David Arthurs

Acoustic Excitation by Flow in T-Junctions

The flow-acoustic nature of sharp-edged T-junctions is investigated experimentally. In this paper, the pipes forming the T-crossbar are referred to as the branches and the pipe forming the central stem of the T-shape is referred to as the main pipe. Four test cases are studied corresponding to: (a) T-junction with flow from each branch into the main pipe; (b) T-junction with flow from one branch into the main pipe, the other branch being closed; (c) T-junction with flow from the main pipe into the two branches, which is the reverse flow situation of the first case; and (d) T-junction with flow from the main pipe into one branch and the other branch is closed, which is the second case with reverse flow. It is found that the flow at the T-junction can excite the pipe acoustic modes to varying degrees, depending on the flow direction and piping configuration. For cases (c) and (d), the dimensionless pressure amplitude of the acoustic mode reaches a maximum at a Strouhal number similar to that of the turbulence broadband peak measured in the separation bubble downstream of the T-junction corner. Cases (a) and (b) exhibit a different type of flow-acoustic coupling. In both cases, the maximum acoustic pressure occurs at a Strouhal number which is different from that observed in the separation bubble. In addition, the pulsation amplitude is substantially stronger than that observed in cases (c) and (d). Detuning the branches weakens the resonance intensity, especially in case (c), which exhibits the strongest acoustic resonance.

Flow Induced Acoustic Resonances of an Annular Duct With Co-Axial Side Branches

This paper investigates the aeroacoustic response of an annular duct with co-axial side branches, and examines the effect of several passive countermeasures. The investigated geometry is inspired by the design of the Roll Posts in the Rolls-Royce LiftSystem® engine, which is currently being developed for the Lockheed Martin Joint Strike Fighter aircraft. The effects of design parameters, such as diameter ratio, branch length ratio and thickness of the annular flow on the frequency and resonance intensity of the first acoustic mode are studied experimentally. Numerical simulations of the acoustic mode shapes and frequencies are also performed. The annular flow has been found to excite several acoustic modes, the strongest in all cases being the first acoustic mode, which consists of an antisymmetric quarter wavelength along the length of each branch. The ratios of the branch length and diameter, with respect to the main duct diameter, have been found to have strong effects on the frequency of the acoustic modes. The effects of passive measures to weaken the acoustic coupling between the co-axial branches have been also investigated. These include detuning the co-axial branches, by decreasing the length of one branch, and installing a pair of splitter plates in the annular flow duct.Copyright

Flow-Sound Interaction Mechanisms in the Wake of Two Side-By-Side Cylinders

The phenomenon of flow-excited acoustic resonance, where periodic flow oscillations are enhanced by a resonant sound field, is a design concern in many engineering applications such as in heat exchangers, piping systems, and cavity flows. This study experimentally examines the phenomenon of flow-excited acoustic resonance for two side-by-side cylinders in a duct with cross-flow. This geometry has been investigated for three cylinder spacing ratios, defined as the center-to-center distance between the cylinders normalized by their diameter, of T/D = 1.25, 1.46 and 2.5, and for a range of acoustic pressure amplitudes. Intermediate and small spacing ratios have been given special attention, as these cases have been found to exhibit bistable flow in the wake in the absence of acoustic resonance. Phase-locked PIV measurements reveal that the self-excited sound field produces a strong oscillatory flow pattern in the cylinder wakes, which is symmetric for large spacing ratios and high acoustic amplitudes, but remains bistable for small spacing ratios, even during very intense acoustic resonances. The aeroacoustic sources and sinks within the flow field, computed using Howes theory of vortex sound, are compared for the range of spacing ratios and acoustic pressure amplitudes examined in this study.

Self-Excited Oscillations of Two Opposing Planar Jets

The self-excited oscillation generated by two opposing planar jets is investigated experimentally. Strong flow oscillations resulting in intense acoustic tone generation are observed for wide ranges of jet flow velocity and distance between the opposing jet exits. A study of phase-locked particle image velocimetry (PIV) complemented with sound measurements is performed to clarify the self-excitation mechanism and the oscillation pattern(s) of the opposing jets.

Identification of Aeroacoustic Sources in a T-Junction

This paper experimentally investigates the flow-sound interaction mechanisms in a T-junction combining the flow from its two co-axial side-branches into the central branch. The T-junction has a sudden area expansion at each side-branch entrance. Flow separation at these area expansions forms free shear layers which are shown to excite the acoustic mode(s) of the branches over several ranges of flow velocity, each of which results from the coupling of the acoustic mode with a different shear layer oscillation mode. Phase-locked particle image velocimetry is utilized to detail the unsteady flow field over the acoustic cycle for the oscillation mode which resulted in the strongest acoustic resonance. Finite element analysis is used to characterize the excited acoustic mode shape and its associated particle velocity field. In-depth analysis of the flow-sound interaction mechanism inside the T-junction is performed by means of Howe’s acoustic analogy. It is concluded that the flow-sound interaction mechanism in the entrance region of the T-junction produces a spatially alternating pattern of acoustic energy generation and absorption. This alternating pattern of energy exchange between the flow and sound fields results in a minimal amount of net acoustic power being generated in the entrance region. However, the increasing orthogonality between the acoustic particle streamlines and the flow streamlines near the exit of the T-junction at its center results in the majority of the generated sound power which sustains the acoustic resonance.Copyright

The Effect of Fluid-Resonant Coupling in High-Speed Impinging Planar Jet Flows

This study investigates the effect of fluid-resonant coupling, i.e. the coupling between unstable modes of an impinging jet with resonant acoustic modes occurring between the nozzle and the impingement surface, on the self-excited oscillations of high-speed impinging planar jet. In order to investigate this phenomenon, a series of experiments have been performed using a high-speed impinging planar jet with varying nozzle thickness (h) and impingement distance (xo), for a single Mach number in the compressible flow regime. The test results reveal that the jet oscillation is controlled by a fluid-dynamic mechanism for small impingement distances, where the unstable mode of the jet is controlled by the impingement ratio. At larger impingement distances, the response is dominated by a fluid-resonant mechanism, in which the various hydrodynamic modes of the jet couple with different resonant acoustic modes occurring between the nozzle and the impingement surface. Within the fluid-resonant regime the system produces acoustic tones that are excited predominantly as a function of the impingement distance, with the nozzle thickness and impingement ratio having only minor effects on the tone frequency. Flow visualization images show that the same unstable mode is excited for multiple nozzle thicknesses at a constant impingement distance, despite the wide variations in associated impingement ratio.Copyright

Aeroacoustics of Opposing Planar Air Jets

The self-excited response of two opposing planar air jets is experimentally examined as a function of both flow velocity of the jets, as well as the impingement distance, i.e. the distance z between them. This initial study is performed for a single jet thickness of h = 2 mm and a spanwise aspect ratio of 50. Two sets of experiments have been performed, with the first set of measurements having been obtained over a range of impingement ratios from z/h = 7 to z/h = 130 for several moderate Mach numbers (M = 0.44, 0.58 & 0.73), whereas the second set of measurements has been performed by varying the jet Mach number at fixed impingement ratios.The opposing planar jets are found to produce intense acoustic tones over the entire range of impingement ratio and flow velocity examined in this study, showing robust oscillation behavior even for very large distances between the jets. Acoustic tone frequency has been found to increase linearly with the jet exit velocity at constant impingement ratio and to decrease with the impingement distance at constant jet velocity. The flow field over the oscillation cycle has been investigated by means of phase-averaged PIV measurements technique, which has shown that the high noise levels and acoustic tones are produced by a pronounced flapping oscillation of the opposing jet columns.Copyright

Effect of Sound on the Wake of Side-by-Side Cylinders in a Duct

The phenomenon of flow-excited acoustic resonance, where periodic flow oscillations are enhanced by a resonant sound field, is a design concern in many engineering applications such as in heat exchangers, piping systems, and cavity flows. This study experimentally examines the phenomenon of flow-excited acoustic resonance for two side-by-side cylinders in a duct with cross-flow. This geometry has been investigated for three cylinder spacing ratios, defined as the center-to-center distance, normalized by the diameter, of T/D = 1.25, 1.46 and 2.5, and for a range of acoustic pressure amplitudes. Intermediate and small spacing ratios have been given special attention, as these cases have been found to exhibit bistable flow in the wake in the absence of acoustic resonance. Phase-locked PIV measurements reveal that the self-excited sound field produces a strong oscillatory flow pattern in the cylinder wakes, which is symmetric for large spacing ratios and high acoustic amplitudes, but remains bistable for small spacing ratios, even during very intense acoustic resonances. The aeroacoustic sources and sinks within the flow have been computed using Howe’s theory of vortex sound, and the distribution of these sources will be compared for the range of spacing ratios examined in this study.Copyright

Numerical and Experimental Investigation of Flow-Acoustic Resonance of Side-by-Side Cylinders in a Duct

The phenomenon of flow-excited acoustic resonance is a design concern in many engineering applications, especially when wakes of bluff bodies are encountered in ducts, piping systems, heat exchangers, and other confined systems. In this article, the case of self-excited acoustic resonance of two side-by-side cylinders in a duct with cross-flow is investigated both numerically and experimentally. A single spacing ratio between the cylinders, T/D = 2.5, is investigated, where D is the diameter of the cylinders and T is the center-to-center distance between them. The numerical investigation is performed using a finite-volume method at a Reynolds number of 30,000 to simulate the unsteady flow field, which is then coupled with a finite element simulation of the resonant sound field. The experimental investigation is performed using phase-locked Particle Image Velocimetry (PIV) during the occurrence of flow-excited acoustic resonance. The results of both methods reveal that the flow-excited acoustic resonance produces a strong oscillatory flow pattern in the cylinder wakes with strong in-phase vortex shedding being synchronized by the excited acoustic resonance. The distribution and strength of the aeroacoustic sources and sinks within the flow field have been computed by means of Howe’s theory of aerodynamic sound for both the experimental and numerical cases, with the results of the two methods comparing favorably, showing similar trends in the oscillating flow fields, and very similar trends in the distribution of net acoustic power.Copyright

Self-Sustained Oscillations of High Speed Impinging Planar Jets

High speed impinging jets are frequently used in a variety of industrial applications including thermal and coating control processes. These flows are liable to the production of very intense narrow band acoustic tones, which are produced by a feedback mechanism between instabilities in the jet free shear layer which roll up to form large scale coherent structures, and pressure fluctuations produced by the impingement of these structures at the impingement surface. This paper examines tone generation of a high speed planar gas jet impinging normally on a flat, rigid surface. Experiments are performed over the complete range of subsonic and transonic jet flow velocities for which tones are generated, from U0 = 150m/s (M≈0.4) to choked flow (U0 = 343m/s, M = 1), and over the complete range of impingement distance for which tones occur. The effect of varying the jet thickness is also examined. The behavior of the planar impinging jet case is compared to that of the axisymmetric case, and found to be significantly different, with tones being excited at larger impingement distances, and at lower flow velocities. The Strouhal numbers associated with tone generation in the planar case are on average an order of magnitude lower than that of the axisymmetric case when using similar velocity and length scales. The frequency behavior of the resulting tones is predicted using a simple feedback model, which allows the identification of the various shear layer modes of the instabilities driving tone generation. Finally, a thorough dimensionless analysis is performed in order to quantify the system behavior in terms of the appropriate scales.Copyright