• Influence of instability modes on cooling performance in hypersonic boundary layer with slot injection

      Cerminara, Adriano; Hermann, Tobias; Ifti, Hassan Saad; Deiterding, Ralf; Sandham, Neil; McGilvray, Matthew (Elsevier, 2020-12-14)
      A combined numerical-experimental investigation is presented with focus on the effects of boundary-layer instabilities and transition on the wall cooling performance in a Mach 5 low-enthalpy flow over a flat plate, with coolant injection achieved through a row of slots. The numerical study has been performed through direct numerical simulation (DNS) of the compressible Navier-Stokes equations, and is supported by results from linear stability analysis (LST) for the considered boundary layer. The experiments have been conducted in the High Density Tunnel (HDT) of the Oxford Thermofluids Institute, and include several blowing ratio conditions of injected air for the same freestream conditions. Surface heat transfer and pressure measurements, film effectiveness measurements, and Schlieren images are presented. The analysis links the wall cooling performance to the growth of imposed unstable boundary layer modes. Results indicate that 2D and 3D unstable modes, pertaining to the class of first instability modes, exist in the laminar boundary layer, and that imposition of these modes at different amplitudes leads to different states of the boundary layer, which we refer to as a perturbed state and a transitional state for medium and high amplitude respectively. As confirmed by comparison with experimental data, the perturbed and transitional states of the boundary layer significantly affect the wall cooling performance, providing an increase of the wall heat flux that results in a reduction of the beneficial effects of cooling.
    • A mesoscopic modelling approach for direct numerical simulations of transition to turbulence in hypersonic flow with transpiration cooling

      Cerminara, Adriano; Deiterding, Ralf; Sandham, Neil (Elsevier, 2020-11-09)
      A rescaling methodology is developed for high-fidelity, cost-efficient direct numerical simulations (DNS) of flow through porous media, modelled at mesoscopic scale, in a hypersonic freestream. The simulations consider a Mach 5 hypersonic flow over a flat plate with coolant injection from a porous layer with 42 % porosity. The porous layer is designed using a configuration studied in the literature, consisting of a staggered arrangement of cylinder/sphere elements. A characteristic Reynolds number Rec of the flow in a pore cell unit is first used to impose aerodynamic similarity between different porous layers with the same porosity, ∈, but different pore size. A relation between the pressure drop and the Reynolds number is derived to allow a controlled rescaling of the pore size from the realistic micrometre scales to higher and more affordable scales. Results of simulations carried out for higher cylinder diameters, namely 24 µm, 48 µm and 96 µm, demonstrate that an equivalent Darcy-Forchheimer behaviour to the reference experimental microstructure is obtained at the different pore sizes. The approach of a porous layer with staggered spheres is applied to a 3D domain case of porous injection in the Darcy limit over a flat plate, to study the transition mechanism and the associated cooling performance, in comparison with a reference case of slot injection. Results of the direct numerical simulations show that porous injection in an unstable boundary layer leads to a more rapid transition process, compared to slot injection. On the other hand, the mixing of coolant within the boundary layer is enhanced in the porous injection case, both in the immediate outer region of the porous layer and in the turbulent region. This has the beneficial effect of increasing the cooling performance by reducing the temperature near the wall, which provides a higher cooling effectiveness, compared to the slot injection case, even with an earlier transition to turbulence.