Page 54 - 48Fundamentals of Compressible Fluid Mechanics

16 CHAPTER 1. INTRODUCTION

Mach’s revolution-
ary experiment demonstrated
the existence of the shock
wave as shown in Figure
1.7. It is amazing that
Mach was able to photo-
graph the phenomenon us-
ing the spinning arm tech-
nique (no wind tunnel was
available at that time and Fig. 1.7: The Photo of the bullet in a supersonic flow that
most definitely nothing that Mach made. Note it was not taken in a wind tunnel
could take a photo at super-
sonic speeds. His experiments required exact timing. He was not able to attach the
camera to the arm and utilize the remote control (not existent at that time). Mach’s
shadowgraph technique and a related method called Schlieren Photography are
still used today.
Yet, Mach’s contributions to supersonic flow were not limited to experi-
mental methods alone. Mach understood the basic characteristics of external su-
personic flow where the most important variable affecting the flow is the ratio of
the speed of the flow 48 (U) relative to the speed of sound (c). Mach was the first
to note the transition that occurs when the ratio U/c goes from being less than 1 to
greater than 1. The name Mach Number (M) was coined by J. Ackeret (Prandtl’s
student) in 1932 in honor of Mach.

1.3.5.3 John William Strutt (Lord Rayleigh)

A researcher with a wide interest, started stud-
ies in compressible flow mostly from from a
mathematical approach. At that time there
wasn’t the realization that the flow could be
choked. It seems that Rayleigh was the first who
realized that flow with chemical reactions (heat
transfer) can be choked.
Lord Rayleigh was a British physi-
cist born near Maldon, Essex, on November
12, 1842. In 1861 he entered Trinity College
at Cambridge, where he commenced reading
mathematics. His exceptional abilities soon en-
abled him to overtake his colleagues. He grad-
uated in the Mathematical Tripos in 1865 as Se-
nior Wrangler and Smith’s Prizeman. In 1866 he
Fig. 1.8: Photo of Lord Rayleigh
obtained a fellowship at Trinity which he held un-
48 Mach dealt with only air, but it is reasonable to assume that he understood that this ratio was applied
to other gases.