Billowing Hot Air: A Brief Analysis of Fluid Dynamics in Exhaust Systems

[J. Fast]

Careful. There is fully developed laminar flow, transition to turbulence, and fully developed turbulent flow.

I was just about to go into a long-winded conversation about turbulent flow as you seemed interested in the concept, but then I realized I dedicated an entire section of my thesis to describing turbulence at a brief level, so I'll let it do the talking.

http://etd.auburn.edu/etd/bitstream/...pdf?sequence=2

Section 1.3, pages 4-7.

I realize that pretty pictures are most helpful to visualize things, so if you're interested in seeing turbulence in action, check out pages 67-71. There are more pictures without commentary in Appendix D.

As far as the open turbos comment, do you mean no intake pipe attached to the compressor side? Could you link to Matt's thread? It makes sense that he would see high power gains by adding an intake pipe. The pipe helps to straighten the flow. That's not to say it's not turbulent or has no vorticity, just that the y-component of each molecule's velocity vector is decreased dramatically. Since energy must be conserved, the x-component must necessarily increase, leading to greater momentum about the x-axis, which is exactly what you want (flow perpendicular to the compressor blades). Without the intake pipe, you get air literally spilling over the compressor inlet, causing local pockets of separation before the compressor blades, leading to very high drag.

Good writeup and congrats on the thesis. It was a very interesting read, Tyla! Thanks for the explanation, it's much appreciated! Can you explain the intake setup with the increaing to decreasing conical section?

Are they putting a filter in there right at the inlet for some sort of magnus effect? I never really thought about it before but here's what I'm thinking after looking at the intake design someone posted. Please forgive me as it's been years and years since I've read any physics related stuff but I remember when labbing with bernoulli's we placed a golf ball, a filter, and a propeller in a pipe and changed it's orientation from dead center. I want to say it was a magnus effect lab or something maybe?

From what I remember, we produced an air colum in a pipe and inserted an object. When we spun the object perpendicular to the air column it changed the upwash and down wash characteristics and actally split the colum into smaller colums with different local velocitys. Some colums were very fast, and others were much slower. Depending on how close the object was to the shoulder of the air colum in the pipe, and how fast it was spinning, you could predict and actually pinpont where the colums were going to hit in the down wash.

Here's what I'm thinking now that someone pointed this out and turbo tech is evolving with stepped compressor wheel designs. I'm thinking adding a ball or conical object in the bounded air colum (an intake pipe in this case) will create a predictable magnus effect that will allow the split air colums to catch different steps in a compressor wheel? Say the upwash is split, one side has decreased pressure and increased velocity. Well, the other side must have increased pressure and decreased velocity to balance the effect, right? They will eventually balance back out if given enough pipe length but if you take that distance away then they can be split and used as split columns to spool different sections of a stepped compressor wheel? Not all the blades are the same angle, surface area, or length so that's Just a thought... Right on Tyla, I really dig physics man! Thanks for the read... Good stuff.

Cheers!

[BigTyla]

Thanks J.Fast!

Lots of questions above. I'll respond tomorrow after sleep.

[BigTyla]

Good writeup and congrats on the thesis. It was a very interesting read, Tyla! Thanks for the explanation, it's much appreciated! Can you explain the intake setup with the increaing to decreasing conical section?

Are they putting a filter in there right at the inlet for some sort of magnus effect?

I can't imagine that being the case. The Magnus effect only applies for a freely-spinning body. The filter in this pipe is bounded.

Here's what I'm thinking now that someone pointed this out and turbo tech is evolving with stepped compressor wheel designs. I'm thinking adding a ball or conical object in the bounded air colum (an intake pipe in this case) will create a predictable magnus effect that will allow the split air colums to catch different steps in a compressor wheel? Say the upwash is split, one side has decreased pressure and increased velocity. Well, the other side must have increased pressure and decreased velocity to balance the effect, right? They will eventually balance back out if given enough pipe length but if you take that distance away then they can be split and used as split columns to spool different sections of a stepped compressor wheel? Not all the blades are the same angle, surface area, or length so that's Just a thought.

Unfortunately this isn't easily predicatble, and in fact the flow separation resulting from the Magnus effect will cause extreme efficiency losses in the turbocharger. It would be best leaving the air as it is.

Stepped compressor design is nothing new. In fact it's been used in turbojets and turbofans for quite some time. What I'd be interested in is how much more flow this wheel design results in compared to a traditional wheel design. In other words, how much increased surface area scales with increased flow and if there might be a way to optimize this value.

I suppose we need an intake and turbocharger design thread soon!