  We report on probability-density-functions (PDF) of the mass density in
numerical simulations of highly compressible hydrodynamic flows and the
corresponding structure formation of Lagrangian particles advected by the
flows. Numerical simulations were performed with $512^3$ collocation points and
2 million tracer particles integrated over several dynamical times. We propose
a connection between the PDF of the Lagrangian tracer particles and the
predicted log-normal distribution of the density fluctuations in isothermal
systems.
