An Optical Instrument for Measuring Fluctuating Velocities in Highly Turbulent Flows

1978 ◽  
Vol 29 (2) ◽  
pp. 98-113 ◽  
Author(s):  
M. Gaster ◽  
J.F.M. Maybrey
Keyword(s):  
Turbulent Flows ◽  
Light Sources ◽  
Optical Instrument ◽  
Surface Reflection ◽  
Power Spectral ◽  
Optical Layout ◽  
Wide Range ◽  
Laser Anemometer ◽  
Sampling Zone ◽  
Highly Turbulent Flows

SummaryFlow measurement by optical devices of various types, particularly those involving laser light sources, have received considerable attention over the last few years. Different schemes employing a wide range of optical layouts have evolved and the resulting signals have been processed in a number of ingenious ways. We report new experimental work on an optical instrument that can be considered as the forerunner of the laser anemometer in the belief that in certain circumstances this particular optical layout offers some real advantages over the majority of laser anemometers. One important advantage of this system is the ease with which both the shape and size of the sampling zone can be independently controlled. Another is the ability to position the sampling region very close to a boundary without having to contend with the surface reflection difficulties that often prevent such measurements being made with laser optics. The instrument measures the velocities of small particles suspended in the fluid in much the same way as the laser anemometer. In unsteady flows this results in a series of velocity estimates generated at random time instants. These intermittent samples of the velocity are used to form power spectral density estimates by methods recently developed for the analysis of randomly sampled records (Gaster and Roberts, 1975 & 1976). This method of analysis could well be applied to the processing of the signals generated by laser anemometers operating in the burst counter mode.

scholarly journals Laser Anemometry Techniques for Turbine Applications

1987 ◽  
Author(s):  
Mark P. Wernet ◽  
Lawrence G. Oberle
Keyword(s):  
Turbulent Flows ◽  
Time Of Flight ◽  
Nasa Lewis Research ◽  
Acceptance Angle ◽  
Flight System ◽  
Laser Anemometry ◽  
Laser Anemometer ◽  
Highly Turbulent Flows ◽  
Anemometer System ◽  
Comparison Measurements

Laser anemometry offers a nonintrusive means for obtaining flow field information. Our current research efforts at NASA Lewis Research Center are focused on instrumenting a warm turbine facility with a laser anemometer system. In an effort to determine the laser anemometer system best qualified for the warm turbine environment, we compared the performance of a conventional laser fringe anemometer and a two spot time-of-flight system with a new, modified time-of-flight system, called a Four Spot laser anemometer. The comparison measurements were made in highly turbulent flows near walls. The Four Spot anemometer uses elliptical spots to increase the flow acceptance angle to be comparable to that of a Laser Fringe Anemometer. Also, the Four Spot uses an optical code that vastly simplifies the pulse detection processor. The results of the comparison measurements will exemplify which laser anemometer system is best suited to the hostile environment typically encountered in warm rotating turbomachinery.

The measurement of the spectra of highly turbulent flows by a randomly triggered pulsed-wire anemometer

1976 ◽  
Vol 77 (3) ◽  
pp. 499-509 ◽  
Author(s):  
M. Gaster ◽  
L. J. S. Bradbury
Keyword(s):  
Turbulent Flows ◽  
Sampling Rate ◽  
Spectral Information ◽  
Mean Velocity ◽  
Spectral Estimates ◽  
Power Spectral ◽  
Average Sampling ◽  
Highly Turbulent Flows ◽  
Velocity Turbulence

The pulsed-wire anemometer has enabled velocity measurements to be made in a variety of unsteady turbulent flows. So far the use of the instrument has been confined to the determination of mean velocity, turbulence intensity and probability density. Here we show how spectral information can also be obtained. The instrument provides estimates of the velocity by measuring the time of flight of a heated flow tracer. Periodic samples of velocity can be generated by driving the anemometer with a regular train of pulses, but unfortunately it is not possible to pulse the heater at a rate greater than about 50 or so times per second, without risk of burning out the wire. This limits the spectral information that can be obtained to situations where all the energy occurs at frequencies below 25 Hz. The random sampling scheme used here avoids the aliasing problem inherent in periodic sampling and enables estimates of power spectral density to be formed up to frequencies many times the average sampling rate. This technique is used to obtain spectral estimates of the velocity fluctuations that arise at various locations in the wake of a flat plate.

Lattice Boltzmann for Turbulence Modeling

2018 ◽  
Author(s):  
Sauro Succi
Keyword(s):  
Turbulent Flows ◽  
Lattice Boltzmann ◽  
Turbulence Modeling ◽  
Real Life ◽  
Practical Interest ◽  
Reynolds Numbers ◽  
Full Resolution ◽  
To Come ◽  
Highly Turbulent Flows ◽  
Main Ideas

This chapter introduces the main ideas behind the application of LBE methods to the problem of turbulence modeling, namely the simulation of flows which contain scales of motion too small to be resolved on present-day and foreseeable future computers. Many real-life flows of practical interest exhibit Reynolds numbers far too high to be directly simulated in full resolution on present-day computers and arguably for many years to come. This raises the challenge of predicting the behavior of highly turbulent flows without directly simulating all scales of motion which take part to turbulence dynamics, but only those that fall within the computer resolution at hand.

A Parallel HOFD Scheme for Direct Numerical Simulation of Turbulent Magnetically Driven Flows

2001 ◽  
Author(s):  
X. Ai ◽  
B. Q. Li
Keyword(s):  
Numerical Simulation ◽  
Finite Difference ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Parallel Machines ◽  
Higher Order ◽  
Electrically Conducting ◽  
Wide Range ◽  
Order Scheme ◽  
Higher Order Scheme

Abstract Turbulent magnetically flows occur in a wide range of material processing systems involving electrically conducting melts. This paper presents a parallel higher order scheme for the direct numerical simulation of turbulent magnetically driven flows in induction channels. The numerical method is based on the higher order finite difference algorithm, which enjoys the spectral accuracy while minimizing the computational intensity. This, coupled with the parallel computing strategy, provides a very useful means to simulate turbulent flows. The higher order finite difference formulation of magnetically driven flow problems is described in this paper. The details of the parallel algorithm and its implementation for the simulations on parallel machines are discussed. The accuracy and numerical performance of the higher order finite difference scheme are assessed in comparison with the spectral method. The examples of turbulent magnetically driven flows in induction channels and pressure gradient driven flows in regular channels are given, and the computed results are compared with experimental measurements wherever possible.

Reduced heart rate variability during sleep in long-duration spaceflight

2013 ◽  
Vol 305 (2) ◽  
pp. R164-R170 ◽  
Author(s):  
D. Xu ◽  
J. K. Shoemaker ◽  
A. P. Blaber ◽  
P. Arbeille ◽  
K. Fraser ◽  
...  
Keyword(s):  
Heart Rate ◽  
Heart Rate Variability ◽  
Substantial Reduction ◽  
Space Station ◽  
Power Spectral Analysis ◽  
Power Spectral ◽  
Long Duration ◽  
Fractal Properties ◽  
Wide Range ◽  
Simultaneous Movement

Limited data are available to describe the regulation of heart rate (HR) during sleep in spaceflight. Sleep provides a stable supine baseline during preflight Earth recordings for comparison of heart rate variability (HRV) over a wide range of frequencies using both linear, complexity, and fractal indicators. The current study investigated the effect of long-duration spaceflight on HR and HRV during sleep in seven astronauts aboard the International Space Station up to 6 mo. Measurements included electrocardiographic waveforms from Holter monitors and simultaneous movement records from accelerometers before, during, and after the flights. HR was unchanged inflight and elevated postflight [59.6 ± 8.9 beats per minute (bpm) compared with preflight 53.3 ± 7.3 bpm; P < 0.01]. Compared with preflight data, HRV indicators from both time domain and power spectral analysis methods were diminished inflight from ultralow to high frequencies and partially recovered to preflight levels after landing. During inflight and at postflight, complexity and fractal properties of HR were not different from preflight properties. Slow fluctuations (<0.04 Hz) in HR presented moderate correlations with movements during sleep, partially accounting for the reduction in HRV. In summary, substantial reduction in HRV was observed with linear, but not with complexity and fractal, methods of analysis. These results suggest that periodic elements that influence regulation of HR through reflex mechanisms are altered during sleep in spaceflight but that underlying system complexity and fractal dynamics were not altered.

Fluctuating Forces in U-Tubes Subjected to Internal Two-Phase Flow

2005 ◽  
Author(s):  
Jean-Luc Riverin ◽  
Michel J. Pettigrew
Keyword(s):  
Two Phase Flow ◽  
Periodic Structures ◽  
Internal Flow ◽  
Resonance Phenomenon ◽  
Dimensionless Frequency ◽  
Phase Flow ◽  
Two Phase ◽  
Power Spectral ◽  
Wide Range ◽  
Periodic Components

Severe in-plane vibrations were observed in a series of 20-mm dia. PVC vertical U-tubes of different elbow geometries subjected to air-water internal flow. An experimental study was undertaken to investigate the excitation mechanism. Vibration response, excitation forces and fluctuating properties of two-phase flow were measured over a wide range of flow conditions. The experimental results show that the observed vibrations are due to a resonance phenomenon between periodic momentum flux fluctuations of two-phase flow and the first modes of U-tubes. The excitation forces consist of a combination of narrow-band and periodic components, with a predominant frequency that increases proportionally to flow velocity. For a given void fraction, the force spectra for various flow velocities and elbow geometries coincide generally well on a plot of the normalized power spectral density as a function of a dimensionless frequency. The predominant frequencies of excitation agree with recent results on the characteristics of periodic structures in two-phase flow.

Assessment of Certainty of Ventilation Processes Mathematical Modeling

2015 ◽  
Vol 725-726 ◽  
pp. 1255-1260
Author(s):  
Tamara Daciuk ◽  
Vera Ulyasheva
Keyword(s):  
Mathematical Modeling ◽  
Turbulent Flows ◽  
Turbulence Models ◽  
Field Measurements ◽  
Heat Emission ◽  
Emission Sources ◽  
Wide Range ◽  
Calculation Results ◽  
Definition Of ◽  
Semiempirical Models

Numerical experiment has been successfully used during recent 10-15 years to solve a wide range of thermal and hydrogasodynamic tasks. Application of mathematical modeling used to design the ventilation systems for production premises characterized by heat emission may be considered to be an effective method to obtain reasonable solutions. Results of calculation performed with numerical solution of ventilation tasks depend on turbulence model selection. Currently a large number of different turbulence models used to calculate turbulent flows are known. Testing and definition of applicability limits for semiempirical models of turbulence should be considered to be a preliminary stage of calculation. This article presents results of test calculations pertaining to thermal air process modeling in premises characterized by presence of heat emission sources performed with employment of different models of turbulence. Besides, analysis of calculation results and comparison with field measurements data are presented.

Calculation of Developing Turbulent Flows in a Rotating Pipe

1991 ◽  
Vol 113 (1) ◽  
pp. 34-41 ◽  
Author(s):  
G. J. Yoo ◽  
R. M. C. So ◽  
B. C. Hwang
Keyword(s):  
Turbulent Flows ◽  
Reynolds Stress ◽  
Circumferential Strain ◽  
Three Dimensional ◽  
Wall Region ◽  
Boundary Layer Flows ◽  
Viscous Effects ◽  
Wide Range ◽  
Turbulence Closures ◽  
Rotating Boundary

Internal rotating boundary-layer flows are strongly influenced by large circumferential strain and the turbulence field is anisotropic. This is especially true in the entry region of a rotating pipe where the flow is three dimensional, the centrifugal force due to fluid rotation is less important, and the circumferential strain created by surface rotation has a significant effect on the turbulence field near the wall. Consequently, viscous effects cannot be neglected in the near-wall region. Several low-Reynolds-number turbulence closures are proposed for the calculation of developing rotating pipe flows. Some are two-equation closures with and without algebraic stress correction, while others are full Reynolds-stress closures. It is found that two-equation closures with and without algebraic stress correction are totally inadequate for this three-dimensional flow, while Reynolds-stress closures give results that are in good agreement with measurements over a wide range of rotation numbers.

scholarly journals Effect of light on aggregational behavior of planarian Dugesia tigrina

2017 ◽  
Author(s):  
Jaemin Lee
Keyword(s):  
Defense Mechanisms ◽  
Uv Light ◽  
Infrared Light ◽  
Light Sources ◽  
Initial Exposure ◽  
Dugesia Tigrina ◽  
Wide Range ◽  
Photophobic Response ◽  
Negative Effect ◽  
Effect Of Light

Introduction. Planarians are renowned for their regenerative ability due to pluripotent stem cells, as well as their peculiar photophobic response. However, few facts are known about their aggregational behavior. This study aims to reveal the effect of light on aggregational behavior. Reynierse (1966) suggested that light has a negative effect on the formation of aggregations. However, one of his objectives for aggregational behavior was inappropriate. This study reevaluated the effect of existence of light on aggregational behavior, as well as ascertained the effect of wavelength on the formation of aggregations. Methods. In this study, the ratio of individuals participating in aggregations was measured as a criterion to determine aggregational behavior. Aggregational behavior was measured after two hours from the initial exposure to different light sources. The behaviors under white LED light and under shade were compared, as well as the behaviors under five different light sources: infrared lamp, red, green, blue LED, and ultraviolet lamp. Results. The existence of light interfered the formation of aggregations (t-test, p < 0.0001), which supports the former study of Reynierse. Also, aggregational behavior differed under different wavelengths (ANOVA, p < 0.0001). Except for the infrared light which emitted a wide range of wavelengths, the behavior showed hierarchy: decreasing aggregational behavior in accordance with decreasing wavelength. UV light has the most significant negative effect on the formation of aggregations. Discussion. Exposure to light caused negative effects on performing aggregational behavior. Participation in aggregations appears to be influenced by photophobic response, especially under lights of short wavelength. Disintegrating aggregations under exposure to lights can potentially bring evolutionary benefit. This behavior possibly makes the aggregating planarians altogether exposed to a higher risk or predation, considering that they lack defense mechanisms. Planarians can lower the risk and continue the populations by disintegrating the aggregational behavior under the existence of UV and lights of higher wavelength, which are indicatives of daytime. Understanding aggregational behavior of animals of a lower order would give better insight on general herding behavior, and potentially help interpreting more complex behaviors of higher animals.

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