Numerical simulation of the slamming phenomenon of a wave-piercing trimaran in the presence of irregular waves under various seagoing modes

Trimaran vessels have been of great interest to naval architects, due to their large deck space, low resistance at high speeds and stability. Meanwhile, determination of their slamming forces in the presence of highly nonlinear waves has always been a challenge for the structural designers. However, because of novelty and complexity of their hulls, there is insufficient information in this regard, which necessitates suitable effort in filling this gap. Accordingly, in this article, using Flow-3D, a commercial computational fluid dynamics code, seakeeping of a wave-piercing trimaran is simulated in the presence of irregular waves via standard Bretschneider spectrum in sea state 5 in various seagoing modes. These modes include two speeds and two waves with encountering angles of head sea and bow quartering sea. For validation purposes, seakeeping of a trimaran vessel and water entry of a wedge-shaped section are investigated and numerical results are compared against experimental and analytical data. Good compliance of the results confirms the accuracy of the proposed numerical model. In the slamming analysis of the considered trimaran, using the relative vertical velocity of the bow section from the seakeeping analysis, the water entry problem is investigated for the most severe slamming mode. Pressure distribution caused by water entry and instantaneous impact pressure, known as structural design pressure, is computed. The exerted slamming pressure on the bottom and bow flare region is determined to be 4300 and 2100 Pa, respectively. Results indicate that during slamming phenomenon, the maximum pressure exerted on the vessel’s floor occurs at the time of impact at which the pressure coefficient is 4.4. Accurate assessment of the vessel’s vertical velocity from the start of water entry until the water surface rise-up, and the utilized technique considered for slamming phenomenon in more realistic sea condition, can be considered as important features of this study.

Statistical studies of duration of low-power solar flares

This paper is a sequel to papers dealing with time parameters of solar flares in the Hα line. Using data from the international flare patrol for 1972–2010, we have determined the mean duration of flares of different importance and classes of area. We have established that 94.6 % of flares last more than 60 min. The duration of 90 % of flares with min is 2.1–3.3 hrs. In rare cases, flares can last about 12 hrs. The duration of powerful solar flares does not exceed 8.3 hrs. We have found that the duration of solar flares depends on features of their development. Flares with one brilliant point in the flare region have the shortest duration; two-ribbon flares and flares exhibiting several intensity maxima have the longest duration. We have confirmed that the duration of flares increases with increasing classes of area and brightness.

Statistical studies of duration of low-power solar flares

This paper is a sequel to papers dealing with time parameters of solar flares in the Hα line. Using data from the international flare patrol for 1972–2010, we have determined the mean duration of flares of different importance and classes of area. We have established that 94.6 % of flares last more than 60 min. The duration of 90 % of flares with min is 2.1–3.3 hrs. In rare cases, flares can last about 12 hrs. The duration of powerful solar flares does not exceed 8.3 hrs. We have found that the duration of solar flares depends on features of their development. Flares with one brilliant point in the flare region have the shortest duration; two-ribbon flares and flares exhibiting several intensity maxima have the longest duration. We have confirmed that the duration of flares increases with increasing classes of area and brightness.

Statistical studies of duration of low-power solar flares

This paper is a sequel to papers dealing with time parameters of solar flares in the Hα line. Using data from the international flare patrol for 1972–2010, we have determined mean duration of flares of different importance and classes of area. We have established that 94.6 % of flares last more than 60 min. The duration of 90 % of flares with min is 2.1–3.3 hrs. In rare cases, flares can last about 12 hrs. The duration of powerful solar flares does not exceed 8.3 hrs. We have found that the duration of solar flares de-pends on features of their development. Flares with one brilliant point in the flare region have the shortest duration; two-ribbon flares and flares exhibiting several intensity maxima have the longest duration. We have confirmed that the duration of flares increases with increasing classes of area and brightness.

Spatially inhomogeneous acceleration of electrons in solar flares

The imaging spectroscopy capabilities of the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) enable the examination of the accelerated electron distribution throughout a solar flare region. In particular, it has been revealed that the energisation of these particles takes place over a region of finite size, sometimes resolved by RHESSI observations. In this paper, we present, for the first time, a spatially distributed acceleration model and investigate the role of inhomogeneous acceleration on the observed X-ray emission properties. We have modelled transport explicitly examining scatter-free and diffusive transport within the acceleration region and compare with the analytic leaky-box solution. The results show the importance of including this spatial variation when modelling electron acceleration in solar flares. The presence of an inhomogeneous, extended acceleration region produces a spectral index that is, in most cases, different from the simple leaky-box prediction. In particular, it results in a generally softer spectral index than predicted by the leaky-box solution, for both scatter-free and diffusive transport, and thus should be taken into account when modelling stochastic acceleration in solar flares.

Waves and oscillations in sunspot atmosphere: a review

The review focuses on recent experimental and theoretical studies of sources of oscillations in the sunspot atmosphere. The results of observations with ground-based and spaceborne instruments are presented. An important role of the cut-off frequency in forming the spatial distribution of narrowband sources of oscillations above sunspot is shown. The alternative techniques for studying the magnetic field structure by using helioseismological data are discussed. The dynamics of propagating wave fronts is studied by applying the pixelized wavelet filtering technique. The height analysis of oscillation parameters is performed. A possibility to initiate flare energy release by MHD waves propagating along magnetic waveguides from sunspots into the flare region is discussed. The attention is paid to processes of the increase in wave activity in sunspots before the flare energy release. A brief description of the theoretical model for oscillations based on the subphotospheric low-frequency resonator is provided.