Radial Velocity Fluctuations Research Articles

TURBULENCE IN A THREE-DIMENSIONAL DEFLAGRATION MODEL FOR TYPE Ia SUPERNOVAE. I. SCALING PROPERTIES

We analyze the statistical properties of the turbulent velocity field in the deflagration model for Type Ia supernovae. In particular, we consider the question of whether turbulence is isotropic and consistent with the Kolmogorov theory at small length scales. Using numerical data from a high-resolution simulation of a thermonuclear supernova explosion, spectra of the turbulence energy and velocity structure functions are computed. We show that the turbulent velocity field is isotropic at small length scales and follows a scaling law that is consistent with the Kolmogorov theory until most of the nuclear fuel is burned. At length scales greater than a certain characteristic scale, turbulence becomes anisotropic. Here, the radial velocity fluctuations follow the scaling law of the Rayleigh-Taylor instability, whereas the angular component still obeys Kolmogorov scaling. In the late phase of the explosion, this characteristic scale drops below the numerical resolution of the simulation. The analysis confirms that a subgrid-scale model for the unresolved turbulence energy is required for the consistent calculation of the flame speed in deflagration models of Type Ia supernovae, and that the assumption of isotropy on these scales is appropriate.

0307 回転同軸二重円筒におけるポリマー一様溶液抵抗低減流れのLDV解析(OS3-1 流れの抵抗低減,オーガナイズドセッション)

Two-component LDV measurement of the turbulent statistics in the drag reducing flow of polymer homogeneous solution between the concentric cylinders was performed. Measurement has been conducted with changing rotating number of inner cylinder and concentration of the polymer solution. The drag reduction rate was achieved about 37 % in our experiment. As a result of LDV measurement, in the drag reducing flow of polymer solution, the mean azimuthal velocity near the wall of inner cylinder is higher than that of the pure water flow. The rms of azimuthal velocity fluctuation of polymer solution in the near-wall region is larger than that of the pure water, while the rms of radial velocity fluctuation of polymer solution is lower than that of the pure water. The Reynolds shear stress of the polymer solution near the wall of the inner cylinder is lower than that of the pure water.

Visualization of aerodynamic noise source around a rotating fan blade

The purpose of this study is to understand the aerodynamic noise source distribution around a rotating fan blade by measuring the noise signal and velocity field around the blade. The local noise-level distribution over the fan blade is measured by microphone arrays, and the flow field is visualized by smoke and phase-averaged PIV measurement. The noise source distribution is examined by cross-correlation analysis between noise signal and velocity fluctuation. It is found that the noise source is located near the rotating fan blade, especially around leading and trailing edges. The separation and reattachment of flow are observed near the leading edge, and the tip vortices and vortex shedding are found near the trailing edge. The cross-correlation distribution of the noise signal and the radial velocity fluctuation shows large magnitude in the correlated regions, which indicates the noise generation by the formation of vortex structure around the blade.

Correlation length of large‐scale solar wind velocity fluctuations measured tangent to the Earth's orbit: First results from Stereo

Proton velocity data from identical plasma instruments on board NASA's two Stereo spacecraft are used to investigate the correlation length of large‐scale radial velocity fluctuations in the solar wind in the direction tangent to the Earth's orbit. The data cover the period from February 2007 through August 2007, near solar minimum in 2007. Estimates of the spatial correlation function are based on simultaneous 10 min averages of the solar wind speed at the two spacecraft that are binned according to the distance between the spacecraft. The results for the correlation function are noisy due, in part, to the modest number of data points in each bin. Linear least squares fits are used to extract the correlation length from noisy data. It is found that the transverse correlation length (along the Y‐direction) is 0.25 ± 0.02 AU which is approximately equal to the longitudinal correlation length (along the X‐direction) 0.28 ± 0.05 AU measured using the same data. Assuming that the autocorrelation time of the velocity signal at one spacecraft is determined by the rate of rigid rotation of solar wind source structures past the Earth‐Sun line, the observed equivalence of longitudinal and transverse integral scales can be understood as a rotation effect. It must be emphasized that the correlation length studied here pertains to large‐scale solar wind fluctuations associated with the large‐scale flow, not to the smaller‐scale fluctuations associated with solar wind turbulence.

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.

On an approach for improving the range resolution of pulsed coherent Doppler lidars

As a result of an analysis of the autocovariance of the complex heterodyne lidar signal, some general-enough inverse techniques (algorithms) are derived for recovering with high range resolution, below the sensing pulse length, of Doppler-velocity profiles in the atmosphere. Unlike our preceding works, it is assumed here that the laser pulses can have arbitrary fluctuating shape. The presence is also supposed of possible regular, arbitrary in form, intrapulse frequency deviations (chirp) and random frequency, phase and radial (Doppler)-velocity fluctuations. The algorithm performance and efficiency are studied and illustrated by computer simulations, taking into account the influence of the chirp and various random factors such as additive noise, pulse-shape fluctuations and radial-velocity fluctuations. It is shown that the algorithms developed allow the Doppler-velocity profiles to be determined with a considerably shorter resolution interval compared with the pulse length, at a reasonable number of signal realizations (laser shots) and appropriate data processing to reduce the statistical error due to the random factors.

Velocity measurements in confined swirling flow of polymer solutions with vortex shedding

A particle image velocimetry system is used to examine the velocity field in the unsteady swirling flow of polyacrylamide (PAA) solutions (PAA 0.5wt.% and 1.0wt.%) with the vortex shedding due to a rotating disk in a cylindrical casing. In our earlier work [Tamano et al., Phys. Fluids 19, 023103 (2007)], the vortex shedding is investigated by the flow visualization technique. In this work, we investigate the velocity field for better understanding of the vortex shedding mechanism and the nonaxisymmetric ring vortex which was observed for the higher Reynolds number compared to that of the axisymmetric ring vortex. It was found that the fluid within the ring vortex formed near the rotating disk rotates with semirigid rotation, where the angular velocity of the ring vortex was about 0.9 times that of the rotating disk. A high shear layer existed at the boundary between the ring vortex and the outer large-scale secondary flow. When the ring vortex was shed, the surrounding large-scale secondary flow rushed into near the rotating axis, so that strong axial flow was observed near the rotating axis. For the nonaxisymmetric ring vortex, azimuthal and radial velocities oscillate periodically with considerably large amplitude. In addition, the joint probability density function showed that the correlation between azimuthal and radial velocity fluctuations varied with the radial locations.

Vortex Shedding in Confined Swirling Flow of Polymer Solutions

A ring vortex shedding phenomenon observed in the confined swirling flows of polyacrylamide (PAA) solutions whose concentrations were 0.5wt% and 1.0wt% was investigated using particle image velocimetry (PIV) system. It was found that the fluid within the ring vortex formed near the rotating disc rorates with semi-rigid rotation, where the angular velocity of ring votex was about 0.9times that of the rotating disc. The high shear layer was found to exist at the boundary between the ring vortex and the outer large-scale secondary flow. After the ring vortex was shed, the surrounding large-scale secondary flow moved into near the rotating axis, so that the strong axial flow was observed near the rotating axis. Periodic azimuthal and radial velocity fluctuations with considerably large amplitude, in which their correlation varied with the radial locations, were seen for the nonaxisymmetric ring vortex which was observed for the higher Reynolds number compared to that of the axisymmetric ring vortex.

A Study of the Effect of Polymer Solution in Promoting Friction Reduction in Turbulent Channel Flow

Abstract In this work, turbulent drag reduction in a pipe is investigated by using laser Doppler velocimetry. The effect of decreasing the friction factor of the flow is obtained by addition of high molecular weight polymers. The mechanism of drag reduction is explained in terms of a stress anisotropy that inhibits the transversal transport of momentum by turbulent fluctuations. Semi-theoretical models based on a nonlinear constitutive equation, which takes into account an extra extensional rate of strain in the flow produced by the local additive orientation, are presented. The semi-theoretical models used to predict the friction factor of the flow in the presence of the polymer have successfully described the experimental measurements. The results have revealed a reduction in the friction factor of 65% for a concentration of 350ppm in volume of polyacrylamide (PAMA) in an aqueous solution. In addition, the flow statistics, such as the axial and radial velocity fluctuations, the normalized autocorrelation functions as well as the power spectra for both velocity fluctuation components, are examined for the Newtonian flow of pure water and the flow of a 120ppm solution of PAMA at the same friction velocity. Next, the results are compared in order to characterize the effect of the additive on the turbulent flow.

Direct Numerical Simulation of Turbulent Flow Around a Rotating Circular Cylinder

Abstract Turbulent flow around a rotating circular cylinder has numerous applications including wall shear stress and mass-transfer measurement related to the corrosion studies. It is also of interest in the context of flow over convex surfaces where standard turbulence models perform poorly. The main purpose of this paper is to elucidate the basic turbulence mechanism around a rotating cylinder at low Reynolds numbers to provide a better understanding of flow fundamentals. Direct numerical simulation (DNS) has been performed in a reference frame rotating at constant angular velocity with the cylinder. The governing equations are discretized by using a finite-volume method. As for fully developed channel, pipe, and boundary layer flows, a laminar sublayer, buffer layer, and logarithmic outer region were observed. The level of mean velocity is lower in the buffer and outer regions but the logarithmic region still has a slope equal to the inverse of the von Karman constant. Instantaneous flow visualization revealed that the turbulence length scale typically decreases as the Reynolds number increases. Wavelet analysis provided some insight into the dependence of structural characteristics on wave number. The budget of the turbulent kinetic energy was computed and found to be similar to that in plane channel flow as well as in pipe and zero pressure gradient boundary layer flows. Coriolis effects show as an equivalent production for the azimuthal and radial velocity fluctuations leading to their ratio being lowered relative to similar nonrotating boundary layer flows.

Force Balance Controlling Radial Gas Holdup Distribution for the Recirculating Turbulent Flow in Bubble Columns

Radial distribution of gas holdup for recirculating turbulent flow in a bubble column is theoretically studied on the basis of time-averaged Navier-Stokes equations for both gas and liquid phases. The basic equations are simplified for the recirculating turbulent flow, and the radial distribution of gas holdup is successfully shown to be parabolic, which is experimentally well-known. On the way of derivation, it is shown that the radial velocity fluctuation of the liquid phase was mainly caused by two adjacent bubbles, and that the radial drug force acting on a bubble is proportional to the Reynolds stress of the gas–liquid multiphase.

Direct measurement of entrainment during nanoparticle synthesis in spray flames

Using phase Doppler anemometry the scale of turbulence and the spray dilution by air entrainment are investigated during nanoparticle synthesis by flame spray pyrolysis (FSP). Ethanol and a solution of 0.5 M zirconium propoxide in ethanol are dispersed and combusted using an external-mixing, gas-assisted atomizer resulting in product ZrO 2 particles of about 11 nm in diameter as determined by nitrogen adsorption at a production rate of 100 g/h. Solution droplet size distributions of the solid cone spray are measured and related to standard correlations of spray atomization. Air entrainment and the radial spread of the expanding jet are determined from the gas velocities in horizontal planes across the spray cone at different heights above the nozzle. The isotropy of the turbulence is investigated using measured axial and radial velocity fluctuations. The turbulent flow is characterized by the integral time and Kolmogorov scales as well as the average shear rates acting on droplets and particles. The flow structure of these spray flames is of major importance as it determines product particle size, polydispersity, morphology, and crystallinity.

Unsteady Aerodynamics of an Aeroengine Double Swirler Lean Premixing Prevaporizing Burner

Lean premixing prevaporizing (LPP) burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large-scale model of a double swirler aeroengine LPP burner at isothermal conditions. A three-dimensional (3D) laser Doppler velocimeter and an ensemble-averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation, which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow-mixing process is highly enhanced. Turbulence acts on a large spectrum of fluctuation frequencies, whereas the large-scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.

Confinement and bursty transport in a flux-driven convection model with sheared flows

Transport and confinement within the resistive-g paradigm are investigated by means of two-dimensional numerical simulations. The system is driven by a constant incoming heat flux at the inner radial boundary. Different confinement and transport states are identified, involving self-sustained sheared poloidal flows. At the onset of turbulent convection the probability distribution functions of pressure and radial velocity fluctuations measured in the centre of the plasma layer have a nearly Gaussian form. Further increasing the heat flux drive these distributions become increasingly non-Gaussian, developing exponential tails. This large-scale intermittency is ascribed to the presence of bursting in the domain averaged convective transport and the fluctuation energy integrals. The quasi-periodic bursts are separated by shear-dominated quiescent periods in which the mean flow energy decreases and the confined heat increases on diffusive timescales. The time-averaged thermal energy confined within the plasma layer shows a power law dependence and significant increase with the injected power over the range of turbulent convection investigated.

Control of an axisymmetric jet using a passive ring

This study examines the development of an axisymmetric jet, which is disturbed by placing a thin wire ring axisymmetrically in the mixing layer close to the nozzle exit (0.1⩽ x/ D⩽0.5). Mixing layer development up to x/ D=5 is examined in detail using X-hotwire measurements, whereas far field development up to x/ D=30 is examined using single hotwire measurements on the jet axis. The introduction of the ring in the mixing layer results in a significant reduction in both the mixing layer growth rate and jet spread rate. The turbulence level in the mixing layer is substantially reduced for both axial and radial fluctuations (≈20% and 15% respectively). In addition to these reductions, the normalised distributions of mean velocity, axial and radial velocity fluctuations as well as the skewness and flatness factor of these fluctuations in the mixing layer all exhibit improved similarity. Variations of the jet Reynolds number (and thus the wire Reynolds number of the ring) and the axial location of the ring yield nearly identical changes to the characteristics of jet mixing layer. Increasing jet Reynolds number reduces the effect of the ring in the region 5< x/ D<30.

Poloidal Structure of the Turbulence and the Turbulent Flux During Edge Plasma Biasing in the Castor Tokamak

The poloidal structure of the turbulence and the turbulent particle flux in the scrape-off layer (SOL) of the CASTOR tokamak are analysed by means of a poloidal ring of 124 probes distributed uniformly along the whole poloidal circumference. Fluctuation measurements are performed in standard ohmic regime as well as in discharges when a biased electrode is inserted either into the SOL or deeper into the confinement region. It is found that in both cases a strongly sheared radial electric field is created in the SOL, which de-correlates the density and radial velocity fluctuations and reduces their levels. Consequently, the turbulent flux is reduced. However, no phase shift is observed between density and radial velocity fluctuations. When the electrode is localized in the confinement region, all the above effects are less pronounced because of a smaller shear. In addition, the increase of the E × B velocity at biasing leads in both cases to the formation of oscillations in the temporal correlation function. These oscillations are specifically associated to the poloidal mode (m = q), which is created for a limited time and rotates poloidally. This mode does not modify the phase between density and radial velocity fluctuations and has no further effect on the turbulent flux.

Experimental research on drag reduction by polymer additives in a turbulent pipe flow

AbstractIn order to investigate the effects of injection position on drag reduction as well as further the effects of polymer additives on turbulent structures, LDA measurements of turbulent pipe flows were conducted. The results show that the amount of drag reduction grows with the increase of the Reynolds number, and injecting the polymer at the centre of pipe is more effective than at the wall. Due to the addition of polymer solution, the axial, radial r.m.s. velocity fluctuations and Reynolds stress decrease over the entire pipe cross‐section, the time auto‐correlation coefficients of axial and radial velocity fluctuation at the centre of pipe decay more slowly, the number of spectrum peaks is decreased, and the peak shifts towards lower wave numbers. The results also reveal that, due to the addition of polymer solution, the large‐scale vortices are enhanced and small‐scale vortices are suppressed.

High-range-resolution velocity-estimation techniques for coherent Doppler lidars with exponentially shaped laser pulses.

On the basis of an analysis of the autocovariance of the complex heterodyne signal, some novel algorithms are derived and investigated for recovering the nonuniform Doppler-velocity coherent-lidar profiles within the lidar resolution interval conditioned by the sensing laser-pulse length. The case of exponentially shaped sensing laser pulses is considered. The algorithm performance and efficiency are studied and illustrated by computer simulations (based on the use of pulse models and real laser pulses), taking into account the influence of additive noise and radial-velocity fluctuations. It is shown that, at some reasonable number of signal realizations used and with appropriate data processing to suppress the noise effects, the Doppler-velocity profiles can be determined with a considerably shorter resolution interval in comparison with that (usually accepted as a lower bound) determined by the pulse length.

Surfactant-induced effects on turbulent swirling flows

We employ digital particle imaging velocimetry (DPIV) to investigate the influence of a drag reducing cationic surfactant additive, cetyltrimethyl-ammonium chloride (CTAC), on turbulent swirling flows generated in a cylindrical vessel either by a rotating disk or a rotating disk fitted with vertical flat blades. The largest concentration of CTAC used in this study (0.05 ≤ C ≤ 0.5 mmol/l) is an order of magnitude smaller than those used in experimental investigations of surfactant induced drag reduction in turbulent pipe/channel flows. Even for such dilute systems, a number of dramatic and intriguing effects are observed. In the case of disk-driven flow, it is shown that the surfactant has a non-monotonic influence on turbulence intensity: both radial and axial root mean square velocity fluctuations first increase with increasing surfactant concentration C, reach a maximum and decrease upon further increase in C. Moreover, the maximum intensity is attained at a concentration that is practically independent of the angular frequency Ω of the disk. For the flow driven by bladed impeller, the introduction of the surfactant leads to flow reversal at the impeller plane for low concentrations. Enhancement in the radial and azimuthal mean velocities is also observed. For relatively larger concentrations (=0.5 mmol/l), a mean flow field that consists of multiple transient mixing pockets emerges as Ω exceeds a critical value. Plausible mechanisms are proposed to explain these observations.

Enhanced Anthocyanin Production from Grape Callus in an Air-Lift Type Bioreactor Using a Viscous Additive-Supplemented Medium

An N-medium containing carboxymethyl cellulose (CMC) was applied to an air-lift type bioreactor culture of grape (Vitis vinifera cv. Bailey alicant A.) callus, and anthocyanin production was investigated. Grape callus grew well at an air flow rate of 80 ml/min and anthocyanin production was significantly increased in the N-medium, reaching 17 mg/l after 7 d of culture. The anthocyanin content of the N-medium was about two times higher than that of the conventional medium without CMC. The effect of air flow rate was also investigated within the range from 40 to 160 ml/min. A twofold increase in anthocyanin content was obtained at all the air flow rates tested in the N-medium. The distribution of grape callus size obtained after 7 d of the bioreactor culture was investigated. The average callus size was 490 mum which was 1.6 times larger than that obtained in the conventional medium. It was found that large calli with a relatively high anthocyanin pigment content were formed in the bioreactor culture using the N-medium. The fluid dynamics in the bioreactor was also investigated at three points (top, middle and bottom) in the bioreactor by laser doppler velocimetry. The average axial velocity of the circulated medium was 0.4 times lower than that of the conventional medium while their average radial velocities were almost the same (zero). The standard deviation of radial velocity fluctuation in the N-medium was also 0.4 times less than that in the conventional medium. These results suggest that turbulent flow occurred in the bioreactor culture using the conventional medium and the degree of turbulent flow decreased significantly when 0.8% CMC was added to the medium to prepare the N-medium. A change of the flow pattern is considered to be the cause of the decrease in hydrodynamic stress, resulting in enhanced pigment production due to the enlargement of the callus.