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Computational Fluid Dynamics

by Van J. Brackin, P.E. BSME

What is CFD? CFD stands for Computational Fluid Dynamics. It's a fancy way of saying that an analytical model can be set up to calculate flows and pressures of fluids interacting with solid surfaces.

We use SOLIDWORKS Flow Simulation to analyze the fluid flow through a wide variety of products. From medical devices to adhesive dispensers. We use this software to predict product performance and identify potential sources of problems. This allows us to optimize the design of products, prior to building and testing physical parts.

With CFD, the knowns include: 1) Your physical model. This is the 3D computer model of the physical geometry you want to analyze, say a garden hose nozzle. 2) The fluid properties. What are you flowing and at what temperature? Air, water, oil, honey, toothpaste, ketchup, blood? Libraries of fluid properties (viscosity, density, compressibility, etc.) have been developed for all of these fluids so that the software can use these fluids to analyze the structure through which they flow. 3) Boundary conditions. These define the known pressures and flow rates where fluid is entering and exiting the model. For instance, if I'm analyzing a garden hose nozzle, I know my inlet pressure is 45 psi, and my outlet pressure is zero. 4) Model properties. These are all of the other input parameters that the model takes into consideration. Things such as how rough the walls are, how fine of a grid to do the calculations over, and how does the fluid temperature interact with the temperature of the solid.

The unknowns are the things you are looking to find. If I input pressure, what is my flow rate? Where in the system do I have the highest and lowest pressures? This tells me about restrictions in the system. What are the velocities inside the system? If the velocities are too high, this could lead to erosion, if too low then stagnation. When the software runs, it solves for all of these values, and more.

The method that CFD software uses to solve fluid simulation problems involves 1) Discretization of the model. This is where the model is divided into "elements" or small blocks which added together approximate the part. 2) Model inputs (boundary conditions and other model inputs) are applied. 3) When the Analysis is run, the computer first makes assumptions about what the solution is. Then it solves a set of Navier-Stokes equations (used to calculate the motion of viscous fluids) over all of the elements. It then compares the new solution to the old solution and sees how far off the previous guess was. It makes adjustments to the model and continues to repeat the calculations until the program "converges" on a solution.

Once the solution is found, the software allows the user to display pressures, velocities, flow rates, temperatures, etc. Normally, this is done by superimposing colors on the model surface which represent the range of values. Hotter colors (yellow, orange, red) represent higher values, while cooler colors (green, blue) represent lower values. In this way, the Engineer can quickly display and determine areas of concern.

Using Computational Fluid Dynamics at Zewski allows us to analyze and improve designs without having to build and test physical models. It helps us answer questions, solve problems, and make products better.