Fluidic Research Articles

Momentum Coefficient Governing Discrete Jet Actuation for Separation Control

Amplitude scaling laws for active control of boundary layer separation using discrete jet forcing are not fully established. Momentum coefficient is the most widely accepted scaling parameter, but accurate measurement and assessment of the employed assumptions are often not documented or even attempted. This work discusses various methods for experimentally determining the momentum coefficient produced by fluidic oscillators. Complementary simulations of the actuator are performed using delayed detached eddy simulations to validate the methods and offer explanations for shortcomings in the experiments. Scaling methods are applied to the experimental study of separation control on the suction side of a type II Glauert airfoil, otherwise known as the NASA hump model. An array of fluidic oscillators is located at and used to control separation over a range of Reynolds numbers () for various actuator sizes and spacings . Integrated surface pressure measurements are used to determine the suitability of various momentum coefficient definitions for scaling the flow control efficacy. It is found that the momentum coefficients obtained using direct measurements of actuator plenum or throat pressure govern scaling with respect to both Reynolds number and actuator size. This is independent of the sweeping frequency in the range surveyed.

Characterisation and comparison of unsteady actuators: a fluidic oscillator and a sweeping jet

PurposeThe purpose of this study is to compare two unsteady actuators: an oscillator and a sweeping jet. Both actuators can produce an oscillating jet of different amplitudes and frequencies without any moving parts, making them an attractive actuator concept. The Coanda effect phenomenon can explain the operating principles of these two unsteady actuators.Design/methodology/approachA numerical study was conducted to compare the amplitudes and frequencies of fluidic and sweeping jet (SJ) oscillators to obtain an efficient actuator to control separated flows at high Reynolds numbers. For this goal, two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations were carried out using computational fluid dynamics (CFD) fluent code to evaluate the actuator performances. The discrete fast Fourier transform method determined the oscillation frequencies.FindingsThe oscillation frequencies gradually increase as the inlet pressure increases. The characteristics and dimensions of the vortices produced in the mixing chamber and feedback loops vary overtime when the injected fluid is swept sideways. The frequencies supplied by the SJ are stronger than those obtained by the fluidic oscillator, which may contribute to improving the aerodynamic performance at a lower power supply cost.Originality/valueThe existence of the splitter in the fluidic oscillator led to the production of separate pulses, which would be useful in various industrial applications, including active control of combustion and mixing processes while other applications such as flow separation control require SJs. With the latter actuator higher and interesting frequencies can be obtained, leading to efficient flow control.

Gravity-driven fluid oscillations in a drinking straw: An alternative approach

Gravity-driven fluid oscillations in a drinking straw: An alternative approach

Unsteady fluidic oscillators for active controlling boundary layer separation in an ultra-high-lift low-pressure turbine

This study used large eddy simulation (LES) to investigate the control mechanism of a fluidic oscillator in the suppression of non-reattachment separation of a boundary layer from an ultra-high-lift, aft-loaded, low-pressure turbine under low Reynolds number. The internal flow characteristics of the fluidic oscillator were examined, with a focus on the influence mechanism of the pulse jet from the fluidic oscillator on boundary layer separation and transition. The effects of different jet angles (θ) on the suppression of separation bubbles were also compared in detail. Finally, the mechanism of interaction between the fluidic oscillator jet and the blade suction surface boundary layer was assessed. It was found that the pulse jet of the fluidic oscillator generated a streamwise vortex pair above the surface of the blade suction, which promoted an exchange of momentum between the boundary layer for the blade suction surface and the main flow, and clearly suppressed boundary layer separation. The streamwise vortex pair at θ=30° showed the characteristics of higher strength, smaller size, slower decay, and longer penetration distance than the results at θ=90°, such that the momentum exchange between the low-energy fluid inside the boundary layer and the main flow is sustainable, which promoted the laminar boundary layer to produce bypass transition and significantly improved the aerodynamic performance of the turbine.

Application of Phase-Averaging Methods to Sweeping Supersonic Jets

This paper examines various methods for phase averaging the flowfield of a sweeping supersonic jet emanating from a scaled fluidic oscillator. Determination of the phase-averaged flowfield is challenging, especially at high oscillation frequencies where there is a jitter in the periodicity caused by its inherent fluid dynamics. Several phase-averaging approaches (including mathematical, signal processing, and phase-lock techniques) applied to time-resolved schlieren images of the flowfield are described. Criteria for evaluation are based on the image distribution per phase bin, the centerline intensity profile of the phase-averaged schlieren image, and the cross correlation of the images with the time-averaged reference image. Examination of the different methods based on the criteria shows that the zero-crossing method is sufficiently accurate and most suitable for the naturally oscillating flowfield investigated in this study. Although the phase-locked approach works well at specific phase angles, overall, it is found unsuitable for the current test conditions. The zero-crossing method applied to a sweeping supersonic jet schlieren reveals shock structure distortion when switching directions in the oscillation cycle. Results indicate that the zero-crossing method described in this study can also be extended to other non-time-resolved quantitative and qualitative flowfield measurement techniques using a simultaneously acquired time-resolved reference signal.

Microbubble generation driven by the oscillation in a self‐excited fluidic oscillator

AbstractThe oscillation‐driven microbubble generation in a self‐excited fluidic oscillator is explored in this work. By investigating the behaviors of bubbles, it is noted that for relatively low liquid flow rate, bubbles move upward directly and breakup does not occur inside the chamber region. Interestingly, the existence of bubbles could destroy the structure of flow field and even affect the oscillation. Nevertheless, for relatively high liquid flow rate, the bubbles would not be able to change the flow field structure of liquid phase effectually. Meanwhile, binary, shear‐off‐induced and a kind of dynamic erosive breakup of the bubbles could occur for the effects of oscillations of liquid main jet. Abundant microbubbles could be produced mainly by the dynamic erosive breakup. The oscillation is only induced by the Coanda effect without any moving parts. Nevertheless, it is able to supply a robust oscillating turbulent field to enhance the breakup of bubbles dramatically.

Flow features of a new fluidic oscillator using time-resolved PIV measurement and 3D numerical simulation

Flow features of a new fluidic oscillator using time-resolved PIV measurement and 3D numerical simulation

Interaction of flow pattern and heat transfer in oscillating heat pipes for hot spot applications

Oscillating heat pipes (OHP) are increasingly used for the thermal management of hot spots. Thereby, a local evaporation zone occurs that is still not fully understood due to complex interactions of flow patterns and heat transfer. This study quantifies the thermal performance and the flow behavior of oscillating heat pipes with a centrally located hot spot heater. The copper OHPs were filled with acetone and tested with different orientations. The filling ratios varied from 0% to 90%, the heat inputs increased from 25 W to 200 W. Our results illustrate that the flow velocity values and the resulting thermal resistance are highly interrelated. Within one channel, there is a wide range of velocity values from 30 mm/s to 1200 mm/s; the thermal resistance decreases with higher average flow velocity. By characterizing the flow regime, we show that a reliable start-up depends on the initial vapor liquid pattern. Depending on the initial pattern, an individual temperature difference is required for the start-up, even for similar operating conditions. The analysis of the stopping behavior shows that a uniform temperature difference of 14 K is required for a stable fluid oscillation. Our study substantiates a strong interrelation between flow pattern and heat transfer in OHPs, which also shows a strong potential for future optimizations of OHP applications.

Numerical and Experimental Study on the Effects of Rheological Behavior of Drilling Fluid on the Performance of the Fluidic Down-the-Hole Hammer

Summary In the field of oil and gas exploration, the fluidic down-the-hole (DTH) hammer has frequently been used to solve the challenges associated with hard rock drilling and excessive friction along the drillstring. Appropriate performance prediction for drilling tools is particularly important for the drilling operation. Previous experiments showed that the performance prediction scheme determined by the water was not accurate for fluidic DTH hammer driven by mud, which is a typical pseudoplastic power-law fluid. Based on the results, compared with water, the rheological behavior of the pseudoplastic power-law fluid can be significant for the flow behavior in the fluidic oscillator with a lower supply flow rate. However, for a normal or higher level of supply flow rate, because of the shear diluting effect of the pseudoplastic power-law fluid, the influence of rheology behavior of drilling mud on the performance of fluidic DTH hammer was not apparent. The temperature of the drilling fluid and barite weighting agent can also affect the performance of the fluidic DTH hammer by affecting the rheological properties of the drilling mud. In this paper, numerical simulation and experiments are used for qualitative discussion and conclusion verification, respectively.

Fluidic Oscillators, Feedback Channel Effect under Compressible Flow Conditions.

Fluidic oscillators are often used to modify the forces fluid generates on any given bluff body; they can also be used as flow, pressure or acoustic sensors, with each application requiring a particular oscillator configuration. Regarding the fluidic oscillators’ main performance, a problem which is not yet clarified is the understanding of the feedback channel effect on the oscillator outlet mass flow frequency and amplitude, especially under compressible flow conditions. In order to bring light to this point, a set of three-dimensional Direct Numerical Simulations under compressible flow conditions are introduced in the present paper; four different feedback channel lengths and two inlet Reynolds numbers = 12,410 and = 18,617 are considered. From the results obtained, it is observed that as the inlet velocity increases, the fluidic oscillator outlet mass flow frequency and amplitude increase. An increase of the feedback channel length decreases the outlet mass flow oscillating frequency. At large feedback channel lengths, the former main oscillation tends to disappear, the jet inside the mixing chamber simply fluctuates at high frequencies. Once the Feedback Channel (FC) length exceeds a certain threshold, the oscillation stops. Under all conditions studied, pressure waves are observed to be traveling along the feedback channels, their origin and interaction with the jet entering the mixing chamber are thoroughly evaluated. The paper proves that jet oscillations are pressure-driven.

Effects of Bending of Fluidic Oscillators on Aerodynamic Performance of an Airfoil with a Flap

The present work investigated the effects of bending the outlet nozzles of fluidic oscillators installed on the NACA0015 airfoil with a flap on the flow control performance and, thus, the aerodynamic performance of the airfoil. The effects of bending on fluidic oscillators have not been reported so far in previous works. The aerodynamic analysis was performed numerically using unsteady Reynolds-averaged Navier-Stokes equations. Three different cases were considered: Case 1 changes only the bending angle with a fixed pitch angle, Case 2 changes only the pitch angle without bending, and Case 3 changes both the bending and pitch angles. Although the bending of the oscillators was introduced inevitably due to a geometrical limitation in the installation, the results indicated that the bending rather improved the lift coefficient and lift-to-drag ratio of the airfoil by improving the characteristics of the fluidic oscillators, such as the jetting angle and peak velocity ratio.

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

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.

Numerical investigation on the control of cryogenic fluid oscillations by applying an external sound source

Self-excited oscillations that occur in the cryogenic helium tube system are seen as thermoacoustic instabilities. To increase the operation stability of the system and suppress the conversion of thermal energy into acoustic waves, one solution where an external sound source driven by a piston is applied to control oscillations in a helium tube system. In this work, a numerical study of minimizing self-excited oscillations is conducted. Oscillation frequency and amplitude of the coupling system of the helium tube system and the external sound source are acquired as functions of the piston speed and frequency. It is found that the suppression of pressure oscillations can be achieved when the driving frequency is far from the self-excited oscillation frequency of the cryogenic helium tube. Increasing the driving speed of the piston gives rise to the coupling oscillation amplitude, and the control strategy leads to 99.2% oscillation amplitude reduction. Moreover, the nonlinear dynamics behavior caused by the coupling of self-excited oscillations and the external sound source is revealed. Synchronization of frequency occurs when the system is coupled to the external sound source oscillating with different frequencies. Our findings could provide a guideline in minimizing self-excited oscillations in the cryogenic helium system.

Flow field of impinging sweeping jets

Sweeping jets are oscillating jets generated by fluidic oscillators, i.e., devices designed to produce an oscillation of the flow without the use of any moving parts (Raghu, 2013). A typical configuration of such devices consists of an expansion chamber connected to a high-pressure supply via a converging nozzle and provided with feedback channels. The oscillating motion in the expansion chamber is triggered by an inherent flow instability and sustained by the flow rate across the feedback channels. Recently, sweeping jets have been studied in flow control applications for noise reduction, separation and circulation control over airfoils, control of resonant cavity oscillations and deflection of jets. The advantageous features of fluidic actuators, among which are the wide range of operating frequencies (up to kHz with meso-scale) and the distributed momentum addition, have also stimulated an increasing interest in their application to electronics cooling. Several recent studies on the convective heat transfer from impinging sweeping jets (e.g., Hossain et al., 2018; Park et al., 2018) have shown that, compared to conventional round jets, they offer higher cooling rates with better uniformity at least for small jet-to-plate spacings.

Global and local heat transfer behaviour of a three-dimensional Pulsating Heat Pipe: combined effect of the heat load, orientation and condenser temperature

• A 3D CLPHP is tested at different orientations and condenser temperatures. • A local heat transfer analysis is performed within the adiabatic section. • The starting from the local heat transfer behaviour. • The PHP global performance working regimes are quantified is independent from gravity at high heat loads. • Regular fluid oscillations in all branches promote a gravity-independent operation. Pulsating heat pipes having non-planar layouts (i.e. three-dimensional layouts) are preferable to the planar geometries when both gravity-independency and compactness are needed. A multi-turn three-dimensional closed loop pulsating heat pipe is tested on ground with the aim of investigating the effects of gravity by means of a quantitative description of the working modes occurring during its operation at different orientations. The viscous effects are furthermore considered by varying the condenser temperature. The studied device is made of an annealed aluminium tube (inner/outer diameter: 3/5 mm), folded in 14 turns and partially filled with methanol (volumetric filling ratio: 0.5). The channels are externally coated with a black high-emissivity paint, thus allowing the measurement of the outer wall temperature in the whole adiabatic section by means of a high-speed medium wave infrared camera. The wall temperature and the fluid pressure are instead monitored by means of thermocouples and pressure transducers directly inserted in the fluid stream. The pulsating heat pipe global performance is first assessed by evaluating the equivalent thermal resistance for every test case, showing that the studied three-dimensional layout is independent of gravity at high power input levels, where the equivalent thermal resistance settles around 0.25 K/W for almost every orientation. The thermographic acquisitions are therefore post-processed to estimate the heat flux locally exchanged between the working fluid and the channels wall, and the resulting local wall-to-fluid heat fluxes are statistically reduced for the quantification of the observed working modes, i.e. pure conduction, start-up, intermittent flow and full activation. At high heat loads to the evaporator, the gravity-independent behaviour of the device is found to be promoted by full activations, i.e. when a regular fluid motion is perceivable in every branch within the adiabatic section. The flow modes are also identified, pointing out that the local heat transfer behaviour is capable of discerning fluid oscillations from net fluid circulations. All the qualitative and quantitative pieces of data are comprehensively presented to give further information regarding the device dependency on the orientation. To deeper investigate the link between local and global quantities, the equivalent thermal resistance is finally compared with the statistical coefficients provided by the local heat transfer analysis.

Flow Oscillating Characteristics of Fluidic Oscillator Pairs Using the Interbridge Method

In this study, fluidic oscillator pairs were devised to include a connection of feedback ports with an interbridge and a removal of inner feedback loops. Each fluidic oscillator pair was designed by aiming synchronization of the sweeping jets. Numerical simulations of the four fluidic oscillator pairs were carried out to observe and evaluate both flow and sweeping characteristics of each fluidic oscillator pair under a nozzle pressure ratio of 3.0. The numerical setup was validated through a comparison between the experimental results and computational simulation results. The results indicate that, based on the interbridge arrangements, the main flows in two fluidic oscillators were synchronized by flow interaction via interbridges. Consistent in-phase jets and out-of-phase jets were obtained. By arranging an inport bridge and an outport bridge between each fluidic oscillator and removing the inner feedback loops, in-phase jets were obtained. By contrast, by arranging an outport bridge between each fluidic oscillator, out-of-phase jets were obtained. Meanwhile, when a chamber bridge was additionally arranged between the recirculation chambers of each fluidic oscillator, increases in the sweeping frequency and flow instability were observed. In addition, the relationship between the recirculation in the recirculation chamber and the sweeping characteristics was quantitatively evaluated.

Strain-Tunable Microfluidic Devices with Crack and Wrinkle Microvalves for Microsphere Screening and Fluidic Logic Gates.

Mechanical instabilities in soft materials have led to the formation of unique surface patterns such as wrinkles and cracks for a wide range of applications that are related to surface morphologies and their dynamic tuning. Here, we report a simple yet effective strategy to fabricate strain-tunable crack and wrinkle microvalves with dimensions responding to the applied tensile strain. The crack microvalves initially closed before stretching are opened as the tensile strain is applied, whereas the wrinkle microvalves exhibit the opposite trend. Next, the performance of crack and wrinkle microvalves is characterized. The design predictions on the bursting pressure of microvalves and others from the theory agree reasonably well with the experimental measurements. The microfluidic devices with strain-tunable crack and wrinkle microvalves have then been demonstrated for microsphere screening and programmable microfluidic logic devices. The demonstrated microfluidic devices complement the prior studies to open up opportunities in microparticle/cell manipulations, fluidic operations, and biomedicine.

Analysis of heat capacity ratio on porous media in oscillating flow

The paper analyzes heat transfer between an oscillating compressible fluid and solid to evaluate the influence of finite thermal capacity on the heat exchanged between these media. This type of problem is often studied in porous media applications by assuming the solid to have its surface at a constant temperature. When this hypothesis fails, in the frequency domain, the analytical solutions of the thermal field involve a complex dimensionless parameter known in the literature as εs. It was introduced in thermo-acoustic applications to model the solid properties of the porous core. In this work, the parameter is revisited by deriving an analytical formulation valid for arbitrarily shaped channels of porous materials, and a physical interpretation of the parameter is proposed in terms of fluid and solid entropy oscillation. CFD-based simulations of the thermal coupling between solid and fluid on three different simplified geometries of porous material have been conducted to confirm this physical interpretation, by comparing numerical and analytical data. Furthermore, from a numerical point of view, the parameter εs has been written as the inverse of a dimensionless Robin boundary condition. In this work, it is verified that the latter can be adopted at the fluid-solid interfaces to reproduce the thermal effects of the solid material, without numerically simulating it, both in frequency and time domain.

Effects of Bent Outlet on Characteristics of a Fluidic Oscillator with and without External Flow

A fluidic oscillator with a bent outlet nozzle was investigated to find the effects of the bending angle on the characteristics of the oscillator with and without external flow. Unsteady aerodynamic analyses were performed on the internal flow of the oscillator with two feedback channels and the interaction between oscillator jets and external flow on a NACA0015 airfoil. The analyses were performed using three-dimensional unsteady Reynolds-averaged Navier-Stokes equations with a shear stress transport turbulence model. The bending angle was tested in a range of 0–40°. The results suggest that the jet frequency increases with the bending angle for high mass flow rates, but at a bending angle of 40°, the oscillation of the jet disappears. The pressure drop through the oscillator increases with the bending angle for positive bending angles. The external flow generally suppresses the jet oscillation, and the effect of external flow on the frequency increases as the bending angle increases. The effect of external flow on the peak velocity ratio at the exit is dominant in the cases where the jet oscillation disappears.

Fully 3D-printed soft robots with integrated fluidic circuitry.

The emergence of soft robots has presented new challenges associated with controlling the underlying fluidics of such systems. Here, we introduce a strategy for additively manufacturing unified soft robots comprising fully integrated fluidic circuitry in a single print run via PolyJet three-dimensional (3D) printing. We explore the efficacy of this approach for soft robots designed to leverage novel 3D fluidic circuit elements-e.g., fluidic diodes, "normally closed" transistors, and "normally open" transistors with geometrically tunable pressure-gain functionalities-to operate in response to fluidic analogs of conventional electronic signals, including constant-flow ["direct current (DC)"], "alternating current (AC)"-inspired, and preprogrammed aperiodic ("variable current") input conditions. By enabling fully integrated soft robotic entities (composed of soft actuators, fluidic circuitry, and body features) to be rapidly disseminated, modified on demand, and 3D-printed in a single run, the presented design and additive manufacturing strategy offers unique promise to catalyze new classes of soft robots.