96 



Figure 40 - Scheme for Computing Viscous Pressure Drag 



1 . It Is assumed that the frlctlonal layer does not disturb seriously 

 the potential flow outside of it. The static pressure p(A') at the boundary 

 point A' of the frlctlonal layer is calculated and, using an essential proper- 

 ty of this layer, the assumption is made that the pressure on the body (Point 

 A) is equal to p(A'). It is claimed that the viscous resistance so computed 



7 



agrees well with experiments. 



2. It is assumed that the body and the frlctlonal wake form a new ef- 

 fective body, for which the pressure distribution may be calculated from con- 

 siderations of potential flow. This procedure leads again to a pressure drop 

 at the stern compared with the calculation applied for the original body.*^" 



Obviously these two methods can be only applied to infinitely long 

 cylinders (two-dimensional case) and to bodies of revolution, for which the 

 boundary layer thickness can be calculated. Thus it seems to be natural that 

 any research on viscous pressure resistance starts with Investigations on 

 these bodies; a body of revolution is obviously a closer approximation to a 

 double model than an infinitely long cylinder. 



As has been pointed out , experimental data are very scarce . The 

 following empirical formulas are quoted: 



1. Two-dimensional case (symmetrical profiles) 



Hoerner's formula for the total viscous drag^^ 



d\4 



-p^= 1 + 2^+ 70(y) a-thickness [1 ] 



2. Axial symmetry (bodies of revolution) 



Hoerner's formula for the total viscous drag^^ 



-^= 1 + 0.5^+ b(ij) d-diaraeter [2] 



