Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

The present direct numerical simulation (DNS) study, the first of its kind, explores the effect that the location of a cylinder, immersed in the turbulent wake of a grid-element, has on heat transfer. An insulated single square grid-element is used to generate the turbulent wake upstream of the heated circular cylinder. Due to fine-scale resolution requirements, the simulations are carried out for a low Reynolds number. Three locations downstream of the grid-element, inside the production, peak and decay regions, respectively, are considered. The turbulent flow in the production and peak regions is highly intermittent, non-Gaussian and inhomogeneous, while it is Gaussian, homogeneous and fully turbulent in the decay region. The turbulence intensities at the location of the cylinder in the production and decay regions are almost equal at 11 %, while the peak location has the highest turbulence intensity of 15 %. A baseline simulation of heat transfer from the cylinder without oncoming turbulence was also performed. Although the oncoming turbulent intensities are similar, the production region increases the stagnation point heat transfer by 63 %, while in the decay region it is enhanced by only 28 %. This difference cannot be explained only by the increased approaching velocity in the production region. The existing correlations for the stagnation point heat transfer coefficient are found invalid for the production and peak locations, while they are satisfied in the decay region. It is established that the flow in the production and peak regions is dominated by shedding events, in which the predominant vorticity component is in the azimuthal direction. This leads to increased heat transfer from the cylinder, even before vorticity is stretched by the accelerating boundary layer. The frequency of oncoming turbulence in production and peak cases also lies close to the range of frequencies that can penetrate the boundary layer developing on the cylinder, and therefore the latter is very responsive to the impinging disturbances. The highest Nusselt number along the circumference of the cylinder is shifted 45 degrees from the front stagnation point. This shift is due to the turbulence-generating grid-element bars that result in the prevalence of intense events at the point of maximum Nusselt number compared to the stagnation point.

HIGH-RESOLUTION TOPOGRAPHY OF MERCURY FROM MESSENGER ORBITAL STEREO IMAGING – THE SOUTHERN HEMISPHERE QUADRANGLES

We produce high-resolution (222 m/grid element) Digital Terrain Models (DTMs) for Mercury using stereo images from the MESSENGER orbital mission. We have developed a scheme to process large numbers, typically more than 6000, images by photogrammetric techniques, which include, multiple image matching, pyramid strategy, and bundle block adjustments. In this paper, we present models for map quadrangles of the southern hemisphere H11, H12, H13, and H14.

Research on Testing Adequacy Criterion of Reactor Physics Code of NESTOR

In view of the characteristics of the physical code Nestor the focus is on the correctness of calculation for which the test adequacy criterion has been established. This is based on structural coverage and the input domain. According to such test adequacy criterion, testing strategies have been applied on the entire testing process. They consist of unit static, unit dynamic, integration, system and regression test strategy. Each strategy is composed of test target, test range, technology and method, entry criterion, completion criterion, test focus and priority. After compared with 11 basic benchmarks from nuclear power plants and calculation result of benchmark programs, the ELEMENT program result is correct and credible; the relative error of result is less than three percent. The ELEMENT testing is adequacy. Its test cases covers fuel grid element types, fuel types, non-combustible grid element types, and control rod computational models. Furthermore, it puts forward a research direction in the future.

Genesis and evolution of velocity gradients in near-field spatially developing turbulence

This paper investigates the dynamics of velocity gradients for a spatially developing flow generated by a single square element of a fractal square grid at low inlet Reynolds number through direct numerical simulation. This square grid-element is also the fundamental block of a classical grid. The flow along the grid-element centreline is initially irrotational and becomes turbulent further downstream due to the lateral excursions of vortical turbulent wakes from the grid-element bars. We study the generation and evolution of the symmetric and anti-symmetric parts of the velocity gradient tensor for this spatially developing flow using the transport equations of mean strain product and mean enstrophy respectively. The choice of low inlet Reynolds number allows for fine spatial resolution and long simulations, both of which are conducive in balancing the budget equations of the above quantities. The budget analysis is carried out along the grid-element centreline and the bar centreline. The former is observed to consist of two subregions: one in the immediate lee of the grid-element which is dominated by irrotational strain, and one further downstream where both strain and vorticity coexist. In the demarcation area between these two subregions, where the turbulence is inhomogeneous and developing, the energy spectrum exhibits the best$-5/3$power-law slope. This is the same location where the experiments at much higher inlet Reynolds number show a well-defined$-5/3$spectrum over more than a decade of frequencies. Yet, the$Q{-}R$diagram, where$Q$and$R$are the second and third invariants of the velocity gradient tensor, remains undeveloped in the near-grid-element region, and both the intermediate and extensive strain-rate eigenvectors align with the vorticity vector. Along the grid-element centreline, the strain is the first velocity gradient quantity generated by the action of pressure Hessian. This strain is then transported downstream by fluctuations and strain self-amplification is activated a little later. Further downstream, vorticity from the bar wakes is brought towards the grid-element centreline, and, through the interaction with strain, leads to the production of enstrophy. The strain-rate tensor has a statistically axial stretching form in the production region, but a statistically biaxial stretching form in the decay region. The usual signatures of velocity gradients such as the shape of$Q{-}R$diagrams and the alignment of vorticity vector with the intermediate eigenvector are detected only in the decay region even though the local Reynolds number (based on the Taylor length scale) is only between 30 and 40.

Global Circuit Model with Clouds

Cloud data from the International Satellite Cloud Climatology Project (ISCCP) database have been introduced into the global circuit model developed by Tinsley and Zhou. Using the cloud-top pressure data and cloud type information, the authors have estimated the cloud thickness for each type of cloud. A treatment of the ion pair concentration in the cloud layer that depends on the radii and concentration of the cloud droplets is used to evaluate the reduction of conductivity in the cloud layer. The conductivities within typical clouds are found to be in the range of 2%–5% of that of cloud-free air at the same altitude, for the range of altitudes for typical low clouds to typical high clouds. The global circuit model was used to determine the increase in columnar resistance of each grid element location for various months in years of high and low volcanic and solar activity, taking into account the observed fractional cloud cover for different cloud types and thickness in each location. For a single 5° × 5° grid element in the Indian Ocean, for example, with the observed fractional cloud cover amounts for low, middle, and high clouds each near 20%, the ionosphere-to-surface column resistance increased by about 10%. (For 100%, fraction—that is, uniformly overcast conditions—for each of the cloud types, the increase depends on the cloud height and thickness and is about a factor of 10 for each of the lower-level clouds in this example and a factor of 2 for the cirrus cloud.) It was found that treating clouds, in the fraction of each grid element in which they were present, as having zero conductivity made very little difference to the results. The increase in global total resistance for the global ensemble of columns in the ionosphere–earth return path in the global circuit was about 10%, applicable to the several solar and volcanic activity conditions, but this is probably an upper limit, in light of the unavailability of data on subkilometer breaks in cloud cover.