Streaming Velocity Research Articles

Evolution of High-redshift Quasar Hosts and Promotion of Massive Black Hole Seed Formation

Abstract High-redshift luminous quasars powered by accreting supermassive black holes (SMBHs) with mass ≳109 M ⊙ constrain their formation pathways. We investigate the formation of heavy seeds of SMBHs through gas collapse in the quasar host progenitors, using merger trees to trace the halo growth in highly biased, overdense regions of the universe. The progenitor halos are likely irradiated by intense H2-photodissociating radiation from nearby star-forming galaxies and heat the interior gas by successive mergers. The kinetic energy of the gas originating from mergers, as well as the baryonic streaming motion, prevents gas collapse and delays prior star formation. With a streaming velocity higher than the rms value, gas clouds in nearly all 104 realizations of merger trees enter the atomic-cooling stage and begin to collapse isothermally with T ≃ 8000 K via Lyα cooling. The fraction of trees that host isothermal gas collapse is 14% and increases with streaming velocity, while the rest form H2-cooled cores after short isothermal phases. If the collapsing gas is enriched to Z crit ∼ 2 × 10−3 Z ⊙, requiring efficient metal mixing, this fraction could be reduced by additional cooling via metal fine-structure lines. In the massive collapsing gas, the accretion rate onto a newly born protostar ranges between 3 × 10−3 M ⊙ yr−1 and 5 M ⊙ yr−1, among which a large fraction exceeds the critical rate suppressing stellar radiative feedback. As a result, we expect a distribution of stellar mass (presumably BH mass) ranging from several hundred to above 105 M ⊙, potentially forming massive BH binary mergers and yielding gravitational-wave events.

The Critical Dark Matter Halo Mass for Population III Star Formation: Dependence on Lyman–Werner Radiation, Baryon-dark Matter Streaming Velocity, and Redshift

Abstract A critical dark matter halo mass (M crit) for Population III stars can be defined as the typical minimum halo mass that hosts sufficient cold-dense gas required for the formation of the first stars. The presence of Lyman–Werner (UV) radiation, which can dissociate molecular hydrogen, and the baryon-dark matter streaming velocity both delay the formation of Population III stars by increasing M crit. In this work, we constrain M crit as a function of Lyman–Werner flux (including self-shielding), baryon-dark matter streaming, and redshift using cosmological simulations with a large sample of halos utilizing the adaptive mesh refinement code enzo. We provide a fit for M crit as a function of these quantities, which we expect to be particularly useful for semi-analytical models of early galaxy formation. In addition, we find (i) the measured redshift dependence of M crit in the absence of radiation or streaming is (1 + z)−1.58, consistent with a constant virial temperature; (ii) increasing the UV background increases M crit while steepening the redshift dependence, up to (1 + z)−5.7; (iii) baryon-dark matter streaming boosts M crit but flattens the dependence on redshift; (iv) the combination of the two effects is not simply multiplicative.

The influence of streaming velocities and Lyman–Werner radiation on the formation of the first stars

ABSTRACT The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures Tvir < 104 K. Cooling in these minihaloes is dominated by molecular hydrogen (H2), and so Population III star formation is only possible in those minihaloes that form enough H2 to cool on a short timescale. As H2 cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale Mmin above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H2 by a high redshift Lyman–Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on Mmin and on Mave (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on Mmin than the LW background, but also that both effects are additive. We also provide a fitting functions quantifying the dependence of Mave and Mmin on the streaming velocity and the strength of the LW background.

Quantitative assessment of parallel acoustofluidic device.

The advantage of ultrasonic fields in harmless and label-free applications intrigued researchers to develop this technology. The capability of acoustofluidic technology for medical applications has not been thoroughly analyzed and visualized. Toward efficient design, in this research, flowing fluid in a microchannel excited by acoustic waves is fully investigated. To study the behavior of acoustic streaming, the main interfering parameters such as inlet velocity, working frequency, displacement amplitude, fluid buffer material, and hybrid effect in a rectangular water-filled microchannel actuated by standing surface acoustic waves are studied. Governing equations for acoustic field and laminar flow are derived employing perturbation theory. For each set of equations, appropriate boundary conditions are applied. Results demonstrate a parallel device is capable of increasing the inlet flow for rapid operations. Frequency increment raises the acoustic streaming velocity magnitude. Displacement amplitude amplification increases the acoustic streaming velocity and helps the streaming flow dominate over the incoming flow. The qualitative analysis of the hybrid effect shows using hard walls can significantly increase the streaming power without depleting excessive energy. A combination of several effective parameters provides an energy-efficient and fully controllable device for biomedical applications such as fluid mixing and cell lysis.

New Experimental Results on Steady Streaming in Oscillating Deformable Spherical Container

The paper presents an experimental study of steady streaming generated by harmonic oscillations of the boundaries of a deformable spherical container. The container is filled with a viscous fluid and its oscillations are produced by means of two linear servo motors, installed symmetrically. The shape of the boundary during deformation is studied by processing images recorded with a high speed camera. The flows are investigated using the particle image velocimetry. In the analysis of results, the focus is made on the relation between the shape of the oscillating boundary, the dimensionless frequency and the streaming velocity. The possibility of controlling the steady streaming pattern in order to affect the convective mass transfer is discussed.

Influence of Ion–Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations

Abstract We explore the physics of the gyroresonant cosmic-ray streaming instability (CRSI) including the effects of ion–neutral (IN) damping. This is the main damping mechanism in (partially ionized) atomic and molecular gas, which are the primary components of the interstellar medium (ISM) by mass. Limitation of CRSI by IN damping is important in setting the amplitude of Alfvén waves that scatter cosmic rays (CRs) and control galactic-scale transport. Our study employs the magnetohydrodynamic (MHD)–particle-in-cell hybrid fluid-kinetic numerical technique to follow linear growth as well as post-linear and saturation phases. During the linear phase of the instability—where simulations and analytical theory are in good agreement—IN damping prevents wave growth at small and large wavelengths, with the unstable bandwidth lower for higher IN collision rates ν in. Purely MHD effects during the post-linear phase extend the wave spectrum toward larger k. In the saturated state, the CR distribution evolves toward greater isotropy (lower streaming velocity) by scattering off of Alfvén waves excited by the instability. In the absence of low-k waves, CRs with sufficiently high momentum are not isotropized. The maximum wave amplitude and rate of isotropization of the distribution function decrease at higher ν in. When the IN damping rate approaches the maximum growth rate of CRSI, wave growth and isotropization are suppressed. Implications of our results for CR transport in partially ionized ISM phases are discussed.

Transverse Momentum Differential Global Analysis of Heavy-Ion Collisions.

The understanding of heavy ion collisions and its quark-gluon plasma (QGP) formation requires a complicated interplay of rich physics in a wealth of experimental data. In this work we compare for identified particles the transverse momentum dependence of both the yields and the anisotropic flow coefficients for both PbPb and pPb collisions. We do this in a global model fit including a free streaming prehydrodynamic phase with variable velocity v_{fs}, thereby widening the scope of initial conditions. During the hydrodynamic phase we vary three second order transport coefficients. The free streaming velocity has a preference slightly below the speed of light. In this extended model the QGP bulk viscosity is small and even consistent with zero.

Bayesian analysis of heavy ion collisions with the heavy ion computational framework Trajectum

We introduce a model for heavy ion collisions named Trajectum, which includes an expanded initial stage with a variable free streaming velocity $v_{\rm fs}$ and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian Emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in [1] to perform a detailed comparison to experimental data from PbPb and $p$Pb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.

Refining pulsar radio emission due to streaming instabilities: Linear theory and PIC simulations in a wide parameter range

Context. Several important mechanisms that explain coherent pulsar radio emission rely on streaming (or beam) instabilities of the relativistic pair plasma in a pulsar magnetosphere. However, it is still not clear whether the streaming instability by itself is sufficient to explain the observed coherent radio emission. Due to the relativistic conditions that are present in the pulsar magnetosphere, kinetic instabilities could be quenched. Moreover, uncertainties regarding specific model-dependent parameters impede conclusions concerning this question. Aims. We aim to constrain the possible parameter range for which a streaming instability could lead to pulsar radio emission, focusing on the transition between strong and weak beam models, beam drift speed, and temperature dependence of the beam and background plasma components. Methods. We solve a linear relativistic kinetic dispersion relation appropriate for pulsar conditions in a more general way than in previous studies, considering a wider parameter range. In doing so, we provide a theoretical prediction of maximum and integrated growth rates as well as of the fractional bandwidth of the most unstable waves for the investigated parameter ranges. The analytical results are validated by comparison with relativistic kinetic particle-in-cell (PIC) numerical simulations. Results. We obtain growth rates as a function of background and beam densities, temperatures, and streaming velocities while finding a remarkable agreement of the linear dispersion predictions and numerical simulation results in a wide parameter range. Monotonous growth is found when increasing the beam-to-background density ratio. With growing beam velocity, the growth rates firstly increase, reach a maximum and decrease again for higher beam velocities. A monotonous dependence on the plasma temperatures is found, manifesting in an asymptotic behaviour when reaching colder temperatures. A simultaneous change of both temperatures proves not to be a mere linear superposition of both individual temperature dependences. We show that the generated waves are phase-coherent by calculating the fractional bandwidth. Conclusions. Plasma streaming instabilities of the pulsar pair plasma can efficiently generate coherent radio signals if the streaming velocity is ultra-relativistic with Lorentz factors in the range 13 < γ < 300, if the background and beam temperatures are small enough (inverse temperatures ρ0; ρ1 ≥ 1, i.e., T0; T1 ≤ 6 × 109), and if the beam-to-background plasma density ratio n1/(γbn0) exceeds 10−3, which means that n1/n0 has to be between 1.3 and 20% (depending on the streaming velocity).

Morphological and kinematical analysis of the double-barred galaxy NGC 3504 using ALMA CO (2–1) data

ABSTRACT We present results obtained from ALMA CO (2–1) data of the double-barred galaxy NGC 3504. With three times higher angular resolution (∼0${_{.}^{\prime\prime}}$8) than previous studies, our observations reveal an inner molecular gas bar, a nuclear ring, and four inner spiral arm-like structures in the central 1-kpc region. Furthermore, the CO emission is clearly aligned with the two dust lanes in the outer bar region, with differences in shape and intensity between them. The total molecular gas mass in the observed region (50 arcsec × 57 arcsec) is estimated to be ${\sim}3. 1\times 10^9 \, {\rm M}_{\odot }$, which is 17 per cent of the stellar mass. We used the Kinemetry package to fit the velocity field and found that circular motion strongly dominates at R = 0.3–0.8 kpc, but radial motion becomes important at R < 0.3 kpc and R = 1.0–2.5 kpc, which is expected due to the presence of the inner and outer bars. Finally, assuming that the gas moves along the dust lanes in the bar rotating frame, we derived the pattern speed of the outer bar to be 18 ± 5 km s−1 kpc−1, the average streaming velocities on each of the two dust lanes to be 165 and 221 km s−1, and the total mass inflow rate along the dust lanes to be $12\, {\rm M}_{\odot }$ yr−1. Our results give a new example of an inner gas bar within a gas-rich double-barred galaxy and suggest that the formation of double-barred galaxies could be associated with the existence of such gas structures.

Acoustic pulsation for heat transfer abatement in supersonic channel flow

The impact of periodic pulsation normal to the flow passage on the heat and momentum transport is studied using direct numerical simulations. The selected test case is the fully developed supersonic turbulent flow at Reynolds numbers Reb=ρ¯bU¯bδ/μ¯wall=3000 and 6000 and Mach number Mcl=U¯cl/c¯cl≈1.5, bounded by isothermal walls. The pulsation is introduced by imposing a time-periodic uniform body-force in the spanwise directions, mimicking two acoustic drivers placed on the side-walls that are operating with a 180° phase difference. Results indicate that the spanwise pulsation at a moderate forcing amplitude, with the spanwise velocity oscillation amplitude W̃cl≈12% of bulk streamwise velocity or W̃cl+=W̃cl/uτ≈2, can create about 8% reduction in the Nusselt number (Nu), as well as in the skin friction coefficient (Cf), if applied at the optimal pulsation period T+=Tuτ2/ν¯wall within [173, 346], where T=2π/ωf. The excitation outside this band fails to achieve such high levels of modulations in heat and momentum transport processes near the walls. Visualizing the instantaneous temperature field reveals a periodic tilting of the near-wall turbulent structures that is translated into a decay in the energy of the streamwise vortices and an increase in the mean spanwise distance of such structures. The effect of the excitation on reducing the turbulent heat flux and Reynolds shear stress is studied, and the resulting streaming temperature and velocity profiles are discussed.

Large-scale Variation in Reionization History Caused by Baryon–Dark Matter Streaming Velocity

Abstract At cosmic recombination, there was supersonic relative motion between baryons and dark matter, which originated from baryonic acoustic oscillations in the early universe. This motion has been considered to have a negligible impact on the late stage of cosmic reionization because the relative velocity quickly decreases. However, recent studies have suggested that the recombination in gas clouds smaller than the local Jeans mass (≲108 M ☉) can affect the reionization history by boosting the number of ultraviolet photons required for ionizing the intergalactic medium. Motivated by this, we performed a series of radiation-hydrodynamic simulations to investigate whether the streaming motion can generate variation in the local reionization history by smoothing out clumpy small-scale structures and lowering the ionizing photon budget. We found that the streaming velocity can add a variation of Δz e ∼ 0.05–0.5 in the end-of-reionization redshift, depending on the level of X-ray preheating and the time evolution of ionizing sources. The variation tends to be larger when the ionizing efficiency of galaxies decreases toward later times. Given the long spatial fluctuation scales of the streaming motion (≳100 Mpc), it can help to explain the Lyα opacity variation observed from quasars and leave large-scale imprints on the ionization field of the intergalactic medium during the reionization. The pre-reionization heating by X-ray sources is another critical factor that can suppress small-scale gas clumping and can diminish the variation in z e introduced by the streaming motion.

Acoustically induced thermal effects on Rayleigh streaming

The present study focuses on acoustically induced thermal effects on Rayleigh streaming inside a resonator. Firstly, we consider the effect of the transverse (or wall-normal) mean temperature gradient on the acoustic streaming flow generated by a standing wave between two parallel plates. Analytical expressions for acoustic quantities are developed and used to express the sources of linear streaming. The influence of a transverse temperature variation on the streaming velocity is clearly identified through a term proportional to the temperature difference and to the square of the half-width of the guide. This term modifies the Rayleigh streaming pattern and may generate an additional vortex. On the other hand, the longitudinal (or wall-parallel) temperature difference is calculated as a cumulated effect of thermoacoustic heat transport in the fluid, heat conduction in the wall and heat convection of the air outside the resonator. At high acoustic levels, heat is significantly convected by the streaming flow and the resulting transverse temperature difference is proportional to the longitudinal temperature difference. Combining these expressions brings out a new criterion parameter for the nonlinear Reynolds number ( $Re_{NL}$ ) characterizing the transition in streaming patterns at high acoustic levels. This result explains previous experimental and numerical observations of the streaming flow dynamics at high acoustic amplitudes, under different temperature boundary conditions, and can provide a powerful prediction tool for streaming pattern transitions.

Excitation of kinetic Alfvén waves by streaming ions in a dusty magnetoplasma with generalized (r, q) distribution function

AbstractWe investigate the parallel streaming effects on the dispersion characteristics of a kinetic Alfvén wave (KAW) in a low β dusty magnetoplasma. To analyse the influence of streaming ions obeying generalized (r, q) distribution function, hot and magnetized electrons, and mobile charged dust, a theoretical approach has been used for the instability analysis by employing two potential theories. A linear kinetic dispersion relation of Alfvén waves is derived, whose solutions are used to interpret the numerical and analytical results. The solutions of dispersion relation indicate that the characteristics of KAWs are transformed when generalized (r, q) distribution function is employed instead of its Maxwellian counterpart. We also found that the unstable modes have a strong dependence on spectral indices r and q, dust parameters, and plasma β. For the excitation of KAWs, the streaming velocity has been observed to be within the sub‐Alfvén range, whereas when it extends to the super‐Alfvén range, the growth rates are significantly suppressed. The observations further show that an ambient magnetic field and superthermal particles inhibit the growth of an electromagnetic wave to a significant degree and have a stabilizing effect on the wave mode, whereas an increasing concentration of low‐energy particles contributes to enhancing growth rates.

Electropumping of nanofluidic water by linear and angular momentum coupling: theoretical foundations and molecular dynamics simulations.

In this article we review the relatively new phenomenon of electropumping in nanofluidic systems, in which nonzero net flow results when polar molecules are rotated by external electric fields. The flow is a consequence of coupling of the spin angular momentum of molecules with their linear streaming momentum. By devising confining surfaces that are asymmetric - specifically one surface is more hydrophobic compared to the other - unidirectional flow results and so pumping can be achieved without the use of pressure gradients. We first cover the historical background to this phenomenon and follow that with a detailed theoretical description of the governing hydrodynamics. Following that we summarise work that has applied this phenomenon to pump water confined to planar nanochannels, semi-functionalised single carbon nanotubes and concentric carbon nanotubes. We also report on the energy efficiency of this pumping technique by comparisons with traditional flows of planar Couette and Poiseuille flow, with the surprising conclusion that electropumping at the nanoscale is some 4 orders of magnitude more efficient than pumping by Poiseuille flow.

Using Plant Cells of Nitellopsis obtusa for Biophysical Education

ABSTRACT Using giant characeaen algae Nitellopsis obtusa in laboratory exercises is proposed to familiarize students with basic concepts of electrophysiology and provide some simple hands-on practice. The described concept experiments present extracellular registration of action potentials (APs) and investigation of cytoplasmic streaming properties. Students are expected to register the propagation velocity of APs (found to be 3.4 ± 1.5 cm/s in N. obtusa), as well as the velocity of cytoplasmic streaming (66.7 ± 9 μm/s). Proposed exercises also concern recovery dynamics of cytoplasmic streaming after a stimulation (recovery time constant τ = 3.7 ± 2.1 min) as well as investigation of an effect of various chemicals (e.g., KCl) on all selected parameters. The experiments endorse characeaen algae as a model system to be routinely explored in education of biophysics and bioelectrical phenomena of the cell.

Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.

Formation mechanism of the nanostructure in laser streaming phenomenon.

Laser streaming is a phenomenon in which liquid streaming is driven directly from the laser through an in situ fabricated nanostructure. In this study, liquid streaming of a gold nanoparticle suspension driven by a pulsed laser was studied using a high-speed camera. The laser streaming formation time, streaming velocity, and relative energy conversion efficiency of laser streaming was measured for different nanoparticle concentrations, focal lens position, laser powers, and laser repetition rates. In addition to the laser intensity, which played a significant role in the formation process of laser streaming, the optical gradient force was found to be an important approach involved in the transport and provision of nanoparticles during the formation of laser streaming. This finding facilitated a better understanding of the formation mechanism of laser streaming and demonstrated the possibilities of a new potential laser etching technique based on nanosecond lasers and nanoparticle suspensions. This result can also expand the application of laser streaming in microfluids and other fields that require lasers to move macroscopic objects at relatively high speeds.

A new class of Ion-acoustic solitons that can exist below critical Mach number

It is commonly believed that ion-acoustic solitons can only exist above the critical Mach number in a plasma system. A new class of ion-acoustic solitons that can exist below the critical Mach number is reported for the first time in a three-component plasma consisting of hot Maxwellian electrons, and two counterstreaming ion beams. The analysis is based on the Sagdeev pseudopotential technique, and considers a simple case of two counterstreaming proton beams with equal density and streaming velocity. Linear stability analysis shows that the slow ion-acoustic modes become unstable due to ion beam instability when the beam velocity normalized with the ion acoustic speed, U0, is in the range of 0.55 ≤ U0 ≤ 1.14. It is shown that when the normalized streaming velocity is below or at a threshold value, Uth = 1.14, only the regular solitons having Mach numbers greater than critical Mach number can exist. However, when the streaming velocity exceeds the threshold value (all modes are stable), both regular and the new class of ion-acoustic solitons can exist. A special case of unequal ion densities and unequal streaming velocities of the counterstreaming beams is considered in , and similar effects are found. Hence, the new class of slow ion-acoustic solitons can exist in the parametric regime where the system is stable to counterstreaming ion beams instability. The results could be useful in the interpretation of slow electrostatic solitary waves (ESWs) observed in the magnetosphere.

Shape and spin of minihaloes – II. The effect of streaming velocities

ABSTRACT Models of the decoupling of baryons and photons during the recombination epoch predict the existence of a large-scale velocity offset between baryons and dark matter at later times, the so-called streaming velocity. In this paper, we use high resolution numerical simulations to investigate the impact of this streaming velocity on the spin and shape distributions of high-redshift minihaloes, the formation sites of the earliest generation of stars. We find that the presence of a streaming velocity has a negligible effect on the spin and shape of the dark matter component of the minihaloes. However, it strongly affects the behaviour of the gas component. The most probable spin parameter increases from ∼0.03 in the absence of streaming to ∼0.15 for a run with a streaming velocity of three times σrms, corresponding to 1.4 km s−1 at redshift z = 15. The gas within the minihaloes becomes increasingly less spherical and more oblate as the streaming velocity increases, with dense clumps being found at larger distances from the halo centre. The impact of the streaming velocity is also mass-dependent: less massive objects are influenced more strongly, on account of their shallower potential wells. The number of haloes in which gas cooling and runaway gravitational collapse occurs decreases substantially as the streaming velocity increases. However, the spin and shape distributions of gas that does manage to cool and collapse are insensitive to the value of the streaming velocity and we therefore do not expect the properties of the stars that formed from this collapsed gas to depend on the value of the streaming velocity. The spin and shape of this central gas clump are uncorrelated with the same properties measured on the scale of the halo as a whole.