Exhaust of Underexpanded Jets from Finite Reservoirs

The response of an underexpanded jet to a depleting finite reservoir is examined with experiments and simulations. An open-ended shock-tube facility with a variable reservoir length is used to obtain images of nitrogen- and helium-jet structures at successive instances during the blowdown from initial pressure ratios of up to 250. The reservoir and ambient pressures are simultaneously measured to obtain the instantaneous pressure ratio. We estimate the time scales for jet formation and reservoir depletion as a function of the specific heat ratio of the gas and the initial pressure ratio. The jet structure formation time scale is found to become approximately independent of the pressure ratio for ratios greater than 50. In the present work, no evidence of time dependence in the Mach disk shock location is observed for rates of pressure decrease associated with isentropic blowdown of a finite reservoir while the pressure ratio is greater than 15. The shock location in the finite-reservoir jet can be calculated from an existing empirical fit to infinite-reservoir jet data evaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however, the present data deviate from a compilation of data for infinite-reservoir jets. A new fit is obtained to data in the lower-pressure regime. The self-similarity of the jet structure is quantified, and departure from similarity is noted to begin at pressure ratios lower than about 15, approximately the same ratio that limits existing empirical fits

Unsteady high-pressure flow experiments with applications to explosive volcanic eruptions

Motivated by the hypothesis that volcanic blasts can have supersonic regions, we investigate the role of unsteady flow in jets from a high-pressure finite reservoir. We examine the processes for formation of far-field features, such as Mach disk shocks, by using a shock tube facility and numerical experiments to investigate phenomena to previously unobtained pressure ratios of 250:1. The Mach disk shock initially forms at the edges of the vent and moves toward the centerline. The shock is established within a few vent diameters and propagates downstream toward the equilibrium location as the jet develops. The start-up process is characterized by two different timescales: the duration of supersonic flow at the nozzle exit and the formation time of the Mach disk shock. The termination process also is characterized by two different timescales: the travel time required for the Mach disk shock to reach its equilibrium position and the time at which the Mach disk shock begins significantly to collapse away from its equilibrium position. The critical comparisons for the formation of steady state supersonic regions are between the two start-up timescales and the termination timescales. We conclude that for typical vulcanian eruptions and the Mount St. Helens directed blast, the Mach disk shock could have formed near the vent, and that there was time for it to propagate a distance comparable to its equilibrium location. These experiments provide a framework for analysis of short-lived volcanic eruptions and data for benchmarking simulations of jet structures in explosive volcanic blasts

Experimental simulations of the May 18, 1980 directed blast at Mount St. Helens, WA

The 1980 directed blast at Mount St. Helens erupted from a high-pressure magma chamber into atmospheric conditions at a pressure ratio of ~150:1, producing a high-velocity dusty gas flow. Decompression from even modestly high pressure ratios (>2:1) produces supersonic flow and thus, this event was modeled as a supersonic underexpanded jet by Kieffer (1981). Steady-state underexpanded jets have a complex geometrical structure in which there is an abrupt, stationary, normal shock wave, called the Mach disk shock. For steady flow, a log-linear relationship between pressure ratio and Mach disk standoff distance, known as the Ashkenas-Sherman relation, is valid for pressure ratios above 15:1 given by x/D=0.67(Rp)^(0.5) where Rp is the pressure ratio, and x/D is the standoff distance normalized to vent diameter. The effects of unsteady discharge from a finite reservoir and application to Mount St. Helens have not been previously investigated. In order to simulate the blast, we use laboratory and numerical experiments of unsteady flow from a finite reservoir to examine jet structure. The reservoir and test section correspond to the magma chamber and ambient atmospheric conditions at Mount St. Helens respectively. We completed a series of laboratory experiments in which we varied the initial pressure ratio, reservoir length and reservoir gas (nitrogen, helium). The numerical simulations show that the Mach disk initially forms close to the vent and then travels downstream to its equilibrium position. The experiments show that as the reservoir pressure continuously decreases during the venting, or “blowdown”, the Mach disk shock continuously moves back toward the reservoir after its formation at the equilibrium position. Results of these experiments indicate that above a pressure ratio of 15:1, the Mach disk standoff distance for unsteady flow falls on the empirical Ashkenas-Sherman curve for steady flow. We present a new relation for the location of the Mach disk shock for pressure ratios below 15:1 given by x/D=0.41(Rp)^(0.66). The results indicate no dependence of the normalized Mach disk location on the finiteness of the reservoir. These results may be of interest not only for high pressure eruptions such as Mount St. Helens, but to low pressure steam eruptions as well because helium is a good analog to steam

Hydrodynamics of a multiple tidal inlet system : Katama Bay, Martha’s Vineyard, MA

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2015Observations, theoretical models, and a numerical model (ADCIRC) are used to investigate the effects of tides, waves, bay bathymetry, and changing inlet geometry on the hydrodynamics of the multiple-inlet Katama system, Martha’s Vineyard, MA. Momentum fluxes from breaking waves drive water into the inlet, nearly stopping the 2 m/s ebb currents during a hurricane. The evolving morphology of Katama Inlet has a dominant effect on tidal distortion and bay circulation. As Katama inlet lengthened, narrowed, and shoaled between 2011 and 2014, the relative effects of friction (observed and simulated) increased greatly, resulting in reduced circulation energy, an increase in the M6 tidal constituent, and changes in velocity asymmetries that are consistent with an evolution from flood to ebb dominance. The effects of changing inlet parameters (e.g., inlet geometry, bay bathymetry, friction, tidal forcing) are quantified via a lumped element model that accounts for the presence of a shallow flood shoal that limits flow from the ocean into the bay. As the difference in depth between inlet and flood shoal increases, the amplitude and phase of the incoming tide are increasingly modified from predictions without a flood shoal, and flows into the bay are further hindered.Thanks to the Office of the Assistant Secretary of Defense for Research and Engineering, the National Science Foundation, NOAA Sea Grant, and the Office of Naval Research for providing funding

Morphological response of variable river discharge and wave forcing at a bar-built estuary

17 USC 105 interim-entered record; under review.The article of record as published may be found at https://doi.org/10.1016/j.ecss.2021.107438Observations of morphological evolution at Carmel River State Beach, Carmel, CA, USA, were made during two winter periods where the estuary underwent transitions from closed to open states episodically during each observation period. However, each winter was climatologically distinct: the first (Dec 2016–May 2017) was a high river discharge year (several events >200 m³ /s) with westerly offshore waves and the second (Dec 2017–May 2018) was a low river discharge year with northwesterly offshore waves. The morphological response of the beach was measured using Structure-from-Motion from both aircraft and unmanned aerial vehicles (UAVs) and shows two distinct seasonal trends. The first (in 2016–2017) indicates rapid (hours) and frequent (days-weeks) migration of the river breach channel across the span of the beach. The second (in 2017–2018) indicates no migration of the initial breach channel, despite multiple breach events. Analysis of the offshore wave energy using the Coastal Data Information Program (CDIP) hindcast model results indicate a stronger longshore wave radiation stress during the migratory breach year. In addition, discharge rates during this year were more than three times stronger than the non-migratory year, indicating a stronger offshore jet from the breach site. These observations support the hypothesis that migration requires both a strong river discharge and a longshore wave radiation stress component.Naval Postgraduate School Naval Research ProgramOffice of Naval Research-CRUSER Progra

Effects of a shallow flood shoal and friction on hydrodynamics of a multiple-inlet system

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 6055–6065, doi:10.1002/2016JC012502.Prior studies have shown that frictional changes owing to evolving geometry of an inlet in a multiple inlet-bay system can affect tidally driven circulation. Here, a step between a relatively deep inlet and a shallow bay also is shown to affect tidal sea-level fluctuations in a bay connected to multiple inlets. To examine the relative importance of friction and a step, a lumped element (parameter) model is used that includes tidal reflection from the step. The model is applied to the two-inlet system of Katama Inlet (which connects Katama Bay on Martha's Vineyard, MA to the Atlantic Ocean) and Edgartown Channel (which connects the bay to Vineyard Sound). Consistent with observations and previous numerical simulations, the lumped element model suggests that the presence of a shallow flood shoal limits the influence of an inlet. In addition, the model suggests an increasing importance of friction relative to the importance of the step as an inlet shallows, narrows, and lengthens, as observed at Katama Inlet from 2011 to 2014.ASD(R&E); NOAA Sea Grant; NSF; ONR2018-01-2

Flow of supersonic jets across flat plates: Implications for ground-level flow from volcanic blasts

We report on laboratory experiments examining the interaction of a jet from an overpressurized reservoir with a canonical ground surface to simulate lateral blasts at volcanoes such as the 1980 blast at Mount St. Helens. These benchmark experiments test the application of supersonic jet models to simulate the flow of volcanic jets over a lateral topography. The internal shock structure of the free jet is modified such that the Mach disk shock is elevated above the surface. In elevation view, the width of the shock is reduced in comparison with a free jet, while in map view the dimensions are comparable. The distance of the Mach disk shock from the vent is in good agreement with free jet data and can be predicted with existing theory. The internal shock structures can interact with and penetrate the boundary layer. In the shock-boundary layer interaction, an oblique shock foot is present in the schlieren images and a distinctive ground signature is evident in surface measurements. The location of the oblique shock foot and the surface demarcation are closely correlated with the Mach disk shock location during reservoir depletion, and therefore, estimates of a ground signature in a zone devastated by a blast can be based on the calculated shock location from free jet theory. These experiments, combined with scaling arguments, suggest that the imprint of the Mach disk shock on the ground should be within the range of 4–9 km at Mount St. Helens depending on assumed reservoir pressure and vent dimensions

Fluctuations and differential contraction during regeneration of Hydra vulgaris tissue toroids

We studied regenerating bilayered tissue toroids dissected from Hydra vulgaris polyps and relate our macroscopic observations to the dynamics of force-generating mesoscopic cytoskeletal structures. Tissue fragments undergo a specific toroid-spheroid folding process leading to complete regeneration towards a new organism. The time scale of folding is too fast for biochemical signalling or morphogenetic gradients which forced us to assume purely mechanical self-organization. The initial pattern selection dynamics was studied by embedding toroids into hydro-gels allowing us to observe the deformation modes over longer periods of time. We found increasing mechanical fluctuations which break the toroidal symmetry and discuss the evolution of their power spectra for various gel stiffnesses. Our observations are related to single cell studies which explain the mechanical feasibility of the folding process. In addition, we observed switching of cells from a tissue bound to a migrating state after folding failure as well as in tissue injury. We found a supra-cellular actin ring assembled along the toroid's inner edge. Its contraction can lead to the observed folding dynamics as we could confirm by finite element simulations. This actin ring in the inner cell layer is assembled by myosin- driven length fluctuations of supra-cellular {\alpha}-actin structures (myonemes) in the outer cell-layer.Comment: 19 pages and 8 figures, submitted to New Journal of Physic

Assimilating Lagrangian data for parameter estimation in a multiple-inlet system

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Ocean Modelling 113 (2017): 131-144, doi:10.1016/j.ocemod.2017.04.001.Numerical models of ocean circulation often depend on parameters that must be tuned to match either results from laboratory experiments or field observations. This study demonstrates that an initial, suboptimal estimate of a parameter in a model of a small bay can be improved by assimilating observations of trajectories of passive drifters. The parameter of interest is the Manning's n coefficient of friction in a small inlet of the bay, which had been tuned to match velocity observations from 2011. In 2013, the geometry of the inlet had changed, and the friction parameter was no longer optimal. Results from synthetic experiments demonstrate that assimilation of drifter trajectories improves the estimate of n, both when the drifters are located in the same region as the parameter of interest and when the drifters are located in a different region of the bay. Real drifter trajectories from field experiments in 2013 also are assimilated, and results are compared with velocity observations. When the real drifters are located away from the region of interest, the results depend on the time interval (with respect to the full available trajectories) over which assimilation is performed. When the drifters are in the same region as the parameter of interest, the value of n estimated with assimilation yields improved estimates of velocity throughout the bay.This work was supported by: Department of Defense Multidisciplinary University Research Initiative (MURI) [grant N000141110087], administered by the Office of Naval Research; the National Science Foundation (NSF); the National Oceanic and Atmospheric Administration (NOAA); NOAA's Climate Program Office; the Department of Energy's Office for Science (BER); and the Assistant Secretary of Defense (Research & Development)

Kemijski profil sedimenata Plominskog zaljeva

Granulometric, chemical, and leaching properties of sediments dredged in the Plomin Bay (Northern Adriatic Sea, Croatia) were investigated in order to asses the risk of remobilisation of heavy metals into the water column. In total 65 samples from 65 sampling sites were taken from different sediment depths within the bay. Analysis of variance confirmed the homogeneity of granulometric and elemental composition of the investigated sediment throughout its volume. Granulometric analysis showed that all samples corresponded to a pelitic fraction (<0.063 mm). Bulk elemental mass fractions in the sediments were similar to literature data on relatively unpolluted areas of the Adriatic Sea. High sedimentation rate caused by constant infl ow of material from the Boljunčica River drainage may be responsible for low levels of heavy metals and negligible infl uence of fl y and bottom ash from a nearby disposal site on the chemical composition of the sediments. In contact with sea water only 0.29 mg kg-1 of V, 0.04 mg kg-1 of Cr, 0.07 mg kg-1 of Ni, 0.33 mg kg-1 of Cu, 0.67 mg kg-1 of Zn and 0.06 mg kg-1 of Pb could be remobilised from sediment material into the water column. However, these values increased three to ten times in case of leaching with organic acids.Granulometrijska i kemijska svojstva te mogućnost otpuštanja teških metala ispitivani su u sedimentima Plominskog zaljeva (Sjeverni Jadran, Hrvatska) u svrhu utvrđivanja rizika od remobilizacije teških metala iz sedimenta u stupac vode. Uzeto je 65 uzoraka s različitih točaka i dubina unutar zaljeva. Analizom varijance potvrđena je granulometrijska i kemijska homogenost cijelog volumena sedimenta, što upućuje na jedan prevladavajući izvor tijekom cijeloga sedimentacijskog razdoblja. Granulometrijskom analizom je utvrđeno da u svim uzorcima prevladava sitnozrnata frakcija (<0,063 mm). Koncentracije elemenata u ukupnim uzorcima sedimenata slične su literaturnim vrijednostima objavljenim za relativno onečišćena područja Jadranskog mora. Velika brzina sedimentacije uzrokovana konstantnim donosom materijala iz slijevnog područja Boljunčice vjerojatan je uzrok niskih koncentracija teških metala i slabo vidljivog utjecaja odlagališta šljake i pepela na sastav sedimenata. U kontaktu s morskom vodom moguća je remobilizacija samo 0,29 mg kg-1 V, 0,04 mg kg-1 Cr, 0,07 mg kg-1 Ni, 0,33 mg kg-1 Cu, 0,67 mg kg-1 Zn i 0,06 mg kg-1 Pb iz sedimenta u stupac morske vode. Ipak ove vrijednosti su tri do deset puta povećane u slučaju izluživanja s pomoću organskih kiselina