Absence Of Turbulence Research Articles

Wave period and flow asymmetry effects on the transition to turbulence in relation to sediment dynamics

The presence or absence of turbulence in near‐bed flows is known to affect patterns of sediment transport. Therefore the accurate prediction of turbulence is important when considering the initiation of motion of medium to coarse sands as such grains may be eroded by conditions similar to those causing turbulence. An experimental study is described to investigate boundary layer stability under nonlinear (asymmetric) oscillatory flows. In addition, the results are combined with data collected elsewhere to better explain the dependence of initial transition to turbulence on bed roughness and oscillatory period. An oscillating trolley system simulating such oscillatory flows at prototype scale is described. The transition to turbulence under asymmetric (nonlinear) oscillatory flow is investigated utilizing visual techniques and the high‐resolution measurement of near‐bed velocity. These data are used in conjunction with the extensive data sets of Li [1954] and Manohar [1955] for linear oscillatory flows, collected using similar equipment and methods. Relationships are presented to describe the transition over smooth and fixed (granular) roughness beds under linear and nonlinear flows. The transition to turbulence is shown to have a strong positive linear correlation with both oscillatory period and grain roughness up to a maximum Reynolds number for linear flows. An additional positive nonlinear relationship with asymmetry is observed; this was proportionately greater with increasing bed roughness (from smooth beds to a uniform bed roughness of 550 μm). These relationships suggest that transition is regulated by a balance between stabilizing and destabilizing forces or conditions, namely, rates of fluid acceleration, the time frame for development of turbulence, and sources of the initial flow perturbation. It is argued that (wave) period dependence observed in threshold of motion data may be explained by a combination of (a) period dependency in the transition to turbulence or of (b) translation of the proposed regulatory mechanisms for the transition to turbulence.

A boundary element method for the calculation of noise barrier insertion loss in the presence of atmospheric turbulence

Atmospheric turbulence is an important factor that limits the amount of attenuation a barrier can provide in the outdoor environment. It is therefore important to develop a reliable method to predict its effect on barrier performance. The boundary element method (BEM) has been shown to be a very effective technique for predicting barrier insertion loss in the absence of turbulence. This paper develops a simple and efficient modification of the BEM formulation to predict the insertion loss of a barrier in the presence of atmospheric turbulence. The modification is based on two alternative methods: (1) random realisations of log-amplitude and phase fluctuations of boundary sources and (2) de-correlation of source coherence using the mutual coherence function (MCF). An investigation into the behaviours of these two methods is carried out and simplified forms of the methods developed. Some systematic differences between the predictions from the methods are found. When incorporated into the BEM formulation, the method of random realisations and the method of MCF de-correlation provide predictions that agree well with predictions by the parabolic equation method and by the scattering cross-section method on a variety of thin barrier configurations.

Avoiding ergodicity and turbulence in [formula omitted] vector fields

We show that analytic R 3 vector fields having the property of being transversal to either analytic functions or foliations F 2 , or parallel to a foliation, are free from ergodicity and turbulence. The absence of turbulence and ergodicity via induced vector fields is also proven.

Relaxation of writhe and twist of a bi-helical magnetic field

In the past few years suggestions have emerged that the solar magnetic field might have a bi-helical contribution with oppositely polarized magnetic fields at large and small scales, and that the shedding of such fields may be crucial for the operation of the dynamo. It is shown that, if a bi-helical field is shed into the solar wind, positive and negative contributions of the magnetic helicity spectrum tend to mix and decay. Even in the absence of turbulence, mixing and decay can occur on a time scale faster than the resistive one provided the two signs of magnetic helicity originate from a single tube. In the presence of turbulence, positively and negatively polarized contributions mix rapidly in such a way that the ratio of magnetic helicity to magnetic energy is largest both at the largest scale and in the dissipation range. In absolute units the small scale excess of helical fields is however negligible.

The kinematics of bend-induced mixing in micro-conduits

The absence of turbulence and the difficulty associated with introducing moving components into micro-fluidic systems make the mixing problem in micro-devices challenging. We studied steady, laminar, incompressible flow through a sequence of conduits with rectangular cross-sections aligned to form 90° with each other. The feasibility of taking advantage of bend-induced vortices to stir the fluid and enhance the mixing process was evaluated theoretically and experimentally. Since at very low Reynolds numbers the bend-induced vortices decay rapidly, it was necessary to utilize a large number of bends to achieve the desired effects. Since it is not practical to directly simulate the flow through a large number of bends, we borrowed Jones et al.’s [J. Fluid Mech. 209 (1989) 335–357] idea of constructing a two-dimensional map to project fluid particles from a cross-section upstream of the bend to a cross-section downstream of the bend. This map was then applied repetitively to trace particle trajectories in various bend arrangements. Under certain conditions, chaotic advection was predicted. A prototype of a stirrer was fabricated with low temperature co-fired ceramic tapes.

Multiwavelength observations of the M15 intermediate-velocity cloud

We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images, William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60 and 100 micron coadded images towards the intermediate velocity cloud located in the general direction of the M15 globular cluster. When combined with previously-published Arecibo data, the HI gas in the IVC is found to be clumpy, with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density (assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic temperature, in the absence of turbulence, of 540 K. There are indications in the HI data of 2-component velocity structure in the IVC, indicative of cloudlets. This velocity structure is also tentatively seen in the CaK spectra, although the SNR is low. The main IVC condensation is detected by WHAM in H-alpha with intensities uncorrected for Galactic absorption of upto 1.3 Rayleigh, indicating that the cloud is partially ionised. The FWHM of the ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial and velocity coincidence of the H-alpha and HI peaks in emission is qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial coincidence over a 0.7 degree field, with low and intermediate-velocity gas, respectively, indicating that the IVC may contain dust.

Modified guidance laws to escape microbursts with turbulence

This paper introduces Modified Altitude‐ and Dive‐Guidance laws for escaping a microburst with turbulence. The goal is to develop a procedure to estimate the highest altitude at which an aircraft can fly through a microburst without running into stall. First, a new metric is constructed that quantifies the aircraft upward force capability in a microburst encounter. In the absence of turbulence, the metric is shown to be a decreasing function of altitude. This suggests that descending to a low altitude may improve safety in the sense that the aircraft will have more upward force capability to maintain its altitude. In the presence of stochastic turbulence, the metric is treated as a random variable and its probability distribution function is analytically approximated as a function of altitude. This approximation allows us to determine the highest safe altitude at which the aircraft may descend, hence avoiding to descend too low. This highest safe altitude is used as the commanded altitude in Modified Altitude‐ and Dive‐Guidance. Monte Carlo simulations show that these Modified Altitude‐ and Dive‐Guidance strategies can decrease the probability of minimum altitude being lower than a given value without significantly increasing the probability of crash.

HI observations of the high-velocity cloud in the direction of M 92

We present wide-field neutral hydrogen (H i) Lovell telescope multibeam, and Dominion Radio Astrophysical Observatory H i synthesis observations, of the high velocity cloud (HVC) located in the general direction of the globular cluster M 92. This cloud is part of the larger Complex C and lies at velocities between ~-80 and -130 km s-1 in the Local Standard of Rest. The Lovell telescope observations, of resolution 12 arcmin spatially and 3.0 km s-1 in velocity, fully sampling a 3.112.6° RA-Dec grid, have found that this part of HVC Complex C comprises two main condensations, lying approximately north-south in declination, separated by ~2° and being parallel to the Galactic plane. At this resolution, peak values of the brightness temperature and H i column density of ~1.4 K and ~51019 cm-2 are determined, with relatively high values of the full width half maximum velocity (FWHM) of ~22 km s-1 being observed, equivalent to a gas kinetic temperature, in the absence of turbulence and geometric effects of ~10 000 K. Each of these properties, as well as the sizes of the clouds, are similar in the two components. The DRAO observations, towards the Northern HVC condensation, are the first high-resolution H i spectra of Complex C. When smoothed to a resolution of 3 arcmin, they identify several H i intensity peaks with column densities in the range 4-71019 cm-2. Further smoothing of these data to 6 arcmin resolution tentatively indicates that parts of the HVC consist of two velocity components, of similar brightness temperature, separated by ~7 km s-1 in velocity, and with FWHM velocity widths of ~5-7km s-1. No IRAS 60 or 100 micron flux is associated with the M 92 HVC. Cloud properties are briefly discussed and compared to previous observations of HVCs.

Anisoplanatic imaging through turbulent media: image recovery by local information fusion from a set of short-exposure images.

A wide-field-of-view white-light imaging experiment with artificially generated turbulence layers located between the extended object and the imaging system is described. Relocation of the turbulence sources along the imaging path allowed the creation of controllable anisoplanatic effects. We demonstrate that the recently proposed synthetic imaging technique [J. Opt. Soc. Am. A 16, 1623 (1999)] may result in substantial improvement in image quality for highly anisoplanatic conditions. It is shown that for multisource objects located at different distances the processing of turbulence-degraded short-exposure images may lead to a synthetic image that has an image quality superior to that of the undistorted image obtained in the absence of turbulence (turbulence-induced image quality enhancement).

Performance of time-frequency representation techniques to measure blood flow turbulence with pulsed-wave Doppler ultrasound

The current processing performed by commercial instruments to obtain the time-frequency representation (TFR) of pulsed-wave Doppler signals may not be adequate to characterize turbulent flow motions. The assessment of the intensity of turbulence is of high clinical importance and measuring high-frequency (small-scale) flow motions, using Doppler ultrasound (US), is a difficult problem that has been studied very little. The objective was to optimize the performance of the spectrogram (SPEC), autoregressive modeling (AR), Choi–Williams distribution (CWD), Choi–Williams reduced interference distribution (CW-RID), Bessel distribution (BD), and matching pursuit method (MP) for mean velocity waveform estimation and turbulence detection. The intensity of turbulence was measured from the fluctuations of the Doppler mean velocity obtained from a simulation model under pulsatile flow. The Kolmogorov spectrum, which is used to determine the frequency of the fluctuations and, thus, the scale of the turbulent motions, was also computed for each method. The best set of parameters for each TFR method was determined by minimizing the error of the absolute frequency fluctuations and Kolmogorov spectral bandwidth measured from the simulated and computed Doppler spectra. The results showed that different parameters must be used for each method to minimize the velocity variance of the estimator, to optimize the detection of the turbulent frequency fluctuations, and to estimate the Kolmogorov spectrum. To minimize the variance and to measure the absolute turbulent frequency fluctuations, four methods provided similar results: SPEC (10-ms sine-cosine windows), AR (10-ms rectangular windows, model order = 8), CWD (wN and wM = 10-ms rectangular windows, σ = 0.01), and BD (wN = 10-ms rectangular windows, α = 16). The velocity variance in the absence of turbulence was on the order of 0.04 m/s (coefficient of variation ranging from 8.0% to 14.5%, depending on the method). With these spectral techniques, the peak of the turbulence intensity was adequately estimated (velocity bias < 0.01 m/s). To track the frequency of turbulence, the best method was BD (wN = 2-ms rectangular windows, α = 2). The bias in the estimate of the −10 dB bandwidth of the Kolmogorov spectrum was 354 ± 51 Hz in the absence of turbulence (the true bandwidth should be 0 Hz), and −193 ± 371 Hz with turbulence (the simulated −10-dB bandwidth was estimated at 1256 Hz instead of 1449 Hz). In conclusion, several TFR methods can be used to measure the magnitude of the turbulent fluctuations. To track eddies ranging from large vortex to small turbulent fluctuations (wide Kolmogorov spectrum), the Bessel distribution with appropriate set of parameters is recommended. (E-mail: cloutig@ircm.qc.ca)

The “Kinetic Stabilizer”: A Simpler Tandem Mirror Configuration?

In the search for better approaches to magnetic fusion it is important to keep in mind the lessons learned in the 50 years that fusion plasma confinement has been studied. One of the lessons learned is that “closed” and “open” fusion devices differ fundamentally with respect to an important property of their confinement, as follows: Without known exception closed systems such as the tokamak, the stellarator, or the reversed-field pinch, have been found to have their confinement times limited by non-classical, i.e., turbulence-related, processes, leading to the requirement that such systems must be scaled-up in dimensions to sizes much larger than would be the case in the absence of turbulence. By contrast, from the earliest days of fusion research, it has been demonstrated that open magnetic systems of the mirror variety can achieve confinement times close to that associated with classical, i.e., collisional, processes. While these good results have been obtained in both axially symmetric fields and in non-axisymmetric fields, the clearest cases have been those in which the confining fields are solenoidal and axially symmetric. These observations, i.e., of confinement not enhanced by turbulence, can be traced theoretically to such factors as the absence of parallel currents in the plasma, and to the constraints on particle drifts imposed by the adiabatic invariants governing particle confinement in axisymmetric open systems. In the past the MHD instability of axially symmetric open systems has been seen as a barrier to their use. However, theory predicts MHD-stable confinement is achievable if sufficient plasma is present in the “good curvature” regions outside the mirrors. This theory has been confirmed by experiments on the Gas Dynamic Trap mirror-based experiment at Novosibirsk, In this paper a new way of exploiting this stabilizing principle, involving creating a localized “stabilizer plasma” outside a mirror, will be discussed. To create this plasma ion beams are injected along the field lines in such a way as to be reflected before they reach the mirrors, thus forming a localized peak in the plasma density. It will be shown that the power required to produce these stabilizing plasmas is much less than the power per meter of fusion power systems that might employ this technique. Use of the Kinetic Stabilizer idea may therefore permit the construction of tandem mirror fusion power systems that are much smaller and simpler than those based on the use of non-axisymmetric fields to achieve MHD stability.

Numerical simulation of the effect of refractive turbulence on coherent lidar return statistics in the atmosphere

We propose an algorithm and the results of a numerical study of random realizations and statistics of a pulsed coherent lidar return that allow for refractive turbulence. We show that, under conditions of refractive turbulence, the relative variance of the lidar return power can exceed unity by a factor of as much as 1.5. Clear manifestations of the turbulent effect of backscattering amplification have been revealed from simulations of space-based lidar sensing of the atmosphere with coherent lidar. Under conditions of strong optical turbulence in the atmospheric boundary layer, as a result of the backscattering amplification effect, the mean lidar return power can exceed the return power in the absence of turbulence by a factor of 3.

Constraints on Stellar Hydrodynamics from Abundance Anomalies of LiBeB and Metals

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the atomic diffusion of all important contributors to opacity. The radiative accelerations and the opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self-consistent stellar evolution models for A and F stars. In A and F stars an iron-peak convection zone appears.The calculated abundance anomalies are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. To reduce the calculated anomalies to the observed ones, an additional source of turbulence (or some other hydrodynamical process) must be introduced. The mixed zone must extend about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out.The LiBeB abundances observed in clusters give additional information. The abundances of the 28 species offer considerable constraints on the models. Various potential turbulence models have been introduced in a stellar evolution code and results of evolutionary calculations for Li gap stars are discussed in the light of the constraints offered by the abundances of LiBeB and metals. The radiative accelerations of LiBeB have also been recalculated taking the effect of changing metal abundances into account. This modifies the expected Li gap in the absence of turbulence.

Nonlinear scattering of crossed ultrasonic beams in the presence of turbulence: Multiple scattering effects

The nonlinear scattering of two finite‐amplitude mutually perpendicular crossed ultrasonic beams—interacting in the presence of turbulence—generates a scattered sum frequency component that radiates outside the interaction region. In the absence of turbulence, virtually no scattering, at the sum frequency, is observed [M. S. Korman and R. T. Beyer, J. Acoust. Soc. Am. 84, 339–349 (1988); 85, 611–620 (1989)]. Here, two primary cw (f1=2.05 MHz and f2=1.95 MHz) beams are generated by 2.54‐cm‐diam circular plane array piston transducer units (T1 and T2). A 4‐MHz receiving unit (R) is of similar construction. All beam axes form a common plane and overlap region with the axis of a submerged circular water jet, which generates the turbulence. With R fixed, T1 and T2 rotate on radius arms—always keeping the beams perpendicular. Symmetry suggests an angle θ*, where θ*=0° defines forward scattering. This geometry allows a nonlinear forward scattering intensity component to exist without concern for a coherent component. Here, multiple scattering effects are needed to predict their results. Supplemental scattering experiments will be presented in an attempt to identify a transition from single (Born approximation) to multiple scattering. [Work supported by Naval Academy Research Council.]

Dynamics of heat and mass transfer through a water-air interface based on measurements of thermal radio emission at a frequency of 60 GHz

We study the dynamics of heat and mass transfer through a water-air interface on the basis of laboratory radiometric measurements of the time dependence of thermal radio emission of water at a frequency of 60 GHz, which is related to air turbulization above its surface by a fan. We recover the dynamics for the temperature profile in water and in a viscous sublayer of air as well as for the heat flux through the water-air interface. The flux components related to evaporation and heat exchange and the rate of evaporation from a unit surface are determined. An equation for determining the thickness of the viscous sublayer in the air from the heat flux value is obtained;this thickness is about 2 mm under the experimental conditions. The process dependence on the water turbulence is established and methods for determining the thermal film thickness are proposed. In the absence of turbulence in unstable stratified water we observed the development of a periodic convective process from the time the critical value of the Rayleigh number was reached. The heat exchange dependence on the presence of petroleum film on the water surface is studied. It is found that for a film thickness of 5 μm the heat exchange rate decreases by a factor of 3 because of the petroleum presence.

Numerical simulation of the dynamics of sea spray over the waves

SeaCluse is a code describing and quantifying the nonlinear interactions between spray droplet concentrations and the scalar fields of water vapor concentration and temperature in the marine atmospheric surface layer as a function of the basic micrometeorological parameters. It is currently developed to simulate most of the dynamics of the evaporating sea spray droplets, their transformations, and their influence on the structure of the marine lower atmosphere. It includes two parts, a “preprocessor” computing the air flow structure and droplet trajectories over the waves in the absence of turbulence and evaporation, and a “main program” computing along the vertical the horizontally averaged budgets of droplet and water vapor concentrations, and sensible heat, including the dynamic and thermodynamic air‐droplet interactions. This paper presents the first simulations, without evaporation, which reveal several characteristic features of the spray droplet dynamics over the waves. The mean air flow induced by the wave motion generates an efficient transport of many droplets up to the wave crest level or slightly higher, where they accumulate. In contrast, turbulence appears to be a rather inefficient process to elevate further the droplets that have nonnegligible weight and inertia (and an efficient process for very small droplets). The residence times in the air Tfly of droplets larger than 90 μm in radius do not increase much, compared to their values in still air or over a flat surface, and they increase with increasing wind speed much less rapidly than the wave height Ho: Tfly ∼ U0.3 while Ho ∼ U2. Above the wave crest, the droplet concentrations are several orders of magnitude smaller than between wave troughs and crests. The precision of the turbulent diffusivity model for droplets therefore appears crucial to correctly infer the spray surface source function from concentration measurements in the atmosphere.

Optimum truncation of a Gaussian beam for propagation through atmospheric turbulence

The mean on-axis far-field (or focal-plane) irradiance of a Gaussian beam that is truncated by a circular aperture in the presence of atmospheric turbulence is considered. In the absence of turbulence, an accurate analytic approximation for the irradiance distribution that is valid within the main central lobe of the beam is presented. Based on this approximation, the mean on-axis far-field irradiance and the corresponding turbulence Strehl ratio for the truncated Gaussian beam are then obtained. By maximization of the on-axis irradiance, the optimum ratio of the beam diameter to the aperture diameter in the presence of turbulence is obtained, and the results for the corresponding maximum on-axis irradiance as a function of the strength of turbulence are presented. In particular, for D/r(0) > 1, where D is the aperture diameter and r(0) is Fried's coherence length, optimum truncation of a Gaussian beam and uniform illumination of a circular aperture (where the same total power isuniformly distributed over the aperture) result in the same on-axis irradiance in the presence of uncompensated turbulence.

On radar interferometric techniques in the situation of volume scatter

A volume scatter situation is assumed, and results expected from radar interferometric techniques are derived. It is shown that fictitious scattering points will appear, with the property that the points for different Doppler frequencies lie along a straight line. This line is not generally in the direction of the wind; only if the scatter is isotropic in azimuth will the line coincide with the wind direction. In the presence of turbulence the effective scattering points still lie along a straight line. However, the scattering point for a particular Doppler frequency will be closer to the zenith than it would be in the absence of turbulence. The results apply if smoothed cross spectra are used. If the cross spectra are not smoothed, the expected effects are harder to predict.

Thermal convection in a nonequilibrium turbulent medium with rotation

The equations of thermal convection in a rotating plane horizontal layer of nonequilibrium turbulent fluid are obtained, the system of equations is linearized and the boundary value problem is formulated. Some general properties of the perturbation spectrum are found and a solution, which includes the classical solution in the absence of turbulence as a limiting case, is obtained.

Effective Ampère force in developed magnetohydrodynamic turbulence.

The interaction between a large-scale uniform weak magnetic field B and a developed small-scale magnetohydrodynamic (MHD) turbulence is studied. It is found that the effective mean Amp\`ere force in the turbulence is given by ${\mathbf{F}}_{\mathit{m}}$=-\ensuremath{\nabla}(${\mathit{Q}}_{\mathit{p}}$${\mathbf{B}}^{2}$/8\ensuremath{\pi})+(B\ensuremath{\cdot}\ensuremath{\nabla})${\mathit{Q}}_{\mathit{s}}$B/4\ensuremath{\pi}. The turbulent magnetic coefficients ${\mathit{Q}}_{\mathit{p}}$ and ${\mathit{Q}}_{\mathit{s}}$ are drastically decreased at large magnetic Reynolds numbers, whereas in the absence of turbulence ${\mathit{Q}}_{\mathit{p}}$=${\mathit{Q}}_{\mathit{s}}$=1. This phenomenon arises due to a negative contribution of the MHD turbulence to the mean magnetic force. This is caused by the generation of magnetic fluctuations at the expense of fluctuations of the velocity field. This effect is nonlinear in terms of the large-scale magnetic field. It is shown here that in turbulence with a mean large-scale magnetic field, a universal ${\mathit{k}}^{\mathrm{\ensuremath{-}}1}$ spectrum of magnetic fluctuations exists; this spectrum is independent of the exponent of the spectrum of the turbulent velocity field. A variant of the renormalization group (RNG) method allows the derivation of the scaling and amplitude of the turbulent transport coefficients: turbulent viscosity, turbulent magnetic diffusion, and turbulent magnetic coefficients. A small parameter in the RNG method is the ratio \ensuremath{\varepsilon}=${\mathit{B}}^{2}$/(8\ensuremath{\pi}${\mathit{W}}_{\mathit{k}}$) of the large-scale magnetic energy density to the energy density ${\mathit{W}}_{\mathit{k}}$=(1/2)\ensuremath{\rho}〈${\mathit{u}}^{2}$〉 of the turbulent velocity field.