HYDRODYNAMICAL RELATIONS 63 



pressures of the order 10,000 Ib./in.^ the increase is very appreciable. 

 A comparison of these results on regular reflections with available 

 experimental evidence on water is made in Chapter 7. The theory is 

 in qualitative agreement with experiment. Other experiments under 

 better defined conditions in air leave little doubt that the theory is 

 essentially correct for regular reflection insofar as it is applicable. 

 (The theory considers only the case of plane shock waves of infinite 



Table 2.5. Pressure increase by normal and oblique reflection at a rigid boundary 

 (from calculations of Polachek and Seeger). 



duration and so can only describe the initial pressure differences near 

 the stationary point for waves of finite duration.) 



C. Mach reflection. There remains to be considered the question 

 of what happens for sufficiently oblique incidence that regular reflection 

 cannot satisfy the physical relations. The theoretical difficulties of 

 this problem are very great and the available experimental data, par- 

 ticularly in water, are not adequate as yet to clarify its various aspects. 



— ^ ^ ^ ^ ^^ ^ -^ >^ ^ 



^igstream Istem 



incident 

 Refiected 



Fig. 2.13 Mach reflection at a rigid boundary. 



The experimental evidence does, however, show that a reflected shock 

 is developed, which meets the incident wave at a point in the fluid, and 

 the two waves join to form a third shock wave extending to the wall. 

 This form of reflection, sketched in Fig. 2.13, is known as irregular or 

 Mach reflection and the third shock is frequently described as the Mach 

 stem (after E. Mach who made early investigations on similar phe- 

 nomena). There is one other feature of the irregular or Mach reflec- 



