Richard A. Jepsen

Erosion Measurements in Linear, Oscillatory, and Combined Oscillatory and Linear Flow Regimes

Abstract Many contaminated sediments and dredged material mixtures of cohesive and non-cohesive sediments occur in wave-dominated environments. In-situ analysis is imperative in understanding the erosion and transport of these sediments. Recent research efforts have developed a flume with unidirectional flow that can measure in-situ sediment erosion with depth (SEDflume). However, the flow regime for the SEDflume has limited applicability to wave-dominated environments. Therefore, a unique device, called the SEAWOLF flume, was developed and used by Sandia National Laboratories to simulate high-shear stress erosion processes experienced in coastal waters where wave forcing dominates the system. The SEAWOLF is capable of testing in-situ or laboratory prepared cores. Erosion rates of cohesive and non-cohesive sediments prepared in the laboratory were determined in oscillatory and combined oscillatory and linear flow regimes. Results of these tests were compared to results from the unidirectional SEDflume. Although maximum shear stresses for oscillatory flows were as high as 7 Pa for the tests, the associated erosion rate for specific sediment over the entire wave cycle were comparable to much lower shear stresses found for constant, linear flows. For example, sediment exposed to a maximum of 7 Pa over a 15 s period resulted in erosion rates similar to results for a constant linear shear stress of 3.4 Pa. Analysis of results for all sediments tested led to a determination of values for an effective shear stress that relates wave-induced erosion to linear flow induced erosion.

Are drop-impact phenomena described by Rayleigh-Taylor or Kelvin-Helmholtz theory?

Drop impact, spreading, fingering, and snap-off are important inmany engineering applications such as spray drying, industrial painting, environmentally friendly combustion, inkjet printing, materials processing, fire suppression, and pharmaceutical coating. Controlling drop-impact instability is crucial to designing optimized systems for the aforementioned applications. Classical Rayleigh-Taylor (RT) theory has been widely used to analyze fingering where instabilities at the leading edge of the toroidal ring form fingers that may ultimately snap off to form small droplets. In this study, we demonstrate the inapplicability of RT theory, in particular because it fails to explain the stable regimes observed under conditions of low air density and the instabilities observed when a drop impacts a pool of equal-density fluid. Specifically, finger instability decreases with decreasing air density, whereas the RT theory suggests that instability should remain unchanged. Moreover, experiments show that fingers form upon impact of a dyed water drop with a water pool, whereas the RT theory predicts noinstability when the densities of the two interacting fluids are equal. Experimental evidence is instead consistent with instability predictions made using the shear-driven Kelvin-Helmholtz theory.

Canaveral ODMDS Dredged Material Erosion Rate Analysis

In this study, the erosion properties of four sediments related to the Canaveral Ocean Dredged Material Disposal Site have been determined as a function of density, consolidation, and shear stress by means of a high shear stress sediment erosion flume at Sandia National Laboratories. Additional analysis was completed for each sediment to determine mineralogy, particle size, and organic content. This was done to support numerical modeling efforts, aid in effective management, and minimize environmental impact. The motivation for this work is based on concerns of dredged material transporting beyond the designated site and estimates of site capacity.

Assessment of Strike of Adult Killer Whales by an OpenHydro Tidal Turbine Blade

Report to DOE on an analysis to determine the effects of a potential impact to an endangered whale from tidal turbines proposed for deployment in Puget Sound.

Diagnostics for liquid dispersion due to a high-speed impact with accident or vulnerability assessment application

The high-speed impact and subsequent dispersion of a large liquid slug is of interest for assessing vulnerability of structures when subjected to such an event. The Weber number associated with such liquid impacts is generally between 105 and 108. Because of the experiment scale and destructive nature of these high-energy impacts, most traditional diagnostics are difficult to implement. Therefore, unique diagnostics were employed in several tests to gather information on impact force, spreading instability, slug break-up, ejection velocity, droplet deformation and spray characteristics. Measurement techniques discussed here include high-speed photometrics, particle image velocimetry (PIV), TrackEye particle analysis, speckle correlation, single-pass schlieren imaging, phase Doppler particle analyzer (PDPA) and load cell measurements as applied to large-scale, high-speed liquid impacts.

Vibrafuge: Re-Entry and Launch Test Simulation in a Combined Linear Acceleration and Vibration Environment

Sandia National Laboratories has developed a new technique for testing in a combined linear acceleration and vibration environment. Amplified piezo-electric actuator assemblies are used in combination with Sandia’s 29-ft centrifuge facility to surpass the load capabilities of previous attempts using traditional mechanical shaker systems. The piezoelectric actuators are lightweight, modular and overcome several limitations presented by a mechanical shaker. They are ‘scalable’, that is, adding more piezo-electric units in parallel or in series can support larger-weight test articles or wider range of displacement/frequency regimes. In addition, the units could be mounted on the centrifuge arm in various configurations to provide a variety of input directions. The design along with test results will be presented to demonstrate the capabilities of the new piezo-electric Vibrafuge.

Development for the Optional Use of Circular Core Tubes with the High Shear Stress Flume

In this study, the erosion rates of four reconstituted sediments in both rectangular and circular sample tubes have been determined as a function of density and shear stress by means of a high shear stress sediment erosion flume at Sandia National Laboratories. This was done to determine if circular cores used in field sampling would provide the same results found using the existing technology of rectangular cores. Two samples were natural, cohesive sediments retrieved from different sites in the Boston Harbor identified as Open Cell and Mid Channel. The other two sediments were medium and coarse grain, non-cohesive quartz sediments. For each sediment type, erosion tests were performed with both rectangular and circular core tubes. For all cores, bulk density was determined as a function of depth and consolidation time. Sediments were eroded to determine erosion rates as a function of density and shear stress for both types of core tubes used. No measurable difference was found between the two core types.

Experimental Splash Studies of Monodisperse Sprays Impacting Variously Shaped Surfaces

Despite numerous studies of the drop impact phenomena, studies of the fundamental mechanisms of how the splash corona and subsequent necking yield splashed droplets, not to mention characteristics of these splashed droplets, remain a subject of great interest. Here, we consider a simple question: After impact, what are the characteristics of splashed droplets? Spatial variations in the fraction of splashed liquid, Sauter mean diameter, and drop-size distribution for water and diesel impacting onto variously shaped rods are reported. Liquid drops of nearly uniform size are continuously injected onto a 2-mm-diameter aluminum cylindrical rod at velocities of up to 17 m/s. The impact face of the rod is flat with angles from θ = 0 to 60° or it has a concave, convex, or conical shape. The experimental results indicate that diesel breaks up more easily than water due to its low surface tension. However, due to increased energy loss through viscous dissipation during drop collapse and spreading, dispersion of diesel drops upon and after impact is less energetic than that of water since diesel droplets do not travel as fast or as far as water droplets. During corona formation, stretching and necking of diesel drops before their snap-off are particularly evident due to diesels high viscosity. Size distribution of splashed diesel droplets is more uniform than that of water near the impact region and water is more uniform further away.

Effects of Arroyo Sediment Influxes on the Rio Grande River Channel near El Paso, Texas

Arroyos that flow into the Rio Grande River channel along the U.S.–Mexico border provide intermittent influxes of sediment that may obstruct the channel and cause overflow as well as sedimentation problems downstream. These phenomena were studied using a recently developed, unique, in situ method for measuring the erosion properties of sediments with depth and at high shear stresses. Results of the investigation confirm that the arroyo sediments can affect the channel of the Rio Grande by introducing sediments that are more difficult to erode compared to those already present. Two sites were mapped and characterized in terms of vegetation and soil distribution. Sediment samples were collected, and erosion rates, mineralogy, and sediment grain-size distributions were determined. Results showed that large flows in both arroyos were capable of obstructing the Rio Grande channel by introducing sediments that were more difficult to erode than the existing channel sediments.

The SEAWOLF Flume: Sediment Erosion Actuated by Wave Oscillations and Linear Flow

Sandia National Laboratories has previously developed a unidirectional High Shear Stress Sediment Erosion flume for the US Army Corps of Engineers, Coastal Hydraulics Laboratory. The flow regime for this flume has limited applicability to wave-dominated environments. A significant design modification to the existing flume allows oscillatory flow to be superimposed upon a unidirectional current. The new flume simulates highshear stress erosion processes experienced in coastal waters where wave forcing dominates the system. Flow velocity measurements, and erosion experiments with known sediment samples were performed with the new flume. Also, preliminary computational flow models closely simulate experimental results and allow for a detailed assessment of the induced shear stresses at the sediment surface.