Turbulence Suppression Research Articles

Study on the flow structure and its effects on the interaction and formation of cryogenic taylor bubbles by PIV technique and POD analysis

AbstractThe flow structures around continuous cryogenic Taylor bubbles in the concentric glass tube with different diameters were investigated using particle image velocimetry (PIV) technique under various inclination angles. Optical distortions produced by tube curvature and the refractive index differences between tube and fluid were corrected using ray tracing method. Based on the PIV results, the proper orthogonal decomposition (POD) method was used to investigate the effect of flow structure on the interaction of Taylor bubbles through the large‐scale turbulent structures of POD modes. Four best‐fitted curves for Taylor bubble interaction for different inclination angles were represented. A correlation for Taylor bubble formation was developed with the consideration of turbulence suppression, tube inner diameter, fluid properties, inclination angle, etc. The results demonstrate that the minimum stable distance attains the smallest value at . The correlation for Taylor bubble formation shows good agreement with the experimental data both in the present study and from its references.

Modeling of bubble coalescence and break-up considering turbulent suppression phenomena in bubbly two-phase flow

New mechanistic bubble coalescence and break-up models considering turbulent suppression phenomena, which can possibly occur in the high liquid velocity condition of turbulent bubbly two-phase flow, are presented. The energy exchange mechanism between a turbulent eddy and interfacial structure was taken into account in the efficiency terms. Numerical simulations of turbulent bubbly flow were conducted in a CFD code to evaluate the newly developed models, in comparison with other advanced models coupled with a bubble-induced turbulent effect for one-group interfacial area transport equation. Local measurements of the bubble characteristics on the bubble size evolution along a vertical pipe flow were performed at KAERI-VAWL test facility using the five-sensor conductivity probe method to provide database for models validation. Results from the calculation clearly show the improvements of the newly developed models.

New Paradigm for Suppression of Gyrokinetic Turbulence by Velocity Shear

The shear in the mean field velocity Doppler shift is shown to suppress the amplitude of electric potential fluctuations by inducing a shift in the peak of the radial wave number spectrum. An analytic model of the process shows that the fluctuation spectrum shifts in the direction where the velocity shear is linearly destabilizing but that nonlinear mixing causes a recentering of the spectrum about a shifted radial wave number at reduced amplitude A model for the 2D nonlinear spectrum is used in a quasilinear calculation of the transport that is shown to accurately reproduce the suppression of energy and particle transport and the Reynolds stress due to the velocity shear.

S051032 界面活性剤の添加による歳差球体内乱流の変調([S051-03]複雑流体の流動現象(3))

Precession is the motion that the spin axis of a rotating object rotates about another axis. A precessing sphere generates and sustains turbulence of a confined fluid. Using particle image velocimetry, we investigate turbulent velocity fields on the equatorial plane of the precessing sphere. With the additions of cationic surfactant(CTAC) and counterion, sodium salicylate(NaSal), turbulence suppression is observed in a central region of the sphere. When themolar concentration of CTAC(C_D) is kept constant and that of NaSal(C_S) increases, in the range of C_S/C_D ≳ 1, turbulence is suppressed, while such a suppression is not observed for C_S/C_D = 0.5. For C_S/C_D ≳ 1, there are fully entangled threadlike micelles in the solution. Moreover, free salicylate ions exist and they promote reconnections of threadlike micelles. This may be the reason why turbulence suppression in a precessing sphere is sustained for a long time in the cases with rich free salicylate ions.

PET高速溶融紡糸過程のオンライン広角X線散乱測定

We report results of on-line wide-angle X-ray scattering (WAXS) measurements for melt spinning process of poly(ethylene terephthalate) (PET). In order to obtain good results, we have conducted experiments by paying attention as much as possible. First of all, a special experimental set-up has been developed by employing a purge box which contains the spin line and the X-ray detector (imaging plate), as well, with helium gas purge, in order to minimize noise level due to air scattering. Furthermore, a high-molecular weight PET sample prepared by solid-state polymerization was used in order to increase the spin line stress, which accommodates us with starting of orientation-induced crystallization at low take-up velocities. A special care to minimize the swaying of the spin line has been taken as well by employing a perforated metal guard inside the spinning tube which covers the spin line from the spinneret to the winder with insertion of the helium gas purge box in the middle. This worked well for suppression of the air turbulence. By such special experimental treatments, we have succeeded in conducting on-line two-dimensional WAXS measurements during the high speed melt spinning of PET fibers with the take-up velocity of 1600m/min. As a high brilliant X-ray source, we utilized SPring-8 (Japan Synchrotron Radiation Research Institute, Harima, Japan). The combination of above mentioned techniques enabled us to obtain crystalline reflections from oriented crystals with only 2min exposure. Nevertheless, we observed no appreciable change in crystallinity and orientation of crystallites along the spin line. It is because the control of positioning the neck point where the crystallization sets in was sensitively influenced by the ambient gas condition. Further special care to overcome this difficulty should be taken in our future study.

Suppression of drift wave turbulence and zonal flow formation by changing axial boundary conditions in a cylindrical magnetized plasma device

For drift wave turbulence, due to charge conservation, the divergence of the parallel current is coupled to the divergence of the perpendicular polarization current, which determines the effective radial momentum flux, i.e., the Reynolds stress. Changes in the current flow patterns also affect the nonlinear energy transfer from smaller to larger scales. Here, we show that by changing the end plate boundary conditions in a cylindrical plasma device, the radial currents through the plasma and hence the net momentum transport and the nonlinear coupling for the inverse energy transfer are strongly modified. The transition to drift wave turbulence and the formation of low frequency zonal flows can be either suppressed with conducting boundaries or enhanced with insulating boundaries.

RANS-based simulation of turbulent wave boundary layer and sheet-flow sediment transport processes

A numerical model coupling the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations with two-equation k−ω turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes. The hydrodynamic model is additionally coupled with bed and suspended load descriptions, the latter based on an unsteady turbulent-diffusion equation, for simulation of sheet-flow sediment transport processes. In addition to standard features common within such RANS-based approaches, the present model includes: (1) hindered settling velocities at high suspended sediment concentrations, (2) turbulence suppression due to density gradients in the water–sand mixture, (3) boundary layer streaming due to convective terms, and (4) converging–diverging effects due to a sloping bed. The present model therefore provides a framework for simultaneous inclusion of a number of local factors important within cross-shore wave boundary layer and sediment transport dynamics. The hydrodynamic model is validated for both hydraulically smooth and rough conditions, based on wave friction factor diagrams and boundary layer streaming profiles, with the results in excellent agreement with experimental and/or previous numerical work. The sediment transport model is likewise validated against oscillatory tunnel experiments involving both velocity-skewed and acceleration-skewed flows, as well as against measurements beneath real progressive waves. Model capabilities are exploited to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity-skewed conditions. For the medium sand grain conditions considered, the model results suggest that streaming effects can enhance onshore sediment transport rates by as much as a factor of two. Moreover, for fine sand conditions streaming (and related convective) effects are demonstrated to potentially reverse the direction of net transport (i.e. from offshore to onshore) relative that predicted in oscillatory tunnel conditions. The developed model is implemented within the popular Matlab environment, and hence may be attractive for both research and educational purposes.

Simulations of ITER in the presence of ITB using the NTV intrinsic toroidal rotation model

Simulations of a standard H-mode International Thermonuclear Experimental Reactor (ITER) scenario in the presence of internal transport barrier (ITB) are carried out using the 1.5D BALDUR integrated predictive modelling code. The intrinsic offset toroidal rotation, which can play an essential role in turbulent transport suppression that results in the ITB formation, is theoretically calculated using a model based on the neoclassical toroidal viscosity (NTV) concept. The core transport in this simulation is a combination of a mixed Bohm/gyro-Bohm anomalous transport model and an NCLASS neoclassical transport model. The boundary condition of the simulations is taken to be at the top of the pedestal where the pedestal value is calculated using the pedestal model based on a combination of pedestal width scaling determined by magnetic/flow shear stabilization and an infinite-n ballooning pressure gradient model. It is found that the predicted intrinsic rotation can result in the formation of ITB, locating mostly between r/a = 0.6 and 0.8 and having a strong impact on the plasma performance in ITER. It is also found that the variations of plasma density and heating power result in a minimal change in toroidal rotation; whereas the increase in plasma effective charge can considerably reduce the toroidal velocity peaking.

Magnetic turbulence suppression by a helical mode in a cylindrical geometry

To study processes involved in a helical structure formation in reversed field pinch devices, the scaling of a turbulent boundary layer width associated with a vortex structure having large shears of magnetic field and flow is obtained for reduced magnetohydrodynamics. The coherent vortex, with its flow and magnetic shears, interacts with Alfvén turbulence, forming a turbulent boundary layer at the edge of the vortex. The layer arises from the balance between turbulence diffusion rates and shearing rates and suppresses the turbulence in the structure. The suppression of turbulence impedes relaxation of the coherent vortex profiles, leading to long coherence times. The scaling of the boundary layer width reveals that both magnetic shear and flow shear can effectively suppress magnetic turbulence.

Three-dimensional flow dynamics of an argon RF plasma with dc jet assistance: a numerical study

Time-dependent three-dimensional numerical simulation based on a large-eddy simulation approach is conducted to ascertain the complicated thermofluid dynamics of an argon radio-frequency (RF) inductively coupled plasma with a direct-current (dc) plasma jet assistance, considering non-uniform densities and properties in time and space as well as turbulence generation and suppression. Using a combination of numerical schemes suitable to capture vortices, the present simulation successfully shows unsteady behaviour of the plasma as well as wave-like interfaces between a high-temperature flow and a low-temperature flow as a result of the balance of fluid-dynamical instability and a viscous diffusion effect. Small cold vortices generated near a dc jet injector are entrained into and merged with vortices generated around the dc jet. Subsequently, they interact with large vortices in an RF induction coil region, which causes a much more complex vortex structure.

A New Analysis of Heat Transfer Deterioration on Basis of Turbulent Viscosity Variations of Supercritical Fluids

A two-dimensional computational fluid dynamics (CFD) code has been used to study the anomalies encountered in convection heat transfer to upward turbulent flows of supercritical fluids in tubes. In this study, the effect of turbulent viscosity variations on heat transfer deterioration (HTD) and the mechanisms involved have been investigated. The results show that the suppression of the flow turbulence which leads to the deterioration of heat transfer can be partially due to the decrease in the turbulent viscosity as a result of density decrease along a heated flow. Before this study the buoyancy and the thermal acceleration effects were called as the main two known mechanisms for the heat transfer deterioration.

Modification of Turbulent Transport with Continuous Variation of Flow Shear in the Large Plasma Device

Continuous control over azimuthal flow and shear in the edge of the Large Plasma Device (LAPD) has been achieved using a biasable limiter which has allowed a careful study of the effect of flow shear on pressure-gradient-driven turbulence and transport in the LAPD. The LAPD rotates spontaneously in the ion diamagnetic direction; positive limiter bias first reduces, then minimizes (producing a near-zero shear state), and finally reverses the flow into the electron diamagnetic direction. Degradation of particle confinement is observed in the minimum shearing state and a reduction in the turbulent particle flux is observed with increasing shearing in both flow directions. Near-complete suppression of the turbulent particle flux is observed for shearing rates comparable to the turbulent autocorrelation rate measured in the minimum shear state. Turbulent flux suppression is dominated by amplitude reduction in low-frequency (<10 kHz) density fluctuations. An increase in fluctuations for the highest shearing states is observed with the emergence of a coherent mode which does not lead to net particle transport. The variations of density fluctuations are fit well with power laws and compare favorably to simple models of shear suppression of transport.

Dynamics of complete turbulence suppression in turbidity currents driven by monodisperse suspensions of sediment

AbstractTurbidity currents derive their motion from the excess density imposed by suspended sediments. The settling tendency of sediments is countered by flow turbulence, which expends energy to keep them in suspension. This interaction leads to downward increasing concentration of suspended sediments (stable stratification) in the flow. Thus in a turbidity current sediments play the dual role of sustaining turbulence by driving the flow and damping turbulence due to stable stratification. By means of direct numerical simulations, it has been shown previously that stratification above a threshold can substantially reduce turbulence and possibly extinguish it. This study expands the simplified model by Cantero et al. (J. Geophys. Res., vol. 114, 2009a, C03008), and puts forth a proposition that explains the mechanism of complete turbulence suppression due to suspended sediments. In our simulations it is observed that suspensions of larger sediments lead to stronger stratification and, above a threshold size, induce an abrupt transition in the flow to complete turbulence suppression. It has been widely accepted that hairpin and quasi-streamwise vortices are key to sustaining turbulence in wall-bounded flows, and that only vortices of sufficiently strong intensity can spawn the next generation of vortices. This auto-generation mechanism keeps the flow populated with hairpin and quasi-streamwise vortical structures and thus sustains turbulence. From statistical analysis of Reynolds stress events and visualization of flow structures, it is observed that settling sediments damp the Reynolds stress events (Q2 events), which means a reduction in both the strength and spatial distribution of vortical structures. Beyond the threshold sediment size, the existing vortical structures in the flow are damped to an extent where they lose their ability to regenerate the subsequent generation of turbulent vortical structures, which ultimately leads to complete turbulence suppression.

Suppression of turbulence with linear velocity profile in high-temperature plasma

Suppression of turbulence with linear velocity profile in high-temperature plasma

Spatio-temporal evolution of the L → I → H transition

We investigate the dynamics of the low(L) → high(H) transition using a time-dependent, one dimensional (in radius) model which self-consistently describes the time evolution of zonal flows (ZFs), mean flows (MFs), poloidal spin-up, and density and pressure profiles. The model represents the physics of ZF and MF competition, turbulence suppression via E×B shearing, and poloidal flows driven by turbulence. Numerical solutions of this model show that the L→H transition can occur via an intermediate phase (I-phase) which involves oscillations of profiles due to ZF and MF competition. The I-phase appears as a nonlinear transition wave originating at the edge boundary and propagates inward. Locally, I-phase exhibits the characteristics of a limit-cycle oscillation. All these observations are consistent with recent experimental results. We examine the trigger of the L→H transition, by defining a ratio of the rate of energy transfer from the turbulence to the zonal flow to the rate of energy input into the turbulence. When the ratio exceeds order unity, ZF shear gains energy, and a net decay of the turbulence is possible, thus triggering the L→H transition. Numerical calculations indicate that the L→H transition is triggered by this peak of the normalized ZF shearing. Zonal flows act as “reservoir,” in which to store increasing fluctuation energy without increasing transport, thus allowing the mean flow shear to increase and lock in the transition. A counterpart of the L → I→H transition, i.e., an L→H transition without I-phase, is obtained in a fast power ramp, for which I-phase is compressed into a single burst of ZF, which triggers the transition. Effects of neutral charge exchange on the L→H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L→H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L→H threshold. Studies of fueling effects on the transition and pedestal structure with an emphasis on the particle pinch are reported.

A statistical analysis of avalanching heat transport in stationary enhanced core confinement regimes

We present a statistical analysis of heat transport in stationary enhanced confinement regimes obtained from flux-driven gyrofluid simulations. The probability density functions of heat flux in improved confinement regimes, characterized by the Nusselt number, show significant deviation from Gaussian, with a markedly fat tail, implying the existence of heat avalanches. Two types of avalanching transport are found to be relevant to stationary states, depending on the degree of turbulence suppression. In the weakly suppressed regime, heat avalanches occur in the form of quasi-periodic (QP) heat pulses. Collisional relaxation of zonal flow is likely to be the origin of these QP heat pulses. This phenomenon is similar to transient limit cycle oscillations observed prior to edge pedestal formation in recent experiments. On the other hand, a spectral analysis of heat flux in the strongly suppressed regime shows the emergence of a 1/f (f is the frequency) band, suggesting the presence of self-organized criticality (SOC)-like episodic heat avalanches. This episodic 1/f heat avalanches have a long temporal correlation and constitute the dominant transport process in this regime.

Influence of active control on STG-based generation of streamwise vortices in near-wall turbulence

AbstractNear-wall streamwise vortices are closely related to the generation of high skin friction in wall-bounded turbulent flows. A common feature of controlled, friction-reduced turbulent flows is weakened near-wall streamwise vortices. In the present study, the streak transient growth (STG) mechanism for generating near-wall streamwise vortices by Schoppa &amp; Hussain (J. Fluid Mech., vol. 453, 2002, pp. 57–108) is employed, and the opposition control proposed by Choi, Moin &amp; Kim (J. Fluid Mech., vol. 262, 1994, pp. 75–110) is imposed during the transient growth process of perturbations to determine how active control affects the generation of quasi-streamwise vortices. In the transient growth stage, when the detection plane is located near the wall (${ y}_{d}^{+ } = 15$), the control can suppress the production of streamwise vorticity by weakening the near-wall vertical velocity; when the detection plane moves away from the wall (${ y}_{d}^{+ } = 28$), the control has the opposite effect. In the vortex generation stage, the control cannot change the dominance of the stretching effect. Controls imposed at different stages reveal the importance of the STG stage in vortex generation. Strengthened out-of-phase control and lessened in-phase control are proposed as an extension of the original opposition-control scheme. Application in a fully developed turbulent channel flow shows that strengthened ${ y}_{d}^{+ } = 10$ control can yield an even higher drag reduction rate than the original ${ y}_{d}^{+ } = 15$ control. Moreover, lessened ${ y}_{d}^{+ } = 28$ control can also achieve drag reduction and turbulence suppression.

Role of Reynolds stress and toroidal momentum transport in the dynamics of internal transport barriers

We study the interplay between intrinsic rotation and internal transport barrier (ITB) dynamics through the dynamic change of the parallel Reynolds stress. Global flux-driven gyrofluid simulations are used for this study. In particular, we investigate the role of parallel velocity gradient instability (PVGI) in the ITB formation and the back transition. It is found that the excitation of PVGI is followed by a change in the Reynolds stress which drives a momentum redistribution. This significantly influences E×B shear evolution and subsequent ITB dynamics. Nonlocal interactions among fluctuations are also observed during the PVGI excitation, resulting in turbulence suppression at the ITB.

Feedback and Outflows

AbstractThe low stellar and gas mass fractions, low galaxy-wide star formation rates (relative to galactic dynamical times) and observations of rapid outflows from galaxies, all suggest that stars and active galactic nuclei violently alter the state of the interstellar and even inter-halo gas in galaxies. I argue that the low galaxy wide star formation rates are not the result of turbulent suppression of star formation on small scale, but rather the result of a balance between dynamical pressure and the force (or rate of momentum deposition) provided by stellar feedback, either in the form of radiation pressure or by supernovae. Galaxy scale winds can also be driven by feedback, either from stars or active galactic nuclei, although the exact mechanisms involved are still not well determined.

Towards a universal criteria for turbulence suppression in dilute turbidity currents with non‐cohesive sediments

Turbidity currents exhibit fascinating physics as their sustained propagation depends on a tight interplay between the suspended sediments and turbulence. If resuspension dominates over deposition the intensity of the flow will increase, while if deposition dominates the flow turbulence can be completely damped inducing rapid settling of sediments and, eventually, flow extinction. This work explores the phenomenon whereby turbulence in a dilute turbidity current with non‐cohesive sediments is abruptly extinguished owing to increased suspended sediment stratification. Three parameters control the flow dynamics: Reynolds number (Reτ), Richardson number (Riτ) and sediment settling velocity ( ). The condition for complete turbulence suppression can be expressed as a critical value for . Based on simulations, limited experiments and limited field data, the critical value appears to have a logarithmic dependence on Reτ.