UNSTEADY BOUNDARY LAYER TRANSITION ON A 11111111111lligium HIGH PRESSURE TURBINE ROTOR BLADE

99-GT-194 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussioeis printed only if the paper is published in an AS ME Journal. ABSTRACT This investigation is aimed at the experimental determination of the location, the extent, and the modes of the laminar-to-turbulent transition processes in the boundary layers of a high pressure turbine rotor blade. The results are based on time-resolved, qualitative wall shear stress data which was derived from surface hotfilm measurements. The tests were conducted in the "Windtunnel for Rotating Cascades" of the DLR in Gottingen. For the evaluation of the influence of passing wakes and shocks on the unsteady boundary layer transition, a test with undisturbed rotor inlet flow was conducted in addition to full stage tests. Two different transition modes led to a periodic-unsteady, multimoded transition on the suction side. In between two wakes, transition started in the bypass mode and terminated as separated-flow transition. Underneath the wakes, plain bypass transition occurred. The weak periodic boundary layer features on the pressure side indicate that this surface was not significantly affected by passing wakes or shocks. The acquired data reveals that the periodically disturbed suction side boundary layer is less susceptible to bubble bursting than the undisturbed flowfield. Thus, these blades may be subjected to higher aerodynamic loads. Accordingly, as in low pressure turbines, the unsteady effects in high pressure turbines may allow for a reduction of the number of rotor blades, with respect to the original design

Half a century of high-latitude fisheries oceanography research on the "recruitment problem" in Northeast Arctic cod (Gadus morhua)

Predicting recruitment in a reliable fashion is a great challenge within operational fisheries advice. Here, we consulted the unique but undercommunicated IMR Cod Larva Project (1975–1990), its spin-offs, placed in an international era of advancements over the last 50 years to glance into the future. Few initiatives of this kind have applied such extensive research approaches, spanning from laboratory, mesocosm, tank, and field studies to process modelling. The “critical period” concept appeared misleading, covering months rather than days of the early life history stages (ELHS) of Northeast Arctic cod. Larval feeding success was strongly modified by improved encounter rates from wind-induced turbulence. In addition, the following maternal effect studies evidenced that the dynamics of stock demography prior to spawning should be upheld to promote recruitment success. Although we now have lower-trophic level models as well as ELHS individual-based models, such models are still insufficiently reflecting the needed spatiotemporal resolution. The same problem applies to climate/circulation models. Nevertheless, this long-lasting research has significantly improved the mechanistic understanding of ELHS dynamics but also of the more predictable adult reproductive parameters. Based on a “to-list list,” we suggest research avenues that should be pursued to further improve our ability predicting recruitment strength in marine fish stocks.publishedVersio

An appraisal of the drivers of Norwegian spring‐spawning herring (Clupea harengus) recruitment

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Drift indices confirm that rapid larval displacement is essential for recruitment success in high-latitude oceans

Larval drift is a key process for successful fish recruitment. We used Norwegian spring-spawning herring (Clupea harengus) as model species to investigate the relationship between larval drift and recruitment. Larval drift indices were derived from simulations based on survey observations between 1993 and 2016. We show that forward simulated larval drift indices have an important positive relation to recruitment success. The relationship demonstrates elevated recruitment when larvae relocate rapidly northwards toward the Barents Sea. Negative or low larval drift indices coincide with only weak recruitment emphasizing limited survival in years with enhanced larval retention. Hence, with this work we combine drift model outcomes refined with survey data indicating that more extensive larval drift is an important component in population dynamics for high-latitude small pelagic fishes. However, larval displacement alone represents only one among many controlling factors but may offer possible predictions of the probability of higher or lower recruitment in the short term. The applicability of the drift indices is adaptable in all world oceans and all marine organisms that occupy planktonic life stages exposed to dynamic ocean currents. The study demonstrates how larval drift indices help to identify larval transport or retention to be crucial for population replenishment.publishedVersio

Environmental influences on Norwegian spring-spawning herring (<i>Clupea harengus</i> L.) larvae reveal recent constraints in recruitment success

The lack of any abundant recruiting year class of Norwegian spring-spawning (NSS) herring between 2005 and 2015 contributed to an approximate reduction of 40% in the spawning-stock biomass since 2009, i.e. from 7 to 4 million tonnes. Warming of the North Atlantic is suggested to contribute to this reduction in recruitment. In the past, a warm phase induced by a positive Atlantic Multidecadal Oscillation (AMO) in the North Atlantic was positively correlated to the NSS herring stock size. Recent unprecedented ocean warming in the Norwegian Sea ecosystem, besides elevated temperatures due to a positive AMO, seems to be outside optimal environmental conditions for early life history stages of NSS herring. We analysed 28 years of survey data using generalized additive models to reconstruct environmental conditions for drifting yolksac and preflexion stage larvae. Our results indicate that strong recruitment years were more likely when the larvae occurred simultaneously with a negative AMO during positive temperature anomalies. The transition from yolksac stages towards preflexion stages occurred while there was a slow increase in water temperature during the larval drift. Weak recruitment years generally occurred when larvae experienced elevated temperatures during the life stage transition under a positive AMO. These results augment evidence that the historical positive relationship between AMO and stock dynamics is reversed between 1988 and 2015. Albeit not implying any specific mechanistic biological interactions, we can assume that the unprecedented warming has modified the ecosystem drivers that negatively affect drifting larvae. Since 2016, the North Atlantic is shifting into a negative AMO phase, possibly resulting in the 10-year recruitment suppression of NSS herring ending soon.publishedVersio

Is it possible to photoperiod manipulate spawning time in planktivorous fish ? A long-term experiment on Atlantic herring

The oocyte development (vitellogenesis) of individual fish is highly dependent upon their physiology which is influenced by both intrinsic and extrinsic factors. Thus, if individuals encounter poor biophysical conditions, they will likely be unable to reproduce. The photoperiod is an external factor that is constant between years but with increasing intra-annual (seasonal) variation polewards. In high-latitude marine environments, several ecological processes are strongly photic entrained, e.g., the planktonic spring bloom. However, it is still unclear whether day length or planktonic peaks (feeding opportunity) is the main timer or regulator behind gametogenesis not only for teleost piscivores but also for planktivores living in these waters. Hence, we experimentally investigated the role of photoperiod steering vitellogenesis in the planktivorous Atlantic herring (Clupea harengus), rearing larvae up to the mature adult stage. We imposed a natural and 6-month offset photoperiod hypothesising that vitellogenesis is entrained by this photic zeitgeber. The results of our experiment clearly demonstrated that herring have a strong photic zeitgeber acting upon vitellogenesis according to the experienced photoperiod. Thus, the Offset Group showed a displacement in vitellogenesis of 6 months. The second hypothesis that feeding opportunities play a clear role in assisting this photic zeitgeber in Atlantic herring could be rejected. This clarification supports that the survival potential of the larvae is the main selection pressure in operation in these respects, i.e., rather than the extent of feeding opportunities of the adults.publishedVersio

Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.publishedVersio

Asynchronized spawning responses of small pelaigic fish to a short-term enviromental change

We provide substantial evidence on how short-term changes in environmental conditions activate and deactivate spawning activities in small pelagic fishes. An ichthyoplankton survey was conducted along the southernmost part of the Canary Current upwelling ecosystem in May 2013, covering the area twice within 20 d. This period coincided with a strong environmental change from a cold productive upwelling regime to a warmer and less productive upwelling relaxation event. This change triggered a shift in spawning activity from European anchovy Engraulis encrasicolus to round/flat sardinella Sardinella spp. We used zero-altered negative binomial regression models with a generalized additive structure based on integrated nested Laplace approximations to link early larval distribution patterns to the 2 different regimes. The models confirmed 2 species-specific temperature spawning windows, suggesting a spawning pause of anchovy during upwelling relaxation while simultaneously activating spawning in sardinella. Observing immediate spawning responses to the 2 environmental regimes underlines the assumption that windows of spawning opportunity are the main drivers of small pelagic fish fluctuations in upwelling regions. The duration of a specific environmental condition can, therefore, increase or decrease the chances for reproductive success. The observations of this study may explain why certain small pelagic fish species can dominate over others during a particular period and might also apply to other upwelling regions of the world oceans where upwelling and relaxation events alternate.publishedVersio

Highly mixed impacts of near-future climate change on stock productivity proxies in the North East Atlantic

Impacts of climate change on ocean productivity sustaining world fisheries are predominantly negative but vary greatly among regions. We assessed how 39 fisheries resources—ranging from data-poor to data-rich stocks—in the North East Atlantic are most likely affected under the intermediate climate emission scenario RCP4.5 towards 2050. This region is one of the most productive waters in the world but subjected to pronounced climate change, especially in the northernmost part. In this climate impact assessment, we applied a hybrid solution combining expert opinions (scorings)—supported by an extensive literature review—with mechanistic approaches, considering stocks in three different large marine ecosystems, the North, Norwegian and Barents Seas. This approach enabled calculation of the directional effect as a function of climate exposure and sensitivity attributes (life-history schedules), focusing on local stocks (conspecifics) across latitudes rather than the species in general. The resulting synopsis (50–82°N) contributes substantially to global assessments of major fisheries (FAO, The State of World Fisheries and Aquaculture, 2020), complementing related studies off northeast United States (35–45°N) (Hare et al., PLoS One, 2016, 11, e0146756) and Portugal (37–42°N) (Bueno-Pardo et al., Scientific Reports, 2021, 11, 2958). Contrary to prevailing fisheries forecasts elsewhere, we found that most assessed stocks respond positively. However, the underlying, extensive environmental clines implied that North East Atlantic stocks will develop entirely different depending upon the encountered stressors: cold-temperate stocks at the southern and Arctic stocks at the northern fringes appeared severely negatively impacted, whereas warm-temperate stocks expanding from south were found to do well along with cold-temperate stocks currently inhabiting below-optimal temperatures in the northern subregion.publishedVersio

Fischlarvendynamiken in Küsten- und Ozeanhabitaten des Kanarenstromökosystems (12 – 23°N)

Understanding processes that regulate dispersal patterns and survival of larval fish in relation to habitat structures as bottom-up drivers in upwelling ecosystems is the objective of this dissertation. To what extent do larval fish assemblages reflect species-specific adaptations to the physical environment in marine ecosystems? That’s only one of several remaining questions that have received much attention in the past, yet remain unanswered for all globally important marine ecosystems. The abundant small pelagic fish species in the southern Canary Current Upwelling Ecosystem (CCUE) provide livelihoods for millions of people, but the physical processes that drive abundance and distribution of these species are not well known. In order to understand vital processes in the CCUE that control the dispersal of larval fish assemblages, four surveys at sea were conducted between 2014 and 2016. The data framework, which was used to answer central research questions, was based on a combination of field work (expeditions with larval fish sampling), in-situ measurements of environmental parameters (salinity, fluorescence, conductivity, temperature, depth – CTD), community analysis (e.g. Cluster Analyses, Discriminant Analyses, Similarity Percentage Analysis (SIMPER), Multivariate Analysis of Variance (MANOVA), Random Forest Models, and Generalized Additive Models (GAMs)), and satellite based remote sensing data (sea surface temperature, sea surface chlorophyll-a, wind regime, absolute dynamic topography and geostrophic current vectors). Results from data collection served as the basis for preparation of my dissertation and are presented in the form of three scientific publications (Chapter I – III). • In Chapter I, a spatiotemporal niche-partitioning of larval habitats was detected for the two most dominant small pelagic species, round sardinella (Sardinella aurita) and European sardine (Sardina pilchardus), constrained by species-specific upwelling regimes. • In Chapter II, active larval vertical migration behaviour was revealed in an offshore upwelling cell. Species that occurred in the inner upwelling cell took advantage of a passive retention to circumvent a loss of larvae into unproductive oceanic regions during an upwelling event. • In Chapter III, two larval fish assemblages were observed, each inhabiting one of two water masses that were trapped by two mesoscale eddies along a front