When performing a computational fluid dynamics (CFD) analysis, engineers set up a virtual model to predict how fluids will flow
and the effects they will have on the structures with which they come into contact.
To analyze the flow pattern within any system, you start with the known physical characteristics of the system:
dimensions of pipes, pumps, intake areas and valves, for example.
You also define the fluid, including viscosity and flow rates.
The values you define and the other decisions and/or assumptions that you make about them are vitally important to the overall accuracy of the simulation.
Here are some of the key questions to consider when setting up and performing a CFD analysis:
Can my CAD design be used directly?
CFD analysis has recently been made much easier through the automatic modeling of fluids based on CAD designs.
While the structures were easily transferable from CAD into FEA, engineers would often have to model the fluid part of the system in FEA.
Now, the fluid is automatically modeled based on the geometry of the structure.
CFD now allows the automatic modeling of the fluid medium based on a CAD solid model.
The user simply specifies the surfaces that bound the region of the fluid medium using a built-in dialog and
ALGOR software automatically creates new parts where CFD analysis will be performed.
This makes modeling fluid flow systems easier and faster.
Shown here is a valve assembly that was modeled in SolidWorks (left);
the user then specified the surfaces for modeling the fluid medium in FEMPRO (center);
and a new part was automatically created where CFD analysis will be performed (right). |
In ALGOR, for example, InCAD technology provides direct CAD/CAE data exchange with most CAD solid modelers including Alibre Design,
Autodesk Inventor, Inovate, IronCAD, KeyCreator, Mechanical Desktop, Pro/ENGINEER, Rhinoceros, Solid Edge and SolidWorks
with full associativity with each design change for Alibre Design, Autodesk Inventor, Inovate, IronCAD, Pro/ENGINEER, Solid Edge and SolidWorks.
Bill King of the Hewlett-Packard (HP) Company in Boise, ID, used ALGOR’s CFD analysis software to optimize flow through the ink feed channel
in the manifold of HP’s inkjet cartridges.
The fluid flow model shown here features a hybrid mesh created directly from a STEP CAD universal file.
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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 in order to accurately capture the interactions.
ALGOR V19 includes a boundary layer mesher for this very purpose.
Boundary layer meshing automatically 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 throughout keeps down computing costs.
ALGOR V19’s boundary layer meshing is used for accurately simulating detailed fluid flow behavior around a fluid boundary.
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Do I need to define constraints for the walls?
In CFD analysis, much of the model must include constraints to represent the fact that a fluid in a particular area is contained (against a wall or other surface).
Rather than forcing the user to define all of these constraints, the software simply constrains all areas of the model
that do not have another type of applied constraint, such as pressure, velocity or an inlet/outlet area.
Again, this cuts down greatly on the amount of time it takes to model properties in CFD.
Can my computer handle CFD analysis?
Large, complex CFD simulations have traditionally challenged desktop computer hardware systems, especially their memory capacities.
One possible solution to the computing crunch problem is the adoption of a 64-bit platform.
ALGOR V19 features support for 64-bit Microsoft Windows® and 32- and 64-bit Red Hat® Linux® operating systems for all analysis types,
including CFD analysis.
But there are other solutions, short of buying a new computer.
Engineers can simply switch to a segregate scheme which analyzes various parts of the global system separately.
This approach is possible using segregate transient and steady solvers for faster runtimes using less computer memory.
The segregated scheme is designed to address large-scale problems, dramatically cutting down on computing time.
Can you monitor results in real time?
In the past, once an analysis had begun, the engineer had to wait until it was finished in order to see the results.
If the modeling was flawed—for example, the fluid was flowing in the wrong direction, it was leaking into an area where it wasn’t supposed to flow
or it wasn’t flowing at the right velocity—you had to wait until the analysis was finished to find out, determine the cause and then remodel the event.
Now, users see the results immediately. Engineers can also restart the process and change parameters at any selected point.
This cuts down on design and analysis time and makes CFD much easier and more effective.
What additional results do I view when evaluating flow results?
Specialized fluid flow results include particle tracking, streamlines and isosurface displays, making the display of fluid flow results clearer and more compelling.
For example, engineers can track a particle from the beginning to end of the flow.
Such particle tracking along with streamlines provide much needed details about the entire fluid system and flow patterns within that system.
Enhanced fluid flow results presentation includes the ability to show streamlines (bottom right)
and particle tracking (top left), making the display of fluid flow results clearer and more compelling.
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You should keep these points in mind when you set up your next CFD analysis.
By using these features, you’ll find that CFD analysis has been greatly improved for ease-of-use, speed and effectiveness.
