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The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO2), with and without O2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivations include CO2 capture applications using O2 and CO2, combustion of biogas that contains CO2 and the use of EGR or H2O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW of nominal power. The oxidant (air-O2, +H2O, +CO2) is introduced in the annular part though a swirler. The fuel (CH4) is fed though the central tube and injected radially at the exit section. The study focused on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O2, H2O and CO2 in the mixture. The fraction of diluents varied from 0 to 20%, O2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements were recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16-20%). O2 enrichment decreases lift-off height and enhances flame stability. Increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements highlight the turbulent coherent structure of the swirling flow and the effect of dilution on axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism.

Transition modeling in complex flow situations including adverse pressure gradient, streamline curvature, and massive flow separation represents one of the key challenges in computational fluid dynamics that greatly affects the flow characteristics in many thermal and fluid sciences applications. Here, we report a comparative study that helps investigate the capability of the curvature-sensitive kT−kL−ω−v2 transition model against the original kT−kL−ω algorithm in predicting the flow behavior surrounding a smooth circular cylinder subjected to Reynolds numbers in the range from 3.9×103 to 3.6×106. A C-program that fully accounts for the model's four transport equations is particularly developed and coupled with the transient solver of ansysfluent. The present simulation enables accurate prediction of the distributions of skin-friction and pressure coefficients along with careful specification of the corresponding drag coefficients and angles of separation and transition. The simulation reveals insignificant variations in the bulk flow behaviors using either model in both sub- and critical flow regimes while a remarkable improvement in the supercritical drag result is achieved using the curvature-sensitive model.

The method of threshold exceedances is used to reliably estimate the outage probability due to per-span polarization dependent loss (PDL). Bit-wise achievable information rate (BW-AIR) data for 10,000 instances of the link PDL are obtained using a simulation model that accurately captures the effect of PDL on both the signal and amplified spontaneous emission noise. The root mean square error between the mean excess function and a theoretical fit to it, and the average squared absolute error are compared for the critical step of determining the threshold above which transformed BW-AIR data are represented by the generalized Pareto distribution (GPD). Four techniques are considered for determining values of the two parameters that specify the GPD. The outage probability is defined in terms of the BW-AIR being less than a threshold value for the generalized mutual information (GMI), as determined by a specified value for the normalized GMI. To thoroughly demonstrate the approach, the dependence of the outage probability on the per-span PDL, the number of spans, and the constellation entropy is considered for 32 Gbaud, dual-polarization 64-ary quadrature amplitude modulation with uniform and probabilistically shaped constellations. The extent to which the outage probability can be reduced by decreasing the constellation entropy is quantified.

The method of threshold exceedances is used to investigate the impact of transceiver noise on the outage probability due to per-span polarization dependent loss (PDL). A simulation model that captures the distributed effect of PDL on the signal and amplified spontaneous emission noise is used to obtain bit-wise achievable information rate (BW-AIR) data for 10,000 instances of the link PDL. The outage probability is defined in terms of the BW-AIR being less than a threshold determined by the forward error correcting code rate. The dependence of the outage probability on the transceiver signal-to-noise ratio is assessed for 32 Gbaud, dual-polarization 64-ary quadrature amplitude modulation with uniform and probabilistically shaped constellations.

Long non-coding RNAs (LncRNAs) are RNA transcripts longer than 200 nucleotides. They are new players in transcriptional regulation and cancer research. LincRNA-p21 is a p53-regulated lncRNA involved in the p53 transcriptional network. It has an important role in regulating cellular proliferation and apoptosis. Chronic lymphocytic leukemia is derived by a typical defect in apoptosis and characterized by clonal proliferation and accumulation of mature B cells. The aim of the present study was to assess the expression pattern of the lincRNA-p21 and investigate its potential role as a new prognostic marker in CLL. The study was conducted on 80 newly diagnosed CLL patients and 80 age- and sex-matched controls. The analysis of LincRNA-p21 and the p53 downstream proapoptotic target genes (MDM2, PUMA, BAX, and NOXA) was performed by real-time PCR. The cytogenetic abrasions and expression of ZAP70 and CD38 were detected by FISH and Flow cytometry, respectively. LincRNA-p21 was significantly downregulated in CLL patients compared to controls. The downstream proapoptotic targets were significantly downregulated in CLL patients and positively correlated with lincRNA-p21. Low expression of lincRNA-p21 was associated with poor prognostic markers (advanced stages of CLL, del 17p13, ZAP70, and CD38 expression), failure of complete remission, shorter progression free survival, and overall survival. Low lincRNA-p21 expression was independently prognostic for shorter time to treatment. Low expression of lincRNA-p21 demarcates a more aggressive form of CLL with poor prognosis. Therefore, it could be considered as a new prognostic marker to predict disease outcome in CLL.

The effect of bandwidth narrowing originating from the cascading of wavelength selective switches (WSSs) on the generalized mutual information is assessed for probabilistically shaped, 28 Gbaud, 64-ary quadrature amplitude modulation (QAM) constellations. The system simulation includes the generation of shaped symbol sequences based on the Maxwell—Boltzmann distribution, cascaded WSS filters, simplified detection, and calculation of the bit-wise achievable information rate (BW-AIR). Results are compared for a noise loading configuration and a more practical distributed noise configuration. The statistical variations in the filter responses are addressed by considering 1000 realizations that result from randomly selected parameters for each of the individual WSS filter responses. Experimental results for a noise loading configuration and emulation of the overall frequency response for a filter cascade exhibit good agreement with corresponding simulation results.

Transition modeling represents one of the key challenges in computational fluid dynamics. While numerical efforts were traditionally devoted to either improving Reynolds-averaged Navier–Stokes-based turbulence modeling or developing scale-resolving simulations, cautious attention has been recently given to field inversion and machine learning techniques. This paper discusses an updated development of field inversion model for transitional flows based on [Formula: see text] shear stress transport model using the continuous adjoint approach, instead of the typical discrete adjoint method. The original model is modified by multiplying the production term of the turbulent kinetic energy equation by a spatially varying discrepancy function [Formula: see text]. The adjoint equations and the relevant boundary conditions are specifically derived and integrated in OpenFOAM. The present model is validated using two zero pressure-gradient and four non-zero pressure-gradient from flow-over-flat-plate T3-series test cases. The gradient descent method is employed in the optimization process to minimize the discrepancy in the calculated shear stress. The inferred solution indicates a smooth transition to turbulence at the reported critical Reynolds numbers. The optimized model significantly improves the predictions of skin-friction coefficients, originally incorporated in the objective function. To demonstrate the usefulness of the present approach, the investigation is further extended to determine both velocity and shear Reynolds-stress profiles, which to our knowledge has not been reported before. Furthermore, a reduction in the percentage error from 50.2% to 7.3% is well observed in the predicted boundary layer thickness considering the laminar regime in the T3C5 test case.

An estimation method for the nonlinear signal-to-noise ratio (SNR) is extended to simultaneously identify the shaping distribution for rate-adaptive probabilistically shaped (PS) 64-ary quadrature amplitude modulation (QAM) constellations. An artificial neural network (ANN) is trained with the fiber nonlinearity induced amplitude noise covariance and phase noise correlation extracted from received symbols. The identification employs estimates of the nonlinear signal-to-noise ratio $(\text{SNR}_{nl})$ and nonlinear system coefficients obtained from the ANN to extract the standardized constellation moments and hence identify the shaping distribution. The data used for training are 504 input–output sets, which are obtained from a simplified MATLAB simulation for a 32 Gbaud dual polarization PS 64-QAM signal considering seven different shaping distributions and a wide range of link configurations with varying fiber lengths and number of wavelength division multiplexed channels. Validation using 180 input–output sets that include five shaping distributions corresponding to a bit rate granularity of 25 Gb/s for bit rates between 200 and 300 Gb/s, exhibits high identification success rates, albeit for a limited set of examples. Reduced success rates are achieved for five shaping distributions corresponding to a bit rate granularity of 12.5 Gb/s for bit rates between 200 and 250 Gb/s. For completeness, the identification success rate is also quantified for the case of all seven shaping distributions in the validation.

The statistical behaviors of bitwise achievable information rates (BW AIRs) and effective signal-to-noise ratios (SNRs) due to polarization-dependent loss (PDL) are experimentally characterized for probabilistically shaped and uniform dual-polarization 64-ary quadrature amplitude modulation with a bit rate of 250 Gb/s. A single-span recirculating loop is used to emulate distributed link PDL. Histograms for the BW AIR and effective SNR following offline signal processing resemble shifted, mirror-image Maxwell probability density functions for both constellations and two different per-loop PDL values. For a transmission distance of 1200 km and a per-loop PDL of 1.3 dB, the outage probabilities for the shaped and uniform constellations are estimated to be $1.1\times 10^{-3}$ and $4.4\times 10^{-1}$ , respectively. Moreover, for an outage probability $\leq$ $4.0\times 10^{-3}$ , the increase in transmission distance due to shaping is reduced by 50% compared to that for PDL free transmission.

An efficient procedure is presented for evaluating the performance of multidimensional modulation formats in terms of the achievable information rate (AIR). It allows the explicit properties of signal constellations to be captured and is applicable to fully loaded dense wavelength-division multiplexed transmission systems. The efficiency of the procedure facilitates formulating multidimensional quadrature amplitude modulation (QAM) constellation subset selection as a combinatorial optimization problem. The attained solutions for the quadrature phase shift keying (QPSK) 8D constellation subset selection suggest that a known 8D power and polarization balanced constellation (PPB constellation, 4 bits/8D symbol) and its different variations are the closest 8D QPSK subsets to the Shannon limit at 4 bits/8D symbol. 8D constellation subset selection of a QPSK constellation at 6 bits/8D symbol allows obtaining an 8D polarization balanced version of polarization-switched QPSK (PB-PS-QPSK). Using the proposed procedure, the performance of these constellations and their nonpolarization balanced counter parts, i.e., dual polarization binary phase shift keying (DP-BPSK) and PS-QPSK, is assessed in terms of the estimated AIR. The results exhibit good agreement with those of full system simulations for a single channel and five channels. Moreover, the impact of QAM constellation subset selection on the system performance is evaluated by comparing the reduction in information rate for a symbol rate of 35 Gbaud as a function of the number of channels. For 41 channels, the PPB constellation outperforms DP-BPSK by 2 Gb/s in information rate for a 12,000 km dispersion-managed (DM) link due to the improved linear and nonlinear constellation properties. Finally, PB-PS-QPSK enables an increase of 1.5 Gb/s in information rate compared to PS-QPSK for a 10,000 km DM link.

The thermoviscous behavior of the oscillating gas within the porous medium of a thermoacoustic refrigerator enables the conversion of sound into heat in the process of typical standing-wave thermoacoustic refrigeration systems. Several nonlinear mechanisms, such as harmonics generation, self-induced streaming and possible turbulence, cause complicated flow behavior and influence the performance of such devices. Few analytical and numerical approximations (Cao et al., 1996; Worlikar and Knio, 1996; Worlikar et al., 1998) describe the flow field and the energy flux density in standing devices comprising stacks of parallel plates, but almost no 3-D simulation has been developed that models the large-amplitude excitations and enables the prediction of the oscillating-flow behavior within a porous medium made of square channels. Here, we build on existing effort (Abd El-Rahman and Abdel-Rahman, 2014) and report a computational fluid dynamics analysis of a Helium-filled half-wavelength thermoacoustic refrigerator. The finite volume method is used, and the solid and gas domains are represented by large numbers of hexahedral elements. The calculations assume a 3-D periodic cell structure to reduce the computational cost and apply the dynamic mesh technique to account for the adiabatically oscillating wall boundaries. The simulation uses an implicit time integration of the full unsteady compressible flow equations along with the conjugate heat transfer algorithm. For the sake of simulation, the geometry and operating conditions closely follow the experimental setup of Lotton et al. (2009) [5] . A typical run involves about 200,000 computational cells, whereas the working frequency and the applied drive ratio are 592.5 Hz and 2.0%, respectively. The stationary system response is predicted and the numerical values are compared to theory. The results show good agreement with theoretical behavior in the linear regime of drive ratios up to 2.0% while a maximum cooling effect of 10 degrees is captured between the stack ends. This simulation provides an interesting tool for understanding the transient flow behavior, mean energy-flux density and flow streaming, and helps building 3-D computational fluid dynamics models for thermoacoustic devices.

The flow transition from laminar to turbulent inside of typical thermoacoustic devices influences the heat-exchange capacities of these devices and dramatically impacts overall performances as well. A few measurements [Eckmann and Grotberg (1991), J. Fluid Mech. 222, 329–350; Hino, Sawamoto, and Takasu (1976). J. Fluid Mech. 75, 193–207] and direct simulations [Feldmann and Wagner (2012). J. Turbul. 13(32), 1–28; Feldmann and Wagner (2016a). New Results in Numerical and Experimental Fluid Mechanics X, pp. 113–122] were reported that describe the transitional oscillatory flows; however, almost no turbulence model has been developed that enables accurate detection and characterization of the reported intermittent turbulent fluctuations. The present work aims to assess the applicability of the k-kL-ω transition model to transitional oscillatory pipe flows. A sinusoidal pressure gradient is introduced into ANSYS finite-volume solver for flow field simulation at different acoustic frequencies, while a friction Reynolds number of 1440 is retained. The stationary turbulent and the laminar oscillatory pipe flows are first considered for validation and model calibration against published LDA measurements [Durst, Kikura, Lekakis, Jovanovic, and Ye (1996). Exp. Fluids 20, 417–428] and DNS results [Feldmann and Wagner (2012). J. Turbul. 13(32), 1–28] in addition to the Sexl's laminar-flow theory [Sexl (1930). Zeitschrift Phys. 61(5), 349–362]. Investigation of the total fluctuation kinetic energy of transitional oscillations reveals the appearance of intermittent fluctuations within the near-wall region at Wo = 13 during deceleration, while fully turbulent oscillations are predicted in the entire pipe domain at Wo = 5. Although the present results are qualitatively in good agreement with reported experimental [Eckmann and Grotberg (1991). J. Fluid Mech. 222, 329–350] and DNS findings [Feldmann and Wagner (2012). J. Turbul. 13(32), 1–28], the velocity profiles show poor agreement with corresponding DNS data during flow acceleration at Wo = 5.The flow transition from laminar to turbulent inside of typical thermoacoustic devices influences the heat-exchange capacities of these devices and dramatically impacts overall performances as well. A few measurements [Eckmann and Grotberg (1991), J. Fluid Mech. 222, 329–350; Hino, Sawamoto, and Takasu (1976). J. Fluid Mech. 75, 193–207] and direct simulations [Feldmann and Wagner (2012). J. Turbul. 13(32), 1–28; Feldmann and Wagner (2016a). New Results in Numerical and Experimental Fluid Mechanics X, pp. 113–122] were reported that describe the transitional oscillatory flows; however, almost no turbulence model has been developed that enables accurate detection and characterization of the reported intermittent turbulent fluctuations. The present work aims to assess the applicability of the k-kL-ω transition model to transitional oscillatory pipe flows. A sinusoidal pressure gradient is introduced into ANSYS finite-volume solver for flow field simulation at different acoustic frequencies, while a friction...

Plasmodium Falciparum (pf) Malaria is one of the life-threatening infections for human red blood cells (RBCs), which deteriorates the topology of the corresponding bi-layer membranes and causes a 10-fold increase in their respective shear modulus during the well-distinguished Ring, Trophozoite and Schizont stages of infection progression. Previous efforts to characterize the bulk shear stiffness of pf-iRBC membranes include both in-vitro stretching tests and few simulations that enabled partial description of the membrane elasticity while assuming a uniform shear modulus. Although these results provided good insights into the axial-deformation of pf-iRBCs, the computed transverse diameter did not show similar agreement with the experimental values. The aim of the present work is to build a computational model that simulates the stretching tests of healthy and infected RBCs to better understand the mechanics of disease progression and its influence on the elastic properties of RBC membranes. For this purpose, a new patching technique is developed to mimic the infection progression through the decomposition of the cell membrane into infected pair patches and quasi-normal membrane segments. The incompressible membranes are modeled using the non-linear hyper-elastic Skalak constitutive model implemented through a VUMAT subroutine within the framework of a 3-D ABAQUS/Explicit finite-element model. In the advanced Schizont stage, a spheroidal-like geometry with uniform shear modulus is assumed, whereas in the other two stages, sizable circular patches of 2.4 and 4 µm in diameters, respectively, with adaptive shear moduli are implemented to replicate the stiffer pair-patches. The Skalak model indicates an 8-fold increase in the bulk shear modulus of the Schizont-cell beside showing better agreement with the published experimental results. Interestingly, for all other intermediate stages of infection, the bulk shear modulus is found to increase linearly with the percent area-infection, whereas nearly-constant shear modulus in the range of 14 µm is obtained for the quasi-normal membrane segments.

The geometry of airfoil sections in small wind turbine blades affect the boundary layer separation, promote the flow transition to turbulence and consequently influence the performance of many types of small wind turbines. However, few optimization results are available for typical small wind turbine blades. Here, we report a particular shape optimization study of the standard SG6043 airfoil that applies the genetic algorithm along with the low-Re XFOIL solver. The numerical model is first validated against available experimental values for the SG6043 airfoil at a Reynolds number of 100, 000. The predicted lift and drag profiles are qualitatively found in good agreement with the experimental values, particularly in the pre-stall regime, provided that the XFOIL's transition exponent is adjusted to 11. The simulation is then extended to a lower Reynolds number of 60, 000 that is consistent with an average tip-speed-ratio λ = 4.0. Four specific scenarios seeking different objective functions are proposed for optimization. While the first two scenarios focus on typical optimum configurations at which the maximum lift-to-drag ratios are realized, the objective function of the latter scenarios considers the maximum arithmetic-mean over multiple consecutive degrees of the corresponding angle of attacks. The preliminary results show remarkable improvement in the range from 24% to 10% in the predicted lift-to-drag ratios in comparison with the original configuration at the expense of the corresponding airfoils' stiffness. This is followed by an elaboration on the entire blade design along with the specification of the resulting power coefficient and annual energy production.

The impact of polarization dependent loss on probabilistically shaped and uniform DP 64-QAM for a bit-wise achievable information rate of 4.68 bits/2D symbol is quantified. For an outage probability of 0.003, the shaping reach increase is reduced by 50%.

A method to stabilize the resonance wavelength of a depletion-type silicon micro-ring resonator modulator during high-speed operation is described. The method utilizes the intrinsic defect-mediated photo-absorption of a silicon waveguide and results in a modulator chip fabrication process that is free of heterogeneous integration (for example using germanium), thus significantly reducing the complexity and cost of manufacture. Residual defects, present after p-n junction formation, are found to produce an adequate photocurrent for use as a feedback signal, while an integrated heater is used to compensate for thermal drift via closed-loop control. The photocurrent is measured by a source-meter, which simultaneously provides a DC bias to the integrated heater during high-speed operation. A drop-port or an integrated extrinsic detector is not needed. This feedback control method is experimentally demonstrated via a computer-aided proportional-integral-differential loop. The resonance locking is validated for 12.5 Gb/s intensity modulation in a back-to-back bit-error-rate measurement. The stabilization method described is not limited to a specific modulator design and is compatible with speeds greatly in excess of 12.5 Gb/s, in contrast to the bandwidth limitation of other stabilization methods that rely on intrinsic photo-carrier generation through non-linear processes such as two-photon-absorption. Further, the use of intrinsic defects present after standard fabrication insures that no excess loss is associated with this stabilization method.

A novel multidimensional geometric shaping technique for QAM formats is proposed. Optimized constellations at 6 bits/4D symbol and 4 bits/8D symbol are obtained by optimized subset selection of DP-16QAM and DP-QPSK constellations, respectively.

The thermal interactions between the stack plates and their neighboring gas particles within the thermal penetration depth in a thermoacoustic resonator convert acoustic energy into heat energy in the process of standing thermoacoustic refrigeration systems. Few numerical approximations describe the flow behavior and energy flux density in standing devices, but almost no simulation results are available for the fully coupled continuity of Navier–Stokes and energy equations. Here, we report a two-dimensional computational fluid dynamics simulation of the nonlinear oscillating flow behavior in a helium-filled half-wavelength thermoacoustic refrigerator. The finite volume method is used, and the solid and gas domains are represented by large numbers of quadrilateral and triangular elements. The calculations assume a periodic structure to reduce the computational cost and apply the dynamic mesh technique to account for the adiabatically oscillating wall boundaries. The simulation uses an implicit time integra...

The mechanical interactions between fibers in a dense random-fiber network transmit stress, cause fiber curvature, and influence fiber orientation in the processing of many types of composites. A few theories describe the mechanics of fiber networks, but almost no simulation results are available. Here, we report a direct numerical simulation of the mechanical behavior of random-fiber networks. The finite element method is used, and each fiber is represented by a small number of 3D beam elements. The calculations assume a periodic structure to avoid boundary effects, but within the unit cell, the fibers are placed randomly. A special algorithm that uses the random sequential adsorption process creates an initial structure of straight, random, nonintersecting fibers from which a unit cell with periodic boundary conditions is built automatically [A. I. Abd El-Rahman and C. L. Tucker III, “Mechanics of random discontinuous long-fiber thermoplastics. Part I: Generation and characterization of initial geometry...

Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass or carbon reinforcing fibers that are initially 10–13 mm long. When an LFT is injection molded, flow during mold filling degrades the fiber length. Here we present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model tracks a discrete fiber length distribution at each spatial node. A conservation equation for total fiber length is combined with a breakage rate that is based on buckling of fibers due to hydrodynamic forces. The model is combined with a mold filling simulation to predict spatial and temporal variations in fiber length distribution in a mold cavity during filling. The predictions compare well to experiments on a glass–fiber/PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into micromechanics models to predict the stress–strain behavior of molded LFT materials.

An efficient procedure is presented for estimating the performance of multidimensional modulation formats. It allows the explicit properties of signal constellations to be captured and is applicable to fully loaded DWDM transmission systems.

One of the major hurdles in optical burst-switched (OBS) networks is the contention problem. In this paper, we tackle this problem by proposing a new contention resolution technique based on control packet buffering. In particular, a rigorous mathematical model is developed to analyze the performance of an OBS network core node employing this technique along with just-in- time one-way reservation protocol. The key idea that makes this proposal novel is that this buffering is implemented in the electronic domain, thus avoiding the complexity that exist in the optical domain solutions. A detailed evaluation of the system performance using both the burst loss probability and the steady-state throughput is presented. Our results reveal that a considerable improvement in the performance is achieved when adopting the proposed technique.

A broadly applicable perturbation-based approach is demonstrated for estimating the impact of intra-channel fiber nonlinearities on system performance. The estimated performance for DP-QPSK, DP-8QAM, and DP-16QAM exhibits good agreement with both simulation and experimental results.

A quasi-one-dimensional characteristic based non-linear model for unsteady compressible flow is used to simulate the flow in single-pass perforated tube silencers. The flow in both short and long concentric tube resonators is simulated for both low and high-pressure excitation. The temporal solution is first transferred into the frequency domain using FFT, and then processed by the two-microphone technique in order to evaluate the transmission loss. Derivations of required equations, which are used to implement the method of characteristics along with the mathematical formulation of length corrections and different boundary conditions, are presented. The results obtained are compared with previously published experimental and numerical work, showing, in most cases, better agreement with the experimental data.

Based on the method of characteristics, a time domain non-linear model for unsteady compressible flow is used to simulate the flow in multi-pass perforated tube silencers. In the present work, the flow in both two and three pass anechoically terminated silencers will be simulated for low sound pressure level excitation. The temporal solution is first transferred into the frequency domain using FFT, and then processed by the two-microphone technique in order to evaluate the transmission loss. Derivation of necessary one-dimensional equations, which are used to implement the method of characteristics along with the mathematical formulation of length corrections and different boundary conditions, are presented. Resorting to the white noise excitation, a significant reduction in the computational effort is achieved compared to the step-by-step frequency excitation. The paper uses data from previously published experimental and numerical work for the sake of comparison. A good agreement is verified for the frequency range where the one-dimensional assumption is justified.

A few linear theories [Swift, J. Acoust. Soc. Am. 84(4), 1145-1180 (1988); Swift, J. Acoust. Soc. Am. 92(3), 1551-1563 (1992); Olson and Swift, J. Acoust. Soc. Am. 95(3), 1405-1412 (1994)] and numerical models, based on low-Mach number analysis [Worlikar and Knio, J. Comput. Phys. 127(2), 424-451 (1996); Worlikar et al., J. Comput. Phys. 144(2), 199-324 (1996); Hireche et al., Canadian Acoust. 36(3), 164-165 (2008)], describe the flow dynamics of standing-wave thermoacoustic engines, but almost no simulation results are available that enable the prediction of the behavior of practical engines experiencing significant temperature gradient between the stack ends and thus producing large-amplitude oscillations. Here, a one-dimensional non-linear numerical simulation based on the method of characteristics to solve the unsteady compressible Euler equations is reported. Formulation of the governing equations, implementation of the numerical method, and application of the appropriate boundary conditions are presented. The calculation uses explicit time integration along with deduced relationships, expressing the friction coefficient and the Stanton number for oscillating flow inside circular ducts. Helium, a mixture of Helium and Argon, and Neon are used for system operation at mean pressures of 13.8, 9.9, and 7.0 bars, respectively. The self-induced pressure oscillations are accurately captured in the time domain, and then transferred into the frequency domain, distinguishing the pressure signals into fundamental and harmonic responses. The results obtained are compared with reported experimental works [Swift, J. Acoust. Soc. Am. 92(3), 1551-1563 (1992); Olson and Swift, J. Acoust. Soc. Am. 95(3), 1405-1412 (1994)] and the linear theory, showing better agreement with the measured values, particularly in the non-linear regime of the dynamic pressure response.

The deformation mechanics of dry networks of large-aspect-ratio fibers with random orientation controls the processing of long-fiber thermoplastics (LFTs) and greatly affects the mechanical properties of the final composites. Here, we generate initial geometries of fiber networks in a cubic unit cell with a fiber aspect ratio of l/d = 100 and fully periodic boundary conditions for later numerical simulation. The irreversible random sequential adsorption (RSA) process is first used to generate a quasi-random structure due to the excluded-volume requirements. In order to investigate the nonequilibrium character of the RSA, a second method, which is similar to the mechanical contraction method (MCM) (Williams and Philipse, 2003, “Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction,” Phys. Rev. E, 67, pp. 1–9) and based on a simplified Metropolis Monte Carlo (MC) simulation is then developed to produce quasi-equilibrium fiber geometries. The RSA packing results (ϕ ≈ 4.423% when using a fiber aspect ratio of 100) are in good agreement with the maximum unforced random packing limits (Evans and Gibson, 1986, “Prediction of the Maximum Packing Fraction Achievable in Randomly Oriented Short-Fibre Composites,” Compos. Sci. Technol., 25, pp. 149–162). The fiber structures were characterized by several distribution functions, including pair-spatial and pair-orientation distributions, based on either the center-to-center distance or the shortest distance between the particles. The results show that the structures generated by the RSA have an easily-detectable long-range spatial correlation but very little orientational correlation. In contrast, the quasi-equilibrium structures have reduced spatial correlation but increased short-range orientational correlation.

Few numerical works simulate the laminar-flow behavior within the diverging passages of typical thermoacoustic jet pumps. The associated boundary-layer separation is mostly delayed by maintaining a slowly increasing flow passage and thereby the corresponding pressure drop and consumed acoustic energy are noticeably reduced. Of equal importance is the effect of the transition to turbulence on the boundary-layer separation and the resulting flow-pattern. To our knowledge, no simulation has been developed that numerically predict and capture the conditionally turbulent flow regime within typical jet pumps. Here, a new finite-volume model is reported that employs the non-linear CFD solver of ANSYS FLUENT. The k-kl-ω transition model is considered and its parameters are particularly adjusted for present application. A sinusoidal pressure oscillation is enforced at one end, while the far-field approximation is considered at the other end to model the non-reflecting boundary condition. An axisymmetric model alon...

Partially polymerized membranes display a striking mechanical transition at low temperature known as the wrinkling transition. Fluorescence and scanning electron microscopy as well as profile measurements using an atomic force microscope revealed the existence of three degrees of wrinkling depending on the degree of the membrane polymerization. At low polymerization the membrane undergoes a cascade of wrinkling to form a folded phase with a characteristic exponent eta equal to 3, at intermediate polymerization, the membrane is in an intermediate-wrinkled phase (similar to the crumpling of an elastic sheet) with eta approximately 2.5, while at high polymerization the membrane undergoes an abrupt "compaction" to the wrinkled-rough phase with eta approximately 2.