The Measurement of Turbulence with the Laser-Doppler Anemometer

Mesure a l'aide d'un anemometre laser Doppler des vitesses moyenne et quadratique moyenne dans un ecoulement d'eau

Measurements in turbulent swirling flow through an abrupt axisymmetric expansion

The effects of concave curvature on turbulent boundary-layer structure are investigated, using flow visualization and two-component laser-Doppler anemometry. Destabilizing curvature amplifies large-scale motions normal to the wall. When the boundary layer entering the curve is free of spanwise non-uniformities, the resulting large-eddy structure does not consist of distinct longitudinal vortices, as suggested by some previous studies. Rather, the visualized flow is dominated by large eddies (inflows and outflows) that have a streamwise extent of only a few boundary-layer thicknesses, are quite unsteady, and do not cause significant spanwise variations in the mean properties of the boundary layer. Mixing across the boundary layer is enhanced by the new eddy structure, bringing high-momentum fluid closer to the wall than in a normal, flat boundary layer and causing a significant increase in skin friction. Spectral results show that increases in turbulence intensities and Reynolds shear stress across the outer layer are due almost entirely to increased energy in low-frequency, large-scale fluctuations.Flow visualization suggests that the large-scale inflows and outflows have strong influence on flow structure in the near-wall region. However, when the local value of the friction velocity, uτ, is used for scaling, near-wall profiles of Reynolds-averaged quantities show relatively minor differences between the flat and concave cases.The response of the boundary layer to the sudden onset of concave curvature is found to involve two overlapping stages. First, a centrifugal mechanism causes higher-velocity eddies near the start of curvature to migrate toward the wall, while lower-velocity eddies migrate away from the wall. These negatively correlated motions produce an increase in the magnitude of the correlation coefficient within a few initial boundary-layer thicknesses (δ0) from the start of curvature. The further development of the layer requires the slower growth and amplification of the large-scale inflows and outflows. This development of the new large-scale eddy structure continues for at least 20 δ0.

Structure of a turbulent boundary layer on a concave surface

This state of-the-art review is published in two parts: Part 1 has already appeared and deals with basic aspects of the subject, while this part sets out details of specific drag reduction applications. These include oil pipelines, oil well fracturing, sewers, open channels, hydropower penstocks and culverts, the hydrotransport of pulverised solids, the effect of drag reducing additives on fluid machinery, marine applications, their use in heating systems, firefighting and jet cutting. The preparation and injection of polymer solutions, the economics of their use, their degradation in flow and the pollution implications of their release into water courses are aslo covered. Wherever possible field test results are included and discussed. The bibliography contains 154 references to published material.

The Effect Of Drag Reducing Additives On Fluid Flows And Their Industrial Applications Part 2: Present Applications And Future Proposals*

This paper reports the experimental results from a detailed investigation of the turbulent flowfields in supersonic shock-wave/boundary-layer interactions at Af=2.25. The interactions were produced by twodimensional compression corners having angles of 8, 13, and 18 deg, the flows being respectively attached, incipiently separated, and separated. Turbulence data from a laser velocimeter and a constant-temperature, hotwire anemometer are presented along with a mean flow survey from wall and pitot-static pressures. Qualitative aspects of the turbulence deduced from spectral analysis and high-speed schlieren pictures are also discussed. It is shown that a large amount of the turbulent energy is contained in large-scale structures that are still observed downstream of the interacting region, despite the severe pressure gradient. The lateral extent of these structures is of the order of one boundary-layer thickness and is roughly half of their longitudinal scale. A low-frequency unsteadiness is associated with the existence of a separation bubble, but does not affect the rest of the flow. The Reynolds shear stress is reduced in the vicinity of the separation bubble where the spatial derivatives of the normal stresses become significant.

The Structure of Turbulence in a Supersonic Shock-Wave/Boundary-Layer Interaction

The biasing of an ensemble of individual realization laser Doppler anemometer data caused by both spatial and temporal variation of the velocity in the probe volume has been analyzed. The analysis shows that the timeand space-average velocity through the probe volume can be calculated with a weighted ensemble of data. Likewise, the root-mean-square velocity in the probe volume can also be calculated with a weighted average. However, the velocity variance in the data ensemble originates from three sources: velocity fluctuations caused by turbulence at the center of the probe volume; spatial variations in the mean velocity across the probe volume; and spatial variations in velocity fluctuations across the probe volume. Since the root-mean-square velocity at the center of probe volume is generally the parameter of interest, the other two sources of velocity variance must be subtracted from the measured value. The results of the analysis are used to reduce LDA data taken in the viscous sublayer of a channel flow of water. Without correction, the mean velocity estimates can be as much as 109/o too high while the root-mean-square estimates are as much as 100% high. The proposed correction techniques brings the LDA data into good agreement with previous hot-film data.

Effect of Finite-Size Probe Volume upon Laser Doppler Anemometer Measurements

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Experimental evaluation of sampling bias in individual realization laser anemometry

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Prediction of heat transfer coefficients in drag reducing turbulent pipe flows

The purpose of this study was to determine the preliminary feasibility for reducing the viscous drag in dredge spoil pipe flows by the addition of a high molecular weight, long-chain polyacrylamide polymer. Flow-rate pressure drop tests were conducted on fresh water, dredge spoil samples, clay slurries, and salt water with and without polymer additions. Results demonstrated that as much as 70% drag reduction can be achieved with the addition of a high molecular weight, long-chain polyacrylamide polymer to dredge spoils; and that for a given pressure drop, the flow rate of spoil can be increased. However, the amount of drag reduction decreases and the shear velocity at which 40% drag reduction occurs increases as the total solids content of the spoil increases. Several additional parameters—the effect of particle size, spoil viscosity, pipe diameter, polymer concentration, polymer type, and polymer degradation—need further study.

Drag Reduction in Dredge-Spoil Pipe Flows

The purpose of this study was to experimentally determine the feasibility of using hydrocyclone separators for the concentration, clarification, and classification of a dredge spoil. Six dredge spoil samples (supplied by the United States Army Corps of Engineers), two clay slurries, and one sand were used to determine the effect of particle size, viscosity of the fluid, and inlet solids concentration on the effectiveness of 10-gpm hydrocyclones. While the clarification and concentration performance of the hydrocyclones was good on low solids content clay slurries, the performance ranged from below average to poor on the spoil samples. The poor capability of the separators to clarify and concentrate these spoils was due to a combination of high solids content, small particle sizes, and highly pseudoplastic behavior of the spoils. The high solids content affects clarification more than it does concentration, and thus, hydrocyclones may be reasonable effective concentrators for the low solids content spoils. Meanwhile, the hydrocyclone proved very successful at recovering sand from the full range of spoils.

William G. Tiederman

Papers

Effect of Finite-Size Probe Volume upon Laser Doppler Anemometer Measurements

Experimental evaluation of sampling bias in individual realization laser anemometry

Prediction of heat transfer coefficients in drag reducing turbulent pipe flows

Drag Reduction in Dredge-Spoil Pipe Flows

Feasibility of Hydrocyclones for Dredge Spoil