Temperature Velocity Fluctuations Research Articles

Flow kinematics of granular materials considering realistic morphology

The study focuses on understanding the influence of particle morphology, interparticle friction and hopper exit width on spatial and temporal variations in velocity during dry granular flow. Using non-convex morphologies (sand, chickpea and rice), the flow characteristics and velocity fluctuations are explored. Funnel flow pattern was evident in angular shapes even in hoppers designed for mass flow discharge. K-means clustering, an unsupervised algorithm, is used to group the spatial variation of velocity inside the hopper. Three flow clusters were identified which indicate the flow transition from funnel to mass flow. The Free Flow Cluster (FFC) height was observed to be least for bulky-shaped particles because of their high sphericity and roundness values. Angular particles were observed to exhibit rapid temporal velocity fluctuations when compared to elongated and bulky shapes. Finally, the pseudo-shape approximations typically used in DEM fail to capture the true interlocking nature in the flow behaviour observed within realistic particle morphologies.

Unsteady interaction of crossflow instability with a forward-facing step

Experiments have been conducted on a swept wing model in a low-turbulence wind tunnel at chord Reynolds number of $2.17 \times 10^{6}$ to investigate the unsteady interaction of a forward-facing step (FFS) with incoming stationary crossflow (CF) vortices. The impact of varying the FFS height on the development and growth of primary and secondary CF disturbances and the ensuing laminar–turbulent transition is quantified through detailed hot-wire anemometry and infrared thermography measurements. The presence of the FFS results in either a critical (i.e. moderate transition advancement) or a supercritical behaviour (i.e. transition advancing abruptly to the FFS location). The arrival of the forced stationary CF vortices at the step is accompanied by their amplification. Unsteady analysis for the critical cases indicates temporal velocity fluctuations following closely the development of the baseline configuration (i.e. agreeing with the development of secondary instabilities). Consequently, laminar breakdown originates from the outer side of the upwelling region of the CF vortices. In contrast, for the supercritical FFS, the laminar breakdown unexpectedly originates from the inner side of the upwelling region. Evidence points to an unsteady mechanism possibly supported by locally enhanced spanwise-modulated shears and the recirculation region downstream of the FFS edge. This mechanism appears to govern the abrupt tripping of the flow in supercritical step cases. The findings in this work provide insight into the unsteady FFS–CF vortex interaction, which is pivotal to understanding the influence of an FFS on the laminar–turbulent boundary-layer transition in swept aerodynamic surfaces.

Spatiotemporal dynamics of a self-propelled system with opposing alignment and repulsive forces.

Effect of concurrent alignment and repulsion is studied in the purview of a confined active matter system using a modified force-based Vicsek model. On alteration of the alignment and the repulsive force parameters, a low alignment random phase, a midrange alignment milling phase, and a high alignment oscillatory phase are identified. Based on the particle aggregations, the milling phase is further classified into three subphases, two of which are spatial patterns: one consisting of compact ring-shaped mills and the other incorporating both rings and clusters. A correlation function based on the inner product of spatial velocity fluctuations of the particles shows a high correlation length for the ringed milling and the rings-clusters hybrid milling state. On analyzing temporal velocity fluctuations of particles through chaos detection techniques, low alignment and high alignment states are indicative of chaos, while the middle order alignment is symbolic of periodicity. The extent of synchronization of the particles' motion is analyzed through a Hilbert transform-based mean frequency approach, leading to the detection of a weak chimera state in the case of the spatial structures. The ringed milling state shows a unique category of weak chimera consisting of multiple oscillator groups showcasing different synchronization frequencies coexisting with desynchronized oscillators.

How tracer particles sample the complexity of turbulence

On their roller coaster ride through turbulence, tracer particles sample the fluctuations of the underlying fields in space and time. Quantitatively relating particle and field statistics remains a fundamental challenge in a large variety of turbulent flows. We quantify how tracer particles sample turbulence by expressing their temporal velocity fluctuations in terms of an effective probabilistic sampling of spatial velocity field fluctuations. To corroborate our theory, we investigate an extensive suite of direct numerical simulations of hydrodynamic turbulence covering a Taylor-scale Reynolds number range from 150 to 430. Our approach allows the assessment of particle statistics from the knowledge of flow field statistics only, therefore opening avenues to a new generation of models for transport in complex flows.

Gravel-Bed Hydrodynamics: Double-Averaging Approach

AbstractThis study focuses on the double-averaged (DA) turbulence characteristics and the local flow characterization in the flow zones over and within the flow-gravel-bed interface. The cumulative running averaged (CRA) value of a turbulence quantity for a given interval (in this study, half of the gravel size) of flow measurement is the prerequisite to determine a complete DA value of that quantity. The CRA turbulence quantities within the interfacial sublayer require a longer distance to become spatially independent. A method is proposed to fit the log law for the DA velocity over the bed and a polynomial law within the interfacial sublayer. The form-induced shear stress is 40% of the DA Reynolds shear stress occurring at the virtual bed level. The spatial velocity fluctuations diminish more rapidly than the temporal velocity fluctuations below the virtual bed level. In the inertial subrange of turbulent kinetic energy (TKE), the eddy size decreases significantly near the virtual bed level, and in the ...

Asymptotic dispersion for two‐dimensional highly heterogeneous permeability fields under temporally fluctuating flow

Temporal fluctuations of water flux have been investigated as a mechanism that strongly enhances transverse dispersion in heterogeneous media. Unfortunately, most results have been obtained by linear stochastic theories on permeability fields of limited variability. Worse, results are inconsistent regarding the impact of fluctuations on longitudinal dispersion, which motivates our work to find the effect of temporal velocity fluctuations on macrodispersion. We perform numerical Monte Carlo simulations for highly variable permeability fields of up to 800 correlation lengths. We find that fluctuations longitudinal to the main flow direction hardly modify macrodispersion because they do not alter the flow lines. Fluctuations transverse to the main flow direction not only increase transverse dispersion, which is well known, but also reduce the longitudinal macrodispersion in a significant and consistent way, which contradicts previous findings. The reduction of the longitudinal dispersion is comparable to the increase of transverse dispersion. Most surprisingly, for high heterogeneity, temporal fluctuations cause total (longitudinal plus transverse) macrodispersion to drop with respect to the steady state one. Enhancement of the transverse macrodispersion comes from both the increase of the transverse velocity variability and Lagrangian correlation. Reduction of the longitudinal macrodispersion results from the reduction of the Lagrangian correlation of the longitudinal velocity. That is, temporal fluctuations reduce longitudinal spreading both by breaking the fastest velocity paths on the plume front and by letting solute bypass the low‐permeability zones that tend to block or trap the solute in steady state flow conditions.

STREAMWISE VORTICES AND REYNOLDS STRESSES OF WALL-BOUNDED TURBULENT SHEAR FLOWS

Reynolds Equation is the most commonly used governing equation in turbulence. However, its application in wall-bounded turbulent shear flows may involve a defect. In general, Reynolds averaging should be ensemble averaging and Reynolds stresses are supposed to express all the actions of turbulence on the mean field. In statistically steady three-dimensional flows, Reynolds stresses are usually defined as correlations of temporal velocity fluctuations so that they cannot contain the influences of steady components of streamwise vortices. This is believed to be one of the reasons why many closure models in RANS meet problems in flows where streamwise vortices play significant roles. In this paper, Spatial-Temporal (S-T) averaged Reynolds stresses were defined, which separates the turbulence actions caused by temporal or spatial velocity fluctuations. DNS data for a fully developed channel flow were then used to check balancing of equations. Comparison showed that the balancing errors in the S-T averaged Reynolds equations were obviously smaller than those in the temporal averaged one, in particular, in the near wall region where the streamwise vortices located. Thus, a combination of traditional model with a supplemental model expressing influences of streamwise vortices might be a way out to improve the turbulence modeling.

Renormalization theory for eddy diffusivity in turbulent transport.

We examine the derivation of eddy-diffusivity equations for transport of passive scalars in a turbulent velocity field. Our main contention is that, in the long-time--large-distance limit, the eddy-diffusivity equations can take very different forms according to the statistical properties of the subgrid velocity, and that these equations depend very sensitively on the interplay between spatial and temporal velocity fluctuations. Such crossovers can be represented in a ``phase diagram'' involving two relevant statistical parameters. Strikingly, the Kolmogorov-Obukhov statistical theory is shown to lie on a phase-transition boundary.

Apparent dispersion in transient groundwater flow

This paper investigates the effects of large‐scale temporal velocity fluctuations, particularly changes in the direction of flow, on solute spreading in a two‐dimensional aquifer. Relations for apparent longitudinal and transverse dispersivity are developed through an analytical solution for dispersion in a fluctuating, quasi‐steady uniform flow field, in which storativity is zero. For transient flow, spatial moments are evaluated from numerical solutions. Ignored or unknown transients in the direction of flow primarily act to increase the apparent transverse dispersivity because the longitudinal dispersivity is acting in a direction that is not the assumed flow direction. This increase is a function of the angle between the transient flow vector and the assumed steady state flow direction and the ratio of transverse to longitudinal dispersivity. The maximum effect on transverse dispersivity occurs if storativity is assumed to be zero, such that the flow field responds instantly to boundary condition changes.

The growth and breakdown of streamwise vortices in the presence of a wall

The growth and breakdown of counter-rotating streamwise vortices, generated on a concave wall via the Goertler instability mechanism, were experimentally studied as a model for comparable eddy structures that exist in transitional and turbulent flat-plate boundary layers. The experiments were conducted in a low-speed open-return wind tunnel, using smoke-wire visualization and multiple-probe hot wires to study the vortices. As low-momentum fluid was removed from the wall, low-speed regions formed between the vortices; these regions grew in the normal direction faster than a nominally Blasius boundary layer and created strongly inflexional normal and spanwise profiles of the streamwise velocity component. Instability oscillations developed on these unstable profiles that scaled with the local shear-layer thickness and velocity difference. The spatial scales of the temporal velocity fluctuations were found to correlate with the velocity gradient in the spanwise (rather than in the normal) direction.