Analysis Of Flow Structures Research Articles

Coherent instability in wall-bounded shear

The mechanism underlying the coherent hairpin process in wall-bounded shear flows is studied. An algorithm for the identification and analysis of flow structures based on morphological operations is presented. The method distils the topology of the flow field into a discrete data set and enables the time-resolved sampling of coherent flow processes across multiple scales. Application to direct simulation data of transitional and turbulent boundary layers at moderate Reynolds number sheds light on the flow physics of the hairpin process. The analysis links the hairpin to an exponential instability which is amplified in the flow distorted by a negative perturbation in the streamwise velocity component. Linear analyses substantiate the connection to an inviscid instability mechanism of varicose type. The formation of packets of hairpins is related to a self-similar process which originates from a single patch of low-speed fluid and describes a recurrence of the dynamics that leads to the formation of an individual hairpin. Analysis of the evolution of several thousand turbulent hairpins provides a statistical characterization of the principal dynamics and yields a time-resolved average of the hairpin process. Comparisons with the transitional hairpin show qualitatively consistent trends and thus support earlier hypotheses of their equivalence. In terms of the causality of events, the results suggest that the hairpin is a manifestation of the varicose instability and as such is a consequence rather than a cause of the primary perturbations of the flow.

Numerical Investigation of a Novel Microscale Swirling Jet Reactor for Medical Sensor Applications

A microscale swirler and corresponding reactor for a recent detection and analysis tool for healthcare applications, Fiber optic-surface plasmon resonance (FO-SPR), is presented in this study. The sensor is a 400 μm diameter needle that works as a detector for certain particles. Currently, the detection process relies on diffusion of particles towards the sensor and hence diagnostic time is rather long. The aim of this study is to decrease that diagnostic time by introducing convective mixing in the reactor by means of a swirling inlet flow. This will increase the particle deposition on the FO-SPR sensor and hence an increase in detection rate, as this rate strongly depends on the aimed particle concentration near the sensor. As the flow rates are rather low and the length scales are small, the flow in such reactors is laminar. In this study, robustly controllable mixing features of a swirling jet flow is used to increase the particle concentration near the sensor. A numerical analysis (CFD) is performed to characterize the flow and a detailed analysis of flow structures depending on the flow rate are reported.

Flow Mechanism and Characteristics of Pressure-Equalizing Film Along the Surface of a Moving Underwater Vehicle

Pressure-equalizing film is a slice of air layer attached to vehicle exterior with nearly uniform inside pressure, similar to ventilated cavity in composition; it is generated through exhaust process of the inside air chamber as vehicle emerges from deep water, and can reduce the lateral force and pitching moment that vertical launched underwater vehicle suffered. In this work, the emerging process of vehicle from water with pressure-equalizing exhaust was numerically calculated to investigate the evolution and flow characteristics of the generated pressure-equalizing film along its surface. Results indicated that during the whole exhaust process, the film can be obviously classified into different sections according to the distribution of phase volume fraction or pressure. The exhaust velocity ratio and flow rate from vehicle interior chamber were also found to increase as vehicle moves. In the analysis of flow structures, vortex structures such as the horseshoe vortex, “detour-separation” vortex, and counter-rotating vortex pair (CVP) can be figured out in the region of the exhaust hole. Under the effect of re-entrant jet, water around the film tail would be entrained upstream then enter the surface film to mix with the pressure-equalizing air. It leads to the happening of the three-dimensional (3D) wall vortex in the flow field.

STRUKTUR ALUR DAN BENTUK KONFLIK YANG MEMBANGUN NOVEL SAMAN KARYA AYU UTAMI

In the novel Saman, it can be found some interesting problems to be analyzed. This paper limits the descriptive analysis of flow and conflict structures and their interrelationships in the meaning of novel. The problem is considered interesting because the path created by the author is not sequential. In addition, the flow in the novel is also interesting because it is different from the conventional rules flow. Saman is a fragment of Ayu Utami's first novel, Laila Tak Mampir di New York. This Fragment won the Jakarta Arts Council Roman contest 1999 and also the recipient of the award for his work which is considered to expand the limit of writing in the community. The structure of Saman's novel flow is not based on the order of life events (in fact). It is intended by the author to have the reader interested in reading the entire contents of the novel. The meaning to be conveyed is to commit the wrong action (affair, leave the duties of priesthood) will lead to other people hurt and will lead to conflict or problems. The form of conflict that built the flow of novel Saman there are two, namely social conflict and inner conflict. Both forms of this conflict arise because this novel raises the problem of infidelity and sexual problems.

Rayleigh type streaming effect on magnetohydrodynamic characteristics of fluidized bed particles

We developed and implemented a theory, involving the propagation of a wave in a magnetic field for boundary layer analysis of flow structures. Our investigation revealed that the position of nodes in a standing wave is a function of the applied magnetic field. Hence, an approximate solution to the acoustical wave problem near a rigid wall was derived using the perturbation theory. Our results revealed that the velocity of the steady flow outside the boundary layer was independent of viscosity but was dependents on magnetic field. The practical implication of the derived result has been presented by discussing one illustration: a case in which an external standing wave is imposed in the transverse direction with respect to the main flow. The flow may be described using the three non-dimensional parameters. Streamline behavior was plotted for the volumetric flow rate analysis of the problem.

Numerical simulation and sensitivity analysis of heat transfer enhancement in a flat heat exchanger tube with discrete inclined ribs

In this work, we propose a numerical study of the thermal-hydraulic performance in a novel flat heat exchanger tube with discrete inclined ribs. Analysis of flow structures shows that four longitudinal swirl flows are induced in the flat tube. These swirl flows lead to adequate fluid mixing between the core flow and the near wall regions, and consequently, improve the heat transfer performance significantly. To determine the effects of parameters such as Reynolds number (300≤Re≤1500), the rib pitch ratio (0.6≤P∗≤1.8), and the rib height ratio (0.06≤H∗≤0.18) on the thermal and flow performance, a sensitivity analysis has been carried out by means of Response Surface Methodology. It is found that a reducing of P∗ and an increase in H∗ causes an increment in the Nusselt number and friction factor. The highest values of the Nusselt number and friction factor are achieved at P∗=0.6 and H∗=0.18 when Re is held at 900. Besides, the Nusselt number is more sensitive to Re and H∗ while the friction factor is more sensitive to P∗. The findings from this work may provide useful guidelines for engineers and researchers to design efficient heat exchangers.

Large-Eddy-Simulation-based analysis of complex flow structures within the volute of a vaneless centrifugal pump

Centrifugal pumps are very common in many fluid handling industrial applications, such as petrochemicals, oil and gas, etc. Although the design practices for centrifugal pumps are well established, efforts are directed towards optimising such systems for better operational efficiencies. In order to optimally design centrifugal pumps, it is beneficial to first understand the complex flow phenomena within different sections of the pump for a variety of operating conditions. This is normally achieved through the use of modern techniques, such as Computational Fluid Dynamics (CFD), where the flow within centrifugal pumps can be numerically modelled and important flow features can be analysed for better understanding of interactions amongst different process variables. CFD offers different turbulence modelling techniques with an aim to predict realistic flow approximations. Large Eddy Simulation (LES) offers a more accurate solution to this, in which the larger eddies are resolved while smaller eddies are modelled; hence predictions using LES are more realistic. Further, in turbulence modelling within centrifugal pumps, it is also important to model the complete interaction amongst different variables rather than a simplistic single blade passage flow analysis. In the present work, the complex blade–tongue interactions and their consequent effects on the pressure fluctuations within the volute have been evaluated. It is seen that the secondary flow features in the near-tongue regions due to blade interactions with the tongue affect the flow characteristics within the volute considerably.

Recent Results from Analysis of Flow Structures and Energy Modes Induced by Viscous Wave around a Surface-Piercing Cylinder

Due to its relevance in ocean engineering, the subject of the flow field generated by water waves around a vertical circular cylinder piercing the free surface has recently started to be considered by several research groups. In particular, we studied this problem starting from the velocity-potential framework, then the implementation of the numerical solution of the Euler equations in their velocity-pressure formulation, and finally the performance of the integration of the Navier-Stokes equations in primitive variables. We also developed and applied methods of extraction of the flow coherent structures and most energetic modes. In this work, we present some new results of our research directed, in particular, toward the clarification of the main nonintuitive character of the phenomenon of interaction between a wave and a surface-piercing cylinder, namely, the fact that the wave exerts its maximum force and exhibits its maximum run-up on the cylindrical obstacle at different instants. The understanding of this phenomenon becomes of crucial importance in the perspective of governing the entity of the wave run-up on the obstacle by means of wave-flow-control techniques.

Discrete particle simulation of solid flow in a melter-gasifier in smelting reduction process

Smelting reduction process is one of the promising technologies that produce liquid iron with advantages in the reduction of costs and adverse effect on environment. In this process, the main reactor of Melter-Gasifier (MG) involves complicated multiphase flow, heat and mass transfer phenomena, in which solid flow plays a dominant role. In this work, the solid flow behavior in an MG is investigated by discrete particle simulation under different conditions. The results show a good agreement with the observations in experiments. It reveals that three stagnant zones are formed: one is at the MG central bottom, and the other two are located on the side walls. The analysis indicates that charging method, MG geometry, and particle-wall frictional force are factors affecting the size of the stagnant zones on the side walls. Rolling friction does not affect solid flow much. Particle scale analysis of solid flow and force structures is also conducted, showing that strong forces exist in the regions with low porosity. The analysis of particle mass fraction shows that at the MG upper part, HCI (hot compacted iron) particles dominate the MG center, and coal particles tend to distribute to the side walls.

Analysis of high-speed train flow structures under crosswind

The flow structures along the length of a High-Speed Train (HST) are experimentally investigated under simulated crosswind conditions. The experiment uses flow visualizations and velocity measurements in a wind tunnel on a HST model at various angles of yaw. Surface visualizations shows the development of a longitudinal vortex, originating at the nose which travels along the leeward side of the train. A hypothesis is presented and tested proposing flow structures may repeat when the Boundary layer (BL) on the roof of the HST reaches maximum development or is “pushed off”. A key motivation is to evaluate the validity of using shortened HST models in a wind tunnel to correctly represent full-length trains. Using this theory, a shortened train model appears unable to completely model full-scale HSTs at realistic yaw angles.

Stall inception induced by the volute tongue at centrifugal compressor inlet

The stall and surge directly impact on the safety and reliability of compressors. The spike-type and modal-type stall inception exist in compressors. At present, few studies pay attention to the stall inception of centrifugal compressor, such as the formation reason for the stall inception and the action by the volute tongue on the stall precursor. This paper investigated the stall characteristics of a high-speed small-flow centrifugal compressor and illustrated the relationship between the volute tongue and the location of stall inception. In addition, the mechanism of stall inception was also clarified. Both the analysis of initial flow structures and the comparison of the frequency spectrum characteristics at different monitoring points show that the spike-type stall occurs at about 115° circumferential position in this centrifugal compressor. The nonaxisymmetric geometry structure of the volute leads to the uneven circumferential pressure distribution. The blockade effect of the volute tongue results in high static pressure area near the volute tongue. The disturbance caused by high static pressure adversely propagates into the diffuser, resulting in the static pressure peak value at different radii. As the pressure peaks adversely migrate to the impeller inlet and induce the leading edge spillover near the corresponding blade, the spike-type stall occurs. Therefore, the volute tongue both induces the stall inception and determines the circumferential position of the stall inception at the centrifugal compressor inlet.

Lift enhancement by bats' dynamically changing wingspan.

This paper elucidates the aerodynamic role of the dynamically changing wingspan in bat flight. Based on direct numerical simulations of the flow over a slow-flying bat, it is found that the dynamically changing wingspan can significantly enhance the lift. Further, an analysis of flow structures and lift decomposition reveal that the elevated vortex lift associated with the leading-edge vortices intensified by the dynamically changing wingspan considerably contributed to enhancement of the time-averaged lift. The nonlinear interaction between the dynamically changing wing and the vortical structures plays an important role in the lift enhancement of a flying bat in addition to the geometrical effect of changing the lifting-surface area in a flapping cycle. In addition, the dynamically changing wingspan leads to the higher efficiency in terms of generating lift for a given amount of the mechanical energy consumed in flight.

Multi-scale analysis of flow structures in fluidized beds with immersed tubes

Multi-scale analysis and non-linear analysis were combined to investigate the hydrodynamics of fluidized beds with and without horizontal tubes. Pressure fluctuations were measured and analyzed employing discrete wavelet analysis, recurrence plot analysis, and recurrence quantification analysis. A systematic procedure was followed to determine wavelet parameters. At low gas velocities, the energy of macro-structures reduces with the addition of the first tube and then increases with the addition of a second tube. However, there is no notable difference at high gas velocities. Determinism is high for the bed without tubes, which can be attributed to the periodic behavior of bubbles. Determinism decreases with the addition of tubes because the breakage of bubbles results in less periodic behavior. The three methods of analysis used in this study captured the effects of immersed tubes on the hydrodynamics of fluidized beds. Recurrence quantitative analysis was found to be a powerful and easy-to-use method that can capture the nonlinear characteristics of fluidized beds much more quickly than conventional methods of nonlinear analysis. This method can thus be effectively used for the online monitoring of hydrodynamic changes in fluidized beds.

A priori and a posteriori investigations for developing large eddy simulations of multi-species turbulent mixing under high-pressure conditions

A Direct Numerical Simulation (DNS) database was created representing mixing of species under high-pressure conditions. The configuration considered is that of a temporally evolving mixing layer. The database was examined and analyzed for the purpose of modeling some of the unclosed terms that appear in the Large Eddy Simulation (LES) equations. Several metrics are used to understand the LES modeling requirements. First, a statistical analysis of the DNS-database large-scale flow structures was performed to provide a metric for probing the accuracy of the proposed LES models as the flow fields obtained from accurate LESs should contain structures of morphology statistically similar to those observed in the filtered-and-coarsened DNS (FC-DNS) fields. To characterize the morphology of the large-scales structures, the Minkowski functionals of the iso-surfaces were evaluated for two different fields: the second-invariant of the rate of deformation tensor and the irreversible entropy production rate. To remove the presence of the small flow scales, both of these fields were computed using the FC-DNS solutions. It was found that the large-scale structures of the irreversible entropy production rate exhibit higher morphological complexity than those of the second invariant of the rate of deformation tensor, indicating that the burden of modeling will be on recovering the thermodynamic fields. Second, to evaluate the physical effects which must be modeled at the subfilter scale, an a priori analysis was conducted. This a priori analysis, conducted in the coarse-grid LES regime, revealed that standard closures for the filtered pressure, the filtered heat flux, and the filtered species mass fluxes, in which a filtered function of a variable is equal to the function of the filtered variable, may no longer be valid for the high-pressure flows considered in this study. The terms requiring modeling are the filtered pressure, the filtered heat flux, the filtered pressure work, and the filtered species mass fluxes. Improved models were developed based on a scale-similarity approach and were found to perform considerably better than the classical ones. These improved models were also assessed in an a posteriori study. Different combinations of the standard models and the improved ones were tested. At the relatively small Reynolds numbers achievable in DNS and at the relatively small filter widths used here, the standard models for the filtered pressure, the filtered heat flux, and the filtered species fluxes were found to yield accurate results for the morphology of the large-scale structures present in the flow. Analysis of the temporal evolution of several volume-averaged quantities representative of the mixing layer growth, and of the cross-stream variation of homogeneous-plane averages and second-order correlations, as well as of visualizations, indicated that the models performed equivalently for the conditions of the simulations. The expectation is that at the much larger Reynolds numbers and much larger filter widths used in practical applications, the improved models will have much more accurate performance than the standard one.

Quantitative analysis of flow structures in compression ramp based on flow visualization

Flow visualization studies on the laminar boundary layer flows over a 25° compression ramp are carried out via NPLS technique in a Mach 3.0 wind tunnel; fine flow structures such as boundary layer, shear layer, and shock waves may be visualized clearly. Based on the visualized data, quantitative analysis is conducted using intermittency, spatial correlations, and fractal theory. The intermittency function γ of the boundary layer and the interaction region is calculated, and the size of coherent structures and the structure angle θ for the boundary layer are obtained, so that the fractal dimension of the boundary layer in streamwise can be provided. Experimental data in the present paper have been compared with the results of Ringuette and Bookey, and the quantitative characteristics of flow structures are discussed in detail.

Computational and experimental analysis of flow structures induced by a plasma actuator with burst modulations in quiescent air

Computational and experimental analysis of flow structures induced by a plasma actuator with burst modulations in quiescent air

Identification of Coherent Flow Structures in Nonequilibrium Plasmas

Nonequilibrium atmospheric-pressure plasmas are at the core of a broad range of energy, medical, environmental, and manufacturing applications. The analysis of coherent flow structures is used in diverse areas of fluids engineering for the study of complex flows, and can lead to greater understanding of nonequilibrium plasma flows, particularly of the dynamics of their degree of thermodynamic nonequilibrium. Three different coherent structure identification criteria are used to analyze simulation results of the flow dynamics from an arc plasma torch obtained using a thermodynamic nonequilibrium model. The results reveal some of the possibilities and challenges of the studied criteria for the analysis of nonequilibrium discharges.

Proper Orthogonal Decomposition Analysis of Flow Structures Generated around Engine Cooling Fan

Proper Orthogonal Decomposition Analysis of Flow Structures Generated around Engine Cooling Fan

The Experimental Analysis of Cavitating Structure Fluctuations and Pressure Pulsations in the Cavitation Station

The experimental analysis of unsteady cavitating flow structures has been performed in the cavitation station in order to evaluate and correlate the static pressure measurements with the cavitation cloud structure dynamics which was quantified using the computer aided visualization technique. The paper describes the experimental method and presents the results of the investigation of the unsteady cavitation flow field, performed in the wake of bluff body. Comparison of the static pressure oscillation and cavitation cloud structures fluctuation caused by cloud shedding in the wake of bluff body, show the interaction between signals. The detailed analysis confirms two distinct types of flow pulses and quantifies the effect of cavitation number on the strength of the pressure and structure pulses.

Numerical investigation of the interphase forces and turbulence closure in 3D square bubble columns

3D time dependent numerical study has been performed to predict the flow hydrodynamics in bubble columns by employing explicit algebraic Reynolds stress (EARSM), re-normalization group (RNG) and RNG bubble induced turbulence (BIT) k–ε models and the computational fluid dynamics (CFD) results are compared with experimental work of Deen (2001). This work shows the comparison between different drag force models (Ishii and Zuber, 1979; Schiller and Nauman, 1935; Grace et al., 1976) and drag coefficient). Moreover, the effects of the lift, virtual mass and suchlike other imperative non drag forces inclusive of wall lubrication as well as the turbulent dispersion have been examined. All the results show good quantitative agreement with experiments in case of turbulent kinetic energy, axial gas and liquid velocity profiles. EARSM k–ε has found to perform better than both the RNG k–ε modalities in estimating the turbulent kinetic energy profiles, but gave poor predictions of axial velocity profiles. Comparatively, RNG and RNG BIT models have successfully predicted the averaged flow field. Whereas the turbulent kinetic energy profiles at some locations, have been slightly under estimated by the RNG models. The inclusion of bubble induced turbulence led to slight improvement in estimation of the turbulent kinetic energy profiles. The advanced performance of RNG models stems in the better estimation of rate of turbulent dissipation and turbulent viscosity. Thus, both RNG and RNG BIT models can be readily utilized for the analysis of average flow fields and turbulent flow structures.