The study of the effect of close placing of two individual elements of roughness with the cylindrical shape on the nature of laminar-turbulent transition were performed. Experiments were performed for a blunt conical model with the radius 9 mm at Mach number M = 5. These double elements of roughness with different heights and divergence angles between elements were allocated at the blunted nose of the tested model. Measurements with a hot-wire anemometer provided information about the averaged and unsteady parameters of the boundary layer in the wake behind the roughness elements. For all types of roughness, we confirmed existence of an effective height of roughness; for the heights above the effective one, we observe deformation of the boundary layer margin and the growth of non-equilibrium in the wake. Process of turbulization behind a double roughness element (similar to the case of single roughness) is accompanied by the generation of longitudinal vortices and by the deformation of the velocity profile. Depending on this deformation, pulsations in the wake either enhance or decline. In contrast to the single roughness configuration, the double element roughness decreases the mass flowrate for a narrow range of angles (and fullness of the boundary layer profile is more significant). Meanwhile, flow turbulization occurs right behind the single element of turbulization, for the case of double element turbulization, the main gain in the mass flowrate occurs with the wake (developing from the boundary between twin elements).Roughness has significant influence on unsteady characteristics of the boundary layer (when the height of roughness element is lower than the effective height). The wake downstream the double elements roughness exhibits the interaction between vortices, and this reduces the effective Reynolds number (compared to the case of single roughness).

中文翻译：

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高超声速边界层中双粗糙度元素后面的尾流湍流

研究了紧密放置两个具有圆柱形状的粗糙度元素对层流湍流过渡特性的影响。对半径为9 mm，马赫数M = 5的钝圆锥模型进行了实验。这些具有不同高度和元素之间的发散角的粗糙度双元素分配在测试模型的钝头处。用热线风速仪进行的测量提供了有关粗糙度元素后面的尾流中边界层的平均和非稳定参数的信息。对于所有类型的粗糙度，我们都确认存在有效的粗糙度高度。对于高于有效高度的高度，我们观察到边界层边界的变形和尾流中非平衡的增长。在双重粗糙度元素后面的湍流过程（类似于单一粗糙度的情况）伴随着纵向涡旋的产生和速度分布的变形。根据这种变形，尾流中的脉动增强或减弱。与单粗糙度配置相比，双元素粗糙度在较窄的角度范围内会降低质量流率（并且边界层轮廓的填充度更为显着）。同时，流动湍流恰好发生在单个湍流元素的后面，对于双元素湍流，质量流量的主要增益随尾流而发生（从双晶元素之间的边界发展）。粗糙度对边界层的不稳定特性有重大影响（当粗糙度元素的高度低于有效高度时）。双元素粗糙度下游的尾流显示出涡旋之间的相互作用，并且这降低了有效雷诺数（与单一粗糙度相比）。