Asymmetric Flow Pattern Research Articles

Numerical Analysis of Macro-Instability Characteristic in a Stirred Tank by Means of Large Eddy Simulations: Off-Bottom Clearance Effect

Low frequency and large scale flow variations, often called macro-instability (MI) phenomena, were studied by means of a combination of large eddy simulations (LES) and sliding mesh (SM) models. Numerical predictions of MI characterictic in a six-bladed Rushton turbine stirred tank were performed by varying the off-bottom clearance at constant impeller rotational speed. The occurrence of MI in this study was identified by two methods: observing visually the flow velocity vector field and analyzing the time-series data of both velocity in the bulk flow and dynamic pressure on the vessel wall. The transient flow field visualization revealed the secondary circulation flow and the asymmetrical flow pattern beside of the mean flow pattern. The high variation intensity of flow pattern variation was clearly identified in the region with smaller space. The flow pattern variation was corroborated with the presence of high amplitude peaks in the low frequency part of the frequency spectrum of flow velocity. The distinct peak that was usually designated as the frequency of MI revealed the characteristic of MI. The physical meaning of the frequency of MI was the periodical appearance of the pronounced flow pattern. The numerical predictions of the frequency of MI in the region below and above impeller studied in this paper were confirmed well with the experimental results reported by Matsuda et al. (2004). The numerical prediction of the dynamic pressure monitored on the tank wall was also in agreement with the flow velocity monitored in the bulk flow for indicating the MI phenomena. This way of monitoring will be useful for the study of MI in multiphase agitated tank in the near future and in the practical application.

Experimental and numerical modelling of sedimentation in a rectangular shallow basin

Numerical simulation of flows in shallow reservoirs has to be checked for its consistency in predicting real flow conditions and sedimentation patterns. Typical flow patterns may exhibit flow separation at the inlet, accompanied by several recirculation and stagnation areas all over the reservoir surface. The aim of the present research project is to study the influence of the geometry of a reservoir on sediment transport and deposition numerically and experimentally, focusing on a prototype reservoir depth between 5 and 15 m as well as suspended sediment transport. A series of numerical simulations is presented and compared with scaled laboratory experiments, with the objective of testing the sensitivity to different flow and sediment parameters and different turbulence closure schemes. Different scenarios are analyzed and a detailed comparison of preliminary laboratory tests and some selected simulations are presented. The laboratory experiments show that suspended sediment transport and deposition are determined by the initial flow pattern and by the upstream and downstream boundary conditions. In the experiments, deposition in the rectangular basin systematically developed along the left bank, although inflow and outflow were positioned symmetrically along the centre of the basin. Three major horizontal eddies developed influencing the sediment deposition pattern. Although asymmetric flow patterns are privileged, a symmetric pattern can appear from time to time. This particular behaviour could also be reproduced by a two-dimensional depth-averaged flow and sediment transport model (CCHE2D). The paper presents numerical simulations using different turbulence closure schemes (k-ɛ and eddy viscosity models). In spite of the symmetric setup, these generally produced an asymmetric flow pattern that can easily switch sides depending on the assumptions made for the initial and boundary conditions. When using the laboratory experiment as a reference, the most reliable numerical results have been obtained with a parabolic depth-averaged eddy viscosity model. This model appeared to be the only one that was able to reproduce the strongly asymmetric flow behaviour observed during the experiments.

Population structure and gene flow reversals in Atlantic salmon (Salmo salar) over contemporary and long‐term temporal scales: effects of population size and life history

Metapopulation dynamics are increasingly invoked in management and conservation of endangered species. In this context, asymmetrical gene flow patterns can be density dependent, with migration occurring mainly from larger into smaller populations, which may depend on it for their persistence. Using genetic markers, such patterns have recently been documented for various organisms including salmonids, suggesting this may be a more general pattern. However, metapopulation theory does not restrict gene flow asymmetry to 'source-sink' structures, nor need these patterns be constant over longer evolutionary timescales. In anadromous salmonids, gene flow can be expected to be shaped by various selective pressures underlying homing and dispersal ('straying') behaviours. The relative importance of these selective forces will vary spatially and for populations of different census size. Furthermore, the consequences of life-history variation among populations for dispersal and hence gene flow remain poorly quantified. We examine population structure and connectivity in Atlantic salmon (Salmo salar L.) from Newfoundland and Labrador, a region where populations of this species are relatively pristine. Using genetic variation at 13 microsatellite loci from samples (N=1346) collected from a total of 20 rivers, we examine connectivity at several regional and temporal scales and test the hypothesis that the predominant direction of gene flow is from large into small populations. We reject this hypothesis and find that the directionality of migration is affected by the temporal scale over which gene flow is assessed. Whereas large populations tend to function as sources of dispersal over contemporary timescales, such patterns are often changed and even reversed over evolutionary, coalescent-derived timescales. These patterns of population structure furthermore vary between different regions and are compatible with demographic and life-history attributes. We find no evidence for sex-biased dispersal underlying gene flow asymmetry. Our findings caution against generalizations concerning the directionality of gene flow in Atlantic salmon and emphasize the need for detailed regional study, if such information is to be meaningfully applied in conservation and management of salmonids.

Dynamically driven mantle flow and shear wave splitting asymmetry across the EPR, MELT area

The southern East Pacific Rise exhibits profound asymmetries in its geophysical character, implying asymmetric melting and flow patterns beneath the plate boundary with inflow from the Pacific superswell region. Shear wave splitting is 0.5–1 seconds larger on the Pacific plate than the Nazca plate. As seismic anisotropy is a measure of mantle strain history, the shear wave splitting signature provides an important constraint on mantle flow patterns. Finite‐element models of mantle flow using rheology that accounts for both dislocation and diffusion creep, coupled with calculations of mineral texturing and subsequent shear wave splitting predictions, show that either a model of dynamically driven flow or a passive thermal anomaly model can provide a reasonable fit to the observed asymmetric shear wave splitting. Dynamic flow produces a thicker anisotropic layer beneath the Pacific plate, while extending the melting region westward, simultaneously explaining shear wave splitting observations and the inferred asymmetric melt production.

A Study of an Asymmetric Flow in an Overexpanded Rocket Nozzle

This paper presents an investigation of the unsteady three-dimensional flow in an overexpanded rocket nozzle during a shutdown transient. Experimental and numerical investigations were carried out. It is known that, in some cases, an asymmetric flow induces a serious lateral force called “side load” during transient operations. The focus of the present study is to clarify characteristics of the asymmetric flow patterns. Results obtained by measurements and computations for the cases of the shutdown transient have shown qualitative agreement. The flow pattern becomes asymmetric during the transition of the separation pattern from restricted shock separation (RSS) to free shock separation (FSS). As a result, a large and impulsive side load was observed. Additionally, during the transition of the separation pattern, a three-dimensional vortex structure appeared on the nozzle wall in a separation bubble.

LATTICE BGK MODEL SIMULATION OF ASYMMETRIC FLOW INSIDE A CONTINUOUS SLAB CASTING MOLD

The incompressible lattice Bhatnager-Gross-Krook (BGK) model of computational fluid dynamics, from which the unsteady incompressible Navier-Stokes equations can be exactly derived with the limit of small Mach number, was established in continuous casting mold. An asymmetric flow pattern in the two-dimensional central plane of continuous slab casting mold was simulated, and the flow pattern is not stationary but changes over frequently if the Reynolds number is larger than 3000 or so. The results are found to be in excellent agreement with previous experimental results.

Control of Asymmetric Flow Fields of Slender Bodies at High Angle of Attack

The wind tunnel experiments is conducted to get inspiration for understanding the mechanism of the asymmetric flow pattern and developing an innovative flow control technique for a slender body at high angle of attack. The bi-stable situation of the side forces is observed, which could be easily switched by a tiny disturbances either from coming flow or from artificial disturbances at nose tip (including manufacturing defect). In turbulent flows the side forces switched randomly between positive and negative. There exists a hysteresis loop of side force with the rolling angle. A rod in front of the slender body is used to change the vortex pattern, which could be kept even the rod is moved out from the stream. A miniature strake attached to the nose tip of the model can be moved to different circumferential position. When the strake is stationary, the hysteresis loop disappears and the side force does not change with the turbulent fluctuation of coming flow. The results from dynamic measurements of section side force indicates that when the strake swung at lower frequency the side force can follow the cadence of the swinging strake. With increasing frequency, the magnitude of the side force decreases. At still high frequency, the side force diminishes to zero. If the strake is swinging, while the middle position can be changed to different circumferential angle ϕs on either left or right side, the side forces can be changed proportionally with the angle ϕs. On the basis of the experimental results, the mechanism of the asymmetry is discussed.

Asymmetric Flow in Symmetric Supercritical Expansions

A theory presented previously by the writer explained the asymmetric flow patterns occurring in symmetric rectangular expansions with subcritical flow. The theory is extended in the present paper to supercritical flows, in which the hydraulic jump is one of the flow features. The theory is shown to agree with available observations. Transverse oscillatory conditions are discussed in terms of a dynamic instability that can accompany static stability. The ranges for which experimental data are available with which to test the theory are identified, making it apparent where additional data would be useful for the full range of the problem. The phenomenological understanding provided by the present paper is important to the development of stabilization measures. In addition to stabilization by submergence, the hydraulic jump should be amenable to stabilization by other referenced methods, including various vane configurations and pipe communication between the eddy regions.

A Study of an Asymmetric Flow in an Overexpanded Rocket Nozzle

This paper presents about an investigation of unsteady three-dimensional flows in an overexpanded rocket nozzle during the shutdown transient. The experimental and numerical investigations are performed. It has been known that, in some case, an asymmetric flow induces the serious lateral force called “side load” during transient operation. The focus of the present paper is the clarification of the characteristics of asymmetric flow patterns during shutdown transient. Each result obtained by measurements and computation shows qualitative agreement. The flow pattern becomes asymmetric during the transition of the separation pattern from restricted shock separation (RSS) to free shock separation (FSS) and result in a large and impulsive side load. Additionally, during the transition, a three-dimensional vortex structure is formed on the nozzle wall in the separation bubble.

Two-Fluid Eulerian Simulation of Bubble Column Reactors with Distributors

A two-fluid Eulerian model has been used to study the effect of various sieve plate gas distributors and gas superficial velocities on the hydrodynamics of three-dimensional flow in bubble columns. Turbulence has been modelled using the k–ε turbulence model. Inclusion of gas distributors showed asymmetric flow patterns which are otherwise smoothened when modelled using a uniform gas source. The asymmetric behaviour increased with an increase in the number of holes giving better mixing. The gas hold-up increased with superficial gas velocity, and was independent of the distributor design at high superficial gas velocities. Comparison of simulation results with the experimental work from the literature has provided a successful validation of the model. This study demonstrates the significance of the real distributor in hydrodynamic studies that has been largely neglected in the past.

Numerical and Physical Simulation of Flow Patterns in a Swirling Flow Tundish

Swirling flow tundish is a new kind of tundish which has shown good effects on flotation of inclusion and reduction of inclusion content. In this paper, studies have been carried out on the flow fields in a one‐strand slab tundish. A full scale model of the flow patterns in the water model tundish was developed using a self‐developed code. RTD curves under different experimental conditions were obtained from both physical and numerical simulations. The effects of the swirling flow chamber geometry and the flowrate on flow patters in the tundish were discussed and compared with results from the numerical simulation. Validation of the self‐developed codes was achieved by comparing the physical and numerical results of the RTD curves and the mean rotational velocities in swirling flow tundish. As a result, significant rotational flow in the swirling flow chamber and asymmetrical flow pattern in the whole tundish were confirmed and the effects of these parameters on dead zone and mean residence time were also obtained. Further and more comprehensive studies are needed to optimize the design and application of such tundishes.

Stability of the crossflow pattern around a slender and influence of disturbance

Topological structure and stability of a slender cross flow is discussed by the stability theory of dynamic system. The inner boundary of flow field was limiting streamline and it was proved that the topological structure connected saddles by limiting streamline is stable. It is proved that the development of slender vortices leads to the change of topological structure about cross flow. And it is the change from stable and symmetrical vortices flow pattern to unstable and symmetrical vortices flow pattern, and then to stable and asymmetrical vortices flow pattern due to little disturbance which leads to the development of asymmetrical slender vortices. The influence of disturbance to flowfield structure was discussed by unfolding theory too.

Nonuniqueness of the Transonic Flow past an Airfoil

The inviscid transonic flow past a symmetric airfoil having a curvature minimum in the middle is numerically investigated. It is shown that at zero angle of attack α both symmetric and asymmetric steady-state flow patterns can exist on a certain freestream Mach number range Mmin <M∞ < Mmax. On this range, the asymmetric flows are stable against small perturbations, whereas the symmetric flows are stable only if M∞ does not coincide with a singular Mach number at which small variations in M∞ or α can result in flow restructuring.

Segregated regions in continuous laminar stirred tank reactors

Using visualization techniques, including acid/base reactions and UV fluorescence, we provide experimental evidence of segregated regions (islands) during mixing of viscous Newtonian fluids under laminar flow conditions in continuous stirred tank reactors (CSTRs). The effect of inlet/outlet stream position and Reynolds number on the dynamics of the mixing processes is examined. Numerical experiments in 3-D map were able to capture the main features of the CSTR flow by perturbing a Batch system using an imposed axial flow. Asymmetric flow patterns produced by off-center positioning of inlet and outlet pipes cause a reduction in size of the segregated region, enlarging the chaotic region and leading to more efficient mixing. Under dynamic inlet flow conditions, the laminar steady flow is perturbed, giving rise to an asymmetric flow pattern that is able to destroy toroidal segregated regions. Counter-intuitively, higher agitation speed (higher Re) did not enhance overall mixing efficiency. Faster agitation stabilized the toroidal regions, making it harder to destroy them. In addition, dynamic mixing protocols are investigated to enhance mixing performance. We demonstrate that time-dependent pumping and stirring protocols are able to efficiently destroy long-lasting toroidal regions.

The congenitally bicuspid aortic valve: how does it function? Why does it fail?

Background The study was intended to investigate the reason why congenitally bicuspid valves (CBAVs), which may function hemodynamically and clinically well, have a high early failure rate. Methods Observations were made on cryopreserved, then thawed human aortic roots containing CBAVs. Valvular function was studied in the left heart simulator using conventional and 500-frames/second cinematography, intravascular ultrasound, by preparation of silicone molds, and by computerized digital modeling. Results The function of the clinically “normal” CBAVs is characterized by (a) excessive folding and creasing, which (unlike in the trileaflet valve) persist throughout the cardiac cycle; (b) extended areas of leaflet contact; (c) significant morphologic stenosis; and (d) asymmetrical flow patterns and turbulence. Conclusions The above features subject the CBAV to abnormally high stresses and may lead to early thickening and eventually calcification and stenosis. The abnormal flow patterns also predilect for dilatation and dissection of the ascending aorta.

Hydraulic aspects of föhn winds in an Alpine valley

AbstractThis study examines the applicability of single‐layer hydraulic theory to Alpine föhn winds in the example of the flow in the Brenner Pass region (Austria/Italy). The south föhn is described as the gap flow over the Pass and along the associated Wipp Valley. Numerical shallow‐water simulations for a wide range of initial conditions, including shallow‐ and deep‐föhn cases, are discussed and compared with selected measurements collected within the Mesoscale Alpine Programme (MAP). The observational analyses are based on Doppler and aerosol‐backscatter lidar data for a specific föhn case, and on surface observations and radiosoundings for all föhn cases observed within the 70‐day MAP Special Observing Period (SOP).Radiosoundings at a location upstream (south) and downstream (north) of the Brenner Pass reveal that the average MAP SOP föhn case had subcritical flow south and nearly critical flow north of the Brenner Pass. The hydraulic model indicates flow transition to a supercritical state near the Pass, a hydraulic jump 2 km further north, mainly subcritical flow in the upper Wipp Valley except near the Brenner and, especially for shallow föhn, a tendency to becoming supercritical in the lower Wipp Valley. The model results suggest that the vertical topographic contraction exerts stronger control for the flow at the Brenner gap than the lateral contraction. In accordance with Doppler lidar measurements, the hydraulic model captures the decrease of the wind speed in the across‐valley direction from east to west in the northern part of the valley. This asymmetric flow pattern is a result of the complex valley geometry rather than of the influence of upper‐level synoptic winds, as suggested in a previous study.For some observed cases the strength of the temperature inversion at the top of the föhn layer decreased across the Brenner Pass, presumably due to entrainment processes. Consequently, the reduced‐gravity decreased as well. There are two major reasons for the only moderate agreement between the hydraulic parameters retrieved from soundings and those provided by the model: the model assumes a constant value of reduced‐gravity, and the calculation of hydraulic parameters for föhn cases without a strong and sharp temperature inversion is ambiguous. Essential flow patterns in the simulated fields, such as the location of the pressure minimum and wind speed maxima, compare well with the surface observations along a valley transect. Indications for a steeply amplified, or breaking, gravity wave resembling a hydraulic jump to the lee of the two mountain ridges are found in lidar observations as well as in the simulations. Copyright © 2004 Royal Meteorological Society

First Hybrid Turbulence Modeling for Turbine Blade Cooling

Introduction G AS turbines require proper cooling mechanisms to protect the airfoils from thermal stresses generated by exposure to hot combustion gases. The problem becomes aggravated by the growing trend to use higher turbine inlet temperatures to generate more power. Thus, film cooling is used as a cooling mechanism, and it works in the form of row of holes located in the spanwise direction, through which cold jets are issued into the hot crossflow. The penetration of cold jets into the main flow creates a complex flowfield. Systematic investigation of such flowfield started in late 1950s. Figure 1 shows the schematic of a single round jet injected in the crossflow at an angle α = 35 deg. The figure also describes the boundary conditions applied at different faces. Even though use of symmetry boundary condition at the hole centerline would reduce the computational time by half, its use is avoided as it prevents the possibility of capturing the unsteady asymmetric vortical flow patterns. This geometry is well accepted by the gas-turbine community and has been extensively studied1 for cooling performance for a wide range of blowing ratios, M = ρ j Vj/ρfsVfs, where ρ and V are density and normal velocity, respectively, for jet j and freestream fs. Goldstein2 correlated film cooling effectiveness η = (Tfs − T )/ (Tfs − Tj ) with the parameter x/Mb, where x is the downstream distance; M is the blowing ratio; b is the slot width; and Tfs, T , and Tj are the temperatures of crossflow, blade, and jet, respectively. Sinha et al.1 carried out experimental work to study the relationship between the fluid-thermal parameters of jet and film cooling effectiveness using a row of inclined holes. The mixing of a jet in a cross stream is a fully three-dimensional phenomenon.3 Amer et al.4 pointed out that the flow predictions are greatly affected by the selection of the turbulence model. Roy5 documented the cooling performance of 12 different arrangements of holes with a combination of blowing ratio M , distance between the holes L , and jet angle α using a upwind-biased finite volume code and standard k–ω turbulence closure model. Garg and Rigby6 resolved the plenum and hole pipes for a three-row showerhead film cooling arrangement with Wilcox’s k–ω turbulence model. Heidmann et al.7 used Reynolds-averaged Navier–Stokes (RANS) to compute the heat transfer for a realistic turbine vane with 12 rows of film cooling holes with shaped holes and plena resolved. Though these studies provide good details of the flow, the anisotropic dynamic nature of the spanwise vortices that affect the film cooling process are more complex than that can be captured by the mixing models used in aforementioned papers. Acharya8 compared the re-

Flow asymmetry associated with astronomical tides: Implications for the residual transport of sediment

Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, generating compound tides and overtides. The present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow‐water effects. In particular, the K1, O1, and M2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon is presented, and an inventory is made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Tidal current asymmetry may induce residual transport of sediment. Analytical expressions are derived which quantify the residual transport of sediment due to the K1, O1, and M2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity. The residual transport induced by the K1, O1, and M2 tides is compared to the residual transport due to the asymmetry associated with the M2 and M4 tides. The former mechanism is particularly relevant in diurnal, or mixed, mainly diurnal tidal regimes, where the K1 and O1 tides are dominant.

DSC solution for flow in a staggered double lid driven cavity

AbstractA benchmark quality solution is presented for flow in a staggered double lid driven cavity obtained by using the wavelet‐based discrete singular convolution (DSC). The proposed wavelet based algorithm combines local methods' flexibility and global methods' accuracy, and hence, is a promising approach for achieving the high accuracy solution of the Navier–Stokes equations. Block structured grids with pseudo‐overlapping subdomains are employed in the present simulation. A third order Runge–Kutta scheme is used for the temporal discretization. Quantitative results are presented, apart from the qualitative fluid flow patterns. The prevalence of rich features of flow morphology, such as two primary vortex patterns, merged single primary vortex patterns, and secondary eddies, makes this problem very attractive and interesting. The problem is quite challenging for the possible existence of numerically induced asymmetric flow patterns and elliptic instability. Important computational issues like consistence, convergence and reliability of the numerical scheme are examined. The DSC algorithm is tested on the single lid driven cavity flow and the Taylor problem with a closed form solution. The double lid driven cavity simulations are cross‐validated with the standard second order finite volume method. Copyright © 2003 John Wiley &amp; Sons, Ltd.

A Study of Side Load in 2-D Nozzles during Shutdown

In the present paper, the experimental and numerical investigations are performed to clarify the mechanism of the side load in rocket nozzles using two types of nozzles-modified ideal nozzle and modified compressed truncated perfect (CTP) nozzle. In the experiment, the flow field is visualized by the Schlieren method and asymmetric flow patterns are observed. In the modified CTP nozzle, two types of the separation patterns are observed. One is "free shock separation" and the other is "restricted shock separation." The numerical simulations predict that the transition of separation pattern from the FSS to the RSS causes a large impulsive side load. It is also shown that the flow in a two-dimensional nozzle can be three dimensional due to flow separation on the side wall.