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We present a unified modeling framework for Time-Based Maintenance (TBM) and Condition-Based Maintenance (CBM) for optimization of replacements in multi-component systems. The considered system has a K -out-of- N reliability structure, and components deteriorate according to a multivariate gamma process with Levy copula dependence. The TBM and CBM models are formulated as Markov Decision Processes (MDPs), and optimal policies are found using dynamic programming. Solving the CBM model requires that the continuous deterioration process is discretized. We therefore investigate the discretization level required for obtaining a near-optimal policy. Our results indicate that a coarser discretization level than previously suggested in the literature is adequate, indicating that dynamic programming is a feasible approach for optimization in multi-component systems. We further demonstrate this through empirical results for the size limit of the MDP models when solved with an optimized implementation of modified policy iteration. The TBM model can generally be solved with more components than the CBM model, since the former has a sparser state transition structure. In the special case of independent component deterioration, transition probabilities can be calculated efficiently at runtime. This reduces the memory requirements substantially. For this case, we also achieved a tenfold speedup when using ten processors in a parallel implementation of algorithm. Altogether, our results show that the computational requirements for systems with independent component deterioration increase at a slower rate than for systems with stochastic dependence.

This article assesses the theory of ‘the subjunctive as a subordination marker’ proposed by Schmitt Jensen (1970a) in light of recent advances in the study of complementation. It argues that the theory is consistent with the functional framework of Givón (2001) where event integration is said to be mirrored in syntactic integration, but that it has to be revised on two points. Firstly, instead of marking syntactic subordination, we propose that the subjunctive is used to prompt a non-referring construal of the subordinate clause in certain constructions. Secondly, we suggest that the subjunctive prompts a conceptually secondary construal of the subordinate clause in other constructions. By proposing a twofold function of the subjunctive, we can keep the main findings of Schmitt Jensen while avoiding potential circularity and shortcomings that the syntactic theory of the subjunctive faces when applied to, e.g., mood alternation.

The important role of unicellular flagellated micro-organisms in aquatic food webs is mediated by their flagella, which enable them to swim and generate feeding currents. The flagellum in many predatory flagellates is equipped with hairs (mastigonemes) that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, the mechanism of such reversal has been attributed to drag-based thrust of individual hairs, neglecting their hydrodynamic interactions. However, at natural densities of hairs, hydrodynamic interactions are important. In fact, using fully resolved three-dimensional computational fluid dynamics, we show here that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing smooth flagella. We reveal the significant role of the curvature of the flagellum, and using model analysis we demonstrate that strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a basis for understanding the diverse flagellar architectures and feeding modes of predatory flagellates.

Ciliates are ubiquitous in the marine environment and important consumers of phytoplankton and flagellates. The feeding on suspended food particles in ciliates is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. Many ciliates are upstream collectors that use a single membranelle band to generate feeding flow, retain food particles, and transport them to the mouth region. To explore a representative upstream collector, we consider the ciliate Euplotes vannus. We determine the clearance rate using particle tracking and estimate the flow-generating force by fitting a point force model to the observed flow. To obtain a mechanistic understanding of feeding flow and particle retention, we use high-speed video microscopy. The cilia move parallel to the membranelle band towards the mouth region in the power strokes, and a metachronal wave propagates away from the mouth region parallel to the band and outwards along the membranelles. A gap, therefore, opens between neighboring membranelles from the inner side of the band where the mouth region is located, and while food particles are retained, water is drawn in and pushed outwards across the membranelle band as the gap closes from the inner side. We suggest a model that rationalizes our observations and quantifies the mechanism by which the membranelle band can both generate feeding flow and retain food particles. The model compares favorably with our observations and suggests a trade-off between the clearance rate and the membranelle gap that determines the lower cutoff in the prey size spectrum. We quantify the trade-off and discuss the ecological significance of our findings.

The important role of flagellates in aquatic microbial food webs is mediated by their flagella that enable them to swim and generate a feeding current. The flagellum in most predatory flagellates is equipped with rigid hairs that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, such reversal has been attributed to drag anisotropy of individual hairs, neglecting their hydrodynamic interactions. Here, we show that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing microswimmers with smooth flagella. Using computational fluid dynamics and model analysis, we demonstrate that long and not too closely spaced hairs and strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a theoretical basis for understanding the diverse flagellar architectures and feeding modes found in predatory flagellates.

Microscopic sessile suspension feeders live attached to surfaces and, by consuming bacteria-sized prey and by being consumed, they form an important part of aquatic ecosystems. Their environmental impact is mediated by their feeding rate, which depends on a self-generated feeding current. The feeding rate has been hypothesized to be limited by recirculating eddies that cause the organisms to feed from water that is depleted of food particles. However, those results considered organisms in still water, while ambient flow is often present in their natural habitats. We show, using a point-force model, that even very slow ambient flow, with speed several orders of magnitude less than that of the self-generated feeding current, is sufficient to disrupt the eddies around perpendicular suspension feeders, providing a constant supply of food-rich water. However, the feeding rate decreases in external flow at a range of non-perpendicular orientations due to the formation of recirculation structures not seen in still water. We quantify the feeding flow and observe such recirculation experimentally for the suspension feeder Vorticella convallaria in external flows typical of streams and rivers.

Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high).

Purpose To determine the prevalence of SARS-CoV-2 at a Danish tertiary referral otorhinolaryngology clinic during the first wave of the COVID-19 pandemic among patients with suspected acute upper airway infection (UAI) and patients operated for head and neck cancer (HNC), respectively. To monitor changes in the number of patient encounters for acute UAI and the number of referrals for the workup of HNC. Trial registration: NCT-04356560 (Clinicaltrials.gov). Methods Prospective enrolled case series of all patients with suspected acute UAI (n = 88) and of patients undergoing surgery for HNC (n = 96), respectively, from March 23rd to May 5th, 2020, at a public tertiary referral otorhinolaryngology clinic in Denmark. SARS-CoV-2 was diagnosed with nasopharyngeal and oropharyngeal swabbing. Patients with suspected acute UAI had symptoms and definitive diagnoses registered in a database. Trends in the number of referrals and patient encounters were retrieved from an electronic patient journal system and analyzed retrospectively. Results Eighty-eight patients with acute UAI were enrolled including 55 men and 34 women, median age of 31 years (range: 10 months to 82 years). One patient (1.1%) tested positive. Among 96 patients operated for HNC, zero tested positive. The number of referrals for HNC workup, and patient encounters for peritonsillar abscesses, decreased markedly in the first 3 weeks. Conclusion The prevalence of SARS-CoV-2 during the first 6 weeks of the first wave was minimal among patients with acute UAI and zero among patients operated for HNC. The decrease in referrals for the workup of HNC may increase time to treatment initiation and patient morbidity. Supplementary Information The online version contains supplementary material available at 10.1007/s00405-020-06514-6.

Importance Otorhinolaryngology is considered one of the medical specialties with a high risk for exposure to corona disease 2019 (COVID-19). Uncontrolled transmission in a hospital department poses a risk to both healthcare workers (HCWs) and patients. Objective To monitor SARS-CoV-2 incidence, transmission, and antibody development among HCWs to identify high risk procedures, pathways, and work areas within the department. Methods Prospective cohort study of HCWs using repetitive oro- and nasopharygeal swab samples, antibody tests, and self-reported symptoms questionnaires at a tertiary referral center in Copenhagen, Denmark. Results 347/361 (96%) HCWs participated. Seven (1.9%) were positive on swab tests and none had symptoms. Fifteen (4.2%) developed antibodies. Only one case of potential transmission between HCWs was identified. Infection rates were low and no procedures or areas within the department were identified as exposing HCWs to a higher risk. Conclusions and relevance Adherence to the surveillance program was high. The low incidence among HCW during the first wave of the COVID-19 pandemic may reflect local transmission and infection control precautions, as well as a low infectious burden in the Danish society.

Many Danish cognition verbs take both indicative and infinitive complements. We examine what the contrast between the two complement types codes. The literature offers two answers. One is that the contrast has to do with coreferentiality: if the cognition verb’s primary argument is coreferential with the primary argument of the complement, the complement tends to be infinitive; if not, the complement must be indicative. The other answer concerns the contrast between propositions and states-of-affairs: the indicative complement designates a proposition, the infinitive complement a state-of-affairs. Corpus studies support both answers. They also support an analysis of indicatives as proposition markers. But infinitives cannot – straightforwardly – be analysed as state-of-affairs markers.

Daily scheduling of surgical operations is a complicated and recurrent problem in the literature on health care optimization. In this study, we present an often overlooked approach to this problem that incorporates a rolling and overlapping planning horizon. The basis of our modeling approach is a Markov decision process, where patients are scheduled to a date and room on a daily basis. Acknowledging that both state and action space are only partially observable, we employ our model using a simulation-based method, where actions are derived from a heuristic search procedure. We test the potential of using this modeling approach on the resulting hospital costs, and number of patients that are outsourced to avoid violating constraints on capacity. Using data from a Danish hospital, we find a distinct improvement in performance when compared to a policy that resembles a manual planner. Further analysis shows that substantial improvements can be attained by employing other simple policies.

Many aquatic microorganisms are attached to solid surfaces while creating feeding flows that bring prey particles to them. To explore the effects of surface proximity and orientation of the flow-generating force, we analyze the low-Reynolds-number flow due to a point force above a plane no-slip surface. The presence of the surface reduces the feeding flow relative to the unbounded situation. We show that the reduction of the flow rate through a circular clearance disk perpendicular to the force and centered at its position is twice as large when the force is perpendicular to the surface as when it is parallel. When the force is perpendicular to the surface, the flow forms a toroidal eddy with closed streamlines, and the resulting flow recirculation may lead to refiltration of water that has already been cleared for prey. We prove that due to the nature of the far-field flow, the shortest recirculation time along a streamline through a circular clearance disk is inversely proportional to the flow rate to the power four. Finally, we discuss the biological advantages and disadvantages of perpendicular and parallel force orientation and the effects of prey diffusion and ambient flow, and we argue that recirculation is irrelevant in the typical perpendicular feeding flow since the recirculation time is long compared to the biologically relevant timescales.