Jet Shear Layer Research Articles

1508 二次元噴流せん断層の不安定波による自励振動 (第2報:せん断層音とエッジ音との関連性)(OS03-2 熱・流体の可視化と計測,オーガナイズドセッション 3:「熱・流体の可視化と計測」)

1508 二次元噴流せん断層の不安定波による自励振動 (第2報:せん断層音とエッジ音との関連性)(OS03-2 熱・流体の可視化と計測,オーガナイズドセッション 3:「熱・流体の可視化と計測」)

4D flow jet shear layer detection method for the measurement of effective orifice area and assessment of aortic stenosis severity

Background Aortic stenosis (AS) is the most common cause of valve replacement and its severity is mainly assessed by transthoracic Doppler echocardiography (TTE) to quantify valve effective orifice area (EOA) as determined by the continuity equation. In a previous study we demonstrated that EOA can be directly determined with 2D flow MRI downstream of the AS using the jet shear layer detection (JSLD) method and the acoustical source term (AST) concept. However, both TTE and 2D flow MRI rely on the measurement of local and single-directional velocities and thus incomplete assessment of the complex post-valve flow dynamics in a significant proportion of patients. 3D CINE phase contrast MRI with 3-directional velocity encoding (4D flow MRI) may improve EOA estimation by the JSLD method coupled with full volumetric coverage of ascending aortic 3D blood flow. The objective of this study was to validate 4D flow MRI based EOA estimation using an in-vitro stenosis phantom and to apply the technique for the in-vivo measurement of the valve EOA compared to the reference standard 2D flow MRI. Methods

The Prediction of Broadband Shock-Associated Noise Including Propagation Effects

An acoustic analogy is developed based on the Euler equations for broadband shock-associated noise (BBSAN) that directly incorporates the vector Green's function of the linearized Euler equations and a steady Reynolds-Averaged Navier-Stokes solution (SRANS) to describe the mean flow. The vector Green's function allows the BBSAN propagation through the jet shear layer to be determined. The large-scale coherent turbulence is modeled by two-point second order velocity cross-correlations. Turbulent length and time scales are related to the turbulent kinetic energy and dissipation rate. An adjoint vector Green's function solver is implemented to determine the vector Green's function based on a locally parallel mean flow at different streamwise locations. The newly developed acoustic analogy can be simplified to one that uses the Green's function associated with the Helmholtz equation, which is consistent with a previous formulation by the authors. A large number of predictions are generated using three different nozzles over a wide range of fully-expanded jet Mach numbers and jet stagnation temperatures. These predictions are compared with experimental data from multiple jet noise experimental facilities. In addition, two models for the so-called ‘fine-scale’ mixing noise are included in the comparisons. Improved BBSAN predictions are obtained relative to other models that do not include propagation effects.

Heat Transfer in an Oblique Jet Impingement Configuration With Varying Jet Geometries

The experimental and numerical heat transfer results in a trapezoidal duct with two staggered rows of inclined impingement jets are presented. The influence of changes in the jet bore geometry on the wall heat transfer is examined. The goal of this project is to minimize the thermal load in an internal gas turbine blade channel and to provide sufficient cooling for local hot spots. The dimensionless pitch is varied between p/djet=3 − 6. For p/djet=3, cylindrical and conically narrowing bores with a cross section reduction of 25% and 50%, respectively, are investigated. The studies are conducted at 10,000≤Re≤75,000. Experimental results are obtained using a transient thermochromic liquid crystal technique. The numerical simulations are performed solving the RANS equations with FLUENT using the low- Re k- ω -SST turbulence model. The results show that for a greater pitch, the decreasing interaction between the jets leads to diminished local wall heat transfer. The area averaged Nusselt numbers decrease by up to 15% for p/djet=4.5, and up to 30% for p/djet=6, respectively, if compared to the baseline pitch of p/djet=3. The conical bore design accelerates the jets, thus increasing the area-averaged heat transfer for identical mass-flow by up to 15% and 30% for the moderately and strongly narrowing jets, respectively. A dependency of the displacement between the Nu maximum and the geometric stagnation point from the jet shear layer is shown.

The visualization of the acoustic feedback loop in impinging underexpanded supersonic jet flows using ultra-high frame rate schlieren

The use of modern ultra-high speed cameras to acquire time-resolved schlieren image sequences of supersonic jet impingement is presented. The use of these cameras, with framerates of up to 1 million frames per second, allows for the first time-resolved visualizations of the impinging jet acoustic feedback loop. The role of upstream travelling acoustic waves in generating perturbations in the jet shear layer at the nozzle exit is also directly observed for the first time. The arrival of the acoustic wave at the nozzle lip generates a sinusoidal variation in density gradient that persists until a distance of $$\frac{x}{d}=0.3$$ . A structure that rapidly evolves into a large-scale vortex ring forms at the trailing edge of this initial instability, first observed at approximately $$\frac{x}{d}=0.25$$ .

A direct numerical simulation study of turbulence and flame structure in transverse jets analysed in jet-trajectory based coordinates

AbstractAn${\mathrm{H} }_{2} / {\mathrm{N} }_{2} $jet in cross-flow (JICF) of air is studied using three-dimensional direct numerical simulation with and without chemical reaction in order to investigate the role of the complex JICF turbulent flow field in the mechanism of fast fuel-oxidant mixing and of aerodynamic flame stabilization in the near field of the jet nozzle. Focus is on delineating the flow/mixing/chemistry conditions that are necessary and/or sufficient to achieve flame anchoring that ultimately enables the formulation of more reliable and precise guidelines for design of fuel injection nozzles. A mixture averaged diffusion formulation that includes the effect of thermal diffusion is used along with a detailed chemical kinetics mechanism for hydrogen–air combustion. A new parametrization technique is used to describe the jet trajectory: solution of Laplace’s equation upon, and then within, an opportune scalar surface anchored by Dirichlet boundary conditions at the jet nozzle and plume exit from the domain provides a smoothly varying field along the jet path. The surface is selected to describe the scalar mixing and reaction associated with a transverse jet. The derived field,$j(\mathbi{x})$, is used as a condition to mark the position along the natural jet trajectory when analysing the variation of relevant flow, mixing and reaction quantities in the present direct numerical simulation (DNS) datasets. Results indicate the presence of a correlation between the flame base location in parameter space and a region of low velocity magnitude, high enstrophy, high mixing rate and high equivalence ratio (flame root region). Instantaneously, a variety of vortical structures, well known from the literature as important contributors to fuel-oxidant mixing, are observed in both inert and reactive cases with a considerable span in length scales. Moreover, instantaneous plots from reactive cases illustrate that the most upstream flame tongues propagate close to the trailing edge of the fuel jet potential core near the jet shear layer vortex shedding position. Some degree of asymmetry with respect to the domain mid-plane in the spanwise direction is observed in the averaged fields, both for the inert and reactive cases.

Shear layer instabilities in low-density transverse jets

The near-field shear layer instabilities forming in round transverse jets of variable (reduced) densities relative to the crossflow are investigated through gas-phase experiments. Jets composed of helium and nitrogen mixtures are injected from a converging nozzle mounted flush with an injection wall into air crossflow, allowing the jet-to-crossflow density ratio S to be varied between 1.00, the equidensity case, and 0.14, at constant jet Reynolds number Re j = 1,800. Jet-to-crossflow momentum flux ratios J are examined in the range $$\infty>J\geq5$$ at incremental values of the density ratio S. The results of single-component hotwire measurements in the jet shear layer indicate that a transition to global instability likely occurs as J is brought below approximately 10, and/or as S is brought below approximately 0.45–0.40. This observation appears to link many previous independent studies of both equidensity transverse jets and low-density free jets, which may become globally unstable under alteration of J and S, respectively. However, the dynamical character of the transition to global instability in the low-density transverse jet displays differences under independent variation of J and S, which may indicate the predominance of different modes.

Large-Eddy Simulation of Underexpanded Supersonic Swirling Jets

A large-eddy simulation of underexpanded supersonic swirling jets issuing into a quiescent environment was carried out for two typical swirl numbers. The corresponding nonswirling jet was also calculated for comparison and validation against the experimental data. The swirling effect on the various fundamental mechanisms that dictate the intricate flow phenomena, including flow features, shock cell structures, jet spreading characteristics and turbulence behaviors, was carefully analyzed, and it is found that the first shock cell length is reduced in the swirling jet in comparison with the nonswirling jet. A recirculation zone is formed in the high swirl number case, and the jet spreads quickly in the radial direction due to the swirling effect. Moreover, intensive turbulent fluctuations are generated along the jet shear layer and at the end of the jet potential core, which will be helpful in the mixing process. The obtained results provide a physical insight into understanding the mechanisms relevant to underexpanded supersonic swirling jets.

High-Speed and High-Reynolds-Number Jet Control Using Localized Arc Filament Plasma Actuators

A class of plasma actuators called localized arc filament plasma actuators for high-speed and high-Reynoldsnumber flow and acoustic control has been developed at the Gas Dynamics and Turbulence Laboratory. Over the past several years, these high-bandwidth (0 to 200 kHz) and individually controlled actuators have been used successfully to excite the jet shear layer, jet column, and azimuthal instabilities in high subsonic and supersonic jets. The focus of this paper is to provide detailed information and sample results highlighting the capabilities and potential of the actuators and the control technique formixing enhancement, noise mitigation, and flow and acoustic diagnostics. The jet, using three different nozzles, is operated over a large range of jet Mach numbers (0.9 to 1.65), stagnation temperature ratios (up to 2.5), and Reynolds numbers (0:2 10 to 1:65 10). Over this space of operating conditions, the jet is found to respond to control with a large range of forcing Strouhal numbers and azimuthalmodes. The results reveal that the jet flowfield and acoustic farfield can be dramatically altered, providing a powerful control tool in these practical high-speed and high-Reynolds-number jets.

Analysis of the Near Field in a Plasma-Actuator-Controlled Supersonic Jet

Simulations are employed to examine the near field of aMach 1.3 jet subjected to eight plasma actuators mounted on the periphery of the nozzle exit. The simulations are validated by comparing several quantities with experiments, including mean and fluctuating streamwise velocities, as well as visualizations of coherent features. The generation, evolution, and breakdown of coherent structures with first (m 1), second (m 2), and third (m 3) azimuthal mode excitation are described and set in the context of previous simulations with axisymmetric and mixed modes at themost amplified jet columnmode (Strouhal number 0.3). Hair-pin-like vortices, observed in instantaneous, as well as phase-averaged data, are noted as key building blocks in the evolution process. The tips/heads arise in the outer region of the jet shear layer and legs extend forward, slightly inclined in the direction of the jet axis where the velocity is higher. The interaction of these structures through self and mutual induction with those generated by adjacent actuators, or later by the same actuator, yields a rich variety of features depending on excitationmode. In the second part of the paper, the effect of Strouhal number on the near field is explored with m 0 and m 1 excitation. The results show that, at higher frequencies, large structure formation is inhibited and peak sound pressure level amplitudes diminish in a manner consistent with experimental results.

On the role of glottis-interior sources in the production of voiced sound

The voice source is dominated by aeroacoustic sources downstream of the glottis. In this paper an investigation is made of the contribution to voiced speech of secondary sources within the glottis. The acoustic waveform is ultimately determined by the volume velocity of air at the glottis, which is controlled by vocal fold vibration, pressure forcing from the lungs, and unsteady backreactions from the sound and from the supraglottal air jet. The theory of aerodynamic sound is applied to study the influence on the fine details of the acoustic waveform of "potential flow" added-mass-type glottal sources, glottis friction, and vorticity either in the glottis-wall boundary layer or in the portion of the free jet shear layer within the glottis. These sources govern predominantly the high frequency content of the sound when the glottis is near closure. A detailed analysis performed for a canonical, cylindrical glottis of rectangular cross section indicates that glottis-interior boundary/shear layer vortex sources and the surface frictional source are of comparable importance; the influence of the potential flow source is about an order of magnitude smaller.

Finite Element Solutions of a Wave Equation for Sound Propagation in Sheared Flows

Aweakdiscretization in the frequency domain of theLilley–Goldstein acoustic analogy equation is presented. This equation is cast into a linear system of two equations that are successively arranged in a convenient form for a Galerkin projection. The method is applied to compute the sound generated by Dirac ring sources of different azimuthal order and radius and propagated through an axisymmetric parallel jet. The effects due to the jet temperature and velocity profile combined with the effects due to the source frequency, radius, and azimuthal order are illustrated through extensive parametric results. It is shown that three-dimensional Green’s functions can be reconstructed by superposition of few axisymmetric solutions of low azimuthal order when the sources are located inside the jet. Finally, tones from annular sources located in the potential core of amixing jet are propagated through the jet shear layer and compared with the tones computed in uniform flow conditions. It is shown that, for conditions that are representative of a propeller rig located in the potential core of a wind-tunnel jet, the shear-layer refraction effects can be corrected by using a thin-layer approximation.

LARGE EDDY SIMULATION OF AN ELLIPTIC JET INJECTED INTO A SUPERSONIC CROSSFLOW

A transverse jet issuing from an elliptic injector into a supersonic crossflow has been investigated using large eddy simulation. The complex flow structures and the relevant flow features are analyzed to exhibit the evolution of shock structures, vertical structures and jet shear layer. A horseshoe vortex is formed in the upstream of the jet and the shock structures exhibit small fluctuations due to the flow interaction. The kidney-shaped counter-rotating vortex pair dominates the flow field in the downstream of the jet. The elliptic jet spreads rapidly in the spanwise direction and then the axis-switching phenomenon occurs. Intense turbulent fluctuations are identified behind the Mach disk because of the large velocity gradients.

S055033 音波制御した大気圧プラズマジェットによる表面改質効果

The aim of this investigation is to reveal how the working gas behavior affects the motion of a microwave discharged atmospheric pressure plasma jet (APPJ) in surface modification process on a flat glass plate. The motion of APPJ with argon was actively controlled by sound wave. The periodic excitation was locally irradiated at the jet shear layer. The behavior of plasma jet and working gas were directly captured by a high speed digital CCD camera for symmetric or asymmetric modes. As a result, the plasma jet is stable under no-excitation condition. On the contrast, periodic meandering motion in the excitation direction was observed under the both of symmetric and asymmetric excitation. The plasma jet amplitude for asymmetric excitation is much larger than that for symmetric excitation.

S053031 マイクロ噴流群による単軸円形噴流の大規模渦構造および混合の能動制御

Large-scale vortical structures and associated flow development in an axisymmetric jet are actively controlled by manipulating the initial shear layer with arrayed miniature jet actuators. The periodic radial miniature jet injections are realized by using a rapid-response pneumatic servovalve, and sinusoidal/pulsed flowing are employed in the present study. The spatio-temporal primary jet structures are investigated through phase-locked particle image verocimetry (PIV). In the pulsed configuration, it is found that intense vortex rings are produced in phase with the periodic control input regardless of the valve-driven frequency f_v examined. When the Strouhal number Stv, which is defined with f_v, is larger than unity, the vortex rings are shed continuously and thus the jet flow development is prompted with low periodic velocity fluctuation. The diameter of the vortices becomes small as Stv is increased, so that the transport range of the jet and ambient fluids can be controlled by changing Stv. In addition, the evolution of counter-rotating streamwise vortices is also confirmed. These streamwise vortices are formed as a result of tilting of the primary jet azimuthal vorticity by the miniature jet impinging on the primary jet shear layer. It is conjectured that they contribute the mixing enhancement in the azimuthal direction.

Unsteady wake vortices in jets in cross-flow

The current flow visualization study investigates unsteady wake vortices of jets in cross-flow in order to (1) advance the understanding of their origin and characteristics and (2) explore various excitation techniques for organizing and accentuating them. An isolated circular jet passed through a nozzle and entered the cross-flow normal to the wall. Free stream velocities up to 5 m/s and jet-to-cross-flow velocity ratio range between 1 and 10 were covered. While mechanical perturbation did not result in any significant periodic organization of the wake vortices, the database obtained for the unperturbed flow provided further insight into their behavior. The key finding was that the wake vortices always originated from the lee-side of the jet where the jet efflux boundary layer and the wall boundary layer intersected. In no case these vortices were seen to form either from the wall boundary layer or the jet shear layer at downstream locations. After formation the wake vortex twists and stretches as it convects downstream with the base still attached to the near-wall region on the jet’s lee side. The top remains connected to the underside of the jet where the tracer particles dissipate due to high turbulence in the shear layer.

Instability wave models for the near-field fluctuations of turbulent jets

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

Numerical Simulation of Single-Stream Jets from a Serrated Nozzle

Hybrid large-eddy type simulations for cold jet flows from a serrated nozzle are performed at an acoustic Mach number Ma ac = 0.9 and Re = 1.03×106. Since the solver being used tends towards having dissipative qualities, the subgrid scale (SGS) model is omitted, giving a numerical type LES (NLES) or implicit LES (ILES) reminiscent procedure. To overcome near wall streak resolution problems a near wall RANS (Reynolds averaged Navier-Stokes) model is smoothly blended to the LES making a hybrid RANS-ILES. The geometric complexity of the serrated nozzle is fully considered without simplification or emulation. An improved but still modest hexahedral multi-block grid with circa 20 million grid points (with respect to 12.5 million in Xia et al., Int J Heat Fluid Flow 30:1067–1079, 2009) is used. Despite the modest grid size, encouraging and improved results are obtained. Directly resolved mean and second-order fluctuating quantities along the jet centerline and in the jet shear layer compare favorably with measurements. The radiated far-field sound predicted using the Ffowcs Williams and Hawkings (FW-H) surface integral method shows good agreement with the measurements in directivity and sound spectra.

Particle image velocimetry study of the impinging height effect on an overexpanded supersonic impinging free jet at Ma = 1.754. Part 1: global coherent structure

Results of an experimental study of an overexpanded axisymmetric supersonic impinging jet at Mach 1.754 with particle image velocimetry (PIV) are presented in this article. The focus of Part I is on the influence of the impinging height on the global coherent flow structure, while in Part II the detailed velocity magnitude, both axial and radial, as well as the position variation of the plate shock will be presented. The PIV results clearly demonstrate the formation of the stagnation bubble as well as the presence of the recirculating flow in it. When the impinging height reaches five times the nozzle exit diameter, the stagnation bubble disappears, which is thought to be the result of the increase in the thickness of the shear layer of the main jet and consequently the decrease in the difference of the stagnation pressure between the centre of the stagnation zone and the impinging point of the main jet shear layer. The comparison between instantaneous and mean PIV measurements demonstrates that there exists highly unsteady behaviour of the impinging jet. The shock wave structure and the unsteady entraining vortex of the nozzle jet and the wall jet are all clearly recorded by PIV. All these results are crucial to understand the effects of the impinging height on the global flowfield coherent structure of the overexpanded axisymmetric supersonic jet impingement.

Mechanism of acoustic radiation from Supersonic Jets Impinging to Inclined Flat Plates

Acoustic wave radiated from supersonic cold jets impinging to inclined flat-plates is investigated numerically with the help of the experimental work. This type of acoustic generation is important for estimating and minimizing the acoustic loading of launch vehicle at lift-off. Through the study on the 45-degree-inclined flat plate located 5 D downstream from the nozzle exit, two noise sources are found to be generated due to the jet impingement: (1) interaction between the vortex of the jet shear-layer and the shock waves appearing at the jet impingement region and at the downstream region, (2) the Mach wave radiated from the large-scale vortex structure of the flow downstream of the plate. The former is similar to the broadband shock-associated noise. Those features are clearly confirmed by applying the proper orthogonal decomposition analysis to the numerical result. Prediction accuracy of 5 dB in far-field OASPL is obtained in the current numerical technique.