scholarly journals On the impact of swirl on the growth of coherent structures

2014 ◽  
Vol 741 ◽  
pp. 156-199 ◽  
Author(s):  
K. Oberleithner ◽  
C. O. Paschereit ◽  
I. Wygnanski
Keyword(s):  
Stability Analysis ◽  
Coherent Structures ◽  
Large Scale ◽  
Vortex Breakdown ◽  
Shear Mode ◽  
Mean Flow ◽  
Swirling Jet ◽  
The Mean ◽  
The Impact ◽  
Helical Mode

AbstractSpatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co- and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time- and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities.

Three-dimensional coherent structures in a swirling jet undergoing vortex breakdown: stability analysis and empirical mode construction

2011 ◽  
Vol 679 ◽  
pp. 383-414 ◽  
Author(s):  
K. OBERLEITHNER ◽  
M. SIEBER ◽  
C. N. NAYERI ◽  
C. O. PASCHEREIT ◽  
C. PETZ ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Coherent Structures ◽  
Vortex Breakdown ◽  
Particle Image Velocimetry Data ◽  
Turbulent Shear ◽  
Mean Flow ◽  
Helical Structures ◽  
Global Mode ◽  
Swirling Jet ◽  
Time Periodic

The spatio-temporal evolution of a turbulent swirling jet undergoing vortex breakdown has been investigated. Experiments suggest the existence of a self-excited global mode having a single dominant frequency. This oscillatory mode is shown to be absolutely unstable and leads to a rotating counter-winding helical structure that is located at the periphery of the recirculation zone. The resulting time-periodic 3D velocity field is predicted theoretically as being the most unstable mode determined by parabolized stability analysis employing the mean flow data from experiments. The 3D oscillatory flow is constructed from uncorrelated 2D snapshots of particle image velocimetry data, using proper orthogonal decomposition, a phase-averaging technique and an azimuthal symmetry associated with helical structures. Stability-derived modes and empirically derived modes correspond remarkably well, yielding prototypical coherent structures that dominate the investigated flow region. The proposed method of constructing 3D time-periodic velocity fields from uncorrelated 2D data is applicable to a large class of turbulent shear flows.

scholarly journals Transitional Flow Dynamics Behind a Micro-Ramp

2019 ◽  
Vol 104 (2-3) ◽  
pp. 533-552
Author(s):  
J. Casacuberta ◽  
K. J. Groot ◽  
Q. Ye ◽  
S. Hickel
Keyword(s):  
Boundary Layer ◽  
Stability Analysis ◽  
Large Scale ◽  
Vortex Pair ◽  
Base Flow ◽  
Transitional Flow ◽  
Mean Flow ◽  
Primary Vortex ◽  
The Mean ◽  
Near Wall

AbstractMicro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

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.

scholarly journals Nonlinear dynamics of large-scale coherent structures in turbulent free shear layers

2015 ◽  
Vol 787 ◽  
pp. 396-439 ◽  
Author(s):  
Xuesong Wu ◽  
Xiuling Zhuang
Keyword(s):  
Nonlinear Dynamics ◽  
Coherent Structures ◽  
Large Scale ◽  
Shear Layers ◽  
Small Scale ◽  
Evolution System ◽  
Mean Flow ◽  
Free Shear Layers ◽  
The Mean ◽  
Scale Turbulence

Fully developed turbulent free shear layers exhibit a high degree of order, characterized by large-scale coherent structures in the form of spanwise vortex rollers. Extensive experimental investigations show that such organized motions bear remarkable resemblance to instability waves, and their main characteristics, including the length scales, propagation speeds and transverse structures, are reasonably well predicted by linear stability analysis of the mean flow. In this paper, we present a mathematical theory to describe the nonlinear dynamics of coherent structures. The formulation is based on the triple decomposition of the instantaneous flow into a mean field, coherent fluctuations and small-scale turbulence but with the mean-flow distortion induced by nonlinear interactions of coherent fluctuations being treated as part of the organized motion. The system is closed by employing a gradient type of model for the time- and phase-averaged Reynolds stresses of fine-scale turbulence. In the high-Reynolds-number limit, the nonlinear non-equilibrium critical-layer theory for laminar-flow instabilities is adapted to turbulent shear layers by accounting for (1) the enhanced non-parallelism associated with fast spreading of the mean flow, and (2) the influence of small-scale turbulence on coherent structures. The combination of these factors with nonlinearity leads to an interesting evolution system, consisting of coupled amplitude and vorticity equations, in which non-parallelism contributes the so-called translating critical-layer effect. Numerical solutions of the evolution system capture vortex roll-up, which is the hallmark of a turbulent mixing layer, and the predicted amplitude development mimics the qualitative feature of oscillatory saturation that has been observed in a number of experiments. A fair degree of quantitative agreement is obtained with one set of experimental data.

Study of the Vortex Breakdown in a Conical Swirler Using LDV, LES and POD

2007 ◽  
Author(s):  
Christophe Duwig ◽  
Laszlo Fuchs ◽  
Arnaud Lacarelle ◽  
Matthias Beutke ◽  
Christian Oliver Paschereit
Keyword(s):  
Flow Pattern ◽  
Coherent Structures ◽  
Large Scale ◽  
Vortex Breakdown ◽  
Numerical Study ◽  
Flame Stabilization ◽  
Turbulent Fluctuation ◽  
Characteristic Frequencies ◽  
The Impact ◽  
Good Agreement

Modeling and understanding the vortex breakdown is a key issue of modern Lean Premixed Combustors. The main difficulty of the problem is the unsteady behavior of this type of flow: Large structures resulting from vortex breakdown and the swirling shear-layers, affect directly the flame stabilization leading to heat-release fluctuations and combustion instabilities. Consequently, one needs to capture and understand turbulent coherent structures dynamics for designing efficient burners. This task is particularly challenging since it deals with capturing coherent motions within a chaotic system and should be done using state-of-the art numerical and experimental techniques. The present work focuses on the experimental and numerical study of iso-thermal vortex breakdown in a conical swirler. Experimental investigations were performed with 2D Laser Doppler Velocimetry (LDV) and Hotwire Anemometry at the outlet of the combustor model. Averaged velocity fields and RMS values are showing a strong central recirculation zone. In addition, characteristic frequencies of the flow have been exhibited showing the strong influence of large scale turbulent fluctuation on the flow pattern. These measurements showed also the impact of different outlet geometries on the strength and position of the coherent structures of the flow. Further, Large Eddy Simulation (LES) has been used to obtain a 4D description of the flow. Comparison with LDV profiles showed a good agreement, indicating that the LES tool captures accurately the flow. The LES results were then processed for capturing and identifying the coherent structures. Firstly, characteristic frequencies were analyzed. Here also a good agreement with the experimental data was achieved. Secondly the cores of the vortices were visualized providing a good insight into the unsteady flow pattern. Finally, Proper Orthogonal Decomposition (POD) was applied to the 4D field in order to identify the contribution of different large scale fluctuation modes. The presence of the Precessing Vortex Core (PVC) corresponding to a pair of helical structures was captured.

scholarly journals Effect of Reynolds number and inflow parameters on mean and turbulent flow over complex topography

2016 ◽  
Vol 1 (2) ◽  
pp. 237-254 ◽  
Author(s):  
Ryan Kilpatrick ◽  
Horia Hangan ◽  
Kamran Siddiqui ◽  
Dan Parvu ◽  
Julia Lange ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Wind Shear ◽  
Large Scale ◽  
Flow Behaviour ◽  
Complex Topography ◽  
Mean Flow ◽  
Effective Roughness ◽  
The Mean ◽  
The Impact

Abstract. A characterization of mean and turbulent flow behaviour over complex topography was conducted using a large-scale (1 : 25) model in the WindEEE Dome at Western University. The specific topographic feature considered was the Bolund Hill escarpment facing westerly winds. A total of eight unique inflow conditions were tested in order to isolate the impact of key parameters such as Reynolds number, inflow shear profile, and effective roughness, on flow behaviour over the escarpment. The results show that the mean flow behaviour was generally not affected by the Reynolds number; however, a slight increase in speed-up over the escarpment was observed for cases with lower inflow roughness. The shape of the inflow wind shear profile also had a minor impact on the mean flow near the escarpment. More significant effects were observed in the turbulent flow behaviour, where the turbulent kinetic energy (TKE) over the escarpment was found be a strong function of inflow roughness and a weak function of the Reynolds number. The local change in the inflow wind shear was found to have the most significant influence on the TKE magnitude, which more closely approximated the full-scale TKE data, a result which had not been previously observed in wind tunnel modelling of this topography.

scholarly journals Investigation of the effect of Reynolds number and inflow parameters on mean and turbulent flow over complex topography

2016 ◽  
Author(s):  
Ryan Kilpatrick ◽  
Horia Hangan ◽  
Kamran Siddiqui ◽  
Dan Parvu ◽  
Julia Lange ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Wind Shear ◽  
Large Scale ◽  
Flow Behaviour ◽  
Complex Topography ◽  
Mean Flow ◽  
Effective Roughness ◽  
The Mean ◽  
The Impact

Abstract. A characterization of mean and turbulent flow behaviour over complex topography was conducted using a large-scale (1:25) model of Bolund Hill in the WindEEE Dome at Western University. The specific topographic feature considered was an escarpment. A total of eight unique inflow conditions were tested in order to isolate the impact of key parameters such as Reynolds number, inflow shear profile and upstream effective roughness, on flow behaviour over the escarpment. The results show that the mean flow behaviour was generally not affected by the Reynolds number, however a slight increase in speed-up over the escarpment was observed for cases with lower upstream roughness. The shape of the inflow wind shear profile also had a minor impact on the mean flow near the escarpment. More significant effects were observed in the turbulent flow behaviour, where the turbulent kinetic energy (TKE) over the escarpment was found be a strong function of upstream roughness and a weak function of the Reynolds number. The local change in the upstream wind shear was found to have the most significant influence on the TKE magnitude, which more closely approximated the full-scale TKE data, and had not been previously observed in wind tunnel modelling of this topography.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

2009 ◽  
Vol 36 (10) ◽  
pp. 1643-1655 ◽  
Author(s):  
Ana Maria Ferreira da Silva ◽  
Habib Ahmari
Keyword(s):  
Flow Velocity ◽  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Flow Depth ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Open Channel Flows ◽  
The Mean

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

scholarly journals Instability wave models for the near-field fluctuations of turbulent jets

2011 ◽  
Vol 689 ◽  
pp. 97-128 ◽  
Author(s):  
K. Gudmundsson ◽  
Tim Colonius
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Microphone Array ◽  
Near Field ◽  
Turbulent Jets ◽  
Instability Wave ◽  
Mean Flow ◽  
Velocity Fluctuations ◽  
The Mean ◽  
Scale Structures

AbstractPrevious work has shown that aspects of the evolution of large-scale structures, particularly in forced and transitional mixing layers and jets, can be described by linear and nonlinear stability theories. However, questions persist as to the choice of the basic (steady) flow field to perturb, and the extent to which disturbances in natural (unforced), initially turbulent jets may be modelled with the theory. For unforced jets, identification is made difficult by the lack of a phase reference that would permit a portion of the signal associated with the instability wave to be isolated from other, uncorrelated fluctuations. In this paper, we investigate the extent to which pressure and velocity fluctuations in subsonic, turbulent round jets can be described aslinearperturbations to the mean flow field. The disturbances are expanded about the experimentally measured jet mean flow field, and evolved using linear parabolized stability equations (PSE) that account, in an approximate way, for the weakly non-parallel jet mean flow field. We utilize data from an extensive microphone array that measures pressure fluctuations just outside the jet shear layer to show that, up to an unknown initial disturbance spectrum, the phase, wavelength, and amplitude envelope of convecting wavepackets agree well with PSE solutions at frequencies and azimuthal wavenumbers that can be accurately measured with the array. We next apply the proper orthogonal decomposition to near-field velocity fluctuations measured with particle image velocimetry, and show that the structure of the most energetic modes is also similar to eigenfunctions from the linear theory. Importantly, the amplitudes of the modes inferred from the velocity fluctuations are in reasonable agreement with those identified from the microphone array. The results therefore suggest that, to predict, with reasonable accuracy, the evolution of the largest-scale structures that comprise the most energetic portion of the turbulent spectrum of natural jets, nonlinear effects need only be indirectly accounted for by considering perturbations to the mean turbulent flow field, while neglecting any non-zero frequency disturbance interactions.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

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