Press Room - What CFD analysts do after work

What CFD analysts do after work

Anders Tenstam, Epsilon HighTech AB, Sweden

About a year ago, I acquired a house, which was built in the 50’s and extended and re-built in the early 70’s. It was built as a holiday home, but had been used as permanent residence for the last 20 years or so. As many of us know, houses built in the 70’s do not meet modern standards of ventilation. Swedish building regulations state that the ventilation in buildings intended for permanent use should have a ventilation rate that ensures the air volume of the house is completely renewed every second hour. Accordingly, the ventilation should be sized so that all ventilation slots in the house sum up to the total volume flow needed. However, when it comes to defining exactly where the ventilation slots should be placed in each room, the decision rests with the ventilation craftsmen and their sometimes intuitive feel for the matter based on rules of thumb that may often be rather crude. This is where the CFD analyst can come in and resolve fluid flow issues that go beyond intuition!

The problem statement boils down to where to place the ventilation slots in each room to fulfil the ventilation norm, not only on the global scale, but also locally. More specifically, the airflow should be circulated every two hours around the whole building.

Of course, it is straightforward to circulate the air throughout, but some areas of the house might be quite stagnant with locally unsatisfactory air quality as a consequence.

After formulating this problem and the subsequent revelation that I am doomed to be a CFD engineer twenty-four hours/day, seven days/week, this is where pro-STAR comes into the picture.

Armed with a folding rule and some drawings of the house, a cartesian shell mesh was created representing each floor. The floor surfaces were extruded to the full room height to construct the volume mesh, and furniture (voids in the fluids) was accounted for by flipping fluid elements into solids. Measuring took longer than mesh generation, the combined process taking only 4-5 hours. After this, a number of different ventilation slot placements could be tested to find the best set of positions to fulfil the task.

In order to find a measure for air quality, a passive scalar was introduced into fluid cells. Simultaneously, “fresh” air (i.e. scalar value zero) was introduced in each ventilation slot. Consequently, a measure for the local “unfreshness” of the air became the scalar concentration throughout the domain, and different set-ups could be compared using this measure, which ultimately led to a satisfactory set-up of ventilation openings that would keep the circulation efficiency at maximum.

After the analysis was completed, out of curiosity I asked a ventilation expert for suitable placement of the ventilation openings in the house, and would you believe it, he actually pinpointed to a few inches the same locations I had found myself through the analyses! To this day, I am not sure exactly which of us made the cleverest analysis, but I do know that I will not lose sleep over this matter in the near future. Case closed.

Next project: Where shall I locate my fire alarm smoke detector…

Fig.1 - The figure shows where the final set-up of inlet vents is located in the model.

Fig.2 - Iso-surfaces of local high scalar concentration (red) and velocity vectors at 120 cm height. Green and blue colour code of vectors is below 5cm/s, which may be considered draft-free.