Firat Yener Testik

Burial and scour around short cylinder under progressive shoaling waves

Abstract The results of a laboratory experimental program aimed at better understanding the scour around and burial of heavy cylindrical objects under oscillating flow on a sandy bed are described. This study was motivated by its application to the dynamics of isolated cobbles/mines on a sandy floor under nonlinear progressive waves, such as that occur in shallow coastal waters beyond the wave-breaking region. In the experiments, nonlinear progressive waves were generated in a long wave tank of rectangular cross-section with a bottom slope. Model mines (short cylinders) were placed on the sandy bottom and the temporal evolution of the bed profile and the velocity field in the near field of the object were observed. Experiments were conducted at relatively high Reynolds numbers for a range of flow conditions, which can be characterized by the Keulegan–Carpenter number and Shields parameter. Depending on the values of these parameters, four different scour regimes around the cylinder including periodical burial of cylinder under migrating sand ripples were observed; they were classified as: (i) no scour/burial, (ii) initial scour, (iii) expanded scour, and (iv) periodic burial cases. A scour regime diagram was developed and the demarcation criteria between different regimes were deduced. Semi-empirical formulae that permit estimation of the scour depth with time, the equilibrium maximum scour depth and length, and conditions necessary for the burial of the cylinder as a function of main external parameters are also proposed.

Flow around a short horizontal bottom cylinder under steady and oscillatory flows

This paper presents the results of an experimental study on flow around a short cylinder placed horizontally on a solid bottom. The aim was to provide a better understanding of flow past blunt objects immersed in the oceanic wave boundary layer. Both steady and oscillatory flows are considered. In the parameter range studied (Reynolds number Re=300–5300, Keulegan–Carpenter number KC=4–31), the near wake was dominated by large periodically shed horseshoe vortices of size comparable to that of the cylinder. In oscillatory flow, horseshoe vortices of opposite sign form periodically on both sides of the cylinder, and they flip from one side of the cylinder to the other with the change of flow direction. This flipping vortex (of one sign) of a given phase of a cycle interacts with the vortex (of opposite sign) that forms in the opposite phase, and the pair is shed away periodically from the cylinder. Flow visualization and particle image velocimetry were used to identify and quantify these flow structures.

Revisiting Low and List (1982): Evaluation of Raindrop Collision Parameterizations Using Laboratory Observations and Modeling

Abstract Raindrop collision and breakup is a stochastic process that affects the evolution of drop size distributions (DSDs) in precipitating clouds. Low and List have remained the obligatory reference on this matter for almost three decades. Based on a limited number of drop sizes (10), Low and List proposed generalized parameterizations of collisional breakup across the raindrop spectra that are standard building blocks for numerical models of rainfall microphysics. Here, recent laboratory experiments of drop collision at NASA’s Wallops Island Facility (NWIF) using updated high-speed imaging technology with the objective of assessing the generality of Low and List are reported. The experimental fragment size distributions (FSDs) for the collision of selected drop pairs were evaluated against explicit simulations using a dynamical microphysics model (Prat and Barros, with parameterizations based on Low and List updated by McFarquhar). One-to-one comparison of the FSDs shows similar distributions; however...

Field Observations of Multimode Raindrop Oscillations by High-Speed Imaging

Abstract Periodic oscillations of raindrops falling at terminal velocity in natural rain are visualized for the first time by high-speed imaging. These images show the existence of an oscillation mode with the same frequency as the fundamental harmonic, but with shape different than that predicted by linear theory. These oscillations cause a lateral drift with a speed of approximately 20%–30% of the drop terminal velocity and without a preferred direction. These experimental observations serve as an insightful illustration of the potential benefit of applying high-speed imaging technology to investigate the dynamical microstructure of rainfall at the raindrop scale.

Adjustment of sand ripples under changing water waves

The results of an experimental investigation on the adjustment of vortex sand ripples under shoaling waves to changing of wave conditions are presented in this paper. A large wave tank was used to generate shoaling waves. Waves with small (S), moderate (M), and large (L) intensities (as specified by the wave paddle excursion) were used to model three basic cases of cyclic variation of wave forcing, namely, M-L-M, L-M-L, and L-S-L. Depending on the forcing transitions (L-M, M-L, or L-S), three main ripple adjustment processes were identified: (i) ripple splitting, (ii) ripple regrowth, and (iii) ripple flattening. Quantitative data on the time evolution of ripple characteristics were collected using the structured light technique. The results of the observations were explained by extending a simplified physical model proposed earlier for ripples under constant wave forcing to the case of changing wave forcing.

Morphodynamics and cobbles behavior in and near the surf zone

Abstract The evolution of an initially flat sandy slope and the dynamics of large objects (cobbles/mines) emplaced on it are studied in a laboratory wave tank under simulated surf conditions. Upon initiation of wave forcing, the initially flat beach undergoes bedform changes before reaching a quasi-steady morphology characterized by a system of sand ripples along the slope and a large bar near the break point. Although the incoming wave characteristics are held fixed, the bottom morphology never reaches a strict steady state, but rather slowly changes due to the migration of ripples and bar transformation. When the wave characteristics are changed, the bedform adjusts to a new quasi-steady state after a suitable adjustment time. Studies conducted by placing model cobbles/mines on the evolving sandy bottom subjected to wave forcing show four distinct scenarios: (i) periodic cobble oscillations with zero mean displacement and small scour around the cobbles, (ii) mean onshore motion of relatively light cobbles, (iii) periodic burial of relatively heavy cobbles when their sizes are comparable to those of sand ripples, and (iv) the burial of relatively large cobbles under the bar, when the bar migrates due to changes of incoming waves. Quantitative data on the characteristics and dynamics of the bedform, including ripple-formation front propagating down the slope, ripple growth and drift, and flow around ripples, are presented. Physical explanations are provided for the observations.

On the concentration structure of high-concentration constant-volume fluid mud gravity currents

An exhaustive laboratory experimental campaign was undertaken in order to elucidate the concentration structure of two-dimensional constant-volume non-Newtonian fluid mud gravity currents. Two sets of experiments were conducted in a lock-exchange tank. The first set of experiments involved measuring the vertical concentration profiles using a siphoning technique; the second set involved auxiliary visual observations. The first set of experiments consisted of 32 experimental runs for four different experimental conditions, with an array of siphoned samples being withdrawn throughout the head and body of the gravity current. From these samples, vertical concentration profiles occurring in constant-volume fluid mud gravity currents were classified and the underlying physical processes that led to the occurrence of observed profiles were discussed. Furthermore, the functional form of the vertical concentration profiles within the head of relatively low-initial-concentration gravity currents was proposed. The relatively high-initial-concentration gravity currents revealed the presence of a lutocline in the current head and body, the presence of which was observed for constant-flux release gravity currents. To our knowledge, this is the first measurement of a lutocline in constant-volume gravity currents. Abrupt transitions, a phenomenon in which the bulk of the suspended sediment in the propagating gravity current drops out, were observed through the concentration profiles and through 15 auxiliary visual experimental runs. It was found that abrupt transitions were caused by the presence of a lutocline. The entrainment of ambient water resulting in the dilution of the gravity current at different concentration contours has been quantified. In a previous work by the authors of this study, it was shown that the initial reduced gravity is directly proportional to the growth rate of the visual area of the two-dimensional current. The analysis of our experimental observations presented in this study, however, showed the initial reduced gravity to be inversely proportional to the growth rate of the area enclosed by concentration contours with higher values than that of the visual area. These seemingly opposing conclusions are rationalized and the considerable practical impacts are discussed.

Toward a Physical Characterization of Raindrop Collision Outcome Regimes

A comprehensive raindrop collision outcome regime diagram that delineates the physical conditions associated with the outcome regimes (i.e., bounce, coalescence, and different breakup types) of binary raindrop collisions is proposed. The proposed diagram builds on a theoretical regime diagram defined in the phase space of collision Weber numbers We and the drop diameter ratio p by including critical angle of impact considerations. In this study,the theoreticalregimediagramisfirstevaluatedagainstacomprehensive dataset for drop collision experiments representative of raindrop collisions in nature. Subsequently, the theoretical regime diagram is modified to explicitly describe the dominant regimes of raindrop interactions in (We, p )b y delineatingthe physical conditions necessary for the occurrence of distinct types of collision-induced breakup (neck/filament, sheet, disk, and crown breakups) based on critical angle of impact consideration. Crown breakup is a subtype of disk breakup for lower collision kinetic energy that presents distinctive morphology. Finally, the experimental results are analyzed in the context of the comprehensive collision regime diagram, and conditionalprobabilities that can be used in the parameterization of breakup kernelsin stochasticmodels of raindrop dynamics are provided.

Mine Burial in the Shoaling Zone: Scaling of Laboratory Results to Oceanic Situations

During the past several years, the Environmental Fluid Dynamics Group at Arizona State University (ASU, Tempe, AZ) conducted a comprehensive laboratory-based research program to elucidate the mechanisms and dynamics of mine burial in noncohesive sediments under shoaling waves on coastal slopes. This paper presents a brief description of this program as well as salient observations and quantitative parameterizations for scour, ripples, and burial that resulted from it (which constitute the ASU mine burial model). Improvements to mine burial predictive capabilities offered by the ASU model are demonstrated by evaluating it vis-a-vis the (legacy) mine burial models that are in common use [Defense Research Agency mine burial environment (DRAMBUIE), Industrie Anlagen Bau Gesellschaft (NBURY, Munchen, Germany), and wave-induced spreadsheet prediction (WISSP)]. To this end, both the legacy and ASU models are discussed briefly and compared with field experimental data obtained during the 2003 Indian Rocks Beach (IRB, FL) experiment. The scour/burial data collected during the IRB campaign, using instrumented mines and diver observations, show that the predictions of the mostly laboratory-based ASU model agree satisfactorily with field observations, both qualitatively and quantitatively.

Turbulent entrainment into fluid mud gravity currents

The entrainment of ambient water into non-Newtonian fluid mud gravity currents was investigated in this study. Constant volume release gravity currents were generated in a lock-exchange tank for a wide range of experimental conditions. A technique similar to the so-called light attenuation technique was used to find the boundary of the current, allowing for the calculation of both temporal and bulk entrainment parameters (in terms of the temporal and bulk entrainment velocities, respectively). It was found that the temporal entrainment velocity is dependent on different parameters in the different propagation phases. The slumping phase begins with an adjustment zone (henceforth, non-established zone) in which the temporal entrainment velocity is not a function of the current front velocity, followed by the established zone in which the temporal entrainment velocity is a function of the current front velocity. This dependence of the temporal entrainment velocity on the current front velocity carries through to the inertia-buoyancy phase. As expected, temporal entrainment velocity in the viscous-buoyancy phase was negligible in comparison to average entrainment velocity in the other phases. It is observed that the temporal entrainment characteristics in the non-established zone is governed by the competition between the entrainment-inhibiting density stratification effects and the entrainment-favouring effects of the Kelvin–Helmholtz billows that are quantified by the Richardson number and the Reynolds number of the gravity current, respectively. In the established zone, Reynolds number effects were observed to dominate over Richardson number effects in dictating temporal entrainment characteristics. A parameterization for the temporal entrainment velocity for non-Newtonian fluid mud gravity currents is developed based upon the experimental observations. This study also found that the bulk entrainment characteristics for the non-Newtonian fluid mud gravity currents can be parameterized by the Newtonian bulk entrainment parameterizations that rely solely on a bulk Richardson number. Interestingly, it was found that the non-Newtonian characteristics of the gravity current have little to no effect on the entrainment of the Newtonian ambient fluid.