Numerical Study on Plane and Radial Wall Jets to Validate the 2D Assumption for an Idealized Downburst Outflow

Turbulent radial and plane wall jets have been extensively investigated both experimentally and numerically over the past few decades. Previous studies mostly focused on the heat and mass transfers involved in jet flows. In this study, a comprehensive investigation was conducted on turbulent radial and plane wall jets, considering both jet spread and velocity decay for different parameters. The numerical results were compared with existing experimental measurements. The comparison focused on the velocity profile, jet spread, and velocity decay, and revealed that the Reynolds stress model (RSM) performs well in the simulation of both radial and plane wall jets. The results show that with a typical ratio of cloud base height to diameter for most downburst events, the effects of nozzle height and Reynolds number on the evolution of the radial wall jet are not significant. Both the jet spread and velocity decay exhibit a clear dependence on the Reynolds number below a critical value. Above this critical value, the plane wall jet becomes asymptotically independent of the Reynolds number. The co-flow was found to have a significant influence on the evolution of the plane wall jet. Comparatively, the jet spread and velocity of the radial wall jet were faster than those of the plane jet. For applications in civil engineering, it is valid to approximate the downburst outflow with a two-dimensional (2D) assumption from the perspective of longitudinal evolution of the flows.

Dynamic characteristics of supersonic turbulent free jets from four types of circular nozzles

The effects of nozzle structures and working pressure on the dynamic characteristics of supersonic turbulent free jets have been investigated numerically. Four types of nozzles (namely Laval, pipe, contraction I, and contraction II, respectively) and four pressure conditions (namely K = 0.8, 1, 1.5, and 2, respectively) were considered. A Standard k-ε model was utilized for the calculation of the supersonic turbulent free jets. Validation of the model was performed on the Laval jet by comparing it with the experiment and large-eddy simulation (LES). A perfect agreement was achieved in terms of the centerline and radial axial velocity profiles. The jets issuing from the Laval and the pipe had a longer potential core and a larger centerline axial velocity with the same outlet momentum. The length of the potential core was proportional to the working pressure, but variations of the centerline axial velocity decay rate were inverse for all nozzles. The effects of nozzle structures and work pressure on the spreading rates of the jets were insignificant. No obvious change trend could be observed on the kinematic and geometric virtual origins. The study can provide references for the nozzle and working pressure selection in practical application.

Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on Malta

<p>EGU Abstract</p><p> </p><p><strong>Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on Malta</strong></p><p> </p><p>The Maltese archipelago, a group of 5 small island sits in the Central Mediterranean Sea, some 90-100 km directly south of Sicily.  It is ideally located to capture evidence of major events through the Mediterranean Sea. Its eastern seaboard, in particular, is able to record tsunamis arising from the Hellenic Arc, some 600 km to the east at elevations up to ~ 20 m asl.  We here study extreme wave signatures at Zonqor in SE Malta (the main island), on a strip of coastal terrain unsullied by urbanisation on which tsunami signatures are abundant and well preserved. </p><p> </p><p>The Zonqor coastline displays an exceptional range of geomorphic signatures of extreme sea wave events. This study brings together evidence acquired from field survey, analysis of time-sequential aerial and satellite imagery, and hydrodynamic modelling to investigate the histories of boulder groups identified by their intrinsic and contextual characteristics.</p><p> </p><p>Clear differences are revealed between the distribution of boulders recently moved (Recent Movers) and those evidently of considerable age (Ancient Movers). Tracking the movement of boulders by aerial photography since 1957, and satellite imagery and field observations more recently, confirms that storms of surprisingly frequent interval are capable of driving complex boulder movements. This includes the lifting of boulders of up to 7 m in length. Scrutiny of the ancient boulders, including extreme weathering features is indicative of longterm in-situ post transportation residence. It also reveals fascinating landward-facing (reverse) imbrication indicative of a very powerful return flow, cautiously suggesting  tsunami(s) as the agent of their emplacement.   </p><p> </p><p>A novel method, including due attention to the Froude number, is developed for depicting velocity decay profiles of hypothetical design waves, thereby overcoming some of the limitations of the Nott approach. Applied here, the wave run-up context further sets the ancient movers apart from their recent mover companions. </p><p> </p><p>The combined evidence implies a palimpsestic landscape where storm waves are regular geomorphic agents that add to and rework the distribution of boulders close to the shoreline, whilst over long time periods the boulder landscape becomes reset by tsunami, a concept that is of value to agencies in Malta responsible for coastal safety, planning and  management.                                                   </p><p> </p><p> </p><p>Derek Mottershead</p><p>01/2021</p>

High-speed turbulent gas jets: an LES investigation of Mach and Reynolds number effects on the velocity decay and spreading rate

The aim of this work is the investigation of Mach and Reynolds numbers effects on the behaviour of turbulent gas jets in order to gain new insights into the fluid dynamic process of turbulent jet mixing and spreading. An in-house solver (Flow-Large Eddy and Direct Simulation, FLEDS) of the Favre-filtered Navier Stokes equations has been used. Compressibility has been analyzed by considering gas jets with Mach number equal to 0.8, 1.4, 2.0 and 2.6, and Re equal to 10,000. As concerns the influence of Re on gas jets, four cases have been investigated, i.e. $$\mathrm{Re} = 2500$$ Re = 2500 , 5000, 10,000 and 20,000, with Mach number equal to 1.4. The results show that, in accordance with previous experimental and numerical studies, the potential core length increases with Mach number. As regards the velocity decay and the spreading rate downstream of the potential core, compressibility effects are not relevant except for the jet with Mach number of 2.6. The normalized turbulent kinetic energy along the centerline as a function of the normalized streamwise distance shows a similar peak at the end of the potential core for all jets, except for the case with Mach number of 2.6. By increasing Re, the length of the potential core decreases up to the same value for all Re higher than 10,000. In the region downstream of the potential core, the velocity decay decreases as Re number increases from 10,000 to 20,000, whereas, for lower values of Re, the influence is almost negligible.

Component Velocities and Turbulence Intensities within Ship Twin-Propeller Jet Using CFD and ADV

This study presents the decays of three components of velocity for a ship twin-propeller jet associated with turbulence intensities using the Acoustic Doppler Velocimetry (ADV) measurement and computational fluid dynamics (CFD) methods. Previous research has shown that a single-propeller jet consists of a zone of flow establishment and a zone of established flow. Twin-propeller jets are more complex than single-propeller jets, and can be divided into zones with four peaks, two peaks, and one peak. The axial velocity distribution is the main contributor and can be predicted using the Gaussian normal distribution. The axial velocity decay is described by linear equations using the maximum axial velocity in the efflux plane. The tangential and radial velocity decays show linear and nonlinear distributions in different zones. The turbulence intensity increases locally in the critical position of the noninterference zone and the interference zone. The current research converts the axial momentum theory of a single propeller into twin-propeller jet theory with a series of equations used to predict the overall twin-propeller jet structure.

Radial Wall Jet Flow Over Sigmoidal Surfaces

Radial wall jet flows across flat smooth surfaces have been studied for decades. These studies show that the radial velocity of these jets decays inversely with distance from the nozzle with modest contribution from friction (Poreh, et al., 1967; Rajaratnam, 1976). However, the extent to which flat surface results apply to curved surfaces remains unclear. In this paper we explore the influence of settled particle bed slope on radial wall jet velocity profiles. Jet flows over particle beds often introduce curvature in the particle bed profile, but the influence of the developed curvature on the velocity profile has not been explored. We model the step change in thickness as a sigmoidal curve of variable steepness and use conservation of momentum to evaluate the velocity profile for steady fixed beds. We find that surface curvature has a significant influence on the velocity decay coefficients, provided there is a slip velocity in the vicinity of the particle bed interface, which is strictly true for particle surfaces. We show that the velocity profile attenuates because of curvature. Indeed, conservation of momentum predicts conditions where the forward momentum of the flow is directed completely upward. The solution identifies two new dimensionless groups that determine whether a curved surface is sufficient to block radial flow and force flow vertically.

Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on a Complex Rocky Shoreline of Malta

The Żonqor coastline, southeast Malta, displays an exceptional range of geomorphic signatures of extreme coastal events. This paper brings together evidence acquired from a field survey, analysis of time-sequential imagery, and hydrodynamic modelling to investigate the histories of boulder groups identified by their intrinsic and contextual characteristics. Clear differences are revealed between the distribution of boulders recently moved and those of considerable age. Tracking the movement of boulders since 1957 confirms that storms of surprisingly frequent interval are capable of complex boulder movements, including lifting of megaclasts. Scrutiny of the ancient boulders, including weathering features and fascinating landward-facing (reverse) imbrication, cautiously suggests tsunami as the agent for their emplacement. A novel method is developed for depicting the velocity decay profiles of hypothetical waves, which overcomes some of the limitations of the Nott approach. Applied here, the wave run-up context further sets the ancient movers apart from their recent mover companions. The combined evidence implies a palimpsestic landscape where storm waves are regular geomorphic agents that add to and rework the distribution of boulders close to the shoreline, but over long time periods the landscape becomes reset by tsunami—a concept that is of value to agencies in Malta responsible for coastal safety, planning and management.

Experimental and Computational Study of Archery Arrows Fletched with Straight Vanes

The aerodynamic characteristics of archery arrows fletched with two types of straight vanes, for which the area is different, were studied. The arrows’ pitching moment (CM), lift (CL) and drag (CD) coefficients were measured in the 60 × 60 cm Magnetic Suspension and Balance System (MSBS) from JAXA. At a Reynolds number of Re = 1.2 × 104, the values of CD were 1.56 and 2.05 for the short and large vanes, respectively. In a second experimental procedure, the arrows’ deceleration in free flight was measured by inserting an acceleration sensor inside their shafts. For shots with an initial velocity of around 56.4 ms−1, a velocity decay of around 8% was measured. A turbulent–laminar boundary layer transition during free flight was found for shots with an average Re = 1.8 × 104. Lastly, through numerical computations, the area difference of the two vanes was analyzed to verify the importance of CM and CL during the arrows’ flights.