Sengupta Uttiya, Nemati Hassan, Boersma Bendiks J, Pecnik Rene
Department of Process and Energy, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, Netherlands.
Flow Turbul Combust. 2017;99(3):909-931. doi: 10.1007/s10494-017-9872-4. Epub 2017 Nov 21.
This work investigates fully developed turbulent flows of carbon-dioxide close to its vapour-liquid critical point in a channel with a hot and a cold wall. Two direct numerical simulations are performed at low Mach numbers, with the trans-critical transition near the channel centre and the cold wall, respectively. An additional simulation with constant transport properties is used to selectively investigate the effect of the non-linear equation of state on turbulence. Compared to the case where the pseudo-critical transition occurs in the channel center, the case with the pseudo-critical transition close to the cold wall reveals that compressibility effects can exist in the near-wall region even at low Mach numbers. An analysis of the velocity streaks near the hot and the cold walls also indicates a greater degree of streak coherence near the cold wall. A comparison between the constant and variable viscosity cases at the same Reynolds number, Mach number and having the same isothermal wall boundary conditions reveals that variable viscosity increases turbulence near the cold wall and also causes higher velocity gradients near the hot wall. We also show that the extended van Driest transformation results in a better agreement of the velocity profile with the log-law of the wall compared to the standard van Driest transformation. The semi-locally scaled turbulent velocity fluctuations and the turbulent kinetic energy budgets on the hot and the cold sides of the channel collapse on top of each other, thereby establishing the validity of Morkovin's hypothesis.
本研究探讨了在具有热壁和冷壁的通道中，接近其气液临界点的二氧化碳充分发展的湍流。在低马赫数下进行了两次直接数值模拟，分别在通道中心附近和冷壁附近发生跨临界转变。还进行了一次具有恒定输运特性的模拟，以选择性地研究非线性状态方程对湍流的影响。与伪临界转变发生在通道中心的情况相比，伪临界转变靠近冷壁的情况表明，即使在低马赫数下，近壁区域也可能存在压缩性效应。对热壁和冷壁附近的速度条纹进行分析，也表明冷壁附近的条纹相干程度更高。在相同雷诺数、马赫数且具有相同等温壁边界条件下，对恒定粘度和可变粘度情况进行比较，结果表明可变粘度会增加冷壁附近的湍流，并且在热壁附近会导致更高的速度梯度。我们还表明，与标准的范德瑞斯特变换相比，扩展的范德瑞斯特变换使速度剖面与壁面对数律的一致性更好。通道热侧和冷侧的半局部尺度湍流速度波动和湍动能收支相互重叠，从而证实了莫尔科文假设的有效性。
