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The quantification of submarine and intertidal groundwater discharge (SiGD) or purely submarine groundwater discharge (SGD) from coastal karst aquifers presents a major challenge, as neither is directly measurable. In addition, the expected heterogeneity and intrinsic structure of such karst aquifers must be considered when quantifying SGD or SiGD. This study applies a set of methods for the coastal karst aquifer of Bell Harbour in western Ireland, using long-term onshore and offshore time series from a high-resolution monitoring network, to link catchment groundwater flow dynamics to groundwater discharge as SiGD. The SiGD is estimated using the Bpollution flushing model^, i.e. a mass-balance approach, while catchment dynamics are quantified using borehole hydrograph analysis, single-borehole dilution tests, a water balance calculation, and cross-correlation analysis. The results of these analyses are then synthesised, describing a multi-level conduit-dominated coastal aquifer with a highly fluctuating overflow regime draining as SiGD, which is in part highly correlated with the overall piezometric level in the aquifer. This concept was simulated using a hydraulic pipe network model built in InfoWorks ICM [Integrated Catchment Modeling] ® version 7.0 software (Innovyze). The model is capable of representing the overall highly variable discharge dynamics, predicting SiGD from the catchment to range from almost 0 to 4.3 m 3 /s. The study emphasises the need for long-term monitoring as the basis for any discharge studies of coastal karst aquifers. It further highlights the fact that multiple discharge locations may drain the aquifer, and therefore must be taken into consideration in the assessment of coastal karst aquifers.

Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.

Karstified carbonate aquifers may receive significant recharge contributions from losing streams, hence, the knowledge about surface water-groundwater (SW-GW) interactions is crucial with regard to water management (e.g., source protection zone delineation). The dynamics of SW-GW interactions may depend on factors such as the relative water levels between streams and aquifers, resulting in a temporal variation of exchange, which imposes complexity to the understanding of such dynamics. This study highlights the use of high-resolution time series and multiresolution analysis to help to gain insights into such complex dynamics. Wavelet coherence is applied on hourly time series of rainfall, stream, and spring discharges of a low-lying karstified spring catchment to yield a correlation in the time-frequency domain. This analysis provides comprehensive information on the overall impact of the river on the spring, which is supported by the cross-correlation function, as well as by more detailed information, including time-variant influences such as a threshold level of influence. Field observations of turbidity sampling at the spring appear to support this interpretation. This innovative approach relies on basic hydrological parameters, water level, or discharge, and is therefore applicable to many other systems with such existing time series. Article impact statement: Wavelet coherence yields highresolution information on surface water-groundwater interaction.

Karstified carbonate aquifers are highly heterogeneous systems characterized by multiple recharge, flow, and discharge components. The quantification of the relative contribution of these components, as well as their numerical representation, remains a challenge. This paper identifies three recharge components in the time and frequency domain. While the analysis in the time domain follows traditional approaches, the analysis of the power spectrum allows frequencies associated with specific spectral coefficients and noise types to be distinguished more objectively. The analysis follows the presented hypothesis that the different frequency-noise components are the result of aquifer heterogeneity transforming the random rainfall input into a sequence of non-Gaussian signals. The distinct signals are then numerically represented in the context of a semidistributed pipe network model in order to simulate recharge, flow, and discharge of an Irish karst catchment more realistically. By linking the power spectra of the modeled recharge components with the spectra of the spring discharge, the information usually gained by classical performance indicators is significantly widened. The modeled spring discharge is well matched in the time and frequency domain, yet the different recharge dynamics explain the signal of the aquifer outlet in different noise domains across the spectrum. This study demonstrates the conjunctive use of frequency analysis in conceptualization of a hydrological system together with modeling and evaluation.

Several different approaches have been developed to model the specific characteristics of karst aquifers, taking account of their inherent complex spatial and temporal heterogeneities. This paper sets out the development of a semidistributed modelling approach for applications in an Irish karst context using urban drainage software. The models have proven to be very useful for different studies, with examples given for the ecohydrology of ephemeral karst lakes, extreme groundwater-flood alleviation, karst network investigation, submarine groundwater discharge, and quantification of different recharge and flow components. The limitations of the approach are also highlighted, in particular not being able to simulate diffuse infiltration and flow paths explicitly across the groundwater catchment. Hence, a more distributed, finite-difference modelling approach using MODFLOW Unstructured Grid (USG) with the newly developed Connected Linear Network (CLN) process is then compared against the semidistributed approach on the same karst catchment. Whilst it has proven difficult to achieve the same levels of model performance in simulating the spring flows in the distributed model compared to the semidistributed model, the ability to interrogate the flow paths at any point on the three-dimensional aquifer is demonstrated, which can give new insights into flows (and potential contaminant transport) through such complex systems. The influence of the proximity of highly transmissive conduits on the flow dynamics through the much-lower transmissive matrix cells in which the network is embedded has been particularly investigated.

In order to prevent hot-gas ingestion into the rotating turbo machine’s inside, rim seals are used in the cavities located between stator- and rotor-disc. The sealing flow ejected through the rim seal interacts with the boundary layer of the main gas flow, thus playing a significant role in the formation of secondary flows which are a major contributor to aerodynamic losses in turbine passages. Investigations performed in the EU project MAGPI concentrate on the interaction between the sealing flow and the main gas flow and in particular on the influence of different rim seal geometries regarding the loss-mechanism in a low-pressure turbine passage. Within the CFD work reported in this paper static simulations of one typical low-pressure turbine passage were conducted containing two different rim seal geometries, respectively. The sealing flow through the rim seal had an azimuthal velocity component and its rate has been varied between 0–1% of the main gas flow. The modular design of the computational domain provided the easy exchange of the rim seal geometry without remeshing the main gas flow. This allowed assessing the appearing effects only to the change of rim seal geometry. The results of this work agree with well-known secondary flow phenomena inside a turbine passage and reveal the impact of the different rim seal geometries on hot-gas ingestion and aerodynamic losses quantified by a total pressure loss coefficient along the turbine blade. While the simple axial gap geometry suffers considerable hot-gas ingestion upstream the blade leading edge, the compound geometry implying an axial overlapping presents a more promising prevention against hot-gas ingestion. Furthermore, the effect of rim seals on the turbine passage flow field has been identified applying adequate flow visualisation techniques. As a result of the favourable conduction of sealing flow through the compound geometry, the boundary layer is less lifted by the ejected sealing flow, thus resulting in a comparatively reduced total pressure loss coefficient over the turbine blade.

The sealing of the machine’s inside against hot-gas ingestion is commonly provided by blowing relative cold compressor air radially out through the turbine wheelspace. Rim-seals located inside the wheelspace are primarily designed to keep the required amount of sealing at a minimum. A further possible function of the rim-seal follows from the desire to reduce the aerodynamic losses contributed by the interaction of the emerging sealing flow with the boundary layer of the incoming main flow. Investigations performend in the EU project MAGPI concentrate on the interaction between the sealing flow and the main gas flow and in particular on the effect of different rim seal designs regarding the loss-mechanism in a low-pressure turbine passage. Two different rim seal designs inside a linear low-pressure turbine cascade rig have been analysed in detail. Both, the simple axial gap and the more complex compound design were investigated under the influence of different sealing mass flow rates. Furthermore, a configuration without any cavity in the main gas flow served as a reference case. Extensive measurements of the total pressure loss over the turbine blade have been conducted by means of a five-hole probe. Additionally, the blade loading has been measured at several blade heights. A considerable increase of total pressure losses was observed due to the presence of a cavity with any rim seal design, even for no sealing flow. Higher sealing mass flow rates intensified this effect which becomes manifested in a strengthening of the secondary flows downstream the cascade. Experiments revealed also significant differences in loss-increment depending on the rim seal design used. Deeper insight into the interaction of the flows close to the rim seal is given by results of Laser-Doppler-Velocimetry measurements. The rounded shape of the compound design, which implies an axial overlapping, represents a promising prevention against hot-gas ingestion. While the axial gap design is characterized by higher losses, it also suffers considerable hot-gas ingestion in front of the blade leading edge. A parametric study regarding a possible optimization of the axial gap design is presented in this work.