Background Flow Velocity Research Articles

Quasi-geostrophic monopoles in sheared zonal jets and multiple-jet flows

This work continues our earlier studies of the interaction between a monopolar vortex and a sheared zonal flow in the framework of a 1.5-layer quasi-geostrophic model, based on numerical experiments with singular vortices. Earlier examination of flows with shears of fixed sign showed that the interaction depends strongly on the latitudinal distribution of the gradient of background potential vorticity b(y) (y being the latitude). The latitude y0 at which b(y) changes sign turns out to be of particular importance. In the vicinity of y0, under certain conditions, there arises the zonal-strip region, which attracts (repels) prograde (retrograde) vortices. This effect is examined here for the zonal flows in the form of individual jets as well as for the systems of alternating zonal jets; in all these cases, the background-flow velocity shear and the parameter b(y) can change sign depending on y. It is shown that the vortex drifts to the nearest latitude y0 on the prograde side of the zonal flow, and the meridional speed of the trapped vortex almost vanishes, but its zonal speed is directed westward and approaches the Rossby-wave drift velocity.

Volumetric lattice Boltzmann method for pore-scale mass diffusion-advection process in geopolymer porous structures

Porous materials present significant advantages for absorbing radioactive isotopes in nuclear waste streams. To improve absorption efficiency in nuclear waste treatment, a thorough understanding of the diffusion-advection process within porous structures is essential for material design. In this study, we present advancements in the volumetric lattice Boltzmann method (VLBM) for modeling and simulating pore-scale diffusion-advection of radioactive isotopes within geopolymer porous structures. These structures are created using the phase field method (PFM) to precisely control pore architectures. In our VLBM approach, we introduce a concentration field of an isotope seamlessly coupled with the velocity field and solve it by the time evolution of its particle population function. To address the computational intensity inherent in the coupled lattice Boltzmann equations for velocity and concentration fields, we implement graphics processing unit (GPU) parallelization. Validation of the developed model involves examining the flow and diffusion fields in porous structures. Remarkably, good agreement is observed for both the velocity field from VLBM and multiphysics object-oriented simulation environment (MOOSE), and the concentration field from VLBM and the finite difference method (FDM). Furthermore, we investigate the effects of background flow, species diffusivity, and porosity on the diffusion-advection behavior by varying the background flow velocity, diffusion coefficient, and pore volume fraction, respectively. Notably, all three parameters exert an influence on the diffusion-advection process. Increased background flow and diffusivity markedly accelerate the process due to increased advection intensity and enhanced diffusion capability, respectively. Conversely, increasing the porosity has a less significant effect, causing a slight slowdown of the diffusion-advection process due to the expanded pore volume. This comprehensive parametric study provides valuable insights into the kinetics of isotope uptake in porous structures, facilitating the development of porous materials for nuclear waste treatment applications.

A numerical analysis of background flow velocity effects on long-term post-injection migration of CO2 plumes in tilted storage aquifers

A numerical analysis of background flow velocity effects on long-term post-injection migration of CO2 plumes in tilted storage aquifers

Analytical Model for the Generation of Vorticity Due to Inhomogeneous Friction on the Underlying Surface

In a number of recent publications, attention is drawn to the heterogeneity of the underlying surface as a factor that can contribute to the initiation and intensification of tornadoes. The paper proposes an analytical model for the generation of vorticity under the influence of horizontally inhomogeneous friction. The resulting vorticity is proportional to the background flow velocity and to the horizontal gradient of the drag coefficient transverse to this flow; dependence on other factors is relatively weak. Numerical estimates show the possibility of efficient vorticity generation.

Experimental Study on the Effects of Waves and Current on Ice Melting

Ice melting plays a crucial role in ocean circulation and global climate. Laboratory experiments were used to study the dynamic mechanisms of the influence of waves and currents on ice melting. The results showed that under near stable air temperature and water temperature conditions, the ice melting rate was significantly greater with waves than that without waves, as well as the higher the wave height, the greater the melting rate. This is related to the increase in the contact area between ice and water by waves. Further research was carried out to observe the flow field at different locations on the ice bottom, ice sides, and behind the ice by particle image velocimetry (PIV) and dyeing experiments. At different flow velocities, the changes in the side melting rate and bottom melting rate were not the same. Meltwater is attached to the bottom in the form of plume at low background flow velocity, which leads to the slowness of the heat exchange between the ice with a higher ambient temperature. Therefore, the melting of the ice bottom and the ice side were slower at low flow velocity. At high background flow velocity, there is strong shear instability and vortex at the ice bottom and behind the ice. The dissipation and mixing effects caused by vortices accelerate the melting of the ice bottom and the ice back. The thermodynamic factors, such as air temperature and water temperature, had significant impacts in the experiments. Further research needs to improve the accuracy of temperature control of experimental equipment. Computational fluid dynamics and sensitive tests of numerical simulation may also be carried out on ice melting.

A numerical solver for investigating the space charge effect on theelectric field in liquid argon time projection chambers

This paper reports the development of a numerical solver aimed to simulate the interaction between the space charge (i.e. ions) distribution and the electric field in liquid argon time projection chamber (LArTPC) detectors. The ion transport equation is solved by a time-accurate, cell-centered finite volume method and the electric potential equation by a continuous finite element method. The electric potential equation updates the electric field which provides the drift velocity to the ion transport equation. The ion transport equation updates the space charge density distribution which appears as the source term in the electric potential equation. The interaction between the space charge distribution and the electric field is numerically simulated within each physical time step. The convective velocity in the ion transport equation can include the background flow velocity in addition to the electric drift velocity. The numerical solver has been parallelized using the Message Passing Interface (MPI) library. Numerical tests show and verify the capability and accuracy of the current numerical solver. It is planned that the developed numerical solver, together with a Computational Fluid Dynamics (CFD) package which provides the flow velocity field, can be used to investigate the space charge effect on the electric field in large-scale particle detectors.

Geostrophic Eddies Spread Near-Inertial Wave Energy to High Frequencies

Abstract The generation of broadband wave energy frequency spectra from narrowband wave forcing in geophysical flows remains a conundrum. In contrast to the long-standing view that nonlinear wave–wave interactions drive the spreading of wave energy in frequency space, recent work suggests that Doppler-shifting by geostrophic flows may be the primary agent. We investigate this possibility by ray tracing a large number of inertia–gravity wave packets through three-dimensional, geostrophically turbulent flows generated either by a quasigeostrophic (QG) simulation or by synthetic random processes. We find that, in all cases investigated, a broadband quasi-stationary inertia–gravity wave frequency spectrum forms, irrespective of the initial frequencies and wave vectors of the packets. The frequency spectrum is well represented by a power law. A possible theoretical explanation relies on the analogy between the kinematic stretching of passive tracer gradients and the refraction of wave vectors. Consistent with this hypothesis, the spectrum of eigenvalues of the background flow velocity gradients predicts a frequency spectrum that is nearly identical to that found by integration of the ray tracing equations.

Importance of Background Vorticity Effect and Doppler Shift in Defining Near‐Inertial Internal Waves

AbstractNear‐inertial waves contain a significant fraction of the ocean's internal wave energy and can propagate long distances from their source before dissipating. However, a varying background flow velocity can alter the wave propagation in two ways. The background vorticity modifies the lower bound of the wave frequency bandwidth while Doppler shifts alter the wave intrinsic frequency. Both effects complicate the identification of the waves and the quantification of their energy content. This study analyses the output of a realistic simulation of the North Pacific using adaptive frequency filters to isolate the effects of vorticity and Doppler shift on the apparent wave energy. Spectral filters neglecting background vorticity effects results in apparent near‐inertial energy being underestimated by 40% in anticyclonic structures and overestimated by 100% in cyclonic structures. The asymmetry in energy bias is reinforced when both background vorticity and Doppler shift effects are omitted from the frequency filters.

Diffraction of shock waves through a non-quiescent medium

An investigation of shock diffraction through a non-quiescent background medium is presented using both experimental and numerical techniques. Unlike diffracting shocks in quiescent media, a spatial distortion of the shock front occurs, producing a region of constant shock angle. An example of this process arises in the exhaust from a pulse-detonation combustor. As the background velocity is increased, such as through the inclusion of a converging nozzle at the exhaust, the spatial distortion becomes more apparent. Numerical simulations using a compressible Euler solver demonstrate that the distortion is not due to the geometrical influence of the nozzle, but rather is a function of the magnitude of the background flow velocity. The distortion is studied using a modified geometrical shock dynamics formulation which includes the background flow and is validated against experiments. A simple model is presented to predict the shock distortion angle in the weak-shock limit. Finally, the axial decay behaviour of the shock is investigated and it is shown that the advection of the shock by the background flow delays the arrival of the head and tail of the expansion characteristic at the centreline. This leads to an increase in the rate of decay of the shock Mach number as the background flow velocity is increased.

Urban street canyon flows under combined wind forcing and thermal buoyancy

In this work, we investigate buoyancy-driven flows within urban street canyon cavities of three aspect ratios under simultaneous inertial wind forcing. The main aim of this work is to enhance the understanding of induced urban airflow patterns under non-isothermal conditions through experimental investigation, which to date are relatively scarce. The experimental results can be used for corresponding computational fluid dynamics simulations. Scaled-down models of typical street-canyon cavity geometries were deployed inside a water channel, where different ambient atmospheric conditions were simulated using dimensional analysis and similarity criteria. Three model street-canyon cavities were examined with height-to-width (aspect) ratios of 2/3, 1, and 2. The thermal buoyancy forcing was applied by means of differential heating between the two canyon side antagonistic walls for a given background flow velocity well-above the canyon height. The non-dimensional parameter B was used to quantify the influences of buoyancy and inertial forcing on the urban-canyon flow, as well as factoring in the geometrical aspect of the street canyon. The particle image velocimetry technique was used to acquire velocity vector fields across the middle vertical planar cross section of the urban street canyon. The results showed that the canyon aspect ratio affects the resulting flow field; however, a main vortical structure is present in all the visualized flow patterns with flow direction always being consistent with that of an uprising flow along the canyon heated wall.

Jet propulsion of a squid-inspired swimmer in the presence of background flow

Inspired by recent studies of a squid-like swimmer, we propose a three-dimensional jet propulsion system composed of an empty chamber enclosed within a deformable body with an opening. By prescribing the body deformation and jet velocity profile, we numerically investigate the jet flow field and propulsion performance under the influence of background flow during a single deflation procedure. Three jet velocity profiles, i.e., constant, cosine and half cosine, are considered. We find that the maximum circulation of the vortex ring is reduced at a higher background flow velocity. This is because stronger interaction between the jet flow and background flow makes it harder to feed the leading vortex ring. Regarding thrust production, our analysis based on conservation of momentum indicates that with the constant profile the peak thrust is dominated by the time derivative of the fluid momentum inside the body, while momentum flux related thrust accounts for the quasi-steady thrust. For the cosine profile, its peak is mainly sourced from momentum flux associated with the unsteady vortex ring formation. No prominent thrust peak exists with the half cosine profile whose thrust continuously increases during the jetting. For all the three jet velocity profiles, added-mass related thrust attributed to body deformation enhances the overall thrust generation non-negligibly. Under the present tethered mode, the background flow has negligible influence on the thrust attributed to momentum flux and momentum change of the fluid inside the body. However, it indeed affects the over pressure-related thrust but its effect is relatively small. The overall thrust declines due to the significantly increased drag force at large incoming flow speed despite the rise of added-mass related thrust. Unsteady thrust involving vortex ring formation becomes more important in the overall thrust generation with an increased background flow velocity, reflected by larger ratios of the unsteady impulse to jet thrust impulse.

Electrokinetic energy conversion in nanochannels grafted with pH-responsive polyelectrolyte brushes modelled using augmented strong stretching theory.

In this paper, we develop a theory to quantify the electrokinetic energy conversion in electrolyte-filled nanochannels grafted with pH-responsive polyelectrolyte (PE) brushes. A pressure-driven flow drives the mobile electrolyte ions of the electric double layer (EDL) supported by the charged PE brushes leading to the generation of a streaming current, a streaming electric field and eventually an electrical energy. The salient feature of this study is that the brushes are described using our recently developed augmented Strong Stretching Theory (SST) model. In all the previous theoretical studies on liquid transport in PE-brush-grafted nanochannels, the brushes have either been assumed to be of constant height (independent of salt concentration or pH) or modelled using the Alexander-de-Gennes model that considers uniform monomer distribution along the brush height. Such simplifications have meant that the salt and the pH dependence of the brush height, the monomer distribution, and the resulting electrostatics have not been appropriately accounted for in the transport calculations. This paper addresses these limitations and provides a much more detailed description of the brushes while capturing the corresponding electrokinetic energy conversion. The results establish that the presence of the PE brushes ensures a localization of the average EDL charge density away from the grafting surface, thereby enabling the migration of the EDL ions with a larger background flow velocity; as a consequence, there is an enhancement of the streaming current, streaming electric field, and the resulting electrical energy generation under certain grafting densities of the PE brushes.

Effect of acoustic wave on the enhancement of moisture adsorption of silica-gel

In this study the enhancement effect of adsorption mass transfer by acoustic wave is investigated to understand how the acoustic wave influences the adsorption performance. The experimental data were obtained to show the behavior of silica-gel adsorption with/without acoustic field. The experimental results showed the adsorption speed with acoustic wave became higher than that without acoustic wave although the equilibrium adsorption amount was kept to be the same. From the viewpoint of acoustic wave conditions, the adsorption speed was increased with increasing the velocity amplitude, while the pressure amplitude had no effect on the enhancement. The maximum adsorption speed attained five times as high as the case without acoustic wave when the velocity amplitude was 1.35 m/s. Background air flow velocity is also an influential factor. The experimental results support that the non-dimensional ratio of the velocity amplitude to the background flow velocity is essential to describe the enhancement effect. Based on the observed data, an empirical formula was estimated to express the relationship between the enhancement effect and the non-dimensional velocity amplitude. The liner regression showed the good agreement and gave the critical non-dimensional velocity amplitude of 1.9 over which acoustic wave is effective to amplify the mass transfer rate of adsorption.

Experimental investigation of the thermal dispersion coefficient under forced groundwater flow for designing an optimal groundwater heat pump (GWHP) system

Mechanical thermal dispersion has often been neglected or underestimated in the simulation of heat transport in porous media, e.g., by using zero or the default value in simulators, or by using the scaling law for solute dispersivity as a thermal dispersivity value. However, large amounts of water usually injected in groundwater heat pump (GWHP) systems may increase the groundwater flow velocity much faster than natural flow and thus change the importance of mechanical dispersion in heat transport. In this study, to investigate the effects of water injection on the flow field, thermal dispersion coefficient, and associated heat transport process, a laboratory experiment using two different heat sources as tracers was performed at various background flow velocities (Re < 0.52). The analysis results from analytical and numerical models indicate that injected water increases both flow velocities and thermal dispersion coefficients, especially near the injection well, and thus makes the effect of mechanical dispersion on heat transport very important even at low background flow velocity. This result was also found in the field-based modeling results, but the radius of hydraulic and thermal effects was larger. In particular, thermal dispersivity on a field scale is known to increase depending on the scale of measurement and the degree of aquifer heterogeneity. Therefore, to ensure the efficiency and sustainability in field applications such as GWHP systems, it is necessary to evaluate site-specific thermal dispersivity through field experiments.

Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (Te) and its fluctuations (δTe). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%–15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s–1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s–1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed.

Prediction of acoustic absorption performance of a perforated plate with air jets

The present study focuses on the prediction of acoustic absorption performance of a perforated plate with air jets by theoretical calculations. In addition, we experimentally measured the flow rate, internal pressure, acoustic pressure, and transfer function using an acoustic impedance tube. The normal incidence absorption coefficient was calculated from the measured transfer function using transfer function methods. We investigated the influences of background air space, flow velocity, thickness, aperture rate, and aperture diameter of a perforated plate on the acoustic absorption characteristics. The frequency characteristics of the acoustic absorption coefficient showed a maximum value at a local frequency. As the background air space increased, the peak frequency of acoustic absorption characteristics decreased. As the flow velocity passing through the apertures increased, the peak level of the acoustic absorption coefficient also increased. The theoretical results agreed well with the experimental ones qualitatively.

Onsager symmetry from mesoscopic time reversibility and the hydrodynamic dispersion tensor for coarse-grained systems.

Onsager reciprocity relations derive from the fundamental time reversibility of the underlying microscopic equations of motion. This gives rise to a large set of symmetric cross-coupling phenomena. We here demonstrate that different reciprocity relations may arise from the notion of mesoscopic time reversibility, i.e., reversibility of intrinsically coarse-grained equations of motion. We use Brownian dynamics as an example of such a dynamical description and show how it gives rise to reciprocity in the hydrodynamic dispersion tensor as long as the background flow velocity is reversed as well.

Diffraction of acoustic-gravity waves in the presence of a turning point

Acoustic-gravity waves (AGWs) in an inhomogeneous atmosphere often have caustics, where the ray theory predicts unphysical, divergent values of the wave amplitude and needs to be modified. Unlike acoustic waves and gravity waves in incompressible fluids, AGW fields in the vicinity of a caustic have never been systematically studied. Here, asymptotic expansions of acoustic gravity waves are derived in the presence of a turning point in a horizontally stratified, moving fluid such as the atmosphere. Sound speed and the background flow (wind) velocity are assumed to vary gradually with height, and slowness of these variations determines the large parameter of the problem. It is found that uniform asymptotic expansions of the wave field in the presence of a turning point can be expressed in terms of the Airy function and its derivative. The geometrical, or Berry, phase, which arises in the consistent Wentzel-Kramers-Brillouin approximation for AGWs, plays an important role in the caustic asymptotics. In the dominant term of the uniform asymptotic solution, the terms with the Airy function and its derivative are weighted by the cosine and sine of the Berry phase, respectively. The physical meaning and corollaries of the asymptotic solutions are discussed.

Vorticity in analog gravity

In the analog gravity framework, the acoustic disturbances in a moving fluid can be described by an equation of motion identical to a relativistic scalar massless field propagating in curved space-time. This description is possible only when the fluid under consideration is barotropic, inviscid, and irrotational. In this case, the propagation of the perturbations is governed by an acoustic metric that depends algebrically on the local speed of sound, density, and the background flow velocity, the latter assumed to be vorticity-free. In this work we provide a straightforward extension in order to go beyond the irrotational constraint. Using a charged—relativistic and nonrelativistic—Bose–Einstein condensate as a physical system, we show that in the low-momentum limit and performing the eikonal approximation we can derive a d’Alembertian equation of motion for the charged phonons where the emergent acoustic metric depends on flow velocity in the presence of vorticity.

Minimally destructive, Doppler measurement of a quantized flow in a ring-shaped Bose–Einstein condensate

The Doppler effect, the shift in the frequency of sound due to motion, is present in both classical gases and quantum superfluids. Here, we perform an in situ, minimally destructive measurement, of the persistent current in a ring-shaped, superfluid Bose–Einstein condensate using the Doppler effect. Phonon modes generated in this condensate have their frequencies Doppler shifted by a persistent current. This frequency shift will cause a standing-wave phonon mode to be ‘dragged’ along with the persistent current. By measuring this precession, one can extract the background flow velocity. This technique will find utility in experiments where the winding number is important, such as in emerging ‘atomtronic’ devices.