Panagiotis Prinos

This study examines the collision of two counterflowing gravity currents (GCs) in a stratified environment using large-eddy simulation. A half-depth lock-exchange configuration with linearly stratified ambient fluid is used to study symmetric collision (i.e., GCs with the same densities and heights). Numerical experiments on collision of both subcritical and supercritical GCs are performed for weak and strong stratification. The focus is on the investigation of both collision and post-collision phases, with emphasis on the height and the vertical velocity of the collided fluid, the bore velocity during the post-collision phase, and the mixing and its efficiency. The results indicate that stronger stratification causes the GCs to become subcritical and decelerate before collision. In contrast, for weaker stratification, GCs become supercritical and the collision resembles that in a non-stratified environment. Subcritical collision is characterized by oscillatory vertical motion of the collided fluid, which can suppress bore formation. Supercritical collision is distinguished by clearly defined phases and bore development during the post-collision phase, similar to that in a non-stratified environment. The qualitative difference between the collision of subcritical and supercritical GCs is due to the occurrence of internal waves, which emerge during GC propagation before collision and interfere in the middle of the tank, determining collision dynamics. Mixing during collision is also affected by stratification strength. While stronger stratification suppresses turbulence and mixing, weaker stratification enhances turbulence and broadens the mixing region, leading to higher mixing with reduced mixing efficiency.

Pressure management is a fundamental and highly effective method for the management of real losses in water distribution networks and therefore reducing non-revenue water. In this work, a methodology is developed to assess leakages in the water distribution networks of small settlements. The settlement of Agios Germanos in the Municipality of Prespes is selected as a representative case study. The hydraulic modeling of the water distribution network in the study area is used to assess the hydraulic behavior of the existing infrastructure in its current state of operation and to find critical locations to install the necessary measuring equipment (pressure sensors, flow meters, water level sensors, and pressure reducing valves). This equipment is used to calibrate the hydraulic model, estimate leakages, and manage them effectively. Minimum night flow analysis is utilized to assess leakages in the studied network based on measurements of the hydraulic parameters from the equipment installed. The effects of pressure management on leakages are then examined by assessing the relationship between the pressure and leak flow rate in the selected settlement.

In this work, vegetation effects on the characteristics of radiation-induced natural convection (isotherms, circulation patterns, exchange flow rate) in sloping water bodies are investigated numerically. The water body consists of (a) a sloping vegetated region (with a bottom slope equal to 0.1) and (b) a deep region with a horizontal bottom. The vegetation of porosity 0.85 (typical of aquatic plants found in lakes) has a length equal to the length of the sloping region. It can block (totally or partially) the radiation and as a result a non-uniform (differential) heating is developed along the free surface of the water body. The Volume-Averaged Navier–Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. The radiation-induced natural convection in a water body with only a sloping region (with no vegetation) is also considered for validation purposes since numerical and scaling analysis results are available in literature. The results indicate significant vegetation effects on the thermal and flow patterns especially for vegetation which blocks completely surface heating.

In this work, diurnal heating, by solar radiation, and cooling in a sloping water body with vegetated shallow regions are investigated numerically. The sloping water body consists of a vegetated region with a bottom slope equal to 0.1 and a deep region with a horizontal bottom. The Volume-Averaged Navier-Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. The latter has porosity equal to 0.85 (typical of aquatic plants found in lakes) and a length equal to the total length of the sloping region. At the top free surface, a time-dependent thermal forcing is applied, which is reduced in the vegetated region. Shallow water is considered with a maximum water depth being less than the penetration depth of the solar radiation. The non-vegetated sloping water body is also considered for validation purposes. The results (isotherms, streamlines, exchange flow rate), after ten full thermal forcing cycles, indicate significant vegetation effects on the daytime and night circulation and the exchange flow rates.

The expansion of urban areas and the increasing frequency and magnitude of intense rainfall events are anticipated to contribute to the widespread escalation of urban flood risk across the globe. To effectively mitigate future flood risks, it is crucial to combine a comprehensive examination of intense rainfall events in urban areas with the utilization of detailed hydrodynamic models. This study combines extreme value analysis techniques applied to rainfall data ranging from sub-hourly to daily durations with a high-resolution flood modelling analysis at the building level in the centre of Thessaloniki, Greece. A scaling procedure is employed to rainfall return levels assessed by applying the generalised extreme value (GEV) distribution to annual maximum fine-temporal-scale data, and these scaling laws are then applied to more reliable daily rainfall return levels estimated by means of the generalised Pareto distribution (GPD), in order to develop storm profiles with durations of 1 h and 2 h. The advanced flood model, CityCAT, is then used for the simulation of pluvial flooding, providing reliable assessments of building-level exposure to flooding hazards. The results of the analysis conducted provide insights into flood depths and water flowpaths in the city centre of Thessaloniki, identifying major flowpaths along certain main streets resulting in localised flooding, and identifying around 165 and 186 buildings highly exposed to inundation risk in the study area for 50-year storm events with durations of 1 h and 2 h, respectively. For the first time in this study area, a detailed analysis of extreme rainfall events is combined with a high-resolution Digital Terrain Model (DTM), used as an input into the advanced and fully featured CityCAT hydrodynamic model, to assess critical flowpaths and buildings at high flood risk. The results of this study can aid in the planning and design of resilient solutions to combat urban flash floods, as well as contribute to targeted flood damage mitigation and flood risk reduction.

There are no studies specifically aimed at characterizing and quantifying drag forces on finite cylinder arrays in the mixing layer of compound channel flows. Addressing this research gap, the current study is aimed at characterizing experimentally drag forces and drag coefficients on a square-cylinder array placed near the main-channel/floodplain interface, where a mixing layer develops. Testing conditions comprise two values of relative submergence of the floodplain and similar ranges of Froude and bulk Reynolds numbers. Time-averaged hydrodynamic drag forces are calculated from an integral analysis: the Reynolds-averaged integral momentum (RAIM) conservation equations are applied to a control volume to compute the drag force, with all other terms in the RAIM equations directly estimated from velocity or depth measurements. This investigation revealed that, for both tested conditions, the values of the array-averaged drag coefficient are smaller than those of cylinders in tandem or side by side. It is argued that momentum exchanges between the flow in the main channel and the flow in front of the array contributes to reduce the pressure difference on cylinders closer to the interface. The observed drag reduction does not scale with the normalized shear rate or the relative submersion. It is proposed that the value of the drag coefficient is inversely proportional to a Reynolds number based on the velocity difference between the main-channel and the array and on cylinder spacing.