# Computational Fluid Dynamics ∇ Fluid Simulation

The scientific field of computational fluid dynamics (CFD), also known as fluid simulation, is related to the numerical analysis of fluid flows or stress analysis in solids.

The basic idea of the numerical analysis is as follows: Use valid and conserved partial differential equations. Discretize the continuous volume into single discrete volume elements and solve the partial differential equations for each volume element until the mathematical system is converged. The discretized volume is also named numerical mesh. The shapes of the volumes can be of arbitrary type.

While solving the conserved equations at each volume element, a detailed solution in 3D is obtained. That means all calculated quantities can be visualized in 3D at each position. Therefore, computational fluid dynamics calculations can be used for optimization purposes. Furthermore, the fluid simulation analysis allows one to get a fundamental and detailed insight into the physical phenomena. One of the main advantages is the calculation of different fields that are not measurable such as turbulent quantities or hard and expensive to measure.

## CFD Simulations

Nowadays, CFD simulations are present in many engineering applications and scientific topics. To get an impression in which fields the computational fluid dynamics is used, a shortlist is presented:

• In-plant constructions CFD analysis is used explicitly to calculate and investigate into DeNOx systems (emission reduction systems). Here, the mixing of the DeNOx reactant and the flue gas is calculated. Also, the uniformity index in front of the catalysts is evaluated. Furthermore, different optimizations are performed to lower the pressure drop within the exhaust gas duct system and much more such as heat-recovery systems and silencer analysis.
• Sound analysis getting more and more critical regarding emission limitations. Explicitly in plant constructions, the silencer is used to reduce noise emissions. There are ongoing developments to couple the fluid flow and the noise generation.
• Chemical and process engineering topics are using CFD simulations to investigate combustion, catalyst, chemical reaction, and multi-phase processes.
• Medicine, pharmacy, and biology. The analysis of dust-free rooms are investigated here (probably using particle loaded approaches). Furthermore, fluid flows through vessels are analyzed, or the airflow during intake into the lung is calculated. Also, in biology, the water transport of roots is investigated.
• In building service engineering, CFD analysis is used to investigate heating and cooling techniques. This field is known as »heat ventilation and air condition« (HVAC). Furthermore, comfort calculations are performed before buildings are constructed. Commonly the norm DIN EN ISO 7730 is used for those analyses.
• Foundry industries use CFD simulations to investigate into the casting and solidification process. Both fields are highly complex. Here, different analysis is performed, such as thermal stress calculation, segregation effects, hot-topping effects, particle loaded flows, and clogging of nozzles. The casting simulations are incredibly complicated, and many phenomena take place simultaneously..
• Computer science and semiconductor technology. Here, different analysis is performed. Especially, thermal stress analysis is critical in semiconductors as well as cooling scenarios for CPU's, GPU's, and other electronic components.
• Furthermore, CFD simulations are commonly used to investigate particle loaded flows, injection of liquids into gas streams, evaporation, heat-transfer, fluid-flows in general, hydraulics, combustion, chemical process, and to understand the physics in detail. The field in which computational fluid dynamics can be applied is almost unlimited.

All above mentioned application topics are versatile and therefore, CFD analysis can be used in a lot of under-disciplines.

## Physical Effects and Optimization

CFD simulations are not used only to optimize the design or different parameters. The analysis of the numerical results also allows engineers and scientists to get a fundamental understanding of the fluid and particle processes that happen in the analyzed geometry. Especially in scientific research, the essential knowledge of the underlying physical phenomena is critical to develop new models or formulations.

Furthermore, optimization algorithms can be used to automatically optimize different parameters. These parameters are changed based on an individual object function (or different object functions). For example, the free software package DAKOTA allows one to use different optimization algorithms on the fly.

## Visualization of the numerical quantities

During the numerical evaluation of the equations, a bunch of numerical data is saved. The visualization of those data is analyzed after the calculation during the so-called post-processing step. Different applications can be used, such as ParaView. It allows one to get a detailed insight into the numerical data while various manipulations are possible. Adding glyphs, streamlines, contour plots, or threshold filters are common practice. The visualization allows one to get relations and correlations of different phenomena that can be used further improve the design, or sometimes new insights can be achieved.

The analysis and preparation of the numerical data can be done in 3D at each position one wants to check. It allows the visualization of non-measurable quantities, and thus, fundamental correlations can be derived.

As mentioned before, the visualization options are unlimited. Thus, CFD simulations are unique due to their representation of the calculated data. At the same time, the numerical analysis is much cheaper compared to experimental investigations.

The video above shows the visualization of the moving water column (top) and the interface between water and air (top). Furthermore, the velocity field is shown. The high turbulent and complex flow field can be visualized easily.

## Validation of the numerical results

Numerical simulations are based on various assumptions, models, and simplifications, which usually have to be validated and verified for new topics. This ensures that the numerically obtained values ​​are reliable and correct. The validation of the results is particularly necessary when new models are developed and then integrated into software packages.

Validating the results ensures that the calculated data is reliable. Many years of experience in the field of CFD analysis helps with the consideration of the numerical data; especially in the plausibility analysis.

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