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# How to Construct an Accurate Flow Domain for CFD Analysis

February 6, 2015 By: Hsin-Hua Tsuei

A CFD analysis, similar to all other engineering analyses (FEA, Emag, etc.), starts with geometry, which often comes from a CAD model. The first step of a CFD analysis is to construct a flow domain in or around that geometry. For a CFD analysis with an external flow domain, or a flow domain that is built around your geometry, it is more straightforward.

To construct an external flow domain, you literally make a box (or any other shape) large enough to contain the object of interest, like a car, an airplane, etc., so as not interfere with flowfield adjacent to the object, and to have valid boundary conditions representing free stream and ambient conditions. Then, use the volume of the bounding box, subtract the geometry from the CAD model, which leaves you with the flow domain for the external flow analysis. If external flow analysis is the focus of your project, just make sure the “internal” components of the CAD model, such as the radiator in a car, are not considered. Otherwise, the resultant flow domain can contain many features and solid surfaces not needed for the external flow analysis.

With internal flow, flow domain generation can get a bit complicated. CAD models often only include solid parts, since the flow passages are the invisible volume in a real machine assembly. One of the first steps in any flow domain construction for a CFD analysis is to “find” where these invisible volumes are in the CAD model of interest. Flow will “creep” into any volume or tight spacing that is not occupied by a solid, such as gap between two solid parts, engineering clearances, etc. Thus, sculpting the flow volume from the complex solid parts in the CAD model requires not only careful understanding of the flow path, but sometimes a vivid imagination. You will need to identify potential flow volumes not clearly seen in the CAD model.

Model of a Ball Valve

## ballvalve1.png

This process is like making a plastic injection mold – pour invisible material into the solid CAD model, then break the solid parts to take the formed plastic part out. The molded part is like is the internal flow domain of interest. Very often, some part of the flow volume is “missed” in the process of building the flow domain, which can cause misleading results.  For example, in the case of an internal flow domain featuring many gaps or clearances, if one or more of the clearances is not constructed correctly, the final pressure drop may be a fraction of what it should be. If this error occurs, the CFD results will not compare favorably to test data – not because of the inaccuracy of the analysis itself, but because  part of the complex internal flow passage is not being factored in as part of the analysis.

Ball Valve Flow Domain

## ballvalve2.png

Another common scenario is that a part of the complex internal flow passage is not constructed or connected correctly, and  could result in flow blockage in this passage. The flow is then forced to find alternative paths to propagate through the system, which can lead to unexpected flow acceleration in other areas, pressure rise, etc. If a conjugate heat transfer (CHT) analysis is performed, the flow blockage can lead to insufficient cooling for certain areas. Dead zones and hot spots will appear in the CFD results, but not in the test data.

So before accepting the validity and accuracy of CFD results, always carefully examine if the flow domain has been constructed correctly.  After all, the flow domain is the beginning of any CFD analysis. If we start off down the wrong path in any type of analysis, it will be difficult and time consuming to recover!