The energy required for space heating amounts to approximately 68% of the total energy demand of existing buildings in Europe. The heat requirement of a building, and thus its carbon emission, can be lowered by optimizing the supply and return temperature of the heating system. A lower supply temperature enables a wider variety of transition pathways towards sustainable heating with reduced carbon emissions. However, the minimum supply temperature that guarantees acceptable indoor temperatures in existing dwellings during design weather conditions is still unknown. In this study, we determine the minimum supply temperature by fitting a 2 R–2C model to hourly measurement data. The measurement data is obtained from a representative set of 220 existing gas-fired dwellings in the Netherlands. The heating system of each dwelling was equipped with a pulse flowmeter and temperature sensors on both the supply and return side. Additionally, data was collected from the thermostat in the main living room and the gas boiler. The data was supplemented with weather data from a nearby weather station. The data-driven model shows that the minimum supply temperature can be lower than 55 °C for 60% of the dwellings during design weather conditions (i.e., −10 °C in the Netherlands). Moreover, the minimum supply temperature is poorly correlated with general building properties, such as the building typology, construction period or specific annual space heating demand (kWh/(m²yr)). On the contrary, the ratio between the required and installed heat output of the radiators in the heating system is a promising parameter to predict the minimum design supply temperature of an individual dwelling that guarantees an acceptable indoor temperature during design weather conditions.

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We present an experimental study on the gas flow field development over a plunging breaking wave prior to impact on a vertical wall. The variability of wave impact pressure over repeated measurements is well known (Bagnold, 1939). The formation of instabilities on the wave crest are postulated to be the main source of impact pressure variability (Dias and Ghidaglia, 2018). However, the mechanism that results in wave impact pressure variability and the influence of the gas phase in particular are relatively unknown. The velocity field of the gas phase is measured with particle image velocimetry, while simultaneously the local free surface is determined with a stereo-planar laser-induced fluorescence technique. The bulk velocity between the wave crest tip and the impact wall deviates from the mass conservation estimate based on the velocity profile between the wave crest and the impact wall. This is caused by a significant increase of the local gas velocity near the wave crest tip. The non-uniformities in the seeding concentration accumulate near the wave crest tip and reduce the accuracy of the velocity measurements. However, the bulk velocity estimate is significantly improved with a fit of the velocity profile that is based on a potential flow over a bluff body. Additionally, the development of vortex is observed and quantified for two typical measurements with either a disturbance on the wave crest or a smooth wave crest. The circulation development is comparable to the formation and separation of a vortex ring, which results in a saturated vortex that separates from the wave crest (Gharib et al., 1998). Furthermore, the impact location of the wave tip is altered by the formation of secondary vortices. The secondary vortex enhances the lift locally and alters the trajectory of the wave crest tip, which may result in additional variability of the wave impact pressure.@en

The prospect of stricter national and international emission standards for the shipping industry are a driving force in the search for alternative shipping fuels, such as liquefied natural gas (LNG). However, new challenges arise with the widespread use of LNG. For example, there is a desire to use LNG cargo containment systems at lower filling levels. These filling levels are strictly limited to prevent the movement of liquid inside the containment system, which is known as sloshing. Sloshing inside a cargo containment system can result in extreme wave impact events with the potential to cause structural damage. Therefore, a fundamental understanding of these extreme wave impact events is required before studying increasingly complex phenomena.   The study of wave impacts on a wall has been an active area of research for decades. Moreover, the impact of waves upon structures is relevant for many fields such as ocean, coastal, and maritime engineering. The generation of repeatable waves in a laboratory environment is not trivial (Bagnold, 1939). Small changes in the experimental conditions, such as the water depth and the wave generation method, result in significant impact pressure variability. The impact pressure variability is even observed in carefully repeated wave impact experiments with minimal variability of the input parameters. For these measurements, the source of the impact pressure variability is thought to be the instability development on the wave crest. However, the mechanism that is responsible for the formation of these instabilities is still largely unknown.   The aim of this work is to gain insight in the sources of wave impact pressure variability. This is accomplished using direct measurements of the liquid free surface and particle image velocimetry of the surrounding air. The measurements are limited to a single wave (i.e., with a fixed steering signal and water depth) at atmospheric conditions, because of the complexity of the experimental measurements. A plunging breaking wave with a large gas pocket is generated that impacts on a vertical wall. The compression of the large gas pocket induces a significant gas flow between the wave crest and the vertical impact wall, which results in the formation of instabilities on the wave crest.   Quantitative measurements of the liquid free surface are obtained with an extension of the planar laser induced fluorescence (PLIF) method. The newly developed scanning stereo-PLIF measurement technique uses a stereo-camera set-up with a self-calibration procedure adapted for free surface flows. Thereby, the stereo-PLIF technique enables measurements of a free surface over a two-dimensional domain (e.g., y=f(x,z,t)). The system is versatile with a minimal influence on the fluid properties and the measurement domain can be scaled as needed.  A repeatable plunging breaking wave is created in the wave flume of the Hydraulic Engineering Laboratory at the Delft University of Technology. The wave encloses a gas pocket as it approaches the vertical impact wall. Initially, the plunging breaking wave is globally comparable to waves that do not impact on a vertical wall. The aspect ratio of the cross-sectional area of the gas pocket remains relatively constant at Rx/Ry = 1.6 ( ∼√3). Furthermore, the wave velocity (√gh0) and wave tip velocity (1.2√gh0) are initially similar to that of a plunging breaking wave. On the other hand, the trajectory of the wave tip is altered compared to that of a typical plunging breaking wave.   The trajectory of the wave tip is globally similar over repeated wave impact measurements. However, moments before impact the wave tip is deflected by the gas expelled from the gas pocket. The deflection of the wave tip introduces significant variation between the repeated measurements. On close inspection, the wave tip resembles a liquid sheet, that is destabilized by an initial Kelvin-Helmholtz instability (Villermaux et al., 2002). The flapping liquid sheet accelerates the wave tip, which triggers the development of a Rayleigh-Taylor instability. This results in approximately equally spaced liquid filaments (i.e., liquid fingers) over the spanwise direction of the wave. The spanwise wavelength depends on the density ratio (ρa/ρl) and surface tension, which was previously shown to be a source of wave impact pressure variability.  Additionally, particle image velocimetry measurements are performed to determine the interaction between the liquid and gas during a wave impact event. The global gas flow is similar to that of a plunging breaking wave, where a vortex develops on the leeward side of the wave. The vortex consistently separates from the breaking wave and lingers at the back of the breaking wave in the stagnant air. The development of circulation is typical for a vortex that eventually separates at a universal time scale denoted by the formation number (Gharib et al., 1998). However, a typical formation number can not be defined in this particular case, due to the simultaneous change of both the length and velocity scales.  The velocity profile between the wave tip and the vertical impact wall resembles that of a flow past a bluff body. A fit of the measured velocity profile agrees well with the velocity derived from mass conservation. Interestingly, the velocity close to the wave tip is approximately 2 times higher than the bulk velocity estimate. The high velocity close to the wave tip can thus result in an earlier onset of instability development compared to estimates based on the bulk velocity. Furthermore, the flow tends to separate close to the tip just before impact on the vertical wall. The effect of this flow separation on the impact pressure variability depends on the global wave shape prior to impact. For the case with a disturbance on the wave crest, the secondary vortex that forms close to the wave tip tends to break up. On the other hand, if the wave crest is smooth the secondary vortex remains attached. The attached secondary vortex increases the lift on the wave tip, which results in a significant deflection of the wave tip. Consequently, the development of secondary vortices close to the wave tip results in wave impact pressure variability, as the typical deflection is larger than the membrane diameter of a contemporary pressure transducer.   Local phenomena such as flow separation and the development of instabilities define the variability of the peak pressure during wave impacts. On the other hand, the global characteristics of an air-water wave impact on a vertical wall can be retrieved with pressure impulse models (Cooker et al., 1995). The maximum wave impact pressure is relevant for LNG containments systems and wave energy converters. Consequently, numerical models that aim to quantify wave impact pressure variability require accurate models of both the gas phase and the development of free surface instabilities. @en

We present an experimental study on the variation in wave impact location and present a mechanism for the development of free surface instabilities on the wave crest for repeatable plunging wave impacts on a vertical wall. The existence of free surface instabilities on an impacting wave is well known, but their characteristics and formation mechanism are relatively unknown. The development of the global wave shape is measured using a visualization camera, whereas the local wave shape is measured with an accurate stereo-planar laser-induced fluorescence technique. A repeatable wave is generated with negligible system variability. The global wave behavior resembles that of a plunging breaker, with a gas pocket cross-sectional area defined by an ellipse of constant aspect ratio. The variability of the local wave profile increases significantly as it approaches the wall. The impact location varies by ∼0.5% of the wave height or more than a typical pressure sensor diameter. Additionally, the wave tip accelerates to a velocity of 1.5√gh0 compared to the global wave velocity of 1.2√gh0. The difference in impact location and velocity can result in a pressure variation of ∼25%. A mechanism for instability development is observed as the wave tip becomes thinner and elongates when it approaches the wall. A flapping liquid sheet develops that accelerates the wave tip locally and this triggers a spanwise Rayleigh–Taylor instability.@en

In this work, we extend a planar laser-induced fluorescence method for free surface measurements to a three-dimensional domain using a stereo-camera system, a scanning light sheet, and a modified self-calibration procedure. The stereo-camera set-up enables a versatile measurement domain with self-calibration, improved accuracy, and redundancy (e.g., possibility to overcome occlusions). Fluid properties are not significantly altered by the fluorescent dye, which results in a non-intrusive measurement technique. The technique is validated by determining the free surface of a hydraulic flow over an obstacle and circular waves generated after droplet impact. Free surface waves can be accurately determined over a height of L= 100 mm in a large two-dimensional domain (y(x, z) = 120 × 62 mm²), with sufficient accuracy to determine small amplitude variations (η≈ 0.2 mm). The temporal resolution (Δt= 19 ms) is only limited by the available scanning equipment (f= 1 kHz rate). For other applications, this domain can be scaled as needed. Graphic abstract: [Figure not available: see fulltext.].

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A quasi three-dimensional stereo-camera measurement technique has been devised that is able to measure liquid free surface profiles, by scanning a light sheet. The technique will be applied to study the variability of impact pressure observed during wave impact in a newly developed experimental set-up. The set-up imposes severe optical (single window) and accessibility (autoclave) constraints. The devised measurement technique is required to measure liquid free surface profiles over a domain of (X,Y,Z) = (100, 100,100) mm domain with an accuracy of 1 mm. The performance of the devised measurement technique is evaluated using a convential side-view measurement (Buckley et al 2017) in the water tunnel of the Laboratory for Aero- and Hydrodynamics at the Delft University of Technology. A free surface profile is generated by flow over a bump geometry (Gui et al 2014), which provides a repeatable and increasingly complex free surface profile. The free surface profile is determined for three different cases over a domain of (X,Y,Z) = (170, 100, 62) mm with an averaged systematic error of 2.7 ± 1.2 mm. The observed error is systematic and implies that the edge detection procedure is not robust enough. In future work the systems measurement frequency and edge detection procedure will be improved. @en