I had a technical issue on the product I was designing - an airbag. In this case the inflator released its gas into a long steel tube which distributed the gas where I needed it to. The tube had pierced holes along its lengh and the end was open. It appeared that I wasn't getting the gas where I needed it to go, but it can be hard to tell. So, I took a tube and put a bunch of pressure taps along the lengh. By measuring the pressure I could estimate the mass flow at each location and ergo know how much of the gas was going where. These were real parts. Plus there was video in which you could see the gas coming out of the holes.<br><br>
So, my managment decided to "help" me on the issue and gave me a guy to do CFD (computational fluid dynamics) on the system. Basically a math model. The following is a converstation with this guy after he's developed (I use this term loosely) his model:<br><br>
CFD Guy: I did the model and 95% of the gas is coming out the end of the tube and only 5% is being diverted through the holes along its length.<br><br>
Me: Here's my pressure tap data and several airbag deployments. You'll note that the front part of the airbag inflates which means there IS a significant amount of gas coming out of the pierced holes. This is backed up by this pressure tap data which shows that 40% of the gas exits the first set of pierced holes. Maybe we should look at some of you assumptions. Are you assuming laminar flow?<br><br>
CFD Guy: It's impossible for any gas to come out of those holes. Your data is wrong, my math model is right. All the gas is coming out the end.<br><br>
Me: Look at this deployment; the area of the airbag that's being inflated by the gas coming out of the end of the tube is not inflating. This indicates that there isn't much gas getting to the end.<br><br>
CFD Guy: You must be mistaken. My model says that all the gas comes out the end.<br><br>
Me: Ok. Thanks for all your help!