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Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
The engine efficiency of gas turbines is partly limited by the temperature restrictions of the turbine blade material. In film cooling, cold air is ejected through spanwise rows of small holes on the blade surfaces directly into the oncoming hot boundary layer. This creates a thin layer (or film) of colder air which shields the blade surface from the hot combustion gas. An ejection which is simply inclined with the boundary-layer flow direction develops a counter-rotating vortex pair, termed kidney vortex. In contrast, the flow field of an additionally spanwise-tilted ejection (compound-angle) is known to create just one dominant vortex. By a combination of two subsequent compound-angle ejections, a vortex pair (anti-kidney-vortex) can be established featuring the opposite rotational direction of a kidney-vortex. A reference experiment has shown that such an anti-kidney-vortex is beneficial for the cooling effectiveness.
Two issues arise from this experiment: First, it was performed at ambient conditions. In contrast, numerical simulation allows moving easily to turbine-like conditions in terms of flow temperature and mass-flux ratio. Second, experiment and simulation show that the film requires a downstream distance of more than 15 hole diameters (of broadening in the spanwise direction) to close the hot gaps. However, it is desired to guarantee that a maximum temperature is not exceeded anywhere at the blade surface.
During the HPC-Europa2 visit, the mixed 2nd and 4th order accurate finite-volume code NSMB is used to perform parallel computations based on the Navier-Stokes equations for compressible flows. Large-Eddy simulations (LES) using the approximate deconvolution model (ADM) are carried out to simulate the film-cooling flow which consists of a hot turbulent boundary layer along a flat plate and two injected cold jets. Previously, it was shown that the turbulence properties of the boundary layer are essential to obtain a good match between simulations and experiments. We use the synthetic-eddy method (SEM) to generate a turbulent inflow. The chosen setup is able to reproduce the corresponding physical experiment reasonably well.
First, the influence of a coarsened grid on the film-cooling flow was examined. Although there were quantitative discrepancies in the results, the qualitative data did not change, allowing us to study the influence of certain parameters using fewer grid cells. Second, the temperature ratio and the mass-flux ratio were increased to realistic values.
Finally, in order to optimize cooling we varied the compound-angle of the coolant ejection at realistic conditions. Studying the cooling effectiveness at the wall reveals that an increase of the compound-angle accelerates the broadening in the spanwise direction. Furthermore, the spanwise-averaged film-cooling effectiveness suggests that with increasing magnitude of the compound-angle the peak of the averaged effectiveness increases (from 0.3 to 0.4), the position of this peak moves upstream and the streamwise decline of the effectiveness strengthens. Hence, it might be more beneficial to choose a smaller compound-angle than to obtain a rapid spanwise spreading since already a reduction of the blade temperature by 50 K (equivalent to an effectiveness change of roughly 0.07 for realistic conditions) can double the blade lifetime.
The hydrodynamic state of the interstellar medium (ISM) heated and randomly stirred bysupernovae (SNe) is investigated. We use a three-dimensional non-ideal hydrodynamic ISMmodel in a domain extending 0.5 × 0.5 kpc horizontally and 2 kpc vertically to explore therelative importance of various physical and numerical effects on the multi-phase, turbulentISM. We include both Type I and II SNe, the latter occurring only in dense regions. First weinvestigate the role of the thermal instability in the temperature range 300–6100K, comparingresults obtained for two different cooling functions, one susceptible to the instability, the otherstable. The presence of thermal instability in the system is mainly visible as the tendency ofthe gas to avoid the relevant temperature range, as it quickly evolves towards either colderor warmer phases. Nevertheless, the formation of dense structures for both cooling functionsappears to be dominated by expanding and colliding supernova remnants, rather than by thethermal instability. Part of this effect is likely due to the particular choices of the coolingfunction coefficients, producing different relative amounts of thermal and kinetic energies inthe two cases investigated. As with all other simulations of this kind, the transport coefficientsadopted here are many orders of magnitude larger than are realistic. Here we explore theeffects of changing thermal diffusivity (the Prandtl number), in order to isolate the reliableaspects of the solutions. We find that the total energy contained in the system increases asthermal conductivity decreases (i.e., the Prandtl number increases). This decrease is reflectedin the vertical structure of the disk, with a smaller disc scale height at lower Prandtl number.The hot gas has a significantly larger volume filling factor as the Prandtl number increases.The effect of reduced thermal diffusivity is not only to increase temperature gradients and thethermal energy stored in the system, but also to increase the turbulent pressure by supportingthe hot phase. The purely divergent SN forcing is found to produce significant amounts ofvorticity. The relative vorticity is around 0.6–0.7 for the highest Prandtl numbers explored,Pr=40, and is observed to diminish almost by a factor of two for the lowest Prandtl numbersstudied, Pr=1. Rotation laws with angular velocity decreasing or increasing outwards areinvestigated, enabling us to separate the contributions to kinetic helicity due to rotation andshear. When angular velocity decreases outwards, these two contributions partly cancel eachother, resulting in a smaller net helicity.
We have successfully constructed a model of a section of a typical galactic disc including many essential physical components on the scale of a few parsecs, such as stellar gravity, rotational shear and supernovae remnants. The numerical challenge is to handle a number of physical variables, which span a wide range of magnitudes. This requires very short time steps (of order a few years). To study the physical processes of interest, requires the code to run over much longer times (of order hundreds of Myrs or longer). To accurately compute the small scale features of the interstellar medium requires a resolution of no greater than 4 parsecs, but the large scale structure about the galactic disc requires a domain larger than 500 x 500 x 2000 parsecs. Consequently the simulations requires a large number of processors running in parallel for many weeks. We have run a variety of simulations with different parameters, some over 500 Myrs and many up to 100 Myrs.
So far we have submitted our first paper arising from this work to the Monthly Notices of the Royal Asrtonomical Society and we have additional papers in preparation, which will require further development and additional runs of our model.
The main goal of this work has been to obtain the Renner-Teller (RT) coupled-channel (CC) dynamics of the reaction, NH(a1Δ) + D(2S), considering the four following channels: NH(a1Δ) + D(2S)--> N(2D) + HD(1Σg+) (depletion) (1)
NH(a1Δ) + D(2S) --> ND (a1Δ) + H(2S) (exchange) (2)
NH(a1Δ) + D(2S) --> NH(X3Σ-) + D(2S) (quenching) (3)
NH(a1Δ) + D(2S) --> ND(X3Σ-) + D(2S) (exchange+quenching) (4)
We have used the best available potential energy surfaces and have obtained initial-state-resolved probabilities, cross sections, rate constants and branching ratios via the time dependent real wave-packet method(1) (TDRWP) and flux analysis.
The two electronic states of NHD (X2B1 and A2A1) are the degenerate components of a linear 2Π state, thus giving rise to Renner-Teller(2) (RT) rovibronic nonadiabatic interactions allowing to change the electronic state. The RT effect is responsible to exchange and exchange + quenching reactions, so these reactions are not allowed under the Born-Oppenheimer approximation. Thus, the propagation of the RWP starts in the A2A1 excited potential energy surface(3) (PES), and when it reaches the H-N-D collinear arrangements it becomes possible to change the electronic state (jump into the X2B1 PES(3)) through the RT effect. Moreover, the NH2 X2B1 state has a deep minimum that traps the RWP for long time, increasing in this way the probability of nonadiabatic electronic transition (change of the electronic state).
Finally, the part of the RWP corresponding to reaction channels (1), (2), (3) and (4) is determined and the probability of each one of them is calculated, as a function of the initial conditions.
All the work detailed here is just the continuation of our previous project concerning the Renner-Teller dynamic and kinetic of atom+diatom reactions(5),(6).
(1) S. K. Gray and G. G. Balint-Kurti, J. Chem. Phys. 108, 950 (1998)
(2) C. Petrongolo, J. Chem. Phys. 89, 1297 (1988)
(3) Z.-W. Qu, H. Zhu, R. Schinke, L. Adam, and W. Hack, J. Chem. Phys. 122, 204313 (2005)
(4) S. Akpinar, P. Defazio, P. Gamallo and C. Petrongolo, J. Chem. Phys. 129, 174307 (2008)
(5) P. Gamallo and P. Defazio, J. Chem. Phys. 131, 44320 (2009)
(6) P. Defazio, P. Gamallo, M. González, S. Akpinar, B. Bussery-Honvault, P. Honvault and C. Petrongolo, J. Chem. Phys. 132, 104306-1 (2010)
The first thing we had to do was to test our parallel code in the CINECA machines. Some problems arise from this first step due to poor flexibility of IBM compiler included in the CINECA sp6 machine. Several test were performed and a lot of cpu time was wasted with the aim of solving the problem. Thus, we checked the code running some jobs and comparing the results with others obtained using other computers. The success of this step allowed us to begin the dynamic study of the reactions indicated in the previous section.We want to thank CINECA responsible to allow us to increase the cpu time for finishing all the work expected in the project. Later on, we checked the convergence of the RT-CC-RWP (NH(a1Δ)+D(2S)) calculations verifying a large number of numerical parameters (e.g., rotational basis, mesh, number of iterations, etc.). Once the results were well converged, we performed the RWPs propagations for the title reaction.The different initial conditions investigated have been the following: NH (v0=0, j0=2,3,4) and J=0,1,2,…,40, and K0=0,1,…,min(j,J). Because the CC method has been used, the RWP has been propagated so many times as given by the number of possible K final values (J+1), using a single processor for each propagation. A total of 80000 iteration steps are required to reach convergence and due to this the propagations of the RWPs have been very time demanding.At present, we are carrying the analysis of all this amount of data using the flux and the asymptotic methods to obtain the probability of the four reaction channels. These probabilities will be the basis to obtain both the cross section and the rate constants for all processes. These rate constants will be compared with the experimental data available in the literature.