Large Dynamic Pressure Research Articles

A hydrodynamic mechanism of meteor ablation

Context. Hydrodynamic conditions are similar in a molten meteoroid and a liquid drop in a high-speed airflow. Despite the fact that the latter is well-studied, both experimentally and theoretically, hydrodynamic instability theory has not been applied to study the fragmentation of molten meteoroids.Aims. We aim to treat quasi-continuous spraying of meteoroid melt due to hydrodynamic instability as a possible mechanism of ablation. Our objectives are to calculate the time development of particle release, the released particle sizes and their distribution by sizes, as well as the meteoroid mass loss law.Methods. We have applied gradient instability theory to model the behaviour of the meteoroid melt layer and its interaction with the atmosphere. We have assumed a spherical meteoroid and that the meteoroid has a shallow entry angle, such that the density of the air stream interacting with the meteoroid is nearly constant.Results. High-frequency spraying of the molten meteoroid is numerically simulated. The intermediate and final size distributions of released particles are calculated, as well as the meteoroid mass loss law. Fast and slow meteoroids of iron and stone compositions are modelled, resulting in significant differences in the size distribution of melt particles sprayed from each meteoroid. Less viscous iron melt produces finer particles and a denser aerosol wake than a stony one does.Conclusions. Analysis of the critical conditions for the gradient instability mechanism shows that the dynamic pressure of the air-stream at heights up to 100 km is sufficient to overcome surface tension forces and pull out liquid particles from the meteoroid melt by means of unstable disturbances. Hence, the proposed melt-spraying model is able to explain quasi-continuous mode of meteoroid fragmentation at large heights and low dynamic pressures. A closed-form solution of the meteoroid ablation problem is obtained due to the melt-spraying model usage, at the meteoroid composition, initial radius and velocity being given.

Geoeffective Properties of Solar Transients and Stream Interaction Regions

Interplanetary Coronal Mass Ejections (ICMEs), their possible shocks and sheaths, and co-rotating interaction regions (CIRs) are the primary large-scale heliospheric structures driving geospace disturbances at the Earth. CIRs are followed by a faster stream where Alfvenic fluctuations may drive prolonged high-latitude activity. In this paper we highlight that these structures have all different origins, solar wind conditions and as a consequence, different geomagnetic responses. We discuss general solar wind properties of sheaths, ICMEs (in particular those showing the flux rope signatures), CIRs and fast streams and how they affect their solar wind coupling efficiency and the resulting magnetospheric activity. We show that there are two different solar wind driving modes: (1) Sheath-like with turbulent magnetic fields, and large Alfven Mach ( $M_{A}$ ) numbers and dynamic pressure, and (2) flux rope-like with smoothly varying magnetic field direction, and lower $M_{A}$ numbers and dynamic pressure. We also summarize the key properties of interplanetary shocks for space weather and how they depend on solar cycle and the driver.

Flight Dynamics and Control Using Folding Wingtips: An Experimental Study

This paper presents an experimental investigation on using FOLDing wingtips sERving as cONtrol effectorS (FOLDERONS) for a mini Unmanned Aerial Vehicle (UAV). A representative off-the-shelf mini-UAV with a conventional configuration was selected. The main theme of this paper is to utilise FOLDERONS as a control effector (mainly in roll) to augment the control authority of conventional control surfaces. Furthermore, the impact of actuation rate on the effectiveness of FOLDERONS is assessed. The paper describes the preliminary and detailed design and sizing of the morphing wing. In addition, the manufacturing of the wing system and its integration with the UAV are addressed. Wind-tunnel testing in the RJ Mitchell wind-tunnel at the University of Southampton was performed. Both static (straight and sideslip) and dynamic (straight flight) tests are conducted at a range of airspeeds and Angles Of Attack (AOAs). The impact of folding wingtips on the lateral and directional stability is analysed. The main finding of this paper is that FOLDERONS are effective (especially at large dynamic pressure and AOAs) in controlling the lateral and directional stability. Finally, this study shows that FOLDERONS cannot fully replace conventional ailerons especially at low dynamic pressures, and their strong dependence on the AOA makes them prone to a roll reversal phenomena when the wing (and FOLDERONS) is operating at negative AOAs.

Synthesizing ocean bottom pressure records including seismic wave and tsunami contributions: Toward realistic tests of monitoring systems

AbstractThe present study proposes a method for synthesizing the ocean bottom pressure records during a tsunamigenic earthquake. First, a linear seismic wave simulation is conducted with a kinematic earthquake fault model as a source. Then, a nonlinear tsunami simulation is conducted using the sea bottom movement calculated in the seismic wave simulation. By using these simulation results, this method can provide realistic ocean bottom pressure change data, including both seismic and tsunami contributions. A simple theoretical consideration indicates that the dynamic pressure change caused by the sea bottom acceleration can contribute significantly until the duration of ~90 s for a depth of 4000 m in the ocean. The performance of a tsunami monitoring system was investigated using the synthesized ocean bottom pressure records. It indicates that the system based on the hydrostatic approximation could not measure the actual tsunami height when the time does not elapse enough. The dynamic pressure change and the permanent sea bottom deformation inside the source region break the condition of a simple hydrostatic approximation. A tsunami source estimation method of tFISH is also examined. Even though the synthesized records contain a large dynamic pressure change, which is not considered in the algorithm, tFISH showed a satisfactory performance 5 min after the earthquake occurrence. The pressure records synthesized in this study, including both seismic wave and tsunami contributions, are more practical for evaluating the performance of our monitoring ability, whereas most tsunami monitoring tests neglect the seismic wave contribution.

Parametric study of the solar wind interaction with the Hermean magnetosphere for a weak interplanetary magnetic field

The aim of this study is to simulate the interaction of the solar wind with the Hermean magnetosphere when the interplanetary magnetic field is weak, performing a parametric study for all the range of hydrodynamic values of the solar wind predicted on Mercury for the ENLIL + GONG WSA + Cone SWRC model: density from 12 to 180cm−3, velocity from 200 to 500km/s and temperatures from 2·104 to 18·104K, and compare the results with a real MESSENGER orbit as reference case. We use the code PLUTO in spherical coordinates and an asymmetric multipolar expansion for the Hermean magnetic field. The study shows for all simulations a stand off distance larger than the Mercury radius and the presence of close magnetic field lines on the day side of the planet, so the dynamic pressure of the solar wind is not high enough to push the magnetopause on the planet surface if the interplanetary magnetic field is weak. The simulations with large dynamic pressure lead to a large compression of the Hermean magnetic field modifying its topology in the inner magnetosphere as well as the plasma flows from the magnetosheath towards the planet surface.

Influence of soil liquefaction on the seismic response of a subway station in model tests

In this work, several shaking table tests are conducted to investigate the seismic responses of a large subway station in various liquefied soil conditions. Different liquefaction states are modeled for the soil foundation by inputting different earthquake intensities. According to the test results, when the lateral soil layer surrounding the underground structure is partly liquefied, the cumulative residual deformations of the subway station are much greater than those in either non-liquefied or completely liquefied ground, which is mainly as a result of the large unrecoverable lateral deformation of the partially liquefied ground. Additionally, when the surrounding lateral soil layer is completely liquefied, the subway station structure experiences significant uplift. When only the soil layer near the upper wall is liquefied, large residual dynamic soil pressures on the sidewall are observed at the end of the test. In addition, the strain distribution rules of the underground structure are clearly altered in the presence of different states of soil liquefaction. These new findings can be used as a basis for evaluating the seismic risk of large underground structures in liquefiable ground.

Cometary impact effects at the Moon: Implications for lunar swirl formation

Relatively recent cometary impacts at the Moon could leave unique traces of their origins: high impact velocities and volatile abundances, combined with the presence of a dust- and ice-laden coma, may thermally and mechanically process the lunar surface in ways distinct from the impact of an asteroid. Here we analytically and numerically assess the consequences of a cometary impact at the Moon by considering the combined effects of a collision by the nucleus and inner coma. Our results show that cometary impacts entrain the finest fraction of lunar soil grains (<10μm) over regional scales (∼100–1000km), produce large masses of vaporized material, and likely generate transient magnetic fields that could exceed the Earth’s surface field strength by a factor of 104. This combination of processes is consistent with a mechanism to generate lunar swirls: the diffuse, meandering disturbances in brightness and regolith texture that curl across much of the lunar far-side and are also commonly (but not exclusively) associated with magnetic anomalies. Previous observations of swirl features indicate that bright regions also possess a peculiar, altered regolith structure, which can be produced by the removal of fine soil grains. Regional scouring by an impacting comet explains both the structure and albedo variations: large dynamic pressures entrain the smallest grains within a near-surface flow of dusty plasma, disrupting the backscattering, “fairy-castle” structure of lunar soils in equilibrium with the airless environment. The resulting surface is brightened by compaction of the previously open, porous macrostructure. Darker lanes observed within swirl regions are interpreted as possible melt and/or vapor deposits. Finally, the intense magnetic fields generated during high-speed cometary impacts provide an explanation for correlations between swirl locations and magnetic anomalies.

AUTOMATIC DETECTION ALGORITHM OF DYNAMIC PRESSURE PULSES IN THE SOLAR WIND

Dynamic pressure pulses (DPPs) in the solar wind are a significant phenomenon closely related to the solar-terrestrial connection and physical processes of solar wind dynamics. In order to automatically identify DPPs from solar wind measurements, we develop a procedure with a three-step detection algorithm that is able to rapidly select DPPs from the plasma data stream and simultaneously define the transition region where large dynamic pressure variations occur and demarcate the upstream and downstream region by selecting the relatively quiet status before and after the abrupt change in dynamic pressure. To demonstrate the usefulness, efficiency, and accuracy of this procedure, we have applied it to the Wind observations from 1996 to 2008 by successfully obtaining the DPPs. The procedure can also be applied to other solar wind spacecraft observation data sets with different time resolutions.

Ascent Trajectories of Hypersonic Aircraft: Operability Limits Due to Engine Unstart

A generic waverider-type hypersonic aircraft that undergoes an ascent trajectory has been modeled using a first-principles reduced-order model. Two types of operability limits are added that represent boundaries on the aircraft trajectory map (of vehicle altitude versus Mach number). These boundaries are associated with engine unstart and ram–scram transition. The predicted unstart boundary is to be avoided; the ram–scram transition is a condition through which the aircraft must fly, but it is useful for the control system to know when this transition is approached to account for possible sudden changes in thrust and moments. The model shows that unstart occurs if the aircraft flies too high, too slow, or at too great of an acceleration. The unstart limit can be avoided by selecting a trajectory having sufficiently large dynamic pressure or a low vehicle acceleration. Optimizing these factors avoids an excessive value of the fuel–air ratio that is required for trim. The model also identifies an engine inlet geometry that avoids unstart. To assess the model, the computed results are compared to some available experiments.

Instability regimes in the primary breakup region of planar coflowing sheets

AbstractThis article investigates the appearance of instabilities in two planar coflowing fluid sheets with different densities and viscosities via experiments, numerical simulation and linear stability analysis. At low dynamic pressure ratios a convective instability is shown to appear for which the frequency of the waves in the primary atomization region is influenced by both liquid and gas velocities. For large dynamic pressure ratios an asymptotic regime is obtained in which frequency is solely controlled by gas velocity and the instability becomes absolute. The transition from convective to absolute is shown to be influenced by the velocity defect induced by the presence of the separator plate. We show that in this regime the splitter plate thickness can also affect the nature of the instability if it is larger than the gas vorticity thickness. Computational and experimental results are in agreement with the predictions of a spatio-temporal stability analysis.

Dynamic inelastic response of strain rate sensitive ductile plates due to large impact, dynamic pressure and explosive loadings

This article studies the significant amount of literature that has been published recently on the dynamic response of plating subjected to large dynamic loadings. A theoretical method of analysis for mass impacts, dynamic pressure pulses and impulsive velocity or blast loadings on circular, square and rectangular plates is presented for the idealisation of a rigid, perfectly plastic material. This theoretical analysis caters for the influence of finite-displacements and is developed further to predict the response for plates made from a strain rate sensitive material. Relatively simple equations are presented for the maximum permanent transverse displacements which give good agreement with the corresponding experimental data, and therefore, can be used for design purposes, safety calculations, security studies, hazard assessments and forensic investigations.

Damping and Inertia Coefficients for Two Open Ends Squeeze Film Dampers With a Central Groove: Measurements and Predictions

Aircraft engine rotors are particularly sensitive to rotor imbalance and sudden maneuver loads, since they are always supported on rolling element bearings with little damping. Most engines incorporate squeeze film dampers (SFDs) as means to dissipate mechanical energy from rotor vibrations and to ensure system stability. The paper quantifies experimentally the forced performance of a SFD comprising two parallel film lands separated by a deep central groove. Tests are conducted on two open ends SFDs, both with diameter D = 127 mm and nominal radial clearance c = 0.127 mm. One damper has film lands with length L = 12.7 mm (short length), while the other has 25.4 mm land lengths. The central groove has width L and depth 3/4 L. A light viscosity lubricant flows into the central groove via three orifices, 120 deg apart and then through the film lands to finally exit to ambient. In operation, a static loader pulls the bearing to various eccentric positions and electromagnetic shakers excite the test system with periodic loads to generate whirl orbits of specific amplitudes. A frequency domain method identifies the SFD damping and inertia force coefficients. The long damper generates six times more damping and about three times more added mass than the short length damper. The damping coefficients are sensitive to the static eccentricity (up to ∼ 0.5 c), while showing lesser dependency on the amplitude of whirl motion (up to 0.2 c). On the other hand, inertia coefficients increase mildly with static eccentricity and decrease as the amplitude of whirl motion increases. Cross-coupled force coefficients are insignificant for all imposed operating conditions on either damper. Large dynamic pressures recorded in the central groove demonstrate the groove does not isolate the adjacent squeeze film lands, but contributes to the amplification of the film lands’ reaction forces. Predictions from a novel SFD model that includes flow interactions in the central groove and feed orifices agree well with the test force coefficients for both dampers. The test data and predictions advance current knowledge and demonstrate that SFD-forced performance is tied to the lubricant feed arrangement.

Heavy-ion induced desorption yields of amorphous carbon filmsbombarded with4.2 MeV/ulead ions

During the past decade, intense experimental studies on the heavy-ion induced molecular desorption were performed in several particle accelerator laboratories worldwide in order to understand and overcome large dynamic pressure rises caused by lost beam ions. Different target materials and various coatings were studied for desorption and mitigation techniques were applied to heavy-ion accelerators. For the upgrade of the CERN injector complex, a coating of the Super Proton Synchrotron (SPS) vacuum system with a thin film of amorphous carbon is under study to mitigate the electron cloud effect observed during SPS operation with the nominal proton beam for the Large Hadron Collider (LHC). Since the SPS is also part of the heavy-ion injector chain for LHC, dynamic vacuum studies of amorphous carbon films are important to determine their ion induced desorption yields. At the CERN Heavy Ion Accelerator (LINAC 3), carbon-coated accelerator-type stainless steel vacuum chambers were tested for desorption using $4.2\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ ${\mathrm{Pb}}^{54+}$ ions. We describe the experimental setup and method, present the results for unbaked and baked films, and summarize surface characterizations such as secondary electron yield measurements, x-ray photoemission spectroscopy, and scanning electron microscopy studies. Finally, we present a high-energy scaling of lead-ion induced desorption yields from the $\mathrm{MeV}/\mathrm{u}$ to $\mathrm{GeV}/\mathrm{u}$ range.

Material surface damage by quasistationary compression plasma flow action

In many experiments plasma guns have been used to study the possibility of lowering the erosion due to disruptions and edge localized modes (ELMs) of great interest for the International Thermonuclear Experimental Reactor (ITER) safe operation. Modification of silicon single crystal surfaces by the action of supersonic compression plasma flow (CPF) generated by magnetoplasma compressor (MPC) has been studied. MPC plasma flow parameters (1 MJ/m2 in 0.1 ms) simulated transient peak thermal loads during Type I ELMs and disruptions. Analysis of the target’s erosion, brittle destruction, melting processes and dust formation has been performed. The layer with some of regular structures (rhombic on the Si (111) and rectangular on Si (100) surface) can be separated from the underlying bulk, being ejected as the blocks from the surface. These surface phenomena are results of specific conditions during CPF interaction with target surface and differential stresses produced in a near surface layer. High plasma flow energy density, large dynamic pressure, thermodynamic parameters gradients and induced magnetic field on treated surfaces cause rapid heating and melting of the surface layer, prolonged existence of the molten layer and fast cooling and recrystallisation, as well as surface fracturing.

Effects on the distant geomagnetic tail of a fivefold density drop in the inner sheath region of a magnetic cloud: A joint Wind–ACE study

Using a serendipitous configuration of the ACE and Wind spacecraft, we monitor the response of the distant geomagnetic tail (∼ −220 R E ) to an abrupt, approx. fivefold pressure drop (from ∼19.0 to ∼3.5 nPa) at the front boundary of a magnetic cloud (MC) on November 20, 2003. The interplanetary data are from ACE in orbit around the L1 point. The far-tail observations are from Wind, which was nominally in the magnetosheath, separated from the Sun–Earth line by ∼ 40 R E . The magnetic field in the innermost sheath region of the MC had a large B y ( ∼ 30 nT ) and substantial and variable flows lateral to the Sun–Earth line. There was also a significant northward field (∼35 nT), unique in the vicinity of this MC. These extreme values are reached in a filament forming the earliest relic of material accreted by the MC en route to Earth. The effects resulting from these on the far geomagnetic tail are: (1) expansion, (2) tail twisting, and (3) tail tilting. These extreme conditions were in part responsible for a crossing by Wind of a neutral sheet which is tilted by ∼85° to the ecliptic. Further, Wind made two successive excursions deep into the geomagnetic tail, in the first of which a tailward flow burst of ∼1200 km/s was observed. The dayside part of the interaction of the sudden and large dynamic pressure drop with the bow shock is studied with a local 3D MHD simulation. This work is a contribution to the area ICME/MC-sheaths–magnetosheath interactions.

Development of a Couette–Taylor flow device with active minimization of secondary circulation

A novel Taylor-Couette experiment has been developed to produce rotating shear flows for the study of hydrodynamic and magnetohydrodynamic instabilities which are believed to drive angular momentum transport in astrophysical accretion disks. High speed, concentric, corotating cylinders generate the flow where the height of the cylinders is twice the radial gap width. Ekman pumping is controlled and minimized by splitting the vertical boundaries into pairs of nested, differentially rotating rings. The end rings and cylinders comprise four independently driven rotating components which provide flexibility in developing flow profiles. The working fluids of the experiment are water, a water-glycerol mix, or a liquid gallium alloy. The mechanical complexity of the apparatus and large dynamic pressures generated by high speed operation with the gallium alloy presented unique challenges. The mechanical implementation of the experiment and some representative results obtained with laser Doppler velocimetry in water are discussed.

Review of heavy-ion induced desorption studies for particle accelerators

During high-intensity heavy-ion operation of several particle accelerators worldwide, large dynamic pressure rises of orders of magnitude were caused by lost beam ions that impacted under grazing angle onto the vacuum chamber walls. This ion-induced desorption, observed, for example, at CERN, GSI, and BNL, can seriously limit the ion intensity, luminosity, and beam lifetime of the accelerator. For the heavy-ion program at CERN's Large Hadron Collider collisions between beams of fully stripped lead ($^{208}\mathrm{Pb}^{82+}$) ions with a beam energy of $2.76\text{ }\text{ }\mathrm{TeV}/\mathrm{u}$ and a nominal luminosity of ${10}^{27}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ are foreseen. The GSI future project FAIR (Facility for Antiproton and Ion Research) aims at a beam intensity of ${10}^{12}$ uranium ($^{238}\mathrm{U}^{28+}$) ions per second to be extracted from the synchrotron SIS18. Over the past years an experimental effort has been made to study the observed dynamic vacuum degradations, which are important to understand and overcome for present and future particle accelerators. The paper reviews the results obtained in several laboratories using dedicated test setups, the mitigation techniques found, and their implementation in accelerators.

Silicon surface modification by quasistationary compression plasma flow

Modification of silicon single crystal surface by the action of nitrogen quasistationary compression plasma flow (CPF) generated by magnetoplasma compressor is studied. It was found that during single pulse surface treatment regular fracture features are obtained on the Si (111) and Si (100) surface in the target central part. Some of these regular structures can become free from the underlying bulk, formed as blocks ejected from the surface. Also, oriented silicon periodic structures are produced in the target periphery part. These surface phenomena are results of specific conditions during CPF interaction with silicon surface. High plasma flow energy density, large dynamic pressure, thermodynamic parameters gradients and induced magnetic field on treated surface cause rapid heating and melting of surface layer, as well as surface fracturing, long existence of molten layer and fast cooling and recrystalisation.

Ring current ions and radiation belt electrons during geomagnetic storms driven by coronal mass ejections and corotating interaction regions

Superposed epoch analyses of ring current (RC) ions and radiation belt (RB) electrons are conducted for geomagnetic storms with Dst &lt; −100 nT during solar cycle 23. As a new approach, the storms are categorized into three groups in terms of the solar wind structures; storms with Dst &gt; −130 nT associated with corotating interaction regions (CIRs), storms with Dst &gt; −130 nT driven by coronal mass ejections (CMEs), and CME‐driven great storms with Dst &lt; −130 nT. The flux enhancements of RB electrons and RC ions at L &gt; 3.5 during the recovery phase largely depend on the solar wind structures; CIRs are significantly more effective for the evolution of the outer belt than are CMEs because of the existence of a series of particle injections driven by Alfvén waves within the fast coronal hole stream following the CIR. On the other hand, the flux enhancements of RB electrons and RC ions at L ≤ 3.0 largely depend on the strength of storms; only particular CMEs with a huge motional electric field and large dynamic pressure are effective in increasing the RB electrons and RC ions at L ≤ 3.0.

Post-earthquake investigation on several geosynthetic-reinforced soil retaining walls and slopes during the Ji-Ji earthquake of Taiwan

This paper gives an overview on the application of geosynthetic-reinforced soil structures in Taiwan. Taiwan has an unique topography and geotechnical conditions that rendered a less conservative and more challenging design compared to that of North America, Europe and Japan. The Ji-Ji (Chi-Chi) earthquake of 1999 gave an opportunity to examine the behavior of reinforced soil structures. The performance of several modular-block reinforced soil retaining walls and reinforced slopes at the vicinity of the fault was evaluated. Reinforced structures performed better than unreinforced soil retaining walls. The failure cases were highlighted and the cause of failure was identified. The lack of seismic design consideration could be a major cause of failure. The compound failure mode, the inertia force of the blocks, and the connection stiffness and strength relative to the large dynamic earth pressure, were among major items that would warrant further design consideration.