Spectral algorithm for the analysis of flows in grooved channels

A grid‐less, spectrally accurate algorithm for the analysis of flows in grooved channels is presented. The algorithm is based on the immersed boundary conditions concept, where the boundary conditions are submerged inside the computational domain and are treated as internal constraints. When grooves' ridges are orthogonal to the flow direction (transverse grooves), the flow remains two‐dimensional. As the grooves rotate away from this direction, the flow becomes three‐dimensional. An auxiliary coordinate system is defined in such a way that one of its axes is aligned with the grooves. It is shown that the governing equations expressed in this system decouple into a two‐dimensional flow across the grooves and a flow in the direction along the grooves, resulting in improved solution efficiencies. Fourier series are used for discretization in the direction transverse to the grooves and Chebyshev expansions for the direction across the channel. Special solvers that take advantage of the matrix structure have been implemented providing a significant acceleration of computation and reduction of memory requirements. Various tests have been conducted in order to illustrate the performance of the algorithm, to show its spectral accuracy and to characterize the effects of various numerical and physical parameters. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat transfer enhancement in grooved channels due to flow bifurcations

The flow bifurcation scenario and heat transfer characteristics in grooved channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element methods for increasing Reynolds numbers in the laminar and transitional regimes. The Eulerian flow characteristics show a transition scenario of two Hopf bifurcations when the flow evolves from a laminar to a time-dependent periodic and then to a quasi-periodic flow. The first Hopf bifurcation occurs to a critical Reynolds number Rec₁ that is significantly lower than the critical Reynolds number for a plane-channel flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω₁ and m· ω₁+n·ω₂, respectively, with m and n integers. Friction factor and pumping power evaluations demonstrate that these parameters are much higher than the plane channel values. The time-average mean Nusselt number remains mostly constant in the laminar regime and continuously increases in the transitional regime. The rate of increase of this Nusselt number is higher for a quasi-periodic than for a periodic flow regime. This higher rate originates because better flow mixing develops in quasi-periodic flow regimes. The flow bifurcation scenario occurring in grooved channels is similar to the Ruelle-Takens-Newhouse transition scenario of Eulerian chaos, observed in symmetric and asymmetric wavy channels.     

Numerical study of self-sustained oscillatory flows and heat transfer in channels with a tandem of transverse vortex generators

A numerical investigation was conducted into channel flows with a tandem of transverse vortex generators in the form of rectangular cylinders. The oscillatory behavior of the flow is studied. Data for heat transfer and flow losses are presented for 100≤Re≤400 and cylinder separation distances 1≤S/H≤4. The results are obtained by numerical solution of the full Navier-Stokes equations and the energy equation. Self-sustained flow oscillations are found for Re>100. Alternate and dynamic shedding of large vortex structures from the cylinders is observed by visualization of the numerically determined flow field. A heat transfer enhancement up to a factor 1.78 compared to plane channel flow is observed. Received on 16 July 1997     

Study of the damping characteristics of sectional flight vehicles of different length

The dependence of the damping characteristics of axisymmetric tri-sectional flight vehicles executing plane oscillations about the zero angle of attack on their geometric parameters is investigated on the supersonic range of uniform oncoming air flow Mach numbers. Systematic data are obtained using the least laborious approach, namely, by calculating the steady-state inviscid flow past equivalent bodies determined from the principle of the local similarity of force interaction. Ranges of the vehicle design parameters on which the dependence of the damping moment coefficient in pitch on the length of the conical or cylindrical-conical stabilizer of the vehicle is nonmonotonic are found. Typical viscosity effects on the characteristics under study are estimated using a method based on the assumptions of the linear theory for finite-thickness bodies. It is established that the damping coefficient increases monotonically with decreasing Reynolds number; moreover, even transition to oscillation antidumping regimes is possible when the sign of the damping coefficient changes.Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 6, 2004, pp. 153–162. Original Russian Text Copyright © 2004 by Antonets and Shmanenkov.     

The reflection of a plane shock wave from a body with groove

When a plane shock wave impinges on bodies with grooves and when a supersonic stream of gas flows past such bodies a complicated flow pattern develops. In a number of cases oscillations of the bow wave [1–3] and an anomalous heating of the gas in the groove [4–6] have been observed. Unsteady reflection of shock waves from bodies with grooves and the processes occurring inside the grooves have been investigated comparatively little.Translated from Izvestiya Akademii Nauk SSSR, Hekhanika Zhidkosti 1 Gaza, No. 5, pp. 180–186, September–October, 1935.The authors wish to thank V. I. Ivanov for carrying out the calculations.     

Near wall characteristics of flow over grooved circular cylinder

An experimental investigation was made of the flow over a grooved circular cylinder of different aspect ratios. Based on the drag coefficient, Strouhal number and mean shear stress, three flow regimes of subcritical, critical and supercritical were found, all of which are below the subcritical Reynolds number of a smooth cylinder. Boundary layer characteristics within the different flow regimes were measured. The shift of the boundary layer away from the grooves and an estimated change in the virtual origin are used to establish the similarity of the flow characteristics of grooved and smooth cylinders.Now at Development and Planning Division, Hong Kong Electric Co. Ltd., Hong Kong     

Influence of periodic excitation on self-sustained vibrations of one disk rotors in arbitrary length journals bearings

Interaction of forced and self-sustained vibrations of one disk rotor is described by nonlinear finite-degree-of-freedom dynamical system. The shaft of the rotor is supported by two journal bearings. The combination of the shooting technique and the continuation algorithm is used to study the rotor periodic vibrations. The Floquet multipliers are calculated to analyze periodic vibrations stability. The results of periodic motions analysis are shown on the frequency response. The quasi-periodic motions are investigated. These nonlinear vibrations coexist with the periodic forced vibrations.     

Shock wave attenuation in partially confined channels

An approximate analytical solution is presented for the attenuation of planar shock waves in channels with perforated walls. The problem is considered as quasi-one-dimensional. Good agreement is found between the theoretical results and available experimental data regarding the rate of shock wave attenuation within the range of initial shock Mach numbers between 1.1 and 4 and perforation ratios between 4.5 × 10^–3 and 0.53. A correlation for the discharge coefficient of a single hole perforation is presented which gives quantitatively good agreement with particular experimental observations.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

爆轰波在突扩通道中传播的数值模拟研究

建立了描述甲烷 空气混合物爆轰波传播的单步化学反应爆轰模型 ,通过数值模拟研究了在二维突扩通道中爆轰波的强度变化和各种波行为。结果表明 :爆轰波在进入突扩通道初始阶段的衍射使爆轰波局部向爆燃转变 ;爆炸波在壁面发生马赫反射形成的高温高压区域将直接诱导自持爆轰波的重新形成。     

甲烷-空气爆轰波传播的数值模拟

研究了甲烷-空气混合物爆轰波在二维突扩通道中传播的各种复杂行为。结果表明：爆轰波在进入突扩通道的初始阶段有局部向爆燃转变的现象；爆炸波在壁面发生马赫反射形成的高温高压区域将直接驱动自持爆轰波的重新形成。     

不同坡口形状铍环激光钎焊温度场和应力场分析

本文对不同坡口形状镀环焊接过程进行了数值模拟,分析了坡口形状和焊接速度对铍环钎焊温度场和应力场的影响,结果表明:开坡口后铍环焊接的熔深和熔宽增大,而热影响区的焊接残余应力相对降低,特别是对于降低铍和AlSi12熔合面处的应力集中具有明显的效果;同时,坡口角度的增加主要对铍环热影响区的应力分布也有一定的影响.因此,铍环接头处开坡口不仅能够降低焊缝区的残余应力,还能减少铍材熔化引起的开裂.     

Comparative investigation of operational performance characteristics of axially grooved and arterial heat pipes

The thermal performance characteristics of an axially grooved heat pipe (AGHP) and an arterial heat pipe (ArHP) sharing a similar external configuration are investigated. A mathematical model is developed to predict the capillary heat transfer limit for both heat pipes. The meniscus attachment point, contact angle and liquid–vapor interfacial shear stress are taken into account in this model. In particular, for predicting the ArHP dry-out, a novel model is proposed by introducing two different failure mechanisms. The results of the mathematical model are experimentally verified.     

Flow characteristics of rectangular open channels with compound vegetation roughness

An idealized parallel flow caused by a lateral bed roughness difference due to the partial vegetation across a channel is investigated. Similar to the flow in a compound channel, there are mixing layers adjacent to the interface between the vegetation and the non-vegetation lanes, and a lateral momentum exchange occurs between the slow-moving water in the former lane and the fast-moving water in the latter lane. Under a uniform flow condition, the three-dimensional (3D) instantaneous velocities of two cases with different discharges and water depths are measured with a 16MHz acoustic Doppler velocimeter (micro ADV). The longitudinal variation of the streamwise velocity and the vertical variation of the Reynolds stress are analyzed. A quadrant analysis is carried out to investigate the outward and inward interaction, ejection, and sweep phenomenon caused by the vegetation variation across the channel. The results show that the flow characteristics in the vegetation lane are similar to those in an open channel fully covered with submerged vegetation, and the flow characteristics in the smooth non-vegetation lane are similar to those in a free open channel. For the cases studied here, the width of the mixing region is about 10% of the channel width, and the mixing region is mainly on the non-vegetation half.     

Stress analysis of strain wave gear drives with four different geometries of wave generator

Meccanica - The comparison of strain wave gear drives with four different geometries of the wave generator is presented. The commonly used elliptical geometry of the wave generator is compared with...     

Surface wave scattering by a material filled rectangular impedance groove in a reactive plane

The problem of TE-polarized surface wave scattering from a rectangular impedance groove located on an infinite reactive plane which is filled with dielectric material is considered for a rather general case where the impedances of the horizontal and vertical sides of the groove have different values. The multiple interactions up to the second order between the edges of the groove are obtained to yield diffracted field. The diffraction problem is first reduced into a modified Wiener-Hopf equation and then solved approximately. The solution contains branch-cut integrals and two infinite sets of constants satisfying two infinite systems of linear algebraic equations. The approximate analytical evaluations of the corresponding integrals as well as the numerical solutions of the linear algebraic equation systems are obtained for various values of the parameters such as the surface reactance of the guiding plane, the vertical and horizontal wall impedances of the groove, the permittivity of the material loading, the width and the height of the groove which permit one to study the effect of these parameters on the diffraction phenomenon.     

Computation of the base pressure in ejector nozzles of different length with zero coefficient of ejection

A simple method is proposed for the analysis of the base pressure in ejector nozzles with arbitrary cowling length by using an empirical criterion of attachment in a broad range of variation of the Mach number of the main nozzle.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 164–166, November–December, 1976.     

Analysis of self-sustained oscillation sources in segmented flows

The analysis of self-sustained oscillations in segmented flow generated through porous walls has been carried out over a wide range of velocity levels; in fact, we studied a cold gas flow induced by injection through different wall injecting blocks. We have attempted in this study to analyse the potential unstable development occurring in solid propellant rocket motors. We lay emphasis upon the phenomenon of wall vortex shedding insofar as it conduces to acoustic mode resonance in the whole chamber, within whose confines impingement of such structures generates a source of noise. It is on account of segmented flow that the thin shear layer develops and that the aforementioned vortex shedding comes to induce aero-acoustic coupling. Subsequent experimental results highlight a link in such flows between these two noise sources - they also allow one to observe a pronounced form of selectivity in the energy transfer, i.e. in longitudinal acoustic mode amplification, which has an attested effect upon all of the pressure oscillations in the chamber.