The gases are considered incompressible when Mach number is

The gases are considered incompressible when Mach number is Correct Answer less than 0.2

Explanation

Gases are considered to be compressible only if its Mach number (M) is greater than 0.3. Hence, for the Mach number, less than 0.3 gases are considered incompressible as density change in the fluid is insignificant.

Note:

Based on Mach Number, Gases are classified as:

• M < 0.3: incompressible flow; change in density is negligible
• 0.3 < M < 0.8: subsonic flow; density changes are significant, but shock waves do not appear
• 0.8< M < 1.2: transonic flow; shock waves appear and divide the subsonic and supersonic regions of the flow; Transonic flow is characterized by mixed regions of locally subsonic and supersonic flow
• 1.2 < M < 3.0: supersonic flow; flow field everywhere is above acoustic speed; Shock waves appear and across the shock wave, the streamline changes direction discontinuously
• 3.0 < M: hypersonic flow; where the temperature, pressure, and density of the flow increase almost explosively across the shock wave