The characteristics of the incoming turbulence are known to significantly affect the aerodynamic performance of immersed bodies and highly contribute to the overall losses. Due to the broad applicability of free-stream turbulence for defining initial and boundary conditions used in CFD, giving necessary attention to the generation of synthetic turbulent fields is essential for the more accurate development process. The present paper describes a synthetic homogeneous isotropic free-stream turbulence generation method. The method is validated using direct numerical simulations of a doubly periodic streamwise evolving channel, representing the free stream. It is demonstrated that input specification of turbulence intensity and turbulence integral length scale can reproduce realistic and self-consistent turbulence structures of the desired spectrum. By varying numerical and physical parameters, we show free-stream turbulence's spatial and temporal development. This method can be used as an inlet boundary for different types of flow found in industrial applications, like wings and turbine blades.

Free-stream turbulence (FST) and its effect on boundary-layer transition is a complex multiscale problem. Under action of FST, elongated streamwise streaky structures are generated inside the boundary layer, and their amplitude and wavelength are crucial for the transition onset. While turbulence intensity is strongly correlated with the transitional Reynolds number, characteristic length scales of the FST are often considered to have a slight impact on the transition location. Conversely, a recent experiment by Fransson & Shahinfar (2020) shows significant effects of FST scales. They found that, for low values of turbulence intensity, an increase in length scale advances transition, which agrees with literature. However, for high turbulence intensities, an increase in length scale postpones transition. Here, we aim at physically understanding and verifying the results of Fransson & Shahinfar (2020) by performing a series of high-fidelity simulations. These results provide understanding why the FST integral length scale affects the transition location differently depending on intensity. Knowing this relation is crucial for the development of transition models, which are commonly used in turbomachinery and aeronautics. A correct transition point is essential as all boundary layer properties, including friction and heat-transfer coefficients, drastically change from laminar to turbulent, and thus fundamentally affects the complete flow.

The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths toward a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behavior of the low-pressure turbine blade. In the simulations, the flow past only one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a super-position of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu = 0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in the case of low free-stream turbulence on the suction side, the Kelvin-Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Gortler instability.

In the present work, the laminar-turbulent transition of the flow evolving arounda low-pressure turbine blade has been investigated. Direct numerical simulationshave been carried out for two different free-stream turbulence intensity (FSTI)levels to investigate the role of free-stream oscillations on the evolution of theblade boundary layer. Emphasis is posed on identifying the mechanisms drivingthe formation and breakup of coherent structures in the high FSTI case andhow these processes are affected by the leading-edge receptivity and/or bythe continuous forcing in the blade passage. Proper orthogonal decomposition(POD) has been adopted to provide a clear statistical representation of theshape of the structures. Extended POD projections provided temporal andspanwise correlations that allowed us to identify dominant temporal structuresand spanwise wavelengths in the transition process.The extended POD analysis shows that the structures on the pressure sideare not related to what happens at the leading edge. The results on the suctionside show that the modes defining the leading edge and the passage basescorrelate with coherent structures responsible for the transition. The mostenergetic mode of the passage basis is strongly related to the most amplifiedwavelength in the boundary layer and breakup events leading to transition.Modes with a smaller spanwise wavelength belong to the band predicted byoptimal disturbance theory, they amplify with a smaller gain in the rear suctionside, and they show the highest degree of correlation between the passage regionand the rear suction side.

In the present work the evolution of the boundary layer over a low-pressureturbine blade is studied by means of direct numerical simulations. The set-upof the simulations follows the experiments by Lengani et al. (2017), aimingto investigate the unsteady flow field induced by the rotor-stator interaction.The free-stream flow is characterized by high level of free-stream turbulenceand periodically impinging wakes. As in the experiments, the wakes are shedby moving bars modeling the rotor blades and placed upstream of the turbineblades. To include the presence of the wake without employing an ad-hoc model,we simulate both the moving bars and the stationary blades in their respectiveframes of reference and the coupling of the two domains is done throughappropriate boundary conditions. The presence of the wake mainly affects thedevelopment of the boundary layer on the suction side of the blade. In particular,the flow separation in the rear part of the blade is suppressed. Moreover, thepresence of the wake introduces alternating regions in the streamwise direction ofhigh- and low-velocity fluctuations inside the boundary layer. These fluctuationsare responsible for significant variations of the shear stress. The analysis of thevelocity fields allows the characterization of the streaky structures forced inthe boundary layer by turbulence carried by upstream wakes. The breakdownevents are observed once positive streamwise velocity fluctuations reach theend of the blade. Both the fluctuations induced by the migration of the wakein the blade passage and the presence of the streaks contribute to high valuesof the disturbance velocity inside the boundary layer with respect to a steadyinflow case. The amplification of the boundary layer disturbances associatedwith different spanwise wavenumbers has been computed. It was found thatthe migration of the wake in the blade passage stands for the most part of theperturbations with zero spanwise wavenumber. The non-zero wavenumbers arefound to be amplified in the rear part of the blade at the boundary betweenthe low and high speed regions associated with the wakes.

In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.

The transition from laminar to turbulent flow is a long-standing research subjectin the field of fluid mechanics. A basic necessity for such studies is a distinctionbetween laminar and turbulent flow. In particular, a binary segmentation isdesired where laminar and turbulent regions behave consistently over time.Previous work in this regard yield inconsistent results, or are restricted to 2Dmanifolds thereby neglecting the three-dimensional nature of the problem. Inthis paper, we present a novel use of feature-based methods to segmenting a3D time-dependent flow into regions of laminar and turbulent behavior. It isbased on the extraction of local extrema from a scalar field such as spanwisevelocity. It turns out that the existence of a large number of extrema in aregion is a good indicator for turbulence. We derive a density function fromthe extracted extrema using a Kernel Density Estimate (KDE) and thresholdit to achieve a binary segmentation into laminar and turbulent regions. Ourmethod shows consistent results and enables the domain scientists to study thethree-dimensional aspects of the laminar-turbulent transition that were difficultto assess before.