Po-Wen Hwang

Transition Characteristics of Flowfield in a Simulated Solid-Rocket Motor

The e ow characteristics in a two-dimensional porous-walled duct simulating a solid-propellant rocket motor are numerically computed to investigate the effects of viscosity, compressibility, and ine ow turbulence ( w) on the e ow transitions. The e nite volume technique is used to solve the time-dependent compressible Navier‐ Stokes equations with a subgrid-scale turbulence model, and the numerical e uxes are computed using a modie ed Godunov scheme. In addition to computed axial mean velocity and turbulence intensity proe les, the axial variations of skin friction coefe cient and the transverse location of peak turbulence intensity are used to identify the mean-e ow transition and turbulence-intensity transition, respectively. In particular, a new way of identifying turbulence-intensity transition by the use of the power spectrum of velocity e uctuations is presented for the e rst time in the present study. The minimum centerline Mach number for the onset of mean-e ow transition is obtained as the compressibility is considered alone. The critical values of w for the onset of turbulence-intensity transition and mean-velocity transition advance as well as for the concurrence and delay between the two transitions are also determined to illustrate why some researchers could observe only a single transition whereas others observed two transitions.

Large-eddy simulations of turbulent reacting flows in a chamber with gaseous ethylene injecting through the porous wall

Abstract Large-eddy simulations were performed to study the turbulent reacting flows in a simulated solid-fuel combustion chamber. The time-dependent axisymmetric compressible conservation equations were solved directly without using subgrid-scale turbulence models. The combustion process considered was a one-step, irreversible, and infinitely fast chemical reaction and the pyrolizing solid fuel was simulated by gaseous ethylene injected through a porous wall for a practical range of fuel blowing velocity encountered in solid-fuel combustion chambers for the first time. The numerical code used the finite-volume technique which involved alternating in time the second-order, explicit MacCormacks and Godunovs methods. Characteristic-based boundary conditions were applied on inflow and outflow boundaries, which allowed outlet boundary conditions to be nonzero gradients and, in turn, a practical length of computational domain to be realized. The effects of combustion on the large-scale unsteady flow structure and the mean flameholder recirculation zone were documented in terms of the density contours, vorticity dynamics, streamlines, mean-velocity vector fields, temperature profiles, flame position, and fuel blowing velocity. A comparison of the distributions of instantaneous and mean mass fractions of reactants shows that the present method appropriately reveals the effects of large-scale turbulent motions on combustion. Furthermore, the present large-eddy simulations have achieved a significant improvement in predicting the mean effective reattachment length over the previous calculations incorporating with turbulence models. The physical insight regarding the decrease of the mean effective reattachment length with combustion was also addressed.

Compressibility effects and mixing enhancement in turbulent free shear flows

Computational simulations have been performed to study compressible, spatially developing turbulent free shear layers with various velocity regimes—subsonic/subsonic, supersonic/subsonic, and supersonic/supersonic— for convective Mach numbers in the range of 0.14-1.28. The numerical code used the finite volume technique and a modified Godunovs scheme. The computed results for the supersonic/subsonic case are first compared with experimental axial mean-velocity profiles, vorticity thickness, and turbulence parameters. Mixing layers with various velocity regimes are then calculated to investigate compressibility effects on the evolution of large-scale structures through the flow visualization of the vorticity field, growth rate of the vorticity thickness, and vorticity dynamics analysis. Various forcing frequencies are applied at the inflow boundary to examine mixing enhancement for free shear layers with higher convective Mach numbers. It is found for the first time that the growth rate of supersonic/supersonic free shear layers increases markedly when the forced layers move up and down with time instead of forming vortex roll-up and pairing.

Heat Conduction in Multiply Adjoined Anisotropic Media with Embedded Point Heat Sources

In this article the direct domain-mapping technique is applied in the boundary element method (BEM) to investigate the heat conduction in composites consisting of multiple anisotropic media with embedded point heat sources. By use of a linear coordinate transformation, the physical domain is mapped to an auxiliary plane for 2D or space for 3D, where the heat conduction is considered isotropic. However, the interfaces of adjoined materials with dissimilar properties will overlap or separate in the mapped plane or space. For the use of the subregioning technique in BEM to solve such problems, the thermal equilibrium condition for interfaces is developed to account for boundary distortions. In the mapped plane or space, not only the locations but also the strength of heat sources are transformed accordingly. After the problem is solved in the mapped plane or space, the obtained numerical solution is thereafter interpolated and transformed back to the one in the physical domain.

Electromagnetic shielding effectiveness and functions of stainless steel/bamboo charcoal conductive fabrics

Following technological advancements, there is a growing population of cellular phone and computer users. However, these electronic instruments cause electromagnetic waves, negatively influencing users’ health or precision instruments’ malfunction. Therefore, shielding electromagnetic wave becomes an important matter. In this study, stainless steel wires and bamboo charcoal roving are made into conductive yarn with 6 turns/cm by ring spinning machine. On a 14-gauge automatic horizontal knitting machine, the resulting yarn is then knitted into stainless steel/bamboo charcoal conductive fabrics and then evaluated for the electrical property and functions. According to experimental testing, electromagnetic shielding effectiveness (EMSE) of the fabrics increases with an increase in stainless steel content and number of lamination layers. In particular, when laminated at an angle of 0°/45°/90°/−45°/0°/45°, the fabrics have an EMSE of above 30 dB at an incident frequency between 2010 and 2445 MHz. The far infrared emissivity increases with bamboo charcoal content, reaching the maximum of 0.9 ɛ, when the fabric was made by one-cycle polyethylene terephthalate (PET)/stainless steel/bamboo charcoal plied yarn in the first feeder and four-cycle PET/bamboo charcoal plied yarn in the second feeder.

Numerical visualization and residence time determination of turbulent reacting duct flow with mass bleed and a backstep on one wall

A numerical experiment was performed to study turbulent reacting duct flows with a backstep and mass bleed on one wall. The time-dependent two-dimensional compressible conservation equations were solved with a subgrid-scale turbulence model. The combustion process considered was a one-step, irreversible, and infinitely fast chemical reaction. The numerical code used the finite-volume technique, which involved alternating in time the second-order, explicit MacCormacks and modified Godunovs schemes. Computed mean-velocity profiles are compared with existing experimental data, and good agreement is attained. Various numerical visualization techniques are implemented to reveal the evolution of large-scale vortical structures, temperature and species contour maps, and paths of fuel and air particles. The numerical time lines experimental allows the particle residence time in the flame-holding recirculation zone to be determined directly by the Lagrangian method through residence time probability density function for the first time. Moreover, a compact exponential expression for the particle number decay rate is provided to correlate the numerical data. A comparison of the Eulerian method previously used by all the researchers to indirectly find the residence time with the present Lagrangian method is further performed. An insight into the size of the detecting area and its relation to the uncertainty associated with the residence time determination using the Eulerian method are then provided.

Flammability Limits and Probability Density Functions in Simulated Solid-Fuel Ramjet Combustors

A numerical analysis was performed to study turbulent reacting e ows in solid-fuel ramjet combustors. The time-dependent axisymmetric compressible conservation equations were solved with a subgrid-scale turbulence model. The combustion process considered was a one-step, irreversible, and ine nitely fast chemical reaction. The numerical code used the e nite volume technique, which involved alternating in time the second-order, explicit MacCormack’ s and modie ed Godunov’ s schemes. Computed temperature proe les are compared with existing experimental data and the previous calculations, incorporating the k-e turbulence model and two-layer near-wall treatment. The superiority of the present predictions is demonstrated. Flammability limits in solid-fuel ramjet combustors have been determined using the characteristic exhaust velocity. Moreover, a compact correlation relating the minimum step height to the combustor diameter, air mass e ow rate, and equivalence ratio for sustained combustion is provided for practical design reference. Probability density function (PDF) proe les of temperature and mixture fraction are also analyzed to understand the thermal structure of turbulent e ames, and to provide information for examining the applicability of the conserved-scalar approach incorporated PDF models to turbulentreacting e ows.

Structure design and property evaluation of silver/stainless steel composite fabric

This study presents a fabrication method for functional commingled yarns and prepared conductive knitted fabrics for shielding electromagnetic waves and electrostatic discharge. Stainless steel filament was used as core yarn, polyethylene terephthalate filament or silver yarn was used as wrapped yarn producing polyethylene terephthalate/polyethylene terephthalate/stainless steel filament or silver/silver/stainless steel filament commingled yarns via filaments hollow spindle spinning system and then knitting into silver/stainless steel composite fabric. The effects of cycle number and metal content on air permeability, surface resistance, and electromagnetic shielding properties of resultant knitted fabrics were discussed. Besides, influences of number of layers and lamination angle on electromagnetic shielding were also investigated intensively. The result shows that, conductive composite fabrics made by silver/silver/stainless steel filament commingled yarns and 450D polyethylene terephthalate plied filaments had higher surface resistance of 3.4 Log(Ω/sq) and 5.6 Log(Ω/sq), respectively, in coursewise and walewise directions. Electromagnetic shielding varied with number of layer, lamination angle, cycle number, and metal content. When six layers of conductive knitted fabrics were laminated with 45°, electromagnetic shielding reached 15 dB at 1–3 GHz frequency. The highest air permeability, 317.6 cm3/cm2/s, occurred at single-layer conductive composite fabric.

Manufacturing Technique and Properties Evaluation of Ni-Coated Copper Composite Fabrics

The existence of the electromagnetic radiation may lead to diseases, which also includes cancer and cause the repellence of electrical compatibility. The textiles which have electromagnetic shielding effectiveness become more important in modern life. In the research, the PET/ Ni-coated Copper composite yarn were made by the wrapping machine, which the core yarn is Ni-coated Copper wire and the wrapped yarn is PET filament. After that, the composite yarn is fabricated by the automatic sampling loom into woven fabrics and had the tests of mechanical properties and electromagnetic shielding effectiveness. The test results revealed that the EMSE of the PET/Ni-Cu complex woven fabrics is 32.28dB, which the test frequency is 900 MHz, laminated layer number is 3 and the laminated angles are 0°/45°/90°, respectively.

Numerical Visualization of Vortical Structures in a Spatially Evolving Turbulent Compressible Mixing Layer

Large-eddy simulations are performed to numerically visualize the generation of streamwise vortical structures and its interaction with spanwisely rolled-up coherent vortical structure during the spatial development of a turbulent supersonic/subsonic mixing layer at convective Mach numberMc=0.51. Time-dependent three-dimensional compressible conservation equations were solved with a subgrid-scale turbulence model. The numerical code used the finite-volume technique, which adopted alternately in temporal discretization the second-order, explicit MacCormack’s and modified Godunov’s schemes. Both transverse and spanwise perturbations were imposed initially for promoting the formation of spanwise rollers and counter-rotating streamwise vortices, respectively. Numerical visualizations are presented in terms of time-sequence isopressure surfaces and vorticity contours of spanwise and streamwise components. The results show that the spatial growth of three-dimensional vortical structures, in particular, the formation of chain-link-fence type structures, is adequately captured by the present computations. Vorticity dynamics is further analyzed, for the first time, to identify the dominant roles played by the convection effect followed by the vortex stretching effect on affecting the evolution of streamwise and spanwise vortical structures, respectively, forMc<0.6.