Kinetic Pressure Research Articles

On how finite β and three-dimensional magnetic perturbations affect the instability of toroidal ion temperature gradient mode

Abstract The effects of finite β (the ratio of plasma kinetic pressure to magnetic pressure) and three-dimensional (3D) magnetic perturbations (MPs) on the instability of toroidal ion temperature gradient (ITG) mode are studied in this work. The expression of ion magnetic drift frequency ω_di modified by the effects from both finite β and 3D MPs is firstly derived based on the local 3D equilibrium model. Then, under the assumptions of adiabatic electrons and localized mode structure around the outboard mid-plane (ϑ_p=0), the dispersion equation of the long wavelength toroidal ITG mode with considering the parallel ion dynamics is derived and solved. The results show that the distribution of ω_di around the outboard mid-plane, including both ω_(di,0)=├ ω_di ┤|_(ϑ_p=0) and ω_(di,0)^''≡├ (d^2 ω_di \/dϑ_p^2 )┤|_(ϑ_p=0) (twist parameter quantifying the degree of concave or convex of ω_di), is the key for affecting the toroidal ITG mode instability. The diamagnetic effects from finite β and the effects from 3D MPs can suppress the instability by reducing |ω_(di,0) |. Via reducing |ω_(di,0)^'' | under the prerequisite of unchanged sign of ω_(di,0)^'', there also exist stabilization effects on the instability from 3D MPs and the modification of local magnetic shear by finite β effects. In addition, the stabilization effects induced by reducing |ω_(di,0)^'' | are closely associated with the global magnetic shear. The mechanisms for the effects of finite β and 3D MPs on the instability of toroidal ITG mode revealed in this work are helpful to the comprehensive understanding of the relationship between internal kink mode induced non-axisymmetric flux surface distortion and internal transport barrier physics in tokamak plasmas.

Large-eddy simulation of tip clearance cavitating flow around a hydrofoil

Large-eddy simulations of tip clearance cavitating flow between a National Advisory Committee for Aeronautics 0012 hydrofoil and the endwall have been performed to investigate the effects of cavitation on vortical structures, turbulence statistics and hydrofoil performance within the tip clearance region. The wall-adapting local eddy-viscosity model is employed to compute the subgrid-scale stress, and the Zwart-Gerber-Belamri cavitation model is used to predict the cavitating flow. The simulation results show that cavitation has insignificant effects on the tip clearance flow structures and the wandering motion of the tip leakage vortex (TLV). Detailed analysis of turbulence statistics indicates that the turbulent kinetic energy and pressure fluctuation inside the TLV core are significantly increased under the effect of cavitation. In addition, it is found that cavitation enhances the unstable wall-normal motion of the TLV, leading to the enhancement of turbulence near the TLV core. The hydrofoil performance affected by cavitation is also investigated, showing that the lift and drag forces acting on the hydrofoil is substantially reduced due to the cavitation.

Vertical Energy Fluxes Driven by the Interaction between Wave Groups and Langmuir Turbulence

Abstract Data from an air–sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave–current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long-wave theory.

A compact, low-pressure manifold with uniform flow at low Reynolds number

Manifolds that distribute fluid into or that collect fluid from a multiplicity of streams are ubiquitous. We introduce a new theory for manifold design to produce uniform flow among their streams. By constructing a tapered header region that feeds uniformly spaced diffuser channels (constraint A), flow uniformity can be achieved with less than a quarter of the footprint of bifurcating manifolds, provided that diffuser channels are arrayed in triangular form (constraint B) with a design-specific angle that satisfies a compatibility condition between its header and diffuser (constraint C). The associated theory harnesses creeping-flow hydraulics to induce a constant header pressure-gradient, in contrast with past theory that relied on the interplay between kinetic energy, pressure, and viscous losses to uniformize header pressure at finite Reynolds number. Experiments using dye-based flow visualization from manifolds incorporating these three design constraints are shown to produce uniform flows, while designs that violate any of the three constraints produce flow that is biased toward the manifold's ends or its center. Our experiments and three-dimensional simulations of such uniformizing manifolds show maximum deviations from uniformity of ∼10% for Reynolds number as high as ∼10. As expected from creeping-flow theory, simulations confirm that such flow uniformity is facilitated by a uniform header-pressure gradient. Finally, the associated uniformizing manifold is shown to produce lower hydraulic resistance than a rectangular manifold circumscribed around it. In addition to the theory's embodiment in the specific form tested here, it is readily applicable to a variety of header and diffuser-channel cross-sectional types.

Intramolecular steric and relaxation analysis of nitrogen clusters

Electron deformation density analysis were performed for ten nitrogen clusters NX (X = 4–12,14), where multiple types of fragmentations were considered for each cluster as interaction between a single isolated nitrogen molecule and the rest of cluster. Two components of electronic deformation density including steric or kinetic energy pressure and orbital relaxation were analyzed to reveal deformation density distributions and the corresponding displaced charges as quantitative description. Results show dominance of relaxation over steric interaction from both qualitative and quantitative displaced charges standpoints. Topological analysis of deformation density is also accomplished and critical points and steric paths were identified.

Momentum and energy considerations in self-aerated free-surface flows

In high-velocity free-surface flows, the interactions between the fast-flowing flow and the atmosphere can lead to strong air–water mixing through the free-surface. The flow resistance may be derived from momentum and energy considerations. This was undertaken through some theoretical development based upon an extension of traditional integral approaches to air–water flows, with the re-analyses of detailed air–water measurements in both prototype spillways and large-size physical models. Both the momentum and energy approaches yielded close results in terms of the Darcy-Weisbach friction factor. Based upon the same data sets, the absence of correction coefficients was found to under-estimate the residual energy, compared to the detailed calculations using the relevant air–water kinetic energy and pressure correction coefficients. The finding has basic design implications, which are discussed.

Heat transfer and thermal–hydraulic evaluations of cross-circular grooved rectangular flow ducts depending on rectangular baffle design parameters

The cross-grooved rectangular flow ducts are largely used for plate heat exchangers. In this work, so as to better the heat transfer in cross-grooved rectangular ducts with circular grooves, rectangular baffles were located in the flow duct, and the influences of the rectangular baffle angles and heights on the features of heat transfer were numerically performed in detail. Equations of Navier–Stokes and energy were resolved by employing a program of Ansys-Fluent with k–ε turbulence model as steady and three-dimensional. Air employed as working fluid has a temperature of inlet 293 K while the circular groove’s wall temperature is 373 K. Rectangular baffles have various angles of 30°, 60°, and 90°, and heights of 0.25 H, 0.5 H, and 0.75 H. The mean Nusselt number (Num), temperature, turbulence kinetic energy (TKE), pressure, and PEC (Performance Evaluation Criterion) number of the flow duct with rectangular baffles were evaluated by comparing it with the cross-circular grooved channel non-baffle. Besides, the contours of velocity, turbulence kinetic energy, temperature, and velocity vector were exhibited for the cross-circular grooved rectangular channels with different baffle angles and height arrangements. Results were matched with experimental and numerical outcomes of the study found in the literature, and it was observed that they were fairly coherent. For Re = 6000, the number of Num increases by 180.48% in the duct with a 90° angle and 0.75 H baffle height compared to the duct non-baffle, and for 30° baffle angle, the PEC value at 0.25 H baffle height is 66.88% higher than that in the 0.5 H.Graphical Compared to the temperature contour distribution in the non-baffle state, the temperature gradient increases due to the turbulence intensity created by the inserting of baffles to the circular grooved ducts. When the baffle height is increased to 0.75 H, the increase in fluid temperatures toward the end of the ducts is the clearest evidence of this situation. Thus, while cooling increases in circular grooves, the temperature gradient in the duct above the circular grooved section gets better, especially in the case of 0.75 H height and 90° baffle angle. Temperature distribution contours of the ducts for non-baffle and with baffle of 90° angle and 0.75 H height for Re = 6000.

Magnetic Fields of the Starless Core L 1512

We present JCMT POL-2 850 μm dust polarization observations and Mimir H-band stellar polarization observations toward the starless core L 1512. We detect the highly ordered core-scale magnetic field traced by the POL-2 data, of which the field orientation is consistent with the parsec-scale magnetic fields traced by Planck data, suggesting the large-scale fields thread from the low-density region to the dense core region in this cloud. The surrounding magnetic field traced by the Mimir data shows a wider variation in the field orientation, suggesting there could be a transition of magnetic field morphology at the envelope-scale. L 1512 was suggested to be presumably older than 1.4 Myr in a previous study via time-dependent chemical analysis, hinting that the magnetic field could be strong enough to slow the collapse of L 1512. In this study, we use the Davis–Chandrasekhar–Fermi method to derive a plane-of-sky magnetic field strength (B pos) of 18 ± 7 μG and an observed mass-to-flux ratio (λ obs) of 3.5 ± 2.4, suggesting that L 1512 is magnetically supercritical. However, the absence of significant infall motion and the presence of an oscillating envelope are inconsistent with the magnetically supercritical condition. Using a virial analysis, we suggest the presence of a hitherto hidden line-of-sight magnetic field strength of ∼27 μG with a mass-to-flux ratio (λ tot) of ∼1.6, in which case both magnetic and kinetic pressures are important in supporting the L 1512 core. On the other hand, L 1512 may have just reached supercriticality and will collapse at any time.

RESEARCH OF TRIANGULAR BAFFLE PLACEMENT EFFECT ON THE HEAT TRANSFER AND FLOW FEATURES IN CROSS-CIRCULAR GROOVED TRIANGULAR CHANNELS

In this study, heat transfer performance and fluid flow properties for cross-circular grooved triangular ducts having different location angles and heights of the triangular baffles are studied numerically. The ANSYS Fluent program is applied to solve the equations of energy and the Navier-Stokes equations by employing the <i>k</i>-ε turbulence model in computational calculations. While the inlet temperature of the working fluid (air) is 293 K, the surface temperature of the lower circular grooved channels is taken constant at 373 K. Triangular baffles have different angles of 30°, 60°, and 90° and heights of 0.25H, 0.5H, and 0.75H. The range of the Reynolds number (Re) employed in this study is 1000-6000. Numerical results agreed with a 3.53% deviation according to empirical work that existed in the literature. The attained outcomes are presented as mean Nusselt number (Nu<sub>m</sub>), fluid temperature, turbulence kinetic energy, pressure, and performance evaluation criterion (PEC) variations for each triangular baffle angle and height. Contour distributions of the temperature, velocity, turbulence kinetic energy, and velocity vector are also evaluated for distinct Re numbers and triangular baffle arrangements. For Re = 3000, the Nu<sub>m</sub> number in the channel with a 90° baffle angle and 0.75H baffle height is 136.73% higher than that in the channel with 0.25H baffle height. In the channel with 0.25H baffle height, the PEC value is 91.91% higher in the 30° baffle angle condition than in the 90° one.

The effects of rectangular baffle angles and heights on heat transfer and pressure drop performance in cross-triangular grooved rectangular flow ducts

The improvements in heat exchangers mainly focus on rising energy efficiency. The cross-grooved rectangular flow ducts are largely used for plate heat exchangers. In this study, to enhance the heat transfer and hydraulic-thermal performance in the cross-grooved rectangular ducts with triangular grooves, rectangular baffles were located in the duct, and the influences of the rectangular baffle angles and heights on the features of the heat transfer and pressure drop were numerically analyzed in detail. The equations of Navier-Stokes and energy were solved by employing the Ansys-Fluent program with the k-ε turbulence model as steady and three-dimensional. Air used as working fluid has an inlet temperature of 293 K while the triangular groove’s wall temperature is 373 K. Rectangular baffles have various angles of 30°, 60°, and 90°, and heights of 0.25H, 0.5H, and 0.75H. The mean Nusselt number (Num), dimensionless temperature (T*), turbulence kinetic energy (TKE*), pressure (p*), and performance evaluation criterion (PEC) number of the ducts with rectangular baffles were evaluated by comparing with the cross-triangular grooved duct without baffle. Also, the contours of turbulence kinetic energy, temperature, and velocity vector were exhibited for the cross-triangular grooved rectangular ducts with different arrangements of baffle angles and heights. Results were matched with experimental and computational outcomes of the study found in the literature, and it was observed that they were fairly compatible. The increment rate of Num at 90° baffle angle and 0.75H height is 197.56 % higher for Re = 6000 compared to 0.25H. Besides, for Re = 1000, the PEC number is 84.50 % higher at 0.25H baffle height and 30° angle than at 90°.

Kinetic temperature and pressure of an active Tonks gas.

Using computer simulation and analytical theory, we study an active analog of the well-known Tonks gas, where active Brownian particles are confined to a periodic one-dimensional (1D) channel. By introducing the notion of a kinetic temperature, we derive an accurate analytical expression for the pressure and clarify the paradoxical behavior where active Brownian particles confined to 1D exhibit anomalous clustering but no motility-induced phase transition. More generally, this work provides a deeper understanding of pressure in active systems as we uncover a unique link between the kinetic temperature and swim pressure valid for active Brownian particles in higher dimensions.

Structure of a Quasi-parallel Shock Front

The aim of this study is to compare observations of the magnetic field structure of observed quasi-parallel collisionless shock fronts with the results obtained analytically. A two-fluid analytic model of the shock front structure was derived under the assumptions that the shock is stationary and planar. The ion and electron kinetic pressures were assumed to be scalar, and polytropic state equations were used. The results of this analytical approach show that the shock magnetic field has an oscillatory structure. Venus Express (VEX) observations of the Venusian bow shock have been used to validate these theoretical findings. The Venusian bow shock and corresponding foreshock are significantly smaller than those of Earth. Thus, observations of the underlying structure of the quasi-parallel shock at Venus are not masked by the presence of high-amplitude waves and nonlinear structures originating in the foreshock. It is shown that the structure of the shock front, as observed by VEX, has a very strong similarity to the structure obtained analytically.

Influence of trailing edge flap length and deflection angle on the performance of the multiphase pump

At medium to high gas content rates, the gas-liquid separation phenomenon and the stagnation behaviors of the gas masses occurring in the impeller channels are essential factors affecting the hydraulic performance of multiphase pumps. This paper designs a split trailing edge flap suitable for the multiphase pump to improve the hydraulic performance in the impeller by varying the flap deflection angle and length. The model’s reliability was assessed by comparing the experimental outcome with the external characteristics of the experimental data. Based on the Eulerian-Eulerian multiphase model and the SST k‐ω turbulence model, the effects of different flap lengths on external characteristics, the turbulent kinetic energy and pressure distribution are analyzed. The study results show that when the length of the separated trailing edge flap is 0.25 l0 and the deflection angle is 5°, the transporting efficiency of the multiphase pump is the highest at IGVF=50% with an efficiency increase of 3.40% compared to the original model. Its optimal flap length scheme effectively reduces the radial pressure gradient and the inverse pressure gradient in the impeller channel, which suppresses the gas blockage phenomenon to a certain extent.

Thermohydraulic performance assessment of a solar air heater with equilateral‐triangular, trapezoidal, and square sectional ribs on the absorber plate: A comparative study

AbstractHeat transfer systems have been employed in numerous industrial and residential settings to convert and recover heat. One effective approach for enhancing heat transfer rates involves incorporating artificial roughness in the form of regularly spaced ribs on the heated surface. In this particular study, the researchers conducted numerical simulations to analyze the turbulent heat and flow properties within a rectangular duct of a solar air heater (SAH). The duct featured three different types of transverse rib shapes: equilateral‐triangular, trapezoidal, and square sectional ribs. The governing equations have been solved numerically using a finite‐volume technique via ANSYS FLUENT. For solving the transport equations of turbulent kinetic energy and its dissipation rate, the RNG k– turbulence model was utilized. At a broad range of Reynolds numbers (6000 ≤ Re ≤ 18,000), relative roughness height (), and a fixed relative roughness pitch (), the Nusselt number (), friction factor (), and thermal performance factor (TPF) have been examined. The maximal Nu was obtained for equilateral‐triangular ribs, while the minimum friction factor was seen for square sectional ribs. The highest TPF of 1.591 was achieved with equilateral‐triangular ribs at and e/Dh = 0.032. Contours of kinetic energy, turbulent intensity, velocity, and pressure produced significant findings for comprehending the SAH's physical phenomena.

Experimental and numerical simulation-based investigation for thermal characteristic of frustum roughened solar air heater

ABSTRACT This research paper aims to investigate the effect of frustum shape roughness geometry on the thermal characteristic (Nusselt no. and friction factor) of solar air heaters (SAH). The study includes experimental and numerical investigation of a solar air heater equipped with frustum roughened absorber plate under varying operating conditions. Frustum shape roughness element produced more disturbances and greater mixing of primary and secondary flow that resulted in greater augmentation in thermal characteristics of SAH. The investigation was conducted in varying Reynolds number range of 2500 -12,500. Dimensionless parameter for Frustrum shaped roughness is varied as relative frustum height (e/Dh) 0.013–0.054, relative frustum pitch (p/e) 8–14, Relative frustum height to base diameter (e/d1) 0.37–0.75 and relative frustum diametral ratio (d1/d2) 1–3. To save time and resources a three-dimensional numerical simulation was also performed. This simulation helped to visualize flow field behavior with the help of streamline, temperature, turbulence kinetic energy and pressure contours. Maximum performance for roughened system was obtained at optimum value of flow and geometric configuration within the range of investigated parameters. The geometric orientation of proposed roughness geometry produced tremendous rise in Nusselt number (Nu) with an acceptable rise in friction (f). Maximum enhancement in Nu for varying p/e, e/Dh, e/d1, and d1/d2 was, 4.58, 5.21, 6.62 and 6.09 times and that of friction was 5.59, 4.69, 4.92 and 4.66 times respectively over conventional smooth SAH.

How accurate are flux-tube (local) gyrokinetic codes in modeling energetic particle effects on core turbulence?

Flux-tube (local) gyrokinetic codes are widely used to simulate drift-wave turbulence in magnetic confinement devices. While a large number of studies show that flux-tube codes provide an excellent approximation for turbulent transport in medium-large devices, it still needs to be determined whether they are sufficient for modeling supra-thermal particle effects on core turbulence. This is called into question given the large temperature of energetic particles (EPs), which makes them hardly confined on a single flux-surface, but also due to the radially broad mode structure of EP-driven modes. The primary focus of this manuscript is to assess the range of validity of flux-tube codes in modeling fast ion effects by comparing radially global turbulence simulations with flux-tube results at different radial locations for realistic JET parameters using the gyrokinetic code GENE. To extend our study to a broad range of different plasma scenarios, this comparison is made for four different plasma regimes, which differ only by the profile of the ratio between the plasma kinetic and magnetic pressure. The latter is artificially rescaled to address the (i) electrostatic limit and regimes with (ii) marginally stable, (iii) weakly unstable and (iv) strongly unstable fast ion modes. These EP-driven modes are identified as Alfvénic ion temperature gradient modes (AITG)/kinetic beta-induced Alfvén eigenmodes (KBAE) via linear ORB5 and LIGKA simulations. It is found that the local flux-tube simulations can recover well the global results only in the electrostatic and marginally stable cases. When the AITG/KBAE becomes linearly unstable, the local approximation fails to correctly model the radially broad fast ion mode structure and the consequent global zonal patterns. According to this study, global turbulence simulations are likely required in regimes with linearly unstable AITG/KBAEs. In conditions with different fast ion-driven modes, these results might change.

Signature of transition in granular flow through apertures

We employ computations to examine transition in granular flow, right above an aperture, in a silo draining under gravity. We show that the energy dissipation rate exhibits a well-defined peak near the aperture. Tracing locations of maximum dissipation rate describes an arch, signifying flow transition. We present a correlation between the transition boundary and outlet using the arguments of self-similarity and geometric similarity, suggesting the spatial non-monotonicity of the dissipation rate to be a signature of flow transition. Utilizing the aforementioned similarity criteria, we also explore the kinetic pressure as a marker of transition in granular flow.

Physics and Chemistry of Radiation Driven Cloud Evolution. [C ii] Kinematics of IC 59, and IC 63

We used high-resolution [C ii] 158 μm mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data of the gas, dust, and polarization to probe the kinematics, structure, and magnetic properties of their photodissociation regions (PDRs). The nebulae are part of the Sh 2-185 H ii region that is illuminated by the B0 IVe star γ Cas. The velocity structure of each PDR changes with distance from γ Cas, which is consistent with driving by the radiation. Based on previous far-ultraviolet (FUV) flux measurements of, and the known distance to, γ Cas, along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (G 0) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances from γ Cas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al., locating the cloud at ∼2 pc from γ Cas at an angle of 58° to the plane of the sky behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70° in front of the star. We do not detect any significant correlation between the orientation of the magnetic field and the velocity gradients of [C ii] gas, which indicates a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be an order of 10 higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.

Dependence of Soft X-Ray Yield on Plasma Focus Parameters and Its Kinetic and Magnetic Pressures

Dependence of Soft X-Ray Yield on Plasma Focus Parameters and Its Kinetic and Magnetic Pressures

Polytropic representation of non-isotropic kinetic pressure tensor for non-ideal plasma fluids in relativistic jets

Non-ideal fluids are likely to be affected by the occurrence of pressure anisotropy effects, whose understanding for relativistic systems requires knowledge of the energy–momentum tensor. In this paper, the case of magnetized jet plasmas at equilibrium is considered, in which both microscopic velocities of constituent particles and the continuum fluid flow are treated as relativistic ones. A theoretical framework based on covariant statistical kinetic approach is implemented, which permits the proper treatment of single-particle and phase-space kinetic constraints and, ultimately, the calculation of the system continuum fluid fields associated with physical observables. A Gaussian-like solution for the kinetic distribution function (KDF) is constructed, in which the physical mechanism responsible for the generation of temperature anisotropy is identified with magnetic moment conservation. A Chapman–Enskog representation of the same KDF is then obtained in terms of expansion around an equilibrium isotropic Juttner distribution. This permits the analytical calculation of the fluid 4-flow and stress–energy tensor and the consequent proof that the corresponding kinetic pressure tensor is non-isotropic. As a notable result, the validity of a polytropic representation for the perturbative non-isotropic pressure contributions is established, whereby directional pressures exhibit specific power-law functional dependences on fluid density.