at transition flow why does fluid statrts curvilieanr path

[gsunil]

i want to know at the critical velocity why the flow starts curving what r the torques acting for this .rotational flow is due to complementary shears due to viscosity.how is iertial forces is a force here how to visualise in action them along with viscous.

 

[BigInch]

Critical flow is where the energy propelling fluid forward is enough to balance the restraining fricton. Forward flow mass x acceleration x distance from wall to center of pipe = friciton x distance from center of pipe to pipe's wall. Force x distance = moment => rotation

Independent events are seldomly independent.