Linear Shaped Charge Semi-analytical Model for Jet Velocity Gradient and Jet Drift.
Abstract not provided.
Garcia, Antonio Roman↗
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Semi-inclusive direct photon+jet and π0+jet correlations measured in p + p and central Au + Au collisions at √sNN = 200 GeV
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73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Gravitational Effects on Flow Instability and Transition in Low Density Jets
Recent experiments have shown that low-density gas jets injected into a high-density ambient gas undergo an instability mode, leading to highly-periodic oscillations in the flow-field for certain conditions. The transition from laminar to turbulent flow in these jets is abrupt, without the gradual change in scales. Even the fine scale turbulent structure repeats itself with extreme regularity from cycle to cycle. Similar observations were obtained in buoyancy-dominated and momentum-dominated jets characterized by the Richardson numbers, Ri = [gD(rho(sub a)-rho(sub j))/rho(sub j)U(sub j)(exp 2) ] where g is the gravitational acceleration, D is the jet diameter, rho(sub a) and rho(sub a) are, respectively, the free-stream and jet densities, and U(sub j) is the mean jet exit velocity. At high Richardson numbers, the instability is presumably caused by buoyancy since the flow-oscillation frequency (f) or the Strouhal number, St = [fD/U(sub j)] scales with Ri. In momentum-dominated jets, however, the Strouhal number of the oscillating flow is relatively independent of the Ri. In this case, a local absolute instability is predicted in the potential core of low-density jets with S [= rho(sub j)/rho(sub a)] < 0.7, which agrees qualitatively with experiments. Although the instability in gas jets of high Richardson numbers is attributed to buoyancy, direct physical evidence has not been acquired in experiments. If the instability is indeed caused by buoyancy, the near-field flow structure of the jet will change significantly when the buoyancy is removed, for example, in the microgravity environment. Thus, quantitative data on the spatial and temporal evolutions of the instability, length and time scale of the oscillating mode and its effects on the mean flow and breakdown of the potential core are needed in normal and microgravity to delineate gravitational effects in buoyant jets. In momentum dominated low-density jets, the instability is speculated to originate in the potential core. However, experiments have not succeeded in identifying the direct physical cause of the instability. For example, the theory predicts an oscillating mode for S<0.62 in the limit of zero momentum thickness, which contradicts with the experimental findings of Kyle and Sreenivasan. The analyses of momentum-dominated jets neglect buoyancy effects because of the small Richardson number. Although this assumption is appropriate in the potential core, the gravitational effects are important in the annular region surrounding the jet, where the density and velocity gradients are large. This reasoning provides basis for the hypothesis that the instability in low Richardosn number jets studied by Kyle and Sreenivasan and Monkewitz et al. is caused by buoyancy. The striking similarity in characteristics of the instability and virtually the identical conclusions reached by Subbarao and Cantwell in buoyant (Ri>0.5) helium jets on one hand and by Kyle and Sreenivasan in momentum-dominated (Ri<1x10(exp -3)) helium jets on the other support this hypothesis. However, quantitative experiments in normal and microgravity are necessary to obtain direct physical evidence of buoyancy effects on the flow instability and structure of momentum-dominated low-density jets. The primary objective of this new research project is to quantify how buoyancy affects the flow instability and structure in the near field of low-density jets. The flow will be described by the spatial and temporal evolutions of the instability, length and time scales of the oscillating mode, and the mean and fluctuating concentration fields. To meet this objective, concentration measurements will be obtained across the whole field using quantitative Rainbow Schlieren Deflectometry, providing spatial resolution of 0.1mm and temporal resolution of 0.017s to 1ms. The experimental effort will be supplemented with linear stability analysis of low-density jets by considering buoyancy. The first objective of this research is to investigate the effects of gravity on the flow instability and structure of low-density jets. The flow instability in these jets has been attributed to buoyancy. By removing buoyancy in our experiments, we seek to obtain the direct physical evidence of the instability mechanism. In the absence of the instability, the flow structure will undergo a significant change. We seek to quantify these changes by mapping the flow field (in terms of the concentration profiles) of these jets at non-buoyant conditions. Such information is presently lacking in the existing literature. The second objective of this research is to determine if the instability in momentum-driven, low-density jets is caused by buoyancy. At these conditions, the buoyancy effects are commonly ignored because of the small Richardson based on global parameters. By eliminating buoyancy in our experiments, globally as well as locally, we seek to examine the possibility that the instability mechanism in self-excited, buoyant or momentum-driven jets is the same. To meet this objective, we would quantify the jet flow in normal and microgravity, while systematically decreasing the Richardson number from buoyancy-driven to momentum driven flow regime. The third objective of this research is to perform a linear stability analysis of low-density gas jets by including the gravitational effects. The flow oscillations in these jets are attributed to an absolute instability, whereby the disturbance grows exponentially at the site to ultimately contaminate the entire flow field. We seek to study the characteristics of both convective and absolute instabilities and demarcate the boundary between them.
Agrawal A. K.↗
Filament Eruptions, Jets, and Space Weather
Previously, from chromospheric H alpha and coronal X‐ray movies of the Sun's polar coronal holes, it was found that nearly all coronal jets (greater than 90%) are one or the other of two roughly equally common different kinds, different in how they erupt: standard jets and blowout jets (Yamauchi et al 2004, Apl, 605, 5ll: Moore et all 2010, Apj, 720, 757). Here, from inspection of SDO/AIA He II 304 A movies of 54 polar x-ray jets observed in Hinode/XRT movies, we report, as Moore et al (2010) anticipated, that (1) most standard x-ray jets (greater than 80%) show no ejected plasma that is cool enough (T is less than or approximately 10(exp 5K) to be seen in the He II 304 A movies; (2) nearly all blownout X-ray jets (greater than 90%) show obvious ejection of such cool plasma; (3) whereas when cool plasma is ejected in standard X-ray jets, it shows no lateral expansion, the cool plasma ejected in blowout X-ray jets shows strong lateral expansion; and (4) in many blowout X-ray jets, the cool plasma ejection displays the erupting-magnetic-rope form of clasic filament eruptions and is thereby seen to be a miniature filament eruption. The XRT movies also showed most blowout X-ray jets to be larger and brighter, and hence to apparently have more energy, than most standard X-ray jets. These observations (1) confirm the dichotomy of coronal jets, (2) agree with the Shibata model for standard jets, and (3) support the conclusion of Moore et al (2010) that in blowout jets the magnetic-arch base of the jet erupts in the manner of the much larger magnetic arcades in which the core field, the field rooted along the arcade's polarity inversion line, is sheared and twisted (sigmoid), often carries a cool‐plasma filament, and erupts to blowout the arcade, producing a CME. From Hinode/SOT Ca II movies of the polar limb, Sterling et al (2010, ApJ, 714, L1) found that chromospheric Type‐II spicules show a dichotomy of eruption dynamics similar to that found here for the cool‐plasma component of coronal X‐ray jets. This favors the idea that Type‐II spicules are miniature counterparts of coronal X‐ray jets. In Moore et al (2011, ApJ, 731, L18), we pointed out that if Type‐II spicules are magnetic eruptions that work like coronal X‐ray jets, they carry an area‐averaged mechanical energy flux of approximately 7x10)(exp 5) erg cm(exp -2) s(exp-1) into the corona in the form of MHD waves and jet outflow, enough to power the heating of the global corona and solar wind. On this basis, from our observations of mini‐filament eruptions in blowout X‐ray jets, we infer that magnetic explosions of the type that have erupting filaments in them are the main engines of both (1) the steady solar wind and (2) the CMEs that produce the most severe space weather by blasting out through the corona and solar wind, making solar energetic particle storms, and bashing the Earth's magnetosphere. We conclude that in focusing on prominences and filament eruptions, Einar had his eye on the main bet for understanding what powers all space weather, both the extreme and the normal.
Moore, Ronald↗
Synthetic Jet Interaction With A Turbulent Boundary Layer Flow
Perhaps one of the more notable advances to have occurred in flow control technology in the last fifteen years is the application of surface-issuing jets for separation control on aerodynamic surfaces. The concept was introduced by Johnston and Night (1990) who proposed using circular jets, skewed and inclined to the wall, to generate streamwise vortices for the purpose of mitigating boundary layer separation. The skew and inclination angles have subsequently been shown to affect the strength and sign of the ensuing vortices. With a non-circular orifice, in addition to skew and inclination, the yaw angle of the major axis of the orifice can influence the flow control effectiveness of the jet. In particular, a study by Chang arid Collins (1997) revealed that a non-circular orifice, yawed relative to the freestream, can be used to control the size and strength of the vortices produced by the control jet. This early work used jets with only a steady injection of mass. Seifert et al. revealed that an unsteady blowing jet, could be as effective at separation control as a steady jet but with less mass flow. Seifert et al. showed that small amplitude blowing oscillations superimposed on a low momentum steady jet Was the most effective approach to delaying separation on a NACA 0015 airfoil at post-stall angles of attack. More recent work suggests that perhaps the most efficient jet control effect comes from a synthetic (oscillatory) jet where the time-averaged mass flux through the orifice is zero, but the net wall normal momentum is non-zero. The control effectiveness of synthetic jets has been demonstrated for several internal and external flow fields used synthetic jet control on a thick, blunt-nosed airfoil to delay stall well beyond the stall angles for the uncontrolled airfoil and with a dramatic increase in the lift-to-drag performance. Amitay et al. used an array of synthetic jets to mitigate flow separation in curved and diffusing ducts. While the control effectiveness of synthetic jet actuators in an application setting has been confirmed through a number of studies, the fluid dynamic mechanism by which control is effected is not well understood. Issues like yaw angle arid velocity ratio that have been studied for steady control jets have not been investigated for synthetic jets. Moreover, the role played by the inherently unsteady nature of a synthetic jet in the interaction with the controlled flow is not known. Some recent work by Rinehart and Glazer and Smith suggests that away from tile immediate vicinity of the jet orifice the flow field is steady in the mean, and that the interaction of a synthetic jet and a boundary layer creates a secondary flow in the boundary layer consisting primarily of streamwise vortices.
Smith, Douglas R.↗
Angular structure of jet quenching within a hybrid strong/weak coupling model
Within the context of a hybrid strong/weak coupling model of jet quenching, we study the modification of the angular distribution of the energy within jets in heavy ion collisions, as partons within jet showers lose energy and get kicked as they traverse the strongly coupled plasma produced in the collision. To describe the dynamics transverse to the jet axis, we add the effects of transverse momentum broadening into our hybrid construction, introducing a parameter K≡qˆ/T3K≡q^/T3 that governs its magnitude. We show that, because of the quenching of the energy of partons within a jet, even when K ≠ 0 the jets that survive with some specified energy in the final state are narrower than jets with that energy in proton-proton collisions. For this reason, many standard observables are rather insensitive to K. We also propose a new differential jet shape ratio observable in which the effects of transverse momentum broadening are apparent. We also analyze the response of the medium to the passage of the jet through it, noting that the momentum lost by the jet appears as the momentum of a wake in the medium. After freezeout this wake becomes soft particles with a broad angular distribution but with net momentum in the jet direction, meaning that the wake contributes to what is reconstructed as a jet. Thus, this effect must be included in any description of the angular structure of the soft component of a jet. We show that the particles coming from the response of the medium to the momentum and energy deposited in it leads to a correlation between the momentum of soft particles well separated from the jet in angle with the direction of the jet momentum, and find qualitative but not quantitative agreement with experimental data on observables designed to extract such a correlation. Generally, by confronting the results that we obtain upon introducing transverse momentum broadening and the response of the medium to the jet with available jet data, we highlight the importance of these processes for understanding the internal, soft, angular structure of high energy jets.
74 ATOMIC AND MOLECULAR PHYSICS↗
Impact of red giant/AGB winds on active galactic nucleus jet propagation
Context. Dense stellar winds may mass-load the jets of active galactic nuclei, although it is unclear on what time and spatial scales the mixing takes place. Aims. Our aim is to study the first steps of the interaction between jets and stellar winds, and also the scales on which the stellar wind mixes with the jet and mass-loads it. Methods. We present a detailed 2D simulation – including thermal cooling – of a bubble formed by the wind of a star designed to study the initial stages of jet-star interaction. We also study the first interaction of the wind bubble with the jet using a 3D simulation in which the star enters the jet. Stability analysis is carried out for the shocked wind structure to evaluate the distances over which the jet-dragged wind, which forms a tail, can propagate without mixing with the jet flow. Results. The 2D simulations point to quick wind bubble expansion and fragmentation after about one bubble shock crossing time. Three-dimensional simulations and stability analysis point to local mixing in the case of strong perturbations and relatively low density ratios between the jet and the jet dragged-wind, and to a possibly more stable shocked wind structure at the phase of maximum tail mass flux. Analytical estimates also indicate that very early stages of the star jet-penetration time may be also relevant for massloading. The combination of these and previous results from the literature suggests highly unstable interaction structures and efficient wind-jet flow mixing on the scale of the jet interaction height. Conclusions. The winds of stars with strong mass loss can efficiently mix with jets from active galactic nuclei. In addition, the initial wind bubble shocked by the jet leads to a transient, large interaction surface. The interaction between jets and stars can produce strong inhomogeneities within the jet. As mixing is expected to be effective on large scales, even individual asymptotic giant branch stars can significantly contribute to the mass-load of the jet and thus affect its dynamics. Shear layer mass-entrainment could be important. The interaction structure can be a source of significant non-thermal emission.
79 ASTRONOMY AND ASTROPHYSICS↗
The cool component and the dichotomy, lateral expansion, and axial rotation of solar X-ray jets
We present results from a study of 54 polar X-ray jets that were observed in coronal X-ray movies from the X-ray Telescope on Hinode and had simultaneous coverage in movies of the cooler transition region (T ∼ 10{sup 5} K) taken in the He II 304 Å band of the Atmospheric Imaging Assembly (AIA) on Solar Dynamics Observatory. These dual observations verify the standard-jet/blowout-jet dichotomy of polar X-ray jets previously found primarily from XRT movies alone. In accord with models of blowout jets and standard jets, the AIA 304 Å movies show a cool (T ∼ 10{sup 5} K) component in nearly all blowout X-ray jets and in a small minority of standard X-ray jets, obvious lateral expansion in blowout X-ray jets but none in standard X-ray jets, and obvious axial rotation in both blowout X-ray jets and standard X-ray jets. In our sample, the number of turns of axial rotation in the cool-component standard X-ray jets is typical of that in the blowout X-ray jets, suggesting that the closed bipolar magnetic field in the jet base has substantial twist not only in all blowout X-ray jets but also in many standard X-ray jets. We point out that our results for the dichotomy, lateral expansion, and axial rotation of X-ray jets add credence to published speculation that type-II spicules are miniature analogs of X-ray jets, are generated by granule-size emerging bipoles, and thereby carry enough energy to power the corona and solar wind.
79 ASTRONOMY AND ASTROPHYSICS↗
On the Two Components of Turbulent Mixing Noise from Supersonic Jets
It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.
Tam, Christopher K. W.↗
Magnetic Untwisting in Most Solar X-Ray Jets
From 54 X‐ray jets observed in the polar coronal holes by Hinode's X‐Ray Telescope (XRT) during coverage in movies from Solar Dynamic Observatory's Atmospheric Imaging Assembly (AIA) taken in its He II 304 Å band at a cadence of 12 s, we have established a basic characteristic of solar X‐ray jets: untwisting motion in the spire. In this presentation, we show the progression of few of these X‐ray jets in XRT images and track their untwisting in AIA He II images. From their structure displayed in their XRT movies, 19 jets were evidently standard jets made by interchange reconnection of the magnetic‐arcade base with ambient open field, 32 were evidently blowout jets made by blowout eruption of the base arcade, and 3 were of ambiguous form. As was anticipated from the >10,000 km span of the base arcade in most polar X‐ray jets and from the disparity of standard jets and blowout jets in their magnetic production, few of the standard X‐ray jets (3 of 19) but nearly all of the blowout X‐ray jets (29 of 32) carried enough cool (T is approximately 105 K) plasma to be seen in their He II movies. In the 32 X‐ray jets that showed a cool component, the He II movies show 10‐100 km/s untwisting motions about the axis of the spire in all 3 standard jets and in 26 of the 29 blowout jets. Evidently, the open magnetic field in nearly all blowout X‐ray jets and probably in most standard X‐ray jets carries transient twist. This twist apparently relaxes by propagating out along the open field as a torsional wave. High‐resolution spectrograms and Dopplergrams have shown that most Type‐II spicules have torsional motions of 10‐30 km/s. Our observation of similar torsional motion in X‐ray jets strengthens the case for Type‐II spicules being made in the same way as X‐ray jets, by blowout eruption of a twisted magnetic arcade in the spicule base and/or by interchange reconnection of the twisted base arcade with the ambient open field. This work was funded by NASA's Heliophysics Division through its LWS TRT Program and its Hinode Project, and by NSF through its Research Experience for Undergraduates Program.
Moore, Ronald↗
Measurement of substructure-dependent suppression of large-radius jets with charged particles in Pb+Pb collisions with ATLAS
Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentum above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The jet $R_\mathrm{AA}$ is measured as a function of jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}$), and the angular separation ($dR_{12}$) of two leading sub-jets. The jet $R_\mathrm{AA}$ gradually decreases with increasing $\sqrt{d_{12}}$, implying significantly stronger suppression of large-radius jets with larger $k_t$ splitting scale. The jet $R_\mathrm{AA}$ gradually decreases for $dR_{12}$ in the range $0.01{-}0.2$ and then remains consistent with a constant for $ΔR_{12} \gtrsim 0.2$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Attached Jet Velocity Profiles in Mixing Tanks
Free jets have been studied in detail over much of the last century, but the theory for offset and attached jets remains incomplete. Attached jets differ from free jets in that they lose momentum to nearby surfaces, attenuating their velocities. The velocity profiles of free circular jets are nearly Gaussian, with quantitative mathematical descriptions derived from first principles by Goertler and Tollmien (Rajaratnam, 1976). In contrast, mathematical descriptions of three-dimensional attached jets from circular nozzles remain much less mature. Agelin-Chaab and Tachie (2011) used particle imaging velocimetry of a three-dimensional attached jet to show that the scaled velocity decays with scaled distance from the nozzle with a power law exponent between -1.15 and -1.21, which is larger in magnitude than that of a free jet. However, quantitative analytical expressions for the velocity profiles of attached jets similar to those of free jets remain elusive. This paper addresses this critical gap. Here we evaluate the velocity profiles of three-dimensional offset jets emerging from circular nozzles that that become attached jets. These jets lose momentum due to interactions with nearby surfaces and are important to evaluating flows in mixing vessels and to suspending solids and trapped gases in radioactive waste tanks. Despite the importance of attached jets, prior insight has been purely experimental, limited to overly simplistic analytical models, or restricted to computationally expensive computational fluid dynamics case studies. We compare the expression of Verhoff (1963) to experimental results to find reasonable quantitative agreement. As stated by Agelin-Chaab and Tachie (2011), “detailed velocity measurements of 3D offset jets are rare.” Such remains the case. This study adds to the literature by providing information at two additional Reynolds numbers (1.43 106 and 1.87 106) and evaluating simple but accurate expressions for velocity profiles. These Reynolds numbers are an order of magnitude higher than prior reports. The semi-empirical stream wise velocity profile perpendicular to the surface proposed by Verhoff (1963) is in approximate agreement with these velocity profiles, which is surprising because these attached jets are three-dimensional instead of two-dimensional as evaluated by Verhoff. However, additional work is necessary to fully describe these profiles quantitatively.
jet, attached jet, offset jet↗
Multiplicity dependence of charged-particle intra-jet properties in pp collisions at $\sqrt{s}$ = 13 TeV
The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy $\sqrt{s}$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η| < 0.9) using the sequential recombination anti-k T algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the trans verse momentum (p T ) interval 5–110 GeV/c. The high multiplicity events are selected by the forward V0 scintilla tor detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet p T in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables z ch and ξ ch are measured for different jet-p T intervals. Jet-p T independent fragmentation of leading jets is observed for wider jets except at high- and low-z ch values. The observed “hump-backed plateau” structure in the ξ ch distribution indicates suppression of low-p T particles. In high-multiplicity events, an enhancement of the fragmen tation probability of low-z ch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more promi nent for low-p T jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet p T . These measurements provide important constraints to models of jet fragmentation.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
ON THE IMPORTANCE OF VERY LIGHT INTERNALLY SUBSONIC AGN JETS IN RADIO-MODE AGN FEEDBACK
Radio-mode active galactic nucleus (AGN) feedback plays a key role in the evolution of galaxy groups and clusters. Its physical origin lies in the kiloparsec-scale interaction of AGN jets with the intracluster medium. Large-scale jet simulations often initiate light internally supersonic jets with density contrast 0.01 < η < 1. Here we argue for the first time for the importance of very light ( η < 0.01) internally subsonic jets. We investigated the shapes of young X-ray cavities produced in a suite of hydrodynamic simulations, and found that bottom-wide cavities are always produced by internally subsonic jets, while internally supersonic jets inflate cylindrical, center-wide, or top-wide cavities. We found examples of real cavities with shapes analogous to those inflated in our simulations by internally subsonic and internally supersonic jets, suggesting a dichotomy of AGN jets according to their internal Mach numbers. We further studied the long-term cavity evolution, and found that old cavities resulted from light jets spread along the jet direction, while those produced by very light jets are significantly elongated along the perpendicular direction. The northwestern ghost cavity in Perseus is pancake shaped, providing tentative evidence for the existence of very light jets. Our simulations show that very light internally subsonic jets decelerate faster and rise much slower in the intracluster medium than light internally supersonic jets, possibly depositing a larger fraction of jet energy to cluster cores and alleviating the problem of low coupling efficiencies found previously. The internal Mach number points to the jet’s energy content, and internally subsonic jets are energetically dominated by non-kinetic energy, such as thermal energy, cosmic rays, or magnetic fields.
79 ASTRONOMY AND ASTROPHYSICS↗
The Stability of Radiatively Cooling Jets. 2: Nonlinear Evolution
We use two-dimensional time-dependent hydrodynamical simulations to follow the growth of the Kelvin-Helmholtz (K-H) instability in cooling jets into the nonlinear regime. We focus primarily on asymmetric modes that give rise to transverse displacements of the jet beam. A variety of Mach numbers and two different cooling curves are studied. The growth rates of waves in the linear regime measured from the numerical simulations are in excellent agreement with the predictions of the linear stability analysis presented in the first paper in this series. In the nonlinear regime, the simulations show that asymmetric modes of the K-H instability can affect the structure and evolution of cooling jets in a number of ways. We find that jets in which the growth rate of the sinusoidal surface wave has a maximum at a so-called resonant frequency can be dominated by large-amplitude sinusoidal oscillations near this frequency. Eventually, growth of this wave can disrupt the jet. On the other hand, nonlinear body waves tend to produce low-amplitude wiggles in the shape of the jet but can result in strong shocks in the jet beam. In cooling jets, these shocks can produce dense knots and filaments of cooling gas within the jet. Ripples in the surface of the jet beam caused by both surface and body waves generate oblique shock "spurs" driven into the ambient gas. Our simulations show these shock "spurs" can accelerate ambient gas at large distances from the jet beam to low velocities, which represents a new mechanism by which low-velocity bipolar outflows may be driven by high-velocity jets. Rapid entrainment and acceleration of ambient gas may also occur if the jet is disrupted. For parameters typical of protostellar jets, the frequency at which K-H growth is a maximum (or highest frequency to which the entire jet can respond dynamically) will be associated with perturbations with a period of - 200 yr. Higher frequency (shorter period) perturbations excite waves associated with body modes that produce internal shocks and only small-amplitude wiggles within the jet. The fact that most observed systems show no evidence for large-amplitude sinusoidal oscillation leading to disruption is indicative that the perturbation frequencies are generally large, consistent with the suggestion that pro- tostellar jets arise from the inner regions (r less than 1 AU) of accretion disks.
Stone, James M.↗
Ballistic Models for Radio Jets in Colliding Galaxies: 3C 278 (NGC 4782/4783)
We present here a general numerical simulation algorithm for ballistic radio jets. The evolution of the jets is determined by their response to mechanical forces (i.e., gravity and ram pressure). Using a previously derived collision model for the interacting pair of elliptical galaxies NGC 4782/4783, we have used this algorithm to simulate the specific two-sided jet morphology seen in the radio source associated with NGC 4782 (3C 278). This is the first time such simulations have been attempted for a galaxy pair whose relative orbit was determined independently from the jet modeling. The general morphological features of the jets in 3C 278 (e.g., position angles, lengths, curvature, and deflection angles) are well matched by the simulations, indicating that our model of the mechanical forces acting on the jets can indeed reproduce most of the basic details of the jet morphology and thereby provide an opportunity to model a combination of detailed optical, radio, and X-ray data for a wide variety of interacting radio-jet galaxies. Our models constrain the initial jet parameters (i.e., the ejection speed, direction, and starting time), the properties of the gaseous medium into which the jets are ejected, and the relative importance of gravitational deflection versus ram pressure bending in influencing the jet morphology. For 3C 278, we find that the effects of ram pressure dominate the structural evolution of the jets. In our best-fit simulation the jet ejection speed is of order 10(exp 4) km/s, to within 50%, suggesting that similar low-luminosity radio jets are also significantly subluminal. We also find that the jets are ejected within approximately 5 deg of the line of sight, the ISM in the nonjet galaxy has a much larger effect on the jet deflection than does the host galaxy, and the jet activity began just over 70 million years ago, roughly 50 million years before the pericenter passage of the two galaxies. Therefore, the onset of nuclear activity appears in 3C 278 to be related to the kinematically observed tidal shock that has been induced in the core of NGC 4782 as a result of its deeply penetrating collision with NGC 4783.
Borne, Kirk D.↗
Buoyancy Effects on Flow Structure and Instability of Low-Density Gas Jets
A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. The primary objective of the proposed research is to study buoyancy effects on the origin and nature of the flow instability and structure in the near-field of low-density gas jets. Quantitative rainbow schlieren deflectometry, Computational fluid dynamics (CFD) and Linear stability analysis were the techniques employed to scale the buoyancy effects. The formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and RMS concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed. The laminar, axisymmetric, unsteady jet flow of helium injected into air was simulated using CFD. Global oscillations were observed in the flow field. The computed oscillation frequency agreed qualitatively with the experimentally measured frequency. Contours of helium concentration, vorticity and velocity provided information about the evolution and propagation of vortices in the oscillating flow field. Buoyancy effects on the instability mode were evaluated by rainbow schlieren flow visualization and concentration measurements in the near-field of self-excited helium jets undergoing gravitational change in the microgravity environment of 2.2s drop tower at NASA John H. Glenn Research Center. The jet Reynolds number was varied from 200 to 1500 and jet Richardson number was varied from 0.72 to 0.002. Power spectra plots generated from Fast Fourier Transform (FFT) analysis of angular deflection data acquired at a temporal resolution of 1000Hz reveal substantial damping of the oscillation amplitude in microgravity at low Richardson numbers (~0.002). Quantitative concentration data in the form of spatial and temporal evolutions of the instability data in Earth gravity and microgravity reveal significant variations in the jet flow structure upon removal of buoyancy forces. Radial variation of the frequency spectra and time traces of helium concentration revealed the importance of gravitational effects in the jet shear layer region. Linear temporal and spatio-temporal stability analyses of a low-density round gas jet injected into a high-density ambient gas were performed by assuming hyper-tan mean velocity and density profiles. The flow was assumed to be non parallel. Viscous and diffusive effects were ignored. The mean flow parameters were represented as the sum of the mean value and a small normal-mode fluctuation. A second order differential equation governing the pressure disturbance amplitude was derived from the basic conservation equations. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the temporal and spatio-temporal results were delineated. A decrease in the density ratio (ratio of the density of the jet to the density of the ambient gas) resulted in an increase in the temporal amplification rate of the disturbances. The temporal growth rate of the disturbances increased as the Froude number was reduced. The spatio-temporal analysis performed to determine the absolute instability characteristics of the jet yield positive absolute temporal growth rates at all Fr and different axial locations. As buoyancy was removed (Fr . 8), the previously existing absolute instability disappeared at all locations establhing buoyancy as the primary instability mechanism in self-excited low-density jets.
Pasumarthi, Kasyap Sriramachandra↗
MOJAVE. X. PARSEC-SCALE JET ORIENTATION VARIATIONS AND SUPERLUMINAL MOTION IN ACTIVE GALACTIC NUCLEI
We describe the parsec-scale kinematics of 200 active galactic nucleus (AGN) jets based on 15 GHz Very Long Baseline Array (VLBA) data obtained between 1994 August 31 and 2011 May 1. We present new VLBA 15 GHz images of these and 59 additional AGNs from the MOJAVE and 2 cm Survey programs. Nearly all of the 60 most heavily observed jets show significant changes in their innermost position angle over a 12-16 yr interval, ranging from 10° to 150° on the sky, corresponding to intrinsic variations of ∼0.°5 to ∼2°. The BL Lac jets show smaller variations than quasars. Roughly half of the heavily observed jets show systematic position angle trends with time, and 20 show indications of oscillatory behavior. The time spans of the data sets are too short compared to the fitted periods (5-12 yr), however, to reliably establish periodicity. The rapid changes and large jumps in position angle seen in many cases suggest that the superluminal AGN jet features occupy only a portion of the entire jet cross section and may be energized portions of thin instability structures within the jet. We have derived vector proper motions for 887 moving features in 200 jets having at least five VLBA epochs. For 557 well-sampled features, there are sufficient data to additionally study possible accelerations. We find that the moving features are generally non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions. Inward motions are rare (2% of all features), are slow (<0.1 mas yr{sup –1}), are more prevalent in BL Lac jets, and are typically found within 1 mas of the unresolved core feature. There is a general trend of increasing apparent speed with distance down the jet for both radio galaxies and BL Lac objects. In most jets, the speeds of the features cluster around a characteristic value, yet there is a considerable dispersion in the distribution. Orientation variations within the jet cannot fully account for the dispersion, implying that the features have a range of Lorentz factor and/or pattern speed. Very slow pattern speed features are rare, comprising only 4% of the sample, and are more prevalent in radio galaxy and BL Lac jets. We confirm a previously reported upper envelope to the distribution of speed versus beamed luminosity for moving jet features. Below 10{sup 26} W Hz{sup –1} there is a fall-off in maximum speed with decreasing 15 GHz radio luminosity. The general shape of the envelope implies that the most intrinsically powerful AGN jets have a wide range of Lorentz factors up to ∼40, while intrinsically weak jets are only mildly relativistic.
79 ASTRONOMY AND ASTROPHYSICS↗