From the BBC news wire I find this article: http://news.bbc.co.uk/2/hi/health/4133093.stm
"The geometry of the nose is highly complex, with no straight lines or simple curves like an aircraft wing and the regime of airflow is not simply laminar or turbulent."
I need to go look up "laminar". Could be my new word for the day.
The team, funded by the Biotechnology and Biological Sciences Research Council, found the human sense of smell relies on a sample of air reaching the olfactory bulb at the top of the nose and that requires a sharp breathe and a high velocity shot of air to reach it.
The geometry of the nose causes the air to move around in the vicinity of the bulb allowing smell to be sensed.
So there is something to "holding your nose" to avoid a smell.