Air-filled soap bubbles for volumetric velocity measurements

Developments in theoretical investigations and experimental techniques are reaching a level of maturity for which it is finally becoming possible to answer some of the most pressing questions in turbulence. The prevailing classical theories all have their strengths and drawbacks based on their respective principal assumptions. To better understand the implications of these assumptions, we have developed a theoryintensive experimental strategy. For these purposes, a laboratory has been established at the Department of Mechanical Engineering, Technical University of Denmark. The objective being to provide the data necessary to test the (bounds of) validity of the existing theories; Most prominently the classical Richardson-Kolmogorov-Batchelor paradigm, but also other generally adopted views such as Rapid Distortion Theory and Equilibrium Similarity. The measurements will be analyzed within a novel theoretical framework that enables not only quantification of the degree to which the small and intermediate scale turbulence behaves according to the existing theories (and their central assumptions), but also unveiling the underlying processes that create the respective state of turbulent flow. The present work will describe the current state of the developments of building up the laboratory.

Section: Full-field Experimentsmentioning

confidence: 99%

“…This was demonstrated by using 300 μm diameter helium filled soap bubbles and the 'Shake the Box' algorithm (Schanz et al (2016)) at kHz repetition rates, c.f. Barros et al (2021).…”

Section: Full-field Experimentsmentioning

confidence: 99%

A novel laboratory pushing the limits for optics-based basic turbulence investigations

Ribergaard¹,

Zhang²,

Abitan³

et al. 2021

“…In addition, Barros et al (2019) clarified the details of complex wake by 3D PIV measurement for the flow around the sphere using micro air-bubbles of 10 to 30 μm as a tracer. The development of a generator capable of releasing a large amount of HFSB for wind tunnel experiments has also been reported by Gibeau & Ghaemi (2018).…”

Section: Introductionmentioning

confidence: 99%

Development of micro soap bubble generator for PIV tracer using home stereolithography 3D printer

Shibata¹,

Yamazaki²,

Matsuda³

2021

A micro soap bubble generator for tracers for PIV measurement was developed using a home stereolithography 3D printer. The nozzle has a coaxial triple pipe structure, and an orifice cap is attached to the nozzle head. The inner diameter of the central pipe is 0.7 mm, and the wall thickness of the central pipe is 0.7 mm. From the comparison of the smoke wire visualization result of the flow around the cylinder placed under the mainstream flow velocity of 3 m/s and the PIV measurement result, it was confirmed that the generated micro soap bubbles have good followability to the flow. Generated bubbles’ particle size was estimated to be Φ0.2 mm at the minimum and Φ6.3 mm at the maximum. The most common was Φ0.9 mm ± 0.1 mm, accounting for more than 50% of the total.

“…Recent 3D PTV methods use several cameras to acquire simultaneous images from a volume of interest that is illuminated by a laser beam or by LEDs. A survey, reported by Barros et al (2021) shows that at sub kHz repetition rate the volume of interest is typically up to the order of 10 2 cm 3 using 1 µm oil droplets, up to 10 4 cm 3 using 300 µm helium filled soap bubbles, or up to about 10 3 cm 3 using 15 µm air filled soap bubbles (henceforth 'soap bubbles'). However, for some investigations of turbulent flows, it is essential to extend further the size of the volume of interest and work at high laser repetition rate.…”

Section: Introductionmentioning

confidence: 99%

Development of an Optical set-up for 3D PIV with a Large Volume

Abitan¹,

Velta²,

Zhang³

et al. 2021

Measurements of 3D volumetric velocity fields are of great theoretical interest with numerous practical applications. These measurements are essential for studying volumetric flows that do not exhibit inherent flow symmetry, such as turbulence or vortex breakdown. In the past decade, several technological innovations facilitated the emergence of 3D-PTV techniques for measuring velocity fields at kHz rate with volumes of interest up to 104 cm3 that contain 300 µm helium-filled soap bubbles. However, when a commercial laser beam with millijoule pulse-energy is expanded and shaped to fill volumes above 102 cm3 for 3D-PTV experiments with 15 µm air filled soap bubbles, one finds that the power density of the laser source is insufficient to generate a signal image. This is because the power density of the laser beam falls inversely with respect to its cross-section area and due to the quadratic dependence of Mie-scattering on the particle diameter. Here, we report of the analysis and development of two optical techniques for extending the volume of measurement in volumetric PTV. In particular, when a volume about 103 cm3 is seeded with 15 µm air-filled soap bubbles and a laser with a pulse energy of few single mJ illuminates it. The first technique uses multi reflections between two opposing parallel mirrors. The second technique is a development of laser scanning PIV for volumetric scanning: The potential to increase the scanned volume is examined by experimenting with an acousto-optic modulator for fast scanning. Furthermore, by employing an off-axis parabolic mirror, we obtain parallel beam scanning, which increases the efficiency and quality of the scanning.