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1. Compressible two-dimensional turbulence: cascade reversal and sensitivity to imposed magnetic field

   https://doi.org/10.1088/1367-2630/ad0172  

   Fouxon, Itzhak ; Kritsuk, Alexei G. ; Mond, Michael ( November 2023 , New Journal of Physics)

   We study the impact of compressibility on two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach numberMaincreases, from inverse atMa = 0, to direct at$Ma=\mathrm{\infty }$. Thus, at$Ma\sim 1$comparable amounts of energy flow from the pumping scale to large and small scales, in accord with previous data. For supersonic turbulence with$Ma\gg 1$the cascade is direct, as in three dimensions, which results in multifractal density field. For that regime ($Ma\gg 1$) we derive a Kolmogorov-type law for potential forcing and obtain an explicit expression for the third order correlation tensor of the velocity. We further show that all third order structure functions are zero up to first order in the inertial range scales, which is in sharp contrast with incompressible turbulence where the third order structure function, that describes the energy flux associated with the energy cascade is non-zero. The properties of compressible turbulence have significant implications on the amplification of magnetic fields in conducting fluids. We thus demonstrate that imposing external magnetic field on compressible flows of conducting fluids allows to manipulate the flow producing possibly large changes even at small Mach numbers. Thus Zeldovich’s antidynamo theorem, by which atMa = 0 the magnetic field is zero in the steady state, must be used with caution. Real flows have finiteMaand, however small it is, for large enough values ofI, the magnetic flux through the disk, the magnetic field changes the flow appreciably, or rearranges it completely. This renders the limitMa → 0 singular for non-zero values ofI. Of particular interest is the effect of the density multifractality, at$Ma\gg 1$which is relevant for astrophysical disks. We demonstrate that in that regime, in the presence of non-zeroIthe magnetic field energy is enhanced by a large factor as compared to its estimates based on the mean field. Finally, based on the insights described above, we propose a novel two-dimensional Burgers’ turbulence, whose three-dimensional counterpart is used for studies of the large-scale structure of the Universe, as a model for supersonic two-dimensional magnetohydrodynamic flows.

    

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2. Accurate near-wall measurements in wall bounded flows with optical flow velocimetry via an explicit no-slip boundary condition

   https://doi.org/10.1088/1361-6501/acf872  

   Jassal, Gauresh Raj ; Schmidt, Bryan E. ( September 2023 , Measurement Science and Technology)

   High fidelity near-wall velocity measurements in wall bounded fluid flows continue to pose a challenge and the resulting limitations on available experimental data cloud our understanding of the near-wall velocity behavior in turbulent boundary layers. One of the challenges is the spatial averaging and limited spatial resolution inherent to cross-correlation-based particle image velocimetry (PIV) methods. To circumvent this difficulty, we implement an explicit no-slip boundary condition in a wavelet-based optical flow velocimetry (wOFV) method. It is found that the no-slip boundary condition on the velocity field can be implemented in wOFV by transforming the constraint to the wavelet domain through a series of algebraic linear transformations, which are formulated in terms of the known wavelet filter matrices, and then satisfying the resulting constraint on the wavelet coefficients using constrained optimization for the optical flow functional minimization. The developed method is then used to study the classical problem of a turbulent channel flow using synthetic data from a direct numerical simulation (DNS) and experimental particle image data from a zero pressure gradient, high Reynolds number turbulent boundary layer. The results obtained by successfully implementing the no-slip boundary condition are compared to velocity measurements from wOFV without the no-slip condition and to a commercial PIV code, using the velocity from the DNS as ground truth. It is found that wOFV with the no-slip condition successfully resolves the near-wall profile with enhanced accuracy compared to the other velocimetry methods, as well as other derived quantities such as wall shear and turbulent intensity, without sacrificing accuracy away from the wall, leading to state of the art measurements in the$y^{+}<1$region of the turbulent boundary layer when applied to experimental particle images.

    

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3. Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

   https://doi.org/10.3847/1538-4357/ad323d  

   Manikantan, Vikram ; Kaaz, Nicholas ; Jacquemin-Ide, Jonatan ; Musoke, Gibwa ; Chatterjee, Koushik ; Liska, Matthew ; Tchekhovskoy, Alexander ( April 2024 , The Astrophysical Journal)

   The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds,that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration (1.2 × 10⁵r_g/c) simulation of a rapidly rotating (a= 0.9) black hole feeding from a thick (H/r∼ 0.3), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of$\dot{L}\propto r^{0.9}$and$\dot{M}\propto r^{0.4}$, respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts.

    

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4. Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

   https://doi.org/10.3847/1538-4357/ac752b  

   Bustard, Chad ; Gronke, Max ( July 2022 , The Astrophysical Journal)

   The Magellanic Stream is sculpted by its infall through the Milky Way’s circumgalactic medium, but the rates and directions of mass, momentum, and energy exchange through the stream-halo interface are relative unknowns critical for determining the origin and fate of the Stream. Complementary to large-scale simulations of LMC-SMC interactions, we apply new insights derived from idealized, high-resolutioncloud-crushingand radiative turbulent mixing layer simulations to the Leading Arm and Trailing Stream. Contrary to classical expectations of fast cloud breakup, we predict that the Leading Arm and much of the Trailing Stream should be surviving infall and even gaining mass due to strong radiative cooling. Provided a sufficiently supersonic tidal swing-out from the Clouds, the present-day Leading Arm could be a series of high-density clumps in the cooling tail behind the progenitor cloud. We back up our analytic framework with a suite of converged wind-tunnel simulations, finding that previous results on cloud survival and mass growth can be extended to high Mach number ($\mathit{}$) flows with a modified drag time$t_{\mathrm{drag}}\propto 1+\mathit{}$and longer growth time. We also simulate the Trailing Stream; we find that the growth time is long (approximately gigayears) compared to the infall time, and approximate Hαemission is low on average (∼ a few milliRayleigh) but can be up to tens of milliRayleigh in bright spots. Our findings also have broader extragalactic implications, e.g., galactic winds, which we discuss.

    

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5. Cosmic-Ray Drag and Damping of Compressive Turbulence

   https://doi.org/10.3847/1538-4357/aceef9  

   Bustard, Chad ; Oh, S. Peng ( September 2023 , The Astrophysical Journal)

   While it is well known that cosmic rays (CRs) can gain energy from turbulence via second-order Fermi acceleration, how this energy transfer affects the turbulent cascade remains largely unexplored. Here, we show that damping and steepening of the compressive turbulent power spectrum are expected once the damping time$t_{\mathrm{damp}}\sim \rho v^2/{\dot{E}}_{\mathrm{CR}}\propto E_{\mathrm{CR}}^{-1}$becomes comparable to the turbulent cascade time. Magnetohydrodynamic simulations of stirred compressive turbulence in a gas-CR fluid with diffusive CR transport show clear imprints of CR-induced damping, saturating at${\dot{E}}_{\mathrm{CR}}\sim \widetilde{ϵ}$, where$\widetilde{ϵ}$is the turbulent energy input rate. In that case, almost all of the energy in large-scale motions is absorbed by CRs and does not cascade down to grid scale. Through a Hodge–Helmholtz decomposition, we confirm that purely compressive forcing can generate significant solenoidal motions, and we find preferential CR damping of the compressive component in simulations with diffusion and streaming, rendering small-scale turbulence largely solenoidal, with implications for thermal instability and proposed resonant scattering ofE≳ 300 GeV CRs by fast modes. When CR transport is streaming dominated, CRs also damp large-scale motions, with kinetic energy reduced by up to 1 order of magnitude in realisticE_CR∼E_gscenarios, but turbulence (with a reduced amplitude) still cascades down to small scales with the same power spectrum. Such large-scale damping implies that turbulent velocities obtained from the observed velocity dispersion may significantly underestimate turbulent forcing rates, i.e.,$\widetilde{ϵ}\gg \rho v^3/L$.

    

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