The aero-thermal behaviour of surface microstructures is of wide relevance, especially given the development of additive manufacturing (AM). Of particular interest is the interaction between fluid flow and heat transfer. In the present work, two contrasting configurations, a flat plate boundary layer and an array of hemispheric micro-structures are examined at three wall-inflow temperature ratios (TR): cooled (T R=0.5), adiabatic (T R=1) and heated wall (T R=1.5). Due to a compensation between fluid viscosity and velocity gradient in the boundary layer, the heat transfer effects may appear deceptively small if judged using the common aerothermal parameters (Cf ,Nu). The authors find instead the local Reynolds number to be more usefully indicative of such aerothermal interaction. The scale-resolving LES simulations at a range of Reynolds numbers show that the cooled wall case is characterized by a markedly earlier transition which takes place at a much lower (by 50%) bulk flow Reynolds number compared to a near-adiabatic case. Furthermore, it is shown that the incompressible flow LES solutions fail to capture the early transition under the same cooling condition. Finally, a regrouping of the non-dimensional parameters (CD, Nu) with TR is proposed leading to more unified characterization for easier scaling of wall heat transfer effects in practical applications.

The injection of air at a controlled pressure into a confined 2 mm thick layer of a dense suspension (cornstarch/water mixture at 57.5 wt%) leads to the formation of branched fractures at a gauge pressure P = 10 kPa (top) and the formation of straight fractures at P = 20 kPa (bottom). The transition occurs when the characteristic width of the branched fractures becomes comparable to the thickness of the layer.