Short-period Oscillations Research Articles

Analysis of drag reduction effects in turbulent Taylor–Couette flow controlled via axial oscillation of inner cylinder

Analysis of drag reduction effects due to axial oscillation of an inner cylinder in a turbulent Taylor–Couette (TC) flow is performed in the present study. The frictional Reynolds number on the inner cylinder is 218, and the non-dimensional oscillating period is varied from 8 to 32. By examining turbulence statistics, we uncover different impacts of the long- and short-period oscillations on the circumferential (θ) and radial (r) velocity fluctuations in large (uθl, url) and small (uθs, urs) scales. One of the most surprising findings is that the short-period oscillation increases the large-scale Reynolds shear stress ⟨uθlurl⟩ by the strong intensification of uθl exceeding the suppression of url. To understand the phenomena, the spectra of each term in the transport equations of the Reynolds normal stresses ⟨uθ′uθ′⟩ and ⟨ur′ur′⟩ are analyzed. First, it is shown that the short-period oscillation weakens the productions of uθs, urs, and url while it enhances that of uθl. In contrast, the long-period oscillation reduces the productions of uθl and url while it mainly intensifies that of urs. Second, the investigations of the pressure–strain terms indicate that the short-period oscillation mainly impedes the inter-component energy transfer originating from the small-scale background turbulence. However, the long-period oscillation benefits the small-scale inter-component energy communication while it hinders the large-scale one. In addition, the inverse energy transfer in the turbulent TC flow is confirmed by inspecting the inter-scale energy transfer terms. The hindrance of the inter-scale energy transfer by the inner-cylinder oscillation plays a non-negligible role in the reduction of the wall friction drag.

Pulsations of microwave emission from a solar flare in a twisted loop caused by intrinsic magnetohydrodynamic oscillations

ABSTRACT We present results revealing microwave pulsations produced in a model of a flaring twisted solar coronal loop, without any external oscillatory driver. Two types of oscillations are identified: slowly decaying oscillations with a period of about 70–75 s and amplitude of about 5–10 per cent seen in loops both with and without energetic electrons, and oscillations with a period of about 40 s and amplitude of a few tens of per cent observed only in loops with energetic electrons for about 100 s after the onset of fast energy release. We interpret the longer-period oscillations as the result of a standing kink mode modulating the average magnetic field strength in the loop, whilst the short-period intermittent oscillations associated with energetic electrons are likely to be produced by fast variations of the electric field, which produces energetic electrons in this scenario. The slowly decaying oscillations can explain the quasi-periodic pulsations often observed in the flaring corona.

A cornucopia of oscillations on the Laptev Sea shelf

The attributes of waves in the Laptev Sea that were present during 2018–2020 across a range of periods, are reported using time series constructed from long-duration, separate-year observations of sea level fluctuations made with two types of seafloor-mounted autonomous pressure recorders together with a single year of temperature measurements. In each case the data were sampled every second. Spectral analyses of the collected time series show that long waves with periods coinciding with the main diurnal harmonics of the tidal potential O1, K1 and semidiurnal S2 predominate in the sea level fluctuations, recognizing that critical latitudes can also strongly influence the generation and propagation of internal tides. Substantial fluctuations were also found corresponding to the main lunar semidiurnal harmonic M2 close to the latitude where the instruments were installed. Sea level fluctuations at a period of about 37.3 h were also apparent, identified as shelf waves using a previously-published model. Moreover, a significant decrease in sea level, up to 1.25 m, was detected on October 28, 2018, which is believed to be a negative storm surge because of the characteristic periods present in the feature. Analysis of water temperature fluctuations over the period range 30 min–24 h revealed peaks in the spectra with periods of several hours. Whilst unverifiable in the present case, similar water movements in the Laptev Sea have been attributed to internal waves arising as a result of the interaction of long surface gravity waves and shelf waves with its islands and coastal terrain, as well as baroclinic instability. Analysis of shorter period oscillations (12 s–120 min) is also carried out, which the authors ascribe to short (high-frequency) internal waves.

VCI-Based Analysis on Spatiotemporal Variations of Spring Drought in China

The analysis of spatiotemporal variations in drought is important for environmental monitoring and agricultural production. In this study, the spring vegetative drought conditions in China were analyzed by using the vegetation condition index (VCI) as an indicator to reveal the drought characteristics in China from 1981–2015. The results suggest that spring vegetative drought (especially moderate drought) occurs frequently in China, and drought conditions have obvious geographical differences and are highly affected by monsoons. The frequency of spring vegetative drought is relatively high in the southern and northern regions, which are greatly affected by monsoons, and is relatively low in the northwestern and Qinghai-Tibet regions, which are less affected by monsoons. During 1981–2015, the spring VCI in China showed an overall upward trend. In addition, the trend was not a single change but a wave-like increasing trend that can be divided into four stages: (1) a stage of slow growth from 1981–1990, (2) a stage of intense fluctuations from 1991–2000, (3) a stage of steady growth from 2001–2010, and (4) a stage of slow descent after 2010. The Mann–Kendall test confirmed that the spring VCI in China was increasing, and the changes in the southern, northwestern, and Qinghai-Tibet regions reached significant levels. The time point of mutation in the southern region was 2000, and that in the northwestern and Qinghai-Tibet regions was 1992. Wavelet time series analysis showed that spring vegetation drought in China has a short-period oscillation of 5–7 years and a long-period oscillation of approximately 23–28 years. The northwestern and Qinghai-Tibet regions, which are less affected by the monsoons, are dominated by long-period oscillations, while the southern and northern regions, which are more affected by the monsoons, are dominated by short-period oscillations.

Tidally perturbed oblique pulsations in the hierarchical triple system V1031 Orionis

Abstract We present Transiting Exoplanet Survey Satellite (TESS) photometry of the triple star system V1031 Ori (AB)C, which exhibits short-period oscillations superimposed on an eclipsing light curve.The high-quality data were analyzed in detail and combined with the published spectroscopic measurements to obtain the fundamental stellar parameters of the program target. The masses and radii of the eclipsing pair (AB) were determined to about $0.5\%$ and $0.7\%$ precision, respectively. We found 23 significant frequencies in two ranges of <2 d−1 and 10.9–12.7 d−1 from the eclipse-subtracted residuals. Among them, 16 in the gravity (g)-mode region may be aliases and artefacts due to imperfect removal of the systematic trends and the binary effects from the TESS data. Seven frequencies in the pressure (p)-mode region were separated by the orbital frequency and split by an average offset of 0.042 ± 0.003 d−1 from their adjacent harmonics. Further, the pulsation amplitudes are clearly modulated with the binary star orbit. The results indicate that the p-mode frequencies are tidally perturbed pulsations from the secondary component and the pulsation axis could be aligned with the tidal axis.

Drag reduction in turbulent Taylor–Couette flow by axial oscillation of inner cylinder

Drag reduction in turbulent Taylor–Couette flows by axial oscillation of inner cylinder is investigated by direct numerical simulation. In the present study, the reference friction Reynolds number is Reτ=210 based on the friction velocity at the inner cylinder in the no control cases and the half gap width. We have obtained the effects of the oscillation period and the radius ratio of the inner to outer cylinders on the drag reduction rate. Our analysis shows that as the radius ratio is getting larger, the maximum drag reduction rate is decreased and the optimal oscillating period is increased. Under the condition of the short oscillating period, a larger radius ratio leads to a lower drag reduction rate. However, when the oscillating period becomes long, the larger radius ratio triggers a higher drag reduction rate. With the help of Fukagata–Iwamoto–Kasagi identity, the wall shear stress has been linked to turbulent motions at different scales. It is found that the long-period oscillations primarily reduce the wall friction drag induced by the large-scale Taylor vortices while the short-period oscillations mainly decrease wall shear stress originating from the small-scale velocity streaks. Visualizations of Taylor vortices and velocity streaks, premultiplied spectra, and the weighted Reynolds shear stress indicate that such different effects are related to the Stokes layer. A thick Stokes layer under the condition of large-period oscillations penetrates to the core region of the flow and the Taylor vortices whose center is located near the middle plane between the cylinders is thus attenuated effectively. On the contrary, the influence range of a thin Stokes layer caused by the short-period oscillation concentrates on the near-wall region, hence, the small-scale velocity streaks there are weakened greatly.

Is the Combination of Linearized Theoretical Analysis and Sand Column Experiment Always Suitable for Estimating the Complex Effective Porosity?

AbstractGroundwater hydrodynamics are significantly affected by the capillary effect when groundwater table approaches sand surface. A complex effective porosity was suggested to replace the specific yield to represent the capillary effect based on varieties of sand column experiments, and its estimation is based on the assumption that the response of time‐varying groundwater table is a simple harmonic motion. Nevertheless, groundwater hydrodynamics always consist of multiple harmonic components, which should be further examined. In this study, we first conducted laboratory sand column experiments with respect to different periodic oscillation conditions. Then, a fast Fourier transformation algorithm was applied to analyze the measured groundwater table, upon which the high order harmonic components under high frequency wave conditions was demonstrated. Subsequently, based on a perturbation approach, we presented a third order analytical solution of time‐varying groundwater table under a simple harmonic oscillation. The gradient descent method was applied to estimate the complex effective porosity by fitting the high order analytical solution to the experimental data. In addition, effects of various parameters, representing different driving head conditions and sediment properties, were testified through parametric studies, which demonstrates high order harmonic components should not be neglected under the conditions of large relative amplitude of driving head, short period oscillation, and the ratio of hydraulic conductivity to dynamic effective porosity being around 1 m/s, for example, in the scenarios of wave‐dominant swash zone. These new findings are of great importance to extend our understandings of the capillary effect on groundwater hydrodynamics.

Variability of the lunar semidiurnal tidal amplitudes in the ionosphere over Brazil

Abstract. The variability in the amplitudes of the lunar semidiurnal tide was investigated using maps of total electron content over Brazil from January 2011 to December 2014. Long-period variability showed that the annual variation is always present in all investigated magnetic latitudes, and it represents the main component of the temporal variability. Semiannual and triannual (two and three times a year, respectively) oscillations were the second and third components, respectively, but they presented significant temporal and spatial variability without a well-defined pattern. Among the short-period oscillations in the amplitude of the lunar tide, the most pronounced ones were concentrated between 7–11 d. These oscillations were stronger around the equinoxes, in particular between September and November in almost all latitudes. In some years, as in 2013 and 2014, for instance, they appeared with a large power spectral density in the winter hemisphere. These observed short-period oscillations could be a result of a direct modulation of the lunar semidiurnal tide by planetary waves from the lower atmosphere and/or due to electrodynamic coupling of E and F regions of the ionosphere.

An Investigation of the Longitudinal Dynamic Stability of a Box-Wing Aircraft

This study was undertaken to identify the longitudinal dynamic characteristics of the Box-Wing Aircraft. Research on the Box Wing aircraft gained traction following increasing demand for an environmental friendly aircraft that would reduce noise and pollutant emissions. The Box-Wing Aircraft is renowned for its potential aerodynamics advantages over conventional aircraft. However, due to its departure from conventional configuration, the stability dynamics of the Box Wing has not been fully explored. This study investigates the longitudinal dynamic stability characteristics of a Box Wing Aircraft at the conceptual design level using the J2 Aircraft Dynamics Software. The study integrates empirical mass estimation methods and aerodynamic data generated from a vortex lattice tool into J2 Aircraft Dynamics Software Suite. This was done to assess the Short Period Oscillation and Phugoid responses of a Box Wing Aircraft and a conventional cantilever wing aircraft. For a 2s step input, the short period oscillation induced on the Box Wing was damped out after 19s resulting in damping ratio of 0.68 and the undamped natural frequency is 1.8 <i>rad/s</i>. In the Phugoid mode, the Box Wing Aircraft gives a damping ratio of 0.006 and undamped natural frequency of 0.07 <i>rad/s.</i> On the other hand, the SPO induced on the reference conventional aircraft settles after 14s producing a damping ratio of 0.76 and undamped natural frequency of 3.12 <i>rad/s</i>. The damping ratio and undamped natural frequency for the Phugoid mode of the reference conventional aircraft were however 0.001 and 0.07 <i>rad/s</i> respectively. These results indicate that the reference conventional cantilever wing aircraft possesses acceptable longitudinal dynamic stability characteristics while the Box Wing Aircraft would require stability augmentation systems to improve its flying and handling qualities.

Characteristics of the upper troposphere wind field according to the satellite measurements and their connection with climatic parameters

The results of the study of the spatio-temporal variability of the main characteristics of jet streams and turbulence zones in the upper troposphere of the Northern Hemisphere according to measurements from European geostationary meteorological satellites for the period 2007 - 2018 are presented. Their relationship with the essential parameters of the Earth’s climate system — the temperature of the troposphere, the area of sea ice, and also large-scale atmospheric phenomena is considered. It has been shown that over the past 12 years there has been a significant increase in the area of regions occupied by relatively weak and moderate turbulence and a slight decrease in areas with strong and very strong turbulence. Based on the cross-wavelet analysis in the time-frequency space, the relationships between the variations in the areas of the turbulent regions and the majority of the characteristics of jet streams are revealed. A close relationship has been established between the characteristics of jet streams and the areas of turbulent zones with the temperature of the upper troposphere. In this case, the effect of temperature on the area of turbulent zones manifests itself indirectly through the characteristics of jet streams. A significant association of variations in the average area of the jet stream and the latitudinal position of its center with the area of sea ice are noted. According to spectral and wavelet analysis of the characteristics of jet streams, along with annual and quasi-biennial oscillations, short-period oscillations are observed with periods in the range of 10-40 days, which can be interpreted as a manifestation of Rossby waves..

Discovery of an ∼30-yr-duration post-nova pulsating supersoft source in the Large Magellanic Cloud

ABSTRACT Supersoft X-ray sources (SSS) have been identified as white dwarfs accreting from binary companions and undergoing nuclear burning of the accreted material on their surface. Although expected to be a relatively numerous population from both binary evolution models and their identification as type Ia supernova progenitor candidates, given the very soft spectrum of SSSs relatively few are known. Here we report on the X-ray and optical properties of 1RXS J050526.3−684628, a previously unidentified accreting nuclear-burning white dwarf located in the Large Magellanic Cloud (LMC). XMM–Newton observations enabled us to study its X-ray spectrum and measure for the first time short-period oscillations of ∼170 s. By analysing newly obtained X-ray data by eROSITA, together with Swift observations and archival ROSAT data, we have followed its long-term evolution over the last 3 decades. We identify 1RXS J050526.3−684628 as a slowly evolving post-nova SSS undergoing residual surface nuclear burning, which finally reached its peak in 2013 and is now declining. Though long expected on theoretical grounds, such long-lived residual-burning objects had not yet been found. By comparison with existing models, we find that the effective temperature and luminosity evolution are consistent with an ∼0.7 M⊙ carbon–oxygen white dwarf accreting ${\sim} 10^{-9}~\rm {M}_{\odot }$ yr−1. Our results suggest that there may be many more undiscovered SSSs and ‘missed’ novae awaiting dedicated deep X-ray searches in the LMC and elsewhere.

Six-Degree of Freedom Mathematical Dynamic Model of a Light-Sport Aircraft

The paper considers the applications of six-degree of freedom mathematical model of a light-sport aircraft and analytically reveals aircraft dynamic response both longitudinal and lateral-directional stability. The model composes of both the kinematics equations of motion and the kinetics equations of aerodynamic forces and moments, and it is known as the aerodynamic model equations. Simulation results show responses of the perturbed dynamic system at the trim condition, and indicate the dynamic stability of both short-period pitching oscillation and phugoid in longitudinal axes. The spiral, roll subsidence, and dutch roll modes in lateral-directional axes are dynamical stable as well. These are essential to understand and evaluate the dynamic behavior, stability, safety and other aspects of the designed aircraft through mathematical model before conducting operational flight test.

On The Determinations of Weather, Seasonal, Sub-Seasonal and Climate Scale Variability and Overall Trends in the Atmosphere and Ocean

The traditional concepts and definitions of multi-scale “weather”, “seasonal variability”, “sub-seasonal variability”, “climate variability”, “trends” and “climate change” for both the global atmosphere and the global ocean are considered. We build upon existing literature and present new evidence that atmospheric and oceanic temporal multi-scale variability are the result of a mix of well-known frequency and amplitude modulated nonlinear and phenomena that occur simultaneously [1-3]. We harvest representative atmospheric temperature and wind data, oceanic temperature and coastal water level from United States (U.S.) and United Kingdom (U.K.) agency archives, collected via in-situ and satellite remotely sensed data and employ a mathematical methodology that can decompose nonlinear data. The data decomposition reveals a continuum of well-defined, modulated, internal modes of oscillations, each with broad spectral peaks and each representative of naturally occurring phenomena. We reveal that the conventional notions of weather and seasonal to subseasonal to climate variability, actually constitute an over-lapping continuum, with shorter period oscillations commuting with longer period oscillations onto overall record length trends. We relate these internal, intrinsic modes of variability to naturally occurring causal agents, from relatively high frequency weather to lower frequency seasonal to sub-seasonal to climate scale variability. Correlative relationships between climate factors reveal causal couplings of the oceanic and atmospheric systems.

Correlation of interfacial perpendicular magnetic anisotropy and interlayer exchange coupling in CoFe/W/CoFe structures

Spin transfer torque magnetic random-access memory is widely considered as a promising candidate for a next generation cache, with a core device of magnetic tunnel junction. The MgO/CoFeB/W/CoFeB/MgO structure as recording layer for magnetic tunnel junctions is of great interest due to features such as strong perpendicular magnetic anisotropy (PMA) and thermal stability. However, the origin of correlation between PMA and interlayer exchange coupling (IEC) in this structure remains unclear. In this paper, we investigate the PMA and IEC in the MgO/CoFe/W/CoFe/MgO structure with first-principles calculation. IEC with long and short period oscillations is observed, which results in much different magnetic anisotropy energies at CoFe/W interface in MgO/CoFe/W/CoFe/MgO structure compared to MgO/CoFe/W structure. Furthermore, we find an oscillation of magnetic anisotropy energies in ferromagnetically coupled MgO/CoFe/W/CoFe/MgO structure. According to k-space and band-decomposed charge-density analyses, this oscillation can be elucidated with quantum well states on spin-up states near Fermi energy within W layers. Thus, the correlation between PMA and IEC is explained by quantum well states. These findings provide comprehensive understanding of PMA and IEC in double-interface structures and pave a new way to modulate PMA with quantum well states in heavy metal layers.

Jump-like inelasticity in sandstone and its effect on the amplitude dependence of P-wave attenuation: An experimental study

The article describes an experimental study, which aims to detect signs of jump-like inelasticity in sandstone and to establish their influence on the P-wave attenuation. The measurements were taken in a sandstone sample using the reflection method at a pulse frequency of 1 MHz and with the strain amplitudes ε1≈ 0.1 microstrain and ε2≈ 0.24 microstrain at a hydrostatic pressure of 10 MPa. Using the high-resolution recording of the acoustic signal, the short-period low-amplitude oscillations were detected in the waveform and were interpreted as manifestations of jump-like inelasticity. These unconventional manifestations have forms of a stress plateau and a stress drop. At all strain amplitudes, with an increasing frequency in the range of (0.5–2.0) MHz, the P-wave attenuation in the sandstone sample monotonously increases and has a peak at a frequency of ∼1.56 MHz. The jump-like process is amplitude-dependent; it introduces changes in the attenuation peak parameters. The simple simulation demonstrated considerable variation in the attenuation peak magnitude (from 0.353 to 0.607) due to this effect. The jump-like deformation process in rocks can presumably be explained by the combined action of the viscoelastic and microplastic mechanisms. The results of this study can be useful for improving the methods of geological interpretation of seismic and acoustic data.

Wavelet Analysis of the Effect of Injection Strategies on Cycle to Cycle Variation GDI Optical Engine under Clean and Fouled Injector

High fluctuation in cyclic variations influences engine combustion negatively, leading to higher fuel consumption, lower performance, and drivability problems. This paper examines the impacts of injection strategies (injection pressure, injection timing and injection duration) on the cyclic variation of gasoline spark ignition (SI) optical engine under clean and fouled injectors. The principal oscillatory modes of the cycle to cycle variation have been identified, and the engine cycles over which these modes may persist are described. Through the wavelet power spectrum, the presence of short, intermediate and long-term periodicities in the pressure signal have been detected. It was noticed that depending on the clean and fouled injector, the long and intermediate-term periodicities may span many cycles, whereas the short-period oscillations tend to appear intermittently. Information of these periodicities could be helpful to promote efficient control strategies for better combustion. The outcomes report that the cyclic variation of the IMEP happens on multiple timescales, and thus, display complex dynamics. As the fouled injector used, mainly intermittent high-frequency fluctuations are recognized.

Storm Tilo over Europe in November 2007: storm surge and impacts on societal and energy infrastructure

Abstract. Storm Tilo on 8–9 November 2007 ranks among the serious winter storms in northern Europe over the past 30 years. Its low pressure centre passed across the northern North Sea, and this led to a cold air outbreak in northwest Europe. Strong north winds across the North Sea contributed to a high storm surge that was serious for coastal regions in eastern England, the Netherlands and Germany. Storm winds and unusually high waves caused shipping accidents and damage to some offshore energy infrastructure. This report presents an outline of the met-ocean conditions and a short overview of storm impacts on societal and energy infrastructure. The progress of the storm surge around the North Sea is analysed using data from the national tide gauge networks. A spectral analysis of the water level data is used to isolate the long period storm surge and short period oscillations (i.e., &lt;4.8 h) from the tidal signal. The calculated skew surge is compared with literature reports for this storm and also with another serious North Sea storm from 31 October–1 November 2006 (Storm Britta). The short period oscillations are compared with the platform and shipping incident reports for the 2 d storm period. The results support previous reports of unusual wave and water level dynamics during some severe regional winter storms.

Characteristics and Drivers of Reference Evapotranspiration in Hilly Regions in Southern China

This paper has adopted related meteorological data collected by 69 meteorological stations between 1951 and 2013 to analyze changes and drivers of reference evapotranspiration (ET0) in the hilly regions located in southern China. Results show that: (1) ET0 in southern China’s hilly regions reaches its maximum in summer and its minimum in winter, and that the annual ET0 shows an increasing trend. ET0 happened abrupt change due to the impact of abrupt meteorological variables changes, and the significant year of mutation were 1953, 1964 and 2008. Most abrupt changes of ET0 in meteorological stations occurred in the 1950s and 1960s. (2) The low value of ET0 was mainly captured in high-altitude areas. Spatially, the ET0 in the east was higher than that in the west. With the exception of a handful of stations, the trend coefficients of ET0 were all positive, exhibiting a gradual rise. Changes in ET0 in the east were much more sensitive than that in the west. Since ET0 was affected by the cyclical changes in relative humidity, short-period oscillations were observed in all these changes. (3) In general, the ET0 was negatively correlated with relative humidity, and positively correlated with temperature and sunshine percentage. ET0 is most sensitive to changes in average temperature, with a sensitivity coefficient of 1.136. ET0 showed positive sensitivity to average temperature and sunshine hours, which were notable in the northeastern, and uniform in the spatial. ET0 showed negatively sensitivity to relative humidity, and the absolute value of sensitivity coefficient in the northwestern is smaller. The highest contribution to ET0 is the average temperature (6.873%), and the total contribution of the four meteorological variables to the change of ET0 is 7.842%. The contribution of average temperature, relative humidity, and sunshine hours to ET0 is higher in the northern and eastern, northern, northern and eastern areas, respectively. Climate indexes (Western Pacific Index (WP), Southern Oscillation Index (SOI), Tropical Northern Atlantic Index (TNA), and El Niño-Southern Oscillation (ENSO)) were correlated with the ET0. In addition, the ET0 and altitude, as well as the latitude and longitude were also correlated with each other.

Development of Seismic Device for Stainless Steel Rectangular Water Tank at Short Period Earthquake

ABSTRACTThe seismic device effective for an existing stainless steel rectangular water tank was developed to prevent the damage of tank by earthquake, especially focusing on the short period earthquake where the impulsive pressure acts on the side wall of tank. The seismic device consisted of a thin high damping rubber plate, sandwiched with two metal plates, which were attached on both the surfaces of the rubber plate. These seismic devices were expected to be set between the base frame of tank and the anchor block.First, a shaking table test of a small water tank (1,000 mm×1,000 mm×1,000 mm) made of stainless steel panels was performed under the several conditions of the number, shape and rubber thickness of the seismic devices. Then, eight seismic devices were set to a large water tank (3,000 mm×3,000 mm×3,000 mm) constructed from nine stainless steel panels for each side wall. These tanks were put on the shaking tables and dynamic shaking tests were performed. The shaking frequency was varied from DC to 12 Hz with a given interval step and the water pressure on the side wall of tank was measured.In this study, by setting the seismic devices to both the small and large water tanks, the natural impulsive frequencies of tanks became a little lower, and the impulsive pressure at the natural frequency became smaller with increasing rubber thickness and with increasing surface pressure. When the HYOGOKEN‐NANBU earthquake waves were given to the large water tank with the seismic devices, the impulsive pressure at the short period oscillation was largely suppressed but the sloshing pressure at the long period oscillation was scarcely enhanced.

Mouth Function Determines the Shape Oscillation Pattern in Regenerating Hydra Tissue Spheres

Hydra is a small freshwater polyp capable of regeneration from small tissue pieces and from aggregates of cells. During regeneration, a hollow bilayered sphere is formed that undergoes osmotically driven shape oscillations of inflation and rupture. These oscillations are necessary for successful regeneration. Eventually, the oscillating sphere breaks rotational symmetry along the future head-foot axis of the animal. Notably, the shape oscillations show an abrupt shift from large-amplitude, long-period oscillations to small-amplitude, short-period oscillations. It has been widely accepted that this shift in oscillation pattern is linked to symmetry breaking and axis formation, and current theoretical models of Hydra symmetry breaking use this assumption as a model constraint. However, a mechanistic explanation for the shift in oscillation pattern is lacking. Using in vivo manipulation and imaging, we quantified the shape oscillation dynamics and dissected the timing and triggers of the pattern shift. Our experiments demonstrate that the shift in the shape oscillation pattern in regenerating Hydra tissue pieces is caused by the formation of a functional mouth and not by shape symmetry breaking as previously assumed. Thus, model assumptions must be revised in light of these new experimental data, which can be used to constrain and validate improved theoretical models of pattern formation in Hydra.