3D Lagrangian Particle Tracking with Multi-Pulse Shake-The-Box in Turbulent Boundary Layer Flows at High Reynolds Numbers

Abstract

The recent introduction of the Multi-Pulse Shake-The-Box technique (MP-STB [3]) extended the capabilities of the Shake-The-Box (STB [4]) 3D Lagrangian particle tracking (LPT) algorithm in terms of applicable flow velocity. In particular, the MP-STB allows for 3D LPT investigations at velocities relevant for industrial and aerodynamic applications in relatively large volumes where the adoption of a time-resolved recording strategy would result in significant compromises in terms of spatial range or resolution (in order to limit the particle displacement between subsequent recordings). Multi-pulse sequences are obtained by synchronizing multiple illumination systems to generate bursts of laser pulses (typically four) where the time separation can be freely adjusted down to less than a microsecond (e.g. for supersonic applications). A 3D imaging system consisting of dual-frame cameras is adopted, where each frame is double-exposed to record the four pulses [2]; the MP-STB technique makes use of an iterative approach to overcome the limitations posed by the short observation time offered by the multi-pulse recording sequence, and allows for the reconstruction of individual four-pulse particle tracks in 3D [3]. The acquisition of long sequences of four-pulse recordings at low frequency (10-15Hz) enables the evaluation of flow statistics. In the present study, the MP-STB technique is employed for the investigation of turbulent boundary layer (TBL) flows at relatively high Reynolds numbers. A TBL at 15 m/s (Reθ ≈ 10; 000) is investigated in the SWG facility at DLR Göttingen; approximately 95; 000 individual tracks are reconstructed within the 80×120×10 mm^3 domain for each four-pulse recording. Spatial gradients, as well as the instantaneous 3D pressure field, are evaluated by means of the FlowFit data assimilation algorithm [1] (fig.1-left). A TBL with adverse pressure gradient (APG) at 29 m/s (Reθ ≈ 30; 000) is investigated in the atmospheric wind tunnel of the University of Armed Forces in Munich; both planar PIV and MP-STB in a 80 × 90 × 7 mm^3 domain are employed; the direct comparison of the results in terms of turbulent fluctuations (fig.1-right) highlights the higher spatial resolution of the LPT approach which, unlike PIV, does not suffer from the signal modulation caused by the finite size of the cross-correlation window. In the final presentation the MP-STB processing technique will be introduced together with the main parameters affecting its performances. Moreover, results regarding the TBL structure and flow statistics (e.g. TBL profiles, two-point correlations) will be presented.