Maintenance Of Turbulence Research Articles

Quantitative experimental research on vortex generation and self-maintenance mechanisms in turbulence

Turbulence is not completely random, but contains organized and multi-scale structures. Vortices have always been recognized as the most important coherent structure in turbulent flow, playing a significant role in the generation, evolution, and maintenance of turbulence. In the present work, the vortex formation and evolution process of a fully developed turbulent boundary layer in a rectangular channel flow is experimentally studied by moving-single frame and long exposure and moving-particle image velocimetry measurements. The Rortex integral (RI) method is proposed for quantitative statistics on the critical vortex core size as well as the accurate rotation strength during the evolution process. In this paper, the vortex regeneration and self-maintenance mechanisms in near-wall turbulent flow are experimentally revealed and quantified by the RI method, to give some revelations for the future research on the turbulence theory. On the one hand, three behaviors of the hairpin vortex regeneration are discovered to play a significant role in turbulence development: (A) hairpin regeneration induced by the interaction between hairpins and packets; (B) auto-generation of multiple hairpin vortex in one packet (secondary and tertiary hairpins); and (C) the merging of hairpin vortices in the packets. On the other hand, the circulation process, which contains a mass of young vortex growth with the parent self-decaying, is verified to sustain and promote the development of turbulence. In consequence, the self-sustaining turbulence theory based on mother–child hairpins generation mechanism is supported by the experimental results.

Compression of turbulent magnetized gas in giant molecular clouds

Interstellar gas clouds are often both highly magnetized and supersonically turbulent, with velocity dispersions set by a competition between driving and dissipation. This balance has been studied extensively in the context of gases with constant mean density. However, many astrophysical systems are contracting under the influence of external pressure or gravity, and the balance between driving and dissipation in a contracting, magnetized medium has yet to be studied. In this paper we present three-dimensional (3D) magnetohydrodynamic (MHD) simulations of compression in a turbulent, magnetized medium that resembles the physical conditions inside molecular clouds. We find that in some circumstances the combination of compression and magnetic fields leads to a rate of turbulent dissipation far less than that observed in non-magnetized gas, or in non-compressing magnetized gas. As a result, a compressing, magnetized gas reaches an equilibrium velocity dispersion much greater than would be expected for either the hydrodynamic or the non-compressing case. We use the simulation results to construct an analytic model that gives an effective equation of state for a coarse-grained parcel of the gas, in the form of an ideal equation of state with a polytropic index that depends on the dissipation and energy transfer rates between the magnetic and turbulent components. We argue that the reduced dissipation rate and larger equilibrium velocity dispersion produced by compressing, magnetized turbulence has important implications for the driving and maintenance of turbulence in molecular clouds, and for the rates of chemical and radiative processes that are sensitive to shocks and dissipation.

Numerical evaluation of the low Reynolds turbulent flow behaviour in a bioreactor

Bioreactors are commonly used in industry, owing to their low energy requirement. These reactors are often operated at reduced flow rates in order to maintain a high retention time, and reduce shear stress on the biomass. An important factor is the mixing between biomass and substrate, and thus the maintenance of turbulence is important. Hence, the aim of this work is the application of the CFD technique to obtain a better understanding of the turbulent behaviour in an anaerobic sequencing batch reactor, with a nominal Reynolds number of about 1,700. This study considered a single-phase approach to evaluate several turbulence models. The obtained results indicate that the flow has little influence on the regions in the transition regime for the conditions evaluated, in spite of the low Reynolds number. Moreover, comparing the use of different models, it was found that the Reynolds stress model does not require longer time for its calculations.

Role of Vorticity in Generation of Pressure Sources and its Implication for Maintenance of Turbulence

The Poisson equation for pressure, together with the evolutions equations for the velocity gradients, reveals the role of vorticity in generation of pressure sources. Specifically, it was shown how a pressure field created by a local source, acting on nearby vorticity, would create new pressure sources. It was further es- tablished that a moving pressure field, which moves with the velocity of its source, but extends well beyond the source location, could lead to generation of fast and slow streaks as wells as contribute to formation of flow structures in the wall region. These processes, which are part of central mechanisms of maintenance of turbulence, suggest that turbulence could be self-sustaining only if the perturbation pressure force could overcome the diffusion effects; the value of friction Reynolds number reflects the balance between the two.

Role of land surface parameterizations on modeling cold-pooling events and low-level jets

Land surface parameterization schemes play a significant role in the accuracy of meso-local scale numerical models by accounting for the exchange of energy and water between the soil and the atmosphere. The role of land surface processes during large-scale cold-pooling events was studied with two land surface schemes (LSMs) in the Advanced Research Weather Forecasting model (ARW). Model evaluation was complex due to the surface and boundary layer interactions at different temporal and spatial scales as revealed by a scale dependent variance analysis. Wavelet analysis was used for the first time to analyze the model errors with specific focus on land surface processes. The ARW model was also evaluated for the formation of a low-level jet (LLJ). It is shown that vertical resolution in the model boundary layer played a significant role in determining the characteristics of LLJ, which influenced the lower boundary layer structure and moisture distribution. The results showed that the simulated low-level jet over southern Georgia was sensitive to the land surface parameterization and led to a significant difference in the boundary layer exchange. The jet shear played a crucial role in the maintenance of turbulence and weak shear caused excessive radiative cooling leading to unrealistic cold pools in the model. The results are important for regional downscaling as the excessive cold pools that are simulated in the model can go unnoticed.

Variability and Maintenance of Turbulence in the Very Stable Boundary Layer

The relationship of turbulence quantities to mean flow quantities, such as the Richardson number, degenerates substantially for strong stability, at least in those studies that do not place restrictions on minimum turbulence or non-stationarity. This study examines the large variability of the turbulence for very stable conditions by analyzing four months of turbulence data from a site with short grass. Brief comparisons are made with three additional sites, one over short grass on flat terrain and two with tall vegetation in complex terrain. For very stable conditions, any dependence of the turbulence quantities on the mean wind speed or bulk Richardson number becomes masked by large scatter, as found in some previous studies. The large variability of the turbulence quantities is due to random variations and other physical influences not represented by the bulk Richardson number. There is no critical Richardson number above which the turbulence vanishes. For very stable conditions, the record-averaged vertical velocity variance and the drag coefficient increase with the strength of the submeso motions (wave motions, solitary waves, horizontal modes and numerous more complex signatures). The submeso motions are on time scales of minutes and not normally considered part of the mean flow. The generation of turbulence by such unpredictable motions appears to preclude universal similarity theory for predicting the surface stress for very stable conditions. Large variation of the stress direction with respect to the wind direction for the very stable regime is also examined. Needed additional work is noted.

On hydrodynamic shear turbulence in stratified Keplerian disks: Transient growth of small-scale 3D vortex mode perturbations

This is a sequel to Paper I (Chagelishvili et al. 2003), where we presented the so-called bypass concept for the onset of turbulence in shearing flows. According to this concept, which was worked out during the last decade by the hydrodynamic community for spectrally stable flows, vortical perturbations undergo transient growth by extracting energy from the shear (a linear process), thereby reaching an amplitude which is sucient to allow for non-linear interactions which, by positive feedback, sustain turbulence. In Paper I we described this transient growth for 2D perturbations in a Keplerian disk; we showed that their kinematics was the same as in plane-parallel flow, and thus that they were not modified by the presence of the Coriolis force. In the present paper, we pursue our goal of applying the bypass scenario to astrophysical disks: we investigate the linear dynamics of 3D small-scale vortical perturbations for single spatial harmonics, in stably stratified, di erentially rotating disks, again in the framework of a nonmodal analysis. We find that these 3D perturbations also undergo substantial transient growth, and that they reach a peak amplitude that is comparable to that of 2D perturbations, as long as their vertical scale remains of the order of the azimuthal scale. When the vertical wave-number exceeds the azimuthal one, the amplification rate is reduced, but this may be more than compensated to by the huge Reynolds number and the high shear rate characterizing astrophysical Keplerian disks. Whereas in 2D the Coriolis force had no impact on the transient growth, in 3D this force somewhat constricts the characteristics of the perturbation dynamics in disk flows, and the initial transient growth is followed by some reduction in amplitude. These dierences are quantitative, rather than of fundamental character. But the 3D case presents two interesting novelties. In plane parallel flow, the perturbations do not decay after their transient amplification, but their energy stays on a plateau before being dissipated through viscous friction. More importantly, especially for the astrophysicist, in disk flow the 3D vortex mode perturbations excite density-spiral waves, whose energy also settles on a plateau before viscous dissipation. These local vortex mode perturbations fit naturally into the bypass concept of hydrodynamic shear turbulence, which was first developed for plane-parallel flows. We submit that these perturbations will also play an important role in the onset and in the maintenance of turbulence in Keplerian disks.

Observations of turbulent boundary-layer interaction with a thunderstorm outflow ? A possible wake region energy source

Laboratory and numerical model simulations of turbulent circulations within the wake regions of thunderstorm outflows have been done with the assumption that there is no turbulence within the ambient airmass. Furthermore, many observational studies have used Doppler radar data that have been filtered so that turbulent structures are reduced in amplitude or eliminated altogether. This study presents unique Doppler radar observations of the collision of a roll-like boundary-layer circulation with a gust flow. The boundary-layer circulation is seen to interact with the circulation within the gust flow head and to reappear within the wake region. It is suggested that the ambient boundary layer may be an energy source for the generation and/or maintenance of turbulence in the wake region.

Turbulent spots in plane Poiseuille flow — Measurements of the velocity field

An experimental study on the development of turbulent spots in plane Poiseuille flow at a Reynolds number of 1600 has been carried out with the aim of achieving a better understanding of the transition to and maintenance of turbulence at low Reynolds numbers. Spots were triggered by a loudspeaker-induced jet of high velocity. The initial disturbance was found to undergo a first stage of rapid expansion, in which sharp internal shear layers form at locations away from the symmetry plane and precede the transition to turbulence. After this initial stage, a nearly self-similar structure develops with the typical features of a turbulent spot. The general features, turbulent properties, and spanwise spreading of the spot were investigated and compared both to previous experimental data and to numerical simulations. High-frequency fluctuations are absent at the front of the spot, whereas the turbulence is apparently self-sustained at the rear and displays features similar to fully turbulent Poiseuille flow at much higher Reynolds numbers. The waves accompanying the wing tips of the spot extend well within the spot, and reach amplitudes far in excess of those previously found outside the spot. The high level of random fluctuations in this part of the spot indicates that the breakdown of the waves is important for the spanwise propagation of the turbulent region.

On the generation and maintenance of turbulence in the interstellar medium

view Abstract Citations (100) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the generation and maintenance of turbulence in the interstellar medium. Fleck, R. C., Jr. Abstract The broad spectrum of turbulent motions observed in the interstellar medium may be produced by the shearing action of differential galactic rotation. A steady state eddy distribution is maintained as energy cascades down the eddy hierarchy to smaller-scale motions. The characteristic decay time for interstellar turbulence is found to be 50-billion yr. Objections frequently raised against the presence of long-lived turbulent motions in the interstellar medium are therefore invalid, since these arguments usually presuppose that there is no source of fresh turbulent energy. Publication: The Astrophysical Journal Pub Date: June 1981 DOI: 10.1086/183573 Bibcode: 1981ApJ...246L.151F Keywords: Galactic Rotation; Interstellar Matter; S Waves; Turbulent Flow; Vortices; Eddy Viscosity; Flow Characteristics; Kolmogoroff Theory; Steady State; Astrophysics full text sources ADS |

Further comments on ?radiation, evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere?

Further comments on ?radiation, evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere?

Comments on ?radiation, evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere?

Comments on ?radiation, evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere?

Radiation, evaporation and the maintenance of turbulence under stable conditions in the lower atmosphere

An expression is derived relating the critical flux Richardson number with the critical (gradient) Richardson number. In contrast to an earlier analysis by Townsend (1958), which is restricted to the atmosphere well outside the earth's boundary layer, the present treatment is intended specifically for turbulent flow in the lower atmosphere and it takes account of the effect of evaporation on the stability. The effect of radiation on the rate of destruction of the mean square of the temperature fluctuations is obtained by considering the radiative flux divergence in a stratified atmosphere and by using a simple functional relationship to represent empirical emissivity data.

A visual investigation of the wall region in turbulent flow

The objective of this study is to investigate for turbulent flow the fluid motions very near a solid boundary, and to create a physical picture which relates these motions to turbulence generation and transport processes. An experimental technique was developed which permitted detailed observations of the regions very near a pipe wall, including the viscous sublayer, without requiring the introduction of any injection or measuring device into the flow. This technique involved suspending solid particles of colloidal size in a liquid, and photographing their motions with a high-speed motion picture camera moving with the flow. To provide greater detail, the field of view was magnified.Fluid motions were observed to change in character with distance from the wall. The sublayer was continuously disturbed by small-scale velocity fluctuations of low magnitude and periodically disturbed by fluid elements which penetrated into the region from positions further removed from the wall. From a thin region adjacent to the sublayer, fluid elements were periodically ejected outward toward the centreline. Often there was associated with these events a zone of high shear at the interface between the mean flow and the decelerated region that gave rise to the ejected element. When the ejected element entered this shear zone, it interacted with the mean flow and created intense, chaotic velocity fluctuations. These ejections and resulting fluctuations were the most important feature of the wall region, and are believed to be a factor in the generation and maintenance of turbulence.