The effects of compressibility on the quiescent momentum core (QMC) and its thermal counterpart, the quiescent thermal core (QTC), are studied using direct numerical simulation datasets of compressible channel flows with the semilocal friction Reynolds number at the channel centerline ${\text{Re}}_{\tau c}^{*}\approx 340\text{–}1270$ and the bulk Mach number $M_b=1.5$ and 3.0. The applicability of the traditional criterion for identifying the QMC interface in incompressible flows (i.e., velocity contours of 95% of the mean channel centerline velocity, $0.95U_c$) is thoroughly examined, followed by a detailed analysis of the Reynolds and Mach number dependence of QMC characteristics. The results show that the QMC in compressible flows retains fundamental similarities to that in incompressible flows at comparable ${\text{Re}}_{\tau c}^{*}$, with low streamwise turbulence intensity and nearly constant mean flow properties within the core. However, the intermittency factor increases slightly at $M_b=3.0$, suggesting a reduced probability of the presence of QMC in the inner region of the channel at higher Mach numbers. Conditional averaging reveals a sharp change in streamwise velocity and a local maximum in spanwise vorticity at the QMC interface, with the prograde shear layer identified as the primary structure responsible for the vorticity concentration. Using the same methodology applied for the QMC, the threshold for identifying the QTC interface is determined as temperature contours of 99% of the mean channel centerline temperature ($0.99T_c$). The QTC is characterized by a thermal interfacial layer with a strong temperature gradient and a jump in temperature across its interface. Geometric analysis reveals that the QTC interface is located closer to the wall, resulting in a significant increase in its thickness compared to that of the QMC. Both the QMC and QTC interfaces exhibit fractal dimensions of $D\approx 1.3$, consistent with that of turbulent/nonturbulent interfaces. These findings highlight the structural similarities between the QMC and the QTC, providing insight into momentum and energy transport in compressible turbulent flows.

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• Channel flow
• Compressible turbulence
• Turbulence
• Turbulence simulations
• Coherent structures