Engineering Precursors to Forewarn the Onset of an Impending Combustion Instability

2014 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Time Scales ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Additional Measure ◽  
Statistical Measures ◽  
Random Manner ◽  
Low Amplitude ◽  
Measured Pressure

The present study aims at arming an operator of fielded turbulent combustors with a repertoire of mathematical measures for real-time monitoring to forewarn the onset of impending combustion instability. In turbulent combustors, the route to high-amplitude, periodic, combustion-driven oscillations from conditions of low-amplitude, chaotic, combustion noise happens through an intermediate regime in flow conditions where the measured pressure fluctuations display bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random manner amidst chaotic fluctuations. This loss of chaos from combustion noise to combustion instability can be quantified to serve as a precursor to impending instability. The recurrence properties of intermittent burst oscillations can be quantified using dynamical systems theory by tracking the distribution of the aperiodic segments in the measured signals. Several statistical measures may be constructed through such recurrence quantification that provide robust early warning signals to an impending instability. Further, the transition to combustion driven oscillations leads to a collapse of the number of relevant time scales involved in the problem. This collapse in time scales can be quantified using generalized Hurst exponents which serve as an additional measure that captures the onset of an impending combustion instability well in advance. The various patent pending measures illustrated in this paper serve as precursors due to the inevitable presence of an intermittent regime of burst oscillations in turbulent combustors.

Multifractality in combustion noise: predicting an impending combustion instability

2014 ◽  
Vol 747 ◽  
pp. 635-655 ◽  
Author(s):  
Vineeth Nair ◽  
R. I. Sujith
Keyword(s):  
Scale Invariance ◽  
Prognostic Value ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Warning Signals ◽  
Stochastic Background ◽  
Turbulent Flow Field ◽  
Low Amplitude

AbstractThe transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined, combustion environments was studied experimentally in a laboratory-scale combustor with two different flameholding devices in a turbulent flow field. We show that the low-amplitude, irregular pressure fluctuations acquired during stable regimes, termed ‘combustion noise’, display scale invariance and have a multifractal signature that disappears at the onset of combustion instability. Traditional analysis often treats combustion noise and combustion instability as acoustic problems wherein the irregular fluctuations observed in experiments are often considered as a stochastic background to the dynamics. We demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures such as Hurst exponents that can act as early warning signals to impending instability in fielded combustors.

scholarly journals Role of Shear Layer Instability in Driving Pressure Oscillations in a Backward Facing Step Combustor

2016 ◽  
Author(s):  
Srinivas K. Kirthy ◽  
Santosh Hemchandra ◽  
Seunghyuck Hong ◽  
Santosh Shanbhogue ◽  
Ahmed F. Ghoniem
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Acoustic Pressure ◽  
Combustion Instability ◽  
High Amplitude ◽  
Open Loop ◽  
Hydrodynamic Modes ◽  
Pressure Oscillations ◽  
Layer Mode ◽  
Low Amplitude

This paper presents a global hydrodynamic stability analysis of flow fields in a backward facing step combustor, assuming weakly non-parallel flow. The baseline experiments in a ‘long’ combustor of length of 5.0 m shows the presence of two combustion instability states characterized by coherent low and high amplitude acoustic pressure oscillations. The analysis is performed for Propane-air mixtures at three values of ϕ = 0.63, 0.72 and 0.85 which correspond to quiet, low amplitude and high amplitude instability states in the long combustor experiments. Base flow velocity and density fields for the hydrodynamic stability analysis are determined from time averaged PIV measurements made after the length of the duct downstream of the step has been shortened to eliminate acoustic pressure oscillations. The analysis shows that the shear layer mode is self-excited for the ϕ = 0.72 case with an oscillation frequency close to that of the long combustor’s fundamental acoustic mode. We show from an analysis of the weakly forced, variable density Navier-Stokes equations that self-excited hydrodynamic modes can be weakly receptive to forcing — suggesting that the low amplitude instability in the long combustor is due to semi-open loop forcing of heat release oscillations by the shear layer mode. At ϕ = 0.85 the flow is hydrodynamically globally stable but locally convectively unstable. Spatial amplification of velocity disturbances by the convectively unstable flow causes high amplitude combustion instability in the long combustor case. These results show that combustion instability can be sustained by acoustic and hydrodynamic modes being either strongly coupled, resulting in fully closed loop forcing, or weakly coupled, resulting in semi-open loop forcing of the flame by a self-excited hydrodynamic mode.

Intermittency route to thermoacoustic instability in turbulent combustors

2014 ◽  
Vol 756 ◽  
pp. 470-487 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Bluff Body ◽  
Combustion Instability ◽  
Signal Amplitude ◽  
High Amplitude ◽  
Operating Conditions ◽  
Combustion Noise ◽  
Continuous Measure ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Amplitude Fluctuations

AbstractThe dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained.

scholarly journals Response to Comment on “Circadian rhythms in the absence of the clock gene Bmal1”

Science ◽  
2021 ◽  
Vol 372 (6539) ◽  
pp. eabf1941
Author(s):  
Sandipan Ray ◽  
Utham K. Valekunja ◽  
Alessandra Stangherlin ◽  
Steven A. Howell ◽  
Ambrosius P. Snijders ◽  
...  
Keyword(s):  
Circadian Rhythms ◽  
Clock Gene ◽  
Statistical Significance ◽  
Statistical Measures ◽  
Statistical Algorithm ◽  
Low Amplitude ◽  
Lines Of Evidence

Abruzzi et al. argue that transcriptome oscillations found in our study in the absence of Bmal1 are of low amplitude, statistical significance, and consistency. However, their conclusions rely solely on a different statistical algorithm than we used. We provide statistical measures and additional analyses showing that our original analyses and observations are accurate. Further, we highlight independent lines of evidence indicating Bmal1-independent 24-hour molecular oscillations.

Frequency Response Analysis of Piecewise Nonlinear Vibration Isolator

2005 ◽  
Author(s):  
Patrick Stahl ◽  
G. Nakhaie Jazar
Keyword(s):  
Low Frequency ◽  
High Amplitude ◽  
Relative Displacement ◽  
Response Analysis ◽  
Frequency Response Analysis ◽  
State Behavior ◽  
Vibration Isolators ◽  
Short Period ◽  
Secondary Suspension ◽  
Low Amplitude

Non-smooth piecewise functional isolators are smart passive vibration isolators that can provide effective isolation for high frequency/low amplitude excitation by introducing a soft primary suspension, and by preventing a high relative displacement in low frequency/high amplitude excitation by introducing a relatively damped secondary suspension. In this investigation a linear secondary suspension is attached to a nonlinear primary suspension. The primary is assumed to be nonlinear to model the inherent nonlinearities involved in real suspensions. However, the secondary suspension comes into action only during a short period of time, and in mall domain around resonance. Therefore, a linear assumption for the secondary suspension is reasonable. The dynamic behavior of the system subject to a harmonic base excitation has been analyzed utilizing the analytic results derived by applying the averaging method. The analytic results match very well in the transition between the two suspensions. A sensitivity analysis has shown the effect of varying dynamic parameters in the steady state behavior of the system.

scholarly journals Loss of Chaos in Combustion Noise as a Precursor of Impending Combustion Instability

2013 ◽  
Vol 5 (4) ◽  
pp. 273-290 ◽  
Author(s):  
Vineeth Nair ◽  
Gireehkumaran Thampi ◽  
Sulochana Karuppusamy ◽  
Saravanan Gopalan ◽  
R. I. Sujith
Keyword(s):  
Combustion Instability ◽  
Combustion Noise

Spinning Behaviour of Diametral Acoustic Modes in Deep Axisymmetric Cavities With Chamfered Edges

2014 ◽  
Author(s):  
Oleksandr Barannyk ◽  
Peter Oshkai
Keyword(s):  
Pressure Fluctuations ◽  
High Amplitude ◽  
Pronounced Change ◽  
Axisymmetric Cavity ◽  
Acoustic Modes ◽  
Splitter Plates ◽  
Flow Oscillations ◽  
Axisymmetric Cavities ◽  
Spinning Behaviour ◽  
Stationary Behaviour

Spinning behaviour of diametral acoustic modes associated with self-sustained flow oscillations in a deep, axisymmetric cavity located in a long pipeline was investigated experimentally. High-amplitude pressure fluctuations resulted from the excitation of the diametral acoustic modes by the fully-turbulent flow in the pipeline. The unsteady pressure was measured at three equally spaced azimuthal locations at the bottom of the cavity. This arrangement allowed calculation of the azimuthal orientation of the acoustic modes, which were classified as stationary, partially spinning or spinning. Introduction of shallow chamfers to the upstream and the downstream edges of the cavity resulted in changes of azimuthal orientation and spinning behaviour of the acoustic modes. In addition, introduction of splitter plates in the cavity led to pronounced change in the spatial orientation and the spinning behaviour of the acoustic modes. The short splitter plates changed the behaviour of the dominant acoustic modes from partially spinning to stationary, while the long splitter plates enforced the stationary behaviour across all resonant acoustic modes.

Application of instantaneous rotations to S‐wave vertical seismic profiling

Geophysics ◽  
1997 ◽  
Vol 62 (5) ◽  
pp. 1365-1368
Author(s):  
M. Boulfoul ◽  
Doyle R. Watts
Keyword(s):  
High Amplitude ◽  
Seismic Profile ◽  
P Wave ◽  
Petroleum Exploration ◽  
S Wave ◽  
Amplitude Variation With Offset ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Seismic Models ◽  
Low Amplitude

The petroleum exploration industry uses S‐wave vertical seismic profiling (VSP) to determine S‐wave velocities from downgoing direct arrivals, and S‐wave reflectivities from upgoing waves. Seismic models for quantitative calibration of amplitude variation with offset (AVO) data require S‐wave velocity profiles (Castagna et al., 1993). Vertical summations (Hardage, 1983) of the upgoing waves produce S‐wave composite traces and enable interpretation of S‐wave seismic profile sections. In the simplest application of amplitude anomalies, the coincidence of high amplitude P‐wave reflectivity and low amplitude S‐wave reflectivity is potentially a direct indicator of the presence of natural gas.

Variability of the dynamics of somatometric parameters in schoolchildren with different nutritional status (longitudinal study)

2020 ◽  
Vol 15 (5) ◽  
pp. 68-72
Author(s):  
V.L. Gritsinskaya ◽  
◽  
V.P. Novikova ◽  
A.I. Khavkin ◽  
◽  
...  
Keyword(s):  
Nutritional Status ◽  
High Amplitude ◽  
Physical Development ◽  
Growth Spurt ◽  
Pubertal Growth Spurt ◽  
Pubertal Growth ◽  
Normal Limits ◽  
Study Participants ◽  
Two Peaks ◽  
Low Amplitude

Objective. To identify specific features of pubertal growth spurt in adolescents depending on their nutritional status in prepuberty. Patients and methods. We analyzed the dynamics of height and weight in 645 children (331 boys and 314 girls) aged between 8 and 16 years. All study participants were divided into three groups depending on whether their weight and height at the age of 8 years were within the normal limits given in the ‘WHO Growth Reference 2007’: children with physical development; underweight children; and overweight children. Results. The dynamics of somatometric parameters during pubertal growth spurt varied between children with different nutritional status. Underweight boys demonstrated prolonged and low-amplitude pubertal growth pattern; in boys with normal physical development, the growth spurt was usually shorter and had high amplitude. In overweight boys, the pubertal growth spurt started with higher annual increase in height, had a more pronounced amplitude, and was shorter than in peers (р < 0.001 ÷ р < 0.05). Both underweight girls and girls with normal physical development demonstrated low-amplitude pubertal growth spurt lasting for two years. Overweight girls had two peaks of pubertal growth spurt, which usually started earlier than in other girls (р < 0.001 ÷ р < 0.01). Conclusion. Our findings can be used as a guide for predicting pubertal spurt in children during medical examinations, determining adequate physical activity in physical education classes at school and in sports sections. Key words: children, nutritional status, pubertal growth spurt

Oxygen Consumption by the Sea Anemone Calliactis Parasitica (Couch)

1980 ◽  
Vol 88 (1) ◽  
pp. 367-374
Author(s):  
A. E. BRAFIELD
Keyword(s):  
Oxygen Consumption ◽  
Oxygen Concentration ◽  
Low Frequency ◽  
High Amplitude ◽  
Total Oxygen ◽  
Strip Chart ◽  
Calliactis Parasitica ◽  
Cycle Lengths ◽  
Mean Cycle ◽  
Low Amplitude

Oxygen consumption by Calliactis parasitica, measured in a continuousflow polarographic respirometer, yielded a slope of 0·92 when plotted against body weight on log scales. This high value is discussed in terms of the sea anemone's basically laminate nature. Strip-chart records of the oxygen concentration of water which had just passed a specimen of Calliactis commonly showed rhythmic fluctuations, either of low amplitude and high frequency or high amplitude and low frequency (mean cycle lengths 11 and 34 min respectively). The fluctuations are explained in terms of rhythmic muscular contractions which irrigate the enteron for respiratory purposes. Analysis of the slow fluctuations indicates that the endoderm is responsible for about 18% of the total oxygen consumption. The oxygen concentration of water in the enteron, measured and recorded continuously, was 4–27% of the air-saturation level. These strip chart records also frequently showed rhythmic fluctuations (mean cycle length 12 min), apparently resulting from the muscular contractions.

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