Sec. 4 1.1 2 



GENERAL LIQUID-FLOW FORMULAS 



27 



If the total pressures on the sohd surface are to be 

 emphasized the preferred method is to draw a 

 hue outside the solid surface, everywhere equi- 

 distant from it at a distance which represents 

 conveniently the ambient pressure p„ , or that of 

 the undisturbed liquid at infinity. A variation of 

 this method is illustrated in diagram 2 of Fig. 

 2.B, where the atmospheric pressure Pa on the 

 hydrofoil section corresponds to the ambient 

 pressure p„ of Fig. 41. J. 



In addition, of course, the pressure coefficients 

 may be plotted on the usual x-y coordinates, 

 using the length along the x-axis of the body as 

 the a;-coordinate. This method is illustrated in 

 Fig. 2.V, depicting the velocity and pressure 

 variation at a stem section on a ship. A third 

 method is to use the developed distance along the 

 boundary or contour of the body (or an expansion 

 of it) as the distance basis, erecting the velocity 

 and pressure vectors normal to this contour line, 



as in the right-liand portion of diagram 2 of 

 Fig. 41. J. 



The pressure relationships are frequently ex- 

 pressed and plotted as fractions or as multiples 

 of the ram pressure 0.5p[/J = q. The dynamic 

 pressure variations caused by body or liquid 

 motion are then referred to as so many "q's," 

 plus or minus. For the previous example the 

 — Ap at the point A is — 0.5625?. 



This variation of the customary pressure 

 coefficient or Euler number is sometimes used 

 when investigating the proper location for a 

 pitot-type velocity indicator or a speed meter. 

 As the entire velocity head in the liquid is con- 

 verted to pressure head at the nose orifice of the 

 pitot tube, this velocity is measured as a pressure 

 when the region under consideration is being 

 surveyed. By relating (1) the pitot pressure 

 observed at any selected point in the field, as 

 registered on an instrument mounted in a fixed 



TABLE 41. c — Velocity Ratios and Pressure Coefficients 

 The data given here are plotted from Eq. (2.xvi), 



Ap _ Ap 



= 1 



T^Ul 



(^)' 



for potential flow in an ideal liquid. They apply to a liquid (or a fluid) of any mass density. 



