General Overview


Computational fluid dynamics (CFD) is a general term that describes an application of numerical analysis and data structures to solve and analyze problems that involve flows, heat exchange and interactions of liquids and gases with each other.
CFD-Simulation can provide the following results:
  • 3d-fields (2d-fields) of variables like pressure, velocity, turbulence, temperature, phase fraction;
  • streamlines and particle trajectories;
  • averaged and integral values like drag and lift coefficients, heat flux, pressure drop.
Pressure Field
Velocity Vectors
Temperature Field
Streamlines

Steps in CFD-Simulation


The whole process of the CFD-Simulations can be divided into the following parts: Geometry Creation, Mesh Creation, Pre-processing, Calculation and Post-processing:
  • Geometry creation - establishment of the 3d-model of the investigated object using customers' plans and drawing;
  • Mesh creation - a division of the established 3d-model into millions (usually not more than 30 million in total) of small numerical elements. In other words, it is a discretization of the numerical space. Consequently, during calculation, it will be assumed that within a volume of each numerical element all the calculated variables (pressure, velocity, temperature) are constants.
  • Pre-processing - definition of all physical models, properties, initial and boundary conditions that will be used in calculation;
  • Calculation - an iterative recomputing of all the numerical elements' variables (pressure, velocity, temperature)  under the influence of the boundary conditions.
  • Post-processing - visualization of the scalar or vector fields of variables, streamlines, surface and contour plots; evaluation of the surface-integral and volume-integral values.
Geometry and mesh creation need around a half of the whole project time (up to 60 % of the project time). Definition of the boundary conditions needs another 10 %, Calculation requires 20 %, rest 10 % belong to the results evaluation.

Competence


Currently, I am able to complete the following tasks:
  • simulation of the air flows inside and outside of the buildings;
  • simulation of the fluid flows inside the mechanical equipment;
  • tasks that involve particle distributions in fluids;
  • simulation of the mixing processes and chemical reactions (gas burners);
  • transient simulations with changing boundary conditions and geometry.

Quality


Reliableness of the results obtained with CFD-Simulation depends on several factors:
  • the correctness of the boundary, initial conditions, and accepted simplifications;
  • right choice of the turbulence, radiation, multiphase etc. models, which can recreate the specific physical processes;
  • numerical scheme and relaxation factors;
  • mesh of elements that is used for calculation. In simulations, only hexahedral meshes are used to improve stability and quality of the results (an example is shown on the right).
However, even if all mentioned requirements are met, it will not guarantee that the simulation results are valid. Currently, the only way to prove the model is to simulate a similar case, where the results are already measured or reliably assessed. Model validation is the first step of each CFD-Project. It is not needed only in case if a similar simulation was already done before.

Software


Till current time I has been working with the following software:
  • Ansys CFX
  • Ansys Fluent
  • FDS
  • OpenFOAM
Experience shows that the reliableness of the results depends more on the engineer skills than on software. The advantage of the commercial packages is the speed, with which a certain task can be fulfilled. However, with the increase of the task's difficulty, the speed difference between a commercial and an open-source is decreased.
Tasks for private customers will be fulfilled using OpenFOAM.

Experience


During my 6 years of working experience I have simulated the following tasks:
  • Operational security of the recooling plant on the top of the data center in emergency cases, Zurich area (service project)
  • Cold air downdraught in the canteen with high ceiling and inductive air outlets,  Basel (service project)
  • Drag and lift coefficients of the trawling doors, Island (research task)
  • Cold air downdraught in the canteen with high ceiling and floor convectors,  Ebikon (service project)
  • Ceiling heating panels in the administrative building, Basel (service project)
  • Mechanical ventilation of the supermarket customer area, Zurich (service project)
  • Accumulation of the snow on the cable railway station, Zermatt (service project)
  • Air movement in the atrium of the Chocolate Center, Kilchberg (service project)
  • Schemes of transient natural ventilation of the room (research project)
  • Thermal state of the ceramic reactive engines of the microsatellite on the earth orbit (service project)
  • Thermal state of the nanosatellite on earth orbit (research project)

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