In direct and large eddy simulations, very small space steps are used close to the solid walls in order to resolve the boundary-layer structures. Due to the restrictive CFL stability criteria of explicit time-stepping schemes, the maximum allowable time step is also very small, leading to high computational costs, notably for converging flow statistics. The use of an implicit integration scheme may overcome this limitation at the price of an increased computational cost per step. Moreover, the most commonly used fully implicit schemes induce higher errors due to the necessary approximations and poor dispersion and dissipation properties. As a compromise, a fourth-order implicit residual smoothing scheme (IRS4), successfully validated for a finite volume solver in Cinnella and Content (2016); Hoarau and Cinnella (2020), has been introduced in a multiblock high-order finite-difference solver. Several improvements are proposed to ensure better dissipation properties, a more efficient treatment of physical boundaries and an accurate and stable parallel multiblock implementation. For moderate CFL numbers, a similar accuracy as the explicit method is obtained with substantial savings in terms of computational time.@en

An internal unsteady compressible flowfield of a pulsed jet actuator (PJA) is simulated by unsteady Reynoldsaveraged Navier–Stokes (URANS) and large-eddy simulation (LES) frameworks. ThePJA’s feedback loops are short enough to have non-negligible jet switching time compared to the oscillation period. The results show that within a range of inlet pressure (which is the only control parameter) where the jet goes from subsonic to supersonic regime, the oscillation frequencies and the exit velocity waveform predicted by LES are in better agreement with the measured data than those predicted by URANS. Although URANS is able to capture the flowfield inside the feedback loops, it fails to predict the exit jet dynamics, unlike LES. This indicates the importance of a proper resolution of turbulence in the interaction region where jet switching occurs. Using LES data, it is found that the total jet switching time remains constant as the inlet pressure changes. It is also found that only the part of the jet switching period that ends at the first moment when the jet attaches to the opposite wall is involved in the oscillation period. Therefore, a new lumped model is proposed to estimate the oscillation frequency, which also explains the trend observed for the low inlet pressure values.@en

In this paper, computations of transonic (M = 0.85) flow over an open cavity are performed through use of a high-order solver employing different Delayed Detached-Eddy Simulation (DDES) strategies: DDES, DDES with shear-layer-adapted subgrid length scale (DDES-SLA), and Improved DDES with SLA (IDDES-SLA). The cavity is considered to have no physical lateral walls, instead the computational domain inside the cavity is terminated in that direction by periodic or slip wall conditions. Different lateral domain sizes are studied. The length-to-depth ratio of the cavity is taken constant at 5 which matches that of the classical M219 cavity, and hence the simulated results are compared with available computational and experimental data for it. The results show that mean and turbulent flow fields at the midspan plane could be captured reasonably without lateral walls in the cavity, when the slip or periodic conditions are applied at least one depth separation. In addition, the use of the SLA length scale, which accelerates the transition to resolved turbulence in shear layers, helps capturing the Kelvin-Helmholtz instability dominated region, thereby providing better turbulent and acoustic related results than use of the standard one, i.e. the maximum cell dimension. Applying slip wall boundary condition at the lateral terminations of the cavity improves the velocity gradients over the periodic one, towards the aft wall which directly affects the sound levels emanating from the cavity ceiling. The best acoustic results are obtained by IDDES-SLA among all the tested, showing good agreement with those of the reference studies. The absence of viscous lateral walls in the cavity does not seem to have an impact on the sound levels except the region close to the front wall.@en