Basics of Fluid Mechanics, 2014a

11.3. SPEED OF SOUND 383
consider a two dimensional situation where a particle is moving from the left to the right.
A particle movement creates a pressure change which travels toward outside in equal
speed relative to the particle. Figure 11.3 depicts an object with three different relative
velocities. Figure 11.3(a) demonstrates that the whole surroundings is influenced by
the object (depicted by red color). While Figure 11.3 (b) that there small zone a head
object that is “aware” if the object arriving. In Figure 11.3 (c) the zone that aware of
the object is practically zero.
(a) Object travels at 0.005 of
the speed of sound
(b) Object travels at 0.05 of the speed of
sound
(c) Object travels at 0.15 of the speed of sound
Fig. -11.3. Moving object at three relative velocities. The gray point in the first circle is the
initial point the object. The finial point is marked by red circled with gray filled. Notice that
the circle line thickness is increase with the time i.e the more green wider circle line thickness.
The transition from the blue fresher lines to the green older lines is properly marked.
In fact, when the object velocity is about or larger than the speed of sound then
the object arrive to location where the fluid does not aware or informed about the object.
The reason that in gas the compressibility plays significant role is because the ratio of
the object or fluid velocity compared to speed of sound. In gases the speed of sound
is smaller as compare to liquid and defendtly to solid. Hence, gases are media where
compressebilty effect must be considered in realtionshp compressebilty. There are some
how defined the Mach cone as the shape of object movement approaching to one. This
shape has angle and it related to Mach angle.