Characterizing evapotranspiration over a meadow ecosystem on the Qinghai-Tibetan Plateau

To characterize evapotranspiration (ET) over grasslands on the Qinghai-Tibetan Plateau, we examined ET and its relevant environmental variables in a Kobresia meadow from 2002 to 2004 using the eddy covariance method. The annual precipitation changed greatly, with 554, 706, and 666 mm a(-1) for the three consecutive calendar years. The annual ET varied correspondingly to the annual precipitation with 341, 407, and 426 mm a(-1). The annual ET was, however, constant at about 60% of the annual precipitation. About 85% annual ET occurred during the growing season from May to September, and the averaged ET for this period was 1.90, 2.23, and 2.22 mm/d, respectively for the three consecutive years. The averaged ET was, however, very low (< 0.40 mm/d) during the nongrowing season from October to April. The annual canopy conductance (gc) and the Priestley-Taylor coefficient (a) showed the lowest values in the year with the lowest precipitation. This study first demonstrates that the alpine meadow ecosystem is characterized by a low ratio of annual ET to precipitation and that the interannual variation of ET is determined by annual precipitation.

Short-term variation of CO2 flux in relation to environmental controls in an alpine meadow on the Qinghai-Tibetan Plateau

[1] The alpine meadow ecosystem on the Qinghai-Tibetan Plateau may play a significant role in the regional carbon cycle. To assess the CO2 flux and its relationship to environmental controls in the ecosystem, eddy covariance of CO2, H2O, and energy fluxes was measured with an open-path system in an alpine meadow on the plateau at an elevation of 3,250 m. Net ecosystem CO2 influx (Fc) averaged 8.8 g m(-2) day(-1) during the period from August 9 to 31, 2001, with a maximum of 15.9 g m(-2) day(-1) and a minimum of 2.3 g m(-2) day(-1). Daytime Fc averaged 16.7 g m(-2) day(-1) and ranged from 10.4 g m(-2) day(-1) to 21.7 g m(-2) day(-1) during the study period. For the same photosynthetic photon flux density (PPFD), gross CO2 uptake (Gc) was significantly higher on cloudy days than on clear days. However, mean daily Gc was higher on clear days than on cloudy days. With high PPFD, Fc decreased as air temperature increased from 10degreesC to 23degreesC. The greater the difference between daytime and nighttime air temperatures, the more the sink was strengthened. Daytime average water use efficiency of the ecosystem (WUEe) was 8.7 mg (CO2)(g H2O)(-1); WUEe values ranged from 5.8 to 15.3 mg (CO2)(g H2O)(-1). WUEe increased with the decrease in vapor pressure deficit. Daily albedo averaged 0.20, ranging from 0.19 to 0.22 during the study period, and was negatively correlated with daily Fc. Our measurements provided some of the first evidence on CO2 exchange for a temperate alpine meadow ecosystem on the Qinghai-Tibetan Plateau, which is necessary for assessing the carbon budget and carbon cycle processes for temperate grassland ecosystems.

西藏高原草原化嵩草草甸生态系统呼吸及碳平衡

The soil respiration and net ecosystem productivity of Kobresia littledalei meadow ecosystem was investigated at Dangxiong grassland station, one grassland field station of Lhasa Plateau Ecosystem Research Station. Soil respiration and soil heterotrophic respiration were measured at the same time by using Li6400-09 chamber in growing season of year 2004. The response of soil respiration and its components, i.e. microbial heterotrophic respiration and root respiration to biotic and abiotic factors were addressed. We studied the daily and seasonal variation on Net Ecosystem carbon Exchange (NEE) measured by eddy covariance equipments and then the regression models between the NEE and the soil temperature. Based on the researches, we analyzed the seasonal variation in grass biomass and estimated NEE combined the Net Ecosystem Productivity with heterogeneous respiration and then assessed the whether the area is carbon source or carbon sink. 1.Above-ground biomass was accumulated since the grass growth started from May; On early September the biomass reached maximum and then decreased. The aboveground net primary production (ANPP) was 150.88 g m~" in 2004. The under-ground biomass reached maximum when the aboveground start to die back. Over 80% of the grass root distributed at the soil depth from 0 to 20cm. The underground NPP was 1235.04 g m"2.. Therefore annual NPP wasl.385X103kg ha"1, i.e.6236.6 kg C ha"1. 2. The daily variation of soil respiration showed single peak curve with maximum mostly at noon and minimum 4:00-6:00 am. Daily variations were greater in June, July and August than those in September and October. Soil respiration had strong correlation with soil temperature at 5cm depth while had weaker correlation with soil moisture, air temperature, surface soil temperature, and so on. But since early September the soil respiration had a obviously correlation with soil moisture at 5cm depth. Biomass had a obviously linearity correlation with soil respiration at 30th June, 20th August, and the daytime of 27th September except at 23lh October and at nighttime of 27th September. We established the soil respiration responding to the soil temperature and to estimate the respiration variation during monsoon season (from June through August) and dry season (May, September and October). The regression between soil respiration and 5cm soil temperature were: monsoon season (June through August), Y=0.592expfl()932\ By estimating , the soil daily respiration in monsoon season is 7.798gCO2m"2 and total soil respiration is 717.44 gCC^m" , and the value of Cho is 2.54; dry season (May, September and October), Y=0.34exp°'085\ the soil daily respiration is 3.355gCO2m~2 and total soil respiration is 308.61 gCC^m", and the value of Cho is 2.34. So the total soil respiration in the grown season (From May to October) is 1026.1 g CO2IT1"2. 3. Soil heterogeneous respiration had a strong correlation with soil temperature especially with soil temperature at 5cm depth. The variation range in soil heterogeneous respiration was widely. The regression between soil heterogeneous respiration and 5cm soil temperature is: monsoon season, Y=0.106exp ' 3x; dry season, Y=0.18exp°"0833x.By estimating total soil heterotrophic respiration in monsoon season is 219.6 gCC^m"2, and the value of Cho is 3.78; While total soil heterogeneous respiration in dry season is 286.2 gCCbm"2, and the value of Cho is 2.3. The total soil heterotrophic respiration of the year is 1379.4kg C ha"1. 4. We estimated the root respiration through the balance between soil respiration and the soil heterotrophic respiration. The contribution of root respiration to total respiration was different during different period: re-greening period 48%; growing period 69%; die-back period 48%. 5. The Ecosystem respiration was relatively strong from May to October, and of which the proportion in total was 97.4%.The total respiration of Ecosystem was 369.6 g CO2 m" .we got the model of grass respiration respond to the soil temperature at 5cm depth and then estimated the daytime grass respiration, plus the nighttime NEE and daytime soil respiration. But when we estimated the grass respiration, we found the result was negative, so the estimating value in this way was not close. 6. The estimating of carbon pool or carbon sink. The NPP minus the soil heterogeneous respiration was the NEE, and it was 4857.3kg C o ha"1, which indicated that the area was the carbon sink.

Multiscale Coupling in Complex Mechanical Systems

Multiscale coupling attracts broad interests from mechanics, physics and chemistry to biology. The diversity and coupling of physics at different scales are two essential features of multiscale problems in far-from-equilibrium systems. The two features present fundamental difficulties and are great challenges to multiscale modeling and simulation. The theory of dynamical system and statistical mechanics provide fundamental tools for the multiscale coupling problems. The paper presents some closed multiscale formulations, e.g., the mapping closure approximation, multiscale large-eddy simulation and statistical mesoscopic damage mechanics, for two typical multiscale coupling problems in mechanics, that is, turbulence in fluids and failure in solids. It is pointed that developing a tractable, closed nonequilibrium statistical theory may be an effective approach to deal with the multiscale coupling problems. Some common characteristics of the statistical theory are discussed.

内蒙古草原温度梯度上3个典型群落的能量平衡研究

通量测量点的能量收支总是表现出不平衡，即使在地势平坦、植被分布均一、稀疏植被下垫面的情况下也有约30%的能量失衡状况。能量平衡闭合 (EBC) 问题在验证涡度相关系统质量方面，得到了广泛的关注。实验在内蒙古草原3个地点，通过涡度相关系统附近移动能量平衡系统的测定手段，采用能量平衡余项法和最小二乘 (OLS) 线性回归法，研究了土壤热通量、净辐射及可供能量空间变异对能量平衡闭合的影响。 结果显示，EBC 在三个研究点的平均余项为8~19 W m-2，OLS 斜率为0.83~0.96。EBC 在土壤湿润情况的站点要高于干旱站点。 土壤热通量的空间变异三站点平均为白天48 W m-2 (占同时间 Rn 的13%)，夜间15 W m-2 (34%)，平均29 W m-2 (24%)。通过8个工作站的测量，这个变异会造成9% (从0.93到1.01) 的 OLS 斜率差别。夜间的能量平衡不闭合可以由土壤热通量的空间变异解释。如果在本研究的的三个草原站点上忽略了土壤热通量，则会造成较大的余项 (峰值时110 W m-2) 产生，从而使 OLS 斜率增大23%。特别是通量板埋置在地面以下30 mm处时，上层的土壤热储部分占到全部土壤热通量的50%，这不仅影响到 EBC 的大小，更起到调节土壤热通量与“真实的波形” 相一致的作用。如果该部分热储被忽略掉，EBC 余项会增加60 W m-2，OLS 斜率也会变化 (减少) 9%。用大尺度多点测量与涡度塔附近的小尺度测定相比较，后者表现出稍高的闭合率，即 OLS 斜率增加4%。 相对于土壤热通量，净辐射的空间变异较小，三站点平均为白天17 W m-2 (5%)，夜间7 W m-2 (13%)，平均 12 W m-2 (5%)。可以引起3% (从0.88到0.91) 的 OLS 斜率差异。研究结果还表明，风速校正应该在 Q7.1 净辐射仪中应用，校正后的结果与 CNR1 的结果在白天有吻合较好，但在其它时段仍有较大差异，特别是在夜间，风速校正基本不起作用，使得两种仪器间差异达20 W m-2。比较表明，风速校正可以提高白天 Rn 的6%，仅降低夜间0.3%。因此，无论是用余项法还是用 OLS 线性回归法，在比较使用不同仪器的站点间的闭合状况时 (本研究的结果适用于草地 Q7.1 与 CNR1 间的比较)，可以用9：00-15：00 h 时段的数据进行比较，这样可以避免因使用不同仪器的差异所造成的影响。用该时段的数据进行比较，仪器间的差异余项法小于6 W m-2，OLS 法小于3%。 受可供能量空间变异影响，三个站点平均 EBC 的不确定性为白天66 W m-2 (19%)，夜间23 W m-2 (50%)，平均42 W m-2 (36%);或者改变 OLS 斜率11%。用最大值和最小值来衡量，EBC 最大不确定性，正午时在站点 I、II 和 III 中分别为81，114和91 W m-2。故在探讨能量平衡和能量平衡闭合问题时，必须充分考虑到这种不确定性，否则会产生偏差，或者得出错误结论。 研究还表明，即使考虑到白天所有可供能量的最大不确定性，仍然不能使能量平衡闭合。中午 (12：00 h)，站点 I，II 和 III 仍然有14±15，48±12和47±14 W m-2 的失衡不能够归因于可供能量的空间不确定性。因此，其它影响因素也需进行细致的探讨。 在两站点不同测量深度土壤热通量结果的差异性比较实验中，无论在站点 I 还是 III，均表现出一致的结论，即随通量板布置深度加深，其测量结果会越高，与浅层布置的相比，差别可高达150 W m-2。深层土壤热通量的计算仍是个难题，需进一步研究。 在不同植被结构对净辐射测定影响的实验中发现，随刈割强度增加，一天中大部分时间净辐射均减少。正午依次为413，395和388 W m-2。无论正午还是全天合计，重度刈割地点的净辐射均比不刈割对照处理少6%，而且，在整个生长季也少6%，约合40,000 W m-2。测量高度不同，不同处理间对测定结果影响不同：刈割处理中，由于下垫面较均一，结果相差不显著;而对照则表现出较高的差异，用两配对样本T检验表明差异达到极显著 (P<0.000，9:30-15:00 h data)。当使用不同新旧程度的 domes 时，对净辐射结果会产生明显的影响。新 domes 的测量结果白天明显高，晚上明显低，使用了11个月的旧 domes，峰值时，白天低估25 W m-2，晚上高估10 W m-2。说明该差异在进行能量平衡闭合计算时，不能忽略。而全用新的和全用旧的进行比较，晚上仅有2-3 W m-2差异。 考虑生态系统中非生物因子对干扰条件下生物多样性动态和功能的影响，有助于更精确地阐明生物多样性-稳定性功能的关系。为此，设计了一个单因子刈割实验——内蒙古地区一种广泛存在的土地利用方式。主要目的是研究不同强度刈割影响下，微气候变量特别是能量平衡各分量和群落结构的变化及二者的关系。连续4年刈割，占第一位的优势种明显由低矮半灌木冷蒿 (Artemisia frigida) 取代了高大丛生禾草克氏针茅 (Stipa kylovii)。重度刈割下，针茅的盖度、生物量和丛重，群落叶面积、绿色生物量、凋落物量和群落高度一致低于轻度刈割/不刈割处理。微气候由于群落特征的这些变化也呈规律性变化。与对照相比，重度刈割降低了生长季土壤含水量的47.5%，但中午和日均土壤表面温度分别增加了7.4和1.2 °C，并且增加地表下2 cm土壤温度日较差 (日最高与最低温度之差) 4.2 °C。刈割处理由于凋落物少、反射强而表现出较低的净辐射，但土壤热通量显著提高，表现为土层加热和冷却快。因此，重度刈割处理较对照降低了可供能量8%，约合52,000 W m-2 。不同刈割强度间来看，NPP 或 LAI 与土壤热通量和净辐射的比值 (G/Rn) 以及波文比 (H/LE) 间呈负相关。重度刈割处理感热通量显著提高，但潜热通量在处理间差异不显著，表明未刈割处理虽然冠层伸展大，但是并没有导致更大的水分亏缺。未刈割处理增加了抵抗物种改变的能力，而刈割处理在连续一年一割的第四年显著增加了物种数，可能与因刈割影响而导致的群落结构与微气候的改变有关。本研究表明，未刈割处理可以减轻高温干旱季节的高温和干旱胁迫，表现出对环境变化的高抵抗性。未刈割处理的凋落物层和较高的垂直结构所形成的遮荫，可以形成一个阻挡蒸发的篱笆，这是维持其水分的保证。因此，为了恢复退化草原生态系统功能，需要修复能导致微气候变化的植物群落结构，否则难以成功。 本研究立足于原创性的实验研究，在中国特有的自然草原生态系统上开展，结合不同温度梯度的三地区涡度相关系统进行了能量平衡闭合的移动比较实验，以及结合常用土地利用方式的定点能量平衡实验。在翔实的数据基础上，为涡度相关方法的陆地表面能量平衡失衡问题提供了解释。增加了对两个主要能量流——土壤热通量和净辐射空间变异规律的认识，研究对于能量平衡和湍流通量相关研究是有价值的。在三个代表性地区首次利用多个净辐射仪和土壤热通量板的结果与三个标准的涡度相关系统进行了比较，这类量化失衡原因的相关研究应受到高度重视并进一步拓展，以提高对能量失衡的认识，进而推进水热和碳循环研究向更深层次发展。

三种方法测定高寒草甸生态系统蒸散比较

利用涡度相关技术（Eddy covariance technique）、小型蒸渗仪（Mini—lysimeter）和波文比-能量平衡法（BREB）对2005年和2006年夏季（7—8月份）青藏高原海北高寒草甸生态系统的昼间蒸散（E）变化进行了对比观测研究。在观测期间，存在能量不闭合现象，涡度相关系统测定的湍流通量相当于有效能量的73％。3种不同方法测定的蒸散量之间具有较好的相关性，涡度相关系统与小型蒸渗仪测定的蒸散量相关系数达0．96，与波文比法的结果相关系数为0．95。然而，波文比法计算的蒸散量最大，比涡度相关系统的观测值高43％；小型蒸渗仪法的测定值次之，比涡度相关法的观测值高19％；涡度相关法测算的蒸散值最小。研究结果表明，利用涡度相关技术测定该高寒草甸生态系统的潜热通量，可能会过小评价该生态系统的蒸散量。

青藏高原高寒灌丛生态系统二氧化碳通量研究

本论文以青藏高原东北部海北地区高寒灌丛（Alpine Shrub）生态系统为研究对象，利用微气象观测系统及涡度相关（Eddy Covariance）技术，自2003年1月1日至2005年12月31日对该类广布于青藏高原的典型高寒草地类型进行长期连续观测。在对生态系统CO2净交换（NEE）以及群落叶面积指数（LAI）、生物量等生物学指标和光合有效辐射（PAR）、温度、土壤水分、脉冲性降水事件等主要环境因子进行连续监测的基础上，重点分析和探讨了海北地区高寒灌丛生态系统净生态系统CO2交换(NEE)在时、日、月及年际尺度上的变化模式，生长季与非生长季高寒灌丛生态系统CO2净交换特征，高寒灌丛生态系统大气CO2源/汇年际差异，土壤温度、昼夜温差、光合有效辐射、脉冲性降水事件等主要环境因子影响。从而，揭示了不同时间尺度下的高寒灌丛生态系统NEE变化规律，阐明主要环境因子对生态系统NEE的影响，明确了该生态系统大气CO2源/汇状况及其季节分布模式；同时，也为青藏高原区域尺度的高寒草地生态系统CO2通量研究和碳收支的估算提供科学依据和基础数据，对进一步揭示我国乃至亚洲陆地生态系统的碳收支状况有着重要意义。主要研究结果概括为以下几个方面： 1、海北地区高寒灌丛生态系统净生态系统CO2交换时动态特征存在很大的季节性差异，暖季小时NEE变化振幅大，CO2净吸收的极值一般出现在午间，最大吸收量为1.7 g CO2 m-2 h-1左右。夜间为CO2净释放，净生态系统交换值较为稳定（0.5~ 0.9 g CO2 m-2 h-1）；冷季日变化振幅极小，除14:00~18:00时一定量CO2释放外，其余时段通量均很小。 2、从日平均净生态系统CO2交换来看，6~9月日平均NEE一般为负值（CO2净吸收），2003~2005年6~9 月间日平均NEE分别为-5.65 g CO2 m-2 d-1、-6.08 g CO2 m-2 d-1和-4.81 g CO2 m-2 d-1；而10~12月及翌年1~5月期间日平均NEE通常为正值（CO2净释放），该时段3年高寒灌丛日平均净生态系统CO2交换分别为1.91 g CO2 m-2 d-1、1.90 g CO2 m-2 d-1和2.19 g CO2 m-2 d-1。2003~2004年高寒灌丛生态系统CO2净释放维持天数分别为249 d、 254 d和264 d，2003年净释放维持天数最少，而净吸收维持天数2005年最少（101d）。2003、2004和2005年全年日平均CO2净吸收分别为0.611 g CO2 m-2 d-1、0.759 g CO2 m-2 d-1和0.167 g CO2 m-2 d-1。 3、就季节差异而言，2003、2004和2005年整个生长季节高寒灌丛平均CO2日净生态系统交换分别为-3.99 g CO2 m-2 d-1、-4.59 g CO2 m-2 d-1、-3.27 g CO2 m-2 d-1。7、8月生长季节CO2净吸收的最高，2003、2004、2005年7月和8月份高寒灌丛生态系统CO2净吸收分别为222 g CO2 m-2 和224 g CO2 m-2、355 g CO2 m-2和216 g CO2 m-2、263 g CO2 m-2和186 g CO2 m-2。在相对短暂的生长季节海北地区高寒灌丛生态系统表现出显著的大气CO2净吸收能力，2003、2004和2005年生长季节高寒灌丛生态系统CO2净吸收量分别为610 g CO2 m-2、701 g CO2 m-2和500 g CO2 m-2。相对于温度等环境因子，高寒灌丛生态系统生长季白昼NEE小时变化规律更受光合有效辐射变化的影响。 4、2003～2005年非生长季节日平均NEE分别为1.83 g CO2 m-2、2.01 g CO2 m-2和2.07 g CO2 m-2。4月和10月是非生长季节CO2净释放的最高月份，2003、2004和2005年全月净释放量为105 g CO2 m-2和77 g CO2 m-2、105 g CO2 m-2和117 g CO2 m-2及105 g CO2 m-2和138 g CO2 m-2，2003～2005年整个非生长季CO2净释放分别为CO2为388 g CO2 m-2、425 g CO2 m-2和439 g CO2 m-2。非生长季节海北地区高寒灌丛生态系统NEE小时变化与5 cm土壤温度存在极显著的正相关关联，表明在非生长季节土壤温度是影响青藏高原高寒灌丛生态系统NEE的重要环境因子。 5、从生态系统CO2源/汇特征来看，海北地区高寒灌丛生态系统2003、2004和2005年全年净CO2固定总量分别为223 g CO2 m-2 a-1、277 g CO2 m-2 a-1和61 g CO2 m-2 a-1，3年平均CO2值为187 g CO2 m-2 a-1。在为期3年的研究时段海北地区高寒灌丛生态系统表现为弱的大气二氧化碳的汇。 6、高寒灌丛群落表观光合量子产额（a）和表观最大光合速率（Pmax）受叶面积指数的影响。在6～9月份期间，由于LAI的不同，a和Pmax值差异明显，7、8月份较高而6月和9月明显较低。海北地区高寒灌丛生态系统a和Pmax值高于西藏当雄地区高寒草甸生态系统，但低于平原地区相关生态系统。 维持天数2005年最少（101d）。2003、2004和2005年全年日平均CO2净吸收分别为0.611 g CO2 m-2 d-1、0.759 g CO2 m-2 d-1和0.167 g CO2 m-2 d-1。 3、就季节差异而言，2003、2004和2005年整个生长季节高寒灌丛平均CO2日净生态系统交换分别为-3.99 g CO2 m-2 d-1、-4.59 g CO2 m-2 d-1、-3.27 g CO2 m-2 d-1。7、8月生长季节CO2净吸收的最高，2003、2004、2005年7月和8月份高寒灌丛生态系统CO2净吸收分别为222 g CO2 m-2 和224 g CO2 m-2、355 g CO2 m-2和216 g CO2 m-2、263 g CO2 m-2和186 g CO2 m-2。在相对短暂的生长季节海北地区高寒灌丛生态系统表现出显著的大气CO2净吸收能力，2003、2004和2005年生长季节高寒灌丛生态系统CO2净吸收量分别为610 g CO2 m-2、701 g CO2 m-2和500 g CO2 m-2。相对于温度等环境因子，高寒灌丛生态系统生长季白昼NEE小时变化规律更受光合有效辐射变化的影响。 4、2003～2005年非生长季节日平均NEE分别为1.83 g CO2 m-2、2.01 g CO2 m-2和2.07 g CO2 m-2。4月和10月是非生长季节CO2净释放的最高月份，2003、2004和2005年全月净释放量为105 g CO2 m-2和77 g CO2 m-2、105 g CO2 m-2和117 g CO2 m-2及105 g CO2 m-2和138 g CO2 m-2，2003～2005年整个非生长季CO2净释放分别为CO2为388 g CO2 m-2、425 g CO2 m-2和439 g CO2 m-2。非生长季节海北地区高寒灌丛生态系统NEE小时变化与5 cm土壤温度存在极显著的正相关关联，表明在非生长季节土壤温度是影响青藏高原高寒灌丛生态系统NEE的重要环境因子。 5、从生态系统CO2源/汇特征来看，海北地区高寒灌丛生态系统2003、2004和2005年全年净CO2固定总量分别为223 g CO2 m-2 a-1、277 g CO2 m-2 a-1和61 g CO2 m-2 a-1，3年平均CO2值为187 g CO2 m-2 a-1。在为期3年的研究时段海北地区高寒灌丛生态系统表现为弱的大气二氧化碳的汇。 6、高寒灌丛群落表观光合量子产额（a）和表观最大光合速率（Pmax）受叶面积指数的影响。在6～9月份期间，由于LAI的不同，a和Pmax值差异明显，7、8月份较高而6月和9月明显较低。海北地区高寒灌丛生态系统a和Pmax值高于西藏当雄地区高寒草甸生态系统，但低于平原地区相关生态系统。

Large Eddy Simulation of Complex Turbulent Flows: Physical Aspects and Research Trends

In the current paper, we have primarily addressed one powerful simulation tool developed during the last decades-Large Eddy Simulation (LES), which is most suitable for unsteady three-dimensional complex turbulent flows in industry and natural environment. The main point in LES is that the large-scale motion is resolved while the small-scale motion is modeled or, in geophysical terminology, parameterized. With a view to devising a subgrid-scale(SGS) model of high quality, we have highlighted analyzing physical aspects in scale interaction and-energy transfer such as dissipation, backscatter, local and non-local interaction, anisotropy and resolution requirement. They are the factors responsible for where the advantages and disadvantages in existing SGS models come from. A case study on LES of turbulence in vegetative canopy is presented to illustrate that LES model is more based on physical arguments. Then, varieties of challenging complex turbulent flows in both industry and geophysical fields in the near future-are presented. In conclusion; we may say with confidence that new century shall see the flourish in the research of turbulence with the aid of LES combined with other approaches.

Derivation Of A Nonlinear Reynolds Stress Model Using Renormalization Group Analysis And Two-Scale Expansion Technique

Adopting Yoshizawa's two-scale expansion technique, the fluctuating field is expanded around the isotropic field. The renormalization group method is applied for calculating the covariance of the fluctuating field at the lower order expansion. A nonlinear Reynolds stress model is derived and the turbulent constants inside are evaluated analytically. Compared with the two-scale direct interaction approximation analysis for turbulent shear flows proposed by Yoshizawa, the calculation is much more simple. The analytical model presented here is close to the Speziale model, which is widely applied in the numerical simulations for the complex turbulent flows.

Vertical structure and evolution of the Luzon Warm Eddy

Eddies are frequently observed in the northeastern South China Sea (SCS). However, there have been few studies on vertical structure and temporal-spatial evolution of these eddies. We analyzed the seasonal Luzon Warm Eddy (LWE) based on Argo float data and the merged data products of satellite altimeters of Topex/Poseidon, Jason-1 and European Research Satellites. The analysis shows that the LWE extends vertically to more than 500 m water depth, with a higher temperature anomaly of 5A degrees C and lower salinity anomaly of 0.5 near the thermocline. The current speeds of the LWE are stronger in its uppermost 200 m, with a maximum speed of 0.6 m/s. Sometimes the LWE incorporates mixed waters from the Kuroshio Current and the SCS, and thus has higher thermohaline characteristics than local marine waters. Time series of eddy kinematic parameters show that the radii and shape of the LWE vary during propagation, and its eddy kinetic energy follows a normal distribution. In addition, we used the empirical orthogonal function (EOF) here to analyze seasonal characteristics of the LWE. The results suggest that the LWE generally forms in July, intensifies in August and September, separates from the coast of Luzon in October and propagates westward, and weakens in December and disappears in February. The LWE's westward migration is approximately along 19A degrees N latitude from northwest of Luzon to southeast of Hainan, with a mean speed of 6.6 cm/s.

Burst Detection in Turbulent Channel Flows Based on Large Eddy Simulation Databases

Reliable turbulent channel flow databases at several Reynolds numbers have been established by large eddy simulation (LES), with two of them validated by comparing with typical direct numerical simulation (DNS) results. Furthermore, the statistics, such as velocity profile, turbulent intensities and shear stress, were obtained as well as the temporal and spatial structure of turbulent bursts. Based on the LES databases available, the conditional sampling methods are used to detect the structures of burst events. A method to deterimine the grouping parameter from the probability distribution function (pdf) curve of the time separation between ejection events is proposed to avoid the errors in detected results. And thus, the dependence of average burst period on thresholds is considerably weakened. Meanwhile, the average burst-to-bed area ratios are detected. It is found that the Reynolds number exhibits little effect on the burst period and burst-to-bed area ratio.

Some Key Technics of Drop Tower Experiment Device of National Microgravity Laboratory (China) (NMLC)

Drop tower is an important ground based facility for microgravity science experiment. The technical performances of the drop tower NMLC are advanced compared with similar facilities in the US, Germany and Japan. The main components such as drop capsule, deceleration devices, release mechanism present its advantages and creativities.

Large-Eddy-Simulation of 3-Dimensional Rayleigh-Taylor Instability in Incompressible Fluids

The 3-dimensiqnal incompressible Rayleigh-Taylor instability is numerically studied through the large-eddy-simulation (LES) approach based on the passive scalar transport model. Both the instantaneous velocity and the passive scalar fields excited by sinu