Rms Fluctuations Research Articles

Temperature measurement of a turbulent buoyant ethylene diffusion flame using a dual-thermocouple technique

High-frequency temperature measurements were carefully conducted for a 15 kW buoyant turbulent ethylene diffusion flame over a 15.2 cm diameter gas burner with air co-flow. A dual-thermocouple probe, consisting of two fine-wire thermocouples with 25 μm and 50 μm wire diameters, was used to determine a compensated turbulent gas temperature. A sensitivity analysis shows that temperatures resolved using this dual-thermocouple technique are less sensitive to changes in thermocouple bead size, therefore, uncertainty is greatly reduced even when soot deposition on the thermocouple bead occurs in sooty flames. Mean and root-mean square (rms) fluctuations of gas temperature were recorded in a two-dimensional plane across the flame centerline. The mean gas temperature monotonically decreases away from the flame centerline at most flame heights, except for 1 diameter above the burner, where a temperature dip is observed. The rms temperature peaks shift from the edge of the flame to the center as the height increases. This is due to the enhanced mixing between fuel and air, which is further shown using probability density functions of the local gas temperature. A systematic temperature dataset with high spatial resolution is established for sooty flames, which is valuable for future soot and radiation model validation.

The Galactic Halo Contribution to the Dispersion Measure of Extragalactic Fast Radio Bursts

Abstract A new model of the Milky Way (MW) halo component of the dispersion measure (DM) for extragalactic sources, such as fast radio bursts, is presented in light of recent diffuse X-ray observations. In addition to the spherical component of isothermal gas (kT ∼ 0.3 keV) in hydrostatic equilibrium with the Galactic gravitational potential, our model includes a disk-like nonspherical hot gas component to reproduce the directional dependence of the observed X-ray emission measure (EM). The total gas mass (1.2 × 1011 M ⊙) is dominated by the spherical component, and is consistent with the total baryon mass of the MW expected from the dark matter mass and the cosmic baryon-to-dark-matter ratio. Our model predicts a mean halo DM of , with a full range of 30– over the whole sky. The large scatter seen in the X-ray EM data implies a ∼0.2 dex (rms) fluctuation of the MW halo DM. We provide an analytic formula to estimate the MW halo DM of our model along any line of sight, which can be easily used to compute the total MW component of DM toward extragalactic sources, in combination with existing DM models of the warm ionized medium associated with the Galactic disk.

Role of the N-terminus in human 4-hydroxyphenylpyruvate dioxygenase activity.

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is a key enzyme in tyrosine catabolism, catalysing the oxidation of 4-hydroxyphenylpyruvate to homogentisate. Genetic deficiency of this enzyme causes type III tyrosinaemia. The enzyme comprises two barrel-shaped domains formed by the N- and C-termini, with the active site located in the C-terminus. This study investigated the role of the N-terminus, located at the domain interface, in HPPD activity. We observed that the kcat/Km decreased ∼8-fold compared with wild type upon removal of the 12 N-terminal residues (ΔR13). Interestingly, the wild-type level of activity was retained in a mutant missing the 17 N-terminal residues, with a kcat/Km 11-fold higher than that of the ΔR13 mutant; however, the structural stability of this mutant was lower than that of wild type. A 2-fold decrease in catalytic efficiency was observed for the K10A and E12A mutants, indicating synergism between these residues in the enzyme catalytic function. A molecular dynamics simulation showed large RMS fluctuations in ΔR13 suggesting that conformational flexibility at the domain interface leads to lower activity in this mutant. These results demonstrate that the N-terminus maintains the stability of the domain interface to allow for catalysis at the active site of HPPD.

Anisotropy of turbulent fluctuations of velocity and third-order moments in the near wake of a circular cylinder

Turbulent structure of the near wake behind a circular cylinder in a cross flow at Re = 3900 was studied experimentally. Instantaneous velocity vector fields in the wake were measured using Smoke Image Velocimetry (SIV). The profiles of velocity components, their RMS fluctuations and Reynolds stresses were compared with direct numerical simulation and experimental data obtained by other authors. The anisotropy of turbulent fluctuations in different zones of the wake was analyzed. Third-order moments of velocity fluctuations in the near wake behind the cylinder were measured for the first time ever.

Investigation of the Wall Scalar Fluctuations Effect on Passive Scalar Turbulent Fields at Several Prandtl Numbers by Means of Direct Numerical Simulations

Abstract We investigate the effect of the wall-scalar fluctuations on passive scalar turbulent fields for a moderate Reynolds number Rτ = 395 and for several Prandtl numbers ranging from the very low value Pr = 0.01 to the high value Pr = 10 by means of direct numerical simulation (DNS) simulations. Several cases of plane channel flows are considered. Case I is a channel flow heated on both walls with a constant imposed heat flux qw. We consider for this case two different types of boundary conditions. For the first one, the isoscalar boundary condition θw = 0 is imposed at the wall implying that its fluctuation and therefore its rms scalar fluctuations θrms=⟨θ′θ′⟩ is zero at the wall whereas in the second type, θw is not prescribed to a fixed value so that it is fluctuating in time at the wall leading to nonzero rms fluctuations. In this latter case, as the heat flux is maintained constant in time at the wall, the fluctuating heat flux q′w reduces to zero at the wall. For illustration purpose, in addition to case I, we also consider case II, which is a plane channel heated only from one wall but cooled from the other one at the same rate taking into account of the freestream scalar boundary condition at the wall θ′w≠0 with q′w=0. The distributions of the mean scalar field, root-mean-square fluctuations, turbulent heat flux, correlation coefficient, turbulent Prandtl number, and Nusselt number are examined in detail. Moreover, some insights into the flow structure of the scalar fields are provided. As a result of interest, it is found that the mean scalar field ⟨θ⟩ is not affected by the scalar fluctuations at the wall. But owing to the different boundary conditions applied at the wall, significant differences in the evolution of the rms scalar fluctuations θrms are observed in the immediate vicinity of the wall. Surprisingly, the maximum rms intensity remains almost unchanged in the near wall region whatever the type of boundary condition is applied at the wall. In addition, the turbulent heat fluxes that play a major role in heat transfer are found to be independent of the wall scalar fluctuations. This study demonstrates that the impact of the wall scalar fluctuations is appreciable mainly in the near wall region. This outcome must be taken into account when simulating industrial flows with thermal boundary conditions involving different fluid/solid combinations.

Investigation of slug-churn flow induced transient excitation forces at pipe bend

Numerical simulations of two-phase flow induced fluctuating forces at a pipe bend have been carried out to study the characteristics of multiphase flow induced vibration (FIV). The multiphase flow patterns and turbulence were modelled using the volume of fluid (VOF) method and the k−ϵ turbulence model respectively. Simulations of seventeen cases of slug and churn flows have been carried out showing the effects of superficial gas and superficial liquid velocities. The simulations results show good agreement of the volume fraction fluctuation frequencies of slug and churn flows with the reported experiment. In addition, the vibration characteristics of the excitation force have been accurately captured. The simulation results show that the predominant frequency of fluctuations of force decreases and the RMS of force fluctuation increases with the increase of superficial gas velocity. On the other hand, both predominant frequency and the RMS of force fluctuations increases with the increase of superficial liquid velocity. Increase of gas fraction narrows the range of frequency ranges, while increasing the liquid expands the frequency ranges of force fluctuations.

Numerical Computation of Turbulent Flow Fields in a Fan-stirred Combustion Bomb

ABSTRACT This work presents a numerical study of the initial turbulent flow characteristics prior to ignition in a fan-stirred combustion vessel. The moving mesh methodology has been applied to account for the rotational movement of eight fans mounted within the vessel. The transient, highly-resolved numerical simulations resolve the 3D turbulent flow field in the whole vessel and complement 2D measurements of the flow field. The calculated turbulence intensities yield reasonably good agreement with measured data, showing a nearly linear increase with the rotation speed of the fans ω. The same applies for the spectra of the turbulent kinetic energy. The rms of velocity fluctuations is almost constant and the same in each direction in the core region with a diameter of approx. 2 cm, confirming the homogeneity and isotropy of the generated turbulent flow field in the fan-stirred bomb, as observed in previous experiments. The calculated integral length scale agrees well with the measured value, which increases with ω or the turbulent Reynolds number Ret , respectively. The calculated rates of decay of the Kolmogorov and Taylor lengths with Ret have shown a quantitatively good agreement with the hypothesis proposed by Kolmogorov for isotropic turbulence, which justifies the validity and reliability of the numerical simulations. The present study serves as a reference for assessing the uncertainties given by common 2D measurement techniques, which allows a more accurate analysis of the turbulence characteristics and the turbulence-flame interactions in a fan-stirred combustion vessel.

Consistent description of mesoscopic transport: Case study of current-dependent magnetoconductance in single GaN:Ge nanowires

The so called phase-coherence length ${l}_{\ensuremath{\varphi}}$ and its relation to the geometrical dimensions of a sample determine the electronic transport regime. Different approaches are established for extracting ${l}_{\ensuremath{\varphi}}$ from magnetotransport data of mesoscopic systems and need to be cross-checked by using experimental data on the same model system in order to ensure an overall consistent theoretical description. Suitable model systems for testing this consistency are single GaN:Ge nanowires. Their magnetoconductance at low temperatures exhibits universal conductance fluctuations as well as weak localization effects. We find that the values of ${l}_{\ensuremath{\varphi}}$ obtained from the established analysis of the magnitude of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) decrease with increasing measurement current, whereas the corresponding values of ${l}_{\ensuremath{\varphi}}$ determined by the analyses of the correlation field ${B}_{\text{C}}$ and the weak localization effect yield the same value for ${l}_{\ensuremath{\varphi}}$ independent of the measurement current used. We apply and modify the existing theoretical framework for bias-dependent differential conductance fluctuations, in order to explain the decrease of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) with increasing current density in our dc measurements. This leads to the same values of ${l}_{\ensuremath{\varphi}}$ independent of the analysis approach applied to the same set of data.

ISW in ΛCDM or something else?

ABSTRACT We investigate a correlation between the Planck’s CMB temperature map and statistics based on the space density of quasars in the SDSS catalogue. It is shown that the amplitude of the positive correlation imposes a lower limit on the amplitude of the Integrated Sachs–Wolfe effect independent of the quasar bias factor. Implications of this constraint for the ISW effect in the Λ cold dark matter (ΛCDM) model are examined. Strength of the correlation indicates that the rms of temperature fluctuations associated with the quasars distributed between 1500 and 3000 Mpc likely exceeds $11{\!-\!}12\, \mu$K. The signal seems to be related to an overall space distribution of quasars rather than to a few exceptionally dominant structures like supervoids. Although, the present estimates are subject to sizable uncertainties, the signal apparently exceeds the model predictions of the ISW effect for the standard ΛCDM cosmology. This conclusion is consistent with several other investigations that also claim some disparity between the observed ISW signal and the theoretical predictions.

Using Mobile Health Technology to Improve Well-Being in Caregivers of Patients with Bipolar Disorder: Poster Abstract Submission

A piloted, partially premixed, liquid-fueled swirl burner is operated at high pressure (1 MPa). High-speed (6 KHz) stereoscopic PIV is used to investigate the characteristics of the stagnation line separating the pilot jet and the central recirculation zone (CRZ) with varying pilot-main ratio and global equivalence ratio. The mean curvature of the stagnation line displayed a large spatial scale pattern that was present for all operating conditions. All three components of velocity, in-plane shear, and swirling strength are conditioned upon the instantaneous stagnation line. Mean distributions of the velocity normal to the stagnation line show that velocity is oriented towards the CRZ when the stagnation line is found nearer the centerline of the combustor. The conditioned out-of-plane velocity (w) shows a distinct concentration of large mean and fluctuation RMS values towards the center of the measurement domain. Varying fuel flow does not significantly change this spatial structure, only the magnitudes of the w statistics. The in-plane shear stress was the largest for the pilot biased condition as a stronger shear layer develops. For the leanest flame, large fluctuation RMS values of shear stress were confined to a region where the pilot jet begins to interact more heavily with the main jet. Operating with less pilot fuel flow enhanced the mean conditional swirling strength indicating that the pilot shear layer was shedding more intense eddies. Disregarding spatial relations, a scatter plot of w, shear stress, and swirling strength displayed trends between the variables. The largest swirling strength values coincide with highest magnitude shear stresses and the widest range of w. These conditioned statistics highlight how certain aspects of the combustor flow field are invariant with fuel distribution. This is desirable for aeropropulsive combustors that must maintain stable ignition from a range of conditions from landing/take-off to cruise.

Constraints on non-resonant photon-axion conversion from the Planck satellite data

The non-resonant conversion of Cosmic Microwave Background (CMB) photons into scalar as well as light pseudoscalar particles such as axion-like particles (ALPs) in the presence of turbulent magnetic fields can cause a unique, spatially fluctuating spectral distortion in the CMB. We use the publicly available Planck temperature maps for the frequency channels (70–545 GHz) to obtain the first ALP distortion map using 45% clean part of the sky. The 95th percentile upper limit on the RMS fluctuation of ALP distortions from the cleanest part of the CMB sky at 15 arcmin angular resolution is 18.5 × 10−6. The RMS fluctuation in the distortion map is also consistent with different combinations of frequency channels and sky-fractions.

Enhancement of efficiency in femtosecond optical parametric oscillators using group-velocity-matching in long nonlinear crystals

The authors describe the exploitation of group-velocity-matching in femtosecond optical parametric oscillators (OPOs) for enhanced down-conversion efficiency into the mid-infrared (mid-IR). We demonstrate the concept in a femtosecond OPO based on a long MgO:PPLN crystal, for the first time, by utilizing group-velocity-matching between pump and idler pulses. Taking advantage of the wide phase-matching bandwidth when pumped near 1 µm enables the use of a 42-mm-long crystal, resulting in an oscillation threshold as low as 5 mW, pump depletion of 78%, and an idler quantum conversion efficiency up to 48% into the mid-IR. Using 80–100 fs pump pulses tunable across 997–1070 nm, we have generated idler radiation across 3132–4273 nm (2340–3193 cm−1) with spectral bandwidth of 140–180 nm, providing up to 65 mW of average power at 80 MHz repetition rate. The near-IR signal is tunable across 1392–1568 nm, with up to 76 mW of average power in transform-limited pulses of ∼400–600 fs duration without dispersion compensation. Preferential operation at group-velocity-matched wavelengths leads to intrinsically high passive power stability with <1% rms fluctuation over 1 h for both signal and idler. With the capability for rapid pump tuning in the mid-IR, this OPO represents a viable source for spectroscopic applications, which we demonstrate using CH4 gas. Extension to other quasi-phase-matched nonlinear crystals is also discussed.

Influence of sound-absorbing coatings on the development of disturbances in a flowing mixture of vibrationally excited gases

In the present paper, we study the development of disturbances on a solid (continuous) plate and on a plate covered with a sound-absorbing coating, both installed at an angle of attack in a hypersonic flow (M∞ = 8.44) of a vibrationally excited mixture of carbon dioxide with nitrogen. The rms fluctuations of pressure were measured on the surface of the plates in a short-duration IT-302M wind tunnel of ITAM SB RAS. The numerical simulation was performed on the basis of solving the two-dimensional unsteady Navier-Stokes equations and a two-temperature model of relaxation flows. In modeling a real porous coating, a skeletal model was used. The model is formed by square elements arranged in staggered order with a distance between elements equal to the diameter of the pores in the sound-absorbing material used in the experiment. Data on the dynamics of the disturbance development on the solid plate and on the plate with the sound-absorbing coating under different conditions of the incident flow are reported. The effect of various parameters of the sound-absorbing coating (depth, length, and location on the plate) was studied. It is shown that the sound-absorbing coating significantly (up to 50 %) suppresses the intensity of pressure fluctuations on the plate surface as compared with the solid surface. A good agreement between the calculated and experimental data on the pressure pulsations on the surface of the plates is demonstrated.

Giant number fluctuations in dry active polar fluids: A shocking analogy with lightning rods.

It has been shown [H. Chaté et al., Phys. Rev. E 77, 046113 (2008) and F. Ginelli, Eur. Phys. J.: Spec. Top. 225, 2099 (2016)] that the hydrodynamic equations of dry active polar fluids (i.e., moving flocks without momentum conservation) imply giant number fluctuations. Specifically, the rms fluctuations ⟨(δN)2⟩ of the number N of active particles in a region containing a mean number of active particles ⟨N⟩ scale according to the law ⟨(δN)2⟩=K'⟨N⟩ϕ(d) with ϕ(d)=710+15d in d ≤ 4 spatial dimensions. This is much larger than the "law of large numbers" scaling ⟨(δN)2⟩=K⟨N⟩ found in most equilibrium and nonequilibrium systems. In this paper, it is demonstrated that giant number fluctuations also depend singularly on the shape of the box in which one counts the particles, vanishing in the limit of very thin and very fat boxes. These fluctuations arise not from large density fluctuations-indeed, the density fluctuations in polar ordered dry active fluids are not in general particularly large-but from long ranged spatial correlations between those fluctuations. These are shown to be closely related in two spatial dimensions to the electrostatic potential near a sharp upward pointing conducting wedge of opening angle 3π4=135° and in three dimensions to the electrostatic potential near a sharp upward pointing charged cone of opening angle 37.16°. This very precise prediction can be stringently tested by alternative box counting experiments that directly measure the direction dependence, as well as the scaling with distance, of the density-density correlation function.

Stable temperature regulation for TES testing below 200mK employing improved Cohen-Coon method

Transition Edge Sensor (TES) is a promising low-temperature detector technology for high-resolution X-ray spectroscopy. Hot Universe Baryon Surveyor (HUBS) is a satellite concept proposed in China to address so-called “missing baryon problem”, which has serious implications on the formation and evolution of galaxies. At the heart of HUBS is a soft X Ray spectrometer based on TES operating below 100 mK. In the developing phase, the characterization of TES requires high accuracy and stable temperature regulation. Typically, 10 μK temperature fluctuation would be needed over the temperature range of 50 mK to 200 mK. To achieve this accuracy, we designed and built a cold plate in a dilution refrigerator, which regulates temperature on the cold plate with a PID controller. To improve the stability of temperature regulation, Cohen-Coon method is employed and optimized experimentally. Over the desired temperature range, the temperature of the cold plate can be regulated stably and accurately. At typical transition temperatures of our TES devices, temperature fluctuation is well controlled, e.g., within 8 μK (rms) fluctuation at 100 and 200 mK.

Study of the Interaction of Quercetin and Taxifolin with β-Lactoglobulin by Fluorescence Spectroscopy and Molecular Dynamics Simulation

The interaction between quercetin and taxifolin with β-lactoglobulin (BLG) was investigated via various methods, including fluorescence spectroscopy, molecular docking and molecular dynamics (MD) simulation. The results have demonstrated that quercetin binds BLG with an affinity higher than that of taxifolin, which is attributed to the nonplanar C-ring and steric hindrance effect in taxifolin. The synchronous fluorescence spectra shows that quercetin and taxifolin do not induce conformational changes of BLG. Molecular docking studies have demonstrated that several amino acids are involved in stabilizing the interaction. Analysis of the MD simulation trajectories shows that the root mean square deviation (RMSD) of various systems reaches equilibrium. Time evolution of the radius of gyration shows as well that BLG and BLG-flavonoid complexes are stable within 5 ns. In addition, analyzing the RMS fluctuations, one can suggest that the structure of the ligand binding site remains rigid during the simulation. The secondary structure of BLG is preserved upon interaction with these flavonoids.

Lean partially premixed turbulent flame equivalence ratio measurements using laser-induced breakdown spectroscopy

The creation of a more stable flame along with the extension of flammability limits under lean mixture combustion was the main motivation to develop a new burner design, which has been investigated in this research. The current burner configuration was utilized to create a wide range of higher turbulent intensities and to produce different degrees of mixture inhomogeneity, which acted to promote minimum pollution, highest performance and higher flame stability. The burner stability assessment was investigated using two types of fuel: natural gas (NG) and liquefied petroleum gas (LPG). They were tested under different degrees of partial premixing, and two turbulence generator disks for lean mixture at an equivalence ratio of φ = 0.8 were used. Following this, the Laser Induced Breakdown Spectroscopy (LIBS) technique was utilized to characterize and quantify the impact of changing the disk slit diameter on the distributions profiles of equivalence ratio or mixture fraction for a NG/air partially premixed flame. A series of homogeneous NG/air mixtures with different equivalence ratios were used to obtain the correlations between the measured emission lines of LIBS spectra and the global flame equivalence ratio. Consequently, the emission spectral lines ratios of H/N, H/O and C/N + O were utilized to predict the equivalence ratio distributions. The results demonstrated that for all of the mixing lengths, NG/air mixture with larger disk generator diameter yielded the maximum burner stability, whilst the LPG/air mixture with a larger disk generator diameter resulted in the minimum burner stability. Furthermore, the flame associated with the larger disk slit diameter had a uniform local equivalence ratio distribution and lower RMS fluctuation profiles of equivalence ratio in comparison to the lower disk slit diameter.

Constraining density fluctuations with big bang nucleosynthesis in the era of precision cosmology

We reexamine big bang nucleosynthesis with large-scale baryon density inhomogeneities when the length scale of the density fluctuations exceeds the neutron diffusion length ($\sim 10^7-10^8$ cm at BBN), and the amplitude of the fluctuations is sufficiently small to prevent gravitational collapse. In this limit, the final light element abundances can be determined by simply mixing the abundances from regions with different baryon/photon ratios without interactions. We examine gaussian, lognormal, and gamma distributions for the baryon/photon ratio, $\eta $. We find that the deuterium and lithium-7 abundances increase with the RMS fluctuation in $\eta $, while the effect on helium-4 is much smaller. We show that these increases in the deuterium and lithium-7 abundances are a consequence of Jensen's inequality, and we derive analytic approximations for these abundances in the limit of small RMS fluctuations. Observational upper limits on the primordial deuterium abundance constrain the RMS fluctuation in $\eta $ to be less than $17\%$ of the mean value of $\eta $. This provides us with a new limit on the graininess of the early universe.

Structure conditioned velocity statistics in a high pressure swirl flame

A piloted, partially premixed, liquid-fueled swirl burner is operated at high pressure (1 MPa). High-speed (6 KHz) stereoscopic PIV is used to investigate the characteristics of the stagnation line separating the pilot jet and the central recirculation zone (CRZ) with varying pilot-main ratio and global equivalence ratio. The mean curvature of the stagnation line displayed a large spatial scale pattern that was present for all operating conditions. All three components of velocity, in-plane shear, and swirling strength are conditioned upon the instantaneous stagnation line. Mean distributions of the velocity normal to the stagnation line show that velocity is oriented towards the CRZ when the stagnation line is found nearer the centerline of the combustor. The conditioned out-of-plane velocity (w) shows a distinct concentration of large mean and fluctuation RMS values towards the center of the measurement domain. Varying fuel flow does not significantly change this spatial structure, only the magnitudes of the w statistics. The in-plane shear stress was the largest for the pilot biased condition as a stronger shear layer develops. For the leanest flame, large fluctuation RMS values of shear stress were confined to a region where the pilot jet begins to interact more heavily with the main jet. Operating with less pilot fuel flow enhanced the mean conditional swirling strength indicating that the pilot shear layer was shedding more intense eddies. Disregarding spatial relations, a scatter plot of w, shear stress, and swirling strength displayed trends between the variables. The largest swirling strength values coincide with highest magnitude shear stresses and the widest range of w. These conditioned statistics highlight how certain aspects of the combustor flow field are invariant with fuel distribution. This is desirable for aeropropulsive combustors that must maintain stable ignition from a range of conditions from landing/take-off to cruise.

The kinematics of boundary layer transition on a long circular cylinder impinged by a fully turbulent round jet

We experimentally examine the impingement of a fully turbulent round jet (with diameter Dj) on a long circular cylinder (with diameter D) in the crossflow plane coinciding with the jet axis for D/Dj = 2.5 and ReDj = 20,000. Particular focus is placed on the kinematics of boundary layer transition that occurs and subsequently leads to a second thermal peak on the cylinder surface downstream of the primary thermal peak near the stagnation point when placed inside the jet’s potential core. To this end, spectral analyses of wall shear stress data and time-resolved velocity data within both laminar and turbulent boundary layers have been performed. The present study demonstrates that the root mean square (rms) fluctuation of the stream-wise velocity component increases along both boundary layers due to the propagation of external perturbations from coherent structures that are shed from the jet exit, which incites the transition of the laminar boundary layer. The transition causes the local heat transfer elevation that interrupts the monotonic decrease from the primary thermal peak whereas it plays no direct part in forming the second thermal peak. Instead, the second thermal peak occurs at a delayed downstream azimuth angle from the transition where the rms velocity fluctuations of the boundary layer flow reach their peak at the dominant frequency, equivalent to that of the coherent structures of the jet.