Cross-Correlation Velocimetry for Measurement of Velocity and Temperature Profiles in Low-Speed, Turbulent, Nonisothermal Flows

1992 ◽  
Vol 114 (2) ◽  
pp. 331-337 ◽  
Author(s):  
V. Motevalli ◽  
C. H. Marks ◽  
B. J. McCaffrey
Keyword(s):  
Cross Correlation ◽  
Temperature Profiles ◽  
Fire Plume ◽  
Mean Velocity ◽  
Low Speed ◽  
Nonisothermal Flows ◽  
Degree Of Confidence ◽  
The Mean ◽  
Temperature Time ◽  
Better Than

A technique utilizing thermocouple pairs as sensors to measure velocity and temperature profiles in low-speed, turbulent, nonisothermal flows is described here. In this technique, Cross-Correlation Velocimetry (CCV), the temperature-time records from a pair of thermocouples, one downstream of the other, are cross-correlated to determine the flow’s preferred mean velocity while temperature is measured directly. The velocity measurements have undergone extensive verification using hotwire, pitot tube, and Laser-Doppler Velocimetry to determine the degree of confidence in this technique. This work demonstrates that the CCV technique is quite reliable and can measure the mean preferred component of the convective velocity with better than ±5 percent certainty. Application of this technique to the measurement of velocities in a ceiling jet induced by a fire plume is briefly presented here.

Measurements with a pulsed-wire and a hot-wire anemometer in the highly turbulent wake of a normal flat plate

1976 ◽  
Vol 77 (3) ◽  
pp. 473-497 ◽  
Author(s):  
L. J. S. Bradbury
Keyword(s):  
Turbulent Flow ◽  
Flat Plate ◽  
Flow Direction ◽  
Operating Conditions ◽  
Turbulent Intensity ◽  
Hot Wire ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean ◽  
Better Than

This paper describes an investigation into the response of both the pulsed-wire anemometer and the hot-wire anemometer in a highly turbulent flow. The first part of the paper is concerned with a theoretical study of some aspects of the response of these instruments in a highly turbulent flow. It is shown that, under normal operating conditions, the pulsed-wire anemometer should give mean velocity and longitudinal turbulent intensity estimates to an accuracy of better than 10% without any restriction on turbulence level. However, to attain this accuracy in measurements of turbulent intensities normal to the mean flow direction, there is a lower limit on the turbulent intensity of about 50%. An analysis is then carried out of the behaviour of the hot-wire anemometer in a highly turbulent flow. It is found that the large errors that are known to develop are very sensitive to the precise structure of the turbulence, so that even qualitative use of hot-wire data in such flows is not feasible. Some brief comments on the possibility of improving the accuracy of the hot-wire anemometer are then given.The second half of the paper describes some comparative measurements in the highly turbulent flow immediately downstream of a normal flat plate. It is shown that, although it is not possible to interpret the hot-wire results on their own, it is possible to calculate the hot-wire response with a surprising degree of accuracy using the results from the pulsed-wire anemometer. This provides a rather indirect but none the less welcome check on the accuracy of the pulsed-wire results, which, in this very highly turbulent flow, have a certain interest in their own right.

Stokes Mechanism of Drag Reduction

2005 ◽  
Vol 73 (3) ◽  
pp. 483-489 ◽  
Author(s):  
Promode R. Bandyopadhyay
Keyword(s):  
Drag Reduction ◽  
Attenuation Factor ◽  
Viscous Sublayer ◽  
Wall Region ◽  
Mean Velocity ◽  
Turbulence Suppression ◽  
Maximum Drag Reduction ◽  
The Mean ◽  
Stokes Second Problem ◽  
Better Than

The mechanism of drag reduction due to spanwise wall oscillation in a turbulent boundary layer is considered. Published measurements and simulation data are analyzed in light of Stokes’ second problem. A kinematic vorticity reorientation hypothesis of drag reduction is first developed. It is shown that spanwise oscillation seeds the near-wall region with oblique and skewed Stokes vorticity waves. They are attached to the wall and gradually align to the freestream direction away from it. The resulting Stokes layer has an attenuated nature compared to its laminar counterpart. The attenuation factor increases in the buffer and viscous sublayer as the wall is approached. The mean velocity profile at the condition of maximum drag reduction is similar to that due to polymer. The final mean state of maximum drag reduction due to turbulence suppression appears to be universal in nature. Finally, it is shown that the proposed kinematic drag reduction hypothesis describes the measurements significantly better than what current direct numerical simulation does.

A Review of Timing Accuracy across the Global Seismographic Network

2021 ◽  
Author(s):  
Adam T. Ringler ◽  
Robert E. Anthony ◽  
David C. Wilson ◽  
Dan Auerbach ◽  
Scott Bargabus ◽  
...  
Keyword(s):  
Cross Correlation ◽  
Timing Accuracy ◽  
Design Goal ◽  
Use Phase ◽  
The Mean ◽  
Time Periods ◽  
The Difference ◽  
Seismographic Network ◽  
Earthquake Data ◽  
Better Than

Abstract The accuracy of timing across a seismic network is important for locating earthquakes as well as studies that use phase-arrival information (e.g., tomography). The Global Seismographic Network (GSN) was designed with the goal of having reported timing be better than 10 ms. In this work, we provide a brief overview of how timing is kept across the GSN and discuss how clock-quality metrics are embedded in Standard for Exchange of Earthquake Data records. Specifically, blockette 1001 contains the timing-quality field, which can be used to identify time periods when poor clock quality could compromise timing accuracy. To verify the timing across the GSN, we compare cross-correlation lags between collocated sensors from 1 January 2000 to 1 January 2020. We find that the mean error is less than 10 ms, with much of the difference likely coming from the method or uncertainty in the phase response of the instruments. This indicates that timing across the GSN is potentially better than 10 ms. We conclude that unless clock quality is compromised (as indicated in blockette 1001), GSN data’s timing accuracy should be suitable for most current seismological applications that require 10 ms accuracy. To assist users, the GSN network operators have implemented a “gsn_timing” metric available via the Incorporated Research Institutions for Seismology Data Management Center that helps users identify data with substandard timing accuracy (the 10 ms design goal of the GSN).

Stokes’ Mechanism of Drag Reduction

2003 ◽  
Author(s):  
Promode R. Bandyopadhyay
Keyword(s):  
Drag Reduction ◽  
Attenuation Factor ◽  
Viscous Sublayer ◽  
Wall Region ◽  
Mean Velocity ◽  
Turbulence Suppression ◽  
Maximum Drag Reduction ◽  
The Mean ◽  
Stokes Second Problem ◽  
Better Than

The mechanism of drag reduction due to spanwise wall oscillation in a turbulent boundary layer is considered. Published measurements and simulation data are analyzed in light of Stokes’ second problem. A kinematic vorticity reorientation hypothesis of drag reduction is first developed. It is shown that spanwise oscillation seeds the near-wall region with oblique and skewed Stokes vorticity waves. They are attached to the wall and gradually align to the freestream direction away from it. The resulting Stokes’ layer has an attenuated nature compared to its laminar counterpart. The attenuation factor increases in the buffer and viscous sublayer as the wall is approached. The mean velocity profile at the condition of maximum drag reduction is similar to that due to polymer. The final mean state of maximum drag reduction due to turbulence suppression appears to be universal in nature. Finally, it is shown that the proposed kinematic drag reduction hypothesis describes the measurements significantly better than what current Direct Numerical Simulation does.

Mean velocity of optically detected intraaxonal particles measured by a cross-correlation method

1976 ◽  
Vol 54 (6) ◽  
pp. 859-869 ◽  
Author(s):  
R. S. Smith ◽  
Z. J. Koles
Keyword(s):  
Cross Correlation ◽  
Correlation Method ◽  
Nerve Fibers ◽  
Mean Velocity ◽  
Myelinated Nerve ◽  
Optical Signals ◽  
Accuracy And Precision ◽  
Small Collection ◽  
The Mean ◽  
Optically Detected

A method which uses the cross correlation of optical signals is described for the determination of the mean velocity of somatopetally moving particles within nerve fibers. The method was validated by simulation experiments and by comparing the results with those obtained by averaging collections of velocities of individual particles. The significant contribution of the method is that it allows objective and rapid serial evaluations of mean particle velocity within individual nerve fibers with good accuracy and precision. A series of results from normal myelinated nerve fibers from Xenopus laevis is presented. Considerable variation (up to 50%) in mean velocity was found between individual nerve fibers. The mean of all determinations indicates that the mean velocity of somatopetally moving particles in axons with diameters > 10 μm is in the region of 1.14 μm/s at a temperature of 22–24 °C. The findings are compared with the small collection of such determinations which have been reported in the literature.

scholarly journals AERODYNAMICS AND AEROACOUSTICS SURVEY FOR A LOW SPEED SUBSONIC JET OPERATING AT MACH 0.25

2014 ◽  
Vol 13 (2) ◽  
pp. 33
Author(s):  
A. R. Proença ◽  
O. De almeida ◽  
R. H. Self
Keyword(s):  
Pitot Tube ◽  
Hot Wire ◽  
Mean Velocity ◽  
Low Speed ◽  
Free Jet ◽  
Acoustic Signature ◽  
Total Noise ◽  
Subsonic Regime ◽  
The Mean

The purpose of this work is to study and characterize, in laboratory, the aerodynamics of a free jet operating at subsonic regime and identify its acoustic signature. This study aims to analyze the fundamental role of turbulent flow structures in the total noise produced at different Mach numbers. This work is focused at low speed subsonic jets operating at Mach number 0.25. The research is done by analyzing the data obtained in experiments using Pitot tube, hot-wire anemometer and acoustic measurements. This work also describes the experimental procedures for each step of analysis, as well as the characteristics of jet noise facility. The data from measurements with Pitot tube is used to study the mean velocity profiles. The average properties are also analyzed with anemometry system, likewise used to study the turbulent intensity of eleven axial lines, ranging from the center line to the edge of the nozzle (lipline). These results are compared with the literature and is verified the accuracy of hot-wire anemometers for turbulent intensities lower than 15%. A database with the sound pressure level as a function of frequency is constructed from experiments serving as data for further numerical analysis to solve this problem.

An Unsteady Flow Wind Tunnel

1974 ◽  
Vol 25 (2) ◽  
pp. 81-90 ◽  
Author(s):  
J H Horlock
Keyword(s):  
Wind Tunnel ◽  
Unsteady Flow ◽  
Free Stream ◽  
Flow Direction ◽  
Axial Flow ◽  
Mean Velocity ◽  
Low Speed ◽  
The Mean ◽  
Low Speed Wind Tunnel

SummaryA novel low-speed wind tunnel which produces unsteady “gust” flows is described. The walls are sinusoidal in shape and are moved in the flow direction with a velocity Wwless than the mean velocity Wmof the free stream. The tunnel is useful for testing isolated aerofoils and aerofoils in cascade in non-convective gusts (Ww< Wm) so that comparisons with predictions by thin aerofoil theory may be made. However, it does not simulate precisely the unsteady flow that occurs in axial flow turbomachines.

scholarly journals Measurements of debris flow velocity through cross-correlation of instrumentation data

2005 ◽  
Vol 5 (1) ◽  
pp. 137-142 ◽  
Author(s):  
M. Arattano ◽  
L. Marchi
Keyword(s):  
Debris Flow ◽  
Cross Correlation ◽  
Time Lag ◽  
Ground Vibration ◽  
Front Velocity ◽  
Correlation Technique ◽  
Ultrasonic Sensors ◽  
Mean Velocity ◽  
Different Types ◽  
The Mean

Abstract. Detection of debris flow occurrence can be efficiently obtained through different types of sensors. A pair of ultrasonic sensors placed at a known distance from each other along a torrent have been used as a method to obtain mean front velocity of debris-flows, in addition to their use as detectors of debris flow occurrence. Also seismic and acoustic sensors have been employed to measure debris-flow front velocity and discharge in the same manner. In order to obtain velocity measurements, however, these methods require the presence of a well identifiable and defined main front in the debris flow wave. The time lag between the recordings of the front of the wave at two consecutive stations allows an estimation of its mean velocity. When a well-defined front is not present and no recurrent feature can be found along the wave, the measurement of velocity may prove difficult. The cross-correlation technique may help identifying the mean velocity of the flow in such cases. In fact, cross correlation allows to determine the mean time lag elapsed between the recording of two sets of data of the same event at different positions. This technique may be also used to measure velocity using signals coming from different types of sensors, for instance where a ground vibration detector has been placed along a torrent where an ultrasonic sensor was already present or viceversa. An application has been made using field data recorded through seismic and ultrasonic sensors in a small instrumented catchment in the Italian Alps (Moscardo Torrent).

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