A Framework to Predict Combustion Noise and Instability: Case Study of a Partially Premixed Flame in a Backward-Facing Step Combustor

2017 ◽  
Author(s):  
Ashwin Kannan ◽  
S. R. Chakravarthy
Keyword(s):  
Large Scale ◽  
External Source ◽  
Acoustic Energy ◽  
Combustion Noise ◽  
Backward Facing Step ◽  
Multi Scale ◽  
Acoustic Feedback ◽  
Partially Premixed ◽  
Spatio Temporal ◽  
Partially Premixed Flame

Incompressible large eddy simulations coupled with acoustics are performed to predict combustion noise and instability in a partially premixed based backward facing step combustor. The computational analysis adopts a simultaneous multi-scale spatio-temporal framework for flow and acoustics such that the flow/acoustics varies at a shorter/longer length scale and a longer/shorter time scale respectively. This engenders flow dilatation and acoustic Reynolds stress (ARS) as the external source terms in the acoustic energy and flow momentum respectively. Numerical results are presented for three cases, at a particular Reynolds number, wherein two of them constitute acoustically coupled (coupled long duct case) and its uncoupled counterpart (no acoustic feedback). The third corresponds to a shorter combustor length (coupled short duct case). These three cases contrast the strong acoustic feedback in the short duct case, both of which are compared with the acoustically uncoupled LES that is common to them. It is found that combustion occurs predominantly in the large-scale vortical structures in the coupled long duct case due to enhanced mixing between the reactants brought about by the strong acoustic feedback (ARS). Thus, the present work is able to not only distinguish between the flow and acoustic processes, but also handle both combustion noise and instability within the same framework.

Effect of Acoustic Feedback on Lagrangian Coherent Structures in a Backward Facing Step Combustor With a Partially Premixed Flame

2017 ◽  
Author(s):  
Ramgopal Sampath ◽  
S. R. Chakravarthy
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Pressure Oscillation ◽  
Velocity Fields ◽  
Premixed Flame ◽  
Time Resolved ◽  
Acoustic Feedback ◽  
Acoustic Characterization ◽  
Partially Premixed ◽  
Partially Premixed Flame

The thermoacoustic oscillations of a partially premixed flame stabilized in a backward facing step combustor are studied at a constant equivalence ratio in long and short combustor configurations corresponding to with and without acoustic feedback respectively. We perform simultaneous time-resolved particle image velocimetry (TR-PIV) and chemiluminescence for selected flow conditions based on the acoustic characterization in the long combustor. The acoustic characterization shows a transition in the dominant pressure amplitudes from low to high magnitudes with an increase in the inlet flow Reynolds number. This is accompanied by a shift in the dominant frequencies. For the intermittent pressure oscillations in the long combustor, the wavelet analysis indicates a switch between the acoustic and vortex modes with silent zones of relatively low-pressure amplitudes. The short combustor configuration indicates the presence of the vortex shedding frequency and an additional band comprising the Kelvin Helmholtz mode. Next, we apply the method of finite-time Lyapunov exponent (FTLE) to the time-resolved velocity fields to extract features of the Lagrangian coherent structures (LCS) of the flow. In the long combustor post transition with the time instants with dominant acoustic mode, a large-scale modulation of the FTLE boundaries over one cycle of pressure oscillation is evident. Further, the FTLEs and the flame boundaries align each other for all phases of the pressure oscillation. In the short combustor, the FTLEs indicate the presence of small wavelength waviness that overrides the large-scale vortex structure, which corresponds to the vortex shedding mode. This behaviour contrasts with the premixed flame in the short combustor reported earlier in which such large scales were found to be seldom present. The presence of the large-scale structures even in the absence of acoustic feedback in a partially premixed flame signifies its inherent unstable nature leading to large pressure amplitudes during acoustic feedback. Lastly, the FTLE boundaries provide the frequency information of the identified coherent structure and also acts as the surrogate flame boundaries that are estimated from just the velocity fields.

scholarly journals Why a Large-Scale Mode Can Be Essential for Understanding Intracellular Actin Waves

Cells ◽  
2020 ◽  
Vol 9 (6) ◽  
pp. 1533 ◽  
Author(s):  
Carsten Beta ◽  
Nir S. Gov ◽  
Arik Yochelis
Keyword(s):  
Large Scale ◽  
Mass Conservation ◽  
Coarse Grained ◽  
Cellular Functions ◽  
Multi Scale ◽  
Spatio Temporal ◽  
Actin Waves ◽  
Inhibitor Type ◽  
Activator Inhibitor Type

During the last decade, intracellular actin waves have attracted much attention due to their essential role in various cellular functions, ranging from motility to cytokinesis. Experimental methods have advanced significantly and can capture the dynamics of actin waves over a large range of spatio-temporal scales. However, the corresponding coarse-grained theory mostly avoids the full complexity of this multi-scale phenomenon. In this perspective, we focus on a minimal continuum model of activator–inhibitor type and highlight the qualitative role of mass conservation, which is typically overlooked. Specifically, our interest is to connect between the mathematical mechanisms of pattern formation in the presence of a large-scale mode, due to mass conservation, and distinct behaviors of actin waves.

scholarly journals Multi-Scale Spatio-Temporal Feature Extraction and Depth Estimation from Sequences by Ordinal Classification

Sensors ◽  
2020 ◽  
Vol 20 (7) ◽  
pp. 1979
Author(s):  
Yang Liu
Keyword(s):  
Large Scale ◽  
Depth Map ◽  
Depth Estimation ◽  
Ordinal Classification ◽  
Multi Scale ◽  
3D Computer Vision ◽  
Error Metrics ◽  
Temporal Features ◽  
Spatio Temporal ◽  
Deep Learning Network

Depth estimation is a key problem in 3D computer vision and has a wide variety of applications. In this paper we explore whether deep learning network can predict depth map accurately by learning multi-scale spatio-temporal features from sequences and recasting the depth estimation from a regression task to an ordinal classification task. We design an encoder-decoder network with several multi-scale strategies to improve its performance and extract spatio-temporal features with ConvLSTM. The results of our experiments show that the proposed method has an improvement of almost 10% in error metrics and up to 2% in accuracy metrics. The results also tell us that extracting spatio-temporal features can dramatically improve the performance in depth estimation task. We consider to extend this work to a self-supervised manner to get rid of the dependence on large-scale labeled data.

Combustion Instabilities of Separated Stratified Swirling Flames With Different Degrees of Premixedness

2019 ◽  
Author(s):  
Xinyao Wang ◽  
Xiao Han ◽  
Xin Hui ◽  
Chi Zhang ◽  
Heng Song ◽  
...  
Keyword(s):  
Large Scale ◽  
Convective Motion ◽  
Dominant Frequency ◽  
Oscillation Period ◽  
Premixed Flame ◽  
Combustion Instabilities ◽  
Swirling Flames ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
Point Fast Fourier Transform

Abstract The effects of premixedness degrees on combustion instabilities of separated stratified swirling flames have been investigated experimentally in the Beihang Axial Swirler Independently-Stratified (BASIS) burner. The degree of premixedness is modulated by the fuel split between two injection positions in the outer stream. In the spectra of pressure oscillations, both the dominant frequency and amplitude of partially premixed flames are positively correlated with fuel split ratios. The partially premixed flame is found to feature a large-scale periodic convective motion based on CH* chemiluminescence images, which have been analyzed under different fuel split ratios by a point-to-point Fast Fourier Transform (FFT) method. The development of above convective motion is explained by combining the variation of pressure and heat release in the oscillation period. Local Rayleigh index maps show that the driving factor of combustion instability for the partially premixed flame mainly comes from the upstream of the combustor. Finally, thermoacoustic network analysis is applied to predict observed frequencies under both perfectly and partially premixed conditions. The supposed additional convective time due to equivalence ratio fluctuations and the elongated flame region for the partially premixed flame is validated by its longer time delay in the sensitivity analysis of the n-τ flame model.

scholarly journals Documentary data and the study of past droughts: a global state of the art

2018 ◽  
Vol 14 (12) ◽  
pp. 1915-1960 ◽  
Author(s):  
Rudolf Brázdil ◽  
Andrea Kiss ◽  
Jürg Luterbacher ◽  
David J. Nash ◽  
Ladislava Řezníčková
Keyword(s):  
Large Scale ◽  
State Of The Art ◽  
Drought Indices ◽  
Documentary Evidence ◽  
Climatic Trends ◽  
Instrumental Observations ◽  
Spatio Temporal ◽  
Epigraphic Evidence ◽  
Administrative Evidence

Abstract. The use of documentary evidence to investigate past climatic trends and events has become a recognised approach in recent decades. This contribution presents the state of the art in its application to droughts. The range of documentary evidence is very wide, including general annals, chronicles, memoirs and diaries kept by missionaries, travellers and those specifically interested in the weather; records kept by administrators tasked with keeping accounts and other financial and economic records; legal-administrative evidence; religious sources; letters; songs; newspapers and journals; pictographic evidence; chronograms; epigraphic evidence; early instrumental observations; society commentaries; and compilations and books. These are available from many parts of the world. This variety of documentary information is evaluated with respect to the reconstruction of hydroclimatic conditions (precipitation, drought frequency and drought indices). Documentary-based drought reconstructions are then addressed in terms of long-term spatio-temporal fluctuations, major drought events, relationships with external forcing and large-scale climate drivers, socio-economic impacts and human responses. Documentary-based drought series are also considered from the viewpoint of spatio-temporal variability for certain continents, and their employment together with hydroclimate reconstructions from other proxies (in particular tree rings) is discussed. Finally, conclusions are drawn, and challenges for the future use of documentary evidence in the study of droughts are presented.

scholarly journals Public Awareness: What Climate Change Scientists Should Consider

2020 ◽  
Vol 12 (20) ◽  
pp. 8369
Author(s):  
Mohammad Rahimi
Keyword(s):  
Climate Change ◽  
Social Media ◽  
Large Scale ◽  
Public Awareness ◽  
Connected Network ◽  
Multi Scale ◽  
Valuable Addition ◽  
Scientific Approach ◽  
Design Solutions ◽  
Mitigate Climate Change

In this Opinion, the importance of public awareness to design solutions to mitigate climate change issues is highlighted. A large-scale acknowledgment of the climate change consequences has great potential to build social momentum. Momentum, in turn, builds motivation and demand, which can be leveraged to develop a multi-scale strategy to tackle the issue. The pursuit of public awareness is a valuable addition to the scientific approach to addressing climate change issues. The Opinion is concluded by providing strategies on how to effectively raise public awareness on climate change-related topics through an integrated, well-connected network of mavens (e.g., scientists) and connectors (e.g., social media influencers).

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