Multi-exposed recordings for 3D Lagrangian particle tracking with Multi-Pulse Shake-The-Box

Abstract

The recent introduction of the Multi-Pulse Shake-The-Box (MP-STB) method opened the possibility of extending 3D Lagrangian particle tracking (LPT) to the investigation of high-speed flows, where long time-resolved sequences of recordings are currently not available due to the limited acquisition frequency of high-speed systems. The MP-STB technique makes use of an iterative approach to overcome the limitations posed by the short observation time offered by a multi-pulse recording sequence. Multi-pulse sequences are typically obtained by synchronizing multiple illumination systems in order to generate bursts of laser pulses where the time separation can be freely adjusted down to less than a microsecond. Several strategies can be adopted for the recording of multi-pulse sequences; a dual camera system can be adopted in order to separate the single pulses onto the camera frames (either by means of polarization or timing), while the use of multi-exposed frames allows for the employment of a single imaging system, largely reducing the complexity and cost of the experimental setup. The main strategies to generate multi-pulse recording sequences are presented here; the application and performances of the MP-STB method are discussed based on the analysis of experimental data from the investigation of three turbulent boundary layer flows at velocities ranging from 10 to approximately 30 /. Results show the capability of the MP-STB technique in reconstructing accurate track fields which can be exploited both to describe instantaneous flow structures and to produce highly spatially resolved statistics by means of ensemble average in small bins. The iterative reconstruction and tracking strategy for MP-STB can be successfully adapted to the case of multi-exposed frames. Results suggest that, despite the increase in particle image density resulting from the double-exposed particle images, the adoption of multi-exposed recordings has the potential to become the technique of choice for the recording of multi-pulse sequences suitable for Lagrangian particle tracking in high-speed flows.

Particle radial distribution function and relative velocity measurement in turbulence at small particle-pair separations

“…Using iterative particle reconstruction (IPR) with the recently emerged multipulse shake-the-box (STB) algorithm brings particle position uncertainties down to 0.15 pixels (Novara et al. 2019), but when the pixel scale is small, this can be challenging to achieve if the experimental set-up experiences any slight vibrations.…”

“…We implemented our small- measurement strategy using the multipulse STB tracking algorithm (Novara et al. 2019; Sellappan, Alvi & Cattafesta 2020) based on STB (Schanz, Gesemann & Schröder 2016) implemented in DaVis 10.1 by LaVision GmbH (Göttingen, Germany), followed by an in-house particle-pair mismatch rejection code described in § 4. The STB particle tracking algorithm works by triangulating particle positions using an array of cameras and includes a unique approach to refine the particle position using the particle images, which makes it advantageous for small- measurements.…”

“…We chose based on the suggestion of as a suitable choice for the recording of multiexposed images for STB (Novara et al. 2019; Sellappan et al. 2020), and based on minimizing to reduce uncertainty at small due to interpolation error (see § 7).…”

The flow around a surface mounted cube: a characterization by time-resolved PIV, 3D Shake-The-Box and LBM simulation

“…These surface imperfections, typically in the μm-range, lead to modal and non-modal growth of the induced disturbances and then to (premature) boundary layer transition during flight. The experimental investigation of transitional flow with growing small-scale disturbances in thin boundary layers around wings at transonic velocities has recently become feasible with micro 3D STB measurement methods (Novara et al 2019). In this context, the present work can be seen as a preparatory step, taken so as to allow such laminarity studies in future research.…”

“…The 3D STB and (with limitations) the TR-PIV techniques, in principle, can be adapted to flows with much higher Reynolds numbers and flow conditions that currently are beyond to reach of DNS. The employed measurement techniques are capable of very high dynamic temporal and spatial range, at least for onepoint statistics of the velocity vector data in small fields of view (Beresh et al 2018) or by multi-pulse STB (Novara et al 2016(Novara et al , 2019. For example, in compressible and swept wing aerodynamics there is continued research interest in the laminar-transitional flow conditions when the surface mounted obstacle heights H are significantly smaller than the BL thicknesses, thus mimicking surface imperfections and roughness caused by limitations in manufacturing capabilities in aerodynamic vehicle production.…”