Computational Fluid... What?!

Computational Fluid Dynamics (CFD) is a field of analysis used to predict behavior of fluids. The Navy uses CFD to predict turbulence in water from its submarines and redesigns the shape of the hulls, etc. to reduce this noise generating turbulence. More recently, in our industry, CFD has been used to model airflow in laboratories or flow of contaminants from rooftop exhaust ducts. For more specialized applications CFD has been used to model “displacement ventilation” schemes for large, glass enclosed lobbies or comfort conditions using high performance air diffusers.

One of my masters degrees from Drexel University specialized in CFD analysis. For those who may be interested, this blog provides an overview of what CFD is and what can it do for your project. My most recent work with CFD was analysis of Desflourane emissions and concentrations for a Post-Anesthesia Care Unit (PACU) in a large hospital. After surgery, patients are brought to the PACU where they continue to off-gas the anesthesia in their bodies. It usually takes a couple of hours for the majority of the anesthesia to leave a patient. CFD was used to model the anesthesia gas emissions from a patient’s breath while laying on a stretcher.

Given the amount of anesthesia off-gassing that occurs current AIA/DHHS guidelines recommend that PACUs do not have recirculating type HVAC systems. In this particular case the hospital had a plan to upgrade the existing HVAC systems; however, wanted to be sure that the new system would result in gas levels below fairly strict thresholds designated by OSHA. Accordingly I modeled the current PACU using CO2 as a tracer gas to confirm the base model’s accuracy. From there I input the actual anesthesia gas parameters and new HVAC system and reviewed the resulting contaminant field along with other thermal and air distribution comfort parameters. The great thing about CFD analysis is the instant results and the various graphic methods of displaying everything from air velocity vectors, contamination levels, temperature, etc. Through this analysis I was able to provide some feedback on modifications to the HVAC system as well as providing the hospital some greater assurance that this highly disruptive and costly project would generate the results they needed.

For those of you considering CFD analysis as a tool to help predict the end result of design decisions for critical or unusual applications it is important to recognize the limitations of CFD analysis. For wind flow modeling of buildings or large scale contaminant streams the tried and true method of creating scale models and placing them in a wind tunnel is still used predominantly. RWDI, located in Canada, has been the premier firm for this type of modeling for years; although now, they supplement their “physical models” with CFD analysis for greater accuracy and confirmation of results. One of the downsides to CFD; however, is the problem of “boundary conditions.” This is the “boundary” where the model ends. For a specific room... this is easy; you stop at the walls. For spaces with open interconnections to others or outdoor conditions, CFD can often provide misleading results. In those situations using a firm that has specific experience with your particular issue is the best strategy

And finally, for those wishing for more depth on the subject, you can get a flavor of the math involved by clicking on the PowerPoint presentation titled CFD Math. At the root of computational fluid dynamics is a simplification of the Navier-Stokes equation which describes generalized fluid flow. The main problem with the Navier-Stokes equation is that it requires partial differential calculus and is cannot be solved by direct means except in certain limited situations. In the early 1930s CFD was developed to the point where it could be used successfully and the advent of computers finally brought this analysis technique into the mainstream of the scientific and engineering community. Essentially you run the calculation over and over again until the results converge which indicates a successful solution. The art and expertise of a CFD practitioner is understanding what CFD method works best in a particular scenario and how to manage boundary conditions such that the results can be trusted.