Compressibility Effects on Boundary-Layer Transition Induced by an Isolated Roughness Element
Compressibility Effects on Boundary-Layer Transition Induced by an Isolated Roughness Element
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DOI：
10.2514/1.j050186
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发表时间：
2010-12
期刊：
影响因子：
2.5
通讯作者：
J. Redford;N. Sandham;G. Roberts
中科院分区：
文献类型：
--
作者：
J. Redford;N. Sandham;G. Roberts
直接数值模拟已用于研究具有孤立粗糙度元素的平板上高速边界层向湍流的转变。粗糙度元素由连续凸块组成，其高度约为边界层厚度的一半。已在对应于绝热或冷却壁条件的两个固定壁温度的雷诺数范围内运行了 3 马赫和 6 马赫的模拟。在每种情况下，都观察到低动量流体远离壁的提升，导致形成分离的剪切层。施加声学扰动以刺激该层的不稳定性，并且在除了马赫数 6 的冷壁条件之外的所有情况下，在最高雷诺数的模拟中观察到湍流的破裂。粗糙度单元后面的流向涡量的大小随着粗糙度高度雷诺数的增加而增加，并且对马赫数和壁温相对不敏感。与发夹涡形成相关的斯特劳哈尔数通常低于不可压缩流中的斯特劳哈尔数。发现计算域内过渡的临界粗糙度高度雷诺数随着马赫数的增加而增加，并且对于冷却壁条件而言更高。发现基于粗糙度高度雷诺兹和马赫数以及壁温的相关性可以将层流与所研究的流动条件范围的过渡情况分开。
Direct numerical simulation has been used to study the transition to turbulence of a high-speed boundary layer over a flat plate with an isolated roughness element. The roughness element consists of a continuous bump that is approximately half the boundary-layer thickness in height. Simulations at Mach 3 and Mach 6 have been run over a range of Reynolds numbers for two fixed wall temperatures corresponding to either an adiabatic or a cooled wall condition. In each case a lift-up of low momentum fluid away from the wall is observed leading to the formation of a detached shear layer. Acoustic disturbances are imposed to stimulate the instability of this layer and breakdown to turbulence is observed in the simulations at the highest Reynolds number in all cases except for the Mach 6 cold wall condition. The magnitude of the streamwise vorticity behind the roughness element increases with roughness height Reynolds number and is relatively insensitive to Mach number and wall temperature. Strouhal numbers associated with hairpin vortex formation are generally lower than in incompressible flow. The critical roughness height Reynolds number for transition within the computational domain is found to increase as the Mach number increases and is higher for the cooled wall condition. A correlation based on roughness height Reynolds and Mach numbers and wall temperature is found to separate laminar from transitional cases for the range of flow conditions studied.