Carlo Brunelli

Synthetic Eddy Methods (SEM) are a family of techniques used to generate realistic inflow data for Large Eddy Simulation (LES) simulation in fluid dynamics. This is one of the easiest and less memory-consumption inflow generators method. The package **SyntheticEddyMethod.jl** implements the SEM and recreates a statistically correct and coherent turbulent flow.

The generation of inflow data for spatially developing turbulent flows is one of the challenges that must be addressed prior to the application of LES to industrial flows and complex geometries. The specification of realistic inlet boundary conditions is widely acknowledged to play a significant role in the accuracy of a numerical simulation. Reynolds Averaged Navier Stokes (RANS) approach needs only mean profiles for the velocity and turbulence variables, making the definition of inflow data relatively simple but the results less accurate. The generation of inflow data is much more difficult in **large-eddy** and **direct-numerical simulations**. The SEM has to be adequately tuned in order to be dependent on the turbulent intensity and to trigger the transitions to turbulence over the tested geometries in order to reproduce the experimental data. It is not advisable to use a generic white noise for creating turbulence mostly because it generates incredibly high local gradients and so the turbulence is going to be completely dissipated. 
This package in a few lines allows the user to obtain a time-dependent statistically correct and coherent turbulent flow. The problem is initialized by the user setting the dimension of the inlet, the mesh size and turbulence intensity.
In the program logic, the eddies, which are coherent fluid structures, resembling the concept of vortex, are generated in a **virtual box**. This is a computational domain placed in such a way that the surface of the inlet is contained within it. In this environment, eddies are just translated, and the properties of the flow are computed on the inlet surface. 
The instantaneous flow field is saved in a cache object and is updated when the user wants to advance of one time-step. Due to the customizable size of the **virtual box**, is it possible to use this package also in case of curved surfaces. 
The flow properties are computed only on the points of the inlet surface. Each eddy is characterised by a single coordinate set defining the centre and an intensity level (+1, -1).
The creation of synthetic eddies relies upon mathematical and statistical definitions. More advanced users can also select between different mathematical functions adopted to generate the eddies (at the moment just the tent and Gaussian, more can be easily coded).
It has been realized in Julia because, at the moment of the development, such a method was implemented only in other programming languages. In comparison with other similar projects (in Python or Fortran), there is less code and the use is straightforward. Furthermore, it is really close to the mathematical notation and it makes it easier to understand by future contributors.
Finally, the results of the package have been tested. The spectral analysis shows that the flow field generated shows the expected turbulence characteristics. The final achievement will be to couple the turbulence generation with the FEM solver for incompressible flows.

In conclusion, this package can be widely adopted in the field of fluid dynamics for obtaining more realistic and accurate results.

Package repository: https://github.com/carlodev/SyntheticEddyMethod.jl

the new XGBoost wrapper

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SyntheticEddyMethod.jl for fluid dynamics

In field of aerodynamic is often required to test different geometries, most of them are standard airfoils. The aim is usually to identify promising geometry for a specific application. Many operations repeat mechanically, and are tedious and time consuming. In specific, the mesh creation is a fundamental but routine task, similar but not identical for all the airfoil and all the test cases. This is where GmshAirfoil.jl comes in handy.

It allows the user to create in a few clicks a .geo file containing all the information for the open-source widely used GMSH software to create properly structured meshes. Before exporting the mesh, the user can visualize the preview, and modify almost all the parameters (number of divisions and or the growth ratio in the inlet, airfoil surface or shear ...) and see the new results in real-time. The package also adds the physical tag (inlet, outlet, airfoil and limits) to the entities relieving the workload from the user. The mesh created is compatible with the most popular CFD software (Fluent, STARCCM+). In reality, this package, was born to create airfoil meshes that are compatible with the FEM package Gridap.


The primary purpose of this package is to create a .geo that can be read by GMSH. It has been decided to write a .geo file and not directly the .msh file for three main reasons. Firstly, the .geo file is easy to read and interpret. In this way, the user can perform some easy changes directly on it, without the need to dig deeply into the code. Secondly, it can be imported into GMSH, all the parameters can be modified so the user can visually verify that the mesh is good enough for its purpose. Lastly, the API of GMSH for Julia at the moment does not provide all the features that are needed. The package is simple but quite powerful, it provides basically only two functions:
-	from_url_to_csv: the user can browse on the airfoiltools.com website looking for the profile to analyze. It is the biggest database of airfoil online. As the name of the function suggests, it creates a .csv file where the airfoil points are stored. This function comes in handy when the user does not have the file with the points of the profile.
-	main_create_geofile: is heart of the package. It allows to create the .geo file. Here its plenty of optional argument that the user can specify to achieve a results closer to his needs:
- Reynolds number: this is used to compute the boundary layer characteristics over the airfoil. An optimal combination of meshing parameters is defined by employing some empiric formulas and setting some constraints on the number of cells in the boundary layer and the growth ratio. In the boundary layer, a sufficient number of cells is needed (at least 30 in a turbulent case), a growth ratio G between 1.05 and 1.2, and we have to ensure that y+ is approximately 1.
- First layer height: sometimes this is the value that the user wants to ensure. Usually, it comes from previous simulations or other authors work.
- Chord: is set c=1 by default, but this value can be overrided.
- Dimensions: 2D or 3D.
 - Elements: TRI/TETRA or QUAD/HEXA, can be the shape of the single element.
We have not specified yet because this mesher is defined as "smart". As shows in the previous section, it automatically compute the optimal mesh characteristic requiring no additional work form the user. Furthermore, it is able to distinguish the trailing edge (rear part), the leading edge, the suction side (top part) and pressure side (bottom part). It automatically fixes two point close to the trailing edge, one on the suction and one on the pressure side, in order to create the inlet region.

It can detect the shape of the trailing edge, that can be sharp or not, and so the mesh will be different. Meshing the trailing edge has always been troublesome for engineers due to its strong curvatures and strange shapes. 

For a 3D mesh, the user can specify the boundaries surfaces normal to x-axis to be periodic.

The mesh, once the .geo file is open in GMSH, is highly customizable. The user can specify the number of nodes in the inlet section, over the airfoil, over the vertical lines, in the shear, as well as the geometrical growth of the cells. The user can also define the length of the domain. Furthermore, it creates a refinement region close to the airfoil in order to customize the parameters to have a better-refined mesh.
The user can also specify the angle of attack of the profile, which is a fundamental parameters for evaluating airfoil performance. By default it is set to zero. Increasing the AoA leads the airfoil to rotate, as well as the shear and refinement mesh.
Class Shape Transformation (CST) is also implemented. It allows increasing the number of points defining the profile.

Carlo Brunelli

Talks:

SyntheticEddyMethod.jl for fluid dynamics

Airfoil meshing automatization with AirfoilGmsh.jl

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