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71.
72.
Bendiks Jan Boersma 《Fluid Dynamics Research》2004,35(6):425-447
In this paper we study the sound field produced by a turbulent round jet with a Mach number of 0.6 based on the centerline velocity and the ambient speed of sound c∞. The turbulent flow field is found by solving the fully compressible Navier–Stokes equations with help of high-order compact finite difference schemes. It is shown that the simulated flow field is in good agreement with experiments. The corresponding sound field has been obtained with help of the Lighthill equation using two different formulations for the Lighthill stress tensor Tij. In the first formulation of Tij the fluctuating density is taken into account. In the second formulation the density is assumed to be constant. As an additional check we have also performed an acoustic calculation using a formulation in which a homogeneous wave equation is solved. The boundary conditions for this homogeneous wave equation are obtained from the numerical simulation of the Navier–Stokes equation. The results obtained with both formulations of the Lighthill stress tensor are nearly identical. This implies that an incompressible formulation of the conservations laws could be used to predict jet noise at low Mach numbers. 相似文献
73.
The need to predict flow and heat transfer problems requires a flexible and fast tool able to simulate complex geometries
without increasing the complexity of the flow solver architecture. Here we use a finite volume code that uses a direct solver
with pressure correction. A new immersed boundary method (IBM) is used for a geometry consisting of a square body in a flow.
The method is applied to flow cases with and without heat transfer. The obstacle simulated in the domain is implemented by
local forcing of the flow with a procedure that adjusts locally the shear stress at the position of the object in conjunction
with a non-penetration condition on the body walls. This approach has already been successfully applied by Breugem and Boersma
(Phys. Fluids 17:15, 2005). We extend it for the case of heat transfer between body and flow. Comparison with other methods has been carried out as
well. However, the proposed method can not be simply extended to immersed boundaries not aligned with the grid. 相似文献
74.
75.
[Structure: see text] Cotrimerization of arynes and electron-deficient alkenes selectively affords dihydrophenanthrenes or ortho-olefinated biaryls, depending on the catalytic system employed. Our findings support a mechanism involving benzyne and a metallacycloheptadiene as key intermediates. 相似文献
76.
77.
Hogerheide MP Ringelberg SN Janssen MD Boersma J Spek AL van Koten G 《Inorganic chemistry》1996,35(5):1195-1200
The structural characterization of two new sodium phenolate complexes, containing ortho-amino substituents, enables the influence of intramolecular coordination on the aggregation of sodium phenolate complexes to be determined. Crystals of hexameric [NaOC(6)H(4)(CH(2)NMe(2))-2](6) (1a) are monoclinic, space group P2(1)/c, with a = 11.668(4) ?, b = 18.146(4) ?, c = 14.221(5) ?, beta = 110.76(3) ?, V = 2815.5(16) ?(3), and Z = 2; R = 0.0736 for 2051 reflections with I > 2.0sigma(I). Complex 1a contains a unique Na(6)O(6) core, consisting of two face-fused cubes, with the ortho-amino substituent of each phenolate coordinating to a sodium atom. In addition, two of the phenolate ligands have an eta(2)-arene interaction with an additional sodium atom in the core. Crystals of dimeric [(NaOC(6)H(2)(CH(2)NMe(2))(2)-2,6-Me-4)(HOC(6)H(2)(CH(2)NMe(2))(2)-2,6-Me-4)](2) (2b) are triclinic, space group P&onemacr;, with a = 10.0670(8) ?, b = 10.7121(7) ?, c = 27.131(3) ?, alpha = 92.176(8) degrees, beta = 99.928(8) degrees, gamma = 106.465(6) degrees, V = 2752.1(4) ?(3), and Z = 2; R = 0.0766 for 5329 reflections with I > 2.0sigma(I). Dimeric complex 2b contains two phenolate ligands, which bridge the two sodium atoms, each coordinating with one ortho-amino substituent to a sodium atom, while the second available ortho-amino substituent remains pendant. The coordination sphere of each sodium atom is completed by a (neutral) bidentate O,N-coordinated parent phenol molecule. The second ortho-amino substituent of this neutral phenol is involved in a hydrogen bridge with its acidic hydrogen. On the basis of these two new crystal structures and previously reported solid state structures for sodium phenolate complexes, it is shown that the introduction of first one and then two ortho-amino substituents into the phenolate ligands successively lowers the degree of association of these complexes in the solid state. In this process, the basic Na(2)O(2) building block of the molecular structures remains intact. 相似文献
78.
Direct numerical simulations of an axisymmetric jet with off-source volumetric heat addition are presented in this paper.
The system solved here involves a three-way coupling between velocity, concentration and temperature. The computations are
performed on a spherical coordinate system, and application of a traction free boundary condition at the lateral edges allows
physical entrainment into the computational domain. The Reynolds and Richardson numbers based on local scales employed in
the simulations are 1000 and 12 respectively. A strong effect of heat addition on the jet is apparent. Heating causes acceleration
of the jet, and an increased dilution due to an increase in entrainment. Further, the streamwise velocity profile is distorted,
and the cross-stream velocity is inward for all radial locations for the heated jet. Interestingly, the maximum temperature
is realized off-axis and a short distance upstream of the exit of the heat injection zone (HIZ). The temperature width is
intermediate between the scalar and velocity widths in the HIZ. Normalized rms of the concentration and temperature increases
in the HIZ, whereas that of streamwise, cross-stream and tangential velocities increases rapidly after decreasing. Both mass
flux and entrainment are larger for the heated jet as compared to their unheated counterparts. The buoyancy flux increases
monotonically in the HIZ, and subsequently remains constant. 相似文献
79.
Avishek Paul Eliza M. Warszawik Mark Loznik Dr. Arnold J. Boersma Prof. Dr. Andreas Herrmann 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(46):20508-20512
Current bacterial RNA switches suffer from lack of versatile inputs and are difficult to engineer. We present versatile and modular RNA switches that are trans-encoded and based on tRNA-mimicking structures (TMSs). These switches provide a high degree of freedom for reengineering and can thus be designed to accept a wide range of inputs, including RNA, small molecules, and proteins. This powerful approach enables control of the translation of protein expression from plasmid and genome DNA. 相似文献
80.
C. Tesauro B. J. Boersma M. A. Hulsen P. K. Ptasinski F. T. M. Nieuwstadt 《Flow, Turbulence and Combustion》2007,79(2):123-132
The mechanism of drag reduction in turbulent flows due to polymers has been investigated with help of a direct numerical simulation.
In particular, we consider the interaction between turbulent velocity fluctuations and polymers in terms of elastic energy
that can be stored in the polymer. To this end all the terms of the elastic energy budget have been computed. The most interesting
term is the production of elastic energy due to turbulent fluctuations, because it describes the interaction between polymers
and turbulence. Although this term appears to be small in the average, it turns out that it can reach very large values instantaneously
and intermittently, and the energy transfer from polymer to turbulence is located in very well defined areas inside the channel.
This implies that locally there is a strong interaction between the polymer and the turbulent flow structure, and this strong
interaction is mostly seen in areas of high velocity fluctuations. 相似文献