Flows over airfoils and blades in rotating machinery, for unmanned and micro-aerial vehicles, wind turbines, and propellers consist of a laminar boundary layer near the leading edge that is often followed by a laminar separation bubble and transition to turbulence further downstream. Typical RANS turbulence models are inadequate for such flows. Direct numerical simulation (DNS) is the most reliable, but is also the most computationally expensive alternative. This work assesses the capability of Immersed Boundary (IB) methods and Large Eddy Simulations (LES) to reduce the computational requirements for such flows and still provide high quality results. Two-dimensional and three-dimensional simulations of a laminar separation bubble on a NACA-0012 airfoil at Re_c=5×10⁴ at 5° of incidence have been performed with an IB code and a commercial code using body fitted grids. Several Subgrid Scale (SGS) models have been implemented in both codes and their performance evaluated. For the two-dimensional simulations with the IB method the results show good agreement with DNS benchmark data for the pressure coefficient C_p and the friction coefficient C_f but only when using dissipative numerical schemes. There is evidence that this behavior can be attributed to the ability of dissipative schemes to damp numerical noise coming from the IB. For the three-dimensional simulations the results show a good prediction of the separation point, but inaccurate prediction of the reattachment point unless full DNS resolution is used. The commercial code shows good agreement with the DNS benchmark data in both two and three-dimensional simulations, but the presence of significant, unquantified numerical dissipation prevents a conclusive assessment of the actual prediction capabilities of very coarse LES with low order schemes in general case.

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We investigate a passive flow-control technique for the interaction of an oblique shock generated by an 8.8° wedge with a turbulent boundary-layer at a free-stream Mach number of Ma_∞=2.3 and a Reynolds number based on the incoming boundary-layer thickness of Reδ0=60.5×103 by means of large-eddy simulation (LES). The compressible Navier-Stokes equations in conservative form are solved using the adaptive local deconvolution method (ALDM) for physically consistent subgrid scale modeling. Emphasis is placed on the correct description of turbulent inflow boundary conditions, which do not artificially force low-frequency periodic motion of the reflected shock. The control configuration combines suction inside the separation zone and blowing upstream of the interaction region by a pressure feedback through a duct embedded in the wall. We vary the suction location within the recirculation zone while the injection position is kept constant. Suction reduces the size of the separation zone with strongest effect when applied in the rear part of the separation bubble. The analysis of wall-pressure spectra reveals that all control configurations shift the high-energy low-frequency range to higher frequencies, while the energy level is significantly reduced only if suction acts in the rear part of the separated zone. In that case also turbulence production within the interaction region is significantly reduced as a consequence of mitigated reflected shock dynamics and near-wall flow acceleration.

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We evaluate the suitability of micro vortex generators for the passive flow control of shock-wave/turbulent boundary layer interactions. For this purpose, implicit large eddy simulations using the adaptive local deconvolution method are performed. The flow configuration consists of an oblique shock with deflection angle β = 9.5°, impinging on a turbulent boundary layer at Ma∞ = 2.31 and Re = 67.4·10³. Analysis focuses on the assessment of the relative displacement between the control devices and the shock system, the complex flow structure behind the devices and the low-frequent motions of the separated region.

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Flows over airfoils and blades in rotating machinery, for unmanned and micro-aerial vehicles, wind turbines, and propellers consist of a laminar boundary layer near the leading edge that is often followed by a laminar separation bubble and transition to turbulence further downstream. Typical RANS turbulence models are inadequate for such flows. Direct numerical simulation (DNS) is the most reliable but is also the most computationally expensive alternative. This work assesses the capability of LES to significantly reduce the resolution requirements for such flows and still provide results of DNS quality. Two flows are considered. A flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble and a 2-D flow on a NACA-0012 airfoil. By employing several different codes we conclude that accurate LES are possible using O(1%) of the DNS resolution and that the numerical dissipation plays a significant role in LES of such flows.

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We investigate a passive flow-control technique for the interaction of an oblique shock generated by an 8.8_∞ wedge with a turbulent boundary layer at a free-stream Mach number of M = 2.3 by means of large-eddy simulation. The control configuration studied combines suction applied inside the separation zone and injection acting upstream of the interaction region through a passive-channel concept. We present results for three configurations, which differ in the suction location within the recirculation zone while the injection position is kept constant. Besides the analysis of mean flow properties, the main focus lies on the turbulence evolution within the interaction region and the effect of the control devices on the low-frequency dynamics of the reflected shock movement.

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Results of a large-eddy simulation (LES) of a supersonic turbulent boundary layer flow along a compression-expansion ramp configuration are presented. The numerical simulation is directly compared with an available experiment at the same flow conditions. The compression-expansion ramp has a deflection angle of β=25°. The flow is characterized by a free-stream Mach number of Ma_∞=2.88 and the Reynolds number based on the incoming boundary layer thickness is Reδ0=132840. The Navier Stokes equations for compressible flows are solved on a cartesian collocated grid. About 32.5×10⁶ grid points are used to discretize the computational domain. Subgrid scale effects are modeled implicitly by the adaptive local deconvolution method (ALDM). A synthetic inflow-turbulence technique is used, which does not introduce any low frequency into the domain, therefore avoiding any possible interference with the shock/boundary layer interaction system. Statistical samples are gathered over 800 characteristic time scales δ₀/U_∞. The numerical data are in good agreement with the experiment in terms of mean surface-pressure distribution, skin-friction, mean velocity profiles, velocity and density fluctuations. For the first time the full compression-expansion ramp configuration was taken into account. The computational results confirm theoretical and experimental findings on fluctuation-amplification across the shockwave/boundary layer interaction region and on turbulence damping through the interaction with rarefaction waves. The LES provide evidence of the existence of Görtler-like structures originating from the recirculation region and traveling downstream along the ramp. An analysis of the wall pressure field clearly shows the presence of a low frequency motion of the shock and strong influence of the Görtler-like vortices on the wall pressure spectra.

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We report on an experimental and computational effort to study the interaction of a compressible turbulent boundary layer with an oblique shock wave. A wide range of shock intensities has been considered in the experiments through a variation of the free-stream Mach number. Detailed mean flow and surface measurements have been carried out for the Ma = 3 configuration together with the employment of a high-speed Schlieren technique. A series of numerical computations have been also carried out in order to highlight the main features of the flow. Reynolds averaged Navier-Stokes computation have been performed on both 2-D and 3-D configurations showing the influence of three-dimensional effects on the considered flow. A detailed Large Eddy Simulation has been performed on the Ma = 3 configuration in order to investigate the unsteady features of the shock boundary layer interaction phenomenon.@en

We present a loosely-coupled approach for the solution of the thermo-fluid-structure interaction problem, based on Dirichlet-Neumann partitioning. A cartesian grid finite volume scheme, with conservative interface method is used for the fluid and a finite-element scheme for the thermo-structure problem. Special attention is given to the transfer of forces, temperatures and to the structural positions. The structural surface is represented by a level set function in the fluid code. The velocity and temperature field required for the coupling are interpolated from structural values on the zero-contour level set surface. Data transfer between the two codes is performed via message passing interface. The proposed method is tested for a cooling-process of a heated metal bar by mean of an external laminar boundary layer flow. Results show that the presented approach is able to handle the complexity of the three-field problem.

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The unsteady behaviour in shockwave turbulent boundary layer interaction is investigated by analysing results from a large eddy simulation of a supersonic turbulent boundary layer over a compressiona-expansion ramp. The interaction leads to a very-low-frequency motion near the foot of the shock, with a characteristic frequency that is three orders of magnitude lower than the typical frequency of the incoming boundary layer. Wall pressure data are first analysed by means of Fourier analysis, highlighting the low-frequency phenomenon in the interaction region. Furthermore, the flow dynamics are analysed by a dynamic mode decomposition which shows the presence of a low-frequency mode associated with the pulsation of the separation bubble and accompanied by a forwarda-backward motion of the shock.

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Results of a large-eddy simulation (LES) of a supersonic turbulent boundary layer flow along a compression-expansion ramp configuration are presented. The numerical simulation is directly compared with an available experiment at the same flow conditions. The compression-expansion ramp has a deflection angle of β = 25°. The flow is characterized by a free-stream Mach number of Ma∞ = 2.88 and the Reynolds number based on the incoming boundary layer thickness is Reδ0 = 132840. The Navier Stokes equations for compressible flows are solved on a cartesian collocated grid. About 32.5 × 106 grid points are used to discretize the computational domain. Subgrid scale effects are modeled implicitly by the adaptive local deconvolution method (ALDM). A synthetic inflow-turbulence technique is used, which does not introduce any low frequency into the domain, therefore avoiding any possible interference with the shock/boundary layer interaction system. Statistical samples are gathered over 1000 characteristic time scales δ0/U∞. The numerical data is in good agreement with the experiment in terms of mean surfacepressure distribution, skin-friction, mean velocity profiles, velocity and density fluctuations. The computational results confirm theoretical and experimental results on fluctuationamplification across the interaction region. In the wake of the main shock a shedding of shocklets is observed. Results show the development of Görtler-like vortices in the reattachment region. The LES provide a reliable and detailed flow information, which helped to improve considerably the understanding of shock-boundary-layer interaction.@en