Three-dimensional detonation cellular structures in rectangular ducts using an improved CESE scheme

The three-dimensional premixed H_2-O_2 detonation propagation in rectangular ducts is simulated using an in-house parallel detonation code based on the second-order space-time conservation element and solution element(CE/SE) scheme.The simulation reproduces three typical cellular structures by setting appropriate cross-sectional size and initial perturbation in square tubes.As the cross-sectional size decreases,critical cellular structures transforming the rectangular or diagonal mode into the spinning mode are obtained and discussed in the perspective of phase variation as well as decreasing of triple point lines.Furthermore,multiple cellular structures are observed through examples with typical aspect ratios.Utilizing the visualization of detailed three-dimensional structures,their formation mechanism is further analyzed.     

On some natural and model 2D bimodal random cellular structures

The topological and metric properties of a few natural 2D random cellular structures, namely an armadillo shell structure and young soap froths, which are formed from two classes of cells, large and small, have been characterized. The topological properties of a model generated from a Kagome tiling, which mimics such random binary structures, have also been exactly calculated. The distribution of the number of cell sides is bimodal for the structures investigated. In contrast to the classical Aboav-Weaire law for homogeneous 2D random cellular structures, nm(n), the mean total number of edges of neighbouring cells of cells with n sides does not vary linearly with n. Only the nm(i, n) (i=1,2) determined separately for every class of cells are linear in n for all investigated structures. Topological properties and correlations between metric and topological properties are finally compared with the predictions of various literature models. Received: 24 December 1997 / Revised: 7 April 1998 / Accepted: 20 April 1998     

Identification of lagrangian coherent structures in the turbulent boundary layer

Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vortices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is implemented to get the ensemble-averaged inclination angle θ _R of typical LCS. θ _R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y ⁺=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y ⁺=100. The ensemble- averaged convection velocity U _c of typical LCS is finally calculated from temporal-spatial correlation analysis of FTLE field. It is found that the wall-normal profile of the convection velocity U _c(y) accords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the downstream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond. Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001)     

Effect of wall temperature on hypersonic turbulent boundary layer

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer.     

Effect of riblets on nonlinear disturbances in the boundary layer

Results of experimental investigations of the nonlinear stage of sinusoidal and varicose instability of a streaky structure, which leads to multiplication of streaky structures and origination of coherent structures (such as Λ-structures), are presented. Riblets suppress the intensity of streaky structures, stabilize the flow against the development of the secondary high-frequency instability of streaky structures, and, for this reason, delay spatial turbulization of the flow. The results of these investigations can be useful for understanding the flow structure in such situations and for possible controlling of the coherent structures aimed at flow stabilization. This work was supported by the President of the Russian Federation (Grant No. NSh-964.2003.1) and by the Russian Foundation for Basic Research (Grant No. 05-01-00034).     

Effect of overlapping Debye spheres on structures of 2D dusty plasmas

In dusty plasmas, overlapping Debye spheres around dust grains could produce an attractive force between them. Its effects on static structures of two-dimensional (2D) dusty plasmas are studied here by using molecular dynamics simulations. Results, in terms of the equilibrium radial distribution function, are compared with those deduced from purely repulsive Debye-Hückel or Yukawa potential for different Coulomb-coupling and screening parameters. The effect of the attractive force is found quite noticeable for usual experimental conditions, and becomes more pronounced for larger screening parameter κ. In particular, it is observed that for large κ the attractive force is dominant, and dust grains tend to aggregate and form patterns with scattering voids.     

Strategies for using cellular automata to locate constrained layer damping on vibrating structures

It is often hard to optimise constrained layer damping (CLD) for structures more complicated than simple beams and plates as its performance depends on its location, the shape of the applied patch, the mode shapes of the structure and the material properties. This paper considers the use of cellular automata (CA) in conjunction with finite element analysis to obtain an efficient coverage of CLD on structures. The effectiveness of several different sets of local rules governing the CA are compared against each other for a structure with known optimum coverage—namely a plate. The algorithm which attempts to replicate most closely known optimal configurations is considered the most successful. This algorithm is then used to generate an efficient CLD treatment that targets several modes of a curved composite panel. To validate the modelling approaches used, results are also presented of a comparison between theoretical and experimentally obtained modal properties of the damped curved panel.     

Effect of rounded edged dimple arrays on the boundary layer development

The effect of a turbulent boundary layer subjected to a series of rounded edged shallow dimple arrays with dimple depth ratios, d/D of 4%, 8% and 12% were experimentally studied. Measurements show the existence of a higher flow speed region at the center of each dimple. The spanwise distribution of the mean wall shear stress immediately downstream of the centers of the last row of dimples does not vary with dimple depth, and is about 45% over that without the dimple array. Turbulence measurements and surface flow visualization shows that the flow over the shallowest dimple differs from the deeper dimples. Flow separation observed with the deeper rounded edged dimples produce similar flow structures as those from sharp edged dimples reported in the literature. However flow separation is not observed when d/D=4% but instead two other higher speed regions either side accompany the central flow. The effects of the dimples are rapidly suppressed by the flat surfaces between of the dimples, and the flow rapidly reverts back to an unmanipulated flat boundary layer flow in these areas.     

Turbulent structures dependent on tidal currents in the bottom boundary layer of the Venice lagoon

Summary The time series of horizontal and vertical turbulent velocity fluctuationsu′, w′ have been recorded by means of an electromagnetic currentmeter in proximity of the bottom of a channel feeding the Venetian Lagoon. Simultaneous surface gradients have been recorded at two tide gauge stations, one upstream and the other downstream of the chosen test site. The time series ofu′, w′ andu′ w′ values have been analysed using standard digital methods and, for each record, spectra, cross-spectra, cospectra, quadrature spectra, phase and coherence ofu′ andw′ have been computed. This analysis allows us to determine temporal and spatial dimensions of turbulent structures that give the greatest contribution to Reynolds stress ( , where ρ is the water density). These structures that seem to be dependent on longitudinal surface gradients are primarily responsible for vertical momentum transport and, consequently, for the lift-up and transport of sediments. Statistic distributions ofu′, w′ andu′ w′ values indicate that the greatest turbulent structures are those withu′ w′<0 and withu′<0 predominating.

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Riassunto Registrazioni delle componenti fluttuanti, orizzontali e verticali,u′ andw′ della velocità di corrente di marea sono state eseguite con un correntometro eletrromagnetico in prossimità del fondo di un canale alimentatore della laguna di Venezia. Contemporaneamente sono stati rilevati i gradienti di livello tra due stazioni mareografiche a monte e a valle del luogo prescelto. Le serie temporali diu′, w′ eu′ w′ sono state analizzate con metodi digitali standard e per ogni record sono stati ricavati spettri, spettri incrociati, spettri di coerenza e quadratura, funzioni di fase e di coerenza. L'analisi ha consentito di determinare le dimensioni temporali e spaziali delle strutture turoolente che danno il maggiore contributo allo stress di Reynolds e che, essendo responsabili del trasferimento verticale della quantità di moto, hanno primaria importanza nei meccanismi di sollevamento dei sedimenti. Si è cercata inoltre una relazione tra l'intensità delle strutture turbolente e i valori del gradiente di livello. Le curve di distribuzione statistica diu′, w′ eu′ w′ con i relativi valori di asimmetria e curtosi hanno confermato che esistono strutture turbolente di grandi dimensioni e che sono caratterizzate dau′ w′<0, prevalentemente conu′<0.

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Резюме Регистрируются временные последовательности флуктуаций горизонтальных и вертикальных турбулентнех скоростейu′, w′ с помощью электромагнитной гидрометрической трубки вблизи дна канала, питающего Венецианскую Лагуну. Также регистрируются одновременные поверхностные градиенты надвух станцнях для калибровки прилива, одна вверх по течению и другая вниз по течению для заданного местоположения. Анализируются временные последовательностиu′, w′ и значенияu′, w′, используя стандартные численные методы. Вычисляются фаза и когерентностьu′ иw′ для всех зарегистрированных спектров. Этот аналиэ позволяет определить временные и пространственные размеры турбулентных структур, которые дают наибольший вклад в напряжение Рейнольдса ( , где р есть плотность воды). Эти структуры, которые, по-видимому, зависят от продольных поверхностных градиентов, ответственны, главным образом, за перенос вертикального импульса и, следовательно, за подъем и перенос осадков. Статистипеские распределенияu′, w′ изначенияu′, w′ показывают, что наибольшие турбулентные структуры имеютu′ w′<0 и преобладают структуры сu′<0.

     

Effect of grain size distribution on the grain boundary electrical response of 2D and 3D polycrystals

We study the influence of grain size distributions of 2D and 3D polycrystals on the electrical response of grain boundaries by means of numerical calculations based on Kirchhoff networks. The GB contribution to the overall conductivity is compared to that of the bricklayer model in order to estimate the validity of such model. We thus quantify the error committed when using the BLM to extract the intrinsic GB resistivity of 2D and 3D real polycrystals from impedance measurements.     

Sources of sensitivity losses in ultrafast 2D NMR

Recent ultrafast techniques make it possible to obtain nD NMR spectra in a single scan. However, an important sensitivity decrease is observed when the excitation duration is increased, which is necessary to improve resolution. A detailed, theoretical and experimental study of sensitivity losses in ultrafast experiments is carried out on the example of J-resolved spectroscopy. The importance of molecular diffusion effects during both encoding and acquisition phases is shown by numerical simulations and experimental results. Other possible sources of signal-to-noise decrease are also considered, such as transverse relaxation, homonuclear J-couplings or chemical shift effects.     

Geometric modeling and analysis of detonation cellular stability

A geometric model with a low computational complexity capable of simulating detonation behavior in physical systems is proposed. In support of the geometric model development, a series of cylindrical 1D simulations with a variable size initiation kernel are performed in hydrogen-oxygen mixtures. From these 1D simulations a detonation cell stabilization mechanism is identified. The stabilization mechanism is predicated on the size of the gap between the pressure and temperature fronts at the point where the average pressure front velocity along one cell length is equal to the CJ velocity. This gap, in a multidimensional detonation, is the ignition kernel of a subsequent blast, and dictates the formation of the subsequent cell. Serial analysis of blasts in this context leads to a unique stable blast kernel size for any mixture, which, within the uncertainty of the initial kernel state, can predict the experimental cell length for mixtures considered in this study. Using a tabulation of the 1D simulations as an input, a formulation and sample results of the geometric model are shown. The geometric model can reproduce both qualitative and quantitative features of experimental detonation cellular structure.     

Determination of the boundary losses in lasers

A breakdown of the detrimental loss factor of a laser into losses within the rod (due to scattering and inactive absorption) and boundary losses (the losses in the resonator due to divergence of the laser beam) is suggested. Formulas are derived for taking account of the boundary loss factor by introducing the effective reflection coefficient of the laser resonator mirrors. The theoretical and experimental relations for variations in the boundary losses resulting from increases in resonator length are compared. It is shown that the boundary losses represent the major portion of the total detrimental losses in a laser.     

Experimental study of mean and pulsation flow characteristics in the 2D/3D supersonic boundary layer behind flat roughness elements

For the first time, detailed experiments aimed at evaluation of the influence of flat microroughness elements (adhesive tape stickers attached to the streamlined surface) on the mean and pulsation flow characteristics in flat-plate and swept-wing boundary layers were carried out under comparable dimensional conditions. At Mach number M = 2, the sticker shape was found to have almost no influence on the amplitude of mass-flow pulsations in the wake of sticker. The relative receptivity of 3D boundary layer to stationary disturbances was obtained to be approximately 1.5 times higher than the receptivity of 2D boundary layer. It was found that the presence of a sticker at the line of the source of controlled disturbance changes the pulsation spectra in 2D and 3D boundary layers in comparison with unperturbed flow. In the wake behind the center of a sticker, the flow can be most unstable.     

Effect of the Tollmien — Schlichting wave on averaged parameters of the boundary layer

The nonlocal equations for two harmonics of the Tollmien — Schlichting wave and averaged parameters of the incompressible boundary layer on a plate are derived. Computed results show that the wave gives rise to a friction increase on the wall, and this increase becomes substantial when the ratio of the amplitudes of the second and main harmonics reaches 0.5.     

Effect of injection dynamics on detonation wave propagation in a linear detonation combustor

The effect of reactant injection and mixing on detonation wave propagation is studied in a self-excited, optically-accessible linear detonation combustor operated with natural gas and oxygen. Fuel injection and mixing processes are captured with 100 kHz planar laser induced fluorescence (PLIF) measurements of acetone tracer injected into the fuel stream. Measurements are acquired at multiple orthogonal planes downstream of the reactant injection site to investigate the three-dimensional mixing field in the chamber. The fuel distribution field is correlated with simultaneously acquired OH${}^{*}$ chemiluminescence measurements that provide a qualitative indication of heat release in the combustor. These measurements are used to provide quantitative information of the fuel injector recovery process and its impact on detonation wave structure across a range of equivalence ratios. While significant differences in the detonation wavefront are observed with change in equivalence ratio, the characterization of the fuel refill process into the chamber after the passage of the detonation wave highlights some key generalizable features. The time available for fuel recovery is consistently between 12 – 19% of the detonation wave period across an equivalence ratio range of 0.83 – 1.48. A linear correlation between injector recovery times and the ratio of the average detonation wave pressure amplitude relative to the pressure drop across the fuel injector is observed. Instantaneous and phase-averaged measurements of acetone-PLIF with the time-coincident OH${}^{*}$ chemiluminescence images provide qualitative information of wave structure and injection dynamics along with quantification of fuel injector recovery, a key metric that drives combustor operation and performance. These measurements significantly enhance the ability to obtain detailed information on the intra- and inter-cycle spatiotemporal evolution of the reactant refill process and its coupled effects on the detonation wave structure and propagation.     

正弦波沟槽对湍流边界层相干结构影响的TR-PIV实验研究

利用高时间分辨率粒子图像测速(time-resolved particle image velocimetry, TR-PIV)技术,在不同雷诺数下对光滑壁面和二维顺流向、三维正弦波(two/three dimensional, 2D/3D)沟槽壁面湍流边界层流场进行了实验测量,从不同沟槽对湍流边界层相干结构影响的角度分析了其减阻的机理.对比不同壁面的各阶统计量结果发现:沟槽降低了壁面摩擦阻力,存在减阻效果,正弦波沟槽的减阻率增大.运用相关函数、λ_(ci)检测准则等方法提取了不同壁面湍流边界层发卡涡和低速条带等典型相干结构的空间拓扑形态,结果表明:两种沟槽壁面的相干结构在流向和法向上的空间尺度均有不同程度的减小,且相干结构与主流之间的倾角趋于更小,流体在法向上的运动及结构的抬升受到明显抑制,发卡涡诱导喷射和扫掠的能力降低,从而影响了湍流中能量与动量的输运过程及湍流的自维持机制,且相比于2D沟槽, 3D正弦波沟槽作用效果更为明显.在同一雷诺数下,随着距离壁面法向位置的增加,不同壁面湍流边界层低速条带的展向间距都变宽;但同一法向位置处2D/3D沟槽壁面湍流边界层低速条带的间距与光滑壁面的相比更宽,沟槽的存在有效抑制了低速条带在展向上的运动,使得低速条带更稳定.