Effect Of Acoustic Excitation Research Articles

Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number

The effectiveness of acoustic excitation as a means of flow control at high Reynolds number turbulent flows is investigated numerically by using improved delayed detached eddy simulations (IDDES). Previous studies on low Reynolds number laminar flows have shown that acoustic excitation can substantially suppress flow separation for specific effective frequency and amplitude ranges. However, the effect of acoustic excitation on higher Reynolds number turbulent flow separation has not yet been explored due to limitations on appropriate fidelity computational methods or experimental facility constraints. Therefore, this paper addresses this research gap. A NACA0015 airfoil profile at 1 × 106 Reynolds number based on the airfoil chord length is used for the investigations. Acoustic excitation is applied to the baseline flow field in the form of transient boundary conditions at the computational domain inlet. A parametric study revealed that the effective sound frequency range shows a Gaussian distribution around the frequency of the dominant disturbances in the baseline flow. A maximum of ∼43% increase in lift-to-drag ratio is observed for the most effective excitation frequency F+=1.0 at a constant excitation amplitude of Am=1.8%. The effect of excitation amplitude follows an asymptotic trend with a maximum effective excitation amplitude above which the gains are not significant. A fully reattached flow is observed for the highest excitation level considered (Am=10%) that results in ∼120% rise in airfoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows, thereby paving the way for application in realistic flow scenarios observed in aircraft and gas turbine engine flow fields.

Effects of acoustic excitation on pinch-off flame structure and NOx emissions in H2/CH4 flame

The flame structure and exhaust emissions of pinch-off flames, in which flames are separated under acoustic excitation conditions, were studied. Flame structure analysis was performed using OH∗ chemiluminescence measurements, and exhaust emissions analysis was performed with a gas analyzer. Structure analysis of the pinch-off flame for acoustic excitation was performed basis the OH∗ images, and various structures were confirmed according to the forcing frequency and velocity perturbation intensity. To analyze the correlation between the flame structure and NO x emission, the flame residence time and emission index of NO x (EINO x ) were analyzed according to the Strouhal number. The flame residence time and EINO x decreased as the velocity perturbation intensity increased; analyzing the NO x emission characteristics is limited when based only on the flame residence time. 1/2-power scaling was followed by EINO x analysis and the Strouhal number was normalized to the flame residence time. • Investigation of dynamic and exhaust emission characteristics of pinch-off flames. • Pinch-off flame structure is defined by detailed NO x emissions analysis. • Flame height decreases relative to an increase in the velocity perturbation. • Reduction of flame height reduces the flame residence time. • The τ res alone is insufficient for analyzing NO x emissions.

Active cooling of photovoltaic (PV) cell by acoustic excitation in single-dimpled internal channel

Solar energy is widely advocated by many countries which is the most popular renewable energy. To get the higher efficiency of the photovoltaic (PV) cell, enhancing cooling performance of the coolant channel is important. In this study, the effect of acoustic excitation was analyzed to enhance the cooling performance of internal channel imprinted with single-dimple for solar PV cell system. Various acoustic-excitation frequencies were compared for the optimized cooling performance of dimple-imprint channel. All experiments were conducted under laminar-flow conditions. The experiments indicated that a Strouhal number of 1.1 corresponding to a similar frequency of the vortex shedding within the dimple cavity was most-effective to enhance cooling performance. By analyzing the local heat/mass transfer distributions, the effect of acoustic excitation can be clearly noticed within the dimple cavity where vorticities formed. In the aft-plateau region, heat/mass transfer gradually decreased and the effect of acoustic excitation was minimal. Overall, the area-averaged heat/mass transfer was enhanced by 68% at St = 1.1. Furthermore, when the acoustic excitation was adopted, the area-averaged heat/mass transfer was enhanced by 26–35% in other frequency cases. It was concluded that an appropriate acoustic-excitation frequency could effectively improve the cooling performance of a single-dimple imprinted channel. In the end, the efficiency of PV cell will be enhanced by adopting the acoustic-excitation with dimple imprint cooling channel.

Effect of acoustic excitation on fiber-reinforced polypropylene and the influence on melt viscosity

The paper describes a novel technological approach to influencing the rheological properties of thermoplastic materials exposed to acoustic energy. The flow behavior of polypropylene with different mass percentages of glass fibers is investigated in a parallel plate rheometer under high-frequency longitudinal excitation. The influence of oscillation frequency on the melt viscosity is explained by means of shear thinning criteria. The dependence of the oscillation shape using sinusoidal excitation on shear thinning as a function of different fiber reinforcement percentages is also investigated. A phenomenological view describes the mutually influencing parameters with regard to different material compositions and different excitation frequencies over the time course of the rheometric measurement. Interacting relationships are analyzed and discussed and the potential of the actuator system to influence the plastic melt is worked out. Based on this, a technological approach follows which describes the transfer of an oscillating mold surface to plastics processing methods, which, especially in the case of energy-intensive injection molding technology, leads to the expectation of possible resource efficiency in energy and material.

Acoustic Perturbation Toward Jet Flame – Variation of Flame Shapes Mode

Effect of acoustic excitation on jet diffusion flames caused by an energy transfer delivered by the source (speaker) to the receiver (jet flame) through a medium of air. Using the first law of thermodynamics, which is the expression of the conservation of energy principle. Energy cannot be created nor be destroyed during a process; it can only change from one form to another. From this principle, the energy transfer from the sound waves to the jet flames causing the flames to change in both macroscopics and microscopic. The macroscopic effects related to the motion and the flame of shapes cause by the acoustic disturbance. This paper shows the macroscopics effects can clearly been seen when the flame shapes start to split in two branches during the transfer of energy. This effects clearly correspond with the periodically changes of acoustic frequency. By using ImageJ, a Java-based image processing program to detailed out the changes of structure correspond by the increase of acoustic frequencies.

Jet preferred mode vs shear layer mode

The effect of acoustic excitation on a low Reynolds number jet with constant centerline velocity u0 but varying velocity profile u(y) is investigated experimentally by particle imaging velocimetry. Different initial conditions at the nozzle orifice are here used with the intent to characterize the relation between the jet preferred mode fp and the natural shear layer mode fn. The jet response to acoustic excitation is described in terms of the centerline velocity decay and the downstream increase in momentum thickness. This study intends to shed some light onto the duality of the two most fundamental flow instabilities and their controversial dependency on each other.

Effects of Acoustic Excitation on the Combustion Instability of Hydrogen-Methane Lean Premixed Swirling Flames.

Lean premixed flames are useful for low nitrogen oxide (NOx) emissions but more prone to induce combustion instability in gas turbines. Combustion instability of a lean premixed swirling flame (LPSF) with hydrogen–methane was investigated experimentally. The effects of hydrogen addition on combustion instability with equivalence ratios 0.75–1 were investigated with acoustic frequencies (90–240 Hz) and acoustic amplitudes (the ratio of velocity fluctuation to an average velocity of 0–0.5), respectively, which are characterized by the gain and phase of the flame describing function (FDF). The evolution of vortex and the flame morphologies were observed by the particle image velocimetry (PIV), intensified charge-coupled device (ICCD), photomultiplier tube (PMT), and Cassegrain optical systems. The global and local heat release fluctuations of the LPSF were shown by CH*/OH* chemiluminescence and temperature measurements. Results show that the FDF features maximum and minimum gain values in the acoustic frequency range of 90–240 Hz and reaches local maximum peaks at 110 and 180 Hz and local minimum peaks at 160 Hz. It can also be observed that varying velocity amplitudes (0–0.5) have greater effects on the gain and phase of FDF than changing equivalence ratios (0.75–1) for lean swirling flames. Higher velocity amplitudes more effectively intensified the compression of the flame length, which enhanced the mixing of the high-burning gas and the unburned gas, and then heat release fluctuations increased. However, it is more interesting that the effects of hydrogen addition on the combustion instability of the LPSF show a completely opposite phenomenon due to acoustic frequency under all experimental conditions. The FDFs were compared at typical frequencies of 140 and 180 Hz, and it was found that combustion instability enhanced with increasing hydrogen content at 140 Hz while weakened at 180 Hz. The flow field of PIV images shows that it is related to the location and development of vortices in the flame with varying acoustic frequencies. The intensity of OH*/CH* chemiluminescence, local temperature, and heat release rate show the same changing trend with the flame morphology for two acoustic parameters with the increasing hydrogen content in the LPSF. This directly affects the compression and curvature of the LPSF and thereby changes the mixture and temperature of the combustible gas, which influence the heat release fluctuation of the LPSF.

Acoustically Forced Droplet Combustion of Liquid Fuel with Reactive Aluminum Nanoparticulates

ABSTRACTThis experimental study explores the effects of acoustic excitation on burning droplets of liquid ethanol loaded with reactive aluminum nanoparticles (nAl). Continuously fed fuel droplet combustion and flame extinction (blowout) experiments were conducted in the vicinity of a pressure node (or velocity antinode) created in a closed acoustic waveguide, with a range of applied resonant forcing frequencies, pressure or velocity perturbation amplitudes, and particle loading concentrations. Simultaneous phase-locked OH* chemiluminescence and visible images were taken to quantify the influence of nanoparticle concentration on burning rate constant (K) and flame-acoustic coupling dynamics. In the presence of velocity perturbations, nAl-laden droplets were observed to burn for longer periods of time than they burned in the absence of acoustics, likely resulting from suppression of particle agglomerate formation and expulsion. It was found that at higher forcing frequencies, there were relatively greater enhancements in K values of nAl-loaded droplets than for lower frequencies. Interestingly, while at higher forcing amplitudes, the burning rate constant increased for a given loading concentration, that increase tended to be reduced at higher loading concentrations. Phase-locked imaging demonstrated that the presence of nAl increased the Rayleigh index (G) as compared with neat fuels under the same forcing conditions. Higher amplitude excitation leading to extinction showed that the addition of nAl could significantly increase the mean extinction strain rate, in some cases by up to 44%.

Flames of Swirling Double-Concentric Jets Subject to Acoustic Excitation at Resonance

The effects of acoustic excitation at resonance on the flame appearances, flame lengths, flame temperatures, and combustion product concentrations of combusting swirling dual-disk double-concentric jets were studied. The Reynolds number of the annular swirling air jet was varied, while it was fixed at 2500 for the central propane jet. The central fuel jet was acoustically forced by a loudspeaker, which was installed using downstream longitudinal irradiation. The central jet pulsation intensities were measured by a calibrated, one-component hot-wire anemometer. The instantaneous full-length and close-up flame images were captured to identify the characteristic flame modes. Long-exposure flame images were taken to measure the flame lengths. The axial and radial temperature distributions of flames were measured using a homemade, fine-wire R-type thermocouple. The concentrations of combustion products were measured by a gas analyzer. Four characteristic flame modes, blue-base wrinkled flame, yellow-base anchored flame, blue-base anchored flame, and lifted flame, were observed in the domain of central jet pulsation intensity and annular swirling jet Reynolds number. The lifted flame, which was formed at large central jet pulsation intensities, presented characteristics of a premixed flame due to significant mixing induced by violent, turbulent flow motions. It was short and stable, with high combustion efficiency and low toxic emissions, when compared with the unexcited flame and other excited characteristic flame modes, which presented characteristics of diffusion flame.

Effect of acoustic excitation on flames of swirling dual-disk double-concentric jets

The flame behavior of acoustically excited combusting swirling double-concentric jets with a dual-disk flow controller at the jet exit was experimentally studied. The fluids for the central and swirling jets were propane and air, respectively. The central jet was acoustically excited using a loudspeaker. The central jet velocities were measured with a one-component hot-wire anemometer to estimate the acoustic excitation behavior. Close-up and full-length short-exposure and time-averaged flame images were captured using traditional photography techniques. The flame temperatures were measured using a homemade L-shaped fine-wire thermocouple. The combustion product concentrations were measured using a gas analyzer. Three characteristic flame modes—yellow-base anchored flame, blue-base anchored flame, and lifted flame—were observed in the domain of the excitation Strouhal numbers and jet pulsation intensities. The lifted flame was blue, very short, highly turbulent, even at low Reynolds numbers, and difficult to blow out. The flame length was significantly reduced, and the flame temperatures were drastically increased when the combusting jets were forced into the characteristic modes of blue-base-anchored and lifted flames. The concentrations of unburned hydrocarbon, carbon monoxide, and nitric oxide fell to negligibly small values. The acoustic excitation was effective in promoting the combustion performance of the swirling double-concentric jet flames.

Dynamics of liquid sheet breakup in the presence of acoustic excitation

We present the results of experimental investigations on the effect of acoustic excitation on the breakup of the liquid sheet formed by oblique impingement of two liquid jets for varying Weber number over a wide range of excitation frequencies (100–2000 Hz) and sound pressure levels (50–120 dB). Visualization studies reveal violent sheet flapping, reduction in the sheet breakup length and width, impact wave amplification and rise in droplet density shed from the sheet. Overall, increase in excitation frequency increases the threshold sound pressure level at which the sheet responds. Reduction in the sheet breakup length and width with frequency is observed at some selected frequencies, which rules out the possibility of resonance. For smooth sheet regime, visual observations show that the dominant sheet breakup mechanism shifts from Rayleigh–Plateau (R–P) to Kelvin–Helmholtz (K–H) instability. It is shown using temporal analysis that the reduction in the sheet breakup length may be attributed to periodic sheet collapse and reformation. Drop shedding frequencies in flapping liquid sheet are seen to synchronize with those excitation frequencies which are lower than or nearly equal to the fundamental frequency without excitation. Acoustic excitation reduces the mean droplet size significantly without altering the drop size distribution pattern that follows a universal Gamma distribution.

Mössbauer spectroscopy under acoustical excitation: thick target effects

A new model of Mossbauer absorption (transmission) spectra with an adequate analysis of the possible effects of acoustic excitation in the thick targets is proposed. In particular, the dependence of the line width of acoustical satellites on the degree of phase correlation of the sound oscillations of resonant nuclei in the target is established by calculations and confirmed in experiment. Such a model is stimulated by an increase in the informativeness of the Mossbauer experiments, using thick samples in ultrasound (US) field, and by possible applications of this research technique. The test measurements of Mossbauer absorption spectra on stainless steel are carried out. The fitting of these spectra confirms the relevance of modifications of the model base of Mossbauer processes in US field.

Nonlinear radial oscillations of micro-bubbles in a compressible UCM medium

ABSTRACTThe nonlinear oscillations of acoustically forced spherical gas bubbles in an upper-convected Maxwell (UCM) compressible fluid are investigated. The nonlinear viscoelastic model used is suitable for large-amplitude excitation of bubbles that cannot be captured by linear models. The effects of acoustic excitation are studied for compressible nonlinear viscoelastic media, which increases the complexity and nonlinearity of the behavior. The Keller–Miksis equation is used to model the dynamics of a single bubble. The constitutive equations of compressible UCM are used for viscoelastic media. These governing equations are non-dimensionalized and coupled to determine the bubble dynamic behavior. The set of derived non-dimensionalized integro-differential equations developed are numerically solved simultaneously using the fourth-order Runge–Kutta method featured by the automatic variable time step-size module. The combined effects of compressibility and viscoelasticity of the fluid on bubble radius are investigated. The results show that the combination of compressibility and nonlinear viscoelasticity for bubble radial oscillations makes forced bubble dynamics more applicable for human needs, especially for large deformations in highly non-Newtonian fluids like industrial polymers or even tissue-like media. It can be seen that compressibility controls the oscillations at higher forcing amplitudes. The relevance and importance of these bubble dynamics to biomedical ultrasound applications and light emissions by sonoluminescence and other industries are evident.

Effect of Acoustic Excitation on R134a/Al2O3 Nanolubricant Mixture Boiling on a Reentrant Cavity Surface

This paper quantifies the influence of acoustic excitation of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a commercial (Turbo-BII-HP) boiling surface. A nanolubricant with 10 nm diameter Al2O3 nanoparticles at a 5.1% volume fraction in the base polyolester lubricant was mixed with R134a at a 1% mass fraction. The study showed that high-frequency ultrasound at 1 MHz can improve R134a/nanolubricant boiling on a reentrant cavity surface by as much as 44%. This maximum enhancement occurred for an applied power level to the fluid of approximately 6 W and a heat flux of approximately 6.9 kW/m2. Applied power levels larger and smaller than 6 W resulted in smaller boiling heat transfer enhancements. In total, five different applied power levels were studied: 0 W, 4 W, 6 W, 8 W, and 12 W. The largest and smallest enhancement averaged over the tested heat flux range were approximately 12% and 2% for the applied power levels of 6 W and 4 W, respectively. In situ insonation at 1 MHz resulted in an improved dispersion of the nanolubricant on the test surface. An existing pool-boiling model for refrigerant/nanolubricant mixtures was modified to include the effect of acoustic excitation. For heat fluxes greater than 25 kW m−2, the model was within 4.5% of the measured heat flux ratios for mixtures, and the average agreement between measurements and predictions was approximately 1% for all power levels.

Visualization of Shear Layer Dynamics in a Transversely Forced Flow and Flame

This study addresses the response of a swirling annular jet flow and flame to transverse acoustic excitation in order to better describe key velocity-coupled processes during transverse combustion instabilities in lean, premixed flames. In particular, visualization and velocimetry techniques provide information about the effects of acoustic excitation on unsteady vortex development in the shear layers. Without acoustic forcing, the shear layers roll up into small vortices, driven by the Kelvin–Helmholtz instability, that convect downstream and grow. In the presence of high-amplitude acoustic forcing, as would be present during a combustion instability, the acoustic oscillations drive shear layer vortices to undergo a strong rollup event. Smoke visualization provides visual evidence of the rollup, while particle image velocimetry measurements show the development and trajectory of this large structure. Finally, planar laser-induced fluorescence of OH shows how the large coherent structure causes flame wrinkling at the base of the flame. The vortex rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and can be the main driver of flame response during combustion instability.

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.

Separation Control by External Acoustic Excitation at Low Reynolds Numbers

At Reynolds numbers approaching those of micro air vehicles (both engineered and natural), the Eppler 387 airfoil (in common with many other smooth profiles) can have multiple lift and drag states at a single wing-incidence angle. Prestall hysteresis and abrupt switching between stable states result from sudden flow reattachment and the appearance of a large separation bubble. It is shown that control of the dynamics can be achieved using external acoustic forcing. Separation control, hysteresis elimination, and more than 70% increase in lift–drag ratio are obtained at certain excitation frequencies and sound pressure levels. The global flow around the wing is effectively modified, and large, stable vortical structures appear in the separated shear layer. Correlation between the effects of acoustic excitation and wind-tunnel resonance shows that the antiresonances in an enclosed chamber correspond to the largest improvement in wing performance. Implications for control and stabilization of small aircraft inside and out of enclosed boxes are considered.

Effects of Acoustic Excitation and Annular Swirl Strength on a Non-Premixed and Swirl-Stabilized Flame

The stabilization of non-premixed flame is often done by coupling an annular swirling flow. Acoustic excitation, however, can improve combustion through fuel jet oscillation but can sometimes blow out the flame. The writers aimed to experimentally investigate the effects of annular swirl strength on acoustically excited non-premixed flame. Visual observations coupled with flame length measurements showed that the flame can be particularly shortened and transformed to blue at a low swirl strength. Flame and flow images obtained through the Mie scattering technique showed that the recirculation bubble enlarged proportionally in accordance with the swirl strength. Jet oscillations were diagnosed by using a laser Doppler anemometer at the central jet. Turbulence and mixing properties obtained from the central jet velocity were statistically compared to identify the best combination of swirl strength and acoustic excitation level. Combustion performance was finely validated through flame length and temperature measurements.

Effect of Acoustic Excitation on Flow Asymmetry over Slender Body at High Angles of Attack

A new kind of flow control method, acoustic excitation, was applied to control flow asymmetry over a slender body at high angles of attack. Results show that acoustic excitation could improve the asymmetric behaviors as long as controlling conditions are chosen appropriately. The development of asymmetric vortices on the leeward along the model axis would be greatly affected by acoustic excitation. Moreover, the fluctuation energy in the boundary layer could be reduced.

A study of influence of acoustic excitation on carbon dioxide capture by a droplet

Carbon dioxide capture by a droplet plays a fundamental role for reducing atmospheric greenhouse effect stemming from anthropogenic activities. To recognize the CO 2 capture dynamics by a quiescent water droplet, a theoretical analysis on the mass transfer of CO 2 from the gas phase to the liquid phase is performed in this work, with emphasis on the effect of acoustic excitation upon the absorption process. The dimensionless fluctuated amplitude and frequency of the acoustic wave are in the ranges of 0.1–0.99 and 0.1–1000, respectively. The analyses suggest that an acoustic wave with smaller amplitude and higher frequency leads to a more significant energy decay in the droplet due to the role of damping played by the droplet. In contrast, pressure excitation along with larger amplitude and lower frequency has a pronounced effect on the mass transfer process, as a result of higher efficiency of energy penetrating into the liquid phase. From the perspective of achieving carbon capture and storage (CCS), the acoustic excitation with the dimensionless frequency of unity is recommended to capture CO 2 by a droplet and the exposure time of the droplet should be controlled at the dimensionless aqueous diffusion time ( τ l ) between 0.31 and 0.36. The present study has provided a useful insight into the design and application of scrubbers for enhancing CO 2 capture by sprays.