S. I. Voropayev

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.

Dynamics of sand ripples and burial/scouring of cobbles in oscillatory flow

Abstract The purpose of this paper is to present the results of a series of laboratory experiments aimed at better understanding (i) the dynamics of sand ripples formed in a wave-induced oscillatory flow and (ii) the influence of sand ripples on the behavior of solid objects (which are free to move) in steady oscillatory flow. This problem is closely related to the migration and burial/scouring of isolated cobbles/mines on a sandy floor by an oscillatory flow such as that occurs in coastal waters beyond the region of wave breaking. The aims of the study were (i) to mimic this process in laboratory experiments and (ii) to develop a physical understanding of the processes involved in the water motion and cobble and sandy surface interaction. The oscillatory flow was created in a long tank of rectangular cross-section using standing waves (first mode) of large amplitudes. Model cobbles (mostly disks) were placed on the sandy bottom and their subsequent behavior, the temporal evolution of the sandy bottom profile and the velocity field in water were observed. The main findings include (i) the demonstration that long-time changes of the bottom topography occur and this may lead to the periodic burial/unburial of cobbles and (ii) the scouring of the bedform surrounding the cobbles is relatively unimportant in the burial/unburial process.

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.

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.

The motion of cobbles in the swash zone on an impermeable slope

The purpose of this communication is to present the results of a series of laboratory experiments aimed at better understanding the dynamics of the motion of large bottom particles (cobbles) in a swash zone. In this region, a thin sheet of water that results from the collapse of a turbulent bore, runs up the beach and can induce the transport of relatively large solid objects in the on-shore direction. The aims of the study were to: (i) mimic this process in laboratory experiments and identify the associated physical processes involved; and (ii) to develop a suitable theoretical model to describe the motion of cobbles. The experiments employed a solid impermeable bottom and were conducted in a long tank of rectangular cross-section. An impulsive hydraulic bore, produced by a dam-break mechanism at one end of the tank, was used to simulate the water motion in the swash zone. Solid objects of simple discoid shape were used to model the cobbles. The results of the laboratory observations were compared with model predictions. In the range of external parameters used for the experiments (size and density of cobbles, propagation velocity and height of the water front, slope and friction at the bottom), a reasonable agreement between the measured and calculated values of the cobble displacement as a function of time was obtained.

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.

Large vortex structures behind a maneuvering body in stratified fluids

When a submerged self-propelled body makes a maneuver, e.g., accelerates, significant momentum is transported to the surrounding fluid. Our experiments show that in a stratified fluid this may lead to the formation of large vortex structures, much larger and different from those produced in the late wake during steady motion. Estimates also show that when an oceanic submerged vehicle changes its velocity by as little as 10% or its direction of motion by 5 degrees, large structures of the size of 1–2 km and with decay times of several days may be expected. Such effects may have potentially important applications and have not been studied previously.

The motion of large bottom particles (cobbles) in a wave-induced oscillatory flow

The purpose of this paper is to present the results of a series of experiments aimed at better understanding the dynamics of the motion of large bottom particles (cobbles) in a wave-induced oscillatory flow. This problem is closely related to the motion of cobbles along the bottom in an oscillatory flow such as occurs in coastal waters beyond the region of wave breaking. The aims of the study were (i) to mimic this process in laboratory experiments and (ii) to develop a physical model to predict cobble movements. The oscillatory flow was created in a long tank of rectangular cross-section using standing waves of large amplitude. Objects of different shapes (spheres and disks) were placed in the flow and their subsequent motion along the tank floor was studied. The results of the observations were compared with the predictions of a theoretical model. For the range of parameters used in the experiments (shape, size, aspect ratio and density of cobbles, amplitude and frequency of the oscillatory flow and bottom friction), reasonable agreement between the measured and calculated values of the cobble displacements as a function of time was obtained.

Dynamics of cobbles in the shoaling region of a surf zone

The motion of large bottom particles (cobbles/mines) was studied in the laboratory under simulated surf conditions. A series of experiments was conducted in a large wave tank, 32×0.9×1.8 m, equipped with a computer-controlled wave maker and a sloping beach. As a first step, a solid impermeable beach with artificial roughness was used in the experiments. Cobbles of different size were placed along the floor and their evolution with time was studied and compared with the model predictions. Onshore and offshore mean motions of cobbles, as well as steady oscillations with zero mean displacement, were observed for different conditions. To explain the results of observations a theoretical model was advanced. The model takes into account all main governing parameters (size and density of cobbles, bottom slope, dynamic and static friction at the bottom, background flow characteristics, etc.). Standard parameterizations were used for a pressure accelerating term, drag, lift and other nonlinear forces. For the range of parameters used in the experiments, satisfactory agreement between the measured and calculated values of the cobble displacements as a function of time was obtained. The model is practically insensitive to the vertical accelerating pressure term but sensitive to the dynamic and static friction. One of the most important variables in the model, which is known with the least accuracy, is the virtual mass coefficient for disk-shaped cobbles moving with variable velocity along a solid boundary.