[en] It is shown that boxiness can be caused by a distribution function sharply biased towards high angular momentum orbits. Some models are presented which are axially symmetric with velocity which drops slowly away from the equatorial plane. Such models can be thought of as very thick hot discs, and it is argued that this is a clue as to the origin of such systems. (author)

[en] We prove the existence of a highly symmetric family of central configurations in which 16 non-negative masses move in concentric circular motions in $p=3$ rings of radii $a_1,a_2,a_3$. On the first ring, there are four bodies of equal masses in a square configuration. On the second ring there are also four bodies of equal masses, each of which located on the bisectors of the angles formed by each pair of the position vectors of two consecutive bodies of the first ring. On the third ring, there are eight bodies of equal masses, each of which is located on the bisectors of the angles formed by each pair of position vectors of two consecutive bodies of the previous two rings. We study the inverse problem, i.e., given three positive radii $a_1,a_2,a_3$ we determine the values of the corresponding non-negative masses $m_1$, $m_2$ and $m_3$, for which the above described configuration is central. We present some particular interesting cases and study some geometrical features of such configurations. We end this paper proposing several lines of possible further generalizations.

[en] Water tunnel experiments were conducted on both equilateral triangular and square arrays with a pitch ratio of 1.5. The tests were run with different array orientations so that the effects of incident flow direction on cross flow induced vibration could be studied. Strouhal numbers of 0.56 and 0.43, based on pitch velocity, were found for the normal triangular and square arrays respectively. No significant resonant amplitudes were observed. The fluidelastic threshold for the normal triangular array was found to be about twice that of the parallel triangular array. However, the stability threshold for the normal triangular array was found to be quite sensitive to incident flow direction suggesting that this higher threshold should not be used in the design of heat exchangers. The square array was found to be rather insensitive to the approach flow direction. Intermediate angles have higher thresholds than the two standard patterns (square and rotated square). (author)

No abstract available

No abstract available

[en] An investigation was made of diffractive self-reproduction of a periodic wave field of two-dimensional arrays is studied. The conditions are found for the existence of this effect and expressions are derived for the self-reproduction distance. The critical gain increment for a phase-locked mode and the selectivity factor of two-dimensional arrays in Talbot cavities are found. It is shown that the efficiency of an array with a triangular occupancy configuration is higher than the efficiency of a square configuration. (modes, cavities, and optical components)

[en] Full text of publication follows: For the purpose of control of fluid-elastic instability, a general criterion for the design of single-phase heat exchanger tube bundles may be inferred utilizing the results obtained through this experimental investigation. It is immediately apparent that the onset of fluid-elastic instability (FEI) is somewhat ambiguous, both because the process is gradual at the initial stages of onset and differs between different geometric and structural (natural frequency of tube array) configurations. The amplitude of vibration slowly increases as the flow velocity increases and at some point grows very quickly. Any attempt to define a specific critical velocity at which FEI occurs will necessarily be a function of what amplitude is defined as the threshold of instability; this naturally depends on the configurations mentioned above. However, general observations that are more useful than the exact critical velocities may be derived here, especially regarding the overall response of the tube array to the imposed changes in configurations and how they may improve the performance of a given tube bundle design or help to choose between competing design proposals. It is clear from the results that a normal square array configuration is far more susceptible to damaging FEI vibrations than a triangular array configuration over a comparable range of operating velocities. Furthermore, the onset of FEI is much more sudden and violent in the normal square case and, hence, significantly more difficult to predict and prevent. The onset of FEI in normal triangular array configurations varies significantly for each of the different tube frequency tunings. However, the critical velocities of each tuning case for the triangular arrays are greater than those measured for the respective values in the normal square array pattern. Observations of the tube motion for the triangular configuration show a very complex pattern emerging. The pattern appears to be almost statistical in nature with the tubes seeming to behave more independently of one another than the square array, indicating that tube-to-tube coupling may have a reduced influence on the tube vibrations. Data has been collected concerning the vortex-shedding phenomena and its influence on the vibration of the tube arrays under consideration. This data directly provides information for the characterization of the vortex-shedding induced vibration based on the various parameters of the study. Additionally, it allows for the investigation of the possible influence of vortex shedding from the tubes on the onset of FEI, since the vortex shedding phenomena typically occurs at a reduced velocity near that of the FEI onset. The energy associated with the vortex shedding is observed to change significantly as a function of the tube array natural frequency, indicating that the fluid-structure interaction depends heavily on the tuning of the tubes, as suspected. In these respects, observation of the vibrations induced by vortex shedding provide valuable information relevant to the entire scope of the experiment. The reduced velocity range over which the vortex phenomena is prevalent seems to be a primary difference between the FEI onset for the square and triangular configurations. (authors)

No abstract available

No abstract available

[en] Individual rods at the edge of the core consist of neutron shielding material, which has a square cross-section. In contrast to the usual cylindrical rod shape, one obtains a better shielding effect against the neutron irradiation from the core. With the greater rod cross-section, the coolant flow is continually throttled axially, so that the reduced heating compared to fuel rods is compensated better in the region of the neutron absorbing material. (orig./HP)