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First CFD with Flowgy

Introduction

First of all, what is Flowgy? Flowgy is a software dedicated for CFD of the nasal cavity with tools for virtual surgery. In this article, I will compare the results that I got with my industrial software (Onshape and Simscale) and the results with Flowgy. I would like to thank Pr Manuel Burgos for loaning me the software and answering all my questions.

Simscale - streamlines - left side
Flowgy® www.flowgy.com - streamlines - left side

I set the max speed at 3,25 m/s in both software. We can see the same thing, most of the airflow passes through the middle meatus. Note that on Flowgy I just showed the airflow in the left nasal cavity to see more clearly what is going on.

Cuts - Velocity

Simscale - cuts view - max velocity 3,5 m/s (red)
Flowgy® www.flowgy.com - cuts view - max velocity 2,5m/s (red)

The main difference in the two views is the max speed, it is set at 3,5 m/s in Simscale and 2,5 m/s in Flowgy because I was not able to set 3,5m/s in Flowgy. But we can roughly see the same thing, the speed of the airflow is very lower in the inferior meatus. It's reassuring to see the same thing because it means that the 3d model I drew of my nasal cavity was quite accurate.

Wall Shear Stress

Now the really interesting thing in Flowgy is that there are other data available, one of them that is particularly interesting in the ENS context is the Wall Shear Stress (WSS). I already mentioned WSS in other articles. I will explain a bit more about what WSS is and why it is particularly interesting data in the ENS context. WSS is the pressure exerted by the airflow to the walls, the unit is the Pascal (Pa). The higher the gradient of velocity near the wall the higher the WSS. It is an important value because it is the WSS that gives the airflow sensation.

Flowgy® www.flowgy.com - left side - Wall Shear Stress - max pressure 1 Pa (red)
Flowgy® www.flowgy.com - cuts view - Wall Shear Stress - max pressure 1 Pa (red)

The pressure higher than 1 Pa is represented in red. We can see that the max WSS is located mostly in the anterior part and also in the middle meatus which corresponds to the velocity of the airflow. So we want to have a better distribution of the WSS between the middle and inferior meats. This is what I will try to do with virtual implants in the next article.

Airflow humidity

Flowgy® www.flowgy.com - cuts view - mini humidity 70% (blue)

In this view is represented the humidity of the airflow, we can of course see that it increases as it passes through the nasal cavity. We can see that part of the airflow is at 92% humidity in the last cut. The values are certainly not very precise. By default in the simulation parameters, the mucosa is at 100% humidity which is a little optimistic given that we often have the mucosa dry because of the ENS. At the same time, the value of the inspired air is set to 20% humidity which is pessimistic because often the outside air is more humid than this. What will be interesting is the comparison before/after the virtual implants so the input parameters are not so important.

Airflow temperature

Flowgy® www.flowgy.com - cuts view - mini temperature 300°K (blue)

Same thing here with the temperature of the airflow, in the simulation parameters the mucosa is set at 309,65°K which is 36,65°C. The inhaled air is set at 21,15°C. The temperature of the mucosa is also maybe a little optimistic in the context of ENS. I think the entrance to the nose is colder than that due to the lack of mucosa. But I can't quantify it and it will depend on the aggressiveness of the turbinectomy. In this illustration, the minimum temperature is set at 300°K (27°C) in blue.

Conclusion, what’s next?

Flowgy gives us really interesting data which will be valuable to compare before and after the addition of virtual implants. I'll talk about it in the next article, but I think it's no longer possible to do nose surgery in 2022 without first having done a virtual surgery to see what it will give in terms of airflow, WSS, etc …
Now I will try to design virtual implants in order to improve airflow distribution, WSS distribution, etc …

3 comments on “First CFD with Flowgy”

  1. Can the change of the shape of the septum contribute to the air flow quality that is introduced, I noticed when I drop my spetum and force it down I have more resistance. Is that a thing or am I just imagining it. I did have turbinate cauterization but it was very minimal, when I saw the scan it was a slight change in size. I know that could be the contributor but I also noticed that my nasal spetum and cartage moved upwards.

  2. Excellent research that is very helpful clinically. I am interested to see if you can recommend the smallest implant that leads to the greatest impact and its proper location for placement.

  3. Hello, are you really Dr Das ?
    It is not a simple question.
    As I said in the article, I designed the implants to have about 3 mm between each wall in order to try to distribute the airflow and the WSS. Smaller implants will surely cause less Wall Shear Stress in the implanted region and of course decrease the cross-sectional area less which can pose a problem for nasal resistance. Also I tried to have a stable cross-sectional area all along the nasal cavity in order to try to distribute the WSS all along. I also think that the implants should be designed according to the case and the symptoms.

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