scholarly journals Lagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current

2019 ◽  
Vol 877 ◽  
pp. 824-843 ◽  
Author(s):  
Marius M. Neamtu-Halic ◽  
Dominik Krug ◽  
George Haller ◽  
Markus Holzner
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Gravity Current ◽  
Three Dimensional ◽  
Lagrangian Coherent Structures ◽  
Flow Measurements ◽  
Vortical Structures ◽  
Entrainment Velocity ◽  
Extraction Algorithm ◽  
Entrainment Process

In this paper, we employ the theory of Lagrangian coherent structures for three-dimensional vortex eduction and investigate the effect of large-scale vortical structures on the turbulent/non-turbulent interface (TNTI) and entrainment of a gravity current. The gravity current is realized experimentally and different levels of stratification are examined. For flow measurements, we use a multivolume three-dimensional particle tracking velocimetry technique. To identify vortical Lagrangian coherent structures (VLCSs), a fully automated three-dimensional extraction algorithm for multiple flow structures based on the so-called Lagrangian-averaged vorticity deviation method is implemented. The size, the orientation and the shape of the VLCSs are analysed and the results show that these characteristics depend only weakly on the strength of the stratification. Through conditional analysis, we provide evidence that VLCSs modulate the average TNTI height, consequently affecting the entrainment process. Furthermore, VLCSs influence the local entrainment velocity and organize the flow field on both the turbulent and non-turbulent sides of the gravity current boundary.

Unsteady measurements in a separated and reattaching flow

1984 ◽  
Vol 144 ◽  
pp. 13-46 ◽  
Author(s):  
N. J. Cherry ◽  
R. Hillier ◽  
M. E. M. P. Latour
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separation Bubble ◽  
Scale Up ◽  
Three Dimensional ◽  
Low Frequency ◽  
Vortical Structures ◽  
Large Scale Structures ◽  
Bubble Length ◽  
Low Frequency Motion

Measurements of fluctuating pressure and velocity, together with instantaneous smoke-flow visualizations, are presented in order to reveal the unsteady structure of a separated and reattaching flow. It is shown that throughout the separation bubble a low-frequency motion can be detected which appears to be similar to that found in other studies of separation. This effect is most significant close to separation, where it leads to a weak flapping of the shear layer. Lateral correlation scales of this low-frequency motion are less than the reattachment length, however; it appears that its timescale is about equal to the characteristic timescale for the shear layer and bubble to change between various shedding phases. These phases were defined by the following observations: shedding of pseudoperiodic trains of vortical structures from the reattachment zone, with a characteristic spacing between structures of typically 60% to 80% of the bubble length; a large-scale but irregular shedding of vorticity; and a relatively quiescent phase with the absence of any large-scale shedding structures and a significant ‘necking’ of the shear layer downstream of reattachment.Spanwise correlations of velocity in the shear layer show on average an almost linear growth of spanwise scale up to reattachment. It appears that the shear layer reaches a fully three-dimensional state soon after separation. The reattachment process does not itself appear to impose an immediate extra three-dimensionalizing effect upon the large-scale structures.

scholarly journals The Development of Fast Response Aerodynamic Probes for Flow Measurements in Turbomachinery

1994 ◽  
Author(s):  
Roger W. Ainsworth ◽  
John L. Allen ◽  
J. Julian M. Batt
Keyword(s):  
Mach Number ◽  
Large Scale ◽  
Flow Direction ◽  
Three Dimensional ◽  
Pressure Sensors ◽  
Fast Response ◽  
Rotor Blades ◽  
Small Scale ◽  
Hot Wire ◽  
Flow Measurements

The advent of a new generation of transient rotating turbine simulation facilities, where engine values of Reynolds and Mach number are matched simultaneously together with the relevant rotational parameters for dimensional similitude (Dunn et al [1988], Epstein et al [1984]. Ainsworth et al [1988]), has provided the stimulus for developing improved instrumentation for investigating the aerodynamic flows in these stages. Much useful work has been conducted in the past using hot-wire and laser anemometers. However, hot-wire anemometers are prone to breakage in the high pressure flows required for correct Reynolds numbers, Furthermore some laser techniques require a longer runtime than these transient facilites permit, and generally yield velocity information only, giving no data on loss production. Advances in semiconductor aerodynamic probes are beginning to fulfil this perceived need. This paper describes advances made in the design, construction, and testing of two and three dimensional fast response aerodynamic probes, where semiconductor pressure sensors are mounted directly on the surface of the probes, using techniques which have previously been successfully used on the surface of rotor blades (Ainsworth, Dietz and Nunn [1991]). These are to be used to measure Mach number and flow direction in compressible unsteady flow regimes. In the first section, a brief review is made of the sensor and associated technology which has been developed to permit a flexible design of fast response aerodynamic probe. Following this, an extensive programme of testing large scale aerodynamic models of candidate geometries for suitable semiconductor scale probes is described, and the results of these discussed. The conclusions of these experiments, conducted for turbine representative mean and unsteady flows, yielded new information for optimising the design of the small scale semiconductor probes, in terms of probe geometry, sensor placement, and aerodynamic performance. Details are given of a range of wedge and pyramid semiconductor probes constructed, and the procedures used in calibrating and making measurements with them. Differences in performance are discussed, allowing the experimenter to choose an appropriate probe for the particular measurement required. Finally, the application of prototype semiconductor probes in a transient rotor experiment at HP turbine representative conditions is described, and the data so obtained is compared with (PD solutions of the unsteady viscous flow-field.

Mixing in Large Scale Tanks: Part I — Flow Modeling of Turbulent Mixing Jets

2004 ◽  
Author(s):  
Si Young Lee ◽  
Robert A. Dimenna ◽  
Richard A. Leishear ◽  
David B. Stefanko
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Operating Conditions ◽  
Liquid Level ◽  
Computational Results ◽  
Validation Data ◽  
Flow Measurements ◽  
Three Dimensional Representation ◽  
Lower Elevation ◽  
Tank Geometry

Flow evolution models were developed to evaluate the performance of the new advanced design mixer pump (ADMP) for sludge mixing and removal operations in one of the large-scale Savannah River Site (SRS) waste tanks, Tank 18. This paper is the first in a series of four that describe the computational model and its validation, the experiment facility and the flow measurements used to provide the validation data, the extension of the computational results to real tank conditions through the use of existing sludge suspension data, and finally, the sludge removal results from actual Tank 18 operations using the new ADMP. A computational fluid dynamics (CFD) approach was used to simulate the sludge removal operations. The models employed a three-dimensional representation of the tank with a two-equation turbulence model, since this approach was verified by both test and literature data. The discharge of the ADMP was modeled as oppositely directed hydraulic jets submerged at the center of the 85-ft diameter tank, with pump suction taken from below. The calculations were based on prototypic tank geometry and nominal operating conditions. In the analysis, the magnitude of the local velocity was used as a measure of slurrying and suspension capability. The computational results showed that normal operations in Tank 18 with the ADMP mixer and a 70-in liquid level would provide adequate sludge removal in most regions of the tank. The exception was the region within about 1.2 ft of the tank wall, based on an historical minimum velocity required to suspend sludge. Sensitivity results showed that a higher tank liquid level and a lower elevation of pump nozzle would result in better performance in suspending and removing the sludge. These results were consistent with experimental observations.

Coherent structures and dominant frequencies in a turbulent three-dimensional diffuser

2012 ◽  
Vol 699 ◽  
pp. 320-351 ◽  
Author(s):  
Johan Malm ◽  
Philipp Schlatter ◽  
Dan S. Henningson
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Low Frequency ◽  
Bulk Velocity ◽  
Time Signal ◽  
Mean Flow ◽  
The Mean ◽  
Dominant Frequencies

AbstractDominant frequencies and coherent structures are investigated in a turbulent, three-dimensional and separated diffuser flow at $\mathit{Re}= 10\hspace{0.167em} 000$ (based on bulk velocity and inflow-duct height), where mean flow characteristics were first studied experimentally by Cherry, Elkins and Eaton (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803–811) and later numerically by Ohlsson et al. (J. Fluid Mech., vol. 650, 2010, pp. 307–318). Coherent structures are educed by proper orthogonal decomposition (POD) of the flow, which together with time probes located in the flow domain are used to extract frequency information. The present study shows that the flow contains multiple phenomena, well separated in frequency space. Dominant large-scale frequencies in a narrow band $\mathit{St}\equiv fh/ {u}_{b} \in [0. 0092, 0. 014] $ (where $h$ is the inflow-duct height and ${u}_{b} $ is the bulk velocity), yielding time periods ${T}^{\ensuremath{\ast} } = T{u}_{b} / h\in [70, 110] $, are deduced from the time signal probes in the upper separated part of the diffuser. The associated structures identified by the POD are large streaks arising from a sinusoidal oscillating motion in the diffuser. Their individual contributions to the total kinetic energy, dominated by the mean flow, are, however, small. The reason for the oscillating movement in this low-frequency range is concluded to be the confinement of the flow in this particular geometric set-up in combination with the high Reynolds number and the large separated zone on the top diffuser wall. Based on this analysis, it is shown that the bulk of the streamwise root mean square (r.m.s.) value arises due to large-scale motion, which in turn can explain the appearance of two or more peaks in the streamwise r.m.s. value. The weak secondary flow present in the inflow duct is shown to survive into the diffuser, where it experiences an imbalance with respect to the upper expanding corners, thereby giving rise to the asymmetry of the mean separated region in the diffuser.

An Experimental Investigation of the Three-Dimensional Flow Within Large Scale Turbine Cascades

1986 ◽  
Author(s):  
Jaromír Jílek
Keyword(s):  
Experimental Investigation ◽  
Large Scale ◽  
Three Dimensional ◽  
Side Wall ◽  
Aerodynamic Characteristics ◽  
Flow Measurements ◽  
Wall Boundary Layer ◽  
Flow Parameters ◽  
Side Walls ◽  
Turbine Cascades

A detailed experimental investigation of the three-dimensional subsonic flow was carried out in a typical nozzle and impulse configuration of plane turbine cascades with a chord length 0.5 m. Flow parameters were measured within the passage and behind the cascade using a five-hole probe. Pressure distribution measurements and flow visualization were made on blade surfaces and side walls. Flow measurements were taken in endwall and airfoil boundary layers for both types of cascades. The influence of the aspect ratio, the inlet side wall boundary layer and the position of traversing planes on aerodynamic characteristics and losses is discussed.

Coherent Structures in a Three-Dimensional Wall Jet

1990 ◽  
Vol 112 (4) ◽  
pp. 462-467 ◽  
Author(s):  
Hisashi Matsuda ◽  
Sei-ichi Iida ◽  
Michio Hayakawa
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Near Field ◽  
Three Dimensional ◽  
Sampling Technique ◽  
Wall Jet ◽  
Structure Model ◽  
Streamwise Vortices ◽  
Lateral Velocity ◽  
Circular Nozzle

The formation mechanism of streamwise vortices in the near field of the three-dimensional wall jet discharging from a circular nozzle along a flat plate is studied experimentally using a conditional sampling technique. Ensemble-averages of the lateral velocity component indicate the presence of large-scale horseshoe-like structures, whose legs are inclined and stretched to form the streamwise vortices in the mixing region of the jet. Based on the present result, a coherent structure model for the near field of the wall jet is proposed.

The Development of Fast Response Aerodynamic Probes for Flow Measurements in Turbomachinery

1995 ◽  
Vol 117 (4) ◽  
pp. 625-634 ◽  
Author(s):  
R. W. Ainsworth ◽  
J. L. Allen ◽  
J. J. M. Batt
Keyword(s):  
Mach Number ◽  
Large Scale ◽  
Flow Direction ◽  
Three Dimensional ◽  
Pressure Sensors ◽  
Fast Response ◽  
Rotor Blades ◽  
Small Scale ◽  
Hot Wire ◽  
Flow Measurements

The advent of a new generation of transient rotating turbine simulation facilities, where engine values of Reynolds and Mach number are matched simultaneously together with the relevant rotational parameters for dimensional similitude (Dunn et al., 1988; Epstein and Guenette, 1984; Ainsworth et al., 1988), has provided the stimulus for developing improved instrumentation for investigating the aerodynamic flows in these stages. Much useful work has been conducted in the past using hot-wire and laser anemometers. However, hot-wire anemometers are prone to breakage in the high-pressure flows required for correct Reynolds numbers. Furthermore, some laser techniques require a longer run-time than these transient facilities permit, and generally yield velocity information only, giving no data on loss production. Advances in semiconductor aerodynamic probes are beginning to fulfill this perceived need. This paper describes advances made in the design, construction, and testing of two and three-dimensional fast response aerodynamic probes, where semiconductor pressure sensors are mounted directly on the surface of the probes, using techniques that have previously been successfully used on the surface of rotor blades (Ainsworth et al., 1991). These are to be used to measure Mach number and flow direction in compressible unsteady flow regimes. In the first section, a brief review is made of the sensor and associated technology that has been developed to permit a flexible design of fast response aerodynamic probe. Following this, an extensive program of testing large-scale aerodynamic models of candidate geometries for suitable semiconductor scale probes is described, and the results of these discussed. The conclusions of these experiments, conducted for turbine representative mean and unsteady flows, yielded new information for optimizing the design of the small-scale semiconductor probes, in terms of probe geometry, sensor placement, and aerodynamic performance. Details are given of a range of wedge and pyramid semiconductor probes constructed, and the procedures used in calibrating and making measurements with them. Differences in performance are discussed, allowing the experimenter to choose an appropriate probe for the particular measurement required. Finally, the application of prototype semiconductor probes in a transient rotor experiment at HP turbine representative conditions is described, and the data so obtained are compared with CFD solutions of the unsteady viscous flow-field.

scholarly journals Lagrangian coherent structures in tropical cyclone intensification

2012 ◽  
Vol 12 (12) ◽  
pp. 5483-5507 ◽  
Author(s):  
B. Rutherford ◽  
G. Dangelmayr ◽  
M. T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Flow Field ◽  
Coherent Structures ◽  
Finite Time ◽  
Potential Temperature ◽  
Three Dimensional ◽  
Lagrangian Coherent Structures ◽  
Moist Convection ◽  
Hyperbolic Structures

Abstract. Recent work has suggested that tropical cyclones intensify via a pathway of rotating deep moist convection in the presence of enhanced fluxes of moisture from the ocean. The rotating deep convective structures possessing enhanced cyclonic vorticity within their cores have been dubbed Vortical Hot Towers (VHTs). In general, the interaction between VHTs and the system-scale vortex, as well as the corresponding evolution of equivalent potential temperature (θe) that modulates the VHT activity, is a complex problem in moist helical turbulence. To better understand the structural aspects of the three-dimensional intensification process, a Lagrangian perspective is explored that focuses on the coherent structures seen in the flow field associated with VHTs and their vortical remnants, as well as the evolution and localized stirring of θe. Recently developed finite-time Lagrangian methods are limited in the three-dimensional turbulence and shear associated with the VHTs. In this paper, new Lagrangian techniques developed for three-dimensional velocity fields are summarized and we apply these techniques to study VHT and θe phenomenology in a high-resolution numerical tropical cyclone simulation. The usefulness of these methods is demonstrated by an analysis of particle trajectories. We find that VHTs create a locally turbulent mixing environment. However, associated with the VHTs are hyperbolic structures that span between adjacent VHTs or adjacent vortical remnants and represent coherent finite-time transport barriers in the flow field. Although the azimuthally-averaged inflow is responsible for the inward advection of boundary layer θe, attracting Lagrangian coherent structures are coincident with pools of high boundary layer θe. Extensions of boundary layer coherent structures grow above the boundary layer during episodes of convection and remain with the convective vortices. These hyperbolic structures form initially as boundaries between VHTs. As vorticity aggregates into a ring-like eyewall feature, the Lagrangian boundaries merge into a ring outside of the region of maximal vorticity.

Sign in / Sign up

Export Citation Format

Share Document