Hydrodynamic Modes Research Articles (Page 1)

Mixing and heat transfer rates are typically enhanced in high-pressure transcritical turbulent flow regimes. This is largely due to the rapid variation of thermophysical properties near the pseudo-boiling region, which can significantly amplify velocity fluctuations and promote flow destabilisation. The stability conditions are influenced by the presence of baroclinic torque, primarily driven by steep, localised density gradients across the pseudo-boiling line; an effect intensified by differentially heated wall boundaries. As a result, enstrophy levels increase compared with equivalent low-pressure systems, and flow dynamics diverge from those of classical wall-bounded turbulence. In this study the dynamic equilibrium of these instabilities is systematically analysed using linear stability theory. It is shown that under isothermal wall transcritical conditions, the nonlinear thermodynamics near the pseudo-boiling region favour destabilisation more readily than in subcritical or supercritical states; though this typically requires high-Mach-number regimes. The destabilisation is further intensified in non-isothermal wall configurations, even at low Brinkman and significantly low Mach numbers. In particular, the sensitivity of neutral curves to Brinkman number variations, along with the modal and non-modal perturbation profiles of hydrodynamic and thermodynamic modes, offer preliminary insight into the conditions driving early destabilisation. Notably, a non-isothermal set-up (where walls are held at different temperatures) is found to be a necessary condition for triggering destabilisation in low-Mach, low-Reynolds-number regimes. For the same Brinkman number, such configurations accelerate destabilisation and enhance algebraic growth compared with isothermal wall cases. As a consequence, high-pressure transcritical flows exhibit increased kinetic energy budgets, driven by elevated production rates and reduced viscous dissipation.

We introduce a regularized fluctuating lattice Boltzmann model (Reg-FLBM) for the D3Q27 lattice, which incorporates thermal fluctuations through Hermite-based projections to ensure compliance with the fluctuation-dissipation theorem. By leveraging the recursive regularization framework, the model achieves thermodynamic consistency for both hydrodynamic and ghost modes. Compared to the conventional single-relaxation-time BGK-FLBM, the Reg-FLBM provides improved stability and a more accurate description of thermal fluctuations. The implementation is optimized for large-scale parallel simulations on graphics processing unit-accelerated architectures, enabling systematic investigation of fluctuation-driven phenomena in mesoscale and nanoscale fluid systems.

<div>The force of the solid contact (<i>F<sub>sc</sub> </i>) between the bearing surface and the shaft surface and the friction force (<i>F<sub>fri</sub> </i>) generated in the crankpin bearing have a great influence on the lubrication performance of the crankpin bearing in the engine. Therefore, the micro-circular texture (MCT) has been proposed and designed on the bearing surface of the crankpin bearing for ameliorating its lubrication performance. To evaluate the effectiveness of MCT in detail, based on the lubricating model of the crankpin bearing under the impaction of external load <i>F</i> <sub>0</sub>, the influence of the density, depth (<i>h<sub>MCT</sub> </i>), and radius (<i>r<sub>MCT</sub> </i>) of MCT on the characteristics of the pressure (<i>p</i>) of oil film, thickness of oil film (<i>h</i>), force of solid contacts, and force of the friction in the crankpin bearing are also investigated, respectively. An algorithmic program written in a MATLAB environment is then applied to simulate the lubrication equations of the crankpin bearing and MCT. Some outstanding results of the study have been achieved as follows: (i) all the results of <i>p</i>, <i>h</i>, <i>F<sub>sc</sub> </i>, and <i>F<sub>fri</sub> </i> with MCT are significantly improved in comparison with those without MCT; thus, the lubrication performance of the crankpin bearing with MCT added is better than that without MCT. (ii) To better improve the lubrication performance of the crankpin bearing, the density of MCT 12 × 6, depth of <i>h<sub>MCT</sub> </i> = 15 μm, and radius of 0.25 < <i>r<sub>MCT</sub> </i> < 0.5 mm should be used for designing MCT. (iii) The MCT in the hydrodynamic lubrication mode improves the lubrication performance of the crankpin bearing lower than the MCT in the mixed lubrication mode. Therefore, MCT should be designed over a portion of the bearing surface in the mixed lubrication mode. This not only reduces the complexity and manufacturing cost of MCT but also improves the lubrication performance of the crankpin bearing.</div>

Studies of heat transfer in cooling systems with natural material coatings have been carried out. The phenomenon of flame spin detonation was observed at an oxidizer excess ratio below unity, with the spraying process being intensified up to sixfold. The coatings demonstrated high reliability compared to other accelerated systems. The maximum specific heat fluxes on the coating range from 2 to 20×10⁶ W/m², with oscillation frequencies reaching 200 Hz. The overheating range of the coating was (20–75) K. The granulometric composition of the materials was obtained, and the hydrodynamic operating modes of the burners were selected. A model was developed for the interaction of a supersonic detonation gas jet of the thermal tool acting normally to the coating. The experimentally determined heat transfer coefficients were found to be 5–6 times higher than those predicted by laminar theory, and several times lower than those predicted by turbulent heat transfer laws. The particle flight time, powder diameter, as well as the ultimate compressive and tensile stresses of the coating were determined. The main practical application of the research is thermal protection through cooling with natural coatings (quartzites, granites, teschenites, marbles, tuffs) for highly forced and high-intensity structures in the fields of energy, metallurgy, and mechanical engineering. The primary industrial implementation of the research is the use of a thermal tool for spraying, processing of rocks, drilling, and cutting of reinforced concrete structures during modernization and reconstruction of enterprises.

The Darcy–Brinkman model at the representative elementary volume scale is utilized to examine the hydrodynamic characteristics of the dam-break wave’s impact on a porous medium. Factors influencing the impact of hydrodynamic processes, including porosity and wave propagation mode, are systematically analysed. The results indicated that the maximum dimensionless water intrusion volume shows a linear relationship with porosity. Under different dam-break wave modes, the impact pressure exhibits distinct pressure evolution curves. For bore-like waves on a wet bed, a double-peak distribution is reported correspond to the impact of the breaking wave and the following main flow. In contrast, for dam-break waves on a dry bed and undular waves on a wet bed, only a single pressure peak can be observed. Finally, the maximum total force of dam-break impact on the upstream surface of the porous medium displays a linear relationship with porosity, for a consistent downstream water depth.

The supersonic wake of a circular cylinder in Mach 3 flow was studied through spectral proper orthogonal decomposition (SPOD) of high-speed focussing schlieren datasets. A wavenumber decomposition of the SPOD eigenvectors was found to be an effective tool for isolating imaging artefacts from the flow features, resulting in a clearer interpretation of the SPOD modes. The cylinder wake consists of both symmetric and antisymmetric instabilities, with the former being the dominant type. The free shear layers that form after the flow separates from the cylinder surface radiate strong Mach waves that interact with the recompression shocks to release significant disturbances into the wake. The wake shows a bimodal vortex shedding behaviour with a purely hydrodynamic instability mode around a Strouhal number of 0.2 and an aeroacoustic instability mode around Strouhal number of 0.42. The hydrodynamic mode, which is presumably the same as the incompressible case, is weaker and decays rapidly as the wake accelerates due to increasing compressibility. The aeroacoustic mode is the dominant shedding mode and persists farther into the wake because of an indirect energy input received through free-stream acoustic waves. A simple aeroacoustic feedback model based on an interaction between downstream propagating shear-layer instabilities and upstream propagating acoustic waves within the recirculation region is shown to accurately predict the shedding frequency. Based on this model, the vortex shedding in supersonic flows over a circular cylinder occurs at a universal Strouhal number (based on approach free-stream velocity and feedback path length) of approximately 0.3.

Abstract We investigate the stability of the causal structure—generated by the introduction of a preferred reference frame—which in turn, is associated with the critical speed of a Landau superfluid and represents the cosmological vacuum. We discuss the acoustic geometry associated with the set of excited states and their relation to the Refractive Index Perturbation (R.I.P.). We show that the presence of the privileged reference frame implies kinematic transformations and velocity compositions that contain, in an upper limit, the properties of Lorentz symmetry and the critical velocity of the superfluid as a lower limit. We treat the critical speed in an Euler reference frame, showing its relation with a convective term, as well as the possibility of deducing a Navier-Stokes equation modified by a quantum potential—linked to the deformation of the momentum generated by the drag of the superfluid with critical speed. Establishing a hydrodynamic reference, where the phase transition occurs, ‘destroys’ superfluids. The conserved properties of this superfluid were studied. The study of the transformations from an Einstein-Euler reference frame to the preferred reference frame allows us to recover the previously defined acoustic geometry. The study of the causal structure of this acoustic geometry indicated the existence of an ergoregion, confirming what we found as a convective term. Thus, we have a stable causal structure, both because of the method defined for acoustic geometries—and because of the fear of Hydrodynamic Modes. Therefore we can categorically state that the preferred reference frame induces a stable causal structure.

With the advancement of high-velocity kinetic energy weapons, the impact velocity encountered by concrete protective structures has evolved from low to high velocity ranges, rendering traditional rigid projectile penetration theories inadequate for accurately describing the physical mechanisms of deformation and erosion coupling during penetration. This study establishes a theoretical analytical framework for penetration dynamics under high-velocity conditions with coupled deformation and erosion effects: the critical velocity threshold distinguishing between rigid projectile penetration and hydrodynamic penetration modes is precisely defined based on the initial impact velocity V0. By integrating empirical mass erosion formulas with cavity expansion theory, a theoretical model encompassing coupled deformation and erosion effects has been developed, incorporating both projectile cross-sectional area evolution and penetration depth prediction. Comparative analysis with published experimental data (small-scale projectiles vertically impacting concrete targets) demonstrates the model’s predictive accuracy, showing maximum errors of 9.5% in critical velocity prediction, 17.89% in projectile cross-sectional area prediction, and 24.4% in penetration depth prediction.

We search for emergent hydrodynamic modes in real-time Hamiltonian dynamics of 2+1-dimensional SU(2) lattice gauge theory on a quasi-one-dimensional plaquette chain, by numerically computing symmetric correlation functions of energy densities on lattice sizes of about 20 with the local Hilbert space truncated at jmax=12. Because of the Umklapp processes, we only find a mode for energy diffusion. The symmetric correlator exhibits transport peak near zero frequency with a width approximately proportional to momentum squared at small momentum, when the system is fully quantum ergodic, as indicated by the eigenenergy level statistics. This transport peak leads to a power-law t−12 decay of the symmetric correlator at late time, also known as the long-time tail, as well as diffusionlike spreading in position space. We also introduce a quantum algorithm for computing the symmetric correlator on a quantum computer and find it gives results consistent with exact diagonalization when tested on the IBM emulator. Finally we discuss the future prospect of searching for the sound modes.

Studies of the maximum heating loads for cooling systems coatings made from natural materials have been conducted. To investigate cooling coatings based on natural materials, an experimental setup including a coating spraying tool has been developed. The conditions for spraying the material onto the heating surface, as well as the design principles for nozzles and combustion chambers, have been established. In the area of the steam-generating surface’s limiting state, shielded from burnout by cooling, these investigations are practically significant. Cooling systems with porous coatings have been developed, which make it possible to prevent the development of fractures in the coatings of chambers and nozzles through the use of thermodynamic and acoustic screens from three heating sources, as well as devices for spraying coatings with detonation high-temperature flares emanating from nozzles and combustion chambers that are cooled by capillary-porous coatings. High-speed filming and holography were used during the investigation. Heat fluxes, temperatures, flow rates, and pressures of liquid and gas flows were measured. The degree of unaccounted flow at different pressures was determined. A model of the interaction of an axisymmetric supersonic detonation jet of gases from the thermal tool normal to the coating has been constructed. Thermodynamic characteristics of oxygen-kerosene burners for the generation of supersonic high-temperature detonation flares by spraying of coatings from powders of natural materials have been established, and the granulometric composition of materials has been determined. Hydrodynamic operating modes of the burners were selected for specific heat fluxes in the range of (2·106 ÷ 2·107) W/m2 from the jet torch into the coating. The oxidizer-to-fuel ratio varied between 0.3÷0.8; the jet torch temperature was (850÷3000)°C; the jet length was (0÷0.16) m; the jet radius was (3÷10) ·10-3 m, and the burner axis angle to the coating was (90÷0) degrees. Capillary-porous and flow-through cooling systems showed high reliability, with the former reducing coolant consumption by up to 80 times.

We study a center-of-mass-conserving Brownian complex Sachdev-Ye-Kitaev model with long-range (power-law) interactions characterized by 1/r^{η}. The kinetic constraint and long-range interactions conspire to yield rich hydrodynamics associated with the conserved charge, which we reveal by computing the Schwinger-Keldysh effective action. Our result shows that charge transport in this system can be subdiffusive, diffusive, or superdiffusive, with the dynamical exponent controlled by η. We further employ a doubled Hilbert space methodology to derive an effective action for the out-of-time-order correlator, from which we obtain the phase diagram delineating regimes where the light cone is linear or logarithmic. Our results provide a concrete example of a quantum many-body system with kinetic constraint and long-range interactions in which the emergent hydrodynamic modes and out-of-time-order correlator can be computed analytically.

The subtle interplay between competing degrees of freedom, anisotropy, and spin correlations in frustrated Kitaev magnets offers an ideal platform to host non-trivial spin freezing and exotic low-energy excitations. We elucidate low-temperature spin freezing as evidenced by thermodynamics, NMR, and inelastic neutron scattering (INS) results in frustrated Kitaev magnets adopting Halperin and Saslow (HS) and spin jam frameworks. The temperature dependence of specific heat (Cm) shows a broad maximum, indicating short-range spin correlations, while the T2 dependence of Cm below the spin-glass temperature (Tg) suggests gapless excitation spectra. The aging and memory effect experiments suggest a non-hierarchical free energy distribution, which differs from the hierarchical organization of conventional spin freezing. The NMR spin-lattice relaxation rate follows a power law behavior below Tg, suggesting exotic spin excitation spectra that are supported by the INS. The INS susceptibility is proportional to the energy transfer, corroborating the emergence of topological spin freezing, which can be explained by the HS hydrodynamic modes. HS modes account for instigating non-Abelian defect propagation, thereby inducing a spin jam state in the low-T regime in Kitaev magnets. Our work captures the essence of topological spin freezing, characterized by macroscopic ground state degeneracy, short-range spin correlations, and linearly dispersive low-energy excitations in frustrated Kitaev magnets.

A low-Mach airflow passing over a cavity can cause aeroacoustic instabilities, resulting in whistling sounds. These instabilities take place when the hydrodynamic mode of the fluid and the acoustic mode of the cavity form constructive feedback exceeding the dissipative losses. The resulting limit cycle has been observed to enable tunable compensation of transmission losses, which is desirable in a broad range of phenomena from lossless non-reciprocal transmission to acoustic cloaking. The general method works by scattering harmonic incident waves with sufficiently large amplitude by a cavity limit cycle oscillating in the weakly non-linear regime, forcing the limit cycle to synchronize with the incident waves. At suitable conditions, the resulting non-linear wave-mode coupling leads to superradiant amplification of the outgoing waves. This work is focused on the tunability of superradiant scattering with regard to the incident wave amplitude and the bifurcation parameter. To investigate the tunability, experiments were performed on a superradiant aeroacoustic meta-atom, which has previously been demonstrated to amplify harmonic acoustic waves. These experimental validations show excellent agreement with corresponding theoretical predictions. As a key result, it is demonstrated that superradiant amplification can be achieved at small incident wave amplitudes, given that the bifurcation parameter is also small enough.

Abstract Heat transfer studies have been conducted for cooling systems with coatings made of natural materials, depending on the parameters of the detonation flame of a thermal tool and the thermophysical properties of natural materials. Cooling systems with porous coatings of mineral media powders (quartzites, granites, teschenites, tuffs, marbles) had been developed, which were applied on a metal surface at temperatures up to (2500 ÷ 3500) °C and flow rates up to 2500 m/s by hot flames emanating from combustion chambers and nozzles. The holography and high-speed filming method has been used in the studies. The cost impact per one thermal tool is at least 200–300 dollars. The phenomenon of spin detonation of a flame at an oxidant excess coefficient of less than one has been recorded; the spraying process was intensified by 2 to 6 times. The coatings have shown high reliability compared to other boosted systems. The maximum specific heat flows on the coating are (from 2 to 20 × 106 W/m2) and the oscillation frequency are up to 200 Hz. The overheating range of the coating was 20 ÷ 75 K. The thermodynamic characteristics of thermal tools have been established in the model and experimentally; the granulometric composition of materials has been obtained; the hydrodynamic operating modes of the burners have been selected (fuel combustion method, jet length, jet angle). The flight time of the particles, the optimal thickness of the coatings, the diameter of the powder, and the limiting compression and tensile stresses of the coating have been determined. Dependences of displacements for coatings under thermal influence have been obtained, which is important for diagnostics and forecasting of plants and prolongation of service life.

Представлены используемые в практической деятельности способы повышения эффективности процессов мембранного разделения жидкостей и наиболее целесообразные области их применения. На примере технологии VSEP показаны теоретические оценки влияния гидродинамических условий на процессы мембранного разделения, в которых мембраны плоскорамного модуля с открытыми напорными каналами не закреплены неподвижно, а подвержены вибрации в виде синусоидальных вращательных колебаний, направленных вдоль рабочей поверхности. Определены основные преимущества таких процессов. Намечены перспективы использования процессов мембранного разделения по технологии VSEP. Обозначена важность продолжения исследований в направлении изучения и моделирования гидродинамических режимов движения жидкостей в напорных межмембранных каналах. The methods used in practice to increase the process efficiency of membrane separation of liquids as well as the most appropriate areas of their application are presented. Using the example of VSEP technology, theoretical estimates of the effect of hydrodynamic conditions on membrane separation processes are shown, where the membranes of a flat-plate module with open pressure channels are not hard-set fixed but subject to vibration in the form of sinusoidal rotary oscillations directed along the working surface. The main advantages of such processes are defined. The prospects for using VSEP technology in membrane separation processes are outlined. The importance of continuing the research trend and simulation of the hydrodynamic modes of liquid movement in pressure intermembrane channels is indicated.

In this paper, we present a detailed analysis of normal modes based on the Boltzmann equation within the mutilated relaxation time approximation (RTA). Using this linearized effective kinetic description, our analysis encompasses a complete order calculation in wavenumber k, extending the conventional hydrodynamic mode analysis to intermediate and short-wavelength regions. Furthermore, our linear mode analysis can provide a natural classification of kinetic modes into collective modes and non-collective single-particle excitations. In the case of an energy-independent relaxation time, the behavior of hydrodynamic onset transitions is recovered (Romatschke in Eur Phys J C 76:352, 2016). However, for the case with an energy-dependent relaxation time, the distinct classification becomes less clear, as the location of hydrodynamic modes is not well separated from non-hydrodynamic modes.

The hydrodynamics of crystals with vacancies is developed on the basis of local-equilibrium thermodynamics, where the chemical potential of vacancies plays a key role together with a constraint relating the concentration of vacancies to the density of mass and the strain tensor. The microscopic foundations are established, leading to Green-Kubo and Einstein-Helfand formulas for the transport coefficients, including the vacancy conductivities and the coefficients of vacancy thermodiffusion. As a consequence of having introduced the chemical potential of vacancies, a relationship is obtained between the conductivities and the Fickian diffusion coefficients for the vacancies. The macroscopic equationsare linearized around equilibrium to deduce the dispersion relations of the eight hydrodynamic modes. The theoretical predictions are confirmed by numerical simulations of the hard-sphere crystal with vacancies. The study explicitly shows that the eighth hydrodynamic mode of nonperfect monatomic crystals is indeed a mode of vacancy diffusion.

Relevance. The problems arising in developing fuel composites and consisting in determining the optimal ratio of combustible components are solved in the study of their physicochemical and operational properties. Particular attention should be paid to ignition and combustion of the air suspension of fuel compositions. Aim. The implementation of these problems is seen in the development of an experimental technique for predicting some indicators of coals and coal waste based on the methodology of analyzing video files air suspension of ignition in the form of a graphical visualization of combustion. Methods. Technique for assessing and predicting some indicators of coals and coal waste obtained on the basis of the methodology of analyzing video files of air suspension ignition in the form of a graphical visualization of combustion. Results and conclusions. The authors have obtained the dependence of the predicted value for determining the air excess coefficient for coals and coal waste. They developed the methodology for graphical visualization of the combustion of coals and coal waste air suspension. The Kuznetsk coal of grade D was studied and its combustion was described in the case of combustion in active hydrodynamic modes. The developed method for assessing and predicting some indicators of coals and coal waste obtained on the basis of the methodology for analyzing video files of air suspension ignition in the form of graphical visualization of the combustion proved its effectiveness. It showed a good distribution of dust-coal particles in a gas-dust cloud throughout the volume, demonstrating stable and efficient combustion of the fuel system. Convective flows in the reaction chamber do not break the forming combustion front. Graphic visualization of combustion showed that during this process, there is a minimum time of destructive processes occurring in coal particles. This indicates efficient heat and mass transfer. For the studied system of Kuznetsk coal of grade D, the optimal fuel-oxidizer ratio α is 1.25. Oxidation reactions in this case occur in a shorter time, which is characterized by an increase in pressure in the reaction volume.

In this paper, we extend the study of holographic superfluids from planar topology to spherical topology, inspired by recent studies on Bose-Einstein condensation (BEC) on shell-shaped geometry. We investigated the superfluid phase transition from normal fluid and its Quasi-Normal Modes (QNMs) on the sphere. It turns out that the critical temperature for the superfluid phase transition on the sphere is higher than that in the planar case. We investigated four different solutions in the backgrounds of large and small black holes. The calculation of free energy selects the most stable solution. Finally, after calculating the quasi-normal modes and their dynamic behavior, we obtained three different channels similar to the planar superfluid case, along with the “first” hydrodynamic excitation mode.

We investigate fluctuations of hot and dense QCD plasma by using the gauge/gravity duality. To this end, we carry out a comprehensive classification and analysis of quasinormal modes of charged black holes in the holographic V-QCD model. It turns out that the Chern-Simons term determined by the flavor anomalies of QCD is strong enough to drive a modulated instability. While such an instability is expected at high densities, we find that the unstable phase extends to surprisingly low densities and high temperatures, close to the region where data from lattice simulations are available. We also analyze the limit of small temperature which is controlled by a quantum critical AdS2 point. We study in detail the signatures of the critical point in the quasinormal mode spectrum, focusing on the interplay between the hydrodynamic modes and other modes. Published by the American Physical Society 2025