Measurements Of Turbulence Structure Research Articles

Velocity Profile and Turbulence Structure Measurement Corrections for Sediment Transport-Induced Water-Worked Bed

When using point measurement for environmental or sediment laden flows, there is well-recognised risk for not having aligned measurements that causes misinterpretation of the measured velocity data. In reality, these kinds of mismeasurement mainly happen due to the misinterpretation of bed orientation caused by the complexity of its determination in natural flows, especially in bedload laden or rough bed flows. This study proposes a novel bed realignment method to improve the measured data benchmarking by three-dimensional (3D) bed profile orientation and implemented it into different sets of experimental data. More specifically, the effects of realignment on velocity profile and streamwise turbulence structure measurements were investigated. The proposed technique was tested against experimental data collected over a water-worked and an experimentally arranged well-packed beds. Different from the well-packed rough bed, the water-worked bed has been generated after long sediment transport and settling and hence can be used to verify the proposed bed-alignment technique thoroughly. During the flow analysis, the corrected velocity, turbulence intensity and Reynolds stress profiles were compared to the theoretical logarithmic law, exponential law and linear gravity (universal Reynolds stress distribution) profiles, respectively. It has been observed that the proposed method has improved the agreement of the measured velocity and turbulence structure data with their actual theoretical profiles, particularly in the near-bed region (where the ratio of the flow measurement vertical distance to the total water depth, z/h, is limited to ≤0.4).

Turbulence Structure Within and Above a Canopy of Bluff Elements

Measurements of turbulence structure in a wind-tunnel model canopy of bluff elements show many of the features associated with vegetation canopies and roughness sublayers but also display features more characteristic of the inertial sublayer (ISL). Points of similarity include the existence of an inflexion point in the space-time averaged streamwise velocity at the canopy top, the variation with height of turbulent second moments and the departure of the turbulent kinetic energy budget from local equilibrium in and just above the canopy. Quadrant analysis shows characteristic dominance of sweep over ejection events within the canopy although sweeps are more frequent than usually seen in vegetation canopies. Points of difference are a u′, w′ correlation coefficient that is closer to the ISL value than to most canopy data, and a turbulent Prandtl number midway between canopy and ISL values. Within the canopy there is distinct spatial partitioning into two flow regimes, the wake and non-wake regions. Both time-mean and conditional statistics take different values in these different regions of the canopy flow. We explain many of these features by appealing to a modified version of the mixing-layer hypothesis that links the dominant turbulent eddies to the instability of the inflexion point at canopy top. However, it is evident that these eddies are perturbed by the quasi-coherent wakes of the bluff canopy elements. Based upon an equation for the instantaneous velocity perturbation, we propose a criterion for deciding when the eddies linked to the inflexion point will dominate flow structure and when that structure will be replaced by an array of superimposed element wakes. In particular, we show that the resemblance of some features of the flow to the ISL does not mean that ISL dynamics operate within bluff-body canopies in any sense.

Double‐diffusive layers in the Adriatic Sea

A microstructure profiler was deployed to make turbulence measurements in the upper layers of the southern Adriatic Sea in the Mediterranean during the Naval Research Laboratory (NRL) DART06A (Dynamics of the Adriatic in Real Time) winter cruise in March 2006. Measurements in the Po river plume along the Italian coast near the Gargano promontory displayed classic double‐diffusive layers and staircase structures resulting from the relatively colder and fresher wintertime Po river outflow water masses overlying warmer and more saline water masses from the Adriatic Sea. We report here on the water mass and turbulence structure measurements made both in the double‐diffusive interfaces and the adjoining mixed layers in the water columns undergoing double‐diffusive convection (DDC). This dataset augments the relatively sparse observations available hitherto on the diffusive layer type of DDC. Measured turbulence diffusivities are consistent with those from earlier theoretical and experimental formulations, suggesting that the wintertime Po river plume is a convenient and easily accessible place to study double diffusive convective processes of importance to mixing in the interior of many regions of the global oceans.

Quasi 3-D measurements of turbulence structure in horizontal air–water bubbly flow

Quasi 3-D measurements of the turbulence structure of air–water bubbly flow in a horizontal tube with 35 mm i.d. are undertaken with two TSI “X”-type hot-film probes. The turbulent fluctuations, u f, v f, w f, in axial, radial and circumferential directions, respectively, and Reynolds stresses − uv and − uw are obtained. It is found that in the lower portion of the tube, the profiles of turbulent fluctuation and Reynolds stress resemble those of single phase flow; whereas in the upper portion of the tube, where the bubble population is high, the turbulence, especially the circumferential fluctuation w f, is substantially enhanced, and the radial turbulence assumes highest value in the radial position −0.7< r/ R<0.5. The magnitudes of Reynolds stresses − uw and − uv in our measurements are in the same level except in the lower portion of the tube where − uw assumes a value close to zero as is the case in single phase flow and vertical air–water bubbly flow.

High‐resolution radar measurements of turbulent structure in the low‐latitude E region

VHF radar observations with high range and time resolutions of 150 m and 2 s were made to study the large‐scale plasma turbulence in the low‐latitude E region over Gadanki. The observations are presented in the form of Doppler spectra, height‐time‐intensity (HTI) maps and height‐time‐Doppler velocity plots. The daytime E region is found to be unstable mostly in the lower part (&lt;100 km) over a narrow altitude range and manifesting as discrete scattering layer, in contrast to large‐scale turbulent structures observed over Jicamarca. During nighttime, however, while thin layered structures similar to that of daytime are dominant in the lower part of the E region, the upper region is found to be unstable over a wide altitude range showing significant large‐scale turbulent structures that are of wave type, and also occasionally plume type resembling that of spread F. The HTI maps show that the large scale structures have vertical wavelengths ranging 1 to 2 km and periods of few tens of seconds. The Doppler velocities associated with the plume type structures are upward and wave‐type peturbations are downward with values mostly in the range of 50–100 m s−1 which are considerably lower to that observed at Jicamarca. The velocity spectral widths ranging 100–150 m s−1 are also found to be significantly lower compared to that reported for Jicamarca. Notwithstanding the differences noted above, the observations presented here seem to conform broadly to that reported for Jicamarca and also to that generated through the nonlocal nonlinear numerical simulation of the gradient drift instability.

High‐resolution radar measurements of turbulent structure in the equatorial electrojet

Observations of the equatorial electrojet were made with unprecedented altitude resolution at the Jicamarca Radio Observatory in January 1990. We obtained high‐quality Doppler spectra with 250‐m and sometimes better altitude resolution at heights from 95 to 120 km during the daytime and early evening with the large 50‐MHz radar. This resolution and coverage revealed a richness of dynamics and structure not previously appreciated, a structure that is sometimes reminiscent of VHF radar maps of equatorial spread F, albeit at smaller primary scale sizes. Individual scattering centers sometimes appear to drift in large swirls over many kilometers in altitude, maintaining their identity for times occasionally as long as a minute or two. On the other hand, we also see rapid variations with altitude (a kilometer or less) and time (a few seconds or less) in the Doppler spectra and signal power.

The behaviour of drag-reducing cationic surfactant solutions

The behaviour of two types of drag reducing surfactant solutions was studied in turbulent flows in pipes of different diameters. Our surfactant systems contained rod-like micelles; they consisted of equimolar mixtures ofn-tetradecyltrimethylammonium bromide,n-hexadecyltrimethylammonium bromide, and sodium salicylate. The structure of the turbulence was studied using a laser-Doppler anemometer in a 50 mm pipe. In the turbulent flow regime both surfactant solutions exhibited characteristic flow regimes. These flow regimes can be influenced by changing the amount of excess salt, the surfactant concentration, or the temperature. Shear viscosity measurements in laminar pipe and Couette flows show the occurrence of the so-called shear-induced state, where the viscosity increases and the surfactant solution becomes viscoelastic. The shape of the turbulent velocity profile depends on the flow regime. In the turbulent flow regime at low Reynolds numbers, velocity profiles similar to those observed for dilute polymer solutions are found, whereas at maximum drag reduction conditions more “S-shaped” profiles that show deviations from a logarithmic profile occur. An attempt is made to explain the drag reduction by rod-like micelles by combining the results of the rheological and the turbulence structure measurements.

Structure of rough-wall turbulent boundary layers

Recent experiments have shown that, in rough-wall turbulent boundary layers, drag varies systematically with the spanwise aspect ratio λz (span/height) of roughness elements. In this paper, the effect of λz on turbulence structure has been examined. Based on λz, the roughness in a transversely grooved surface (λz≫1) is the opposite extreme of model plant canopies (λz≪1) studied in wind tunnels, whereas sandgrain is an intermediate type [λz=O(1)]. Second-, third-, and fourth-order turbulence moments have been measured in turbulent boundary layers over transversely grooved and smooth surfaces and compared with available turbulence structure measurements over other types of surfaces. The near-wall turbulence structure is found to vary with λz. The instantaneous motions involved in the flux of shear stress near the wall in smooth and transversely grooved surfaces are opposite in sign to those in three-dimensional roughness. The former is explained in terms of hairpin vortices alone while the latter group is modeled to have an additional vortex, viz., the so-called necklace vortex which straddles a three-dimensional roughness element near its base.

Laser-doppler measurements of turbulence structure in a drag-reducing pipe flow with polymer injection.

Aqueous solutions of polyethylene oxide were injected into a pipe flow through a small tube at the center of a pipe. The turbulent characteristics in drag-reducing flow with polymer injection were measured by means of a Laser Doppler Velocimeter (LDV). The experimental results were compared with measurements both in a Newtonian fluid (water) flow and in a premixed flow with 300 ppm homogeneous polymer solution. The experimental results suggested that the polymer injection caused a thickening of the buffer layer, enlargement of macroscale turbulent eddy and suppression of fine turbulent eddy. A difference in turbulent characteristics between premixed and polymer injected systems was observed in the distributions of turbulent macroscale, skewness factor and flatness factor. The enlargement of macroscale was more significant in the turbulent core region in the case of polymer injection experiments. The measurements of skewness and flatness factors showed that the low-speed fluid element caused by the ejection process passed more clearly the measuring position near the outer boundary of the buffer layer in the case of polymer injection experiments.

Flow over the summit of an isolated hill

Observations of the mean flow and turbulence statistics over the summit of an isolated, roughly circular hill, Nyland hill, are presented, Nyland hill rises 70 m above the surrounding terrain and has a base diameter of about 500 m. The summit of the hill is very smooth and allows representative measurements to be made close to the surface. The flow speed 8 m above the summit is increased by a factor of 2 over the upstream speed 8 m above level terrain, and flow separation occurs in the lee of the hill. The mean velocity profile over the summit shows an increase in velocity with height up to about 2 m and then a near constant velocity between 2 and 16 m. The flow perturbation relative to the upstream profile is thus a maximum at about 2 m. The measurements of turbulence structure show how the influence of the hill depends on the length scale of the turbulent eddies involved. Scales greater than the scale of the hill are modified through the flow speed-up whilst scales shorter than the hill suffer complex changes. The short-scale turbulence over the summit is only in local equilibrium in the lowest fraction of a metre. Above this equilibrium region, there is a complex adjustment towards the rapid distortion dynamics which appear to dominate at heights above about 8 m. The detailed results are compared with previous studies and available theories.

Laser-Doppler anemometer measurements of turbulent structure in drag-reducing fibre suspensions

A laser-Doppler anemometer (LDA) was used to measure turbulent velocities in drag-reducing fibre suspensions. Measurements of streamwise velocities (and, in one case, the circumferential velocity as well) were made in flow through a straight pipe at x/d = 190, and at Reynolds numbers in the range 1.4 × 104–5.3 × 104. The fibres used were chrysotile asbestos of high aspect ratio (∼ 106), at a concentration of 300 w.p.p.m. They were dispersed in an aqueous solution of a surfactant (0.5% by weight Aerosol OT). In some experiments, the fibre suspensions were supplemented by a drag-reducing polymer (Separan AP30) at a concentration of 150 w.p.p.m. A complete experiment involved passing a quantity of fibre suspension through the apparatus a number of times (at a given Reynolds number) and measuring the velocity distribution across the pipe during each pass. As the amount of drag reduction generally declined with the number of passes (i.e. due to fibre degradation), this provided a convenient way of varying the percentage drag reduction as an experimental parameter. Results were obtained for mean velocity and intensity profiles, autocorrelations, and one-dimensional energy spectra. The mean period of turbulent bursts was determined by measuring autocorrelations with short sampling times.At the lowest Reynolds number (Re = 1.4 × 104), drag reductions of about 70% were obtained during the first two passes. This was accompanied by a reduction in the streamwise intensity below the level obtained in the surfactant solution alone. (Note: The opposite behaviour is found in drag-reducing polymer solutions, where intensity levels are larger than those in the solvent alone.) A measurement of the r.m.s. circumferential velocity showed an increased level (relative to surfactant alone) during this part of the experiment. During further passes, there was a transition to ‘polymer-like’ behaviour, with increased streamwise intensity, which subsequently declined with pass number (and hence drag reduction) towards the result for surfactant alone. This effect had previously been found in preliminary experiments at Re = 9 × 103 (McComb &amp; Chan 1979). Repetition of the experiment a Re = 1.4 × 104, with the addition of Separan AP30, confirmed the existence of this transition from ‘fibre-like’ to ‘polymer-like’ drag reduction. In this case, the drag reduction was smaller (at about 60%), but the mixed suspension was much more resistant to degradation, with transition occurring at the ninth pass. However, such behaviour was not found at higher Reynolds numbers (Re = 3.2 × 104 and 5.3 × 104), in fibre suspensions where increased streamwise intensities occurred, even at high levels of drag reduction (about 70%).Anomalous streamwise autocorrelations were found during ‘fibre-like’ drag reduction but in the ‘polymer-like’ regime they were very similar to those measured in polymer solution, and showed characteristically increased lengthscales. On the other hand, energy spectra were found to be anomalous in all cases and showed an energy deficit at lengthscales of the same order as the fibre length. Finally, mean bursting periods were found to be much increased, with the increases being about the same as those in polymer solutions at the same Reynolds number and percentage drag reduction.

Laser-Doppler anemometer measurements of turbulent structure in non-Newtonian fluids

A laser-Doppler anemometer was used to measure the radial variation of axial turbulent velocity in the flow of water and aqueous polymer solution through a round pipe. Results were obtained for the mean velocity the turbulent intensity, and the one-dimensional energy spectrum. The polymers used were polyacrylamide (Separan AP 273; in a concentration range 25-200 WPPM). Both polymers are well known as effective additives to reduce turbulent drag, when dissolved in water to give a solution concentration of a few PPM. The mean velocity and turbulent intensity were both found to be increased by the polymer additives, with the magnitude of the effect depending on the polymer concentration This was in agreement with the results of other investigations. At low additive concentrations, the energy spectra were found to be identical to those for water flows; this also was in agreement with previous results. At higher concentrations, energy spectra showed noticeable attenuation in the largest (dissipation) range of wavenumbers. There was a corresponding enhancement of the small-wavenumber (energy-containing) range. This seemed to be a threshold effect with onset at a concentration of between 50 PPM and 100 PPM for Polyox WSR 301 and between 25 PPM and 50 PPM for Separan AP 273.

Local drag reduction due to injection of polymer solutions into turbulent flow in a pipe. Part II: Laser‐doppler measurements of turbulent structure

AbstractAqueous solutions of polythyleneoxide (Polyox WSR‐301) were injected into a pipe flow through either a small tube at the center line or an annular slot in the wall. The solution contained polymer at an injection concentration of 1,000 wppm. Injection into water flow with a Reynolds number Re = 3.5 × 104 was at a rate which gave a mean polymer concentration of 5.0 wppm in the water flow. A laser‐Doppler anemometer (LDA) was used to measure the streamwise turbulent velocity at various radial positions and at several stations downstream from the injection point.Results were obtained for mean velocity and intensity profiles; autocorrelations; and one‐dimensional energy spectra. The mean bursting period was determined using the “short‐sampling‐time” autocorrelation method. Changes in all these quantities due to polymer injection were found to depend on the amount of local drag reduction at that particular downstream station but were independent of the local polymer concentration at the measuring point.

Measurements of turbulence structure in the boundary layer on a rough surface

Mean products of velocity fluctuations up to fourth order have been measured in a wind tunnel at the trailing edge of a flat plate, one side of which was covered with floor-sanding paper to produce a fully rough surface. This set-up permits easy comparison of structural parameters in smooth-wall and rough-wall boundary layers. The Reynolds-stress profiles and second-order parameters are closely the same on the rough and smooth surfaces; in particular the decrease in Reynolds shear stress near the rough surface, encountered by several other laboratory workers, was not found in the present results. The triple products are spectacularly altered for a distance of up to 10 roughness heights from the rough surface, and imply a large net rate of transport of turbulent energy and shear stress towards the surface. Comparison with other published data shows that the behaviour of this modified region depends on roughness geometry as well as on the roughness height itself; for example, the mean cube of the normal-component fluctuation remains positive (energy transport away from the surface) over sand or gravel roughness but goes negative, like the other energy-transport terms, over crop canopies.

Full-coverage film cooling. Part 1. Three-dimensional measurements of turbulence structure

Hydrodynamic measurements were made with a triaxial hot wire in the full-coverage region and the recovery region following an array of injection holes inclined downstream, at 30° to the surface. The data were taken under isothermal conditions at ambient temperature and pressure for two blowing ratios: M = 0·9 and M = 0·4. (The ratio M = ρjetUjet/ρ∞U∞, where U is the mean velocity and ρ is the density. Subscripts jet and ∞ stand for injectant and free stream, respectively.) Profiles of the three mean-velocity components and the six Reynolds stresses were obtained at several spanwise positions at each of five locations down the test plate.In the full-coverage region, high levels of turbulence kinetic energy (TKE) were found for low blowing and low TKE levels for high blowing. This observation is especially significant when coupled with the fact that the heat transfer coefficient is high for high blowing, and low for low blowing. This apparent paradox can be resolved by the hypothesis that entrainment of the mainstream fluid must be more important than turbulent mixing in determining the heat transfer behaviour at high blowing ratios (close to unity).In the recovery region, the flow can be described in terms of a two-layer model: an outer boundary layer and a two-dimensional inner boundary layer. The inner layer governs the heat transfer.

Survey and new measurements of turbulent structure near the wall

A survey of recent measurements of turbulent structure near the wall which are applicable to the phenomenon of drag reduction by polymer additives is presented. Knowledge of the vorticity in the wall region provides a framework for understanding turbulent structure and the effects of polymer additives. New measurements using a small, calibrated streamwise vorticity probe are described. Near the wall where the turbulent production and dissipation are a maximum the streamwise vorticity is a maximum and is highly intermittent. New measurements of the Reynolds stress have also been made with a very small, computer calibrated hot-wire array. Intermittent small scale structures producing rapid oscillations of the hot-wire signals are detected near the wall which are too small to be resolved by the array. Their spatial scale is much less than the Kolmogoroff length. It is suggested that they are associated with streamwise vorticity.