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Prandtl-Glauert method : ウィキペディア英語版
Prandtl–Glauert transformation

The Prandtl–Glauert transformation is a mathematical technique which allows solving certain compressible flow problems by incompressible-flow calculation methods. It also allows applying incompressible-flow data to compressible-flow cases.
== Mathematical formulation ==

Inviscid compressible flow over slender bodies is governed by linearized compressible small-disturbance potential equation:〔Kuethe, A.M. and Chow, C.Y., Foundations of Aerodynamics, Wiley, 1976〕

\phi_ \,+\, \phi_ \,+\, \phi_ \;=\; M_\infty^2 \phi_ \;\;\;\; \mbox

together with the small-disturbance flow-tangency boundary condition.

V_\infty \, n_x \,+\, \phi_y \, n_y \,+\, \phi_z \, n_z \;=\; 0 \;\;\;\; \mbox

M_\infty is the freestream Mach number, and n_x, n_y, n_z are the surface-normal vector components. The unknown variable is the perturbation potential \phi(x,y,z), and the total velocity is given by its gradient plus the freestream velocity V_\infty which is assumed here to be along x.

\vec \;=\; \nabla \phi + V_\infty \hat \;=\; (V_\infty + \phi_x) \,\hat \,+\, \phi_y \,\hat \,+\, \phi_z \,\hat

The above formulation is valid only if the small-disturbance approximation applies
,〔Shapiro, A.H., Compressible Fluid Flow I, Wiley, 1953〕

| \nabla \phi | \ll V_\infty

and in addition that there is no transonic flow, approximately stated by the requirement that the local Mach number not exceed unity.

\left(+ (\gamma+1) \frac \right ) M_\infty^2 \;<\; 1

The Prandtl-Glauert (PG) transformation uses the Prandtl-Glauert Factor
\beta \equiv \sqrt. It consists of scaling down all y and z dimensions and angle of attack by the factor of \beta, and the potential by \beta^2.

\begin
\bar &=& x \\
\bar &=& \beta y \\
\bar &=& \beta z \\
\bar &=& \beta \alpha \\
\bar &=& \beta^2 \phi
\end

The small-disturbance potential equation then transforms to the Laplace equation,

\bar_} \,+\, \bar_} \,+\, \bar_} \;=\; 0 \;\;\;\; \mbox

and the flow-tangency boundary condition retains the same form.

V_\infty \, \bar_____

This is the incompressible potential-flow problem about the transformed geometry with surface normal vector components
\bar___ or its gradient components
\bar___ \;=\;
-\frac \frac}}

which is known as Göthert's Rule 〔Göthert, B.H. Plane and Three-Dimensional Flow at High Subsonic Speeds (Extension of the Prandtl Rule). NACA TM 1105, 1946.〕

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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