Thermally-stratified Boundary Layer Research Articles

Gustiness in thermally-stratified urban turbulent boundary-layer flows and the influence of surface roughness

Gustiness is examined for the wind speed, fluctuations, turbulence intensities and fluxes for a real urban topography. Using large-eddy simulation (LES) and an ensemble sampling approach, which allows a more comprehensive characterisation of the urban morphological features, a wide range of boundary-layer stabilities is considered: the bulk Richardson number, Rb ​∈ ​[−0.41, 0.82]. Ratios of the proposed gustiness statistics, G, over the conventional time-averaged flow and turbulence statistics are maximised for z/Have ​≲ ​1 (where Have is the mean building height). The strong linear scaling of G with the plan-area index (λp) for neutral stratification is found to persist for stably- and unstably-stratified flows (R2 ~ 0.8). By contrast, the non-dimensionalised building-height variability, σH/Have, and the effective frontal-area index, λf^≡λfHave/σH, are argued to be of more appropriateness as scaling parameters for G compared to their original forms, σH and λf. While the sensitivity of G to Rb is well defined at greater heights, the influence of surface inhomogeneity may be strong enough to oppose the effect of thermal stratification in the lower surface layers. Qualitative differences in the sensitivities to the boundary-layer stability are narrowly distinguishable amongst the zeroth-, first- and second-order gustiness statistics. The results are relevant to the understanding of urban wind hazards in thermally-stratified boundary layers.

J0550204 逆圧力勾配を伴う温度成層境界層の乱流熱伝達特性に関するDNS研究

J0550204 逆圧力勾配を伴う温度成層境界層の乱流熱伝達特性に関するDNS研究

Large-eddy simulation of plume dispersion under various thermally stratified boundary layers

Abstract. Contaminant gas dispersion in atmospheric boundary layer is of great concern to public health. For the accurate prediction of the dispersion problem, the present study numerically investigates the behavior of plume dispersion by taking into account the atmospheric stability which is classified into three types; neutral, stable, and convective boundary layers. We first proposed an efficient method to generate spatially-developing, thermally-stratified boundary layers and examined the usefulness of our approach by comparing to wind tunnel experimental data for various thermal boundary layers. The spreads of plume in the spanwise direction are quantitatively underestimated especially at large downwind distances from the point source, owing to the underestimation of turbulence intensities for the spanwise component; however, the dependence of the spanwise spreads to atmospheric stability is well represented in a qualitative sense. It was shown that the large-eddy simulation (LES) model provides physically reasonable results.

307 前方ステップを通る温度成層境界層の乱流挙動(OS3-2 熱および物質輸送現象とその応用2,OS3 熱および物質輸送現象とその応用)

307 前方ステップを通る温度成層境界層の乱流挙動(OS3-2 熱および物質輸送現象とその応用2,OS3 熱および物質輸送現象とその応用)

F225 DNSによる温度成層を伴う空間発展乱流境界層の研究(乱流・DNS)

This paper presents turbulent structures of thermally-stratified boundary layer using results of direct numerical simulations (DNSs) for a spatially developing boundary layer. In the previous study (Hattori et al., 2007), we indicated fundamental turbulent statistics of boundary layer flow with various thermal stratifications by means of DNS. DNSs show fascinating results, in which the counter-gradient diffusion phenomena (CDP) in turbulent momentum and heat transfer are observed in the strongly-stratified stable boundary layer It is well-known that the counter-gradient diffusion phenomenon disturbs the transport of turbulent quantities due to reductions of the effective diffusivities for momentum and heat. However, it is not yet fully understood how and why CDP occur in strongly-stratified stable turbulent flows. The objectives of this study are to investigate turbulent structure in specially developing thermally-stratified turbulent boundary layers.