress/rho) * y/nu
y = height of the cell near to the wall
nu = kinematic viscosity.

1_ simply type ‘yPlusLES’