Section 5


A Note on Isentropic Flow ‘Perpetual Motion’




Since isentropic flow is so efficient – that is to say it can be made  almost 100%  ‘loss free” -  it  naturally invites speculation as to the possibility of a ‘perpetual motion’  or ‘quasi-perpetual motion’ flow model. However, the long standing prohibition against heat engine or thermodynamic perpetual motion as a violation of the second law of thermodynamics, naturally creates a  well founded caution – or even resistance to-  such possibility in other areas of physics.


However, one other example of quasi -perpetual  motion occurs in outer space, where loss-free motion of a space satellite through a frictionless space in the absence of any substantial gravitational or other force field,  results in quasi-perpetual orbital motion under Newton’s Second Law,  and is today accepted without notice.


It seems well worthwhile, therefore, to examine the motion of isentropic gas flows  in a bit more detail from this standpoint.



 Frictionless Isentropic Flow


Ideal  isentropic flow is frictionless, so that its accelerations and decelerations are ideally lossless. In practice  isentropic accelerations are nearly ideal with very small  pressure- head losses due to friction, turbulence or viscous effects ;  however, low loss decelerations  are more difficult to achieve  [12,3]. 


Invention No. 1 in a Perpetual Motion Mode.


In Section 2 an isentropic air motor is described with an extremely large amplification of flow power in a  De Laval converging  nozzle.The resulting available power is so many times greater than the necessary mass flow air power from a conventional low efficiency vacuum pump that perpetual  motion from a feed back of amplified flow power is obvious as a self- sustaining motion. However, if flow losses are considered, the operation becomes one of a  decline in power towards  an eventual cessation of the flow. This is best seen from an analysis of mass flow rate  rather than from that of flow power.


For example, let us take a vacuum source sustaining a mass flow rate of 0.074 kg/s for a power input of 1690 watts. This flow rate corresponds to an air flow  power  of flow of only about 100 watts (or a vacuum source efficiency of 100/1690 = 5.9%).


If we pass this mass flow through a De Laval Nozzle  ( Fig. 1) of throat diameter of 1.98 cm the flow will then be sonic at the nozzle  throat. The throat power will be P= ˝ x 0.074 x 3132 = 3625 watts.


Since it only requires 100 watts of flow power to sustain 0.074 kg/s inflow to the nozzle, we then, by inserting a bypass feed-back duct connecting the throat to the system flow exit,  bleed off this amount of power from the throat, and  ( Figure 2) we can then shut down the inefficient vacuum source which requires 1690 watts to deliver 100 watts of flow power and the system will be self sustaining - if there are  no frictional  or other losses. Under this assumption the motion would be self-sustaining indefinitely.


If losses are present, the mass flow will have to decrease. This mass flow decrease is usually observed as  a pressure rise or a flow velocity decrease. For frictional losses, the pressure is related to the velocity squared so that the observed velocity will drop rapidly at first and then much more slowly. Low losses will therefore exhibit  a long slow decline in any self-sustaining  flow velocity.




 In the absence of  any flow losses perpetual flow motion is possible. The practical limits and usefulness of the possibility remain to be fully investigated.




1.  Munson, Bruce, R., Donald F. Young, and Theodore H. Okiishi, Fundamentals of Fluid Mechanics,  Wiley and Sons, New York, 1990.


2. Shapiro, Ascher, H., The Dynamics and Thermodynamics of Compressible Fluid Flow,  2 Vols.,  John Wiley & Sons, New York.1954.


3.  R. Courant and K.O. Friedrichs,  Supersonic Flow and Shock Waves. Interscience, New York, 1948.



Copyright   Bernard A. Power , May 2011



Section Links:


Section 1:  Linear  ( streamline) Flow and Flow Power Amplification


Section 2:  Invention No.1:  A New Isentropic Air Motor and Clean Energy Source


Section 3 To be posted in near future


Section 4:  Flow Acceleration and Centrifugal Force as a Possible Cause of the  Observed Temperature Rise Anomaly in the Ranque-Hilsch Vortex Tube



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