Matthias Kramer

• Dr.-Ing.
• Senior Lecturer at UNSW Canberra

Dams are important for flood mitigation, water supply, and hydroelectricity. Every dam has a water conveyance structure, such as a spillway, to safely release extreme floods when needed. The flows down spillways are often self-aerated and spillway design has typically been investigated in laboratory experiments, which is due to limitations in suita...

Wall shear stress modification in self-aerated flows has been observed in boundary layer type flows mainly in the form of drag reduction. Due to its complexity, drag reduction has predominantly been studied using experimental techniques and empirical analyses, whereas the development of conceptual approaches is limited. In this study, two-phase ope...

Air–water flow turbulence was derived from pseudo-instantaneous velocities measured with a dual-tip phase-detection probe. This new technique is proposed based upon adaptive time windows for cross-correlation analysis combined with robust filtering criteria, allowing computation of velocity time series in highly aerated flows. Each velocity estimat...

Dual-tip phase-detection probes can be used to measure flow properties in gas-liquid flows. Traditionally, time-averaged interfacial velocities have been obtained through cross-correlation analysis of long time-series of phase fraction signals. Using small groups of detected particles, a recently developed adaptive window cross-correlation (AWCC) t...

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure. Such two-phase flows are commonly investigated using phase-detection intrusive probes, yielding velocities that are considered to be directly representative of...

Over the last years, there has been a renewed interest in differentiating various contributions to the air concentration in high Froude-number self-aerated flows, see for example Kramer (2024), comprising entrained and entrapped air. The former is characterized by entrained air packets and bubbles, while entrapped air corresponds to air transported...

This work provides a short clarification on the definition of specific interfacial areas in self-aerated flows, thereby distinguishing between the specific interfacial area related to the volume of an air-water mixture, and the specific interfacial area related to the water volume. This distinction is important when it comes to solving the air-wate...

Water infrastructure forms the backbone of development, being pivotal for water, food and energy security. Both existing and new infrastructure must cope with global climatic challenges and increased human activity. Continuous investment in water infrastructure is crucial, yet in many cases, investments are deferred as they are not perceived as a p...

Over the last decade, plastic pollution in river systems has been identified as a major threat to flora and fauna, as well as to human health. As such, the adequate description of plastic transport processes is of great importance, but there is a knowledge gap in our understanding of the underpinning transport mechanics. This paper studies the infl...

Lagrangian transport models (LTMs) have been widely used in the study of riverine and marine plastic pollution. However, the implementation of boundary condition for LTMs is not well established when compared to conventional Eulerian modelling. Our paper aims to investigate boundary conditions for turbulent particle transport in open-channel flows...

Groundwater vulnerability to pollution, defined as the likelihood of contaminants migrating from the ground surface to the groundwater system, is a critical environmental concern. Traditional assessment methods, often deterministic or empirical, lack a robust probabilistic framework. This study integrates groundwater movement and dynamics into a Ba...

Assessing groundwater vulnerability is essential for understanding the risk of pollutants infiltrating groundwater systems after being introduced at the ground surface. Traditional methods for groundwater vulnerability assessment (GVA) often rely on deterministic or empirical approaches, lacking a robust probabilistic framework. This study introduc...

In recent years, there has been an increasing research interest in the application of remote sensing technology to highly aerated flows, which is because this technology holds the ultimate promise to enable safe and accurate measurements of real-word air-water flows in natural and human made environments. Despite the increasing number of publicatio...

Self-aerated is a fascinating phenomenon that commonly occurs in high Froude-number flows in natural or human made environments. The most common air-water flow measurement instrument to characterize such flows is the intrusive dual-tip phase-detection needle probe, which identifies phase changes around the needle tips due to a change of resistivity...

Over the last decade, fluvial plastics have been identified as major threat to aquatic environments and human health. In order to develop adequate mitigation strategies for plastic pollution, a fundamental process understanding of riverine plastic transport is of significant importance. In this context, the implementation of research findings into...

In this presentation, I am providing an overview of new modelling approaches for air concentration distributions in self-aerated free surface flows

The characterization and the modelling of air concentration distributions in self-aerated high Froude-number free-surface flows has been subject to sustained research interest since the 1970s. Different groups of researchers have conceptualized the air concentration using single-layer or double-layer approaches. Here, it is argued that a double-lay...

The characterisation and the modelling of air concentration distributions in self-aerated free-surface flows has been subject to sustained research interest since the 1970s. Recently, a novel two-state formulation of the structure of a self-aerated flow was proposed by Kramer & Valero ( J. Fluid Mech. , vol. 966, 2023, A37), which physically explai...

The characterisation and the modelling of air concentration distributions in self-aerated free-surface flows has been subject to sustained research interest since the 1970s. Recently, a novel two-state formulation of the structure of a self-aerated flow was proposed by Kramer & Valero [2023 J. Fluid Mech. 966, A37], which physically explains the ai...

In grass-lined spillways, erosion of the canopy layer and the underlying soil by high-velocity flows can ultimately lead to a structural failure, implying rapid and unexpected flooding with potential destruction of environment and property. From a hydrodynamic point of view, canopy loss and soil erosion dynamically change the bed roughness, which l...

High-Froude-number flows become self-aerated when the destabilizing effect of turbulence overcomes gravity and surface tension forces. Traditionally, the resulting air concentration profile has been explained using single-layer approaches that invoke solutions of the advection–diffusion equation for air in water, i.e. bubbles’ dispersion. Based on...

In vegetated flows, hydrodynamic parameters, such as drag coefficient, frontal area and deflected canopy height, influence velocity distributions, mean velocity and flow resistance. Previous studies have focused on flow–structure interaction in sparse vegetation, dense vegetation or transitional canopies, respectively. To date, a unifying approach...

High Froude-number flows become self-aerated when the destabilizing effect of turbulence overcomes gravity and surface tension forces. Traditionally, the resulting air concentration profile has been explained using single-layer approaches that invoke solutions of the advection-diffusion equation for air in water, i.e., bubbles' dispersion. Based on...

Free-surface aeration is a key characteristic of high-velocity flows on spillways. Despite their common use as conveyance structures, there is a research gap in the understanding of flows across spillways with small invert roughness. To address this gap, this study investigated flows down a moderate sloped spillway with four different micro-rough i...

Velocity distributions in supercritical open-channel flows over stepped cavities have traditionally been described using a power-law approach or theoretical solutions of plane mixing layers. These approaches were found to be valid either above the step edges or above/within step cavities, but no generalized model is available. In this study, a four...

Current riverine plastic monitoring best practices mainly consider surface observations, thus neglecting the underlying distribution of plastics in the water column. Bias on plastic budgets estimations hinders advances on modelling and prediction of plastics fate. Here, we experimentally disclose the structure of plastics transport in surface water...

In the Burdekin Basin, Queensland, Australia, groundwater contamination due to agricultural activities has led to concerns over its impacts on globally significant ecosystems such as the Great Barrier Reef. An appropriate method for groundwater vulnerability assessment is essential for the sustainable use of this groundwater resource and its longer...

I would like to share my presentation for the ISHS 2022, where I am talking about recent advances in remote sensing for aerated flows in hydraulic structures. I am presenting a timeline on developments in remote sensing and two selected research outputs, which include large-scale measurements and a new processing technique for distance/displacement...

Grass-lined spillways constitute an environmentally friendly alternative to conventional concrete structures with flat slope and therefore may contribute to water-sensitive urban design. Research into supercritical self-aerated flows down grass-lined spillways has commenced recently, but important knowledge gaps related to air entrainment, flow res...

Measurements in aerated high-velocity spillway flows have been traditionally conducted with point-source instruments. LIDAR is a promising remote sensing technology that may be able to provide more detailed insights into the free-surface properties. So far only one laboratory study has investigated the use of LIDAR technology in air-water flows on...

Grass-lined spillways are flow conveyance structures with environmental benefits. The stability of such spillways has been typically assessed considering soil erosion, but the design of grass-lined spillways based on hydraulic considerations has rarely been conducted. While subcritical flows in channels with grass have been studied extensively, stu...

Grassed waterways are commonly found on floodplains and embankments, including dikes and levees. In addition to flow conveyance, they have invaluable ecological and environmental benefits. Previous research on several types of artificial grass has investigated flow properties, fundamental processes linked with flow vegetation interactions, and wate...

Velocity distributions in supercritical open-channel flows down stepped chutes are typically described using a power-law approach, which is suitable for velocity profiles above step edges. In between step edges, theoretical solutions of plane turbulent mixing layers have successfully been compared with measurements, but no generalized model is avai...

Although the study of microplastics in the aquatic environment incorporates a diversity of research fields, it is still in its infancy in many aspects while comparable topics have been studied in other disciplines for decades. In particular, extensive research in sedimentology can provide valuable insights to guide future microplastics research. To...

The concept of groundwater vulnerability was first introduced in the 1970s in France to recognize sensitive areas in which surface pollution could affect groundwater, and to enable others to develop management methods for groundwater protection against surface pollutants. Since this time, numerous methods have been developed for groundwater vulnera...

Groundwater is a vital resource of water supply throughout the world, and subject to an increasing risk of pollution from many sources (Ouedraogo et al., 2019). This has driven the need to examine the sensitivity of groundwater to surface pollutants, referred to as the groundwater vulnerability (National_Research_Council, 1993). Existing methods of...

The ecological and environmental benefits of grassed spillways present a green solution to the construction of low head conveyance structures. On a grassed spillway, the aggregate of the grass canopy and root structure may alter flow resistance, velocity distribution and other flow properties. While subcritical flows in vegetated open channels have...

Dams are important for flood mitigation, water supply and hydroelectricity. Every dam has a water conveyance structure, such as a spillway, to safely release extreme floods when needed. The flows down spillways are oftentimes self-aerated and spillway design has typically been investigated in laboratory experiments, which is due to limitations in s...

Previous research has shown that the inflow conditions upstream of hydraulic jumps can affect flow aeration, bed pressures and turbulence. However, studies of the effects of inflow conditions on free-surface properties have not been conclusive and have lacked a comparison of partially and fully developed inflow conditions. Detailed measurements of...

Grass-lined spillways represent an environmental friendly solution for flow conveyance in mildly-sloped water ways. The grass linings act as roughness elements that facilitate the stability of the soil and the safe conveyance of flows. Velocity profiles of grass-lined open channel flows have been extensively studied under subcritical flow condition...

Water-related problems affect several billion people’s lives and represent an annual challenge assessed at multitrillion US dollars, which substantiates their core role in the UN Sustainable Development Goals. Preventing direct and indirect impacts associated with water excess or water scarcity events requires expert judgement based on reliable inf...

Dual-tip phase-detection intrusive probes are the most widely used experimental technique for measuring interfacial velocities in highly aerated flows. Recently, we have introduced an adaptive window cross-correlation (AWCC) technique, which allows for the estimation of pseudo-instantaneous velocities and associated velocity statistics. Several pot...

Dual-tip phase-detection intrusive probes are commonly used to characterize gas-liquid flow velocities in hydraulic structures. Traditional post-processing is based upon the cross-correlation analysis of the raw voltage signals of the two probe tips, providing a time-averaged velocity for the full sampling duration. A recent advancement of the trad...

Hydraulic jumps are oftentimes encountered in natural and human-made environments. The transition from supercritical to subcritical flow involves large energy dissipation rates and substantial air entrainment, preventing the use of monophasic flow measurement instrumentation. This paper presents an experimental study of a stable hydraulic jump with...

Hydraulic jumps are oftentimes encountered in natural and human-made environments. The transition from supercritical to subcritical flow involves large energy dissipation rates and substantial air entrainment, preventing the use of monophasic flow measurement instrumentation. This paper presents an experimental study of a stable hydraulic jump with...

Air-water mass transfer in multiphase flows plays a key role in designing hydraulic structures. Air entrainment processes typically occur in spillways, drop shafts, stilling basins or in the casing of hydraulic impulse turbines. Current knowledge of aerated flows mainly relies on experimental model investigations. Herein, three main topis were inve...

Dual-tip phase-detection probes can be used to measure flow properties in gas-liquid flows. Traditionally, time-averaged interfacial velocities have been obtained through cross-correlation analysis of long time-series of phase fraction signals. Using small groups of detected particles, a recently developed adaptive window cross-correlation (AWCC) t...

Highly turbulent free-surface flows are often characterized by air entrainment, resulting in considerable flow bulking and a modulation of energy dissipation. An understanding of air-water flows remains constrained by instrumentation limitations, scale effects and few analytical developments. Whereas past attempts have addressed air-water flow rese...

Turbulent multiphase gas-liquid flows occur in a large variety of natural environments and play an important role in many industrial processes. A common characteristic of these flows is the presence of a dispersed phase, for example air bubbles or water droplets, that are embedded in a continuous phase. In highly-aerated flows, bubbles and droplets...

Hydraulic jumps are oftentimes encountered in natural and human-made environments. The transition from supercritical to subcritical flow involves large energy dissipation rates and substantial air entrainment, preventing the use of monophasic flow measurement instrumentation. This paper presents an experimental study of a stable hydraulic jump with...

The characterization of high-velocity air–water flows is crucial for the design of hydraulic structures including stepped spillways. Recent experimental advances have shown that LIDAR technology can provide free-surface features in fully aerated flows at high spatial and temporal resolution. This manuscript presents the application of LIDAR to the...

Image-based velocimetry captured a great interest in physical modelling of highly-aerated flows, for example in stepped spillways. The present study investigated the performance of a novel filtering technique based upon a detailed systematic comparison with intrusive phase-detection probe data. All measurements were conducted in a large-size steppe...

Stepped spillways are man-made hydraulic structures designed to control the release of flow and to achieve a high energy dissipation. The flow pattern for a given stepped chute geometry can be distinguished into different regimes. Herein, the transition flow regime occurs at a range of intermediate discharges and is characterised by strong hydrodyn...

The present work aims to enhance the energy efficiency of water supply networks by investigating technical and economical feasibility of energy recovery plants at low installed capacities. A cost effective stainless steel machine is investigated in pump and turbine operation and established conversion methods are used for predicting the best effici...

Recent advances in technology have permitted the construction of large dams and spillways. One type, the stepped spillway, is designed to spill floods over the chute with substantial regular energy losses. This article presents an experimental investigation of free-surface instabilities within the non-aerated and aerated region of a gravity-type st...

Following new and innovative concepts of energy recovery which use micro-hydro impulse turbines in drinking water systems, more sophisticated turbine designs become essential to improve energy recovery efficiency. In the case of implementing a hydraulic impulse turbine with tailwater depression in such systems, one major challenge to optimising ove...

The probability of unexpected urban flood hazards is steadily increasing due to global warming and climate change. Consequently, there is a growing need for safety criteria determining the trafficability of inundated roads to ensure a fast and safe evacuation of people in case of such events. In order to determine those criteria, experimental inves...

In the case of impinging water jets or droplets, air entrainment processes are crucial to the casing design of hydraulic impulse turbines in the micro-hydro sector. To initiate first steps towards a precise prediction of the complex, multi-phase casing flow of impulse turbines, single aspects such as the penetration depth of impinging liquid jets h...