Resolvent-based design and experimental testing of porous materials for passive turbulence control

An extended version of the resolvent formulation is used to evaluate the use of anisotropic porous materials as passive flow control devices for turbulent channel flow. The effect of these porous substrates is introduced into the governing equations via a generalized version of Darcy’s law. Model predictions show that materials with high streamwise permeability and low wall-normal permeability (${\varphi }_{\mathit{xy}}=k_{\mathit{xx}}/k_{\mathit{yy}}\gg 1$) can suppress resolvent modes resembling the energetic near-wall cycle. Based on these predictions, two anisotropic porous substrates with ${\varphi }_{\mathit{xy}}>1$ and ${\varphi }_{\mathit{xy}}<1$ were designed and fabricated for experiments in a benchtop water channel experiment. Particle Image Velocimetry (PIV) measurements were used to compute mean turbulence statistics and to educe coherent structure via snapshot Proper Orthogonal Decomposition (POD). Friction velocity estimates based on the Reynolds shear stress profiles do not show evidence of discernible friction reduction (or increase) over the streamwise-preferential substrate with ${\varphi }_{\mathit{xy}}>1$ relative to a smooth wall flow at identical bulk Reynolds number. A significant increase in friction is observed over the substrate with ${\varphi }_{\mathit{xy}}<1$. This increase in friction is linked to the emergence of spanwise rollers resembling Kelvin–Helmholtz vortices. Coherent structures extracted via POD analysis show qualitative agreement with model predictions.     

DNS of passive scalar transport in turbulent channel flow at high Schmidt numbers

We perform DNS of passive scalar transport in low Reynolds number turbulent channel flow at Schmidt numbers up to Sc = 49. The high resolutions required to resolve the scalar concentration fields at such Schmidt numbers are achieved by a hierarchical algorithm in which only the scalar fields are solved on the grid dictated by the Batchelor scale. The velocity fields are solved on coarser grids and prolonged by a conservative interpolation to the fine-grid.

The trends observed so far at lower Schmidt numbers Sc  10 are confirmed, i.e. the mean scalar gradient steepens at the wall with increasing Schmidt number, the peaks of turbulent quantities increase and move towards the wall. The instantaneous scalar fields show a dramatic change. Observable structures get longer and thinner which is connected with the occurrence of steeper gradients, but the wall concentrations penetrate less deeply into the plateau in the core of the channel.

Our data shows that the thickness of the conductive sublayer, as defined by the intersection point of the linear with the logarithmic asymptote scales with Sc^−0.29. With this information it is possible to derive an expression for the dimensionless transfer coefficient K⁺ which is only dependent on Sc and Re_τ. This expression is in full accordance to previous results which demonstrates that the thickness of the conductive sublayer is the dominating quantity for the mean scalar profile.     

Complex dynamics and targeted energy transfer in linear oscillators coupled to multi-degree-of-freedom essentially nonlinear attachments

We study the dynamics of a system of coupled linear oscillators with a multi-DOF end attachment with essential (nonlinearizable) stiffness nonlinearities. We show numerically that the multi-DOF attachment can passively absorb broadband energy from the linear system in a one-way, irreversible fashion, acting in essence as nonlinear energy sink (NES). Strong passive targeted energy transfer from the linear to the nonlinear subsystem is possible over wide frequency and energy ranges. In an effort to study the dynamics of the coupled system of oscillators, we study numerically and analytically the periodic orbits of the corresponding undamped and unforced hamiltonian system with asymptotics and reduction. We prove the existence of a family of countable infinity of periodic orbits that result from combined parametric and external resonance interactions of the masses of the NES. We numerically demonstrate that the topological structure of the periodic orbits in the frequency–energy plane of the hamiltonian system greatly influences the strength of targeted energy transfer in the damped system and, to a great extent, governs the overall transient damped dynamics. This work may be regarded as a contribution towards proving the efficacy the utilizing essentially nonlinear attachments as passive broadband boundary controllers. PACS numbers: 05.45.Xt, 02.30.Hq     

Static Finite Element Validation of a Flexible Micro Air Vehicle

The flexible-wing approach has proven to be a successful method for designing micro air vehicles. The wing’s passive deformation under wind loads can allow for gust rejection, delayed stall, or improved longitudinal stability. As such, an accurate structural model of the flexible wing can provide greater understanding of the aforementioned phenomena. This paper seeks to formulate a static finite element wing model, with a particular emphasis on accuracy. The wing is broken into three different types of elements: beams, plates, and membranes. Individual element types are characterized and validated by constructing simple structures from the appropriate material, and then comparing experimental and numerical deformation fields. Experimental results are found through a visual image correlation system. The elements are then combined to form the complete wing model, which is also validated through experiments. The resulting finite element model is found to be very accurate, able to predict the complicated structural response of a composite wing. Due to observations made during standard wind tunnel testing, the structural response of a typical membrane MAV wing in steady level pre-stall flight is thought to be quasi-static. As such, the finite element model formulated in this work will be indispensable towards future numerical static aeroelastic optimization research efforts aimed at improving the efficiency, agility, and sensitivity of practical micro air vehicles.     

被动桩中土拱效应特征与影响参数研究

采用平面有限元方法,对被动桩在粘性、无粘性土体条件下的土拱形成机理进行了分析。从土拱效应的4个主要方面:土拱形状、桩周土体塑性(拉裂)区的分布、竖向位移等值线及桩后土体残余荷载分担比,探讨了改变桩、土参数,如粘聚力、内摩擦角、弹性模量、泊松比、剪胀角、桩土接触面特性时,土拱效应的变化规律。研究表明,土性不同时,土拱形成过程中土体的受力和破坏形式不同;泊松比、剪胀角、桩土接触特征对土拱效应的影响最明显;强度高、剪胀角大、桩土接触面粗糙或泊松比小的土体,易形成土拱。根据研究结果,指出Tom io Ito排桩塑性绕流土压力理论计算公式尚存不足。     

Assessment of Chemcatcher passive sampler for the monitoring of inorganic mercury and organotin compounds in water

Abstract

Molecularly imprinted polymers (MIPs) for 4-nitrophenol have been successfully prepared by a thermal polymerization method using 4-vinylpyridine (4-VP) and ethylene glycol dimethacrylate (EDMA) as functional monomer and cross-linker, respectively. The obtained materials were evaluated with respect to their selective recognition properties for 4-nitrophenol by HPLC using organic and aqueous eluents. Furthermore, the specific binding sites that have been created during the polymerization process were analyzed via radioligand binding assays. The successful imprinting against 4-nitrophenol provides a new opportunity for advanced separation materials used in environmental analysis.     

需求导向的容量设施竞争选址问题研究

客户意愿与容量限制是竞争设施选址问题中两个重要的影响因素,在考虑客户意愿与设施容量共同作用条件下,建立了最小化企业总成本以及每个客户费用为目标的竞争设施选址问题优化模型,通过设计需求导向服务分配机制解决设施与客户之间服务关系分配问题,结合模拟退火思想提出了求解模型的算法。最后利用数值例子分析了需求导向服务分配机制以及目标权重、预算限额等参数对于选址决策的影响,其中考虑需求导向因素会适当增加企业的总成本,但可以减少客户所付出的费用从而增强对客户的吸引力;另外企业的预算限额对于企业的设施选址决策有着重要的影响,企业所能获取的市场份额与其选址预算限额呈正相关的关系;而客户所需付出的总费用与企业提供服务的总成本两者之间则呈负相关的关系,因此需要通过服务质量与成本之间的权衡实现最理想的选址决策。     

考虑居民选择行为的应急避难场所选址问题研究

突发事件发生后,应急管理部门将面临选择启用哪些应急避难场所供居民避难的决策。与其它设施选址不同,处于有限理性状态下的居民可选择是否去避难以及去哪个避难所避难。为使未被服务的居民数量最小化,考虑居民的有限理性选择行为至关重要。为此,考虑居民避难距离及避难所吸引力等因素,基于竞争选址理论构建避难所对居民的效用函数。在此基础上,运用多项logit模型刻画居民的有限理性选择行为,建立基于有限理性选择行为的选址模型,并与基于效用最优选择行为和基于系统最优原则的选址模型进行对比以验证模型的合理性。为了高效地求解模型,设计了模拟退火算法和遗传算法两种启发式算法。算例分析结果验证了模型和算法的可行性和有效性,并揭示了居民理性程度、选址个数及财政预算均对选址结果有重要影响。     

一类多商品设施选址问题的基于线性松弛解的启发式方法

多商品设施选址问题是众多设施选址问题中一类重要而困难的问题.在这一问题中,顾客的需求可能包含不止一种商品.对于大规模问题,成熟的商业求解器往往不能在满意的时间内找到高质量的可行解.研究了无容量限制的单货源多商品设施选址问题的一般形式,并给出了应用于此类问题的两个启发式方法.这两个方法基于原选址问题的线性规划松弛问题的最优解,分别通过求解紧问题和邻域搜索的方式给出了原问题的一个可行上界.理论分析指出所提方法可以实施于任意可行问题的实例.数值结果表明所提方法可以显著地提高求解器求解此类设施选址问题的求解效率.     

Correlation of precursor and product ions in single-stage high resolution mass spectrometry. A tool for detecting diagnostic ions and improving the precursor elemental composition elucidation

Monitoring of common diagnostic fragments is essential for recognizing molecules which are members of a particular compound class. Up to now, unit resolving tandem quadrupole mass spectrometers, operating in the precursor ion scan mode, have been typically used to perform such analysis. By means of high-resolution mass spectrometry (HRMS) a much more sensitive and selective detection can be achieved. However, using a single-stage HRMS instrument, there is no unequivocal link to the corresponding precursor ion, since such instrumentation does not permit a previous precursor selection. Thus, to address this limitation, an in silico approach to locate precursor ions, based on diagnostic fragments, was developed. Implemented as an Excel macro, the algorithm rapidly assembles and surveys exact mass data to provide a list of feasible precursor candidates according to the correlation of the chromatographic peak shape profile and other additional filtering criteria (e.g. neutral losses and isotopes). The macro was tested with two families of veterinary drugs, sulfonamides and penicillins, which are known to yield diagnostic product ions when fragmented. Data sets obtained from different food matrices (fish and liver), both at high and low concentration of the target compounds, were investigated in order to evaluate the capabilities and limitations of the reported approach. Finally, other possible applications of this technique, such as the elucidation of elemental compositions based on product ions and corresponding neutral losses, were also presented and discussed.