Shock-induced separation of hypersonic transitional boundary layers

在超声速和高超声速下，表面摩擦和表面传热（动能加热）是主要影响。这些都是边界层的结果，边界层是靠近身体表面的一层稀薄的缓速流动，通过粘度的作用而产生。边界层的极限状态要么是平滑流动的层流状态，要么是湍流边界层的随机波动状态。层流边界层的特点，除其他性质外，是低传热和低表面摩擦。在紊流边界层中，典型值可能要大五倍，因此了解边界层的可能状态是至关重要的。过渡现象是由最初的层流边界层转变为湍流边界层的过程。在空气动力学中，过渡本身就是一个定速项目，而过渡的许多方面仍然没有得到很好的理解。高超声速流动的一个特殊特征是过渡区域的范围可能非常大；它可能覆盖身体表面的很大一部分，因此在这个过渡状态下会发生临界流动相互作用。冲击波的作用提供了这样一种关键的相互作用。激波本身几乎是超音速流动不可避免的特征。当气流以这样一种方式转动，使其压力显著提高时，它们就会出现，并且是许多实际配置的特征，例如：吸气式发动机的进气口；控制面挠度；接近气动表面前缘的气流。如果冲击波产生的压力上升足够强，它就会产生所谓的分离流。在这种情况下，边界层不再沿着表面移动；相反，它上升到水面以上，在它下面有一个“被困”的再循环流。这是一个复杂的流程；它可能是不稳定的，除其他因素外，它的特征是表面传热率显著高于附着边界层的传热率。层流边界层比湍流边界层更容易分离，并产生更大的分离带。因此，过渡边界层可能处于中间状态。考虑到过渡边界层实际上可能在层流和湍流状态之间切换，分离的流动可能会对接近的流动的瞬时状态做出强烈反应。因此，本研究的重点是对激波分离过渡边界层的基准实验研究。这是一种通常会发生的相互作用，但由于过渡流的挑战性，迄今为止，无论是在实验上还是在计算上，都只得到了最适度的关注。这里的目标是尽可能高精度地建立过渡边界层和激波系统，并以有助于实验可重复性的方式；这需要实验人员进行最仔细的设计和管理。最终目标是生成高质量的基准数据，以解释基本的流动物理，并为计算模拟生成定义良好的测试用例，并为工程设计数据库做出贡献。

At high supersonic and hypersonic speeds, surface friction and surface heat transfer (kinetic heating) are dominant effects. These are consequences of the boundary layer, the thin layer of retarded flow near the body surface that arises through the action of viscosity. The limit states for a boundary layer are either the smooth-flowing laminar state or the random fluctuations of a turbulent boundary layer. Laminar boundary layers are characterised, amongst other properties, by low heat transfer and low skin friction. In a turbulent boundary layer typical values may be of order five times larger, so that it is critical to understand the likely boundary layer state. The phenomenon of transition is the process by which an initial laminar boundary layer changes to a turbulent one. Transition itself is a pacing item in aerodynamics and many aspects of transition are still not properly understood. A special feature of hypersonic flows is that the transition region may be very substantial in extent; it may cover a significant extent of the body surface so that critical flow interactions then occur in this transitional state. The effect of shock waves provides one such critical interaction. Shock waves, themselves, are an almost inevitable feature of supersonic flows. They arise when the flow is turned in such a manner as to raise its pressure significantly and are features of many practical configurations such as: intakes to air-breathing engines; deflection of control surfaces; the flow approaching leading edges of aerodynamic surfaces. If the pressure rise generated by a shock wave is sufficiently strong it can then generate a so-called separated flow. In this case the boundary layer no longer follows the surface; instead it rises above the surface with a 'trapped' recirculating flow beneath it. This is a complex flow; it may be unsteady and it is characterised, amongst other factors, by surface heat transfer rates that rise significantly above those of the attached boundary layer. Laminar boundary layers are much more readily separated than turbulent boundary layers and produce much larger separation zones. Transitional boundary layers therefore, potentially, occupy a midway state. Given that a transitional boundary layer may, in effect, be switching between laminar and turbulent states, the separated flow is likely to respond strongly to the instantaneous state of the approaching flow.This proposed study is therefore focussed on a benchmark experimental investigation of the separation of a transitional boundary layer by a shock wave. This is an interaction that can commonly occur but which, because of the challenging nature of transitional flows, has so far received only the most modest of attention, either experimentally or computationally. The objective here is to establish both the transitional boundary layer and the shock wave system with as high a precision as possible and in a manner that aids experimental repeatability; this requires the most careful design and management by the experimentalist. The ultimate objective is to generate high quality benchmark data that explains the basic flow physics and produces a well-defined test case for computational simulation and contributes to the engineering design database.