Jet Cavity Research Articles

Drag and heat reduction mechanism in the combinational opposing jet and acoustic cavity concept for hypersonic vehicles

The drag and heat flux reduction mechanism in the hypersonic vehicle has attracted an increasing attention worldwide. In the current study, the two-dimensional axisymmetric Reynolds-averaged Navier–Stokes (RANS) equations and the two equation standard k–ε turbulence model have been utilized to obtain the flow field properties around the hypersonic blunt body, and the combinational opposing jet and acoustic cavity concept has been employed to induce the drag and heat reduction. The numerical method has been validated against the available experimental data in the open literature, and the effects of the wall temperature and Prandtl number on the predicted results have been analyzed, as well as the grid independency analysis. At last, the influences of the cavity location, the length-to-depth ratio of the cavity and the molecular weight of the injectant on the flow field properties have been investigated. The obtained results show that the shock wave stand-off distance has been overestimated, and the locations of the Mach disk and the triple point have been captured accurately. However, the predicted Stanton number is 48.5% less than the experimental data, and its variable trend is the same as that of the experiment. The area of the recirculation zone generated in the vicinity of the Mach disk has a great impact on the drag force reduction, and the injectant with smaller molecular weight can induce more heat flux reduction in the range considered in the current study. The acoustic cavity should not be located in the middle of the jet nozzle in order to obtain the smaller drag force and heat flux, and the cavity with its length-to-depth ratio being even number is beneficial to the drag and heat reduction in the range considered.

Noise generated by ventilated supercavities

The noise generated by ventilated supercavities has been explored experimentally in a water tunnel facility. The most prominent acoustical characteristic is the monopole behavior exhibited by a ventilated supercavity in its pulsating closure regime. The interior cavity pressure and near-field radiated sound are monotonic with a frequency that is related to the speed and length of waves propagating on the supercavity gas/water interface. The cavity interior pressure spectrum level is shown to be related to the near-field and far-field noise spectrum level through spherical spreading of the sound waves from the supercavity interface. As a result, the cavity interior pressure can be used as a measure of the radiated noise. The noise radiated by a pulsating supercavity at the pulsation frequency is at least 40 dB above that radiated by comparable re-entrant jet and twin vortex cavities.

Effect of interaction between spray and attrition jets in a high temperature fluidized bed

A high temperature fluidized bed unit was used with simultaneous liquid injection and jet attrition, using separate nozzles that were located to allow for interactions between the jet cavities they created in the fluidized bed. The effect of jet interactions on liquid–solid contact and particle attrition was studied. When the attrition nozzle was located and inclined so that its jet would interact with the spray jet, there was a large reduction in the formation of large liquid–solid agglomerates, but particle attrition was reduced and the formation of fines was increased. The best results were obtained when the attrition jet hits the spray jet at its base, just downstream of its nozzle tip.

Detached-Eddy Simulations of Synthetic Jets for High-Angles-of-Attack Airfoils

This study investigates the performance of detached-eddy simulation to simulate the flow control by a synthetic jet around an airfoil. The effect of the jet exit angle is focused on. The detached-eddy simulation approach is first validated with a nonblowing, high-angle-of-attack case. Flows around a NACA 0012 airfoil including the synthetic jet cavity and its orifice at 10% chord location are then simulated at 12 deg angle of attack and Reynolds number of . The jet exit angles of 45, 60, 75, and 90 deg have been studied. Massively separated flows were eliminated in all cases. The angles of 45 and 60 deg successfully recovered stably attached flow over the airfoil with small amplitude of the aerodynamic forces, while the flow unsteadiness due to remaining separation is observed at higher jet exit angles, resulting in large amplitude of the drag force.

Suppression of Two-Phase Flow Instabilities in Parallel Microchannels by Using Synthetic Jets

Two-phase flow instabilities in microchannels exhibit pressure and temperature fluctuations with different frequencies and amplitudes. An active way to suppress the dynamic instabilities in the boiling microchannels is to introduce synthetic jets into the channel fluid. Thus, the bubbles can be condensed before they clog the channel and expand upstream causing flow reversal. The present work experimentally investigated the effects of synthetic jets on microchannel flow boiling. An array of synthetic jets was introduced into the microchannel flow. The strength and frequency of the jets were controlled by changing the driving signals of the piezoelectric driven jet actuator. It is found that the bubbles were effectively condensed inside the jet cavity. The boiling flow reversals were notably delayed by the synthetic jets. Meanwhile, the pressure fluctuation amplitudes were suppressed to some extent. It was also observed that synthetic jets can help to uniformize the heat sink temperature distribution.

Experimental Study on the Aerator's Air Hole Area, Cavity Length and Backwater in the Cavity

It cannot be ignored that the concrete surface of the discharge works may be seriously damaged caused by high speed flows. However, aerator installed at the bottom of spillway or tunnel is usually used to solve this problem in engineering. The function of the aeration facilities are influenced significantly by the cavity length and the backwater in the cavity, for the air holes will be blocked if there is a serious backwater in the cavity and so the aeration facilities will be ineffective. This thesis aims to study and analysis the relations among the air hole's area, the cavity length and the backwater depth in different conditions by the model experiments. The experiment results indicate that, the area of the air hole is a key factor to influence the jet cavity after the aerator. Aerator's body size and the chute slope affect cavity size and backwater in it greatly. For the chute with small bottom slope, more attention should be paid to the backwater in aerator's cavity.

Large eddy simulation based studies of jet–cavity interactions in a supersonic flow

Interactions of a cavity flameholder with an upstream injected jet in a Ma 2.52 supersonic flow are investigated numerically. A hybrid RANS/LES (Reynolds-Averaged Navier–Stokes/Large Eddy Simulation) method acting as wall-modeled LES is adopted, for which the recycling/rescaling method is introduced to treat the unsteady turbulent inflow. Patterns of the fluid entrainment into the cavity and escape from the cavity are identified using a scalar-tracing method. It is found that the jet–cavity interactions remarkably enhanced the mass exchange between the fluids in and out of the cavity, resulting in reduced residence time of the cavity fluids. Increasing the distance between the fuel injection and the cavity leading edge tends to attenuate the jet–cavity interactions, leading to weaker mass exchange. Raising the injection pressure appears to enhance the jet–cavity interactions, resulting in a shorter residence time of the cavity fluids. Moreover, the mass decay processes for the fuel and air within the cavity are basically the same while the entrainment processes for the fuel and air into the cavity seem quite different.

Cavity flow regime for spillway aerators

It is well known that the effect of air entrainment for cavitation damage controls is related not only to the air discharge into aerator devices but also the flow regime of the cavity below them. On the basis of the hydraulic characteristics of the flow, the aerator devices were for the first time classified. The theoretical considerations were performed about the jet length and cavity flow regime with the influencing factors. Comparing with the behavior of the flow through the aerator of discharge tunnels, the flow regimes of the cavity below spillway aerators were experimentally investigated, and the empirical expressions were presented to identify the conversions of the cavity flow regimes, including fully filled cavity, partially filled cavity, and net air cavity. Some issues of the design of the aerator devices were suggested in the present work.

Effect of orifice inner lip radius on synthetic jet efficiency

A synthetic jet is formed by periodic oscillation of a fluid through an orifice. The oscillatory motion is driven by a diaphragm or other driver. Previous studies have demonstrated that synthetic jet formation and time-averaged cavity pressure are a function of the orifice shape. Traditionally, the performance of the jet is evaluated with varying configurations of fixed driver input voltage or fixed driver displacement. Neither of these measures accurately reflect the efficiency of the actuator. Defining efficiency as “desired output divided by required input,” these traditional measures may not account for increase in required driving current or force. A sharp inside edge of a thin synthetic jet orifice can result in separated flow and increased momentum flux (due to the decreased flow area) for a fixed driver displacement. This can lead one to believe that efficiency has been improved, when, in reality, much more power was required for the driver. Acoustic power, which is the time-average of volume flow rate through the orifice multiplied by the driving pressure, accurately accounts for the power required to drive the actuator. For any synthetic jet actuator, the power to the driver is the power to the fluid (acoustic power) divided by the driver efficiency. If we assume that the driver efficiency is not a strong function of the load, any change to the acoustic power will result in the same proportional change in the driver input power. This study investigates the efficiency of a round (axisymmetric) synthetic jet actuator as a function of the radius of curvature of the interior edge of the orifice. Simultaneous particle image velocimetry measurements at the jet exit and cavity pressure measurements are used to measure the acoustic power required to generate the jet. The resultant momentum flux of the jet is used as a measure of output of the jet. Results are obtained for a range of displacement amplitudes (or stroke lengths) and radii of curvature, while Reynolds number is held fixed.

Cavity ring-down laser absorption spectroscopy of YBr and YI

The absorption spectra of YBr and YI between 445 and 510 nm have been investigated using the technique of laser vaporization/reaction with free jet expansion and cavity ring down laser absorption spectroscopy. Seven vibronic transition bands of the D1Π–X1Σ+ system of YBr were recorded and studied. Spectra of both Y79Br and Y81Br isotopic molecules were observed and analysed, which yielded vibrational constants for the D1Π state. Eighteen vibronic transition bands of the D1Π – X1Σ+ system of YI have also been recorded and analysed, and accurate vibrational constants were obtained. A comparison of spectroscopic parameters of D1Π state of yttrium monohalides shows the strength of the halogen element as a ligand in splitting the d orbitals of the transition metal atom, Y.

Computational Study of Effects of Nondimensional Parameters on Synthetic Jets

This paper investigates the effects of nondimensional parameters on the characteristics of synthetic jets. Flow inside the synthetic jet cavity and orifice, the flow outside are simulated together using large-eddy simulations (LES). A comparison of the present results and those of the experiment shows that simulating the flow both inside and outside the jet cavity is essential for accurately estimating the velocity and velocity fluctuations of the synthetic jet. LES results under various flow conditions show that strong three-dimensional vortices are generated when the Reynolds number is large, but finer vortex structures form because of stronger vortex interaction as the Strouhal number increases.

Orifice Spillway Aerator: Hydraulic Design

Orifice spillways are in vogue for the dams in the hilly regions where the spillway has to serve the dual function of flood disposal and flushing of sediment through the reservoir. Deep-seated orifice spillways are subjected to cavitation damage as the cavitation index drops below the critical cavitation index of 0.2 because of negative pressures on the profile and high flow velocities. Aerators are provided for mitigating cavitation damage. Design guidelines for aerator of orifice spillways are scanty and not reported much in the literature so far; thus, there still remains gray area in the field of spillway aerator design. The present study investigates the performance of an offset aerator with and without a ramp for deep-seated orifice spillway on a physical and numerical model. Performance of the aerator for varying discharges, heads, and gate openings is studied for varying cavity subpressures. Results with respect to jet length, cavity subpressure and air entrainment coefficients are presented in the form of nondimensional plots. From this study, the nondimensional jet length is in the range of 2 to 35 and increases as the cavity subpressure, approaches atmospheric pressure. It is also found that the air entrainment increases with increase in Froude number, ramp height and cavity pressure. From the present study results, equations for jet length and air entrainment coefficient are developed and are presented for the orifice spillway aerator.

The impact of a translating plunging jet on a pool of the same liquid

An experimental study on the impact of a translating two-dimensional transient jet on an initially quiescent liquid pool is studied experimentally using high-speed cinematic visualization and particle image velocimetry methods. Six jet conditions (covering a range of jet thicknesses, velocities and inclination angles relative to vertical) are considered, with measurements performed over a range of horizontal translation speeds for each jet condition. For all conditions studied herein, the jet penetrates into the pool and forms two craters – one upstream and one downstream of the jet. Gravity acts to close these craters, which after a short time pinch off at intermediate depths, thereby entrapping cavities of air. The translation speed of the jet is found to have a dramatic effect on the cavity shapes, pinch-off depths and pinch-off times. A simple theory based on a potential flow and a hydrostatically driven collapse is used to model this flow, and the resulting jet tip trajectories and cavity shapes compare favourably with the experimental data.

The impact of a translating plunging jet on a pool of the same liquid

An experimental study on the impact of a translating two-dimensional transient jet on an initially quiescent liquid pool is studied experimentally using high-speed cinematic visualization and particle image velocimetry methods. Six jet conditions (covering a range of jet thicknesses, velocities and inclination angles relative to vertical) are considered, with measurements performed over a range of horizontal translation speeds for each jet condition. For all conditions studied herein, the jet penetrates into the pool and forms two craters – one upstream and one downstream of the jet. Gravity acts to close these craters, which after a short time pinch off at intermediate depths, thereby entrapping cavities of air. The translation speed of the jet is found to have a dramatic effect on the cavity shapes, pinch-off depths and pinch-off times. A simple theory based on a potential flow and a hydrostatically driven collapse is used to model this flow, and the resulting jet tip trajectories and cavity shapes compare favourably with the experimental data.

CONSTRAINING THE OUTBURST PROPERTIES OF THE SMBH IN FORNAX A THROUGH X-RAY, INFRARED, AND RADIO OBSERVATIONS

Combined Spitzer, Chandra, XMM-Newton, and VLA observations of the giant radio galaxy NGC 1316 (Fornax A) show a radio jet and X-ray cavities from AGN outbursts likely triggered by a merger with a late-type galaxy at least 0.4 Gyr ago. We detect a weak nucleus with an SED typical of a low-luminosity AGN with a bolometric luminosity of 2.4x10^42 erg/s. The Spitzer IRAC and MIPS images show dust emission strongest in regions with little or no radio emission. The large infrared luminosity relative to the galaxy's K-band luminosity implies an external origin for the dust. The dust mass implies that the merger spiral galaxy had a stellar mass of 1-6x10^10 M_sun and a gas mass of 2-4x10^9 M_sun. Chandra images show a small ~15"(1.6 kpc) cavity coincident with the radio jet, while the XMM-Newton image shows two large X-ray cavities lying 320"(34.8 kpc) east and west of the nucleus, each approximately 230"(25 kpc) in radius. The radio lobes lie at radii of 14.3'(93 kpc) and 15.6'(101 kcp), more distant from the nucleus than the detected X-ray cavities. The relative morphology of the large scale 1.4 GHz and X-ray emission suggests they were products of two distinct outbursts, an earlier one creating the radio lobes and a later one producing the X-ray cavities. Alternatively, if a single outburst created both the X-ray cavities and the radio lobes, this would require that the morphology is not fully defined by the 1.4 GHz emission. For the more likely two outburst scenario, we use the buoyancy rise times to estimate an age for the more recent outburst of 0.1 Gyr and use the associated PV work done by the expanding plasma to create the X-ray cavities to estimate the outburst's energy of 10^58 ergs. The present size and location of the large radio lobes implies that the AGN outburst that created them happened ~0.4 Gyr ago and released ~5x10^58 ergs. (abridged)

Experimental study of whistle noise generated by insert edge and oil separator model of refrigeration cycle

The compressed air experiments are conducted to investigate the whistle noise radiated from the oil separator component of refrigerant cycle system. Two types of insert edge and a height-adjustable oil separator model are adopted. The acoustic characteristic of flow through plain top insert edge is more likely to be broadband. Flow through ramp top insert edge would induce whistle peaks at Mach number higher than 0.1197 and the oil separator model works like an acoustic amplifier. Moreover, the Strouhal number jumps are captured inside oil separator model and two mechanisms are presented to clarify the phenomena. First, the whistle noise at Modes A1, A3, B1 and B3 is regarded as coupling of shear layer instability with resonance acoustic modes of oil separator model. Second, the whistle noise at Modes A2 and B2 is regarded to be the feedback loop of flow–acoustic interaction in fluctuated shear layer and defined as jet–cavity interaction tones.

ERRATUM: “RADIATIVE EFFICIENCY AND CONTENT OF EXTRAGALACTIC RADIO SOURCES: TOWARD A UNIVERSAL SCALING RELATION BETWEEN JET POWER AND RADIO POWER” (2008, ApJ, 686, 859)

We present an analysis of the energetics and particle content of the lobes of 24 radio galaxies at the cores of cooling clusters. The radio lobes in these systems have created visible cavities in the surrounding hot, X-ray-emitting gas, which allow direct measurement of the mechanical jet power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. Using these measurements, we examine the ratio between radio power and total jet power (the radiative efficiency). We find that jet (cavity) power increases with radio synchrotron power approximately as P_jet ~ (L_radio)^beta, where 0.35 < beta < 0.70 depending on the bandpass of measurement and state of the source. However, the scatter about these relations caused by variations in radiative efficiency spans more than four orders of magnitude. After accounting for variations in synchrotron break frequency (age), the scatter is reduced by ~ 50%, yielding the most accurate scaling relation available between the lobe bolometric radio power and the jet (cavity) power. We place limits on the magnetic field strengths and particle content of the radio lobes using a variety of X-ray constraints. We find that the lobe magnetic field strengths vary between a few to several tens of microgauss depending on the age and dynamical state of the lobes. If the cavities are maintained in pressure balance with their surroundings and are supported by internal fields and particles in equipartition, the ratio of energy in electrons to heavy particles (k) must vary widely from approximately unity to 4000, consistent with heavy (hadronic) jets.

Computational Study on Effect of Synthetic Jet Design Parameters

Effects of amplitude and frequency of synthetic jet on the characteristics of induced jet are investigated. To estimate effects of the parameters, flow inside the synthetic jet cavity and orifice and the outer flow is simultaneously simulated using large-eddy simulation (LES). Comparison of the present LES result with the experimental data shows that three-dimensional LES of the flow inside the cavity is essential for accurate estimation of the velocity and velocity fluctuation of the synthetic jet. Comparison of the present results under various flow conditions shows that amplitude and frequency can control profiles of time-averaged vertical velocity and fluctuation of the vertical velocity as well as damping rate of the induced velocity and fluctuation.

Cavity Ring Down Laser Absorption Spectroscopy of NiI

The absorption spectrum of NiI between 445 and 510 nm has been investigated using the technique of laser vaporization/reaction with free jet expansion and cavity ring down laser absorption spectroscopy. Two new transitions namely, [21.3]25/2-X25/2 and [21.9]23/2-X25/2 systems were identified and studied. Spectra of both 58NiI and 60NiI isotopic molecules were observed. Equilibrium molecular constants for both electronic states are reported and the equilibrium bond length for the [21.3]25/2 state and the [21.9]23/2 state was respectively determined to be 2.431 and 2.481 A.

Experimental investigation of axisymmetric coaxial synthetic jets

Measurements were made in the near field of piston driven axisymmetric coaxial synthetic jets emanating from an orifice and a surrounding annulus of equal exit areas and cavity volumes. Piston velocity, amplitude, radial spacing between the orifice and the annulus, and exit angles had a strong influence on the dominant features of the flow. Flow visualization revealed three distinct topologies of the jet consisting of expanding, contracting and recirculating regions and doubling of the number of foci inside of the cavity compared to jet from the orifice alone. The direction of the swirl/rotation imposed on the mean flow was also dependent on the direction of the rotation of dominant foci. Interaction between flow from the orifice and the annulus amplified the azimuthal instability of ring vortices due to the periodic axial stretching and compression of the streamwise vortex filaments. Bifurcation of ring vortices into elliptical lobes reported earlier [S.V. Gaimella, V.P. Schroeder, Local heat transfer distributions in confined multiple air impingement, ASME Journal of Electronic Packaging 123 (3) (2001) 165–172] for single cavity jet was also observed in the coaxial jet. The number of cellular structures however was considerably larger than the single jet case. Large excursions of the jets from the plane of symmetry were observed. Power spectra exhibited sub-harmonic distribution of energy due to coalescence of the vortices. Growth of jet width and decay of centerline velocity were strongly influenced by the spacing and forcing frequency.