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Turbulent flows are ubiquitous in astrophysical environments, andunderstanding density structures and their statistics in turbulent media is ofgreat importance in astrophysics. In this paper, we study the density powerspectra, $P_{\rho}$, of transonic and supersonic turbulent flows through oneand three-dimensional simulations of driven, isothermal hydrodynamic turbulencewith root-mean-square Mach number in the range of $1 \la M_{\rm rms} \la 10$.  From one-dimensional experiments we find that the slope of the density powerspectra becomes gradually shallower as the rms Mach number increases. It isbecause the density distribution transforms from the profile with {\itdiscontinuities} having $P_{\rho} \propto k^{-2}$ for $M_{\rm rms} \sim 1$ tothe profile with {\it peaks} having $P_{\rho} \propto k^0$ for $M_{\rm rms} \gg1$. We also find that the same trend is carried to three-dimension; that is,the density power spectrum flattens as the Mach number increases. But thedensity power spectrum of the flow with $M_{\rm rms} \sim 1$ has the Kolmogorovslope. The flattening is the consequence of the dominant density structures of{\it filaments} and {\it sheets}. Observations have claimed different slopes ofdensity power spectra for electron density and cold H I gas in the interstellarmedium. We argue that while the Kolmogorov spectrum for electron densityreflects the {\it transonic} turbulence of $M_{\rm rms} \sim 1$ in the warmionized medium, the shallower spectrum of cold H I gas reflects the {\itsupersonic} turbulence of $M_{\rm rms} \sim$ a few in the cold neutral medium.

Density Power Spectrum of Compressible Hydrodynamic Turbulent Flows