Turbulent Velocity Measurements Research Articles

Turbulence in Helically Corrugated Pipe Flow

Results are reported of experimental measurements of friction factor, mean and turbulent velocities, and Reynolds stresses in fully developed flow through a helically corrugated pipe. Helical corrugations induce a strong rotational component in the mean flow near the wall but this decreases rapidly toward the pipe axis. Turbulence intensities and axial shear are generally smaller in this flow than in nonrotating pipe flow and the reduction is attributed to rotation. Reynolds stress measurements extrapolated to the wall are in agreement with pressure drop measurements. Both show that pipe friction factor is less in helically corrugated pipe than in normally corrugated pipe, other factors being the same. The axial component of the mean flow is well fitted by the two-dimensional semilogarithmic law-of-the-wall for rough pipes. It is also fitted by the defect law but the numerical parameters of the latter behave as in an adverse pressure gradient boundary layer, indicating less shear stress than in nonrotating pipe flows. This is consistent with reduced friction factor.

Drag reduction in fibre suspensions: transitional behaviour due to fibre degradation

IT is known that the addition of small amounts of certain polymers to a fluid can produce spectacular reductions in turbulent friction1,2. It is also possible to reduce friction by suspending macroscopic fibres in a fluid3. Superficially there are many similarities between the two effects but they are believed to involve different mechanisms because, for example, using polymers and fibres together can produce much larger reductions in drag than would be the case with either additive on its own4. We report here measurements of turbulent velocities in a fibre suspension, using laser–Doppler anemometry. We found that the effect of fibres on turbulent structure was not the same as that of polymers. However, when the fibre was degraded by passing it through the system several times, there was a transition to the type of structural change found in polymer solutions.

Structure of Marine Surface Layer Turbulence

Abstract Measurements of turbulent wind velocity, humidity and temperature spectra for stable and unstable stratification in the atmospheric surface layer obtained during an experiment over the North Pacific Ocean are presented. The velocity field appears to be in a state of local isotropy as measured by the ratio of vertical to streamwise velocity spectra Su(n>/ Su(n> at the measurement height of 29 m above the sea surface. Using Monin-Obukhov scaling, spectral shapes for humidity are similar to those for overland temperature. Evidence is presented which suggests that previous departures of marine temperature measurements from Monin-Obukhobzv similarity may be due to humidity sensitivity of salt-spray-contaminated temperature probes. Overland humidity data from the AFCRL-UCSD 1973 Minnesota Experiment (Champagne et al., 1977) were analyzed and also found to exhibit Monin-Obukhov similarity.

Optical studies of turbulence in an internal combustion engine

A relatively simple and reliable technique is described that has allowed spatially and temporally resolved measurements of the charge flow in the cylinder of a motored internal combustion engine to be made. the experiment uses real fringe laser doppler anemometry with single photon counting and digital autocorrelation for data capture. Measurements of mean and turbulent velocities for different positions, engine speeds, throttle settings and engine crank angles are presented. The experimental autocorrelation functions have been processed by curve fitting to a Gaussian velocity profile and by direct Fourier transformation which yields the velocity distribution without prior assumption of its form. The ability of such a technique to process very poor signal to noise ratios makes it a likely choice for measurements in firing engines.

Horizontal Coherence Decay Near Large Mesoscale Variations in Topography

A surface layer experiment is described which includes measurements of turbulent velocities at 2 m above the surface with an army of newly developed drag anemometers. The experiment site is located in central Pennsylvania where mesoscale topographic irregularities exist. The presence of a low mountain ridge near the site affects the estimated lateral scale of turbulence and the fluctuations of the lateral velocity component. A good correlation has been found between the variance spectrum of the lateral (or crosswind) velocity component and an estimate of the lateral Eulerian integral scale of the longitudinal velocity component. This can provide future estimates of the lateral scale from turbulent velocity measurements at a single location. A model for the decay of horizontal coherence which accounts for the stability, roughness and instrument separation has been suggested in a previous paper by Panofsky and Mizuno. The present data compare favorably with this model. The effect of stability on coherence decay is found to have a definite site dependence.

A note on vertical coherence of streamwise turbulence inside and above a model plant canopy

Measurements of longitudinal turbulent velocity were made at pairs of levels inside and above a model plant canopy in the wind tunnel. It was found that above approximately the zero-plane displacement level, the coherence and phase results were similar in many respects to atmospheric data, but that deviations from this behaviour appeared deeper in the model canopy.

Structure of sheared turbulent fields

Small scale turbulent temperature and velocity measurements were made in a variety of turbulent shear flows in the atmosphere and laboratory, in air and in water. Nonzero skewness values of the streamwise temperature gradient were observed in every case, with a sign equal to that of the gradient dotted with the cross product of the mean temperature gradient and mean shear vectors. Large temperature jumps were found across sharp shear zones at the outer boundaries of large scale eddies rotating with the mean vorticity of the shear flow, giving a distinctive ramp-like structure to the temperature signals which seems to be a characteristic feature of scalar fields mixed by sheared turbulence. The streamwise gradient of the vertical velocity in the atmospheric boundary layer over the ocean was found to have positive skewness. Thus, both velocity and temperature fields are locally anisotropic in the same sense as the anisotropic mean quantities.

Turbulent velocity measurements in a model of aortic stenosis

Experimental results from a laboratory model of aortic stenosis are presented. Valves were represented by a number of nozzles, using three area ratios for varying degrees of stenosis severity, and including non-symmetrical shapes. Steady flow tests were run for a range of Reynolds numbers (Re) appropriate to aortic flow. Turbulence intensities downstream of the nozzle were measured in both axial and radial directions, together with profiles of time mean velocity. The maximum normalised intensity was found to be independent of Re (based on nozzle diameter) for values >9000. Non-dimensional power spectra of the maximum intensity showed some Re dependence but were almost independent of nozzle area ratio and shape, except for oblique nozzles. The spectrum shape depended upon radial and axial positon near the stenosis but by six “aorta” diameters downstream the turbulence was almost homogeneous. The spectrum from a pulsatile flow test was very similar to the corresponding steady flow case.

On the relation between CO and gamma-ray observations, cosmic rays, and the thickness of the galactic disk

view Abstract Citations (13) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the relation between CO and gamma-ray observations, cosmic rays, and the thickness of the galactic disk. Wentzel, D. G. ; Jackson, P. D. ; Rose, W. K. ; Sinha, R. P. Abstract Recent CO observations suggest that dense clouds provide a high mean gas density (mostly H2) at approximately 5 kpc from the galactic center. Gamma-ray observations have been explained as resulting primarily from the interaction of cosmic rays with interstellar matter. The relationship between cosmic-ray density and mean gas density is discussed, and it is argued that the CO clouds must be dynamically independent of the gas that is observed in 21-cm line radiation. A plausible upper limit to the cosmic-ray density at 5 kpc from the galactic center is derived which is based on measurements of galactic-disk thickness and hydrogen turbulent velocities from 21-cm line data. An estimate is also made of the 'distributed' gas density which confines the cosmic rays to the disk. This upper limit is approximately five times the cosmic-ray density near the sun. Observations of the thickness of the galactic disk suggest that the cosmic-ray density increases with distributed gas density. Publication: The Astrophysical Journal Pub Date: October 1975 DOI: 10.1086/181928 Bibcode: 1975ApJ...201L...5W Keywords: Cosmic Rays; Galactic Structure; Gamma Rays; Interstellar Gas; X Ray Astronomy; Galactic Nuclei; Gas Density; Numerical Analysis; Solar Atmosphere; Synchrotron Radiation; Turbulence Effects; Astrophysics full text sources ADS |

Measurement of turbulent burning velocity for large turbulent Reynolds numbers

Turbulent burning velocities have been measured in an explosion vessel equipped with four fans driven by air turbines. This arrangement created a central region of uniform isotropic turbulence in which measurements were made of flame speed, turbulent burning velocity and gas velocity just ahead of the flame front. Two techniques were used for such measurements: one involving anemometer measurements of the gas velocity ahead of the flame and the other involving the creation of two kernels of gas originating from two separated, simultaneous sparks. Measurements were made for a range of methane-air and ethylene-air mixtures at an initial pressure of one atmosphere and at different fan speeds. An increase in fan speed resulted in increases in flame speed and turbulent burning velocity, but the gas velocity ahead of the flame showed little change. This last is explained by significant increase in flame thickness with fan speed. A consideration of current ideas of turbulent structure suggests a rational basis for the correlation of burning velocity data is to plot the ratio of turbulent to laminar burning velocity against the turbulent Reynolds number for the unburnt gas. An increase in fan speed gives an increase in the Reynolds number. The experimental results show there is indeed a primary correlation between this ratio and the turbulent Reynolds number. Reference to results of other workers also gives some confirmation of this.

Wakes behind two-dimensional surface obstacles in turbulent boundary layers

By making simple assumptions, an analytical theory is deduced for the mean velocity behind a two-dimensional obstacle (of heighth) placed on a rigid plane over which flows a turbulent boundary layer (of thickness δ). It is assumed thath[Gt ] δ, and that the wake can be divided into three regions. The velocity deficit −uis greatest in the two regions in which the change in shear stress is important, a wall region (W) close to the wall and a mixing region (M) spreading from the top of the obstacle. Above these is the external region (E) in which the velocity field is an inviscid perturbation on the incident boundary-layer velocity, which is taken to have a power-law profileU(y) =U∞(y−y1)n/δn, wheren[Gt ] 1. In (M), assuming that an eddy viscosity (=KhU(h)) can be defined for the perturbed flow in terms of the incident boundary-layer flow and that the velocity is self-preserving, it is found thatu(x,y) has the form$\frac{u}{U(h)} = \frac{ C }{Kh^2U^2(h)} \frac{f(n)}{x/h},\;\;\;\; {\rm where}\;\;\;\; \eta = (y/h)/[Kx/h]^{1/(n+2)}$, and the constant which defines the strength of the wake is$C = \int^\infty_0 y^U(y)(u-u_E)dy$, whereu=uE(x, y) asy→ 0 in region (E).In region (W),u(y) is proportional to Iny.By considering a large control surface enclosing the obstacle it is shown that the constant of the wake flow is not simply related to the drag of the obstacle, but is equal to the sum of the couple on the obstacle and an integral of the pressure field on the surface near the body.New wind-tunnel measurements of mean and turbulent velocities and Reynolds stresses in the wake behind a two-dimensional rectangular block on a roughened surface are presented. The turbulent boundary layer is artificially developed by well-established methods (Counihan 1969) in such a way that δ = 8h. These measurements are compared with the theory, with other wind-tunnel measurements and also with full-scale measurements of the wind behind windbreaks.It is found that the theory describes the distribution of mean velocity reasonably well, in particular the (x/h)−1decay law is well confirmed. The theory gives the correct self-preserving form for the distribution of Reynolds stress and the maximum increase of the mean-square turbulent velocity is found to decay downstream approximately as$ (\frac{x}{h})^{- \frac{3}{2}} $in accordance with the theory. The theory also suggests that the velocity deficit is affected by the roughness of the terrain (as measured by the roughness lengthy0) in proportion to In (h/y0), and there seems to be some experimental support for this hypothesis.

Turbulence effects on enclosed flames

A major factor limiting the development of turbulent flame theory is the dearth of experimental data. Almost all reported measurements of turbulent burning velocity have tended to emphasize the influence of turbulence intensity, but the equally important aspect of turbulence scale (eddy size) has been comparatively neglected. The present work attempts to remedy this situation by providing experimental data on the initiation and development of enclosed, premixed turbulent flames. The test rig is similar to that employed by Lefebvre and Reid[1]. By suitable variation in flow velocity and the geometry of the turbulence-promoting grid it is possible to create at various distances downstream of this grid regions in which turbulence intensity is sensibly constant but where turbulence scale varies over a fairly wide range. Thus measurements of flame speed and other flow parameters can be made under conditions where the effects of turbulence scale and intensity are completely separated. Many such measurements have been made and the results obtained are discussed in relation to current theories on turbulent flame structure and propagation. In particular it is shown that turbulent flame speed increases with scale under conditions of weak turbulence and decreases with increase in scale under conditions of strong turbulence. These two regions are separated by a so-called “transition region,” which occurs when the turbulence intensity is about twice the laminar flame speed. In this transition region turbulent flame speed is independent of both scale and laminar flame speed.

The laser-Doppler velocimeter and its application to the measurement of turbulence

In 1964, Yeh & Cummins demonstrated that coherent light sources could be used for the measurement of steady fluid velocities by observing the Doppler shift in the frequency of light scattered from small particles moving with the fluid. Since 1964 many investigators have attempted to extend this technique to the measurement of turbulent velocity fluctuations.A fundamental limitation on this type of velocimeter is the Doppler ambiguity introduced by the finite transit time of particles through the scattering volume, turbulent velocity fluctuations across the scattering volume, mean velocity gradients and electronic noise. A unified account of the effect of the Doppler ambiguity on the measurement of the instantaneous velocities is presented and results are interpreted using the power spectrum. The influence of the ambiguity on the measurement of other statistical quantities is also examined.Limitations on the spatial and temporal resolution imposed by the finite sampling volume are examined using the power spectrum and criteria for optimization of the response are proposed.An operational laser-Doppler velocimeter is described and measurements of spectra in both laminar and turbulent flow are presented. The experimental results are seen to be in excellent agreement with theoretical predictions.

Mean and Turbulent Velocities for Plane Jet

Expressions for the lateral mean velocity, stream function, mean vorticity, and Reynolds stress in the zone of established flow of a two-dimensional jet have been obtained, based on a Gaussian, axial, mean velocity distribution. Mean and turbulent velocity measurements in a water jet have been carried out using Pitot tube and electromagnetic induction (Magnetohydrodynamic MHD) probes. Based upon the experimental evidence, the following conclusions are made: (1) The mean and turbulent velocity distributions are the same for air and water jets at the same Reynolds number and relative locations; (2) turbulence measurements, using the MHD method, yield the correct velocity distribution; however, the magnitudes must be corrected for the effects of current density terms; (3) the autocorrelation and spectral density functions exhibit two regions of differing character, corresponding to macro and micro turbulent, eddy scales; and (4) the macro length scale increases linearly and the microscale increases in a nonlinear manner with distance from the slot discharge.

A Study of the Spectral Gust Alleviation Factor

A great deal of research effort is going into the measurement of turbulent air velocities in relation to aircraft flight. There is a continuing need to review our knowledge of aircraft responses to air turbulence in order to appreciate the significance of the new information as it becomes available and in order to demonstrate which parameters in the problem are of most importance and therefore deserving of the closest study.At the present stage of aircraft development we can see dramatic changes in aircraft speed, size and shape. Increased aircraft size, speed and height of operation imply larger values of the gust mass parameter μg than hitherto; they also mean that structural flexibility is becoming increasingly significant. Change of shape towards the slender configuration involves us not only in new appraisals of aircraft aerodynamics, steady and unsteady, but also in different modes of structural distortion from those typical of current subsonic turbojet transport aircraft.

Turbulent Energy Balance and Spectra of the Axisymmetric Wake

Axisymmetric turbulent wake behind a sphere in an incompressible fluid has been experimentally investigated from 50 to 300 diam downstream from the sphere at Reynolds numbers from 4000 to 150 000. Mean and turbulent velocity measurements show that the region of self-preservation starts 50 sphere diam downstream, and the virtual origin of the wake is 12 sphere diam downstream. Detailed measurements were made in the self-preserving region of the wake. The three components of the turbulent velocity, turbulent shear, and the mean velocity defect were measured across the wake using the hot-wire anemometer. From the measurements the turbulent energy production, dissipation, convection, and diffusion across the wake were determined. The one-dimensional energy spectra of the three components of the turbulent velocity, their dependence on distance along and across the wake and on Reynolds number have been measured. The large scale motion (low wavenumbers) is dynamically similar and the small scale motion (large wavenumbers) exhibits Kolmogoroff's universal equilibrium. The present spectral measurements are compared with other published measurements of the universal equilibrium spectra in various turbulent flows.

Measurement of turbulent velocities from the Doppler shift in scattered laser light

The local mean values and fluctuations of the velocity of water in turbulent flow through a tube have been determined from the Doppler shifts in the frequency of laser light scattered by small particles in the water. An optical hetrodyne technique was used to measure the shift in frequencies. The limited lifetime of the Doppler shifted signals, due to the finite time of passage of the scattering particles through the laser beam, causes broadening of the frequency spectra representing the fluctuations in turbulent velocity. This broadening, which is analogous to the Doppler radar ambiguity, was investigated with laminar flow and was allowed for in interpreting the spectra observed with turbulent flow. The optics were chosen so that the coherence limits were at all times greater than the beam diameter and did not further shorten the lifetimes of the Doppler signals. The root mean squares of the fluctuations in velocity, calculated from the observed spectra, are in agreement with corresponding data obtained by hot wire anemometry.

Measurements of turbulent velocity and temperature fluctuations in the wake of a sphere.

Decay of mean, variance, and gradient variance of temperature and velocity were measured in the initially heated wake of a sphere in a water tunnel. Dissipation rates of turbulent kinetic energy by viscosity and of thermal variance by conduction within the wake were obtained by differentiation of the measured variance decay and also from direct measurement of the gradient variances. Both dissipation rates were found to decrease with stream wise distance x approximately as #~ 2-4 for up to 60 diam from the sphere. Small-scale temperature and velocity fluctuations were found to be approximately homogeneous for cross sections of the wake normal to the flow. No region of continuous turbulence was found in the sphere wake, even on the wake axis. A velocity spectrum measured on the wake axis normalized using the measured viscous dissipation rate was found to be in good agreement with spectra from other turbulent systems also normalized by Kolmogoroif scaling.

The universal equilibrium spectra of turbulent velocity and scalar fields

Kolmogoroff's (1941) theory of local isotropy and universal similarity predicts that all turbulent velocity spectra are reducible to a single universal curve for the highest wave-numbers and that under certain conditions dimensional analysis may be used to predict spectral shapes. Identical arguments predict that the fine structure of conserved dynamically passive scalar fields mixed by turbulence will also be universally similar.A single-electrode conductivity probe in a bridge circuit was used to measure the spectra and decay of a random homogeneous field of concentration and temperature behind a grid, and a Lintronic constant-temperature hot-film anemometer was used to measure the decay of velocity field. These experimental measurements of absolute turbulent velocity, temperature, and concentration spectra in salt water are here compared with the general predictions of universal similarity and local isotropy theories, as well as a prediction by Batchelor (1959) of the exact large wave-number spectral form for scalar mixing at high Schmidt number (v[Gt ]D. The spectral shapes are found to have the predicted similarity forms, and the data are consistent with Batchelor's predicted spectrum of the scalar field.

Effect of Wind-Tunnel Contraction on Free-Stream Turbulence

The effect of wind-tunnel contraction on free-stream turbulence is determined by passing a well-defined turbulence through three contractions of ratios 4 : 1 , 9 :1 , and 16:1. Turbulent velocity measurements show that, in absolute magnitudes, the longitudinal component decreases and the lateral component increases as the flow accelerates through the contraction. Postand precontraction spectra are measured to get an idea about the distortion of turbulence structure.