Turbulent Flow Region Research Articles

Near-interface flow modeling in large-eddy simulation of two-phase turbulence

The smallest hydrodynamic length scales in two-phase turbulence are located at the interface between phases, or fluids, as a result of two-way coupling phenomena. Typically, these interface-generated scales are several times smaller than the dissipative scales in the surrounding bulk flow identified by Kolmogorov’s 1941 theory. Consequently, to properly capture these interface-generated small scales with sufficiently fine resolutions, the computational cost of performing large-eddy simulations of two-phase turbulent flow increases significantly from its (single-phase) theoretical optimum and toward values on the order of the direct numerical simulation of turbulence. Therefore, to maintain the cost of scale-resolving approaches linear with respect to the Reynolds number, this work investigates the modeling of the small-scale fluid motions in the vicinity of the viscous near-interface region of two-phase turbulent flows. Given the resemblance between the flow structures in the near-interface regions and those found in the boundary layers of turbulent wall-bounded flow, the modeling methodology proposed is inspired by ideas developed for turbulent flows interacting with solid walls, but modified to capture slip-velocity effects between phases. The performance of the approach is a priori assessed by utilizing data from direct numerical simulations of decaying isotropic turbulence laden with droplets of super-Kolmogorov size, demonstrating its computational feasibility and potential for reducing the cost of large-eddy simulation studies of two-phase turbulence.

Экспериментальное исследование двухфазного теплообмена в модели канала охлаждения головки цилиндра поршневого двигателя

Liquid cooling of cylinder and piston parts in highly boosted internal combustion engines is generally accompanied by local phase transition phenomena, such as surface nucleate boiling. The heat transfer coefficient of nucleate boiling is several times higher than that of single-phase convection. In order to efficiently exploit the thermal effect of nucleate boiling in cooling systems, simultaneously preventing emergency supercritical modes, a deeper understanding of boiling physics based on full-scale experiments is required. We conducted experimental investigation of heat transfer in a simulated cooling duct of a piston engine cylinder head, using a bespoke motor-free installation. We studied the effects of velocity, flow character and coolant type on the heat transfer, accounting for the presence of congestion regions. Over the course of the experiment, we simulated thermal conditions characteristic of different heat transfer types: single-phase convection, nucleate boiling, the onset of boiling crisis. We used the experimental data to plot the coolant heat flow density as a function of wall temperature for different measuring points situated inside the stream and the turbulent flow regions (congestion regions). We show that the mature nucleate boiling mode is the most favourable in terms of how uniform the temperature field within a part is. The experimental data obtained during the investigation may be used to verify mathematical simulations in the two-phase heat transfer theory, provided the data have been appropriately processed

Experimental investigations of local friction factors of laminar and turbulent gas flows in smooth micro-tubes

Local friction factors of laminar and turbulent gas flows including choked flows through micro-tubes were experimentally investigated under atmospheric back pressure and variable inlet pressure. The experiments were carried out using two glass micro-tubes with D = 265.7 and 399.9 μm and a fused silica micro-tube with D = 531.2 μm. Two to three pressure tap holes were drilled into the micro-tube wall at intervals of 5 ~ 6 mm to measure local pressures. The local Mach numbers, temperatures and friction factors were obtained by using stagnation temperatures and pressures, local pressures and mass flow rates. The results for wide range of Reynolds numbers and Mach numbers were obtained including choked flows. Both the local Fanning and the local Darcy friction factors were obtained under the assumption of both Fanno flow (adiabatic wall) and isothermal flow, respectively. Since the measured values of the inner relative surface roughness of the micro-tubes were less than 0.02 %, only the effect of compressibility on friction factors was assessed. The difference between Fanning and Darcy friction factors was described and compared with the ff and fd correlation as a function of Mach number. The friction factor difference obtained under the assumption of Fanno and isothermal flows was also compared with the available literature and numerical results. In the turbulent flow region including the choked flow, the local Fanning friction factors under the assumption of Fanno flow nearly coincided with Blasius correlation.

Experimental investigation of the evaporation of suspended mono-sized heptane droplets in turbulence intensities approaching unity

For decades, experimental investigations of turbulent droplet evaporation have generally followed one of two distinct paths: wind tunnel experiments, which generate significant mean flow but low turbulence intensity, and stirred chamber configurations, which yield the opposite characteristics. The present study bridges the gap between these two established techniques by modifying a fan-stirred spherical chamber to incorporate a controllable mean flow while retaining the attributes of highly-energetic homogeneous and isotropic turbulence. Heptane droplets with a fixed initial diameter, d0, of 500 ± 10 µm are suspended on the intersection of two microfibers and evaporated in pockets of high-intensity turbulence (27% < Tu < 103%) at small mean flow droplet Reynolds numbers, Red, of 10 and 50. A rigorous series of particle image velocimetry (PIV) tests mapped the homogeneous and isotropic turbulent flow regions for various combinations of fan speed, and a two-dimensional translating fiber support system allowed the precise spatial placement of the droplet for subsequent evaporation studies. PIV results indicate the generation of isotropic flow regions that satisfy 0.9 ≤ urms/vrms ≤ 1.1 with a minimum diameter of 20d0 at each test condition. The steady-state droplet evaporation rate, K, increases quasi-linearly with turbulence intensity at both Reynolds numbers, with steeper improvement associated with higher Red. A comparison with zero-mean flow data reveals that the mean velocity component remains a significant contributor to the droplet evaporation rate at both tested Reynolds numbers over the explored range of turbulence intensity. Subsequent analysis indicates that the mean flow attenuates the effect of turbulence. The findings are of fundamental and practical interest, as vaporization in high turbulence intensity is a plausible scenario for small droplets subject to a moderate slip velocity.

Numerical Investigation of High-Speed Turbulent Boundary Layers of Dense Gases

High-speed turbulent boundary layers of a dense gas (PP11) and a perfect gas (air) over flat plates are investigated by means of direct numerical simulations and large eddy simulations. The thermodynamic conditions of the incoming flow are chosen to highlight dense gas effects, and laminar-to-turbulent transition is triggered by suction and blowing. In the paper, the behavior of the fully developed turbulent flow region is investigated. Due to the low characteristic Eckert number of dense gas flows ( $$\hbox {Ec}=U_\infty ^2/c_{p,\infty }T_\infty$$ ), the mean velocity profiles are largely insensitive to the Mach number and very close to the incompressible case even at high speeds. Second-order velocity statistics are also weakly affected by the flow Mach number and the velocity spectra are characterized by a secondary peak in the outer region of the boundary layer because of the higher local friction Reynolds number. Despite the incompressible-like velocity and Reynolds-stress profiles, the strongly non-ideal thermodynamic and transport-property behavior of the dense gas results in unconventional distributions of the fluctuating thermo-physical quantities. Specifically, density and viscosity fluctuations reach a peak close to the wall, instead of vanishing as in perfect gas flows. Additionally, dense gas boundary layers exhibit higher values of the fluctuating Mach number and velocity divergence and a larger dilatational-to-solenoidal dissipation ratio in the near-wall region, which represents a major deviation from high-Mach-number perfect gas boundary layers. Other significant deviations are represented by the more symmetric probability distributions of fluctuating quantities such as the density and velocity divergence, due to the more balanced occurrence of strong expansion and compression events.

Numerical Study on Heat Transfer Enhancement in a Curved Channel with Baffles

Curved channel is commonly used in cement mill's journal bearing. In this study, heat transfer enhancement of curved channel having a rectangular cross-section with using inclined shape baffles is numerically investigated. The effects of different parameters of baffles, i.e. attack angle (?= 45°, 60°, and 90°), and the number of baffles (NB=9 and 13 baffles) are examined. The water is selected as working fluid for laminar and turbulent flow region. A standard k-???? turbulence model together with enhanced wall treatment is applied to solve the complex flow in Re of 500–5000. Influences of those parameters on heat transfer and friction performances in terms of Nusselt number, friction factor, Nussellt number enhancement ratio, and thermal performance factor, respectively are studied. The results show that the best condition to achieve maximum heat transfer at angle ???? = 90°, NB=13 and Re=5000 compared with other conditions. Furthermore, the maximum thermal performance factor (TEF) of the curved channel with using baffles is 4.4 at the same condition. This indicated that the geometry of baffles inside curved channel can improve the heat transfer significantly with reasonable increase in friction losses.

Potential of using inert gas flows for controlling the quality of as-grown silicon single crystal

We have improved the well-known Czochralski single crystal silicon growth method by using two argon gas flows. One flow is the main one (15–20 nl/min) and is directed from top to bottom along the growing single crystal. This flow entrains reaction products of melt and quartz crucible (mainly SiO), removes them from the growth chamber through a port in the bottom of the chamber and provides for the growth of dislocation-free single crystals from large weight charge. Similar processes are well known and have been generally used since the 1970s world over. The second additional gas flow (1.5–2 nl/min) is directed at a 45 arc deg angle to the melt surface in the form of jets emitted from circularly arranged nozzles. This second gas flow initiates the formation of a turbulent melt flow region which separates the crystallization front from oxygen-rich convective flows and accelerates carbon evaporation from the melt. It has been confirmed that oxygen evaporated from the melt (in the form of SiO) acts as transport agent for nonvolatile carbon. Commercial process implementation has shown that carbon content in as-grown single crystals can be reduced to below the carbon content in the charge. Single crystals grown with two argon gas flows have also proven to have highly macro- and micro-homogeneous oxygen distributions, with much greater lengths of single crystal portions in which the oxygen concentration is constant and below the preset limit. Carbon contents of 5–10 times lower than carbon content in the charge can be achieved with low argon gas consumption per one growth process (15–20 nl/min vs 50–80 nl/min for conventional processes). The use of an additional argon gas flow with a 10 times lower flowrate than that of the main flow does not distort the pattern of main (axial) flow circumvention around single crystal surface, does not hamper the “dislocation-free growth” of crystals and does not increase the density of microdefects. This suggests that the new method does not change temperature gradients and does not produce thermal shocks that may generate thermal stresses in single crystals.

Investigation of phase-contrast magnetic resonance imaging underestimation of turbulent flow through the aortic valve phantom: experimental and computational study using lattice Boltzmann method.

The accuracy of phase-contrast magnetic resonance imaging (PC-MRI) measurement is investigated using a computational fluid dynamics (CFD) model with the objective to determine the magnitude of the flow underestimation due to turbulence behind a narrowed valve in a phantom experiment. An acrylic stationary flow phantom is used with three insertable plates mimicking aortic valvular stenoses of varying degrees. Positive and negative horizontal fluxes are measured at equidistant slices using standard PC-MRI sequences by 1.5T and 3T systems. The CFD model is based on the 3D lattice Boltzmann method (LBM). The experimental and simulated data are compared using the Bland-Altman-derived limits of agreement. Based on the LBM results, the turbulence is quantified and confronted with the level of flow underestimation. LBM gives comparable results to PC-MRI for valves up to moderate stenosis on both field strengths. The flow magnitude through a severely stenotic valve was underestimated due to signal void in the regions of turbulent flow behind the valve, consistently with the level of quantified turbulence intensity. Flow measured by PC-MRI is affected by noise and turbulence. LBM can simulate turbulent flow efficiently and accurately, it has therefore the potential to improve clinical interpretation of PC-MRI.

Laminar-turbulent intermittency measurement based on the uncalibrated hot-film data

A new technique for the laminar-turbulent intermittency measurement based on the surface hot-film data is presented. The existing techniques require data acquired from the calibrated hot-films which leads to the real wall shear stress values. However, calibration of the hot-films is usually very complex. In the proposed method, a technique based on the probability distribution function (PDF) of the acquired data using the uncalibrated hot-film sensors is presented and evaluated. The PDF is prepared for a reduced form of the quasi-wall shear stress value instead of the real shear stress value one. This leads to a standard normal distribution curve for the PDF in the turbulent flow region and used as an identifier for the turbulent boundary layer detection. This identifier is employed to calculate the intermittency from the output signal of the hot-film. The sensitivity analyses for the present technique along with a detailed discussion of the results are presented. Following a complete review of the previous techniques, the importance, impact, and applicability of the current method are stated.

Opportunity to use inert gas flow for control of qualitative characteristics of the grown silicon single crystals

The process of growing silicon single crystals by the Czochralski method has been improved, which involves the use of two argon streams. 1st, the main flow, 15—20 nl/min, is directed from top to bottom along the growing single crystal. It captures the reaction products of the melt with a quartz crucible (mainly SiO), removes them from the chamber through a nozzle in the lower part of the chamber and provide dislocation-free single crystals from large loads. Similar processes are known and widely used in world practice since the 1970s. 2nd, additional flow, 1.5—2 nl/min, is directed at an angle of 45° to the surface of the melt in the form of jets from nozzles arranged in a ring. This flow initiates the formation of a region of turbulent melt flow, which isolates the crystallization front from convective flows enriched with oxygen, and also enhances the evaporation of carbon from the melt. It is confirmed that the oxygen evaporated from the melt (in the form of SiO) is a «transport» for non-volatile carbon. Carrying out industrial processes showed that the carbon content in the grown single crystals can be significantly reduced, up to values smaller than in the feedstock. In single crystals grown using two argon streams, an increased macro- and micro-uniformity of the oxygen distribution, a significantly larger crystal length with a given, constant oxygen concentration, were also recorded. Achieving a carbon concentration of 5 to 10 times less than in the feedstock is possible with small amounts of argon for melting (15—20 nl/min compared to 50—80 nl/min used in conventional processes. The use of an additional argon flow, which has an outflow intensity 10 times lower than that of the main flow, does not distort the nature of the flow around the single crystal surface (“axial”), does not disrupt the growth of a dislocation-free single crystal, does not increase the density of microdefects, which indicates the absence of changes in temperature gradients and thermal shock leading to thermal stresses in a single crystal.

Effect of Mud Rheology on Cuttings’ Transport in Drilling Operations

Settling of solid particles in drilling fluids represents a major problem, necessitating effective removal of drilled cuttings to the surface of the well to maintain safe and profitable drilling operations. Experiments were conducted using two group of drilling fluids, Newtonian (water, gas-oil, kerosene, and ethyl-glycol) and Non-Newtonian (Carboxyl Methyl cellulose (CMC)) in four different concentrations. Four cutting sizes were used, with diameters 0.212, 0.445, 0.672, and 0.853 cm, taken from the Garraf area of the Nasirya oil fields. The test results showed lower Reynolds numbers within the laminar flow region compared with the Turbulent flow region. The drag coefficients decreased with increasing particle Reynolds number, and small particle sizes gave higher drag coefficients and lower Reynolds numbers, while large particles gave lower drag coefficients and higher Reynolds numbers in both Newtonian and Non-Newtonian fluids. The results indicate that an increase in CMC concentration will decrease NRep and increase the drag coefficient for different particle sizes. For non-Newtonian fluids, the settling velocity decreases with increases in CMC concentration due to increases in the viscous forces that oppose settling of particles, while in Newtonian fluids, the settling velocity increases with increasing particle size due to gravity forces increasing. The low CMC concentrations (low n, and high k) offer higher settling velocities, while larger particle sizes give lower drag coefficients than smaller ones.

VASCULAR STENOSIS DETECTION USING TEMPORAL-SPECTRAL DIFFERENCES IN CORRELATED ACOUSTIC MEASUREMENTS.

Central venous stenosis is often undiagnosed in patients with hemodialysis vascular access, partly due to imaging difficulties. Noninvasive, point-of-care detection could rely on detecting regions of turbulent blood flow caused by blood velocity changes. Here we present flexible microphone arrays for time-correlated measures of blood flow sounds and a new signal processing approach to calculate time correlation between spectral features. Continuous wavelet transform was used to produce an auditory spectral flux analytic signal, which was thresholded to identify systolic start and end phases. Microphone arrays were tested on pulsatile flow phantoms with blood flow rates of 850-1,200 mL/min and simulated stenosis from 10-85%. Measured results showed an inversion in the time onset of systolic spectral content for sites proximal and distal to stenosis for hemodynamically significant stenoses (+22 ms for stenosis<50% and -20 to -38 ms for stenosis>50%). Equivalent blood velocity increases were calculated as 142-155 cm/s in stenotic phantoms, which are within the physiologic range as measured by ultrasound.

Application of computer vision and neural network analysis to study the structure and dynamics of turbulent jets

In this paper we solved the problem of detecting and detailed analysis of the characteristics of coherent structures in non-swirling and swirling turbulent jets from the 3D PIV experiment and DNS modelling database. To solve the problem intellectual approaches of computer vision based on deep neural networks were applied. It is shown that with the use of a generative competitive neural network, it is possible to reconstruct the structure and dynamics of a turbulent flow with an increased spatial resolution, which is important for analysis and interpretation. The approach is based on a modification of the loss function, which minimizes the residual part of the hydrodynamic equations (the continuity equations and the Poisson equation) for the reconstructed data. It is shown that generative models are more effective in the reconstruction of high moments of turbulent flow than conventional methods POD and DMD. Using a fully convolutional neural network, an automatic segmentation of the turbulent jet flow region and the external environment based on instantaneous velocity and pressure fields was carried out. For a swirling jet, the area of reversed flow is identified. It is shown that the complex of the developed machine learning algorithms successfully copes with the localization of coherent structures, the detection of their trajectory, characteristic size and phase propagation velocity. The obtained new fundamental information is important for a deeper understanding of the role of vortex structures in mixing processes in a jet flow. The developed complex of algorithms for the identification and analysis of characteristics of coherent structures can be applied to a database of measurements and numerical simulation of a wide class of hydrodynamic flows.

Inverse energy cascade and vortical structure in the near-wall region of turbulent channel flow

An inverse cascade is found in the energy transfer in scale space of channel flow. The conditional average of the velocity field associated with the inverse cascade or negative subgrid-scale production reveals a long streamwise vortex near the wall with a short vortex in the upstream region.

Influence of Surface Anisotropy on Turbulent Flow Over Irregular Roughness

The influence of surface anisotropy upon the near-wall region of a rough-wall turbulent channel flow is investigated using direct numerical simulation (DNS). A set of nine irregular rough surfaces with fixed mean peak-to-valley height, near-Gaussian height distributions and specified streamwise and spanwise correlation lengths were synthesised using a surface generation algorithm. By defining the surface anisotropy ratio (SAR) as the ratio of the streamwise and spanwise correlation lengths of the surface, we demonstrate that surfaces with a strong spanwise anisotropy (SAR < 1) can induce an over 200% increase in the roughness function ΔU+, compared to their streamwise anisotropic (SAR > 1) equivalent. Furthermore, we find that the relationship between the roughness function ΔU+ and the SAR parameter approximately follows an exponentially decaying function. The statistical response of the near-wall flow is studied using a “double-averaging” methodology in order to distinguish form-induced “dispersive” stresses from their turbulent counterparts. Outer-layer similarity is recovered for the mean velocity defect profile as well as the Reynolds stresses. The dispersive stresses all attain their maxima within the roughness canopy. Only the streamwise dispersive stress reaches levels that are comparable to the equivalent Reynolds stress, with surfaces of high SAR attaining the highest levels of streamwise dispersive stress. The Reynolds stress anisotropy also shows distinct differences between cases with strong streamwise anisotropy that stay close to an axisymmetric, rod-like state for all wall-normal locations, compared to cases with spanwise anisotropy where an axisymmetric, disk-like state of the Reynolds stress anisotropy tensor is observed around the roughness mean plane. Overall, the results from this study underline that the drag penalty incurred by a rough surface is strongly influenced by the surface topography and highlight its impact upon the mean momentum deficit in the outer flow as well as the Reynolds and dispersive stresses within the roughness layer.

Study of the effect of tilting lateral surface angle and operating parameters on the performance of a vertical shell-and-tube latent heat energy storage system

In this paper, a comprehensive mathematical model is developed to study the thermal energy storage performance in vertical shell-and-tube latent heat thermal energy storage (LHTES) systems with different tilting lateral surface angles. This model integrates heat transfer in heat transfer fluid (HTF), HTF tube and phase change materials (PCM). It is validated using experimental data and then applied to investigate effects of tilting lateral surface angles on the enhancement of natural convection in the LHTES system. The results show that increasing the tilting lateral surface angle largely enhances heat transfer in the melting process whilst it has negative effects on the solidification process. The total melting time reduces by 45% as the tilting angle increases from 0° to 7°. An optimal tilting angle of 4° is identified based on the trade-off between melting and solidification processes. Furthermore, the effect of HTF operating parameters on the performance of the LHTES system is evaluated. It shows that the melting and solidification time of the LHTES system with a tilting angle 4° reduces by 22% and 8.5%, respectively, as the HTF flow Reynolds number increases in laminar and transition flow regions. However, increasing the HTF flow Reynolds number does not show significants effect under turbulent flow regions. The variation of the HTF outlet temperature is also investigated. The temperature difference between the HTF inlet and outlet is found small at all studied HTF flow rates, which indicates that assumption of the constant HTF surface temperature boundary in simulations in the literature is reasonable.

Experimental analyses on heat transfer performance of TiO2–water nanofluid in double-pipe counter-flow heat exchanger for various flow regimes

Nanofluids are widely used in heat transfer applications. This article presents the effect of heat transfer and pressure drop of the TiO2–water nanofluids flowing in a double-tube counter-flow heat exchanger with various flow patterns. In this experimental work, performance of TiO2–water nanofluid on heat transfer in three different cases such as laminar, transition and turbulent flow region were analyzed. TiO2 nanoparticles with average diameters of 20 nm dispersed in water with three volume concentrations of 0.1, 0.3 and 0.5 vol% were used as the test fluid. The results show that the heat transfer of nanofluids is higher than that of the base liquid (water) and increased with the increase in Reynolds number and particle concentrations. The heat transfer rate of nanofluid with 0.5 vol% was 25% greater than that of base liquid, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 0.5 vol% was 15% higher than that of base fluid at given conditions. Pressure drop of nanofluid was increased with increase in volume concentration, and it is slightly higher than that of the base fluid.

Liquid Film Thickness in Vertical Circular Pipes Under Flooding Conditions at the Top End

In our previous study, we measured the void fraction α, pressure gradient dP/dz, and countercurrent flow limitation in a vertical circular pipe (diameter D = 20 mm) under flooding conditions at the square top end and working fluids of air and water to obtain the wall friction factor fw and the interfacial friction factor fi based on the annular flow model. The thickness of the falling liquid film δ obtained from the measured α was relatively well expressed by the correlation for the free-falling film, and the obtained fw was well expressed by the Fanning friction factor f for a circular pipe. Measurements of α in vertical pipes under flooding conditions are few. In this study, therefore, we evaluated α and δ from the measured dP/dz under flooding at the square top end reported by Bharathan et al. with D = 50.8 mm and air-water and by Ilyukhin et al. with D = 20 mm and working fluids of steam and water at pressures of P = 0.6 to 4.1 MPa. As a result, we found that δ obtained from the measured dP/dz and the correlation of fw = f were well correlated in terms of the liquid Reynolds number ReL. The obtained δ was well expressed by the Nusselt’s correlation for the free-falling film in the region of laminar flows, but the obtained δ was larger than the Feind’s correlation for the free-falling film in the region of turbulent flows due to the interfacial friction. We also discussed effects of the diameter and fluid properties on the interfacial friction factor fi.

Effects of spiral start number and depth ratio of corrugated tube on flow and heat transfer characteristics in turbulent flow region

Influence of spirally-corrugated tubes on heat transfer (Nu), pressure loss (f) and thermal enhancement factor (TEF) characteristics was numerically investigated. The physical models of spirally-corrugated tubes and also a straight circular tube were built for the comparative study. The predictions were performed for the spirally-corrugated tubes with spiral start numbers (N) of 2, 3, 4 and 5 and depth ratios (DR = e/D) of 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14 and 0.16 at Reynolds numbers of 5000, 8000, 14000 and 20000. The numerical results show that the spiral corrugated tubes created two types of flow structures: main swirl flows (appear around the core) and secondary swirl flows (appear near tube wall) which facilitate heat transfer between the fluid and the tube walls. The overall heat transfer depends on the competition between two effects: the positive effect of the stronger swirl intensity by increasing start number and the negative effect of the difficulty of fluid transfer into grooves by increasing depth ratio. For the investigated range, the corrugated tubes yield Nusselt numbers and thermal performance factors varying from 0.82 to 2.16 times and 0.66–1.2 times of the straight circular tube, respectively. The highest thermal enhancement factor (TEF) of 1.2 is achieved by using the tube with spiral start number of 4 having moderate depth ratio of 0.08 at Re = 5000.

Dynamics of the large-scale structures and associated noise emission in airfoil slats

We investigate the slat narrowband peak noise generating mechanism. Unsteady flow data were generated by a lattice-Boltzmann-based commercial code for four configurations, accounting for variations of the airfoil angle of attack and slat overlap. Comparison with experimental results indicates that the aspects of the flow field relevant for the generation of the narrowband peaks were accurately captured. Frequency-domain proper orthogonal decomposition (POD) is applied to identify dominant large-scale structures in the frequency range dominated by the peaks. The combined use of the two POD metrics, namely, the turbulent kinetic energy in the turbulent flow region and the acoustic pressure in the far field, demonstrated that the structures most correlated with the noise resemble spanwise coherent Kelvin–Helmholtz vortices which dominate the slat cove only at the frequency of the narrowband peaks. Time evolution of the structures educed using the acoustic pressure correlation provides detailed evidence of the hydrodynamic and acoustic steps of a Rossiter-like feedback mechanism between the slat cusp and trailing edge. The combined analysis of results for the different slat configurations provides an explanation for the effect of the slat configuration on the amplitude of the narrowband peaks observed in previous studies, particularly the influence of the main-element suction peak.