Investigating the Effects of Surface Topography and Roughness on Turbine Aerodynamic Performance

为了实现欧洲航空研究咨询委员会(ACARE)2050年减少飞机排放的飞行路径计划，降低飞机发动机的SFC已成为一项日益重要的任务。这一挑战的一部分是要求通过提高机械级效率来提高燃气轮机的整体空气动力效率。在航空发动机中，熵是衡量低效的有用量度，在航空发动机涡轮中产生的大约三分之一的熵与气流和叶片表面之间的“空气动力摩擦”有关。边界层内的熵产生速率的一个主要因素是空气和叶片之间的“表面摩擦”，它强烈地依赖于叶片表面的粗糙度(地形)。当粗糙单元突出通过层流子层时，由粗糙引起的扰动决定了表面摩擦力的增加。在很少考虑粗糙度的情况下，利用中心线平均粗糙度高度Ra的粗略测量作为单一的负责几何参数，确定对表面摩擦的影响的能力受到相关性不足的限制。具有相同Ra的表面可以具有不同的流向粗糙度分布，其中表面类似于起伏的丘陵或陡峭的山脉。这决定了层流亚层是在地形上扰动还是突出。PHD问题的一个额外维度是涡轮叶片中存在压力梯度，特别是吸力面上的扩散区域。该博士学位的目的是加深对表面形貌(粗糙度)和气动损失之间的关系的理解。这项研究的成果之一将是建立可应用于所有叶片表面(即发动机运行的、新型陶瓷基复合材料和制造的)表面形貌影响的关联式。更好地了解表面粗糙度如何影响表面摩擦力不仅对当前的设计很重要，而且对于成功实施新材料和制造工艺也是至关重要的。博士将采取实验和数值相结合的方法。罗登风洞(位于低速惠特尔实验室)中的平板衬垫工作段将被用来模拟放大发动机的典型边界层，从而可以测量表面地形对表面摩擦的影响。将使用一种数值工具来确定哪些表面需要进行3D打印和实验测试。这将是确定相关性的主要方法，这种相关性可以适用于所有真实的涡轮叶片表面。为了扩大研究范围，将进行高保真计算流体力学(特别是大涡模拟)，以确定是否可以捕捉到导致粗糙度气动损失的流体结构。

Reducing the SFC of aircraft engines has become an increasingly significant task in order to achieve Advisory Council for Aeronautics Research in Europe's (ACARE) 2050 flightpath plan to reduce aircraft emissions. A part of this challenge is the requirement to improve the overall aerodynamic efficiency of gas turbines by increasing machine stage efficiencies. Within aeroengines, entropy is a useful measure of inefficiency and approximately one third of the entropy generated within an aeroengine turbine is associated with the "aerodynamic friction" between the air flow and the blade surfaces. A major contributor to the rate of entropy production within a boundary layer is the "skin friction" between the air and blade which depends strongly on the roughness (topography) of the blade surface. When the roughness elements protrude through the laminar sublayer, the perturbations resulting from the roughness determines the rise in skin friction. In the rare instance that roughness is accounted for, the capabilities of determining the impact on the skin friction are constrained by insufficient correlations utilising coarse measurements of the centreline averaged roughness height, Ra, as the singular responsible geometric parameter. A surface with the same Ra can have various streamwise roughness distributions where the surface is akin to rolling hills or steep mountains. This determines whether the laminar sublayer perturbs over the topography or is protruded through. An added dimension to the PhD problem is the presence of a pressure gradient in turbine blades and particularly the region of diffusion on the suction surface. The aim of this PhD is to develop understanding of the relationship between surface topography (roughness) and aerodynamic loss. One of the deliverable of this research will be the development of correlations for the effects of surface topography that can be applied to all blade surfaces (i.e engine-run, novel ceramic matrix composites and as-manufactured). An improved understanding of how surface roughness affects skin friction is not only important to current designs but essential for the successful implementation of new materials and manufacturing processes. The PhD will take an experimental and numerical approach. A flat-plate liner working section in the Rhoden wind tunnel (situated in the Low Speed Whittle Lab) will be used to simulate scaled-up engine representative boundary layer, allowing the impact of the surface topography on skin friction to be measured. A numerical tool will be used to determine which surfaces to be 3-D printed and experimentally tested. This will be the main method used to determine correlations which can be applicable to all real turbine blade surfaces. To broaden the scope of the research, high fidelity computational fluid dynamics (in particular, large eddy simulation) will be performed to see if the fluid-structures responsible for the roughness induced aerodynamic loss can be captured.