Velocity Power Spectra Research Articles

Turbulence spectra in the near-neutral surface layer over the Loess Plateau in China

We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange, suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a height dependency over the three levels.

Protostellar Turbulence Driven by Collimated Outflows

The majority of stars are thought to form in clusters. Cluster formation in dense clumps of molecular clouds is strongly influenced, perhaps controlled, by supersonic turbulence. We have previously shown that the turbulence in regions of active cluster formation is quickly transformed by the forming stars through protostellar outflows, and that the outflow-driven protostellar turbulence is the environment in which most of the cluster members form. Here, we take initial steps in quantifying the global properties of the protostellar turbulence through 3D MHD simulations. We find that collimated outflows are more efficient in driving turbulence than spherical outflows that carry the same amounts of momentum. Gravity plays an important role in shaping the turbulence, generating infall motions in the cluster forming region that balance the outward motions driven by outflows. The resulting quasi-equilibrium state is maintained through a slow rate of star formation, with a fraction of the total mass converted into stars per free fall time as low as a few percent. Magnetic fields are dynamically important even in magnetically supercritical clumps, provided that their initial strengths are not far below the critical value for static cloud support. We find that the mass weighted PDF of the volume density is often, although not always, approximately lognormal. The PDFs of the column density deviate more strongly from lognormal distributions. There is a prominent break in the velocity power spectrum of the protostellar turbulence, which may provide a way to distinguish it from other types of turbulence.

Movement patterns of peak-dose levodopa-induced dyskinesias in patients with Parkinson's disease

The present study characterized involuntary movements associated with levodopa-induced dyskinesias (LID) in patients with Parkinson's disease. We used amplitude, proportional energy, frequency dispersion and sample entropy to determine whether LID movement patterns are truly random, as clinical description seems to suggest, or possess some underlying pattern that is not visible to the naked eye. LID was captured using a magnetic tracker system, which provided 3D rendering of whole-body LID. Patients were instructed to maintain a standing position, with arms extended in front of them. We compared the measurements of the dyskinetic PD group (DPD) with 10 patients without dyskinesias (NDPD) and 10 control subjects. In comparison to the other two groups, movement patterns from the DPD group had significantly higher amplitude, confirming the presence of dyskinesias. In addition, higher frequency components in the power spectrum of velocity were detected, suggestive of higher velocity in LID movement. Furthermore, there was a concentration in narrow frequency bands, which suggested stable oscillatory activity. Finally, sample entropy revealed more regularity in the DPD group. Although not statistically significant, we found that the amplitude from the NDPD group had a tendency to be smaller than those of controls. As well, the spectra were often more dispersed for the NDPD group. In conclusion, the present results suggest that LID cannot be considered as purely random movement since they possess some deterministic pattern of motion. This may provide a way for patients to adapt to these involuntary movements while performing voluntary motor acts.

Experimental analysis of the precessing vortex core in a free swirling jet

An experimental analysis of the precessing vortex core (PVC) instability in a free swirling jet of air at ambient pressure and temperature is performed by means of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Two parametric studies are considered, varying the swirl parameter and the Reynolds number. The range of parameters considered allowed to study conditions of strong precession as well as the inception and settlement of the instability. Mean velocity and standard deviation profiles, power spectral density functions and probability density functions for the axial and tangential velocity components are presented. Average as well as instantaneous PIV maps are considered in the characterization of the flowfield structure and detection of the instantaneous position of the vortex center. Joint analysis of velocity PDFs and power spectra shows that the PVC contribution to the global statistics of the velocity field can be properly separated from the contribution of the true flow turbulence, giving additional insight to the physics of the precession phenomenon. The results obtained in the explored range of conditions indicate that the true turbulence intensity is not dependent on the swirl parameter.

Estimating the turbulent energy dissipation rate in an airport environment

This note reports on the influence of aircraft wake vortices on the estima- tion of the turbulent energy dissipation rate using sonic anemometer measurements near the runway threshold. The wake vortex traces, which are generated at a height of about 65m and subsequently evolve in ground effect, are clearly visible in the velocity components and temperature. The observed temperature increase of 1K appears related to the stably stratified atmospheric surface layer. The dissipation rate is estimated from the longitudinal velocity power spectrum for a sample in a noctur- nal boundary layer with and without a wake vortex signal. In both cases an inertial subrange is found. For the analyzed sample the estimated dissipation rate is a factor of ten larger compared to the undisturbed sample. Implications for operational wake avoidance systems are discussed.

Turbulent Structure of a Stratified Supernova‐driven Interstellar Medium

To study how supernova feedback structures the turbulent interstellar medium, we construct 3D models of vertically stratified gas stirred by discrete supernova explosions, including vertical gravitational fields and parameterized heating and cooling. The models reproduce many observed characteristics of the Galaxy, such as global circulation of gas (i.e., galactic fountain) and the existence of cold dense clouds in the galactic disk. Global quantities of the model such as warm and hot gas filling factors in the midplane, mass fraction of thermally unstable gas, and the averaged vertical density profile are compared directly with existing observations and shown to be broadly consistent. We find that energy injection occurs over a broad range of scales. There is no single effective driving scale, unlike the usual assumption for idealized models of incompressible turbulence. However, >90% of the total kinetic energy is contained in wavelengths shortward of 200 pc. The shape of the kinetic energy spectrum differs substantially from that of the velocity power spectrum, which implies that the velocity structure varies with the gas density. Velocity structure functions demonstrate that the phenomenological theory proposed by Boldyrev is applicable to the medium. We show that it can be misleading to predict physical properties such as the stellar initial mass function based on numerical simulations that do not include self-gravity of the gas. Even if all the gas in turbulently Jeans-unstable regions in our simulation is assumed to collapse and form stars in local free-fall times, the resulting total collapse rate is significantly lower than the value consistent with the input supernova rate. Supernova-driven turbulence inhibits star formation globally rather than triggering it.

Prediction of rotating stall within an impeller of a centrifugal pump based on spectral analysis of pressure and velocity data

Experimental data, which was acquired in two centrifugal pumps and provided by Grundfos A/S, were analysed to determine if rotating stall could be detected from the velocity and pressure time series. The pressure data, which were uniformly acquired in time at high sample rates(10 kHz), were measured simultaneously in four adjacent di.user channels just downstream of the impeller outlet. The velocity data, which were non-uniformly sampled in time at fairly low rates(100 Hz to 3.5 kHz), were acquired either in or downstream of the impeller. Two di.erent methodologies were employed for detection of stall. The first method, which involved direct analysis of raw data, yielded qualitatively useful flow reversal information from the time series for the radial velocity. The second approach, which was based on power spectrum analysis of velocity and pressure data, could detect the onset and identify the frequency of rotating stall to a satisfactory extent in one of the two pumps. Nearly identical stall frequencies were observed in both velocity and pressure power spectra and this rotating stall phenomenon, which occurred at a very low frequency relative to the impeller speed, did not reveal any noticeable degree of sensitivity to the flow rate. In the other pump, where the available data was limited to velocity time series, the power spectrum analysis was successful in detecting stationary stall for a 6 bladed impeller but did not provide conclusive results for the existence of stall in the case of the 7 bladed impeller. Recommendations on the type of experimental data required for accurate detection of stall are provided based upon the present study.

Corrections to Inertial-Range Power Spectra Measured by Csat3 and Solent Sonic Anemometers,1. Path-Averaging Errors

Sonic anemometer path-averaging errors are determined for measurements of inertial-range velocity and temperature power spectra as a function of k1p, where k1 is the streamwise wave number and p is the sonic path length. The attenuation of vertical velocity spectra is found to be quite similar for the CSAT3 and Solent anemometers and to be insensitive to wind direction. The attenuation of the horizontal wind component spectra is noticeably greater for the Solent sonics than for the CSAT3, but those for the CSAT3 have a greater dependence on wind direction. The attenuation of sonic temperature spectra is also found to be quite similar for the CSAT3 and Solent R3 sonics and to be insensitive to wind direction, while that of the Solent R2 is less than that of the other two sonics and has a minor dependence on wind direction.

The far downstream evolution of the high-Reynolds-number axisymmetric wake behind a disk. Part 1. Single-point statistics

The high-Reynolds-number axisymmetric wake behind a disk has been studied for $10 \leq x/D \leq 150$ ($36 \leq x/\theta \leq 552 $) using a rake of 15 hot wires. The disk had a diameter of 20 mm, and the Reynolds number based on the free-stream velocity was 26 400. Mean velocity profiles, root-mean-square profiles, and power spectra are presented. By using regression techniques to fit the velocity profiles it was possible to obtain accurate velocity centreline deficits and transverse length scales to even the last downstream position. Beyond the initial region which extends to $x/D=30$, the data are in excellent agreement with the high-Reynolds-number equilibrium similarity solution.

A223 固体熱伝導評価のための新しい格子振動解析法の提案(マイクロ・ナノスケール7)

We propose a novel technique of molecular dynamics simulation to evaluate the relaxation time of phonons in solids for investigation of solid heat conductivity. The method is to observe relaxation behavior of the power spectrum of atomic velocities after energetically stimulating modes in a specific frequency region. The transient entropy S(t) is defined with the power spectrum based on non-equilibrium statistical mechanics. The obtained S(t) was found to be well fitted to a single exponential function, from which the frequency-dependent relaxation time can be evaluated.

Acoustic radiation from weakly wrinkled premixed flames

This paper describes a theoretical analysis of acoustic radiation from weakly wrinkled (i.e., u ′ / S L < 1 ) premixed flames. Specifically, it determines the transfer function relating the spectrum of the acoustic pressure oscillations, P ′ ( ω ) , to that of the turbulent velocity fluctuations in the approach flow, U ′ ( ω ) . In the weakly wrinkled limit, this transfer function is local in frequency space; i.e., velocity fluctuations at a frequency ω distort the flame and generate sound at the same frequency. This transfer function primarily depends upon the flame Strouhal number St (based on mean flow velocity and flame length) and the correlation length, Λ, of the flow fluctuations. For cases where the ratio of the correlation length and duct radius Λ / a ≫ 1 , the acoustic pressure and turbulent velocity power spectra are related by P ′ ( ω ) ∼ ω 2 U ′ ( ω ) and P ′ ( ω ) ∼ U ′ ( ω ) for St ≪ 1 and St ≫ 1 , respectively. For cases where Λ / a ≪ 1 , the transfer functions take the form P ′ ( ω ) ∼ ω 2 ( Λ / a ) 2 U ′ ( ω ) and P ′ ( ω ) ∼ ω 2 ( Λ / a ) 2 ( Ψ − Δ ln ( Λ / a ) ) U ′ ( ω ) for St ≪ 1 and St ≫ 1 , respectively, where Ψ and Δ are constants. The latter result demonstrates that this transfer function does not exhibit a simple power law relationship in the high frequency region of the spectra. The simultaneous dependence of this pressure–velocity transfer function upon the Strouhal number and correlation length suggests a mechanism for the experimentally observed maximum in acoustic spectra and provides some insight into the controversy in the literature over how this peak should scale with the flame Strouhal number.

Properties of the Solar Acoustic Source Inferred from Nonadiabatic Oscillation Spectra

Severino et al. suggested in 2001 that observed power and cross spectra of medium-degree p-modes in velocity and intensity can be described by splitting the solar background noise into correlated, coherent, and uncoherent components. We account for the nonadiabatic nature of solar oscillations by including the perturbations of the radiative energy flux in our model for the oscillations. Our calculations show the potential to explain the observations without the ad hoc phase differences between velocity and intensity oscillations introduced in the model of Severino et al. The phases and amplitudes of the correlated noise components are obtained by fitting our nonadiabatic model to the SOHO MDI power and cross spectra. These parameters provide information about the p-mode excitation process. We show that the type and location of the source can not be uniquely determined by the properties of the resonant p-modes in power and cross spectra of velocity and intensity oscillations. However, we obtain estimates for the phases and amplitudes of the correlated noise, which we interpret in terms of isolated rapid downdrafts in intergranular lanes. This idea is supported by three-dimensional simulations of the upper solar convection zone.

Spatial and temporal turbulent velocity and vorticity power spectra from sound scattering

By performing sound-scattering measurements with a detector array consisting of 62 elements in a flow between two counter-rotating disks we obtain the energy and vorticity power spectra directly in both spatial and temporal domains. Fast-accumulated statistics and a large signal-to-noise ratio allow us to get high-quality data rather effectively and to test scaling laws in details.

Online versus offline air quality modeling on cloud‐resolving scales

Computational advances now allow air quality forecast models to fully couple the meteorology with chemical constituents within a unified modeling system – online – that allows two‐way interactions. The more common approach is the offline system, which only allows one‐way coupling from the meteorology – sampled at fixed time intervals – to the chemistry. To study the information loss between online and offline numerical forecasts, a next‐generation nonhydrostatic air quality modeling system has been developed that can be used both offline or online. First, a control online air quality simulation is conducted and the meteorology and chemical data are saved at a 10 s time interval. Subsequently, three offline simulations are conducted with meteorological data updates at 10, 30, and 60 min time intervals. Analysis of the wind velocity power spectrum and chemical profiles indicate that the offline simulations are susceptible to large errors in the vertical mass distribution.

Local modification of benthic flow environments by suspension-feeding stream insects.

Larval black flies often exhibit spatially aggregated distributions, and individuals within patches can potentially reduce the supply of suspended food particles to downstream neighbors by modifying local flow characteristics. We used hot-film anemometry to quantify the magnitude and spatial extent of flow modifications downstream from feeding Simulium vittatum larvae in a laboratory flume, and to determine whether temporal patterns of flow variation are related to movements of the larval feeding appendages. Mean velocity 1 mm downstream from feeding larvae was reduced by 75%, and the percent reduction in velocity diminished asymptotically with downstream distance. Reduced velocities were evident as much as 60 mm downstream from, and 3 mm to either side of, larvae. Turbulence intensity (i.e., the SD of the velocity time series) was generally higher in this region relative to control flow conditions. Three results demonstrate the major contribution of the larval feeding appendages (i.e., labral fans) to such flow modification. First, there was a minimal reduction in mean velocity 5 mm downstream from non-feeding larvae (i.e., with closed labral fans), whereas mean velocity at the same location was reduced markedly when larvae were feeding. Second, the power spectrum of the velocity time series exhibited greatest power at frequencies that corresponded to the frequency of labral fan motions. Third, fan flick times accounted for most of the variance in the velocity power spectrum. The large local flow modifications that we documented have potentially important consequences for the feeding performance and growth of individuals located within larval aggregations, and are likely to influence behavioral interactions and spacing patterns.

Likelihood analysis of the Local Group acceleration revisited

We re-examine likelihood analyses of the Local Group (LG) acceleration, paying particular attention to non-linear effects. Under the approximation that the joint distribution of the LG acceleration and velocity is Gaussian, two quantities describing non-linear effects enter these analyses. The first one is the coherence function, i.e. the cross-correlation coefficient of the Fourier modes of gravity and velocity fields. The second one is the ratio of velocity power spectrum to gravity power spectrum. To date, in all analyses of the LG acceleration, the second quantity has not been accounted for. Extending our previous work, we study both the coherence function and the ratio of the power spectra. With the aid of numerical simulations we obtain expressions for the two as functions of wavevector and σ 8 . Adopting WMAP's best determination of σ 8 , we estimate the most likely value of the parameter β and its errors. As the observed values of the LG velocity and gravity, we adopt respectively an estimate of the LG velocity based on the cosmic microwave background, and Schmoldt et al.'s estimate of the LG acceleration from the PSCz catalogue. We obtain β = 0.66 +0.21 -0.07; thus our error bars are significantly smaller than those of Schmoldt et al. This is not surprising, because the coherence function they used greatly overestimates actual decoherence between non-linear gravity and velocity.

Changes in a coflowing jet structure caused by acoustic forcing

In the present work, the changes in the basic turbulence field of an axisymmetric jet in a coflow when acoustic forcing is applied are investigated. The main objective is to discriminate between changes produced by the acoustic forcing and those produced by the presence of solid particles in a two-phase flow. Power spectra of the axial velocities, u(t), are analyzed to reinforce the idea of the existence of a natural frequency. Time-averaged data are used to characterize the basic flow. This basic flow is compared with the flow altered by the acoustic forcing. By smoothing the phase-averaged rms data (mean statistical curve), the general behavior of the instantaneous fluctuations are unveiled and compared with that of the natural (unforced) jet. In this way, it can be seen that rms values in the forced cases are higher than in the unforced ones, due to the contribution of the external forcing. However, once the coherent structures are extinguished, both forced and unforced jets show a similar trend.

Simultaneous Velocity‐Intensity Spectral and Cross‐Spectral Fitting of Helioseismic Data

Solar oscillation parameters, such as the frequency, are usually estimated by fitting the mode profile in the velocity (V) power spectrum. In this paper, the solar oscillation parameters are derived by simultaneously fitting four observational spectra: V and I (intensity) power, I-V phase difference, and I-V coherence using the model of Severino et al. that contains several background components. We show that this model reproduces the observed spectra for l = 15-50 using Global Oscillation Network Group (GONG) data. A study of the model parameters as a function of frequency shows the well-known behavior of the mode amplitude and width. Comparing the oscillation parameters using the multispectral fitting and the V power spectrum alone with an asymmetric profile shows that the oscillation frequency differs by at most 0.2 μHz below 3 mHz but that the background components are needed to adequately describe the spectrum at higher frequencies. The background amplitudes and phases provide information about the solar oscillation excitation mechanism. For example, the coherent correlated background might be associated with the darkening observed in the intergranular lane at the beginning of the seismic event, while the coherent uncorrelated component might be related to seismic events unable to excite the 5 minute oscillations.

What Causesp‐Mode Asymmetry Reversal?

The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have more power on the low-frequency sides, whereas intensity profiles show the opposite sense of asymmetry. Numerical simulations of the upper convection zone have resonant p-modes with the same asymmetries and asymmetry reversal as the observed modes. The temperature and velocity power spectra at optical depth τcont = 1 have the opposite asymmetry, as is observed for the intensity and velocity spectra. At a fixed geometrical depth, corresponding to = 1, however, the temperature and velocity spectra have the same asymmetry. This indicates that the asymmetry reversal in the simulation is produced by radiative transfer effects and not by correlated noise. The cause of this reversal is the nonlinear amplitude of the displacements in the simulation and the nonlinear dependence of the H- opacity on temperature. Where the temperature is hotter the opacity is larger and photons escape from higher, cooler layers. This reduces the fluctuations in the radiation temperature compared to the gas temperature. The mode asymmetry reversal in the simulation is a small frequency-dependent differential effect within this overall reduction. Because individual solar modes have smaller amplitudes than the simulation modes, this effect will be smaller on the Sun.

How Well Can We Infer the Properties of the Solar Acoustic Sources?

Measurements of the p-mode line asymmetry in the solar oscillation velocity power spectrum have been used on several occasions to infer the properties of the acoustic sources. These inferences are based on the assumption that, unlike the observed intensity signal, the velocity signal does not contain a nonresonant (background) component that is correlated with the p-mode signal. Line asymmetry measurements have also been used to draw inferences on the nature of the correlated background signal that is present in intensity observations. By simultaneously modeling the observed velocity and intensity power spectra and the intensity-velocity cross spectrum, we enforce strict observational constraints on the properties of the fitting model. We find that in order to accurately describe the observed data, we have to include a correlated background component in both our models for the V and I signals at low frequencies. Our results also show that we cannot uniquely determine the acoustic source depth for low-frequency waves or the detailed properties of the correlated background signals. It appears that further physical and/or observational constraints are needed before we can obtain this information.