Influence of Reynolds number on net positive suction head of centrifugal pumps in relation to disc friction losses

Determination of Net Positive Suction Head (NPSH) value in the range of best efficiency is important in order to ensure correct operation of centrifugal pumps in networks. Empirical formulas of NPSH determination include the specific speed, the head and coefficients taking into account the pump impeller type. Using recent formulas on disc frictions and correction factors including the viscosity, a new formula of NPSH correction factor is suggested in this study, taking into account the specific speed, the disc friction losses and the flow velocities at impeller inlet. NPSH evaluated for water by Stepanoff formula is divided in kinetic and disc friction heads. The formulation taking into account disc frictions including viscosity, diameters and velocities enables to determine the NPSH correction factor during the pumping of viscous fluids. Simulations on different centrifugal pump models of specific speeds between 20 and 45 min^-1 are conducted. Obtained results show that the NPSH correction factor depends on the specific speed. It is near the unity when Reynolds number is greater than 10⁵. It increases appreciably with Reynolds number values less than 10⁵. It reaches 2.5 for a Reynolds number of 10⁴ and a specific speed of 25 min^-1, and 5 for a Reynolds number of 10³ and a specific speed of 45 min^-1. The established formula is related to those of Stepanoff for water. To validate this formula, a centrifugal pump with a specific speed of 20 min^-1 is tested for water and a viscous fluid with a viscosity of 75 cSt. NPSH correction factor is also compared with those obtained by other authors. This comparison enables the validation of this new formula for the examined range of centrifugal pump models. This new formula facilitates manufacturers and users the NPSH prediction of standard centrifugal pumps whatever the viscosity of the fluid.     

Multi-objective optimization of a low specific speed centrifugal pump using an evolutionary algorithm

This paper describes the shape optimization of a low specific speed centrifugal pump at the design point. The target pump has already been manually modified on the basis of empirical knowledge. A genetic algorithm (NSGA-II) with certain enhancements is adopted to improve its performance further with respect to two goals. In order to limit the number of design variables without losing geometric information, the impeller is parametrized using the Bézier curve and a B-spline. Numerical simulation based on a Reynolds averaged Navier–Stokes (RANS) turbulent model is done in parallel to evaluate the flow field. A back-propagating neural network is constructed as a surrogate for performance prediction to save computing time, while initial samples are selected according to an orthogonal array. Then global Pareto-optimal solutions are obtained and analysed. The results manifest that unexpected flow structures, such as the secondary flow on the meridian plane, have diminished or vanished in the optimized pump.     

叶轮隔舌间隙对离心泵性能和流动噪声影响的试验研究

搭建了离心泵流动诱导噪声测试台,采用四端网络法声学测试模型,试验研究了离心泵性能和流动噪声随流量的变化规律,分析了空化发生时的流动噪声特性。通过研究不同叶轮切割量对模型泵外特性、流动噪声声压级和空化性能的影响,提出叶轮和隔舌之间的最佳间隙值。研究结果表明：在高效区运行时,模型泵进出口流动诱导噪声均随流量先减小,至效率最高工况点达到最小,然后上升;各流量下,随着空化余量的减小,模型泵进口噪声总声压级先缓慢增加,再迅速上升,达到极值后缓慢下降;随着叶轮切割量的增加,模型泵扬程跟叶轮直径的平方成正比,最高效率点向小流量工况偏移,临界空化余量变小;综和性能和流动噪声考虑,模型泵叶轮和隔舌的最佳间隙率为15%;在间隙值小于最佳值时,切割叶轮能显著降低噪声并提高模型泵的临界空化余量,并且对模型泵出口流动噪声的影响比进口明显。     

桥梁断面表面压力分布及Strouhal数的雷诺数效应

为了研究桥梁断面Strouhal数的雷诺数效应,通过调节试验风速来改变雷诺数,并在模型表面布置测压点,进行风洞试验.测量了不同雷诺数下各测点的风压时程,分析了压力梯度及压力系数功率谱及涡脱频率的随雷诺数变化规律,研究了Strouhal数的雷诺数效应.研究发现桥梁断面的压力梯度及压力系数功率谱具有明显的雷诺数效应,高雷诺数下,通过风压频谱寻找涡脱频率的方法不可行.研究表明,桥梁断面的Strouhal数具有较为明显的雷诺数效应.     

Pedestrian fatality risk as a function of car impact speed

Knowledge of the amount of violence tolerated by the human body is essential when developing and implementing pedestrian safety strategies. When estimating the potential benefits of new countermeasures, the pedestrian fatality risk as a function of impact speed is of particular importance. Although this function has been analysed previously, we state that a proper understanding does not exist. Based on the largest in-depth, pedestrian accident study undertaken to date, we derive an improved risk function for adult pedestrians hit by the front of passenger cars. Our results show far lower fatality risks than generally reported in the traffic safety literature. This discrepancy is primarily explained by sample bias towards severe injury accidents in earlier studies. Nevertheless, a strong dependence on impact speed is found, with the fatality risk at 50 km/h being more than twice as high as the risk at 40 km/h and more than five times higher than the risk at 30 km/h. Our findings should have important implications for the development of pedestrian accident countermeasures worldwide. In particular, the scope of future pedestrian safety policies and research should be broadened to include accidents with impact speeds exceeding 50 km/h.     

On the number of overload-induced delay cycles as a function of thickness

In this paper, a brief review of the experimental facts associated with the crack growth delay period following a tensile overload is given. In the retardation process, crack closure is considered to play a dominant role. A scenario for the two crack opening processes which develop as a fatigue crack penetrates the overload plastic zone is presented. This scenario deals with the effect of specimen thickness on the crack opening processes. Consideration is given to the delay distance and to the minimum crack growth rate. A method of analysis for the determination of the number of delay cycles following an overload is then developed. Finally the method of analysis is used to compare with recent experimental results.     

新型比磁化系数测定仪的研制及其数值模拟研究

针对比磁化系数对矿物磁性研究的重要意义和现有比磁化系数测量装置的不足,根据古依法原理研制一种新型比磁化系数测定仪。基于ANSYS Maxwell对其磁系磁场特性数值模拟研究表明:其磁场特性沿磁系中截面呈对称式分布,磁气隙磁力线呈水平方向分布;磁场强度最高值为1.40 T,与试验测定值的相对误差仅为0.7%,数值模拟具有较高可靠性。应用该新型比磁化系数测定仪对五水硫酸铜和钒钛磁铁矿原矿比磁化系数进行系统测定,其试验结果表明:测定值可精确到1.0×10-9m3/kg,满足物质比磁化系数测定需求。     

Force and flow structures of a wing performing flapping motion at low Reynolds number

Summary Aerodynamic force and flow structures of a wing performing a simplified flapping motion that emulates the wing motion of small insects in normal hovering flight are studied, using the method of numerically solving the Navier-Stokes equations. For a typical case (wing rotation-axis is at 0.25 chord position and wing rotation is symmetrical with respect to stroke reversal), large peaks inC _L andC _D are produced near the end of a stroke by the wing-rotation, but the wing-rotation also generates a vortical structure which induces strong downwash velocity, reducing the lift production in the early part of the following stroke; averaging over one flapping cycle,C _L is 28% larger than the steady-state value and is about that needed to support the weight of a small insect. The timing of the wing-rotation at stroke reversal can change the size of the peaks inC _L andC _D and their averages; e.g. in the case of shifting the wing-rotation forward in time by 7.5% of a stroke period, the averageC _L becomes 63% larger than the steady-state value, which is larger than that needed for weight supporting (and might provide extra force for control or maneuvers). When the rotation-axis is moved rearward to the middlechord position, the lift and drag peaks due to the wing-rotation become smaller, and in this case, the wing-rotation generates a vortical structure that tends to prevent the relative motion between positive and negative vorticities, also reducing the lift production in the early part of the following stroke, resulting in a smaller averageC _L .     

Literature review of pedestrian fatality risk as a function of car impact speed

The aim of this review was to evaluate all studies of pedestrian fatality risk as a function of car impact speed. Relevant papers were primarily investigated with respect to data sampling procedures and methods for statistical analysis. It was uniformly reported that fatality risk increased monotonically with car impact speed. However, the absolute risk estimates varied considerably. Without exceptions, papers written before 2000 were based on direct analyses of data that had a large bias towards severe and fatal injuries. The consequence was to overestimate the fatality risks. We also found more recent research based on less biased data or adjusted for bias. While still showing a steep increase of risk with impact speed, these later papers provided substantially lower risk estimates than had been previously reported.     

Attenuation coefficient and propagation speed estimates of intercostal tissue as a function of pig age

Attention coefficient and propagation speed of intercostal tissues were estimated from chest walls removed postmortem (pm) from 15 5.3 /spl plusmn/ 2.3-day-old, 19 31 /spl plusmn/ 6-day-old, and 15 61 /spl plusmn/ 3-day-old crossbred pigs. These ultrasonic propagation properties were determined from measurements through the intercostal tissues, from the surface of the skin to the parietal pleura. The chest walls were placed in a 0.9% sodium chloride solution, sealed in freezer bags, and stored at -15/spl deg/C prior to measurements. When evaluated, chest-wall storage time ranged between 1 and 477 days pm. All chest walls were allowed to equilibrate to 22/spl deg/C in a water bath prior to evaluation. There was an age dependency of the intercostal tissue propagation speed, with the speed increasing with increasing age. The attenuation coefficient of intercostal tissue was shown to be independent of the age of the pig at the discrete frequencies of 3.1 and 6.2 MHz. For pig intercostal tissues, the estimated attenuation coefficient over the 3.1-9.2 MHz frequency range was A = 1.94f/sup 0.90/ where A is in decibels per centimeter (dB/cm) and f is the ultrasonic frequency in megahertz. In order to determine if there was an effect of storage time pm on estimates of attenuation coefficient, a second experiment was conducted. Five of the youngest pig chest walls measured on day 1 pm in the first experiment were stored at 4/spl deg/C prior to the first evaluation then stored at -15/spl deg/C before being measured again at 108 days pm. There was no difference in the estimated intercostal tissue attenuation coefficient as a function of storage time pm.     

Improvements in physiological performance as a function of practice∗

As a result of the limitations inherent in the present technique of estimating work standards, the concept of utilizing psychological and physiological indices instead of time alone in establishing predetermined work standards is evaluated in an exploratory study.

Experiments were conducted on 15 male subjects on a transport and position type repetitive task. The operation was performed single-handedly and with simultaneous motions for over 1000 cycles per subject per condition. Results indicate that :

(1)Physiological performance improved with practice of the designated manual task.

(2)The energy expenditure per cycle for the preferred and non-preferred hand motions is not significantly different from that required in simultaneous motions.

(3)The indication that energy expenditure of simultaneous hand motions is not significantly different from that of single-hand motions supports the classical claims, put forth by neurophysiologists, that the sum of the individual time elements does not equal the time of the whole motion patterns. This encourages the possibility of being able to establish predetermined work standards using physiological indices instead of time.     

Attenuation coefficient and propagation speed estimates of rat and pig intercostal tissue as a function of temperature

Attenuation coefficient and propagation speed of intercostal tissues were estimated as functions of temperature (22, 30, and 37 degrees C) from fresh chest walls from eight 10- to 11-week-old female Sprague-Dawley (SD) rats, eight 21- to 24-week-old female Long-Evans (LE) rats, and ten 6- to 10-week-old mixed sex Yorkshire (York) pigs. The primary purpose of the study was to estimate the temperature dependence of the intercostal tissue's attenuation coefficient so that accurate estimates of the in situ (at the pleural surface) acoustic pressure levels could be made for our ultrasound-induced lung hemorrhage studies. The attenuation coefficient of intercostal tissue for both species was independent of the temperature at the discrete frequencies of 3.1 MHz (-0.0076, 0.0065, and 0.016 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively) and 6.2 MHz (-0.015, 0.014, and 0.014 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively). However, the temperature-dependent regressions yielded a significant temperature dependency of the intercostal tissue attenuation coefficients in SD and LE rats (over the 3.1 to 9.6 MHz frequency range); there was no temperature dependency in York pigs (over the 3.1 to 8.6 MHz frequency range). There was no significant temperature dependency of the intercostal tissue propagation speed in SD rats; there was a temperature dependency in LE rats and York pigs (-0.59, -1.6, and -2.9 m/s/degrees C for SD rats, LE rats, and York pigs, respectively). Even though the attenuation coefficient's temperature dependency was significant from the linear regression functions, the differences were not very great (-0.040 to -0.13, 0.011 to 0.18, and 0.055 to 0.10 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively, over the data frequency range). These findings suggest that it is not necessary to determine the attenuation coefficient of intercostal tissue at body temperature (37 degrees C), but rather it is sufficient to determine the attenuation coefficient at room temperature (22 degrees C), a much easier experimental procedure.     

RF number as a new index for assessing combustion hazard of flammable gases

A new index called RF number has been proposed for assessing the combustion hazard of all sorts of flammable gases and their mixtures. RF number represents the total expectancy of combustion hazard in terms of flammability limits and heat of combustion for each known and unknown compounds. The advantage of RF number over others such as R-index and F-number for classification of combustion hazard has been highlighted.     

Numerical investigation of the effect of the Prandtl number on the temperature recovery and the Reynolds analogy factors in the boundary layer on a plate

We performed a numerical study of the boundary layer on a wall in a supersonic gas flow using a differential turbulence model. The dependences of the recovery factor and the Reynolds analogy factor on the main flow parameters, such as the Reynolds, Mach, and Prandtl numbers, are obtained for a series of the temperature factor values. The results of calculations are approximated by the dependences of the recovery factor and the Reynolds analogy factor on the Prandtl number.     

Development of a miniature cyclone separator operating at low Reynolds numbers as a pre-separator for portable black carbon monitors

In this study, a cyclone separator that can be used as a sampling inlet for portable black carbon (BC) monitors operating at a flow rate of less than 200 mL/min was developed. A prototype was fabricated to evaluate its performance by experiments, and the cut-off size of the cyclone separator was predicted through numerical analysis by applying various turbulence models. The RNG k–ε model was found to be suitable for the analysis of the cyclone separator operating at Reynolds numbers of less than 1000. Cyclone separators were designed through simulation and fabricated for each operating flow rate (50, 100, 150, and 200 mL/min) of a BC monitor, and their performances were experimentally verified. Meanwhile, when the non-dimensional analysis method of the previous study conducted at Reynolds numbers of 1000 or higher was used, the cyclone separator operating at Reynolds numbers of less than 1000 also exhibited a similar linear tendency.     

Variation in number of active vaporization sites as a function of height of fluid layer

A study was made of the variation in the number of active vaporization sites as a function of the height of a layer of boiling fluid above a heat-releasing surface. An explanation of the results is given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 32, No. 1, pp. 13–17, January, 1977.     

新型少齿差行星齿轮装置振动特性分析与实验研究

对某少齿差行星齿轮装置进行结构改进设计,运用动力接触有限元法求得包括啮合刚度激励、误差激励、啮合冲击激励的轮齿啮合内部动态激励。通过建立有限元模态动力分析模型,计算齿轮装置固有频率及振型,综合考虑内外部激励情况下,研究该齿轮装置的结构动态响应及噪声。利用振动测试设备对该齿轮装置进行振动噪声测试。结果表明：分析预测值与实验结果相符。     

Premix PCC materials: mechanical properties as a function of technological factors

Mechanical properties, microstructure, dimensional stability and phase interactions of premix polymer-cement composite materials (PCC) were investigated as a function of polymer content, water/cement (w/c) ratio and preparation and curing procedures of the samples. The investigated PCC materials were made of normal cement mortar with the addition of variable amounts of water-soluble or dispersible epoxy resins and polyelectrolytes of different functionality. Mechanical strength (compressive and flexural), dimensional changes and microstructure after various curing times were then evaluated in comparison with the same parameters for plain mortar samples prepared by standard procedures. The results so far obtained show that a strong reduction of the w/c ratio in PCC materials can be obtained through polymer addition. Hence, a proper design of the mixing and curing procedures, combined with an optimum reduction of the w/c ratio, causes a remarkable mechanical strength increase in the PCC materials investigated.

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Résumé On a étudié les propriétés mécaniques, la microstructure, la stabilité dimensionnelle et les interactions de phase de matériaux prémix des composites ciment/polymères (PCC) en fonction de la teneur en polymères, du rapport eau/ciment, de la préparation et des méthodes de conservation des échantillons. Les matériaux PCC examinés étaient confectionnés de mortiers de ciment normal additionné de taux variables de solution aqueuse ou de résines époxy dispersibles et de polyélectrolytes anioniques ou cationiques. On a ensuite évalué la résistance mécanique (en compression et en flexion), les modifications dimensionnelles et structurelles à différents intervalles de conservation, en les comparant avec les mêmes paramètres pour des échantillons de mortier préparés selon les normes. Jusqu'ici les résultats montrent qu'on peut obtenir une forte réduction du rapport eau/ciment dans les matériaux PCC par addition de polymère. D'ouù il ressort que des méthodes appropriées de dosage et de conservation ainsi qu'une réduction optimale du rapport eau/ciment déterminent une augmentation remarquable de la résistance mécanique dans les matériaux PCC examinés.

     

IRFPA的响应特性及新的非均匀校正算法

分析了红外焦平面阵列探测元的响应模型以及响应特性与入射辐射、积分时间的关系,指出了实际工程中常用的两点黑体辐射定标校正算法的本质在于利用高低温时不同的响应数据计算增益系数与偏置系数.与此相类似,通过调整积分时间也能得到不同的响应数据,因此提出基于积分时间调整的校正算法,利用不同积分时间下的响应数据计算校正时所需的增益系数与偏置系数.实验结果表明,选择合适的积分时间个数后,新方法校正效果优良,可完全用于工程应用,并可克服黑体定标校正算法的不足.