Spectra Of Velocity Fluctuations Research Articles

Statistical Characteristics of Mesoscale Fluctuations of Wind Velocity, Temperature, and Gas Concentrations Obtained from Aircraft Measurements in the Troposphere of the Arctic Region

Abstract The results of airborne measurements and statistical characteristics of mesoscale fluctuations of wind velocity, temperature, and concentrations of gas constituents at different heights of a stably stratified troposphere are presented. The measurements were carried out in September 2022 in the Arctic region of Russia with the aircraft laboratory Tu-134 “Optik.” The obtained spectra and structure functions of the fluctuations are interpreted with the theoretical model of formation of the spectrum of mesoscale wind velocity and temperature fluctuations described in the paper. The presence at high wavenumbers of a steep section in the obtained horizontal wavenumber spectra of the fluctuations of wind velocity and greenhouse gas concentration with a slope close to −3 is discussed. The fluctuation spectra along different slanted tracks of the aircraft crossing the tropospheric layer between altitudes of 1 and 9 km are also obtained and analyzed with the theoretical model.

Assessment of Reynolds number effects in supersonic turbulent boundary layers

Wall-resolved large-eddy simulations (LES) are performed to investigate Reynolds number effects in supersonic turbulent boundary layers (TBLs) at Mach 2.0. The resulting database covers more than a decade of friction Reynolds number Reτ, from 242 to 5554, which considerably extends the parameter range of current high-fidelity numerical studies. Reynolds number trends are identified on a variety of statistics for skin-friction, velocity and thermodynamic variables. The efficacy of recent scaling laws as well as compressibility effects are also assessed. In particular, we observe the breakdown of Morkovin’s hypothesis for third-order velocity statistics, in agreement with previous observations for variable-property flows at low Mach number. Special attention is also placed on the size and topology of the turbulent structures populating the TBL, with an emphasis on the outer-layer motions at high Reynolds number. The corresponding streamwise spectra of streamwise velocity fluctuations show a clear separation between inner and outer scales, where energetic peaks are found at streamwise wavelengths of λx+≈700 and λx/δ0≈6. The spanwise spacing of the outer-layer structures, in turn, is found to be insensitive to the Reynolds number and equal to ∼0.7δ0. It is also found that the integral length-scales in spanwise direction for the temperature, streamwise and spanwise velocity fields appear to progressively collapse with increasing Reynolds number. The modulating influence that the outer-layer structures exert on the near-wall turbulence is also clearly visible in many of the metrics discussed. In addition, the present LES data is further exploited to assess the Reτ-sensitivity of uniform momentum regions in the flow. We find that the resulting probability density function of the number of zones as well as its evolution with Reτ agrees well with incompressible data. This suggests that uniform zones, which have been associated with outer-layer dynamics, are not strongly influenced by compressibility at the considered Mach number.

Species transport in a variable-density turbulent mixing layer considering stratified instability

Mixing of miscible fluids with differing densities in turbulent mixing layers is of great importance in chemical engineering. In addition to the effect of density ratio R between two fluids, stratified instability will introduce additional complexity to turbulent mixing of species. We investigate this issue by performing direct numerical simulations of a stratified mixing layer involving two miscible fluids with R = 1, 3, and 6. We find the stronger decay of turbulence in the high-density stream in comparison with the low-density counterpart, which is attributed to the decrease in the shear rate and the increase in stratified instability. Moreover, it shows the dynamical connections between stably stratified mixing layers and wall-bounded turbulent flows, which can be supported by the hairpin vortical structures, the vertical profile of integral shear parameter S* in the low-density side (S* ≈ 10–30), and an inverse wave-number scaling for streamwise energy spectrum of velocity fluctuation. The statistical analysis of the budget of the mixture fraction variance shows that the turbulent mixing of species is significantly suppressed in the high-density side while enhanced in the low-density one, as a result of the larger reduction of turbulent production, turbulent flux, and a smaller reduction of dissipation in the high-density side. Finally, we find that the dissipation rate of the structure function of the mixture fraction variance is about four times larger than that of mixture fraction variance. These results can provide support for large eddy simulation of stratified turbulent mixing of miscible fluids with differing densities.

Influence of Ridge Spacing, Ridge Width, and Reynolds Number on Secondary Currents in Turbulent Channel Flow Over Triangular Ridges

Most studies of secondary currents (SCs) over streamwise aligned ridges have been performed for rectangular ridge cross-sections. In this study, secondary currents above triangular ridges are systematically studied using direct numerical simulations of turbulent channel flow. The influence of ridge spacing on flow topology, mean flow, and turbulence statistics is investigated at two friction Reynolds numbers, 550 and 1000. In addition, the effects of ridge width on SCs, which have not previously been considered for this ridge shape, are explored. The influence of SCs on shear stress statistics increases with increased ridge spacing until SCs fill the entire channel. One of the primary findings is that, for ridge configurations with pronounced secondary currents, shear stress statistics exhibit clear Reynolds number sensitivity with a significant growth of dispersive shear stress levels with Reynolds number. In contrast to rectangular ridges, no above-ridge tertiary flows are observed for the tested range of ridge widths. Flow visualisations of SCs reveal the existence of corner vortices that form at the intersection of the lateral ridge sides and the smooth-wall sections. These are found to gradually disappear as ridges increase in width. Premultiplied spectra of streamwise velocity fluctuations show strong dependency on the spanwise sampling location. Whereas spanwise averaged spectra show no strong modifications by SCs, a significant increase of energy levels emerges at higher wavelengths for spectra sampled at the spanwise locations that correspond to the centres of the secondary currents.

Near-wall numerical coherent structures and turbulence generation in wall-modelled large-eddy simulation

Near-wall turbulence structures and generation in the wall-modelled large-eddy simulation (WMLES) are revealed. To elucidate the turbulence structures driving a near-wall turbulence generation in the WMLES, flat-plate turbulent boundary-layer flows calculated by the WMLES and direct numerical simulation (DNS) are closely investigated. A conditional-averaging technique is applied to the instantaneous flow fields and the near-wall statistical structures of the ejection and sweep pairs, which produce the turbulence, are revealed to exist even in the WMLES although the structures are non-physically elongated compared with those obtained by the DNS. Since the near-wall turbulence structures in the WMLES are revealed not to be disordered, but to be coherent structures with low- and high-speed fluids alternating in the spanwise direction, it is suggested that the near-wall turbulence generation in the WMLES is explained by the numerically elongated coherent structures. Furthermore, the Reynolds number effects of wall-bounded turbulent flows, i.e. the appearance of the outer peak in the energy spectrum of the streamwise velocity fluctuations at increasing Reynolds numbers, is found not to be reproduced by the WMLES, and the origin of the outer peak is discussed in association with the inner–outer-layer interactions. The near-wall turbulence structures in the WMLES could depend heavily on the computational grids and the numerical methods. Therefore, additional cases varying the grid resolutions and the numerical methods (numerical schemes and sub-grid-scale models) are also conducted to confirm the consistency of the present conclusions.

Excitation of a Separated Flow by a Series of Protuberances Near the Leading Edge of a Model Aerofoil

Abstract This article documents the manifestation of a shear layer under the excitation of a series of hemispherical protuberances near the leading edge of a constant-thickness airfoil. The experiments are performed at a Reynolds number of 1.6 × 105 based on the chord length and inlet velocity, where freestream turbulence is 1.2%. The hotwire and particle image velocimetry data are analyzed to appreciate the flow feature, illustrating the growth of perturbations, vortex dynamics, intermittency, and spectral response. A laminar separation bubble (LSB) appears near the leading edge for a smooth surface, and the shear layer is inviscidly unstable. The evolution of the shear layer significantly changes with a series of protuberances. The breakdown of the shear layer occurs almost immediately, triggering local turbulence resulting in a considerable reduction of the bubble length. However, a separation bubble of varying spanwise lengths is formed in this case. Although the power spectra of velocity fluctuations reveal the selective amplification of frequencies even with protuberances, the immediate augmentation of turbulence followed by faster decay suggests the transient growth of turbulence. The study has documented insight into features of a separation bubble subjected to leading-edge perturbations and might influence future studies on separation control over an airfoil.

Experimental and numerical investigation of a disc-attached cylinder near a wall

An experimental and numerical study is carried out to investigate the flow up- and downstream of a disc-attached cylinder near a wall with a Reynolds number of Re=U0d/ν=5.4⋅104 (U0 is the mean upstream flow velocity, d is the diameter of the cylinder, and ν is the kinematic viscosity). Laser Doppler Velocimetry measurements were conducted to obtain statistical flow parameters such as the mean velocity and turbulent kinetic energy. The experimental results show that the mean flow can be characterized as two-dimensional at a transverse distance of more than d away from the disc. Furthermore, it is shown that the effect of the disc and the cylinder fades away downstream. Analysis of a power spectrum of the vertical velocity fluctuations shows that vortex shedding is disturbed downstream of the disc. The numerical simulation is performed using a Reynolds-Averaged Navier–Stokes model. The turbulent kinetic energy and mean velocities are well-predicted by the numerical model. An increase in the peak amplitudes of the power spectra obtained from the vertical velocity fluctuations is found at a transverse distance of 3.75d from the centerline of the disc for both the experimental and numerical study, caused by vortices being transported by a transverse flow related to the disc.

Resolvent-based analysis of hypersonic turbulent boundary layers with/without wall cooling

The ability of the low-rank approximation of hypersonic turbulent boundary layers with/without wall cooling is examined with the linear resolvent operator in a compressible form. The freestream Mach number of the base flow is 5.86, and the friction Reynolds number is 420. The wall-to-recovery temperature ratio is set as 1.0 and 0.25, respectively, corresponding to an adiabatic wall condition and a cold-wall condition. Different from the resolvent analysis of incompressible turbulent boundary layers, the optimal response mode in the wave-parameter space exhibits a relatively subsonic and a relatively supersonic region [Bae et al., “Resolvent-based study of compressibility effects on supersonic turbulent boundary layers,” J. Fluid Mech. 883, A29 (2020)], divided by the freestream relative Mach number of unity. The features of energy distribution of the optimal response mode in space and scales are examined, and the energy spectra of streamwise velocity and temperature fluctuations, carried by the optimal response mode, are discussed with typical subsets of streamwise and spanwise wavelengths. This reveals the dynamics of the near-wall small-scale and outer larger-scale motions and the distinction in the relatively subsonic/supersonic region. Moreover, the coherent structures, including the velocity and temperature streaks, quasi-streamwise vortices, and large-scale/very-large-scale motions, are identified in the optimal response mode. Special attention is paid to the effects of wall cooling.

Acoustic receptivity in the airfoil boundary layer: An experimental study in a closed wind tunnel

Airfoil trailing edge noise with a tonal frequency at a medium-Reynolds number (from 2 × 10 5 to 3 × 10 5 in this work) is related to periodic fluctuations in the airfoil boundary layer. Acoustic receptivity plays an important role, in that it constructs a feedback loop to induce ladder-structure phenomena and discrete peak frequencies. The present work is devoted to the experimental study of the acoustic receptivity in the airfoil boundary layer by employing a time-resolved particle image velocimetry method. The symmetric vortex shedding process is noticed, and a hysteresis phenomenon is discovered with the increasing and decreasing wind speed. The author applies the Hilbert transform to show a space-wavenumber spectrum of wall-normal velocity fluctuations to locate resonance points, where acoustic pressure resonates with fluctuations in the boundary layer. The results show that the acoustic reception can affect the local velocity to increase and decrease the wavenumber before and after reach points. The trailing edge noise impacts on the airfoil boundary layer to control the system states and follows the same acoustic feedback loop from Arbey and Bataille [“Noise generated by airfoil profiles placed in a uniform laminar flow,” J. Fluid Mech. 134, 33–47 (1983)].

Characteristics of Large-Scale Coherent Structures on Irregularly Arranged Rough-Bed Open-Channel Flows

Large-scale coherent structures (LSCSs) in rough-bed open-channel flow (OCF) are significant in turbulence research. A recent breakthrough is the bimodal feature of LSCSs on regular rough-bed OCF (i.e., LSCSs exhibit two typical motions: large-scale motions (LSMs) and very-large-scale motions (VLSMs)). However, the presence and characteristics of LSMs and VLSMs in irregularly arranged rough-bed OCF remain unclear. Thus, in this study, high-precision indoor flume experiments were performed under typical irregularly arranged rough-bed conditions, and time-resolved particle image velocimetry was used for velocity measurements. Statistical quantities of velocity fluctuations revealed that the friction Reynolds number and roughness exerted a certain modulation on the velocity fluctuating properties. The spectra of velocity fluctuations provided direct and statistical evidence for the presence of LSMs and VLSMs in irregularly arranged rough-bed OCF. VLSMs contributed more than 60% of the streamwise turbulent kinetic energy and 40% of the Reynolds shear stress in the outer region of the irregularly arranged rough-bed OCF, which was slightly higher than that in the smooth-bed or regular rough-bed OCF scenarios. No apparent dependence of the wavelength of VLSMs on the flow submergence (H/d50) was observed in the present irregularly arranged rough-bed OCF, which is in contrast to that reported for regular rough-bed OCF. Furthermore, the relationship between the peak wavelength of VLSMs and the aspect ratio did not strictly follow a linear increase, in contrast to that documented in the literature.

Direct numerical simulations of turbulent pipe flow up to

Well-resolved direct numerical simulations (DNS) have been performed of the flow in a smooth circular pipe of radius $R$ and axial length $10{\rm \pi} R$ at friction Reynolds numbers up to $Re_\tau =5200$ using the pseudo-spectral code OPENPIPEFLOW. Various turbulence statistics are documented and compared with other DNS and experimental data in pipes as well as channels. Small but distinct differences between various datasets are identified. The friction factor $\lambda$ overshoots by $2\,\%$ and undershoots by $0.6\,\%$ the Prandtl friction law at low and high $Re$ ranges, respectively. In addition, $\lambda$ in our results is slightly higher than in Pirozzoli et al. (J. Fluid Mech., vol. 926, 2021, A28), but matches well the experiments in Furuichi et al. (Phys. Fluids, vol. 27, issue 9, 2015, 095108). The log-law indicator function, which is nearly indistinguishable between pipe and channel up to $y^+=250$ , has not yet developed a plateau farther away from the wall in the pipes even for the $Re_\tau =5200$ cases. The wall shear stress fluctuations and the inner peak of the axial turbulence intensity – which grow monotonically with $Re_\tau$ – are lower in the pipe than in the channel, but the difference decreases with increasing $Re_\tau$ . While the wall value is slightly lower in the channel than in the pipe at the same $Re_\tau$ , the inner peak of the pressure fluctuation shows negligible differences between them. The Reynolds number scaling of all these quantities agrees with both the logarithmic and defect-power laws if the coefficients are properly chosen. The one-dimensional spectrum of the axial velocity fluctuation exhibits a $k^{-1}$ dependence at an intermediate distance from the wall – also seen in the channel. In summary, these high-fidelity data enable us to provide better insights into the flow physics in the pipes as well as the similarity/difference among different types of wall turbulence.

Onset of Nonlinear Electroosmotic Flow under an AC Electric Field.

Nonlinearity of electroosmotic flows (EOFs) is ubiquitous and plays a crucial role in ion transport, specimen mixing, electrochemistry reaction, and electric energy storage and utilization. When and how the transition from a linear regime to a nonlinear one occurs is essential for understanding, prohibiting, or utilizing nonlinear EOF. However, due to the lack of reliable experimental instruments with high spatial and temporal resolutions, the investigation of the onset of nonlinear EOF still remains in theory. Herein, we experimentally studied the velocity fluctuations of EOFs driven by an alternating current (AC) electric field via ultrasensitive fluorescent blinking tricks. The linear and nonlinear AC EOFs are successfully identified from both the time trace and energy spectra of velocity fluctuations. The transitional electric field (EA,C) is determined by both the convection velocity (U) and AC frequency (ff) as EA,C ∼ ff0.48-0.027U. We hope the current investigation could be essential in the development of both theory and applications of nonlinear EOFs.

Junction flow inside and around three-row cylindrical group on rigid flat surface

Groups of bluff bodies are widespread in nature and technology. These are the supports of bridge crossings, high-rise buildings in cities, offshore drilling and wind platforms, algae and vegetation in the seas and rivers, forests and other objects. The flow of air or water around such structures has a complex vortex and jet character and requires significant efforts in the process of scientific research to improve the environmental situation and reduce material and technical costs in the process of operating such structures. The purpose of the research is study the features of the generation and evolution of vortex and jet flows near and inside the three-row group of cylinders, which are installed on the rigid flat surface. The results of experimental studies showed that the flow around the group of cylinders had a complex unsteady nature, which is due to the interaction of vortex and jet flows typical flow elements with the three-row cylindrical group, which was located installed on the rigid flat surface. The three-row cylindrical group (31 piles with a diameter of 0.027 m) is a model of a bridge support, which was streamlined at a velocity of 0.06 m/s to 0.5 m/s (Reynolds number Red=(1600–6700) and Froude number Fr=(0.04–0.18)). Visual investigations and measurements of the velocity field were carried out inside and around the three-row structure. The features of the formation and evolution of vortex and jet flows inside and near the cylindrical group were established. Integral, spectral and correlation characteristics of the velocity fluctuation field were obtained. Mean, root-mean-square values of velocity and probability density functions of velocity fluctuations integrally displayed the changes in the velocity field in the spatial and temporal domain in the junction area of grillage and plate. The power spectral densities of velocity fluctuations and mutual correlation functions made it possible to study the features of the generation of the velocity fluctuation field in the frequency domain and its interrelationships in space. It was revealed that the velocity field inside the horseshoe vortex structures was multimodal. The spectral levels of velocity fluctuations at the periphery of the quasistable horseshoe vortex structures were higher than in the cores of these structures. The highest levels of the velocity fluctuation spectra were observed in front of the second lateral cylinder where the interaction of the vortex and jet flows took place. Discrete peaks in the spectral levels of velocity fluctuations are found at the frequencies of formation of large-scale wake vortices and the frequencies of formation of small-scale vortex structures of the shear layer, which are due to the Kelvin-Helmholtz instability. It has been established that the frequency of formation of shear layer vortices is (10–40) times higher than the frequency of formation of wake vortices.

Parabolic resolvent modes for streaky structures in transitional and turbulent boundary layers

Resolvent analysis is a powerful tool in fluid mechanics, both for laminar and turbulent flows. Depending on application, computational cost is still a limiting factor. We develop a parabolic method for the calculation of low-frequency resolvent modes, associated with streaky structures, which can enable more than an order of magnitude speed up compared to usual global calculations. The method is applied to both laminar and turbulent boundary layers, unveiling physical mechanisms such as those associated with streak growth due to free-stream turbulence, in the laminar case, and the double peak in the frequency spectrum of velocity fluctuations, in a turbulent boundary layer.

Viscoelasticity-Induced Instability in Plane Couette Flow at Very Low Reynolds Number

Elasto-inertial turbulence (EIT), a new turbulent state found in polymer solutions with viscoelastic properties, is associated with drag-reduced turbulence. However, the relationship between EIT and drag-reduced turbulence is not currently well-understood, and it is important to elucidate the mechanism of the transition to EIT. The instability of viscoelastic fluids has been studied in a canonical wall-bounded shear flow to investigate the transition process of EIT. In this study, we numerically deduced that an instability occurs in the linearly stable viscoelastic plane Couette flow for lower Reynolds numbers, at which a non-linear unstable solution exists. Under instability, the flow structure is elongated in the spanwise direction and regularly arranged in the streamwise direction, which is a characteristic structure of EIT. The regularity of the flow structure depends on the Weissenberg number, which represents the strength of elasticity; the structure becomes disordered under high Weissenberg numbers. In the energy spectrum of velocity fluctuations, a steep decay law of the structure’s scale towards a small scale is observed, and this can be recognized as a ubiquitous feature of EIT. The existence of instability in viscoelastic plane Couette flow supports the idea that the transitional path toward EIT may be mediated by subcritical instability.

Influence of Roughness Trips on Near- and Far-Field Trailing-Edge Noise

Roughness trips are widely applied in wind-tunnel testing to hasten the laminar–turbulent boundary-layer transition. It is well known that they influence the boundary-layer transition and its development. However, their influence on aeroacoustic quantities (in particular, on the airfoil trailing-edge noise) is not yet fully understood. This paper discusses the relationship between the trip height and the boundary-layer development and radiated trailing-edge noise. Wind-tunnel experiments were performed with zigzag strips and grit elements placed on the surface of a NACA 0012 airfoil. The roughness trip heights ranged from 17 to 214% of the undisturbed boundary-layer thickness at the trip location. The chord-based Reynolds number varied from to . This paper shows that trip heights within 50 to 105% of have little influence on the near field and the trailing-edge noise. However, trips with increase the spectra of velocity and pressure fluctuations in the low-frequency range up to 2.5 and 3 dB, respectively, resulting in an increase of the trailing-edge radiated noise up to 4 dB in a similar frequency range. In summary, this paper shows the importance of carefully defining the roughness trip used to trigger a laminar–turbulent transition in aeroacoustic and aerodynamic measurements.

Assessment of the Precision of Spectral Model Turbulence Analysis Techniques Using Direct Numerical Simulation Data

AbstractThe spectral model turbulence analysis technique is widely used to derive kinetic energy dissipation rates of turbulent structures (ɛ) from different in situ measurements in the Earth's atmosphere. The essence of this method is to fit a model spectrum to measured spectra of velocity or scalar quantity fluctuations and thereby to derive ɛ only from wavenumber dependence of turbulence spectra. Owing to the simplicity of spectral model of Heisenberg (1948), https://doi.org/10.1007/bf01668899 its application dominates in the literature. Making use of direct numerical simulations which are able to resolve turbulence spectra down to the smallest scales in dissipation range, we advance the spectral model technique by quantifying uncertainties for two spectral models, the Heisenberg (1948), https://doi.org/10.1007/bf01668899 and the Tatarskii (1971) model, depending on (a) resolution of measurements, (b) stage of turbulence evolution, (c) model used. We show that the model of Tatarskii (1971) can yield more accurate results and reveals higher sensitivity to the lowest ɛ‐values. This study shows that the spectral model technique can reliably derive ɛ if measured spectra only resolve half‐decade of power change within the viscous (viscous‐convective) subrange. In summary, we give some practical recommendations on how to derive the most precise and detailed turbulence dissipation field from in situ measurements depending on their quality. We also supply program code of the spectral models used in this study in Python, IDL, and Matlab.

Turbulence Measurement of Two Velocity Components in a Boundary Layer by a Two Parallel Hot-Wire Anemometer

A novel hot-wire technique consisting of two parallel hot-wires is proposed. The gap between the two parallel wires is filled with an adhesive, which can be expected to improve the sensitivity to the flow angle at high velocity regions. In the present study, measurement accuracy of the two component velocity fluctuations is appraised experimentally in a flat-plate turbulent boundary layer. As a result, it was verified that the present probe can reproduce not only the DNS database but also the measurements obtained by the standard I- and X-type hot-wire probes. In addition, the proposed probe can be brought closer to the wall than the X-type one. However, it is observed that the turbulent intensity measurements in the streamwise direction are slightly reduced, and the power spectrum of the streamwise velocity fluctuations is attenuated accordingly in a high-frequency range. Thus, fluid-dynamical and thermal effects of the adhesive need to be investigated in more detail in the future.

3D-LDA diagnostics of a swirled flow in the model of an improved four-vortex furnace

The three-component Laser Doppler Anemometry method (3D-LDA) was used to study the internal aerodynamics of an experimental model of a promising furnace with a four-vortex scheme for burning coal fuel. Distributions of the averaged velocity and velocity fluctuations are obtained. There are no the pronounced peaks in the spectrum of velocity fluctuations, so we can speak about the stability of the investigated flow. The studied model is characterized by a high level of velocity fluctuations, provided for effective mixing of the pulverized coal mixture in the combustion chamber of the furnace.

Wake-aerofoil interaction noise control with trailing-edge serrations

The present study experimentally investigates the effective reduction of wake-aerofoil interaction noise in a tandem aerofoil configuration using trailing-edge sawtooth serrations. The far-field noise results clearly show that at the ‘head-on’ wake impingement, the use of sawtooth serrations on the trailing-edge of the front aerofoil can lead to substantial reduction of the wake-aerofoil interaction noise of approximately 10 dB without any noticeable increase at other frequencies and changes to directivity pattern. Subsequently, detailed near-field measurements were carried out to investigate further the flow dynamics associated with the reduction of the interaction noise. The flow field and unsteady loading results suggest that the wake-aerofoil interaction primarily takes place within the first 30% of the chord close to the leading-edge area, while towards the trailing-edge, the flow is influenced more heavily by the boundary layer development. The velocity and wall pressure fluctuation spectra show significant decrease of the energy frequency content of the flow structures over the wake-aerofoil interaction frequency range after the wake impingement, due to the modification of the wake turbulence by trailing-edge serrations. This leads to notable reduction in the unsteady aerodynamic loading, particularly in the lift direction. Further coupled pressure–velocity analyses reveal that the sawtooth serrations modify the energy frequency content associated with the dominant turbulence eddies by spreading it over a wider frequency range. Near-field to far-field pressure coherence also reveals the loss of coherence, and thus abatement of the radiated far-field noise. The experimental results are relevant to practical applications in rotor–stator and outlet guide vanes.