Multiple mapping conditioning of velocity in turbulent jet flames

Multiple mapping conditioning (MMC) has emerged as a new approach to model turbulent reacting flows. This study revises the standard MMC closure for velocity in turbulent jet flows from linearity in the reference space to linearity in the composition space. This modeling amendment ensures that the standard velocity model in conditional moment closure studies can now be used for MMC computation as well. A simplified model for the velocity-dependence of MMC drift coefficients is derived without loss of generality and is implemented for the revised velocity closure. Modeling results have been corroborated against the Direct Numerical Simulation database of a spatially evolving, planar turbulent jet flame. The revised model shows marked improvement over standard MMC closure in predicting velocity statistics close to the nozzle.     

The significance of air velocity and turbulence intensity for responses to horizontal drafts in a constant air temperature of 23°C

Aim: The aim was to prove the significance of air velocity and turbulence intensity on the responses to drafts and if air velocity is adequately weighted in the draft rating model (DR-model) proposed in ISO 7730 (1994). Methods: Seventeen healthy persons (9 women, 8 men, 19–51 yr) took part in 12 randomly arranged 1-h sessions where horizontal drafts were directed towards the dorsolateral body sites of the sedentary persons. Mean air velocity was varied in 4 (  m/s) and turbulence intensity in 3 steps (Tu: <30,≈50,>70%) but were kept constant during the single sessions, whereas air velocities were increased every 15 min in the basic experiments performed for the DR-model. Air temperature was 23°C, humidity varied between 40% and 60%. Subjective perception and annoyance due to drafts were registered every 5 min using a list of prescribed body parts and skin temperature was measured continuously throughout the sessions at the forearm and at the neck. Results: Draft-induced general annoyance (if draft-induced annoyance was stated for at least one body site) and draft-induced local annoyance as stated for the neck and for the forearm increased with air velocity and/or with turbulence intensity. The decrease in skin temperature, however, was only related to air velocity but not to turbulence intensity. Air velocities are obviously not accurately weighted in the draft-rating model (ISO 7730, 1994). As compared to the effects observed here, the effects predicted with the DR-model were smaller in case of mean air velocities of 0.3 m/s and less but greater for 0.4 m/s. Concerning rather sedentary persons it seems that drafts are tolerable as long as mean air velocities do not exceed 0.2 m/s and as long as turbulence intensity remains below 30% in air temperature of 23°C.

Relevance to industry

Drafts are the most annoying climatic factor at many workplaces and assumed to reduce satisfaction with work, to impair performance and perhaps even health (Griefahn et al., Ind. Health 38 (2000b) 30). The limitation of drafts is therefore an important contribution to industrial safety. Using the draft-rating model proposed in ISO 7730 (1994) for the evaluation and for the prediction of draft-induced annoyance, it must be considered that the model underestimates the effects of drafts in case of air velocities of 0.3 m/s which prevail at more than 85% of workplaces in offices and in the industry (Griefahn, 1999; Zhou, Ph.D. Thesis, International Center for Indoor Environment and Energy, Department of Energy Engineering, Technical University of Denmark, Lyngby, 1999).     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

Modeling strategies for unsteady turbulent flows in the lower plenum of the VHTR

Validation simulations are presented for turbulent flow in a staggered tube bank, geometry similar to the lower plenum of a gas-cooled high temperature reactor. Steady 2D RANS results are compared to unsteady 2D RANS results and experiment. The unsteady calculations account for the fact that nonturbulent fluctuations (due to vortex-shedding) are present in the flow. The unsteady computations are shown to predict the mean variables and the total shear stress quite well. Previous workers’ results indicate that 3D simulations are needed to obtain reasonable agreement; present results indicate 2D is sufficient. Best practices are based on requirements for the ASME Journal of Fluids Engineering.     

激光多普勒测速仪测量误差的估计和修正

用激光多普勒测速仪（LDA）测量紊流的平均流速时带有系统误差，迄今为止的有关理论研究是不完整的。本导出了一组准确的理论公式，即适用于有方向识别功能的LDA，也适用于无方向识别功能的LDA。章讨论了平均流速的测量误差及紊流度的测量误差，同时还给出了这些误差的修正方法。     

CFD-based multiscale modelling of bubble-particle collision efficiency in a turbulent flotation cell

In this paper, bubble-particle collision efficiency in a turbulent flow is investigated from a multiscale modelling viewpoint. An integrated CFD-based scheme for the prediction of turbulent bubble-particle collision efficiency is developed. As part of this scheme, the effect of turbulence and the bubble wall effect on bubble-particle collision efficiency are systematically studied using a 3D low turbulent Reynolds number shear-stress-transport turbulence model. Example simulations and comparisons are carried out to illustrate the methodology. The method can also be applied to non-Newtonian slurries.     

Experimental study of the flow in helical circular pipes: Torsion effect on the flow velocity and turbulence

An objective of the present paper is to experimentally clarify the torsion effect on the flow in helical circular pipes. We have made six helical circular pipes having different pitches and common non-dimensional curvature δ of about 0.1. The torsion parameter β0, which is defined by β0 = τ/（2δ）1/2 with non-dimensional torsion r, are taken to be 0.02, 0.45, 0.69, 1.01, 1.38 and 1.89 covering from small to very large pitch. The velocity distributions and the turbulence of the flow are measured using an X-type hot-wire anemometer in the range of the Reynolds number from 200 to 20000. The results obtained are summarized as follows： The mean secondary flow pattern in a cross section of the pipe changes from an ordinary twin-vortex type as is seen in a curved pipe without torsion （toroidal pipe） to a single vortex type after one of the twin-vortex gradually disappears as β0 increases. The circulation direction of the single vortex is the same as the direction of torsion of the pipe. The mean velocity distribution of the axial flow is similar to that of the toroidal pipe at small β0, but changes its shape as β0 increases, and attains the shape similar to that in a straight circular pipe when ,β0 = 1.89. It is also found that the critical Reynolds number, at which the flow shows a marginal behavior to turbulence, decreases as ,β0 increases for small ,β0, and then increases after taking a minimum at ,β0 ≈ 1.4 as ,β0 increases. The minimum of the critical Reynolds number experimentally obtained is about 400 at ,β0 ≈ 1.4.     

Rough Wall Boundary Layer on Plates in Open Channels

Experiments were performed to measure the characteristics of a turbulent boundary layer developing on a rough surface placed in an open channel flow at close proximity to the free surface. Streamwise velocity measurements were made with a one-component laser Doppler velocimeter system at the top of the spherical roughness elements. Measurements at three stations downstream of the plate leading edge show the growth of the boundary layer on the rough wall and its interaction with the exterior open-channel flow and the free surface. Resorting to the turbulence profile provides an alternative definition of the boundary layer thickness. The near-wall flow follows the well-known logarithmic law with a shift due to roughness. In the outer layer, there are two opposing effects: the free surface tends to decrease the wake component while the roughness tends to increase it. The streamwise turbulence intensity is affected by the shear and turbulence in the exterior flow, the effect of the free surface being greater than that of wall roughness.     

On the main flow features and instabilities in an unbaffled vessel agitated with an eccentrically located impeller

The hydrodynamics of an unbaffled vessel stirred by an eccentrically located Rushton turbine is investigated with both Laser Doppler Anemometry and flow visualisation techniques. The flow field is shown to be characterised by a strong circumferential motion which develops itself around two main vortices, one above and one below the impeller, both inclined with respect to the vertical plane. Such vortices are not steady but move periodically very slowly in comparison to the impeller rotational timescale. Accordingly, two low frequency components, whose values are linearly dependent on the impeller rotational speed, are identified across the vessel. The energetic contribution to the turbulent kinetic energy of such flow instabilities is significant so that they should be taken into account when evaluating micro-mixing information from turbulence quantities. Besides, an additional low frequency component is observed and related to vortex shedding phenomena from the flow-shaft interaction which occur in eccentric agitation operation. The flow discharged from the impeller is also measured and discussed.     

Preliminary Measurements on Grid Turbulence in Liquid 4He

A grid has been pulled through a column of liquid helium at speeds as high as 1 m/s and at temperatures as low as 90 mK. A 300 micrometer Ge thermometer with response time of less than 1 ms measured the temperature rise resulting from the decay of the turbulence generated. It is believed that homogeneous, isotropic quantum turbulence was formed, since mesh Reynolds numbers in excess of 100,000 were created. The rates and power spectra of the energy increases detected in the helium after grid-pulls are determined. The results are compared to other quantum and classical results, and to the theory of the Kelvin wave cascade in a viscosity-free fluid.