Unexcited Jet Research Articles

Effect of excitation Strouhal number on velocity field and mixing capability of acoustically excited dual jets

The present study experimentally investigated the effect of the excitation Strouhal number on the velocity field and mixing capability of excited dual jets. The dual jets were acoustically excited by a loudspeaker. The velocity pulsation at the jet exit was examined using a hotwire anemometer. The evolution process of the excited dual jets was captured using the smoke flow visualization technique. The jet spread width was measured from a long-exposure image by using the edge-detection method. The time-averaged velocity fields were measured using the particle image velocimetry (PIV) technique. The mixing capability was examined by tracer-gas concentration detection. Two characteristic flow modes, namely the bifurcated and parallel vortices, were classified in the domain of pulsation intensity and excitation Strouhal number. The excitation Strouhal number dominated the change in the characteristic flow modes. The leading vortices induced jet flow bifurcation in the bifurcated vortices mode, while the vortices broke up early in the parallel vortices mode. The recirculation region, merging length, combined length, jet spread, and later distribution of the dual jets excited in the bifurcated mode were larger than those in the parallel vortices mode. The tracer-gas value in the bifurcated vortices mode was smaller than that in the parallel vortices mode; therefore, the mixing capability in the bifurcated vortices mode was better than that in the parallel vortices mode. The unexcited and excited dual jets were compared to evaluate the effect of acoustic excitation on the flow and mixing characteristics.

Enhancing Cavitation Intensity in Co-Flow Water Cavitation Peening With Organ Pipe Nozzles

Abstract Co-flow water cavitating jets induce compressive residual stress through cavitation impacts produced by the collapse of the cavitation cloud. Co-flow water cavitation peening causes minimal surface alteration when compared with conventional processes such as shot peening, which is a major advantage. However, enhancement of cavitation intensity for co-flow water cavitation peening nozzles is required for practical applications requiring greater process capability. Scaling of co-flow cavitation peening nozzles to achieve greater cavitation intensity requires higher flowrates, thus requiring pumps of higher capacities. In contrast, organ pipe geometry nozzles can enhance cavitation intensity without a significant increase in pump capacity and have been used in deep-sea drilling applications. The objective of this work is to study the effects of organ pipe inner jet nozzle geometry on co-flow water cavitation intensity and peening performance relative to a standard (unexcited) inner jet nozzle geometry through experiments on aluminum alloy Al 7075-T651. Nozzle performance is characterized via extended mass loss and strip curvature tests, high-speed visualization of the cavitation cloud, analysis of impulse pressures, and through-thickness residual stress measurements. It is found that the optimum organ pipe inner jet nozzle geometry enhances the mass loss and peak strip curvature by 61% and 66%, respectively, when compared with the unexcited inner jet nozzle. Residual stress measurements show that the organ pipe inner jet nozzle produces deeper compressive residual stresses in the material than the unexcited inner jet nozzle.

Interactions of large-scale structures in the near field of round jets at high Reynolds numbers

A possible interaction between vortical structures in a round jet shear layer, viz. vortex rings and streamwise vortices, is explored following Davoust et al. (J. Fluid Mech., vol. 709, 2012, pp. 408–444). These authors reported a radial organization of streamwise vorticity in a jet at high diameter-based Reynolds number ( ), contrary to the classically observed azimuthal organization. They hypothesized that the observed weaker vortex rings in such jets could be deformed by streamwise vortices and further reoriented and stretched in the streamwise direction. As this hypothesis was based on the observations of one configuration of a jet flow, our study aims at assessing it by varying the key parameter, i.e. the relative strength of the vortex rings and the streamwise vortices, through forcing, along with various jet configurations. We first analyse a low-Mach-number, transitional jet, using high-speed stereo particle image velocimetry in a cross-sectional plane at two jet exit diameters from the nozzle exit. The axisymmetric mode is acoustically excited at various amplitudes to increase the strength of the rings relative to streamwise vortices, at a Strouhal number ( ) of 0.49, the most energetic frequency in the unforced jet. Starting from a radial array in the unexcited jet, a gradual shift towards an azimuthal configuration is obtained with increasing excitation level. Quantification of the relative strengths of the streamwise vortices and vortex rings confirms that a radial array is observed whenever the streamwise vortices are more intense than the rings, and conversely for the azimuthal configuration. We then extend the analysis to other jet cases in terms of , and state of the exiting boundary layer. We observe that the correlation between the radial or azimuthal organization of streamwise vortices and the relative strengths of the vortical structures holds systematically, confirming the possibility of the proposed interaction. A detailed analysis of the forced jets also sheds light on some interesting effects of acoustic excitation on the vortical organization in round jets.

Development of the Strategy of Active Control of Instability Waves in Unexcited Turbulent Jets

The possibility of controlling instability waves in the mixing layer of a subsonic unexcited jet is studied. These waves can be noise sources in both free jets and jets as parts of configurations. In the study the method of experimental diagnostics of the instability waves in the near field of a jet using an azimuthal multimicrophone array is realized. The data on the near field fluctuations are used for testing the control strategy proposed by the authors. The strategy consists in narrowband sliding filtration of the original signal and the formation of a narrowband controlling action on the basis of the linear principle of signal superposition. The results of the study represent the next step toward the realization of an active control system suppressing natural instability waves in turbulent jets.

Heat transfer enhancement by velocity field excitation for an impinging round jet

ABSTRACTIn this paper the effect of inlet velocity field excitation on heat transfer by an impinging round jet has been investigated using Large Eddy Simulations (LESs). The time-averaged bulk Reynolds number (Re) of the jet for all cases is 23,000 and the dimensionless jet’s outlet-to-target wall distance (H) is two diameters. The Strouhal number (St) corresponding to the preferred mode of the unexcited jet is estimated through LES. The jet is then excited at different frequencies. The amplitude of excitation is kept at 50% of the bulk inlet velocity. It has been found that the excitation of the jet velocity field causes only large-scale excitation. Heat transfer enhancement is found to be possible with inlet velocity field excitation.

Effects of Pulsation Intensity on Flow and Mixing of Swirling Double-Concentric Jets

The effects of pulsation intensity on the flow and mixing characteristics of acoustically excited swirling double-concentric jets were studied experimentally. The central jet was excited with various pulsation intensities. The flow evolution process, jet spread width, velocity instabilities, and jet dispersion characteristics were examined by the flow visualization, edge-detection technique, hot-wire anemometer, and tracer-gas concentration detection, respectively. As the pulsation intensity was increased over 0.16 one vortex ring was induced during each excitation cycle. This induced vortex ring evolved downstream and suddenly broke up into turbulent eddies at a distance downstream from the jet exit. The dispersion and mixing were simultaneously dominated by two mechanisms: 1) the breakup of the induced vortex ring into turbulent eddies, and 2) the near-field entrainment of the induced vortex rings. The former was more important at pulsation intensities smaller than about 0.5, whereas the latter was more prominent at pulsation intensities larger than approximately 0.5. The results of flow visualization as well as the measurements of velocity, fluctuation intensity, turbulence time and length scales, and tracer-gas-concentration distributions showed close correlations in interpreting the flow and mixing characteristics. Exciting the swirling double-concentric jets at a pulsation intensity of 1.0 might increase the mixing index by 90% over the unexcited jets.

Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.

Effects of crossflow on puff and oscillation modes of a pulsed elevated transverse jet

The effects of crossflow on the flow behaviour and mixing characteristics of an acoustically excited elevated transverse jet were investigated experimentally in a wind tunnel. The jet was excited at a frequency around the resonance condition by using a loudspeaker. The temporally varying flow structures of the excited elevated transverse jet in the median plane were captured by a high-speed digital camera. Visual penetration height and spread width were obtained by using an image processing method. The measured tracer-gas concentration revealed the dispersion characteristics. Forcing the transverse jet around the resonance frequency induced two distinct characteristic flow modes, puff and up–down oscillation of the deflected jet. A series of puffs travelling in the shear layer of the deflected jet were formed as the crossflow Reynolds number was lower than about 2300. For crossflow Reynolds numbers larger than 2300, large up–down oscillations of the deflected jet were observed. Acoustic excitation also eliminated the downwash effect induced by the shear of the transverse stream. The deflected transverse jet excited in the puff flow mode generated a significantly larger penetration height and spread width when compared with those in the oscillation flow mode and with the un-excited transverse jet. The measured tracer-gas concentration distributions demonstrated that the excited transverse jet has much better mixing and dispersion characteristics than the un-excited transverse jet. Besides, the puff flow mode has better mixing and dispersion characteristics than those in the oscillation flow mode.

Turbulent Jet Mixing Enhancement and Control Using Self-Excited Nozzles

A new self-excited jet methodology was developed for the mixing enhancement of jet fluid with its surrounding, quiescent, stagnant, or coflowing fluid. The nozzles, of a square or rectangular cross section, featured two flexible side walls that could go into aerodynamically-induced vibration. The mixing of nozzle fluid was measured using planar laser-induced fluorescence (PLIF) from acetone seeded into the nozzle fluid. Overall, the self-excited jet showed enhanced mixing with the ambient fluid; for example, at 390Hz excitation a mixing rate enhancement of 400% at x∕D=4 and 200% at x∕D=20 over the unexcited jet. The mixing rate was sensitive to the excitation frequency, increasing by 60% with the frequency changing from 200 to 390Hz (corresponding to a Strouhal number from 0.052 to 0.1). It was also observed that the mixing rate increased with the coflow velocity. To explain the observed mixing enhancement, the flow field was studied in detail using four-element hot wire probes. This led to the observation of two pairs of counter rotating large-scale streamwise vortices as the dominant structures in the excited flow. Shedding right from the nozzle exit, these inviscid vortices provided a rapid transport of the momentum and mass between the jet and the surrounding fluid at a length scale comparable to half-nozzle diameter. Moreover, the excited jet gained as much as six times the turbulent kinetic energy at the nozzle exit over the unexcited jet. Most of the turbulent kinetic energy is concentrated within five diameters from the nozzle exit, distributed across the entire jet width, explaining the increased mixing in the near field.

Flow Visualization and Characteristics of a Coaxial Jet with a Tabbed Annular Nozzle

Flow visualization and measurements of mean and fluctuating velocities were performed on a coaxial jet with a velocity ratio of 0.6 at a Reynolds number of 3 000 in an open water tank using hot-film anemometry, particle image velocimetry (2D and stereoscopic PIV) and laser-induced fluorescence (LIF). Axisymmetric and streamwise vortical structures were revealed in the near-field of the coaxial jet. The annular nozzle has six vortex generators in order to enhance the streamwise vortices generated in the mixing layer. Furthermore, the annular jet was excited by a shaker in order to enhance the axisymmetric vortices. For the tabbed coaxial jet, jet spreading downstream was greater than for the jet without tabs. The cause of the entrainment increment is the development of axisymmetric and streamwise vortex structures. In the case of excited jets, significant axisymmetric and streamwise vortical structures develop, and the jet width expands from the exit nozzle. Consequently, the flow rate of the excited jet with tabs is larger than that of the unexcited jet without tabs.

Effect of an external excitation on the flow structure in a circular impinging jet

The effect of an external excitation on circular impinging jet flow is studied experimentally. Velocity fields in moderate air flows (Re=10000) are investigated by hot-wire anemometry. The basic flow is excited by a small sinusoidal modulation of the nozzle exit velocity. The phase-averaging technique is used to study the behavior of vortex structures in the jet, specifically rolling-up, pairing, and interaction with the wall. The jet is found to be sensitive to excitation in the frequency range characterized by a Strouhal number, Ste=feD∕U, from 0.3 to 3. Different flow regimes are identified in the excited impinging jet: a periodic flow regime with the same frequency as the excitation, a regime with a frequency corresponding to a subharmonic of the excitation frequency, a regime alternating between these two frequencies, and two border regimes with more complicated behavior. The low-frequency excitation leads to the formation of vortices, which are larger than those occurring in an unexcited jet. Consequently, the near-wall velocity fluctuations are enhanced and the unsteady flow separation induced by vortex impingement is more pronounced. By contrast, excitation at higher frequencies, characterized by value 0.017 of a Strouhal number based on the shear layer thickness θ, Stθ=feθ∕U, leads to the roll-up of small vortices and hence to the attenuation of the near-wall velocity fluctuations. In this case, the flow separation is suppressed because the small vortices are unable to induce it. Finally, the vortex merging process is sensitive to the double-frequency excitation with subharmonic components. For example, a combined excitation at Ste=1.60 and 0.53 leads to the merging of three vortices.

Possibility of Acoustic Control of Excited Turbulent Jets

The results of experimental investigation of the influence of high‐frequency (low‐frequency) acoustic action on a turbulent jet preexcited by sound of low (high) frequency are presented. It is shown that the qualitative character of the action remains the same as that for an unexcited jet. However, the quantitative results here are somewhat different; in particular, the possibilities of suppressing turbulence in the jet in its high‐frequency excitation are reduced in this case.

BIFURCATING AND BLOOMING JETS

▪ Abstract It is classically assumed that the far field of a round turbulent jet discharging into quiescent fluid has a unique behavior characterized only by its momentum flux. However, there is now considerable evidence that different discharge conditions at the jet nozzle exit can give rise to very different far-field flows. Perhaps the most striking examples of these are the bifurcating and blooming jets produced by appropriate combinations of controlled axial and circumferential excitations at the nozzle exit. With the right excitations, a jet can be made to divide into two separate jets (bifurcating jet), each of which carries half the axial momentum and spreads in a manner similar to a single jet. Trifurcating jets can also be produced. Other excitations can produce blooming jets, in which the jet explodes into a shower of vortex rings, producing a far-field flow that is quite unlike a normal unexcited jet. Bifurcating and blooming jets exhibit much greater mixing than normal jets, suggesting possible applications in flow control. This article summarizes our work on bifurcating and blooming jets, which began with our discovery of them in the early 1980s and continued through the mid- 1990s. One of us (D.E.P.) continued exploration of flow control using excited jets, first at the McDonnell Douglas Corporation, and more recently at the Georgia Institute of Technology. The key to flow control is the manipulation of the large vortical structures in the near field of the jet. Ultimately this work, and that of others, led to full-scale testing of jet engine exhaust mixing control. There it was shown that the jet temperature downstream of the engine can be very significantly reduced by application of well-designed and easily implemented excitation at the engine discharge, thereby solving problems encountered during ground operations. Related jet control work by other investigators is included in this review.

音響加振された噴流拡散火炎の噴流構造と燃焼特性

Acoustically excited jet diffusion flame and unburnt jet in quiescent air are investigated experimentally to provide detailed information about combustion characteristics and the jet structure. The hydrogen jet diffusion flame and unburnt jet are acoustically excited by monotone sound wave generated by a loud speaker. For the unexcited jet diffusion flame, the hysteresis behavior in the flame lift-off and reattachment, and the sudden increase in lift-off height are observed. Applied the acoustic excitation using a specific range of frequency, the jet velocity at which the transition from laminar to turbulent flame occurs is lowered significantly. Also, the flame shortens its original length and is widened in the incident direction of the sound wave. Furthermore, the flame with branched tip is observed in narrower region of the Strouhal number and the Reynolds number. It is also found out that branching of the unburnt jet is created in a region of lower Strouhal number and lower Reynolds number than the branched tip flame.

Coherent structure interactions in an unexcited coaxial jet

Based on a double triggering criterion, the axisymmetric and azimuthal structures within the initial region of an unexcited coaxial jet of mean velocity ratio of 0.3 (inner to outer) were recovered. Outer vortices, two trains of inner vortices and their amalgamation are found in the outer and inner mixing regions. Bifurcation of the amalgamated inner vortices occurs within the first two diameters of the jet and is responsible for the occurrence of the azimuthal structures in the inner jet region further downstream. Co-dominance of the symmetrical and azimuthal structures having different convection velocities is observed in the fully merged zone. Spatial separation between vortical structures and their relative vorticities are important parameters in affecting their convection velocities, growth and decay.

Elliptic jets. Part 1. Characteristics of unexcited and excited jets

This paper summarizes experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions, and effects of excitations at selected frequencies and amplitudes. Elliptic jets are quite different from the extensively studied plane and circular jets - owing mainly to the fact that the azimuthal curvature variation of a vortical structure causes its non-uniform self-induction and hence complex three-dimensional deformation. Such deformation, combined with properly selected excitation can substantially alter entrainment and other turbulence phenomena, thus suggesting preference for the elliptic shape in many jet applications. The dominance of coherent structures in the jet far field is evident from the finding that switching over of the cross-section shape continues at least up to 100 equivalent diameters De. The locations and the number of switchovers are strongly dependent on the initial condition, on the aspect ratio, and, when excited, on the Strouhal number and the excitation level. We studied jets with constant exit momentum thickness θe, all around the perimeter, thus separating the effects of azimuthal variations of θe, (typical of elliptic jets) and of the shear-layer curvature. Also investigated are the instability characteristics, and enhanced entrainment caused by bifurcation as well as pairing of vortical structures. We discuss shear-layer and jet- column domains, and find the latter to be characterized by two modes : the preferred mode and the stable pairing mode - similar to those found in circular jets -both modes scaling on the newly-defined lengthscale De. The paper documents some time- average measurements and their comparison with those in circular and plane jets.

Turbulent plane jet excited mechanically by an oscillating thin plate in the potential core.

A thin plate in a jet core region is driven at both ends by stepping motors, which enable control of the frequency and the amplitude of rotational oscillation about a mean position set at a zero angle of attack. The maximum attack angle is nominally 1.8 degrees for the plates at 15 Hz and 27 Hz and zero for the stationary plate. The stationary plate does not affect the jet flow characteristics in the fully developed region at all. The effects of excitation are salient and appear firstly as a periodic deflection of the jet near the nozzle exit, which is amplified downstream and gives notable effects on the development of the jet. The rate of increase in jet half width reaches more than 5 times the value for an unexcited jet at an excitation frequency of 27 Hz.

Controlled excitation of elliptic jets

Studies of unexcited and excited elliptic jets reveal their characteristics to be noticeably different from circular jets, suggesting applications of excited elliptic jets for enhanced mixing and chemical reaction, and control of aerodynamic noise. The near-field turbulence characteristics, jet spread, and locations of switching of major and minor axes of the jet cross section can be drastically altered by forcing. The preferred mode and the stable pairing mode of an elliptic jet scale with the exit equivalent diameter.

Noise generation by a low-Mach-number jet

Using a ‘clean’ jet facility the relationship between the jet flow and its radiation field was studied experimentally in the Mach-number range 0.05 < Mj < 0.20 and a Reynolds-number range 6 × 104 < ReD < 2.3 × 105. The various acoustic source parameters such as strength, frequency and Mach number were varied systematically, and the far-field pressure measured simultaneously. On the basis of these measurements the nature of the sources in the initial shear layer could be characterized. The principal results, equally valid for unexcited and excited jets, are as follows: the acoustic sources are not convected but are located within a confined volume fixed with respect to the nozzle even though they are being generated by moving disturbances in the jet; they are associated with the nonlinear saturation of the unstable wave amplitudes of the shear layer occurring at the vortex-pairing locations; the radiation intensity varies nonlinearly with the source strength and is highly directional, exponential in character.

Speckle velocimetry study of vortex pairing in a low-Re unexcited jet

Application of speckle velocimetry in a round jet (Re=2300) provides instantaneous 2-D velocity fields and vorticity maps of the interaction between unstable vortices. Detailed descriptions of the pairing process and of the large-scale structures observed near the end of the potential core are obtained without excitation of the jet and without the smearing effects of the conditional sampling method.