The Structure of Turbulent Shear Flow By Dr. A. A. Townsend. Pp. xii + 315. 8¾ in. × 5½ in. (Cambridge: At the University Press.) 40s.

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The Structure of Turbulent Shear Flow

[1] Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere – is evident in lower-tropospheric temperatures and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north-south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper-level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea-ice loss, but are also apparent in summer, possibly related to earlier snow melt on high-latitude land. Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.

X-ray imaging and analysis techniques for quantifying pore-scale structure and processes in subsurface porous medium systems

Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.

Small-scale hydrodynamics in lakes

Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments.

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A review of acoustic measurement of small-scale sediment processes

A method is proposed to reduce the noise contribution to mean turbulence parameters obtained by 3D acoustic Doppler velocity profiler measurements. It is based on a noise spectrum reconstruction from cross-spectra evaluations of two independent and simultaneous measurements of the same vertical velocity component over the whole water depth. The noise spectra and the noise variances are calculated and removed for the three fluctuating velocity components measured in turbulent, open-channel flow. The corrected turbulence spectra show a −5/3 slope over the whole inertial subrange delimited by the frequency band of the device, while the uncorrected turbulence spectra have flat high-frequency regions typical for noise effects. This method does not require any hypothesis on the flow characteristics nor does it depend on device-dependent parameters. The corrected profiles of turbulence intensities, turbulent kinetic energy, shear stress, and turbulent energy balance equation terms, such as production, t...

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A Correction Method for Turbulence Measurements with a 3D Acoustic Doppler Velocity Profiler

Mean flow properties and turbulent characteristics of uniform open-channel flow were experimentally studied in a reasonably steep channel with an erodible gravel bed. A recently developed Acoustic Doppler Velocity Profiler (ADVP) was used to obtain instantaneously the information on the flow profiles. From these measurements, the mean velocities, the turbulence-intensities as well as the Reynolds-stress profiles were obtained. The experimental results show that: i) the mean velocity profile can be expressed by the log-law with Br ≃ 8.42 in the inner region; Coles' law can be used to explain the velocity profiles in the entire region with Π ≃ 0.08; ii) the flow resistance coefficient,f, is explained with a theoretical relation; iii) the horizontal turbulence- intensity distribution agrees well with the exponential relation; iv) the vertical turbulence-intensity distribution agrees with the exponential relation only in the outer region; in the inner region, it curves backwards; v) the Reynolds stress are li...

Uniform flow in open channels with movable gravel bed

A monostatic acoustic Doppler velocity profiler (ADVP) for use in hydraulic research is presented including signal treatment algorithms. The 1-MHz ADVP used in the present study has an accuracy better than 2 mms˙s−1 with a range of up to 100 cm for velocities up to 290 cms˙s−1 and a resolution of 256 points in the profile. Results of ADVP measurements in uniform flow in two laboratory open channels with smooth and gravel beds compare favorably with established distribution laws for mean velocity, variance, and Reynold's stress, indicating that the turbulence scales are sufficiently resolved by the instrument. Measurements with the same instrument in a river and a lake illustrate that it is capable of producing excellent results in the field as well. It was demonstrated that, unaffected by water quality, the instrument is a good research tool for nonintrusive and accurate instantaneous profile measurements under turbulent flow conditions.

Acoustic Velocity Profiler for Laboratory and Field Studies

https://hal.archives-ouvertes.fr/hal-00614448/document

A multi-frequency Acoustic Concentration and Velocity Profiler (ACVP) for boundary layer measurements of fine-scale flow and sediment transport processes

[1] During the winters of 1998 and 1999, observations were made of the cascading of cold water from the nearshore, shallow "shelf'' zones and down the sloping sides of Lake Geneva. Cascading starts on the average 10 hours after the onset of surface cooling. The draining cold water descends like a gravity current, and the downslope speed of the head of these slugs of cold water, U, has a mean value of 5.2 cm s(-1), with slugs persisting, on the average, for 8 hours. When the Monin-Obukov length scale at the water surface, L, is negative, implying convection occurs, and (d) over bar/\Lg 1, where d is the mean shelf depth, the nondimensionalized speed of the front of "slugs,'' U/b(1/3) is found to be 1.3 +/- 0.4, where b is the surface buoyancy flux integrated over the time period from one slug to the next. Each slug is unsteady, the head being followed by several fronts in which the temperature of the current decreases and its thickness increases. These fronts travel faster than the mean flow by a factor of r = 1.38 +/- 0.3. Dynamical similarities are found with roll waves observed in turbulent open channel flows. The circulation induced by the cascade is found to give a positive skewness to the time derivatives of near-surface temperature in shallow waters, in contrast with negative values close to the slope. The volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface. The average volume carried by the slug across the 21 m depth contour is about 1.9 times the volume of water in shallower water (i.e., that on the shelf between shore and a depth of 21 m), implying that cascading is an efficient means of flushing shelf water. Integrated around the lake the mean total volume flux amounts to 11.5 the average winter river inflow.

https://folk.uib.no/ngfif/Reprints/JGR_02_cascade.pdf

Ulrich Lemmin

Papers

A Correction Method for Turbulence Measurements with a 3D Acoustic Doppler Velocity Profiler

Uniform flow in open channels with movable gravel bed

Acoustic Velocity Profiler for Laboratory and Field Studies

A multi-frequency Acoustic Concentration and Velocity Profiler (ACVP) for boundary layer measurements of fine-scale flow and sediment transport processes

Winter cascading of cold water in Lake Geneva