Self-excited Flow Oscillations Research Articles

Aeroacoustic Tonal Noise: Theoretical and Practical Approaches for Flow and Sound Analysis

Aeroacoustic tonal noise, often caused by self-excited flow oscillations, represents a significant challenge in various engineering applications. One prominent example is the cavity tone, responsible for undesirable sound emissions from gaps found on the surfaces of vehicles, door gaps, and pantographs of trains. A similar configuration, known as the edge tone, appears in musical instruments. This paper aims at presenting a comprehensive framework consisting of practical and theoretical tools to analyze these complex flows and the resulting emitted sound. These tools can facilitate the development of mitigation strategies and the optimization of design parameters to minimize unwanted noise emissions.

Mechanisms for Absolute Instabilities in Uniform and Nonuniform Density Swirling Flows

In this paper, transitions from convective to absolute instability in post-vortex-breakdown swirling flow with nonuniform density are investigated using linear stability analysis. Addition of swirl to flow in devices of practical interest achieves rapid mixing, but also renders such flows absolutely unstable, leading to self-excited flow oscillations. Through the spatiotemporal analysis, the onset of absolute instability is systematically analyzed in parametric space for uniform and nonuniform density cases. Five unstable azimuthal wavenumbers (, , ) studied show that, although mode first becomes absolutely unstable, it is the mode associated with the precessing vortex core that turns out to be more unstable for high swirl numbers and reduced shear layer thickness. The imposition of density field, representative of reacting flows, suppresses the absolute instability mode by shifting its onset to higher backflow velocities. Further, to investigate the cause of instabilities, a total disturbance energy equation is obtained with various energy feeding/depleting terms. Although the axial shear is the dominant production term, the alignment of pressure redistribution and density variation terms provides the necessary stabilization to weaken the absolute instabilities.

Control of self-excited thermoacoustic oscillations using transient forcing, hysteresis and mode switching

In many combustion devices, strong self-excited flow oscillations can arise from feedback between unsteady heat release and acoustics, resulting in increased vibration and pollutant emissions. Open-loop acoustic forcing has been shown to be effective in weakening such thermoacoustic oscillations, but current implementations of this control strategy require the forcing to be continuously applied. In this proof-of-concept study, we experimentally demonstrate an alternative method of weakening thermoacoustic oscillations in a self-excited combustion system – a laminar premixed flame in a double open-ended tube. Unlike existing methods, the proposed method combines the use of transient forcing with hysteresis and mode switching, thus avoiding the need to continuously supply energy to the control system. Control is achieved by exploiting the fact that most combustors have a multitude of natural thermoacoustic modes, some of which are linearly unstable but some are nonlinearly unstable. By applying open-loop acoustic forcing at an off-resonance frequency and at an amplitude higher than that required for synchronization, we find that the combustor can switch to one of the nonlinearly unstable natural modes (f2) and remain there, even after the forcing is removed. Dynamic mode decomposition of high-speed chemiluminescence videos shows that this mode switching occurs because the flame structure at f2 is more robust than that at the original linearly unstable natural mode. The final unforced state has a thermoacoustic amplitude of just half that of the initial unforced state, even though the Rayleigh index of the former is higher than that of the latter. Although this 50% reduction in thermoacoustic amplitude is not as large as the 95% reduction achieved with asynchronous quenching, it is achieved without the use of continuous forcing. This is a distinct advantage over existing control strategies as it allows the complexity and power requirements of the control system to be reduced. With further development and testing, particularly on turbulent swirling combustors, the proposed control strategy could pave the way for a new class of open-loop control techniques based on transient forcing rather than continuous forcing.

Mean flow stability analysis of oscillating jet experiments

AbstractLinear stability analysis (LSA) is applied to the mean flow of an oscillating round jet with the aim of investigating the robustness and accuracy of mean flow stability wave models. The jet’s axisymmetric mode is excited at the nozzle lip through a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1 % to 100 %. The instantaneous flow field is measured via particle image velocimetry (PIV) and decomposed into a mean and periodic part utilizing proper orthogonal decomposition (POD). Local LSA is applied to the measured mean flow adopting a weakly non-parallel flow approach. The resulting global perturbation field is carefully compared with the measurements in terms of spatial growth rate, phase velocity, and phase and amplitude distribution. It is shown that the stability wave model accurately predicts the excited flow oscillations during their entire growth phase and during a large part of their decay phase. The stability wave model applies over a wide range of forcing amplitudes, showing no pronounced sensitivity to the strength of nonlinear saturation. The upstream displacement of the neutral point and the successive reduction of gain with increasing forcing amplitude is very well captured by the stability wave model. At very strong forcing ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{>}40\, \%$), the flow becomes essentially stable to the axisymmetric mode. For these extreme cases, the prediction deteriorates from the measurements due to an interaction of the forced wave with the geometric confinement of the nozzle. Moreover, the model fails far downstream in a region where energy is transferred from the oscillation back to the mean flow. This study supports previously conducted mean flow stability analysis of self-excited flow oscillations in the cylinder wake and in the vortex breakdown bubble and extends the methodology to externally forced convectively unstable flows. The high accuracy of mean flow stability wave models as demonstrated here is of great importance for the analysis of coherent structures in turbulent shear flows.

Why Nonuniform Density Suppresses the Precessing Vortex Core

Linear stability analysis is applied to a swirl-stabilized combustor flow with the aim to understand how the flame shape and associated density field affects the manifestation of self-excited flow instabilities. In isothermal swirling jets, self-excited flow oscillations typically manifest in a precessing vortex core and synchronized growth of large-scale spiral-shaped vortical structures. Recent theoretical studies relate these dynamics to a hydrodynamic global instability. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density inhomogeneities created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a perfectly premixed steam-diluted detached flame featuring a strong precessing vortex core. The second represents a perfectly premixed dry flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change in instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows for relating the flame position and the resulting density field to the emergence of a precessing vortex core.

An experimental investigation of the flow field between two radial plates

The flow field in a radial diffuser downstream of a radial impeller is highly complex, as the flow is turbulent, unsteady, viscous, and three-dimensional. Depending on the initial conditions at the diffuser inlet, the flow may separate from one or both walls of the diffuser. The objective of this experimental investigation of flow characteristics in a radial vaneless diffuser of a centrifugal blower is to enhance the performance of centrifugal compressors over a wide operating range. Numerical and experimental investigations of rotating stall effects on compressors and blowers have been conducted over the past four decades to achieve this objective. The flow characteristics in a radial diffuser were measured while changing the impeller speed and keeping other parameters such as diffuser radius and width ratio constant. The velocity and pressure distributions in the diffuser were measured along a radial path using a miniature X-wire probe and dynamic as well as static pressure transducers. The axisymmetrical radial and tangential mean velocity distributions and their statistics, as well as pressure distributions, are reported. Some of the results presented in this paper agree with those from earlier work on this subject. For example, the flow angle α (measured at half the diffuser width and from the diffuser inlet to its outlet) did not exceed 65°. This means that α < αc, although a recirculation region was present on the diffuser shroud wall. The critical flow angle αc required to initiate self-excited flow oscillations has been generally reported to be about 78°. Current data also indicate that the flow exiting the impeller is skewed, as revealed by the triple velocity product correlations. The current data are valuable for vaneless diffuser numerical model developers, since there are very limited similar data available in the open literature. The current data support the opinion that the flow separation is not enough to trigger or initiate stall at the diffuser.

Numerical investigation on the self-excited oscillation of wet steam flow in a supersonic turbine cascade

Abstract The self-excited flow oscillation due to specific supercritical heat addition during the condensation process in wet steam turbine is a important issue. With an Eulerian/Eulerian model, the self-excited oscillation of wet steam flow in a supersonic turbine cascade is investigated. A proper inlet supercooling results in the transition from steady flow to self-excited oscillating flow in the cascade of steam turbine. The frequency dependency on the inlet supercooling is not monotonic. The flow oscillation leads to non-synchronous periodical variation of the inlet and outlet mass flow rate. The aerodynamic force on the blade varies periodically due to the self-excited flow oscillation. With the frequency lies between 18.1-80-64 Hz, the oscillating flow is apt to act with the periodical variation of the inlet supercooling due to stator rotor interaction in a syntonic pattern, and results in larger aerodynamic force on the blade. The loss in the oscillating flow increases 20.64% compared with that in the steady flow.

Analysis of Self-Excited Oscillation of Fluid Flow in a Collapsible Tube by Introducing Incremental Theory

This study is to simulate the self-excited oscillation taking place in a flow of the collapsible tube such as a venular vessel or a natural rubber tube. In order to understand the mechanism, one dimensional model is used to formulate the strong interaction between fluid and tube. In flow region, it is assumed that one dimensional incompressible flow with varying widths of tube is considered. The stabilized Galerkin method combined with the predictor multi-corrector algorithms is im plemented. In structure region, the beam element is used for longitudinal deformation of the tube and the experimental formula, tube law, been employed to evaluate the circumferential stiffness. The tube is subjected to very large deformation and unstable, bucking, characteristic. Then, we tried to introduce incremental theory of virtual work principle. The HHT-α method is also used in the timeintegration schemes. By introducing the incremental formulation, significant improvement is obtained. The mechanism and the relationship between pressure and deformation of the self-excited oscillation were shown in these computations of simple model.

Imaging of the self-excited oscillation of flow past a cavity during generation of a flow tone

Flow through a pipeline-cavity system can give rise to pronounced flow tones, even when the inflow boundary layer is fully turbulent. Such tones arise from the coupling between the inherent instability of the shear flow past the cavity and a resonant acoustic mode of the system. A technique of high-image-density particle image velocimetry is employed in conjunction with a special test section, which allows effective laser illumination and digital acquisition of patterns of particle images. This approach leads to patterns of velocity, vorticity, streamline topology and hydrodynamic contributions to the acoustic power integral. Comparison of global, instantaneous images with time- and phase-averaged representations provides insight into the small-scale and large-scale concentrations of vorticity, and their consequences on the topological features of streamline patterns, as well as the streamwise and transverse projections of the hydrodynamic contribution to the acoustic power integral. Furthermore, these global approaches allow the definition of effective wavelengths and phase speeds of the vortical structures, which can lead to guidance for physical models of the dimensionless frequency of oscillation.

自励振動中のコラプシブルチューブ断面形状の可視化

We studied the characteristics of self-excited oscillation of flow in collapsible tube. Velocity profile along the tube was measured using a laser Doppler velocimeter (LDV). The cross section of the tube was visualized using a laser light sheet, and its shape was picturized using a high-speed video camera. Flow rate and pressure at up- and downstreams of the tube were measured using a electromagnetic flowmeter (EMF) and a pressure transducer. The following results were obtained : when difference between pressures at upstream and downstream ends was maximum, cross-sectional area of the tube became minimum. Velocity profile inside the cross section was influenced a great deal by the shape and upstream and downstream shapes of the cross section. In the upstream of the collapsed position flow was blocked, and in its downstream flow separation took place.

Numerical simulation of flow oscillations in stenotic arterial segment

The hypothesis that stenosis-induced flow disturbances may be the cause of vascular sounds was investigated. The onset of flow oscillations in a vessel with a severe constriction was simulated by a computer model based on the axi-symmetric Navier–Stokes equations. Model includes fluid non-linear inertial forces, viscoelastic wall motion, anatomical taper and proximal and distal circulation. Self-excited flow oscillations with different oscillatory patterns were reproduced. A common characteristic of such flow oscillations was a high-frequency component in the audible band (100–500 Hz). Numerical results support the hypothesis that vascular sounds may be generated by high-frequency flow oscillations.

Low-dimensional control of the circular cylinder wake

Many wake flows exhibit self-excited flow oscillations which are sustained by the flow itself and are not caused by amplification of external noise. The archetypal example of a self-excited wake flow is the low Reynolds number flow past a circular cylinder. This flow exhibits self-sustained periodic vortex shedding above a critical Reynolds number. In general, control of such flows requires stabilization of many globally unstable modes; the present work describes a multiple-sensor control strategy for the cylinder wake which succeeds in controlling a simplified wake model at a Reynolds number above that at which single-sensor schemes fail.Representation of the flow field by a finite set of coherent structures or modes, which are extracted by proper orthogonal decomposition and correspond to the large-scale wake components, allows the efficient design of a closed-loop control algorithm. A neural network is used to furnish an empirical prediction of the modal response of the wake to external control forcing. This model avoids the need for explicit representation of the control actuator–wake interaction. Additionally, the neural network structure of the model allows the design of a robust nonlinear control algorithm. Furthermore the controller does not necessarily require velocity field information, but can control the wake using other quantities (for example flow visualization pictures) which characterize the structure of the velocity field. Successful control of a simplified cylinder wake model is used to demonstrate the feasibility of the low-dimensional control strategy.

The Effect of Longitudinal Tension on Flow in Collapsible Tube.

Simulation experiments in vitro related to flow through a vein and an airway of the lung were performed. The effect of longitudinal tension applied to thin-walled silicone rubber tubes on the relationship between the cross-sectional area of the tube and the transmural pressure, and on static and dynamic relationships between pressure drop and flowrate through the tube was investigated using five different kinds of specially designed tubes of the same size with different applied longitudinal tensions. As a result, the following facts were clarified: Applying longitudinal tension to the tube made it more difficult for the tube to be collapsed. The movement of the tube wall along the tube axis during the self-excited oscillation of flow was restricted and the most greatly collapsed portion shifted upstream. The amplitude of the oscillatory pressure drop was much larger than its time-averaged value. The time required for the tube to collapse gradually from a fully open state amounted to 80 to 85% of the period of the oscillation.

Dynamical System Analyses of Aperiodic Flow-Induced Oscillations of a Collapsible Tube

The paper briefly reviews dynamical systems analysis concepts and techniques which were applied to gain knowledge of a particular biomechanical problem, that of self-excited oscillation of flow through the collapsed flexible tubes found in many different parts of the body. The tubes investigated had a relatively thick wall and the through-flow was turbulent. The methods deployed did not suffice to indicate unequivocally the presence of a low-dimensional chaotic attractor amid this high-dimensional or random influence. Chaos has recently been demonstrated in the same system when upstream forcing interacts with otherwise periodic oscillations.

Relationship of Pressure Wave Velocity to Self-Excited Oscillation of Collapsible Tube Flow

o investigate the causes of self-excited oscillations of collapsible tube flow, measurements of dynamic tube deformation were conducted using a laser displacement meter supported on a pulse-motor stage together with an electromagnetic flow meter. The results have clarified that fluid velocity approaches pressure wave velocity, which is estimated from the tube law, at the throat around the stability boundaries and that a supercritical flow and subcritical flow occur alternately during oscillations. By contrast, a linearized lumped parameter model has been developed. The model demonstrates that slight supercritical flow leads to instability of the negative stiffness type and also suggests that negative dynamic resistance induces instability of the negative damping type. Although causes such as unsteady flow separation were not examined here, a self-excited oscillation caused by supercritical flow close to choking has been verified experimentally and theoretically.

Relation of Pressure Wave Velocity to Self-Excited Oscillation of Collapsible Tube Flow.

To investigate causes of self - excited oscillations of collapsible tube flow, measurements of dynamic tube deformation were conducted with a laser displacement meter carried on a pulse-motor stage together with an electromagnetic flow meter. The results have clarified that fluid velocity approaches pressure wave velocity, which is estimated from the tube law, at the throat around the stability boundaries and that supercritical flow and subcritical flow occur alternately during oscillations. On the other hand, a linearized lumped parameter model has been developed. The model explains that slight supercritical flow leads to jnstability of negative stiffness type and also suggests that dynamic negative resistance induces instability of negative damping type. Though the causes such as unsteady flow separation were not examined here, a self-excited oscillation caused by supercritical flow close to choking has been verified experimentally and theoretically.

Measurements of wave speed and compliance in a collapsible tube during self-excited oscillations: a test of the choking hypothesis

Indirect evidence links self-excited oscillation of flow through collapsed tubes with choking, defined by the cross-sectionally averaged fluid speed u reaching the local speed of small pressure waves c. This was tested by measuring both c-u and c as functions of tube cross-sectional area during self-excited oscillation, using small superimposed high-frequency wave packets. The wavespeed c was derived from the local slope of the pressure/area relationship, measured at both high and low frequency, while c-u was taken as the upstream propagation rate of the pressure disturbances. When u = 0, these were shown to agree with each other. The propagation results showed that choking did not occur at high frequency. At the low frequency of the self-excited oscillation the results were less conclusive, because of dispersion and indirect methodology, but choking appeared not to happen at the modest flow rate of the oscillation investigated. Results on the attenuation of the wave packets were successfully explained using a model of the tube throat consisting of two equal and opposite reflection sites.

Self-Excited Oscillation of Transonic Flow Around an Airfoil in Two-Dimensional Channels

A self-excited oscillation of transonic flow in a simplified cascade model was investigated experimentally, theoretically and numerically. The measurements of the shock wave and wake motions, and the unsteady static pressure field predict a closed-loop mechanism, in which the pressure disturbance that is generated by the oscillation of boundary layer separation propagates upstream in the main flow and forces the shock wave to oscillate, and then the shock oscillation disturbs the boundary layer separation again. A one-dimensional analysis confirms that the self-excited oscillation occurs in the proposed mechanism. Finally, a numerical simulation of the Navier–Stokes equations reveals the unsteady flow structure of the reversed flow region around the trailing edge, which induces the large flow separation to bring about the antiphase oscillation.

Unsteady Flow in a Centrifugal Compressor With Different Types of Vaned Diffusers

Experiments were conducted to investigate the characteristics of self-excited flow oscillations in a high-performance centrifugal compressor system with a straight channel radial vaned diffuser. Fast response dynamic pressure transducers on the shroud wall and blade-mounted strain gages were used to identify the onset of the oscillations and their characteristics in space and time. In addition, flow characteristics near the shroud wall were visualized by an oil injection method, showing the extent of upstream directed reverse flow in the impeller range during significant unsteady flow compressor operation. Rotating nonuniform flow patterns were found in a wide range of operating speeds before the occurrence of surge. The number of lobes in the nonuniform flow patterns was dependent on the operating conditions and varied from two to four. Results of this experimental investigation were compared with those obtained from a previous investigation of the same compressor but with a cambered vane diffuser. Considerable similarity between the two configurations was found in the spatial distribution of the unsteady pressure field and in the frequencies of the fluctuations. The stability margin before the occurrence of surge and the operating regimes in which very intense pressure fluctuations were found were however different. In both cases, flow visualization techniques revealed the occurrence of reversed flow near the shroud wall of the impeller. Reverse flow extent up to the leading edge of the splitter blades systematically correlated with the occurrence of a nonuniform pressure pattern rotating with relatively high speed. Low rotational speed pressure patterns were observed when the extent of the reverse flow was up to the leading edge of the long blade. These different flow characteristics can be related to the occurrence of distinct rotating stall cell numbers. This result could be confirmed by unsteady pressure and blade vibration measurements.

Shock-Induced Flow Oscillations in Steam Turbine Diffusers

In exhaust diffusers of steam turbines pressure oscillations may occur at certain operating conditions, due to shock–boundary layer interactions. These self-excited flow oscillations are caused by the high-speed clearance flow between blade tip and outer contour, leading to an excitation of the running blades. The paper describes the behavior of this unsteady flow phenomenon in the diffuser, identified with steady state and unsteady measurements at the wall, along with probe measurements behind the runner blading. A deeper physical understanding was gained by simulation of this flow pattern on the base of the water analogy.