An Ocean Refractometer: Resolving Millimeter-Scale Turbulent Density Fluctuations via the Refractive Index

2006 ◽  
Vol 23 (1) ◽  
pp. 121-137 ◽  
Author(s):  
Matthew H. Alford ◽  
David W. Gerdt ◽  
Charles M. Adkins
Keyword(s):  
Refractive Index ◽  
Density Fluctuations ◽  
Inertial Subrange ◽  
Conductivity Measurements ◽  
Velocity Fluctuations ◽  
Fiber Motion ◽  
Turbulent Spectrum ◽  
Ocean Measurements ◽  
Fiberoptic Sensor ◽  
Better Than

Abstract A fiberoptic sensor has been constructed to measure oceanic density fluctuations via their refractive index signature. The resolution (Δz = 1 mm, Δt = 0.2 ms) and precision (Δn < 10−8, Δρ = 3.4 × 10−5 kg m−3) of the device are far better than other methods and are sufficient to resolve the entire turbulent spectrum. Spectra show the salinity Batchelor rolloff at levels undetectable via conductivity measurements. However, the low-wavenumber portion of the spectrum occupied by the turbulent inertial subrange (≈1 m–1 cm scales) is marred by noise resulting from fiber motion in response to turbulent velocity fluctuations. The technique is described, and the first ocean measurements are reported.

Dynamics of unstably stratified free shear flows: an experimental investigation of coupled Kelvin–Helmholtz and Rayleigh–Taylor instability

2017 ◽  
Vol 816 ◽  
pp. 619-660 ◽  
Author(s):  
Bhanesh Akula ◽  
Prasoon Suchandra ◽  
Mark Mikhaeil ◽  
Devesh Ranjan
Keyword(s):  
Richardson Number ◽  
Shear Flows ◽  
Mixing Layer ◽  
Layer Growth ◽  
Density Fluctuations ◽  
Inertial Subrange ◽  
Growth Equation ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Rt Instability

The dynamics of the coupled Kelvin–Helmholtz (KH) and Rayleigh–Taylor (RT) instability (referred to as KHRT instability or KHRTI) is studied using statistically steady experiments performed in a multi-layer gas tunnel. Experiments are performed at four density ratios ranging in Atwood number$A_{t}$from 0.035 to 0.159, with varying amounts of shear and$\unicode[STIX]{x0394}U/U$ranging from 0 to 0.48, where$\unicode[STIX]{x0394}U$is the speed difference between the two flow streams being investigated and$U$is the mean velocity of these two streams. Three types of diagnostics – back-lit visualization, hot-wire anemometry and particle image velocimetry (PIV) – are employed to obtain the mixing widths, velocity field and density field. The flow is found to be governed by KH dynamics at early times and RT dynamics at late times. This transition from KH-instability-like to RT-instability-like behaviour is quantified using the Richardson number. Transitional Richardson number magnitudes obtained for the present KHRT flows are found to be in the range 0.17–0.56 similar to the critical Richardson numbers for stably stratified free shear flows. Comparing the evolution of density and velocity mixing widths, the density mixing layer is found to be approximately two times as thick as the velocity mixing layer. Scaling of velocity fluctuations is attempted using combinations of KH and RT scales. It is found that the proposed KHRT velocity scale, obtained using the combined mixing-layer growth equation, is appropriate for intermediate stages of the flow when both KH and RT dynamics are comparable. Probability density functions (p.d.f.s) for different fluctuating quantities are presented. Multiple peaks in p.d.f.s are qualitatively explained from the development of coherent KH roll-ups and their subsequent transition into turbulent pockets. The evolution of energy spectra indicates that density fluctuations start to show an inertial subrange from earlier times compared to velocity fluctuations. The spectra exhibit a slightly steeper slope than the Kolmogorov–Obukhov five-thirds law.

scholarly journals Field Localization and Density Cavitation in Low-Beta Plasmas

2021 ◽  
Vol 2021 ◽  
pp. 1-9
Author(s):  
Motilal Rinawa ◽  
Prashant Chauhan ◽  
Sintu Kumar ◽  
Manoj Kumar Singh ◽  
Hari Kumar Singh ◽  
...  
Keyword(s):  
Ponderomotive Force ◽  
Space Plasma ◽  
Density Fluctuations ◽  
Alfven Wave ◽  
Nonlinear Coupling ◽  
Auroral Region ◽  
Filamentous Structure ◽  
Inertial Alfvén Wave ◽  
Turbulent Spectrum ◽  
Field Localization

In the present paper, filamentous structure formation, associated turbulent spectrum, and density cavity formation phenomena have been investigated for low- β plasma β ≪ m e / m i applicable to the auroral region. A set of dimensionless equations governing the dynamics of three dimensionally propagating inertial Alfvén wave (3D-IAW) and perpendicularly propagating magnetosonic wave (PMSW) has been developed. Ponderomotive force due to 3D-IAW has been included in the dynamics of the PMSW. Numerical simulation has been performed to study the nonlinear coupling of these two waves. From the obtained results, we found that the field intensity localization takes place which may further lead to the additional dissipation/turbulence process for particle heating and acceleration in space plasma. The associated turbulent spectrum is obtained with scaling nearly k − 4.28 at smaller scales (in the dissipation range). Relevance of the obtained results with the observations reported by various spacecrafts such as Hawkeye and Heos 2 has been discussed. Also, density fluctuations (depletion) of ∼ 0.10   n 0 are calculated, which are consistent with the FAST spacecraft observation reported.

Velocity fluctuations in a low-Reynolds-number fluidized bed

2008 ◽  
Vol 596 ◽  
pp. 467-475 ◽  
Author(s):  
SHANG-YOU TEE ◽  
P. J. MUCHA ◽  
M. P. BRENNER ◽  
D. A. WEITZ
Keyword(s):  
Reynolds Number ◽  
Relaxation Time ◽  
Fluidized Bed ◽  
Correlation Length ◽  
Volume Fraction ◽  
Low Reynolds Number ◽  
Density Fluctuations ◽  
Velocity Fluctuations ◽  
Similarities And Differences ◽  
Low Reynolds

The velocity fluctuations of particles in a low-Reynolds-number fluidized bed have important similarities and differences with the velocity fluctuations in a low-Reynolds-number sedimenting suspension. We show that, like sedimentation, the velocity fluctuations in a fluidized bed are described well by the balance between density fluctuations due to Poisson statistics and Stokes drag. However, unlike sedimentation, the correlation length of the fluctuations in a fluidized bed increases with volume fraction. We argue that this difference arises because the relaxation time of density fluctuations is completely different in the two systems.

Microscale temperature and velocity spectra in the atmospheric boundary layer

1977 ◽  
Vol 83 (3) ◽  
pp. 547-567 ◽  
Author(s):  
R. M. Williams ◽  
C. A. Paulson
Keyword(s):  
Reynolds Number ◽  
Prandtl Number ◽  
Temperature Fluctuations ◽  
Inertial Subrange ◽  
Velocity Fluctuations ◽  
One Dimensional ◽  
Temperature Spectra ◽  
The Mean ◽  
Velocity Spectra ◽  
The One

High-frequency fluctuations in temperature and velocity were measured at a height of 2 m above a harvested, nearly level field of rye grass. Conditions were both stably and unstably stratified. Reynolds numbers ranged from 370000 to 740000. Measurements of velocity were made with a hot-wire anemometer and measurements of temperature with a platinum resistance element which had a diameter of 0[sdot ]5 μm and a length of 1 mm. Thirteen runs ranging in length from 78 to 238 s were analysed.Spectra of velocity fluctuations are consistent with previously reported universal forms. Spectra of temperature, however, exhibit an increase in slope with increasing wavenumber as the maximum in the one-dimensional dissipation spectrum is approached. The peak of the one-dimensional dissipation spectrum for temperature fluctuations occurs at a higher wavenumber than that of simultaneous spectra of the dissipation of velocity fluctuations. It is suggested that the change in slope of the temperature spectra and the dissimilarity between temperature and velocity spectra may be due to spatial dissimilarity in the dissipation of temperature and velocity fluctuations. The temperature spectra are compared with a theoretical prediction for fluids with large Prandtl number, due to Batchelor (1959). Even though air has a Prandtl number of 0[sdot ]7, the observations are in qualitative agreement with predictions of the theory. The non-dimensional wavenumber at which the increase in slope occurs is about 0[sdot ]02, in good agreement with observations in the ocean reported by Grantet al. (1968).For the two runs for which the stratification was stable, the normalized spectra of the temperature derivative fall on average slightly below the mean of the spectra of the remaining runs in the range in which the slope is approximately one-third. Hence the Reynolds number may not have always been sufficiently high to satisfy completely the conditions for an inertial subrange.Universal inertial-subrange constants were directly evaluated from one-dimensional dissipation spectra and found to be 0[sdot ]54 and 1[sdot ]00 for velocity and temperature, respectively. The constant for velocity is consistent with previously reported values, while the value for temperature differs from some of the previous direct estimates but is only 20% greater than the mean of the indirect estimates. This discrepancy may be explained by the neglect in the indirect estimates of the divergence terms in the conservation equation for the variance of temperature fluctuations. There is weak evidence that the one-dimensional constant, and hence the temperature spectra, may depend upon the turbulence Reynolds number, which varied from 1200 to 4300 in the observations reported.

scholarly journals Turbulent spectra in the solar wind plasma

2009 ◽  
Vol 76 (2) ◽  
pp. 183-191 ◽  
Author(s):  
DASTGEER SHAIKH ◽  
G. P. ZANK
Keyword(s):  
Solar Wind ◽  
Three Dimensional ◽  
Solar Wind Plasma ◽  
Density Fluctuations ◽  
Magnetized Plasma ◽  
Density Spectrum ◽  
Turbulent Spectrum ◽  
Extended Length ◽  
Turbulent Spectra ◽  
Magnetohydrodynamic Fluid

AbstractObservations of interstellar scintillations at radio wavelengths reveal a Kolmogorov-like scaling of the electron density spectrum with a spectral slope of −5/3 over six decades in wavenumber space. A similar turbulent density spectrum in the solar wind plasma has been reported. The energy transfer process in the magnetized solar wind plasma over such extended length scales remains an unresolved paradox of modern turbulence theories, raising the especially intriguing question of how a compressible magnetized solar wind exhibits a turbulent spectrum that is a characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time-dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfvénic, supersonic and strongly magnetized plasma. It is shown that the observed Kolmogorov-like (−5/3) spectrum can develop in the solar wind plasma by supersonic plasma motions that dissipate into highly subsonic motion that passively convect density fluctuations.

scholarly journals An optical metrology system for the measurement of the refractive index of glass

2020 ◽  
Vol 238 ◽  
pp. 06013
Author(s):  
Inês Leite ◽  
Alexandre Cabral
Keyword(s):  
Refractive Index ◽  
Cost Effective ◽  
Uncertainty Budget ◽  
Optical Metrology ◽  
Cmos Camera ◽  
Reliable System ◽  
Hene Laser ◽  
System Components ◽  
Metrology System ◽  
Better Than

The measurement of the refractive index of parallel plated, optically simple, glass samples is a common and fundamental activity in numerous fields of expertise. This work aimed to optimize a known technique to a simple, cost-effective and reliable system to be implemented in a lab environment, with an accuracy in the results better than 10-2. A setup with a 632.8nm HeNe laser, automatic stage and CMOS camera was used and data was acquired with the help of LabVIEW controlling software. All system components were carefully controlled and optimized with the help of an uncertainty budget. Measurements had an associated uncertainty in the range from 10-3 to 10-4.

A test of the role of refractive index in correlations between solvent effects on electronic spectra and solvent effects on reaction equilibria and rates

1984 ◽  
Vol 49 (10) ◽  
pp. 2332-2341 ◽  
Author(s):  
Vojtěch Bekárek
Keyword(s):  
Refractive Index ◽  
Solvent Effects ◽  
Electronic Spectra ◽  
Kinetic Data ◽  
Fluorescence Spectra ◽  
Simple Relation ◽  
Absorption And Fluorescence Spectra ◽  
Induced Changes ◽  
Better Than

80 correlations have been carried out between the medium-induced changes in equilibrium and kinetic data (Y) and the medium-induced changes in positions of maxima in electronic (both absorption and fluorescence) spectra (Δν) of the Kamlet-Taft type indicators. The correlations have been carried out by means of the equations Y=Y0 + k . Δν and Y=Y0 + k' . Δν/f(n2) using only the Kamlet-Taft select solvents (SSS). The equation involving the function of refreactive index (n), f(n2) = (n2 - 1)/(2n2 + 1), is better than the simple relation between Y and Δν in 93% of the systems studied. The problem of HBD acidity of nitromethane and the relation of the A and B solvent characteristics by Swain and the characteristics of dipolarity of medium by Kamlet-Taft are discussed.

Improved Properties of a-SiC:H Alloys with Reduced Density of CH3 Radicals

1992 ◽  
Vol 258 ◽  
Author(s):  
S. S. Camargo ◽  
M. L. de Oliveira ◽  
A. A. Pasa ◽  
C. Gatts
Keyword(s):  
Refractive Index ◽  
Fermi Level ◽  
Carbon Content ◽  
Conduction Band ◽  
Conductivity Measurements ◽  
Hydrogen Atoms ◽  
Optical Gap ◽  
Hydrogen Dilution ◽  
Photo Conductivity ◽  
Reduced Density

ABSTRACTIn this paper we report on the properties of a-SiC:H alloys with reduced density of CH3 radicals obtained by hydrogen dilution of gases. A reduced density of voids, hydrogen content and density of carbon bonded hydrogen atoms, were obtained; while carbon content and density of Si-H bonds are not affected much. A reduction of the optical gap and an increase of the refractive index were observed and related to the reduced densities of CH3 groups and voids. Dark- and photo- conductivity measurements showed that a performance comparable to the best undiluted films may be easily achieved, associated with a shift of the dark Fermi level towards the conduction band.

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