Influence of Discrete Longitudinal Ribs on Harmonic Waves in Cylindrical Shells

The influence of discrete longitudinal ribs on harmonic waves in cylindrical shells is studied by numerical examples. It is shown that the presence of discrete ribs is manifested as an increased number and changed shape of the dispersion curves     

Propagation of harmonic waves along open circular cylindrical shells reinforced with a quasiregular set of discrete longitudinal ribs

We obtain the exact solution describing the propagation of harmonic waves along an open cylindrical shell reinforced with a quasiregular set of discrete longitudinal ribs. Numerical examples are used to examine the effect of discrete ribs on the number and shape of dispersion curves and the effect of the stiffness and inertial characteristics of the ribs on the excitation frequency for given wave parameters     

Propagation of harmonic waves in longitudinally reinforced cylindrical shells with low shear stiffness

The exact solutions of the equations of motion derived under refined theories of shells and ribs based on the Timoshenko model are used to plot dispersion curves for harmonic waves propagating along a cylindrical shell reinforced with discrete longitudinal ribs __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 5, pp. 37–41, May 2006.     

Flow and Heat Transfer in a High-Aspect-Ratio Rib-Roughed Cooling Channel with Longitudinal Intersecting Ribs

This paper describes a detailed numerical investigation of a stationary high-aspect-ratio rib-roughed rectangular cooling channel with longitudinal intersecting ribs near the gas turbine blade trailing edge region. In order to overcome the heat transfer performance degeneration in the highaspect- ratio channel, longitudinal intersecting ribs are arranged on the channel bottom surface. The effect of the number of longitudinal intersecting ribs on the flow and heat transfer is systematically studied in the Reynolds number range Re = 10 000–30 000. The results show that a heat transfer augmentation region exists just downstream the junction between the longitudinal rib and the angled rib due to additional secondary flows. With more longitudinal intersecting ribs, the heat transfer distributions on the channel surfaces are more uniform. Though the pressure loss is also enlarged with an increase in the number of longitudinal intersecting ribs, the overall thermal efficiency increases in the entire range of Reynolds numbers investigated. The configuration with two sets of longitudinal intersecting ribs shows the best overall thermal efficiency.     

Effect of the stiffness of ribs on the vibrations and stability of open cylindrical shells

The effect of the stiffness of ribs on the minimum natural frequencies and critical stresses of axially compressed open cylindrical shells reinforced with a quasiregular set of longitudinal ribs is analyzed by way of numerical examples. It is shown that the earlier discovered phenomenon of abrupt decrease in the minimum frequencies is independent of rib stiffness for certain modes and a small number of ribs     

大开口复合材料加筋壁板稳定性分析

运用大型有限元软件Patran/Nastran分析了大开口复合材料加筋壁板的稳定性,并对不同加筋方式下壁板屈曲特征值和屈曲模态图进行了比较。结果表明:补强提高了大开口复合材料壁板的稳定性,但往往无法达到很好的效果,需要通过加筋改善其稳定性;加筋复合材料壁板稳定性较原有模型有较大提高;加筋大开口复合材料壁板屈曲特征值随筋条距开口中心距离的增加而减小,其屈曲分界线均位于筋条布置处;纵筋大开口复合材料加筋壁板一阶屈曲特征值为2.13,而横筋只达到1.08;纵筋布置对复合材料壁板稳定性影响明显高于横筋布置,可在实际工程应用中适当增加纵筋的布置。     

用经典Hopkinson杆测试弹性模量的初步探讨

采用一维弹性应力波理论,实现了经典Hopkinson杆试验的计算机模拟,研究了试验"测得"(重构)的应力-应变曲线的精确度,并结合具体试验进行了分析.由于经典Hopkinson杆试验的基本计算公式中引入了"应力均匀性"假设,因此在有限应变范围内,重构的应力-应变曲线总是和输入(真实)曲线差异较大.结果表明,采用经典Hopkinson杆测量的弹性模量是不可靠的,因此实践中应慎用.     

Influence of discretely arranged longitudinal ribs on the stability and vibrations of open circular cylindrical shells

Open cylindrical shells reinforced with quasiregular sets of longitudinal ribs and hinged at all edges are considered. The effect of the discrete arrangement and number of ribs on the critical stresses that cause instability under longitudinal compression and on the minimum natural frequencies of vibrations is examined. Numerical results are analyzed     

Developing and fully developed turbulent flow in ribbed channels

Wall-mounted roughness features, such as ribs, are often placed along the walls of a channel to increase the convective surface area and to augment heat transfer and mixing by increasing turbulence. Depending on the relative roughness size and orientation, the ribs also have varying degrees of increased pressure losses. Designs that use ribs to promote heat transfer encompass the full range of having only a few streamwise ribs, which do not allow fully developed flow conditions, to multiple streamwise ribs, which do allow the flow to become fully developed. The majority of previous studies have focused on perturbing the geometry of the rib with little attention to the spatially and temporally varying flow characteristics and their dependence on the Reynolds number. A staggered rib-roughened channel study was performed using time-resolved digital particle image velocimetry (TRDPIV). Both the developing (entry region) and a fully developed region were interrogated for three Reynolds numbers of 2,500, 10,000, and 20,000. The results indicate that the flow was more sensitive to Reynolds number at the inlet than within the fully developed region. Despite having a similar mean-averaged flowfield structure over the full Reynolds number range investigated, the population and distribution of coherent structures and turbulent dissipation within the fully developed region were also found to be Reynolds number dependent. Exploring the time-accurate flow characteristics revealed that in addition to vortices shed from the rib shear layer, the region of the rib wake was governed by a periodic process of bursting of the wake vortices resulting in the intermittent ejection of the inter-rib recirculation region into the core flow. This periodic process was the driving mechanism resulting in mixing and heat transfer augmentation. A quadrant-splitting burst analysis was also performed to determine the characteristic frequency and duration of inter-rib bursting as well as the wake shedding frequency, both of which were determined to be Reynolds number dependent.     

Thermal and friction characteristics of turbulent flow through rectangular and square ducts with transverse ribs and wire-coil inserts

The heat transfer and the pressure drop characteristics of turbulent flow of air through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and with wire-coil inserts have been studied experimentally. Circular duct has also been used. The transverse ribs in combination with wire-coil inserts have been found to perform better than either ribs or wire-coil inserts acting alone. The flow friction and thermal characteristics are governed by duct aspect ratio, coil helix angle and wire diameter of the coil, rib height and rib spacing, Reynolds number and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. It has been found that on the basis of constant pumping power, up to 35% heat duty increase occurs for the combined ribs and wire-coil inserts case compared to the individual ribs and wire-coil inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 20% for the combined enhancement geometry than the individual enhancement geometries.     

Experimental investigation of local heat transfer in a square duct with various-shaped ribs

In the present study, experimental studies are carried out to investigate the heat transfer and friction characteristics in a square duct roughened by various-shaped ribs on one wall. The ribs are oriented transversely to the main stream in a periodic arrangement. Liquid crystal thermography is employed to measure the local and average heat transfer coefficient on the ribbed surface. The rib height-to-duct hydraulic diameter ratio is fixed at 0.1; the rib pitch-to-height ratio varies from 8 to 15 and the test Reynolds number spans from 8,000 to 20,000. The results show that the trapezoidal-shaped ribs with decreasing height in the flow direction (case C) provide the highest heat transfer enhancement factor and are likely to be used to suppress the local hot spot which usually occurs in the region just behind the ribs.     

Influence of rheology and surface properties in the adhesion of uncross-linked pressure sensitive adhesives

We consider the adhesion of a pressure sensitive adhesive on different substrates (Pyrex, stainless steel, Plexiglas). First, we characterize the rheological properties of the adhesive material and compare it with the predictions of Lodge's model. Then we investigate the adhesive properties using a special machine which enables us to peel at 90° with a fixed peeling front and we construct peeling master curves on different substrates. The mechanisms of peeling are analyzed by looking at the peeling front using a video camera. We realize that the flow within the filaments and ribs observed is mainly of elongational type. Also, by looking at the shape of the backing, we find out that most of the energy is spent within the ribs or filaments.To understand the effect of rheology on adhesion, we propose a simple model to predict peeling curves, by assuming that the flow is mainly of elongational type. This explains the high energy regions. At low velocities, surface energies become important and their effect is also analyzed.To conclude, we propose different dimensionless equations which explain the importance of the relevant parameters, via dimensionless numbers. Thus the peeling energy is investigated, as well as the condition which predicts the transition from cohesive to adhesive peeling.     

求解水下纵向加肋无限长非圆柱壳声辐射问题的一种新的半解析方法

基于齐次扩容精细积分法和复数矢径虚拟边界谱方法，利用Fourier积分变换和稳相法，提出了一种具有较高效率和精度的新的求解水下纵向加肋无限长非圆柱壳声辐射问题的半解析方法．考虑了非圆柱壳和肋骨之间同时存在多种相互作用力和力偶矩，较已往很多学者仅计及法向相互作用力更加符合实际．不仅比较了该文方法和精确解计算纵向加肋圆柱壳在集中点力激励下的声辐射计算结果，同时还研究了肋骨数量、大小以及椭圆柱壳横截面椭圆度对声辐射特性的影响．数值计算结果表明该文方法较已有的混合FE-BE法更为有效．     

Heat transfer and flow temperature measurements in a rotating triangular channel with various rib arrangements

In the present study, the regionally-averaged heat transfer coefficients and flow temperature distributions were measured in an equilateral triangular channel with three different rib arrangements (α = 45, 90 and 135°). To measure regionally-averaged heat transfer coefficients in the channel, two rows of copper blocks and a single heater were installed on two ribbed walls. The fluid temperature distributions were obtained using a thermocouple-array. The rotation number ranged from 0.0 to 0.1 with a fixed Reynolds number of 10,000. For the 90° ribs, the heat transfer coefficients on the pressure side surface were increased significantly with rotation, while the suction side surface had lower heat transfer coefficients than the stationary channel. For the angled ribs, rib-induced secondary flow dominated the heat transfer characteristics and high heat transfer rates were observed on the regions near the inner wall for the 45° angled ribs and near the leading edge for the 135° angled ribs.     

Influence of reinforcement and elastic foundation on the vibrations of shallow shells with rectangular planform

A method for determining the natural frequencies and modes of ribbed shallow shells with rectangular planform on an elastic foundation is developed. The method takes into account the discrete arrangement of the ribs. A shallow spherical shell with square planform is considered as example to analyze the effect of the number of ribs and the Winkler and Pasternak coefficients of subgrade reaction on the natural frequencies and modes. It is recommended to take into account the discrete arrangement of ribs when they are few     

A NOVEL SEMI-ANALYTICAL METHOD FOR SOLVING ACOUSTIC RADIATION FROM LONGITUDINALLY STIFFENED INFINITE NON-CIRCULAR CYLINDRICAL SHELLS IN WATER

Based on the extended homogeneous capacity high precision integration method and the spectrum method of virtual boundary with a complex radius vector, a novel semi-analytical method, which has satisfactory computation effectiveness and precision, is presented for solving the acoustic radiation from a submerged infinite non-circular cylindrical shell stiffened by longitudinal ribs by means of the Fourier integral transformation and stationary phase method. In this work,besides the normal interacting force, which is commonly adopted by some researchers, the other interacting forces and moments between the longitudinal ribs and the non-circular cylindrical shell are considered at the same time. The effects of the number and the size of the cross-section of longitudinal ribs on the characteristics of acoustic radiation are investigated. Numerical results show that the method proposed is more efficient than the existing mixed FE-BE method.     

Dispersion Curves for Harmonic Waves Propagating Along the Directrix of a Cylindrical Shell Reinforced with Rings

A method is proposed to study the influence of discrete ribs on the dispersion curves for harmonic waves propagating along the directrix of a simply supported cylindrical shell reinforced with rings. A numerical example is considered to demonstrate the necessity of accounting for this factor, the possibility of using a simplified formulation of the problem, and the necessity of accounting for the asymmetry of rib arrangement about the median surface of the shell     

Heat transfer and pressure drop measurements in rib-roughened rectangular ducts

In the present study, the thermal and hydraulic performance of three rib-roughened rectangular ducts is investigated. The aspect ratio of the ducts was 1 to 8, and the ribs were arranged staggered on the two wide walls. Three rib configurations were tested: parallel ribs and V-shaped ribs pointing upstream or downstream of the main flow direction. For all cases, the rib height-to-hydraulic diameter ratio was 0.06, with an attack angle of 60° and a pitch-to-height ratio of 10. The Reynolds number range was from 1000 to 6000. Liquid crystal thermography was employed in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on the ribbed surfaces. The secondary flows caused by the inclined ribs create a significant spanwise variation of the heat transfer coefficients on the rib-roughened wall with high heat transfer coefficient at one end of the rib and low value at the other. In the streamwise direction between two consecutive ribs, the temperature distribution shows a sawtooth fashion because of flow reattachment. Based on the local heat transfer coefficients, the average Nusselt numbers were estimated as weighted mean values. Isothermal pressure drop data were taken and presented as Fanning friction factors. The ducts are compared to each other by considering both heat transfer and friction factor performance.     

Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater

In the present work, an experimental investigation of convective heat transfer and pressure drop was carried out for the turning portion of a U-channel where the outer wall was equipped with ribs. The shape of the ribs was varied. The investigation aims to give guidelines for improving the thermo-hydraulic performance of a solar air heater at the turning portion of a U-channel. Both the U-channel and the ribs were made in acrylic material to allow optical access for measuring the surface temperature by using a high-resolution technique based on narrow band thermochromic liquid crystals (TLC R35C5 W) and a CCD camera placed to face the turning portion of the U-channel. The uncertainties were estimated to 5 and 7 % for the Nusselt number and friction factor, respectively. The pressure drop was approximately the same for all the considered shapes of the ribs while the dimpled rib case gave the highest heat transfer coefficient while the grooved rib presented the highest performance index.     

Multi-mode wave propagation in ribbed plates. Part II: Predictions and comparisons

This paper addresses dispersion curve numerical computations for specific ribbed plates. Precisely, wave propagation in the direction parallel to the ribs is the main objective. First, analytical calculations are performed for such ribbed plates after which the results are compared to a more general numerical procedure. This procedure reuses a reduced finite element model of the ribbed plate and extracts guided multi-mode propagation parameters. Comparisons of analytical and numerical estimations show very good agreement. Finally, the experimental results obtained in the companion paper are considered. Specifically, experimental and numerical dispersion curves are compared over a wide frequency range. Close concordance is obtained allowing the dispersion curves to be fully interpreted.