Computational Fluid Dynamics: Solving Problems with Fluid Flows

Computational Fluid Dynamics: Solving Problems with Fluid Flows

What is Computational Fluid Dynamics?

When designing a piece of equipment or designing a processing system that involves liquids or gasses, it is often difficult to assess exact specifications without knowing how those liquids and gasses will behave. In these cases where fluid behavior is a major factor in design, an understanding of fluid mechanics—the analysis of how fluids behave in response to forces exerted upon them—is crucial. While testing a virtual prototype utilizing computational fluid dynamics, engineers can analyze: the turbulence in fluid as it flows, force exerted on equipment by gasses, possibilities of changes in state, dispersion, ill effects, cavitation and more.

The stock definition of computational fluid dynamics (CFD) is: a branch of fluid mechanics that uses numerical analysis and data structures to solve and analyze problems that involve fluid flows. To truly make use of this data, engineers employ their knowledge of computational fluid dynamics, and couple the results with physics, industry best practices, operational knowledge or other data to simulate a real world scenario and determine if a course of action or a design is acceptable or unacceptable. CFD allows engineers to predict fluid response in different situations and create simulations to test their predictions. Changing the variables and studying the simulation results can help engineers find the optimal design based on fluid flow.

How Do Engineers Put CFD to Work?

The process of CFD is multi-faceted. First, the engineers determine the total volume of the fluid or gas to be measured. In order to analyze the problem, however, the larger volume must be broken down into manageable pieces. To accomplish this breakdown, engineers use a process called meshing to create a control volume to improve the accuracy of calculations.

As you can see in the illustration above, meshing takes a large volume of a liquid or gas and breaks it down into discrete sub-elements, or control volumes; for example, this process could break down the complete volume of a liquid in a 30’ long x 10” dia. pipe into a million sub-elements. Discretizing the liquid in this way creates a control volume that can be used to calculate fluid behaviors more precisely. This can be thought of as breaking down the volume into very small pieces so that fluid flow can be simulated as it would occur in reality.

Once the meshing is complete, boundary conditions can be put in place and a three-dimensional model can be built for simulations. The engineers will then use this physical model to simulate how a liquid or gas will behave in response to changes in temperature, turbulence, and obstruction, among other conditions. An example of this could be that an oil refinery has a butterfly valve whose body deteriorates over time. The valve can be modeled 3 dimensionally and the hydrocarbon stream flow can be simulated through the valve to gather necessary data to determine root cause of valve failure.

Because the CFD process is so complex, engineers use powerful CFD software, such as Ansys Mechanical CFD (which we use at Matrix). This software allows us to complete the complicated process of defining the problem, creating the mesh, and designing the model. Once our engineers run simulations, this software also completes complicated mathematical equations quickly to help solve the problems and analyze the efficacy of potential solutions.

When is Computational Fluid Dynamics Useful?

There are many situations in which CFD can be useful, but determining whether to use it is more often a decision regarding cost-effectiveness. The use of CFD is always a question of scale: it’s usually most useful where traditional methods and “go-bys” have already proven to be ineffective.

Some examples of problems that Matrix has solved using CFD include:

• A company in the oil, gas, & chemical industry wanted to have an emergency process in place in the event of an ammonia loss of containment. Matrix, through CFD simulation, determined how quickly the client could reduce the amount of ammonia in the air to make it safe for workers to re-enter the building by modeling portions of the facility and simulating a dispersing ammonia cloud in air.
• Another company in the oil, gas, & chemical industry employed a process that needed to cool a gas from 3000°F to 1000°F within a 5 feet run of pipe. Matrix used CFD to simulate flow and design a quenching process (water spray) that dispersed at a specific time and flow to accomplish that precise process.
• A company in the oil & gas industry was working with a substance that would begin to solidify into lumps below a certain temperature. Matrix used a CFD analysis to pinpoint places where the substance would be most likely clump and clog pipes. After the analysis determined potential problem areas, Matrix was able to provide the client with a revised piping design that limited potential for clogging.

CFD can also be very useful to analyze the cause of and provide solutions for water hammer in all liquid flow processes. Water hammer can be a very powerful and damaging force. (You can read more about water hammer in food and beverage applications here: http://matrixti.com/?s=water+hammer . In essence, any process that requires careful analysis or simulation of liquid and gas behaviors can benefit from CFD.

The Benefits of CFD

As you can see from our description of the processes involved with CFD, trying to solve complicated fluid dynamics problems without CFD methods and software can be next to impossible. When presented with a large-scale, complex fluid flow problem, engaging engineers to employ a CFD process can be crucial. CFD can solve many pain points in liquid or gaseous flow issues, including heat transfer simulation, changing fluid properties, cavitation, chemical leaks, water hammer, material pitting or abrasion, flow induced vibration and countless other fluid scenarios. To put it in summary: if there is a potential for a flow induced pitfall or a new design requires the insight of fluid flow simulation, CFD can be the answer.

Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about Computational Fluid Dynamics and Multidiscipline Engineering, contact Chris Mach, PE, Senior Consultant (Team Leader), Process Solutions Department.

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