Machine Design

Preparing for a Fluid Simulation

CFD software is easier to use, faster, and more effective.

Bob Williams
Algor Inc.
Pittsburgh, Pa.

Algor’s CFD software optimized flow through the ink-feed channel in the manifold of Hewlett-Packard inkjet cartridges. The fluid-flow model features a hybrid mesh created directly from a STEP CAD file. InCAD technology in the software also provides direct data exchange with full associativity for most widely used solid modelers.

The sequence shows some of the preprocessing for a CFD simulation. A valve assembly (left) is modeled in SolidWorks. A user has specified the surfaces that bound the fluid region using a built-in dialog (center). Lastly, the software automatically generates a new part for the analysis that defines the fluid volume.

Boundary-layer meshing lets Algor V19 accurately simulate detailed fluid flow around a physical portion of the valve interior.

Results from a flow simulation include particle tracking (top) and streamlines (lower image).

It's probably not news that computational-fluid-dynamics (CFD) software predicts fluid flow within a defined system, along with the effects it will have on structures it contacts. But what is news is that the software has been improved over the last few years. The following questions can help prepare for a simulation and introduce the new capabilities.

What data is needed before starting CFD simulation?
Before starting a simulation of any system, gather a few physical characteristics such as pipe dimensions, pumps pressures, intake areas, and valve geometry. You must also define the fluid, including viscosity and flow rates, and determine the goal of the simulation. What values and characteristics will be expected? These along with other decisions and assumptions are important to the simulation's overall accuracy.

Can CAD models be used directly?
CFD analysis is much easier to complete thanks to the automatic modeling of fluids based on CAD designs. While structural models transferred easily from CAD into FEA, flow analysts previously had to model the fluid part of the system in FEA. Now, the software models the fluid based on the geometry of the structure.


How do I generate accurate meshes?
CFD analysis is complicated by the interaction between fluids and structures. It is important to remember that changes in a fluid's velocity are often greatest near the surfaces of a model. Therefore, a finer mesh is needed in these areas to accurately capture interactions. Analysis packages with CFD capabilities, such as Algor V19, include a boundary layer mesher for this purpose. It creates a fine fluid mesh near the surface of the structure, while keeping a coarser mesh throughout the rest of the volume. This finer meshing improves accuracy, while the coarser meshing minimizes computation times.

Must I define constraints for walls?
Much of the CFD model must include constraints to represent the fact that a wall or other surface holds a fluid in a particular volume. Rather than forcing users to define all walls and surfaces, the software simply constrains all areas not constrained by other conditions such as pressure, velocity, or an inlet or outlet area. This also greatly cuts the amount of time it takes to model properties in CFD.

Can my desktop computer handle CFD analysis?
Large, complex CFD simulations have traditionally challenged desktop computers, especially their memory capacities. One possible solution to the computing crunch comes by buying a 64-bit operating system. A few engineering software programs support 32 and 64-bit Microsoft Windows and Red Hat Linux operating systems for many types of analyses, including CFD. Other solutions, short of buying a new computer, include software with a segregate scheme that separately analyzes parts of the global system. This approach is possible using segregated transient and steady solvers for shorter runtimes that use less computer memory. The segregate scheme is intended for large-scale problems.

Can users monitor results during a simulation?
Today, they can. But not long ago, engineers often had to wait until simulations finished before viewing results. So if the model was flawed with, for example, fluid flowing in the wrong direction or an incorrect inlet velocity, users had to let the analysis go on. Then, they would hopefully spot something unexpected in the results and investigate further to explain it. After correcting the errors, the simulation would be restarted.

Users now see results as they are calculated so they can stop a simulation, change parameters at any selected point, and restart the calculations. This trims design and analysis time, making CFD easier and more effective.

What other results can be calculated?
Specialized fluid-flow results include more than just velocities, pressures, and directions. They also encompass particle tracking, streamlines, and isosurface displays that make results clearer and more compelling. For example, engineers can track a particle from the beginning to end of the flow. Particle tracking along with streamlines provide details about the entire system and flow patterns within it while isosurfaces display a constant characteristic such as pressure.

Algor Inc.,

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