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4 pages, 294 KiB

Open AccessEditorial

Editorial for the Special Issue on “Fluidic Oscillators—Devices and Applications”

by Rene Woszidlo and Oliver Krüger

Fluids 2022, 7(3), 91; https://doi.org/10.3390/fluids7030091 - 1 Mar 2022

Cited by 1 | Viewed by 2284

Abstract

Fluidic oscillators are devices that produce a temporally and/or spatially oscillating output of fluid flow without requiring any moving parts [...] Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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Research

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24 pages, 122882 KiB

Open AccessArticle

Oscillating Wall Jets for Active Flow Control in a Laboratory Fume Hood—Experimental Investigations

by Juergen Liebsch and Christian Oliver Paschereit

Fluids 2021, 6(8), 279; https://doi.org/10.3390/fluids6080279 - 10 Aug 2021

Cited by 3 | Viewed by 5280

Abstract

Wall jets are applied to reduce flow separation and recirculation of the airflow entering the inner space of a laboratory fume hood through its front opening. The flow separation in the hood was further reduced by introducing a self-induced oscillatory motion using fluidic [...] Read more.

Wall jets are applied to reduce flow separation and recirculation of the airflow entering the inner space of a laboratory fume hood through its front opening. The flow separation in the hood was further reduced by introducing a self-induced oscillatory motion using fluidic oscillators. The design of the oscillators integrated in the predetermined contour are based on numerical simulations. The effect of the steady and unsteady wall jet was investigated experimentally using flow visualization, particle image velocimetry (PIV), and containment measurements. The oscillatory wall-jet led to reduction of flow separation and recirculation even at lower injection volume flows. In consequence, the usage of fluidic oscillators for a laboratory fume hood increases the energy efficiency of the system without reducing the safety of the laboratory fume hood. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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13 pages, 9608 KiB

Open AccessArticle

Free Stream Behavior of Hydrogen Released from a Fluidic Oscillating Nozzle

by Anja Fink, Oliver Nett, Simon Schmidt, Oliver Krüger, Thomas Ebert, Alexander Trottner and Bojan Jander

Fluids 2021, 6(7), 245; https://doi.org/10.3390/fluids6070245 - 5 Jul 2021

Cited by 3 | Viewed by 2419

Abstract

The H₂ internal combustion engine (ICE) is a key technology for complete decarbonization of the transport sector. To match or exceed the power density of conventional combustion engines, H₂ direct injection (DI) is essential. Therefore, new injector concepts that meet the [...] Read more.

The H₂ internal combustion engine (ICE) is a key technology for complete decarbonization of the transport sector. To match or exceed the power density of conventional combustion engines, H₂ direct injection (DI) is essential. Therefore, new injector concepts that meet the requirements of a H₂ operation have to be developed. The macroscopic free stream behavior of H₂ released from an innovative fluidic oscillating nozzle is investigated and compared with that of a conventional multi-hole nozzle. This work consists of H₂ flow measurements and injection tests in a constant volume chamber using the Schlieren method and is accompanied by a LES simulation. The results show that an oscillating H₂ free stream has a higher penetration velocity than the individual jets of a multi-hole nozzle. This behavior can be used to inject H₂ far into the combustion chamber in the vertical direction while the piston is still near bottom dead center. As soon as the oscillation of the H₂ free stream starts, the spray angle increases and therefore H₂ is also distributed in the horizontal direction. In this phase of the injection process, spray angles comparable to those of a multi-hole nozzle are achieved. This behavior has a positive effect on H₂ homogenization, which is desirable for the combustion process. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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13 pages, 36618 KiB

Open AccessArticle

Switching Action of a Bistable Fluidic Amplifier for Ultrasonic Testing

by Thorge Schweitzer, Marla Hörmann, Benjamin Bühling and Bernhard Bobusch

Fluids 2021, 6(5), 171; https://doi.org/10.3390/fluids6050171 - 25 Apr 2021

Cited by 8 | Viewed by 2091

Abstract

Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths [...] Read more.

Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths required for concrete structures, the use of traditional piezoelectric transducer requires high power electric systems. In this study, a novel fluidic transducer based on a bistable fluidic amplifier is investigated. Previous experiments have shown that the switching action of the device produces a high-power broadband ultrasonic signal. This study will provide further insight into the switching behaviour of the fluidic switch. Therefore, parametric CFD simulations based on compressible supersonic RANS simulations were performed, varying the inlet pressure and velocity profiles for the control flow. Switching times are analyzed with different methods, and it was found that these are mostly independent of the slope of the velocity profile at the control port. Furthermore, it was found that an inversely proportional relationship exists between flow velocity in the throat and the switching time. The results agree with the theoretical background established by experimental studies that can be found in the literature. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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26 pages, 21386 KiB

Open AccessArticle

Fluidic-Oscillator-Based Pulsed Jet Actuators for Flow Separation Control

by Stephan Löffler, Carola Ebert and Julien Weiss

Fluids 2021, 6(4), 166; https://doi.org/10.3390/fluids6040166 - 20 Apr 2021

Cited by 8 | Viewed by 3407

Abstract

The control of flow separation on aerodynamic surfaces remains a fundamental goal for future air transportation. On airplane wings and control surfaces, the effects of flow separation include decreased lift, increased drag, and enhanced flow unsteadiness and noise, all of which are detrimental [...] Read more.

The control of flow separation on aerodynamic surfaces remains a fundamental goal for future air transportation. On airplane wings and control surfaces, the effects of flow separation include decreased lift, increased drag, and enhanced flow unsteadiness and noise, all of which are detrimental to flight performance, fuel consumption, and environmental emissions. Many types of actuators have been designed in the past to counter the negative effects of flow separation, from passive vortex generators to active methods like synthetic jets, plasma actuators, or sweeping jets. At the Chair of Aerodynamics at TU Berlin, significant success has been achieved through the use of pulsed jet actuators (PJA) which operate by ejecting a given amount of fluid at a specified frequency through a slit-shape slot on the test surface, thereby increasing entrainment and momentum in a separating boundary layer and thus delaying flow separation. Earlier PJAs were implemented using fast-switching solenoid valves to regulate the jet amplitude and frequency. In recent years, the mechanical valves have been replaced by fluidic oscillators (FO) in an attempt to generate the desired control authority without any moving parts, thus paving the way for future industrial applications. In the present article, we present in-depth flow and design analysis which affect the operation of such FO-based PJAs. We start by reviewing current knowledge on the mechanism of flow separation control with PJAs before embarking on a detailed analysis of single-stage FO-based PJAs. In particular, we show that there is a fundamental regime where the oscillation frequency is mainly driven by the feedback loop length. Additionally, there are higher-order regimes where the oscillation frequency is significantly increased. The parameters that influence the oscillation in the different regimes are discussed and a strategy to incorporate this new knowledge into the design of future actuators is proposed. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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20 pages, 5461 KiB

Open AccessArticle

Integration of Fluidic Nozzles in the New Low Emission Dual Fuel Combustion System for MGT Gas Turbines

by Bernhard Ćosić, Dominik Waßmer and Franklin Genin

Fluids 2021, 6(3), 129; https://doi.org/10.3390/fluids6030129 - 21 Mar 2021

Cited by 4 | Viewed by 6960

Abstract

Fluidic oscillators have proven their capabilities and advantages in terms of the generation of oscillating jets without moving parts for many years, mainly in experimental studies. In this paper, the design, development, and integration of fluidic atomizers into the liquid-fuel system of the [...] Read more.

Fluidic oscillators have proven their capabilities and advantages in terms of the generation of oscillating jets without moving parts for many years, mainly in experimental studies. In this paper, the design, development, and integration of fluidic atomizers into the liquid-fuel system of the dual-fuel low NO_X Advanced Can Combustion (ACC) system of the MAN Gas Turbines (MGT) are presented. The two-stage system comprises a pressure-swirl nozzle as a pilot stage and an assembly of four main premixed nozzles, based on fluidic technology. The design and the features of the pilot nozzle are briefly presented, whereas the focus lies on the functionality and layout of the fluidic nozzles. The complete integration, validation, and verification of this innovative liquid-fuel injection unit are presented. The final system features fast fuel-switchovers, low complexity, high reliability, and dry low emissions in liquid-fuel operation. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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16 pages, 20888 KiB

Open AccessArticle

Design of a Fluidic Actuator with Independent Frequency and Amplitude Modulation for Control of Swirl Flame Dynamics

by Amrit Adhikari, Thorge Schweitzer, Finn Lückoff and Kilian Oberleithner

Fluids 2021, 6(3), 128; https://doi.org/10.3390/fluids6030128 - 20 Mar 2021

Cited by 5 | Viewed by 2451

Abstract

Fluidic actuators are designed to control the oscillatory helical mode, called a precessing vortex core (PVC), which is often observed in gas turbine combustors. The PVC induces large-scale hydrodynamic coherent structures, which can considerably affect flow and flame dynamics. Therefore, appropriate control of [...] Read more.

Fluidic actuators are designed to control the oscillatory helical mode, called a precessing vortex core (PVC), which is often observed in gas turbine combustors. The PVC induces large-scale hydrodynamic coherent structures, which can considerably affect flow and flame dynamics. Therefore, appropriate control of this structure can lead to a more stable and efficient combustion process. Currently available flow control systems are designed to control the PVC in laboratory-scale setups. To further develop these systems and find an approach applicable to the industrial scale, a new actuator design based on fluidic oscillators is presented and studied in this paper. This actuator allows for independently adjusting forcing frequency and amplitude, which is necessary to effectively target the dynamics of the PVC. The functionality and flow control of this actuator design are studied based on numerical simulations and experimental measurements. To verify the flow control authority, the actuator is built into a prototype combustor test rig, which allows for investigating the impact of the actuator’s forcing on the PVC at isothermal conditions. The studies conducted in this work prove the desired functionality and flow control authority of the 3D-printed actuator. Accordingly, a two-part stainless steel design is derived for future test conditions with flame. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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23 pages, 5787 KiB

Open AccessArticle

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

by Mehti Koklu

Fluids 2021, 6(2), 74; https://doi.org/10.3390/fluids6020074 - 7 Feb 2021

Cited by 4 | Viewed by 2328

Abstract

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve [...] Read more.

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve substantial control over the separated flow by increasing the upstream suction pressure and downstream pressure recovery. Using the data available in the literature, the performance of the fluidic oscillators was compared to other active flow control (AFC) methods such as steady blowing, steady suction, and zero-net-mass-flux (ZNMF) actuators. Several integral parameters, such as the inviscid flow comparison coefficient, pressure drag coefficient, and modified normal force coefficient, were used as quality metrics in the performance comparison of the AFC methods. These quality metrics indicated the superiority of the steady suction method, especially at lower excitation amplitudes that is followed by the fluidic oscillators, steady blowing, and the ZNMF actuators, respectively. An aerodynamic figure of merit (AFM) was also constructed using the integral parameters and AFC power usage. The AFM results revealed that, for this study, steady suction was the most efficient AFC method at lower excitation amplitudes. The steady suction loses its efficiency as the excitation amplitude increases, and the fluidic oscillators become the most efficient AFC method. Both the steady suction and the fluidic oscillators have an AFM > 1 for the range tested in this study, indicating that they provide a net benefit when the AFC power consumption is also considered. On the other hand, both the steady blowing and ZNMF actuators were found to be inefficient AFC methods (AFM < 1) for the current configuration. Although they improved the flow field by controlling flow separation, the power requirement was more than their benefit. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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Review

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16 pages, 7078 KiB

Open AccessReview

Fluidic Oscillators Mediating Generation of Microbubbles (Survey)

by Václav Tesař

Fluids 2021, 6(2), 77; https://doi.org/10.3390/fluids6020077 - 9 Feb 2021

Cited by 8 | Viewed by 2901

Abstract

If a gas volume is distributed into many microbubbles of a sub-millimetre size, the total gas/liquid surface becomes very large. This increases overall heat and/or mass transport across the sum of surfaces. The paper discusses several applications in which the use of microbubbles [...] Read more.

If a gas volume is distributed into many microbubbles of a sub-millimetre size, the total gas/liquid surface becomes very large. This increases overall heat and/or mass transport across the sum of surfaces. The paper discusses several applications in which the use of microbubbles increases efficiency of various processes, especially in wastewater treatment and in growing microorganisms such as algae, yeast, bacteria, or primitive fungi. The problem of microbubble generation by percolation in aerator is their coalescence into larger bubbles, whatever small are the pores in the aerator in which the microbubbles are generated. The solution of this size discrepancy question was found in agitating the gas flow by a fluidic oscillator prior to its injection through the aerator. The oscillator is a no-moving-part device, simple, inexpensive, resistant to external effects like acceleration or heat, and with long maintenance-free working life. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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