In fluid dynamics, Howarth–Dorodnitsyn transformation (or Dorodnitsyn-Howarth transformation) is a density-weighted coordinate transformation, which reduces variable-density flow conservation equations to simpler form (in most cases, to incompressible form). The transformation was first used by Anatoly Dorodnitsyn in 1942 and later by Leslie Howarth in 1948.^{[1][2][3][4][5]} The transformation of ${\displaystyle y}$ coordinate (usually taken as the coordinate normal to the predominant flow direction) to ${\displaystyle \eta }$ is given by

${\displaystyle \eta =\int _{0}^{y}{\frac {\rho }{\rho _{\infty ))}\ dy,}$

where ${\displaystyle \rho }$ is the density and ${\displaystyle \rho _{\infty ))$ is the density at infinity. The transformation is extensively used in boundary layer theory and other gas dynamics problems.

Keith Stewartson and C. R. Illingworth, independently introduced in 1949,^[6][7] a transformation that extends the Howarth–Dorodnitsyn transformation to compressible flows. The transformation reads as^[8]

${\displaystyle \xi =\int _{0}^{x}{\frac {c}{c_{\infty ))}{\frac {p}{p_{\infty ))}\ dx,}$

${\displaystyle \eta =\int _{0}^{y}{\frac {\rho }{\rho _{\infty ))}\ dy,}$

where ${\displaystyle x}$ is the streamwise coordinate, ${\displaystyle y}$ is the normal coordinate, ${\displaystyle c}$ denotes the sound speed and ${\displaystyle p}$ denotes the pressure. For ideal gas, the transformation is defined as

${\displaystyle \xi =\int _{0}^{x}\left({\frac {c}{c_{\infty ))}\right)^{(3\gamma -1)/(\gamma -1)}\ dx,}$

${\displaystyle \eta =\int _{0}^{y}{\frac {\rho }{\rho _{\infty ))}\ dy,}$

where ${\displaystyle \gamma }$ is the specific heat ratio.