Helium-filled soap bubbles for vortex core velocimetry

Caridi, Giuseppe Carlo Alp; Sciacchitano, Andrea; Scarano, Fulvio

doi:10.1007/s00348-017-2415-x

Cited by 13 publications

(4 citation statements)

References 33 publications

(38 reference statements)

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“…The scattered light of HFSB has been reported to largely exceed that of conventional micron-size PIV tracers, namely 10 4 -10 5 times (Caridi, 2018). This corresponds with a scaling of the particle image peak intensity with the square of the particle diameter, in agreement with Mie theory of scattering, and an imaging regime governed by diffraction.…”

Section: Introductionsupporting

confidence: 58%

On the scalability of PIV experiments with helium filled soap bubbles

Guerra,

Scarano,

Sciacchitano

2023

Preprint

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The scalability of experiments using PIV relies upon several parameters, namely illumination power, camera sensor and primarily the tracers light scattering capability. Given their larger cross section, helium-filled soap bubbles (HFSB) allow measurements in air flows over a significantly large domain compared to traditional oil or fog droplets. Controlling their diameter translates into scalability of the experiment. This work presents a technique to extend the control of HFSB diameter by geometrical variations of the generator. The latter expands the more limited range allowed by varying the relative helium-air mass flow rates. A theoretical model predicts the bubble size and production rate, which is verified experimentally by high-speed shadow visualization. The overall range of HFSB produced in a stable (bubbling) regime varies from 0.16 mm to 2.7 mm. Imaging by light scattering of such tracers is also investigated, in view of controversies in the literature on whether diffraction or geometrical imaging dominate the imaging regime. The light scattered by scaled HFSB tracers is imaged with a high-speed camera orthogonal to the (LED) illumination. Both the total energy collected on the sensor for a single tracer, as well as its peak intensity are found to preserve scaling with the square of the diameter at object magnification of 10-1 or below, typical of PIV experiments. For large-scale volumetric applications, it is shown that varying the bubble diameter allows increasing both the measurement domain as well as the working distance of the imagers at 10 m and beyond. A scaling rule is proposed for the latter.

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Section: Introductionsupporting

confidence: 58%

On the scalability of PIV experiments with helium filled soap bubbles

Guerra,

Scarano,

Sciacchitano

2023

Preprint

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“…The velocity information within the vortex core is, therefore, unreliable, and the current work will not discuss tip vortex core-related properties such as core radius or maximum swirl velocity. It is noted that Caridi et al (2017) achieved a sufficient HFSB seeding of a delta-wing leading-edge vortex. However, the vortex creation differs from the current case, given that Caridi's wing has a smaller inflow velocity and a much larger effective chord length along which the vortex is created.…”

Section: Vortex Identification and Particle Tracksmentioning

confidence: 97%

Experimental study of secondary vortex structures in a rotor wake

et al. 2019

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The operation and aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake, in particular regarding vortex-vortex interactions of hovering rotors. Rotor simulations using modern computational methods have the potential to capture high levels of detail, which recently triggered discussions of secondary vortex braids entangling the primary tip vortices. These structures are highly dependent on the numerical settings and need experimental validation. The current work investigates the wake of a subscale rotor in ground effect by time-resolved and volumetric flow-field measurements using the "Shake-The-Box" technique. Both the Lagrangian tracks of the flow tracers and the derived gradient-based vortex criteria clearly verify the existence of secondary vortices. A post-processing scheme is applied to isolate these vortices in larger data sets. No distinct spatial organization of the structures was observed, but a slightly preferred sense of rotation which agrees to the shear of the wake swirl. The secondary structures were created shortly downstream of the rotor blades, starting at wake ages of approximately 75 • .

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“…A time response of 10 µs was inferred from the measured slip velocity within the deceleration field along the stagnation streamline. The tracing fidelity of HFSB has also been inquired within turbulent boundary layers (Faleiros et al 2018) and in the core of concentrated vortices (Caridi et al 2017). The good accuracy in the measurements of mean velocity and turbulence statistics found in the aforementioned works paves the way to the application of HFSB in a wide range of aerodynamic configurations, e.g., automotive, aerospace (Sciacchitano et al 2018), wind energy (Caridi et al 2016) and sports (Jux et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Generation and control of helium-filled soap bubbles for PIV

Faleiros

Tuinstra²,

Sciacchitano

et al. 2019

Exp Fluids

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The operating regimes of an orifice-type helium-filled soap bubbles (HFSB) generator are investigated for several combinations of air, helium and soap flow rates to establish the properties of the production process and the resulting tracers. The geometrical properties of the bubbles, the production regimes and the production rates are studied with high-speed shadowgraphy. The results show that the bubble volume is directly proportional to the ratio of helium and air volume flow rates, and that the bubble production rate varies approximately linearly with the air flow rate. The bubble slip velocity is measured along the stagnation streamline ahead of a cylinder via particle image velocimetry (PIV), yielding the particle time response from which the neutral buoyancy condition for HFSB is inferred. The HFSB tracing capability approaches that of an ideal tracer (i.e., minimum slip and shortest response time) when the volume flow rate of helium is approximately one thousandfold the soap flow rate. This study provides guidelines for operating HFSB generation systems, intended for PIV experiments.

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Helium-filled soap bubbles for vortex core velocimetry

Cited by 13 publications

References 33 publications

On the scalability of PIV experiments with helium filled soap bubbles

On the scalability of PIV experiments with helium filled soap bubbles

Experimental study of secondary vortex structures in a rotor wake

Generation and control of helium-filled soap bubbles for PIV

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