Components Of Turbulent Kinetic Energy Research Articles

Mid-Channel Braid-Bar-Induced Turbulent Bursts: Analysis Using Octant Events Approach

In a laboratory, a model of a mid-channel bar is built to study the turbulent flow structures in its vicinity. The present study on the turbulent flow structure around a mid-channel bar is based on unravelling the fluvial fluxes triggered by the bar’s 3D turbulent burst phenomenon. To this end, the three-dimensional velocity components are measured with the help of acoustic doppler velocimetry (ADV). The results indicate that the transverse component of turbulent kinetic energy cannot be neglected when analyzing turbulent burst processes, since the dominant flow is three-dimensional around the mid-channel bar. Due to the three-dimensionality of flow, the octant events approach is used for analyzing the flow in the vicinity of the mid-channel bar. The aim is to develop functional relationships between the stable movements that are modelled in the present study. To find the best Markov chain order to present experimental datasets, the zero-, first-, and second-order Markov chains are analyzed using the Akaike information criterion (AIC) and the Bayesian information criterion (BIC). The parameter transition ratio has evolved in this research to reflect the linkage of streambed elevation changes with stable transitional movements. For a better understanding of the temporal behaviors of stable transitional movements, the residence time vs. frequency graphs are also plotted for scouring as well as for depositional regions. The study outcome herein underlines the usefulness of the octant events approach for characterizing turbulent bursts around mid-channel bar formation, which is a precursor to the initiation of braiding configuration.

Numerical investigations of interface instability and turbulent mixing driven by implosion

The interface instability and turbulent mixing of perturbed Air/SF6 interface driven by implosion in spherical convergent geometry are numerically investigated by the use of in-house compressible hydrodynamic code (multi-viscous flow and turbulence). The results revealed the complex evolving laws and physical mechanisms of interface instability and turbulent mixing due to the evolution of complex waves in this case. The RM instability is induced when the perturbed Air/SF6 interface is driven by the incident shock wave. And the perturbed interface accelerates toward the center, thereafter RT instability occurs. Then the perturbed interface slows down and decelerates towards the center, and the RT stabilization appears and suppresses the development of the interface instability and turbulent mixing. When the RT stabilization acts an absolutely dominant role, the growth of the turbulent mixing zone (TMZ) width is restrained completely, and its width in turn reduces. After the rebound of transmitted shock waves, the secondary loading from heavy fluid to light fluid is a combination of quasi-isentropic ramp wave, shock wave and Taylor wave in time series. The loading of quasi-isentropic ramp wave also contributes to the RT stabilization mechanism, the drive of rebound shock wave was seen to have induced the RM instability too, and the impact of Taylor wave resulted in RT instability once again. In the following, the reflected wave moves toward the center and bounces, the third loading process is the same as the secondary one; the TMZ width repeats the same growth laws. The Bell-Plesset (BP) effect which belongs to the conergent geometry can promote the development of TMZ. In the spherical converegent geometry, the competition mechanism among the RM instability, RT instability, BP effect and RT stabilization controls the evolution of interface instability and turbulent mixing. The distributions of turbulent kinetic energy indicate that TMZ evolves asymmetrically along the radial direction. The distributions of three direction components of turbulent kinetic energy and energy spectra also exhibit the evolution of TMZ as strong anisotropy.

Turbulent flow characteristics in a randomly packed porous bed based on particle image velocimetry measurements

An experimental study was undertaken to better understand the turbulent flow characteristics within a randomly packed porous bed. A relatively low aspect ratio bed (bed width to spherical solid phase particle diameter of 4.67) with the fluid phase refractive index matched to that of the solid phase was used to obtain time resolved particle image velocimetry data. Care was taken to assure that data were outside of the wall affected region, and results are based on detailed time dependent velocity vector maps obtained at selected pores. In particular, four pores were identified that display a range of very disparate mean flow conditions which resemble channel-like flow, impinging flow, recirculating flow, and jet like flow. Velocity data were used for a range of pore Reynolds numbers, Repore, from 418 to 3964 to determine the following turbulence measures: (i) turbulent kinetic energy components, (ii) turbulent shear production rate, (iii) integral Eulerian length and time scales, and (iv) energy spectra. The pore Reynolds number is based on the porous bed hydraulic diameter, DH = ϕDB/(1 − ϕ) where ϕ is bed porosity and DB is solid phase bead diameter and average bed interstitial velocity, Vint = VDarcy/ϕ, where VDarcy = Q/Abed, with Q being the volumetric flow rate and Abed the bed cross section normal to the flow. Results show that when scaled with the bed hydraulic diameter, DH, and average interstitial velocity, Vint, these turbulence measures all collapse for Repore, beyond approximately 2800, except that the integral scales collapse at a lower value near 1300–1800. These results show that the pore turbulence characteristics are remarkably similar from pore to pore and that scaling based on bed averaged variables like DH and Vint characterizes their magnitudes despite very different mean flow conditions.

Effect of High-Amplitude Forcing on Turbulent Combustion Intensity and Vortex Core Precession in a Strongly Swirling Lifted Propane/Air Flame

The present work reports stereoscopic particle image velocimetry (PIV) measurements in a strongly swirling nonreacting jet and partially premixed lifted flame. The spatial distributions of the average velocity and components of turbulent kinetic energy were calculated from the measured ensembles of the instantaneous velocity fields. A pronounced bubble-type vortex breakdown was observed for the studied flows. Based on proper orthogonal decomposition (POD) of the PIV data and on estimates of velocity fluctuation spectra by a laser Doppler velocimetry (LDV) probe, it was concluded that the combustion did not fundamentally affect the type of coherent structures in the strongly swirling flow: a pair of secondary helical vortices was induced by a precessing vortex core in both cases. Because a strongly swirling jet flow is usually insensitive to weak forcing, strong perturbations were superimposed on the flow bulk velocity to force the formation of ring-like vortices in the flow and to investigate the possible outcomes on the turbulent combustion process. The forcing frequency was below that of the precession. Based on the CH* chemiluminescence signal, it was observed that the forcing provided an increase in the turbulent combustion rate near the flame onset, as the entrainment of ambient air to the rich mixture must have increased. Moreover, for a forcing amplitude typically above the magnitude of reverse flow inside the bubble-type recirculation zone, a dramatic suppression of the vortex core precession took place in the reacting case. This effect was accompanied by a quasi-periodical vanishing of the recirculation zone due to interaction of the forced ring-like vortices with the lifted flame.

Disruption of the bottom log layer in large-eddy simulations of full-depth Langmuir circulation

AbstractWe report on disruption of the log layer in the resolved bottom boundary layer in large-eddy simulations (LES) of full-depth Langmuir circulation (LC) in a wind-driven shear current in neutrally-stratified shallow water. LC consists of parallel counter-rotating vortices that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. The disruption is analysed in terms of mean velocity, budgets of turbulent kinetic energy (TKE) and budgets of TKE components. For example, in terms of mean velocity, the mixing due to LC induces a large wake region eroding the classical log-law profile within the range $90\lt { x}_{3}^{+ } \lt 200$. The dependence of this disruption on wind and wave forcing conditions is investigated. Results indicate that the amount of disruption is primarily determined by the wavelength of the surface waves generating LC. These results have important implications for turbulence parameterizations for Reynolds-averaged Navier–Stokes simulations of the coastal ocean.

A Comparative Study of Turbulence in Ramp-Up and Ramp-Down Unsteady Flows

DNS of a turbulent channel flow subjected to a step change in pressure gradient are performed to facilitate a direct comparison between ramp-up and ramp-down flows. Strong differences are found between behaviours of turbulence in the two flows. The wall shear stress in the ramp-up flow first overshoots, and then strongly undershoots the quasi-steady value in the initial stage of the excursion, before approaching the quasi-steady value. In a strongly decelerating flow, the wall shear stress tends to first undershoot but then overshoot the quasi-steady value. ‘Slow’ response of turbulence as well as flow inertia is responsible for these behaviours. In the ramp-up flow, the response of turbulence is similar to that observed in uniformly accelerating flows from previous studies, exhibiting a three-stage development. However, the transition between the various stages is more gradual and the responding stage is much longer and slower in the flows considered here. It has been shown that the delay in the near wall region is longer than that in the buffer layer confirming that turbulence response first occurs at the location of peak turbulence production. In a strongly decelerating flow, the response of turbulence exhibits a two-stage development. In both ramp-up and ramp down flows, the energy distribution in the three components of turbulent kinetic energy deviates from that of the steady flow. In a ramp-up flow, more energy is in \(u_1^\prime \) and less in \(u_2^\prime \) and \(u_3^\prime \), whereas the trend is reversed in a ramp-down flow. This is a reflection of the redistribution of turbulence from \(u_1^\prime \) to \(u_2^\prime \) and \(u_3^\prime \).

Flow characterization of a wavy-walled bioreactor for cartilage tissue engineering

Cartilage tissue engineering requires the use of bioreactors in order to enhance nutrient transport and to provide sufficient mechanical stimuli to promote extracellular matrix (ECM) synthesis by chondrocytes. The amount and quality of ECM components is a large determinant of the biochemical and mechanical properties of engineered cartilage constructs. Mechanical forces created by the hydrodynamic environment within the bioreactors are known to influence ECM synthesis. The present study characterizes the hydrodynamic environment within a novel wavy-walled bioreactor (WWB) used for the development of tissue-engineered cartilage. The geometry of this bioreactor provides a unique hydrodynamic environment for mammalian cell and tissue culture, and investigation of hydrodynamic effects on tissue growth and function. The flow field within the WWB was characterized using two-dimensional particle-image velocimetry (PIV). The flow in the WWB differed significantly from that in the traditional spinner flask both qualitatively and quantitatively, and was influenced by the positioning of constructs within the bioreactor. Measurements of velocity fields were used to estimate the mean-shear stress, Reynolds stress, and turbulent kinetic energy components in the vicinity of the constructs within the WWB. The mean-shear stress experienced by the tissue-engineered constructs in the WWB calculated using PIV measurements was in the range of 0-0.6 dynes/cm2. Quantification of the shear stress experienced by cartilage constructs, in this case through PIV, is essential for the development of tissue-growth models relating hydrodynamic parameters to tissue properties.

Direct numerical simulation of turbulent flows in a wall-normal rotating channel

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a wall-normal rotating channel with the rotation number N τ from 0 to 0.12 and the Reynolds number 194 based on the friction velocity of the non-rotating case and the half-height of the channel. Based on the present calculated results, two typical rotation regimes are identified. When in weak rotation regime with 0 < N τ < 0.05, turbulence statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of the spanwise mean flow induced by the Coriolis force increases, but other statistics are suppressed. When in strong rotation regime with N τ > 0.05, all the turbulence statistics decrease as the effect of the Coriolis force plays a dominant role. The budgets of transport equations for the Reynolds stresses are calculated to reveal the effect of the Coriolis force on the dynamic process of turbulent kinetic energy production, dissipation and redistribution. With the increase of N τ in weak rotation regime, the main mechanism for the generation of the streamwise turbulent energy is gradually altered from the shear production effect related to the streamwise mean flow to the energy redistribution due to the pressure strain correlation. Correspondingly, the generation of the spanwise turbulent energy also changes from the energy redistribution effect to the shear production of the spanwise mean flow. In strong rotation regime, the mean flow shear rate is found to be a key factor to the turbulence production and dissipation. The redistribution between the streamwise and spanwise components of turbulent kinetic energy due to the effect of the Coriolis force becomes weak. A remarkable change of the direction of the near-wall vortical structures, nearly in alignment with the absolute mean flow direction, is observed. An attempt to evaluate the mean spacing between the streaky structures and the angle between the wall structures and streamwise direction has been examined based on the two-point correlations of the velocity fluctuations to reveal the change of the near-wall structures.

Extension of the k-ε model for the numerical simulation of the melt flow in induction crucible furnaces

Checking the calculations of turbulent melt flow in induction crucible furnaces that were carried out with various modifications of the two-dimensional (2-D)k-e model using experimental findings has shown basic differences in the distributions of the specific generation of the turbulent energy and the kinetic energy of the turbulence. The discrepancies are explained by the distinctive three-dimensional (3-D) character of the pulsations and the low-frequency fluctuations of the macroscopic toroidal eddy; these are not taken into account in the numerical methods mentioned. With the aid of this 3-D model, the additional component of turbulent kinetic energy involved is estimated, and an approximation formula for the low-frequency component of the specific generation of turbulence is given. This results in an extension of the 2-Dk-e model for a recirculated flow with several toroidal eddies, leading to good qualitative agreement of the characteristics of turbulent flow with the experimental findings. Since the numerical simulation—in agreement with industrial practice and the experiments carried out—demonstrates good effective mixing of the entire flow region, there is thus a possibility for the simulation of aterial transport in the melt of induction crucible furnaces as part of the widespread 2-D computation methods.

Dynamic response of boundary-layer turbulence to oscillatory shear

The temporal response of a well-developed turbulent boundary layer to the superposition of oscillatory shear has been measured experimentally, over a wide range of frequencies. The response is primarily a periodic organization in magnitude of components of the turbulent velocity field at the forcing frequency. Oscillatory production of turbulence arises predominantly as a modulation of the mean production process in the parent boundary layer. Close to the wall, the relative phases of response of components of turbulent kinetic energy indicate that temporal redistribution of turbulent kinetic energy is driven by robust coherent motions of the underlying mean flow. The local directions of redistribution deduced from these measurements indicate a wall impingement (splatting) effect, consistent with characterizations from numerical simulation.

A Higher Order Closure Model for Canopy Flow

Abstract The equations of motion were used to develop a one-dimensional, nonbuoyant mathematical model of air flow within vegetative canopies. The model consists of equations for mean horizontal momentum, Reynolds stress, and for the three components of turbulent kinetic energy with closure achieved by parameterizing the higher order terms. This eliminates the need to model the Reynolds stress directly using an eddy viscosity. The closure schemes rely upon a prescribed length scale and have been used elsewhere in modeling the atmospheric boundary layer free of vegetation. The equations were solved numerically using specified boundary conditions. Using a profile of plant area density for a crop of corn (Zea mays L.) the model predicted mean wind velocity, Reynolds stress and turbulent intensities for the region from the soil surface to twice the canopy height that compare well with experimental measurements (Shaw et al., 1974a,b). The model is believed to overestimate the intensity of turbulence generated ...

Secondary Wind Speed Maxima Inside Plant Canopies

It is noted that wind profiles measured in forest and crop canopies normally contain a secondary maximum or a region of very small shear beneath the level of greatest foliage density. One-dimensional models utilizing a turbulent transport coefficient cannot predict a reversed velocity gradient and, as a result, profile analyses normally imply coefficients that are unrealistic or nonsensical. Examination of the equation for the local rate of change of Reynolds stress u′w′ shows that the velocity gradient can reverse in sign if the divergence of the turbulent transport of stress is of opposite sign and exceeds in magnitude the pressure-velocity gradient correlation. Direct measurements of the turbulent transport of u′w′ in corn (Zea mays L.) indicate that its value is considerably larger than in the air layers above and show that stress is transported downward from the upper parts of the vegetation. A one-dimensional model of canopy flow which solves the equations for momentum, Reynolds stress and the three components of turbulent kinetic energy, without relating the stress to the mean velocity gradient, predicts a weak secondary maximum in the wind profile for a corn canopy.

A computer study of an analytical model of boundary-layer transition.

The results of a computer study of a new analytical model of transition from laminar to turbulent flow are presented. The model is two-dimension al in the mean flow but threedimensional in the disturbance flow. In particular, an onset of transition boundary is derived for the case of fairly large disturbances introduced into a laminar flat-opiate boundary layer at approximately the critical layer. The nature of the upper and lower branches of this onset of transition map is related to the physical processes going on in the modeled boundary layer. It is found that the upper branch of this curve is defined by a balance of dissipative effects and tendency-towards-isotropy effects as they affect the normal component of turbulent kinetic energy in the boundary layer. The lower branch is denned by a balance of diffusive effects and tendency-towards-isotropy effects on the same normal velocity correlation. Within the framework of these observations, the effect of a favorable pressure gradient on transition is discussed. It is suggested that the conventional approach to computing the stability boundaries of laminar flows in the presence of pressure gradient is incorrect in that it does not account for stream tube stretching effects that are shown to be important in a region near the stagnation point of blunt bodies. Finally the nature of a complete transition, as defined by the model, is discussed, and several suggestions for future research are put forth.