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 Journal of Fluid MechanicsJournal Prestige (SJR): 1.591 Citation Impact (citeScore): 3Number of Followers: 186      Hybrid journal (It can contain Open Access articles) ISSN (Print) 0022-1120 - ISSN (Online) 1469-7645 Published by Cambridge University Press  [387 journals]
• Analysis of a civil aircraft wing transonic shock buffet experiment
• Authors: L. Masini; S. Timme, A. J. Peace
Abstract: The physical mechanism governing the onset of transonic shock buffet on swept wings remains elusive, with no unequivocal description forthcoming despite over half a century of research. This paper elucidates the fundamental flow physics on a civil aircraft wing using an extensive experimental database from a transonic wind tunnel facility. The analysis covers a wide range of flow conditions at a Reynolds number of around $3.6\times 10^{6}$ . Data at pre-buffet conditions and beyond onset are assessed for Mach numbers between 0.70 and 0.84. Critically, unsteady surface pressure data of high spatial and temporal resolution acquired by dynamic pressure-sensitive paint is analysed, in addition to conventional data from pressure transducers and a root strain gauge. We identify two distinct phenomena in shock buffet conditions. First, we highlight a low-frequency shock unsteadiness for Strouhal numbers between 0.05 and 0.15, based on mean aerodynamic chord and reference free stream velocity. This has a characteristic wavelength of approximately 0.8 semi-span lengths (equivalent to three mean aerodynamic chords). Such shock unsteadiness is already observed at low-incidence conditions, below the buffet onset defined by traditional indicators. This has the effect of propagating disturbances predominantly in the inboard direction, depending on localised separation, with a dimensionless convection speed of approximately 0.26 for a Strouhal number of 0.09. Second, we describe a broadband higher-frequency behaviour for Strouhal numbers between 0.2 and 0.5 with a wavelength of 0.2 to 0.3 semi-span lengths (0.6 to 1.2 mean aerodynamic chords). This outboard propagation is confined to the tip region, similar to previously reported buffet cells believed to constitute the shock buffet instability on conventional swept wings. Interestingly, a dimensionless outboard convection speed of approximately 0.26, coinciding with the low-frequency shock unsteadiness, is found to be nearly independent of frequency. We characterise these coexisting phenomena by use of signal processing tools and modal analysis of the dynamic pressure-sensitive paint data, specifically proper orthogonal and dynamic mode decomposition. The results are scrutinised within the context of a broader research effort, including numerical simulation, and viewed alongside other experiments. We anticipate our findings will help to clarify experimental and numerical observations in edge-of-the-envelope conditions and to ultimately inform buffet-control strategies.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.906
Issue No: Vol. 884 (2020)

• On the mechanism of open-loop control of thermoacoustic instability in a
laminar premixed combustor
• Authors: Amitesh Roy; Sirshendu Mondal, Samadhan A. Pawar, R. I. Sujith
Abstract: We identify mechanisms through which open-loop control of thermoacoustic instability is achieved in a laminar combustor and characterize them using synchronization theory. The thermoacoustic system comprises two nonlinearly coupled damped harmonic oscillators – acoustic and unsteady heat release rate (HRR) field – each possessing different eigenfrequencies. The frequency of the preferred mode of HRR oscillations is less than the third acoustic eigenfrequency where thermoacoustic instability develops. We systematically subject the limit-cycle oscillations to an external harmonic forcing at different frequencies and amplitudes. We observe that forcing at a frequency near the preferred mode of the HRR oscillator leads to a greater than 90 % decrease in the amplitude of the limit-cycle oscillations through the phenomenon of asynchronous quenching. Concurrently, there is a resonant amplification in the amplitude of HRR oscillations. Further, we show that the flame dynamics plays a key role in controlling the frequency at which quenching is observed. Most importantly, we show that forcing can cause asynchronous quenching either by imposing out-of-phase relation between pressure and HRR oscillations or by inducing period-2 dynamics in pressure oscillations while period-1 in HRR oscillations, thereby causing phase drifting between the two subsystems. In each of the two cases, acoustic driving is very low and hence thermoacoustic instability is suppressed. We show that the characteristics of forced synchronization of the pressure and HRR oscillations are significantly different. Thus, we find that the simultaneous characterization of the two subsystems is necessary to quantify completely the nonlinear response of the forced thermoacoustic system.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.884
Issue No: Vol. 884 (2020)

• The hydroelastic response of a surface-piercing hydrofoil in multiphase
flows. Part 2. Modal parameters and generalized fluid forces
• Authors: Casey M. Harwood; Mario Felli, Massimo Falchi, Nitin Garg, Steven L. Ceccio, Yin L. Young
Abstract: The fluid–structure interactions (FSI) of compliant lifting surfaces is complicated by free-surface and multiphase flows such as cavitation and ventilation. This paper describes the dynamic FSI response of a flexible surface-piercing hydrofoil in dry, wetted, ventilating and cavitating conditions. Experimental modal analysis is used to quantify the resonant frequencies and damping ratios of the fluid–structure system in each flow regime. The generalized hydrodynamic stiffness, fluid damping and fluid added mass are also determined as ratios to the corresponding structural modal forces. Added mass increases with increasing partial immersion of the hydrofoil and decreases in the presence of gaseous cavities. In particular, modal frequencies were observed to increase significantly in fully ventilated flow compared to fully wetted flow. The modal frequencies varied non-monotonically with speed in fully wetted flow. Gaseous cavities reduced the modal added mass and reduced the fluid disturbing force. Modal damping increases non-monotonically with increasing immersion depth. Forward speed causes the fluid damping force to increase with an approximately quadratic functional behaviour, consistent with a series expansion of the Morison equation, although damping identification became increasingly difficult at high flow speeds. The results indicate that fluid damping is greater than the associated structural damping in a quiescent liquid, and increasingly so with increasing immersion, suggesting viscous dissipation as a dominant mechanism. A preliminary investigation of modal vibration as a means of controlling the size and stability of ventilated cavities indicates that low-order modes encourage the formation of ventilation, while higher-order modes encourage the washout and elimination of ventilation.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.871
Issue No: Vol. 884 (2020)

• Flow–acoustic resonance in a cavity covered by a perforated plate
• Authors: Xiwen Dai
Abstract: To explain the large-scale hydrodynamic instability along a cavity-backed perforated plate in a flow duct, a two-dimensional multimodal analysis of flow disturbances is performed. First, a hole-by-hole description of the perforated plate shows a spatially growing wave with a wavelength close to the plate length, but much larger than the period of perforation. To better understand this problem and also cavity flow oscillations, we then combine the travelling mode and global mode analyses of the flow where the plate is represented by a homogeneous impedance. The spatially growing wave is, from a homogeneous point of view, essentially a Kelvin–Helmholtz instability wave, strongly distorted by evanescent acoustic waves near the cavity downstream edge. The phase difference of the unstable hydrodynamic mode at the two edges is found to be a bit larger than $2\unicode[STIX]{x03C0}$ , whereas the upstream-travelling evanescent waves reduce the total phase change around the feedback loop, so that the phase condition of the global mode can still be satisfied. This particular case indicates the significant effects of those evanescent waves on both the amplitude and phase of cavity flow disturbances. The criterion of the global instability is discussed: the loop gain being larger or smaller than unity determines whether the global mode is unstable or stable. A global mode in the stable regime, which has so far received little attention, is explored by investigating the system response to external forcing. It is shown that sound can be produced when a lightly damped flow–acoustic resonance is excited by a vortical wave.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.934
Issue No: Vol. 884 (2020)

• The alignment of vortical structures in turbulent flow through a
contraction
• Authors: Vivek Mugundhan; R. S. Pugazenthi, Nathan B. Speirs, Ravi Samtaney, S. T. Thoroddsen
Abstract: We investigate experimentally the turbulent flow through a two-dimensional contraction. Using a water tunnel with an active grid we generate turbulence at Taylor microscale Reynolds number $Re_{\unicode[STIX]{x1D706}}\sim 250$ which is advected through a 2.5 : 1 contraction. Volumetric and time-resolved tomographic particle image velocimetry and shake-the-box velocity measurements are used to characterize the evolution of coherent vortical structures at three streamwise locations upstream of and within the contraction. We confirm the conceptual picture of coherent large-scale vortices being stretched and aligned with the mean rate of strain. This alignment of the vortices with the tunnel centreline is stronger compared to the alignment of vorticity with the large-scale strain observed in numerical simulations of homogeneous turbulence. We judge this by the peak probability magnitudes of these alignments. This result is robust and independent of the grid-rotation protocols. On the other hand, while the pointwise vorticity vector also, to a lesser extent, aligns with the mean strain, it principally remains aligned with the intermediate eigenvector of the local instantaneous strain-rate tensor, as is known in other turbulent flows. These results persist when the distance from the grid to the entrance of the contraction is doubled, showing that modest transverse inhomogeneities do not significantly affect these vortical-orientation results.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.887
Issue No: Vol. 884 (2020)

• The effect of double diffusion on entrainment in turbulent plumes
• Authors: Maksim Dadonau; J. L. Partridge, P. F. Linden
Abstract: We investigate experimentally the effect of double diffusion in the salt-fingering configuration on entrainment in turbulent plumes. Plumes over a range of source buoyancy fluxes $B_{0}$ and source density ratios $R_{\unicode[STIX]{x1D70C}}$ are examined. When the plumes are double diffusive ( $R_{\unicode[STIX]{x1D70C}}>0$ ) the entrainment coefficient $\unicode[STIX]{x1D6FC}$ is not constant, with an up to 20 % reduction from the value found for single-diffusive plumes, that is, plumes with $R_{\unicode[STIX]{x1D70C}}=0$ . The scale of reduction is found to be in direct relation to the source density ratio and is inversely related to the distance travelled by the plume, indicating that double-diffusive effects decrease as the plume evolves. We propose an explanation for the observed reduction in the entrainment coefficient on the basis of differential diffusion hindering large-scale engulfment at the edge of the plume.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.925
Issue No: Vol. 884 (2020)

• Characteristics of turbulent square duct flows over porous media
• Authors: Kazuhiko Suga; Yuki Okazaki, Yusuke Kuwata
Abstract: Particle image velocimetry measurements have been carried out to assess the fully developed turbulence in square-sectioned porous duct flows. To the bottom duct wall, this study applies two types of porous media whose porosities are approximately 0.8 and ratios of wall-normal to streamwise permeabilities are 0.8 and 7.8. Both over- and under-surface turbulence of the porous layers are discussed at inlet flow Reynolds numbers of $Re\simeq 3500$ and 7500. Cross-sectional secondary flows are detected with an enhanced magnitude of approximately 6 % of the inlet bulk velocity. The secondary flow pattern consisting of four large vortices is observed to be insensitive to the porous structures. Over the porous wall, although turbulence is enhanced by the permeability, it is confirmed that turbulence over and under the porous surfaces is rather insensitive to the wall-normal permeability compared with the streamwise permeability as seen in porous-wall channel flows. In the present range of streamwise permeability Reynolds numbers of $Re_{K_{x}}=2.49{-}6.37$ , the wall-normal fluctuations become dominant once underneath the porous surface while the streamwise ones become dominant again deep inside the porous layer. Applying streamwise–spanwise plane averaging, which covers a 52 % area in the middle of the duct, to the flow quantities, it is confirmed that the correlations between the pore-scale Reynolds number and the log-law parameters are similar to those seen in a wide range of porous-wall channels. The above characteristics are generally the same as those of porous-wall channels in the same range of porosities and permeability Reynolds numbers even with the enhanced secondary flows. However, from the spectral analysis of flows at the porous walls, it is found that, near the symmetry planes, the wavelengths of the Kelvin–Helmholtz waves become a little shorter than those in turbulent porous-wall channels possibly because of the sidewall boundary layers, particularly at low Reynolds numbers.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.914
Issue No: Vol. 884 (2020)

• Effects of flapping-motion profiles on insect-wing aerodynamics
• Authors: Shantanu S. Bhat; Jisheng Zhao, John Sheridan, Kerry Hourigan, Mark C. Thompson
Abstract: Flapping wings of insects can follow various complex-motion waveforms, influencing the flow structures over a wing and consequently the aerodynamic performance. However, most studies of insect-wing models incorporate either simple harmonic or robofly-like motion waveforms. The effects of other waveforms appear to be under-explored. Motivated by this, the present study investigates the individual and combined effects of the sweep- and pitch-motion waveforms for fixed flapping frequency and amplitude of a fruit-fly wing planform. Physical experiments are conducted to directly measure the forces and torques acting on the wing. Interestingly, the sweep waveform is observed to influence the overall variation in the lift coefficient ( $C_{L}$ ), whereas the pitch waveform is observed to influence only the instantaneous $C_{L}$ during stroke reversal. Carefully validated three-dimensional numerical simulations reveal that a change in the strength of the large-scale vortex over the wing as the sweep profile parameter is varied is responsible for the observed variations in $C_{L}$ . An exploration over wide ranges of the sweep and the pitch profile parameters shows that the waveforms maximising the mean lift coefficient are different from those maximising the power economy. Consistent with some previous experiments on robotic insects, the possibility of passive pitch motion is observed at slower pitching rates. Contours of the mean lift coefficient and power economy mapped on the planes of the sweep and the pitch profile parameters can help designers of flapping-wing micro air vehicles in selecting the waveforms appropriate for their design criteria.
PubDate: 2020-02-10T00:00:00.000Z
DOI: 10.1017/jfm.2019.929
Issue No: Vol. 884 (2020)

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