Optimal stirring strategies for passive scalar mixing

We address the challenge of optimal incompressible stirring to mix an initially inhomogeneous distribution of passive tracers. As a quantitative measure of mixing we adopt the $H^{-1}$ norm of the scalar fluctuation field, equivalent to the (square-root of the) variance of a low-pass filtered image of the tracer concentration field. First we establish that this is a useful gauge even in the absence of molecular diffusion: its vanishing as $t --> \infty$ is evidence of the stirring flow's mixing properties in the sense of ergodic theory. Then we derive absolute limits on the total amount of mixing, as a function of time, on a periodic spatial domain with a prescribed instantaneous stirring energy or stirring power budget. We subsequently determine the flow field that instantaneously maximizes the decay of this mixing measure---when such a flow exists. When no such `steepest descent' flow exists (a possible but non-generic situation) we determine the flow that maximizes the growth rate of the $H^{-1}$ norm's decay rate. This local-in-time optimal stirring strategy is implemented numerically on a benchmark problem and compared to an optimal control approach using a restricted set of flows. Some significant challenges for analysis are outlined.Comment: 10 pages, 3 figures. PDFLaTeX with JFM style (included

An experimental investigation of the relationships among race, prayer, and pain

Background and aims Compared to White individuals, Black individuals demonstrate a lower pain tolerance. Research suggests that differences in pain coping strategies, such as prayer, may mediate this race difference. However, previous research has been cross-sectional and has not determined whether prayer in and of itself or rather the passive nature of prayer is driving the effects on pain tolerance. The aim of this study was to clarify the relationships among race, prayer (both active and passive), and pain tolerance. Methods We randomly assigned 208 pain-free participants (47% Black, 53% White) to one of three groups: active prayer (“God, help me endure the pain”), passive prayer (“God, take the pain away”), or no prayer (“The sky is blue”). Participants first completed a series of questionnaires including the Duke University Religion Index, the Coping Strategies Questionnaire-Revised (CSQ-R), and the Pain Catastrophizing Scale. Participants were then instructed to repeat a specified prayer or distractor coping statement while undergoing a cold pressor task. Cold pain tolerance was measured by the number of seconds that had elapsed while the participant’s hand remained in the cold water bath (maximum 180 s). Results Results of independent samples t-tests indicated that Black participants scored higher on the CSQ-R prayer/hoping subscale. However, there were no race differences among other coping strategies, religiosity, or catastrophizing. Results of a 2 (Race: White vs. Black)×3 (Prayer: active vs. passive vs. no prayer) ANCOVA controlling for a general tendency to pray and catastrophizing in response to prayer indicated a main effect of prayer that approached significance (p=0.06). Pairwise comparisons indicated that those in the active prayer condition demonstrated greater pain tolerance than those in the passive (p=0.06) and no prayer (p=0.03) conditions. Those in the passive and no prayer distractor conditions did not significantly differ (p=0.70). There was also a trending main effect of race [p=0.08], with White participants demonstrating greater pain tolerance than Black participants. Conclusions Taken together, these results indicate that Black participants demonstrated a lower pain tolerance than White participants, and those in the active prayer condition demonstrated greater tolerance than those in the passive and no prayer conditions. Furthermore, Black participants in the passive prayer group demonstrated the lowest pain tolerance, while White participants in the active prayer group exhibited the greatest tolerance. Results of this study suggest that passive prayer, like other passive coping strategies, may be related to lower pain tolerance and thus poorer pain outcomes, perhaps especially for Black individuals. On the other hand, results suggest active prayer is associated with greater pain tolerance, especially for White individuals. Implications These results suggest that understanding the influence of prayer on pain may require differentiation between active versus passive prayer strategies. Like other active coping strategies for pain, active prayer may facilitate self-management of pain and thus enhance pain outcomes independent of race. Psychosocial interventions may help religiously-oriented individuals, regardless of race, cultivate a more active style of prayer to improve their quality of life

Passive house ventilation strategies: demand control?

Since insulation levels in a passive house context are very high, energy losses trough ventilation are of relatively high importance in the total energy balance of a passive house concept. Although fully mechanical ventilation systems are at the core of the this concept, enabling both regeneration and cheap space heating, it can still be optimized. With demand controlled systems, excess ventilation can be minimized, thus reducing both redundant ventilation losses and the accompanying electrical loads. This paper reviews the possibilities for a performance based optimization of ventilation systems for passive houses within the context of the Belgian legislatio

Climate based facade design for business buildings with examples from central London

There is a disconnection between commercial architecture and environmental thinking, where green features can be included as part of a strategy for gaining approvals and marketing projects, but those features are not reviewed after completion and occupation of the building and knowledge is not shared. High levels of air conditioning are still considered unavoidable. Elaborate double skin façades and complex motorized shading systems are adopted; often masking an underlying lack of basic environmental thinking. This article returns (in principle) to the physics of comfort in buildings and the passive strategies which can help achieve this with a low energy and carbon footprint. Passive and active façade design strategies are outlined as the basis of a critical tool and a design methodology for new projects. A new architectural sensibility can arise based on modeling the inputs of sunlight, daylight and air temperature in time and space at the early stages of design. Early but sound strategies can be tested and refined using advanced environmental modeling techniques. Architecture and environmental thinking can proceed hand in hand through the design process

Paved with Good Intentions: The Failure of Passive Disability Policy in Canada

It is common in the disability community to speak of unfulfilled aspirations for full citizenship and participation in the mainstream of Canadian society. In Canada, as in much of the developed world, many adults with disabilities remain outside the mainstream, especially in regard to economic opportunities. Unfortunately, many of the disability policies currently pursued by Canadian governments are unlikely to improve this situation, and may in fact make it worse. This paper offers a critical analysis of a common instrument of current disability policy, the passive cash benefit. I will focus, in particular, on the effects of passive transfers on prospects for adults with disabilities to reach their full income potential through employment. I will attempt to establish that passive income support strategies – for adults with disabilities and for low-income people in general – force their intended beneficiaries to sacrifice employment prospects for help with short-term income needs, a trade-off that reinforces poverty and dependency over the longer term

Vibration isolation with smart fluid dampers: a benchmarking study

The non-linear behaviour of electrorheological (ER) and magnetorheological (MR) dampers makes it difficult to design effective control strategies, and as a consequence a wide range of control systems have been proposed in the literature. These previous studies have not always compared the performance to equivalent passive systems, alternative control designs, or idealised active systems. As a result it is often impossible to compare the performance of different smart damper control strategies. This article provides some insight into the relative performance of two MR damper control strategies: on/off control and feedback linearisation. The performance of both strategies is benchmarked against ideal passive, semi-active and fully active damping. The study relies upon a previously developed model of an MR damper, which in this work is validated experimentally under closed-loop conditions with a broadband mechanical excitation. Two vibration isolation case studies are investigated: a single-degree-of-freedom mass-isolator, and a two-degree-of-freedom system that represents a vehicle suspension system. In both cases, a variety of broadband mechanical excitations are used and the results analysed in the frequency domain. It is shown that although on/off control is more straightforward to implement, its performance is worse than the feedback linearisation strategy, and can be extremely sensitive to the excitation conditions

Do Socially Conscious ETFs Match their Active Counterparts?

Over the past decade, Socially Responsible Investing (SRI) has grown at a rapid pace and, by some estimates, now represents a quarter of the $48 trillion in assets under professional management in the United States. At the same time, investors have broadly shifted from active to passive investing strategies. While there is significant research in each of these respective areas, we believe that we are the first to examine whether a socially conscious investor can employ a passive approach or if the constrained nature of SRI necessitates active management. As such, we examine the performance of socially conscious ETFs versus a matched sample of actively managed SRI mutual funds. We find the performance, as a whole, to be insignificantly different between the two groups, suggesting that social investors may be able to follow a passive approach without sacrificing performance

Enhanced granular medium-based tube press hardening

Active and passive control strategies of internal pressure for hot forming of tubes and profiles with granular media are described. Force transmission and plastic deformation of granular medium is experimentally investigated. Friction between tube, granular medium and die as also the external stress field are shown to be essential for the process understanding. Wrinkling, thinning and insufficient forming of the tube establishes the process window for the active pressure process. By improving the punch geometry and controlling tribological conditions, the process limits are extended. Examples for the passive pressure process reveal new opportunities for hot forming of tubes and profiles.Comment: 4 pages, 11 figure

Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip

Vascular plants rely on differences of osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as M\"unch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems to occur via passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the nonlinear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of M\"unch transport, where phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with the hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies