Two-phase gas-liquid flow properties in the hydraulic jump: Review and perspectives

Research on multiphase flows has been strongly improved over the last decades. Because of their large fields of interests and applications for chemical, hydraulic, coastal and environmental engineers and researchers, these flows have been strongly investigated. Although they are some promising and powerful numerical models and new computing tools, computations can not always solve all actual practical problems (weather forecast, wave breaking on sandy beach…). The recent and significant developments of experimental techniques such as Particle Imagery Velocimetry (PIV) and conductivity or optical probes have particularly led scientists to physical modeling that provide series of data used to calibrate numerical models. Flows with time and length scales that were not achievable in the past are now studied leading to a better description of physical mechanisms involved in mixing, diffusion and turbulence. Nevertheless, turbulence is still not well understood, particularly in two-phase flows. In the present chapter, we focus on a classical multiphase flow, the hydraulic jump. It occurs in bedrock rivers, downstream of spillways, weirs and dams, and in industrial plants. It characterizes the transition from a supercritical open-channel flow (low-depth and high velocity) to a subcritical motion (deep flow and low velocities). Experimentally, this two-phase flow can be easily studied. Furthermore, it involves fundamental physical processes such as air/water mixing and the interaction between turbulence and free surface. This flow contributes to some dissipation of the flow kinetic energy downstream of the impingement point, in a relatively short distance making it useful to minimize flood damages. It is also associated with an increase of turbulence levels and the development of large eddies with implications in terms of scour, erosion and sediment transport. These are some of the reasons that make studies on this flow particularly relevant. Although numerical and analytical studies exist, experimental investigations are still considered as the best way to improve our knowledge. After a brief description of the hydraulic jumps, the first part of this chapter aims to review some historical developments with special regards to the experimental techniques and physical modeling (similitude). In the second part, we describe and discuss the basic properties of the two-phase flow including void fraction, bubble frequency, bubble velocity and bubble size. The free surface and turbulence properties are presented as well. In the last part, we develop some conclusions, perspectives and further measurements that should be undertaken in the future

Non intrusive measurement technique for dynamic free-surface characteristics in hydraulic jumps

This paper concerns dynamic free-surface measurements performed in hydraulic jumps with Froude numbers between 3.1 and 8.5 using non intrusive ultrasonic probes. The interest was first focused on the characteristics of mean (d) and turbulent (d’) levels of the air-water interface. Then they were coupled with phase-detection conductivity probes to assess the accuracy of the sensors. This allowed an accurate definition of the exact level detected by the ultrasonic displacement meters. The results showed a regular increase of the mean level over the jump (roller length). A peak of turbulent fluctuation was found on the roller whose amplitude depends upon the Froude number. Comparisons with previous studies showed a good agreement in terms of shapes and roller length estimation. Frequency analysis of the free-surface fluctuations revealed that highest frequencies in the jump are around 4 Hz. Based upon an autocorrelation analysis, the integral time scales of the air/water interface were found to be between 0.03 s and 0.12 s

Interaction between free-surface, two-phase flow and total pressure in hydraulic jump

A hydraulic jump is characterised by intense turbulent flow patterns and substantial flow aeration. The flow turbulence, at both macroscopic and microscopic scales, interacts with the air entrainment process and the free-surface. A series of simultaneous measurements of the free-surface fluctuations, jump toe oscillations, void fraction and total pressure variations allowed for an investigation of the interactions between these characteristics. Experiments were conducted for a range of Froude numbers from 3.8 to 8.5. The total pressure measurements were justified for the air–water flow characterisation of the flow region with a positive time-averaged velocity. The interactions between roller surface deformation, air entrainment and diffusion, velocity variation, flow bulking, and the associated total pressure field modulation highlighted different flow regions, hence flow patterns, in the roller. The jump toe oscillation was found closely linked to the air entrapment at the toe and velocity variation in the shear flow. The instable total pressure distribution was primarily associated with the free-surface fluctuation for the bubbly roller region and with the velocity re-distribution for the lower shear region underneath. The present work provides new information on the physical characteristics of hydraulic jumps and a comprehensive insight into the nature of such complex turbulent two-phase flow

Experimental study of the wake flow behind three road vehicle models

Pollutants are emitted by different sources and it is worthwhile to note that ground vehicles are one of the main contributors to their high concentration levels in urban areas. Among them, ultrafine particles (UFP) with diameter below 100nm and gas such as nitrogen oxides (NOx) are due to engine combustion meaning that this is a key issue for automotive engineering. UFP may be inhaled by breathing and penetrate into the respiratory system. Thus, car passengers, cyclists and pedestrians are exposed to those UFP that have strong impacts in terms of health. UFP increase mortality and diseases leading to huge costs which have been estimated at up to 2.6% of GDP in France (WHO).   Some recent studies have put in evidence that UFP concentrations may be larger inside the cabin than outside. Decreasing the infiltration rates and thus exposure of car passenger during commuting time is then a major goal. Having that in mind, understanding the nanoparticle dispersion in the wake of a car since their emission from the tailpipe is paramount. Characterizing their interactions with the flow will help to improve our knowledge regarding their dynamics. To achieve this goal, the first step is to provide an accurate description of the flow dynamics downstream of a car. This is the target of the present paper.   Measurements are conducted in a wind tunnel whose test section length, height and width are Ls=1m, Hs=0.3m and ls=0.3m, respectively. Velocity measurements are recorded using a 2D LDV system mounted on a 2D traverse system. Three simplified car models known as Ahmed bodies and representative of flow topologies developing downstream of real cars are used, with rear slant angles of ?=0°, 25° and 35°. Their length/height/width are Lc=0.196m, Hc=0.054m and lc=0.073m, respectively. Vehicles are fixed on the bottom of the test section through four circular supports (height 15mm). The blockage coefficient is then below 5% in agreement with the literature to avoid wall effect. For all experiments, the ratio between velocities of the upstream flow and the exhaust gas one is similar to that of a real car moving in an urban area. It leads to a constant upstream velocity of U?=14.3m/s, the corresponding Reynolds number based on the model height being Re=49,500. A preliminary detailed calibration of the test section enables us to identify a homogenous region with a boundary layer thickness less than 12mm and a turbulence intensity level always below 1%. Achieving optimum flow seeding for LDV measurements is known as a key parameter to obtain successful and representative results. Due to the irregular inter-arrival time between measurements, specific data treatment methods must be taken into account. Different methods are available in the literature that avoid velocity statistics bias occurring in the case of homogeneous spatial concentration of seeding. However, there is a lack of information about non-homogenous situations that causes bias through burst seeding occurrences. Here, an innovative data analysis method has been developed to provide reliable and repeatable results whatever the seeding conditions are. According to that, the boundary layer development on the roof has been characterized showing a separation downstream the front edge of models. Then, wake turbulence fields developing downstream of our three models in the same wind tunnel are assessed. Flow topology, recirculation lengths (Lr), boundary layer detachment on rear slants and shear-layer properties are characterized and compared to literature. Among others, our results show that Lr is maximum (Lr=1.5Hc) for ?=0° and minimum (Lr=0.6Hc) for ?=25°. The maximum of turbulence intensity is about 30% for all models whereas their profiles are very sensitive to the rear slant angle

Experimental assessment of characteristic turbulent scales in two-phase flow of hydraulic jump: from bottom to free surface

A hydraulic jump is a turbulent shear flow with a free-surface roller. The turbulent flow pattern is characterised by the development of instantaneous three-dimensional turbulent structures throughout the air–water column up to the free surface. The length and time scales of the turbulent structures are key information to describe the turbulent processes, which is of significant importance for the improvement of numerical models and physical measurement techniques. However, few physical data are available so far due to the complexity of the measurement. This paper presents an investigation of a series of characteristic turbulent scales for hydraulic jumps, covering the length and time scales of turbulent flow structures in bubbly flow, on free surface and at the impingement point. The bubbly-flow turbulent scales are obtained for Fr\ua0=\ua07.5 with 3.4\ua0×\ua010\ua

Wave influence on turbulence length scales in free surface channel flows

In order to predict sediment movements in coastal environments, the interaction between these particles and turbulence should be better understood. Although previous studies have particularly shown the importance of the turbulence length scales on sediment transport for current flows, few measurements have been made on wave/current flows. The purpose of our experiments is to get a better knowledge on wave action on these characteristic length scales. For this study, in the context of a grid-generated turbulence, we aimed to describe evolution of turbulence macro and micro scales in two kinds of free surface flow. Indeed, current and wave/current flows are studied. Two data analysis techniques are used to estimate these characteristic length scales depending on flow conditions. Whereas a well-known energetic method is used for current flow, a specific analysis based on correlation measurements is lead to describe temporal evolution of turbulence length scale over the wave period. As a main result, we show that the free surface causes a vortex stretching for current flow and that turbulence length scales follow a periodic evolution with a frequency which is twice as the swell period. The turbulence length scales also depend on wave period and amplitude