Your knowledge of fluid dynamics does not apply to the trumpet. The trumpet works by getting a standing wave in the instrument started and maintained. The "airflow" is only required to make the lips open and close like a valve to get that standing wave started. read the thread on how a trumpet works that is sticky at the top of this forum to get a real perspective. Your fluid dynamics would only apply if the trumpet were a glorified megaphone. It is not. Changing the diameter of the bore at various places changes the acoustic length and therefore the tuning. The thickness of the metal or moving the braces affects more of what we assume to be "blow" than any other factor. The taper of the leadpipe affects the intonation more than anything else.