Laval number
Physical key figure  

Surname  Laval number  
Formula symbol 


dimension  dimensionless  
definition  


Named after  Gustav de Laval  
scope of application  compressible flows 
The Lavalnumber (after Gustav de Laval ) or Mach number is a similarity measure of gas dynamics . It results from the ratio of the local flow velocity to the critical speed of sound :
It is
 The critical speed of sound is the speed of sound that would result if the flow were to accelerate (or decelerate) to the speed of sound without loss of heat or friction
 the isentropic exponent
 the specific gas constant
 the critical temperature (explanation see below)
 the resting temperature .
The speed of sound is temperature dependent and the temperature decreases in an accelerated gas flow. The speed of sound is therefore subject to changes along the flow path. The condition in which a gas flow flows at the speed of sound is called the critical point . This point is reached in the narrowest crosssection of a Laval nozzle (with a sufficiently large pressure gradient) . The critical temperature prevails there , which is in a fixed ratio to the resting temperature.
Individual evidence
 ^ E. Truckenbrodt: Fluid Mechanics: Basics and Technical Applications . SpringerVerlag, 2013, ISBN 3662415992 , p. 270 ( limited preview in Google Book search).