In this work, the interaction of the separated wake of the front wing with the rear wing of a tandem configuration is investigated for high-speed stall conditions by means of hybrid RANS/LES simulations, using the zonal AZDES method. After a characterization of the transonic buffet on the front wing, the development of the separated turbulent wake behind the wing is investigated. The interaction of the separated wake with the rear wing is then analyzed in detail. The results reveal that there is a strong variation in the wake characteristics over the buffet cycle, caused by the varying amount of separation on the front wing. During the upstream movement of the shock, the flow is largely separated, resulting in a thick wake with strong, high-frequent fluctuations that can be attributed to large turbulent vortices. On the contrary, when the shock travels downstream, there is only a small amount of separation present, resulting in a thin wake with comparatively low fluctuations that are caused by corresponding smaller turbulent vortices. The impact of the wake of the front wing causes a strong variation in the rear wing loading. An oscillation with a comparatively low frequency can be distinguished from high-frequent fluctuations. The low-frequent oscillation is caused by the variation in the downwash behind the front wing as its lift changes during the buffet cycle. The high-frequent fluctuations are due to the impingement of the turbulent structures onto the rear wing. Because both size and frequency of those vortices vary significantly within the buffet cycle, the amplitude and frequency of the lift and surface pressure fluctuations also change accordingly.

Single droplet impacts onto thin wall-films are a common phenomenon in many applications. For sufficiently high impact velocities, the droplet impact process consists of three phases, i.e., initial contact stage, droplet deformation with radial momentum transfer inducing an upward rising lamella, and crown propagation. Here, we present the results of a combined numerical and experimental study focusing on the early dynamics of the impact process. Specifically, the effects of the initial droplet shape, wall-film thickness, and contact line motion are analyzed. Prior to impact, an oblate spheroidal droplet shape was observed. Using direct numerical simulation, we show that the droplet shape affects the impact dynamics only during the first two phases, as it is one of the key parameter influencing the correct prediction of the impact zone. The contact line propagation is described by a square-root-time dependence R¯CL=ατ for both, dry and wetted surfaces. On dry surfaces, the advancement of the contact line is determined by the rolling motion of the truncated droplet. On wetted surfaces, the value of the α-parameter is controlled by two concurrent effects, namely, rolling motion and wall-film inertia. For impact onto thin films, the rolling motion prevails. With increasing wall-film height, the droplet penetrates into the soft substrates and wall-film inertia becomes the controlling factor. These insights into the early impact dynamics on wetted surface are important for the formulation of a unified modeling approach.

Spray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact ($\alpha =$ 60°) of a single droplet on a horizontal and quiescent wall film of the same liquid. A droplet generator with an accelerated needle releases the droplets ($D=$ 1.5 mm) in a controlled trajectory on a thin film ($h_f/D=$ 0.2). The impact process is recorded from two perspectives with two synchronized high-speed cameras. Varying the Weber number within the splashing regime reveals distinct crown morphologies, which are described in detail. For $We<$ 500, a single central finger develops at the front of the crown, with subsequent detachments of secondary droplets. At higher $We$ (>500), a collision of the crown with the wall film shortly after impact introduces disturbances into the rim, leading to two fingers in the middle of the front crown. A further increase in $We$ (>600) intensifies the crown–film interaction, resulting in an early ejection of tiny droplets and a complete breakup of the front rim. The influence of $We$ on the crown morphology during an oblique impact is also compared to the normal impact (90°). This study paves the way for a classification of impact regimes and a comprehensive picture of the oblique impact process, which deserve more investigation.

Understanding the flow dynamics within a thin wall film during droplet impact spreading is crucial. A key open question concerns the dominant forces governing the temporal decay of stagnation point flow strength. This study employs direct numerical simulations to investigate the flow field across a range of viscosities and different wall boundary conditions. Results demonstrate that viscous forces, particularly wall friction, significantly influence the temporal evolution of the stagnation point flow, even at high Reynolds numbers [O(105)], causing deviation from inviscid theory and adherence to an exponential decay. Furthermore, a comparison with an analytical boundary layer model reveals that pressure, gravity, and surface tension forces are negligible during spreading.

Spray impacts occur in several environmental and technical applications. The impact of droplets at different angles onto walls covered with a thin film of the same liquid can be regarded as an elementary process here. Direct Numerical Simulations (DNS) provide an important contribution to the understanding and modeling of the impact outcome, which might be associated with the formation of a crown and ejection of secondary droplets. Thus, we gain detailed information about, e.g. the flow field and shape of the interface, which are not accessible in experiments. This chapter presents a DNS study of the early crown formation mechanisms present at an oblique droplet impact on a thin film, as well as a grid study showing the resolution required to resolve the impact's details. Highly resolved simulations in large domains require continuous development of the numerical solver's efficiency. The performance of different cycles of the multigrid (MG) solver for the solution of the pressure Poisson equation was compared, and a F-cycle was added. Furthermore, we implemented a hybrid MPI and OpenMP parallelisation, which both increases the scaling limit further. Additionally, studies on the strong and weak scaling are conducted. The choice of the F- and V- cycle in the MG-solver and the additional hybrid parallelisation increased the achieved computed cycles per hour (CPH) by a factor of 12.4 compared to the formerly employed setup. The nodes efficiently usable were increased by a factor of 16. Both, the close to linear scaling regime for the strong scaling and the almost constant performance regime for the weak scaling were increased by this factor.