I've heard that in a high velocity nozzle, any random 'fluid particle' experiences an increase in kinetic energy at the cost of internal energy and pressure energy.And I do understand that for any thing to accelerate a net external force is needed(which, is  due to the pressure gradient in this case,yes). But the shouldn't the fluid particle, still be able to change its kinetic energy at the cost of its internal energy? If yes, plz explain.

  • $\begingroup$ Are you asking whether heat can be converted to bulk flow kinetic energy or more generally about the enthalpy being converted to bulk flow kinetic energy? $\endgroup$ – honeste_vivere Nov 18 '15 at 14:19
  • $\begingroup$ I'm asking whether a fluid's bulk flow speed could be changed at the cost of its internal energy. Or to be more precise, is there any way to align a velocity component of each of those molecules to a particular direction in a way that we can in effect get a bulk flow? $\endgroup$ – Goutham Nov 18 '15 at 14:26
  • $\begingroup$ It is generally easy (by comparison) to convert bulk flow kinetic energy to random kinetic energy (i.e., heat) using things like friction, viscosity, etc. It is not, however, easy or feasible to convert random kinetic energy into bulk flow kinetic energy without doing extra work on the system. $\endgroup$ – honeste_vivere Nov 18 '15 at 14:38
  • $\begingroup$ Oh. Thanks. By the way, if we ever manage to convert random to orderly motion would the temperature of the fluid(by definition) change? $\endgroup$ – Goutham Nov 18 '15 at 14:44
  • $\begingroup$ That depends... See my answer here, http://physics.stackexchange.com/a/218643/59023, which shows how to define temperature and bulk flow velocity. $\endgroup$ – honeste_vivere Nov 18 '15 at 15:15

In Newtonian physics, fluids, like anything else, obey conservation of momentum, or $F=ma$.

The Bernoulli principle is just a re-statement of conservation of momentum.

The only thing that can change a parcel of fluid's speed (i.e. accelerate it) is a force. One kind of force is a pressure gradient. Another kind of force is gravity. If the fluid is conductive, electromagnetism can give still another kind of force. Nothing but a force can accelerate the fluid.


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