Arterial stenosis murmurs: an analysis of flow and pressure fields

The flow field distal to an arterial stenosis is simulated by a confined turbulent jet with moderate Reynolds numbers. The wall pressure fluctuations are related to the momentum fluctuations of the jet by the Poisson equation. A Green's function was derived to satisfy the boundary conditions on a cylindrical surface. This allows the solution of the Poisson's equation to include only a volume integral of the fluctuating momentum, weighed by the relative distance between the source and the sensor. The velocity fluctuations on the jet centerline and at the middle of the shear layer were measured using a laser Doppler anemometer. The wall pressure fluctuations were detected simultaneously by an array of nine wall-mounted pressure transducers along the axial direction. Cross correlation performed between the velocity and pressure fluctuations reveals that the pressure fluctuations were mostly imposed by the passage of turbulent eddies with a convective velocity that is a function of the jet exit velocity. The Strouhal number, defined by the frequency of the passing large-scale structure, is a function of the initial conditions only very close to the jet exit. Further downstream, where the effect of the initial conditions is lost, the Strouhal number approaches a constant irrespect of the jet Reynolds number. The contribution of a source near the jet exit to wall pressure fluctuation near the reattachment is rather weak due to the rapidly decaying weighting function in the axial direction. However, for sources located within one nozzle diameter from the sensor, the cross-spectral density function has a high magnitude with maximum coherence where the pressure spectral changes its slope.     

Equations for describing a family of self-similar solutions of the far field of a round submerged jet

The flow of a turbulent round submerged jet in far field has been considered. The modified Navier?Stokes equations have been derived with the aid of known self-similar properties of the time-mean flow as well as the spatial and temporal characteristics of the entire spectrum of the scales of turbulent fluctuations. A technique has been proposed for the numerical computation of these equations to describe a family of self-similar solutions in a cylindrical region with periodic boundary conditions in the longitudinal direction, and the velocity of the jet expansion enters the obtained equations explicitly as a parameter.     

Characterization of fluctuating hydroxyl concentrations in?a?turbulent nonpremixed hydrogen–nitrogen jet flame

Two-point OH time-series measurements using a high-speed, laser-induced fluorescence system have been performed in a turbulent nonpremixed jet flame to obtain both radial and axial space–time correlations. Turbulent OH structures in such flames are found to undergo convection both axially and radially, but OH convection does not satisfy the ‘frozen-turbulence’ hypothesis owing to various turbulent interactions and chemical reactions. While axial OH convection occurs at approximately the local mean bulk velocity, radial convection is largely compromised by strong turbulent mixing along the same direction. The hydroxyl integral length scale can be interpreted as the typical dimension of a convective OH structure, which is axially elongated and becomes more isotropic in the post-flame region. The hydroxyl integral time scale can be interpreted as approximately the ratio of an axial integral length scale to a corresponding local mean flow velocity. In general, macroscale fluctuations of OH are dominated by large-scale turbulence, with little contribution from small-scale turbulence and OH chemistry.     

Laser velocimeter correlation measurements in subsonic and supersonic jets

This paper describes laser velocimeter cross-correlation measurements conducted in the flow field of a jet. The jet conditions were varied over a range of Mach numbers from 0·5 to 1·37, and in the case of the Mach 0·9 jet, tests were also run at an absolute exit temperature equal to 2·32 that of the ambient. From the families of cross-correlation curves, the integral length scales, the convection velocity and the integral time scale in the moving frame were determined. It was found that the axial and radial integral length scales changed almost linearly with the axial distance from the nozzle, at rates which were in agreement with Jones' [1]. The convection velocity and integral time scale also agreed with values previously obtained with hot wire anemometers. The effects of jet exit conditions on the variation of the length scales were not noticeable. However, in the case of the convection velocity and time scale, there was some dependence on the jet conditions.     

Effect of the Berendsen thermostat on the dynamical properties of water

The effect of the Berendsen thermostat on the dynamical properties of bulk SPC/E water is tested by generating power spectra associated with fluctuations in various observables. The Berendsen thermostat is found to be very effective in preserving temporal correlations in fluctuations of tagged particle quantities over a very wide range of frequencies. Even correlations in fluctuations of global properties, such as the total potential energy, are well preserved for time periods shorter than the thermostat time constant. Deviations in dynamical behaviour from the microcanonical limit do not, however, always decrease smoothly with increasing values of the thermostat time constant, but may be somewhat larger for some intermediate values of τ_B, especially in the supercooled regime, which are similar to time scales for slow relaxation processes in bulk water.     

Observation of laser light scattering by a turbulent gas without seeding

An experiment of laser light scattering by a turbulent highly subsonic free jet of air is presented. The originality of the method lies in the fact that no particle seeding is needed. The experimental scattering cross section, which can be interpreted from the three-dimensional spectral density of temperature fluctuations, is compared to the theoretical one obtained by turbulent mixing theories. The scattered field time correlations measured in the heterodyne mode are interpreted by considering the Doppler effect on moving turbulent eddies and compared with theory. The intensity time correlations measured in the homodyne mode are interpreted in terms of local properties of the turbulence.     

Scale-invariant structure of energy fluctuations in real earthquakes

Earthquakes are obviously complex phenomena associated with complicated spatiotemporal correlations, and they are generally characterized by two power laws: the Gutenberg-Richter (GR) and the Omori-Utsu laws. However, an important challenge has been to explain two apparently contrasting features: the GR and Omori-Utsu laws are scale-invariant and unaffected by energy or time scales, whereas earthquakes occasionally exhibit a characteristic energy or time scale, such as with asperity events. In this paper, three high-quality datasets on earthquakes were used to calculate the earthquake energy fluctuations at various spatiotemporal scales, and the results reveal the correlations between seismic events regardless of their critical or characteristic features. The probability density functions (PDFs) of the fluctuations exhibit evidence of another scaling that behaves as a q-Gaussian rather than random process. The scaling behaviors are observed for scales spanning three orders of magnitude. Considering the spatial heterogeneities in a real earthquake fault, we propose an inhomogeneous Olami-Feder-Christensen (OFC) model to describe the statistical properties of real earthquakes. The numerical simulations show that the inhomogeneous OFC model shares the same statistical properties with real earthquakes.     

Flow Characteristics of Flapping Motion of a Plane Water Jet Impinging onto Free Surface

A submerged turbulent plane jet in shallow water impinging vertically onto the free surface will produce a large-scale flapping motion when the jet exit velocity is larger than a critical one. The flapping phenomenon is verified in this paper through a large eddy simulation where the free surface is modeled by volume of fluid approach. The quantitative results for flapping jet are found to be in good agreement with available experimental data in terms of mean velocity, flapping-induced velocity and turbulence intensity. Results show that the flapping motion is a new flow pattern with characteristic flapping frequency for submerged turbulent plane jets, the mean centerline velocity decay is considerably faster than that of the stable impinging jet without flapping motion, and the flapping-induced velocities are as important as the turbulent fluctuations.     

Eulerian spatial and temporal autocorrelations: assessment of Taylor's hypothesis and a model

We present measurements of Eulerian longitudinal velocity autocorrelations in homogeneous, isotropic, high-intensity (～9%) free-stream turbulence behind an active grid. Spatial correlations are measured using particle image velocimetry as well as with two-point hot-wire anemometry (HWA), while temporal correlations are measured using HWA. The temporal correlations are transformed into spatial correlations by using Taylor's ‘frozen’ hypothesis with both the mean as well as instantaneous velocities. A model relating Eulerian spatial and temporal autocorrelations is also used for this purpose. The differences from the measured spatial correlation resulting from the use of Taylor's hypothesis on the temporal correlation is quantified; even at this moderately high level of turbulent intensity, the result from the use of the instantaneous velocity as convection velocity is practically indistinguishable from that obtained using the mean velocity. Use of the model produces a good agreement between the estimates of the spatial correlation function. A relation between Eulerian spatial and temporal integral scales is also derived.     

出口雷诺数对平面射流自保持性的影响

通过实验研究出口雷诺数对平面湍流射流自保持性的影响. 测量的射流来自相同的喷嘴但不同的雷诺数Re（≡U_jh/ν,其中U_j是出口平均速度、h是窄缝出口的厚度和ν是黏性系数）,其变化范围是Re=4582—57735.所得的数据包括沿轴线的平均速度、湍流强度、积分尺度、高阶矩和能谱. 实验发现,随着Re的增大,平面射流发展减慢,平均速度和湍流强 关键词： 平面射流 雷诺数 自保持性     

薄膜自激拍打射流的流动特性实验

在射流喷嘴出口处安装一端固定的柔性薄膜,在流速足够大时,射流和柔性膜相互诱导产生自激拍打作用,针对这一现象,提出了一种新型的自激拍打射流混合技术。在直径D=40 mm的渐缩喷嘴上固定长度L=(0.5～2)D和厚度δ=50μm的氟化乙烯丙烯共聚物(fluorinated ethylene propylene, FEP)薄膜,使用压差计测量了光滑渐缩喷嘴以及薄膜拍打运动所引起的压力损失。通过激光片光源和高速相机进行薄膜运动状态的显示和拍打幅度的测定,探究了拍打幅度受薄膜长度和Re的影响及其变化规律,利用测得的拍打幅度(A)和频率(f)作为Strouhal数(St=fA/U_o,U_o是射流的出口速度)的特征尺寸进行研究。在Re=3×10⁴的条件下,使用热线风速仪测量了在不同薄膜长度下拍打射流轴向速度沿中心线的分布,并对湍流度、概率密度函数等特征进行分析。此外,经过数字迭代滤波后获得射流沿中心线的积分尺度、 Taylor尺度和Kolmogorov尺度等统计量。实验结果显示,拍打射流的湍流度高于自由射流,意味着前者对周围流体具有更强...     

A comparison of experiment and theory for sound propagation in variable area ducts

An experimental and analytical program has been carried out to evaluate sound suppression techniques in ducts that produce refraction effects due to axial velocity gradients. The analytical program employs a computer code based on the method of multiple scales to calculate the influence of axial variations due to slow changes in the cross-sectional area as well as transverse gradients due to the wall boundary layers. Detailed comparisons between the analytical predictions and the experimental measurements have been made. The circumferential variations of pressure amplitudes and phases at several axial positions have been examined in straight and variable area ducts, with hard walls and lined sections, and with and without a mean flow. Reasonable agreement between the theoretical and experimental results has been found.     

Angular intermittency in QCD jets

Using two methods, via fluctuations and correlations, an analytical formula is derived for the factorial multiplicity moments in a QCD jet at the Double Leading Logarithm accuracy. The resulting self-similar dependence on the solid-angle cell size is characteristic of an intermittency behaviour in angular variables. The intermittency indices depend on the diffusion angle through the running of α_S. Physical features of jet fluctuations such as collimation at large angles and saturation at small angles are well described in the perturbative framework. A parameter-free prediction of angular intermittency is proposed for Z⁰ decays into hadrons, assuming hadronparton duality.

     

Computational study of mixing behaviour of a turbulent jet issuing in a uniform counterflow at low velocity ratios

This paper proposes a computational study for the analysis of the velocity and the scalar concentration field of a round turbulent jet flowing into a uniform stream in opposite direction. The investigation is carried out for a range of low jet-to-counterflow velocity ratios; R = 1.3, 1.6, 2.2, 3.1 and 3.4. The Reynolds stress model is applied in numerical simulation to compare obtained results with experimental data from the literature. It is found that predicted results are in good agreement with the experimental data and that the jet fluid decays faster in the presence of a counterflow. The linearity between the penetration distance and the velocity ratio is verified and the axial fluctuating velocities along jet centreline appear to have two distinct peaks, except for the stronger counterflow. The enhanced mixing efficiency of the counterflowing jet is verified through the radial distribution of velocity and scalar concentration at different streamwise stations.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

Effect of Nozzle Shape on Amplitude of Well Acoustic Emitter Generation

An experimental study of the generation of pressure fluctuations in a well acoustic emitter with nozzles of various shapes has been carried out. The effect of a smooth nozzle inlet section on the generation amplitude, the optimum jet length, and the outlet diameter has been studied. The formation of a region of reverse currents connecting to the cavity in the nozzle between the channel wall and the jet periphery has been considered. A significant increase in the generation amplitude produced at the smooth nozzle inlet and the formation of a uniform velocity profile in the nozzle channel have been observed.     

Third-order statistics and the dynamics of strongly anisotropic turbulent flows

Anisotropy is induced by body forces and/or mean large-scale gradients in turbulent flows. For flows without energy production, the dynamics of second-order velocity or second-order vorticity statistics are essentially governed by triple correlations, which are at the origin of the anisotropy that penetrates towards the inertial range, deeply altering the cascade and the eventual dissipation process, with a series of consequences on the evolution of homogeneous turbulence statistics: in the case of rotating turbulence, the anisotropic spectral transfer slaves the multiscale anisotropic energy distribution; nonlinear dynamics are responsible for the linear growth in terms of Ωt of axial integral length-scales; third-order structure functions, derived from velocity triple correlations, exhibit a significant departure from the 4/5 Kolmogorov law. We describe all these implications in detail, starting from the dynamical equations of velocity statistics in Fourier space, which yield third-order correlations at three points (triads) and allow the explicit removal of pressure fluctuations. We first extend the formalism to anisotropic rotating turbulence with ‘production’, in the presence of mean velocity gradients in the rotating frame. Second, we compare the spectral approach at three points to the two-point approach directly performed in physical space, in which we consider the transport of the scalar second-order structure function ?(δq)²?. This calls into play componental third-order correlations ?(δq)²δu?(r) in axisymmetric turbulence. This permits to discuss inhomogeneous anisotropic effects from spatial decay, shear, or production, as in the central region of a rotating round jet. We show that the above-mentioned important statistical quantities can be estimated from experimental planar particle image velocimetry, and that explicit passage relations systematically exist between one- and two-point statistics in physical and spectral space for second-order tensors, but also sometimes for third-order tensors that are involved in the dynamics.     

Monofractal and multifractal analysis of simulated heat release fluctuations in a spark ignition heat engine

We study data from cycle-by-cycle variations in heat release for a simulated spark-ignited engine. Our analyses are based on nonlinear scaling properties of heat release fluctuations obtained from a turbulent combustion model. We apply monofractal and multifractal methods to characterize the fluctuations for several fuel-air ratio values, ?, from lean mixtures to stoichiometric situations. The monofractal approach reveals that, for lean and stoichiometric conditions, the fluctuations are characterized by the presence of weak anticorrelations, whereas for intermediate mixtures we observe complex dynamics characterized by a crossover in the scaling exponents: for short scales, the variations display positive correlations while for large scales the fluctuations are close to white noise. Moreover, a broad multifractal spectrum is observed for intermediate fuel ratio values, while for low and high ? the fluctuations lead to a narrow spectrum. Finally, we explore the origin of correlations by using the surrogate data method to compare the findings of multifractality and scaling exponents between original simulated and randomized data.     

Capillarylike fluctuations at the interface of falling granular jets

We study the interface fluctuations of a granular jet falling under gravity and show that for small scales they are the analog of the thermally induced capillary waves. Experimental results from radial height and velocity fluctuations, static correlation functions and capillary ripple velocities allow us to estimate a granular surface tension. The ultralow interfacial tensions measured (of the order of 100 microN/m) can be rationalized using a simple model.     

Long-range strain correlations in sheared colloidal glasses

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements, and we identify long-range correlations in the fluctuations of microscopic strain and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.