深海ROV铠缆绞车及主动升沉补偿装置设计研究

深海ROV是一种可以在深海进行观察和作业的水下机器人。其研究和开发，对海洋环境的调查，海洋资源的钻探以及军事应用方面都具有重要的意义。深海ROV收放在深海区域收放作业时，作业环境复杂，作业深度大，如果铠缆绞车采用单卷筒结构会存在驱动装置功率过大，卷筒的转动惯量过大，载体出水的瞬间铠缆外层压入内层等问题。另外，海面的海况条件比较恶劣，支持母船受风、浪、涌、流影响而产生较大幅度的升沉运动对有缆水下机器人正常作业和收放操作有较大的影响，ROV中继器的升沉运动甚至可以造成系缆的损害而使机器人本体丢失。为了提高ROV在恶劣海况下的安全性能，保护中继器与载体之间的系缆不受损坏，需要配备升沉补偿装置。所以深海ROV铠缆绞车及其升沉补偿装置的研究与设计，对实现深海ROV的收放及保障系统安全及各项关键技术的进一步研究具有重要的意义。 本文在863计划“基于母船升沉预测的深海装备主动升沉补偿测控单元的研究开发”资助下，结合7000m ROV收放操作和升沉补偿的要求，开展深海ROV铠缆绞车及其主动升沉补偿装置的设计研究。针对深海ROV收放作业时，ROV铠缆绞车需要具有ROV快速收放能力及升沉补偿能力，设计适合于深海ROV收放作业的铠缆绞车。本文重点研究适合于深海ROV铠缆绞车的双绞车结构形式；研究双绞车的受力分析，驱动装置的选型；设计深海ROV铠缆绞车的储存绞车和牵引绞车；设计用于深海ROV铠缆绞车的主动升沉补偿装置。本文的主要内容如下： （1）研究深海ROV铠缆绞车的结构形式。分析了如果深海ROV铠缆绞车采用单卷筒结构存在转动惯量过大，驱动装置功率过大，在出水的瞬间铠缆外层缆压入内层等问题，提出了双绞车结构。分析了双绞车的组成，布置及其作用原理，各部件的组成及作用原理。 （2）研究深海ROV铠缆绞车的受力及所需要的功率。利用微元分析，求解出深海ROV牵引绞车每个绞盘提供的摩擦力，从而求出各绞车驱动装置需提供的最大驱动力矩和功率，作为驱动装置的选型依据。 （3）设计深海ROV铠缆绞车。通过类比以前的绞车系统，理论计算，初步选定储存绞车及牵引绞车各个部件参数。采用SolidWorks进行三维建模后，采用ANSYS进行有限元分析，通过计算机仿真技术克服理论计算的缺陷，对各个部件进行强度校核及结构优化。 （4）设计主动升沉补偿装置。根据升沉补偿装置的技术指标，通过理论计算确定升沉补偿装置各个部分结构形式和参数，并进行了三维建模、通过有限元分析对各个部件进行强度校核，最后分析了超长缆释放时自身的张力对绞车定位的影响。 通过以上研究，设计出符合7000m ROV收放作业要求的深海ROV铠缆绞车及其升沉补偿装置，为7000m ROV的实现和收放技术的研究奠定了良好的基础。

南海大陆边缘地壳结构与盆地形成机制--以珠江口－琼东南盆地为例

South China Sea is located in the convergence of Eurasian plate, the Pacific Ocean plate and Indian Ocean-Australia plate. The total area is about 3,500,000 km2, the geologic structure is complicated, and the structure line cut off reciprocal is the marginal sea taking form by that the seafloor spreads during the middle Oligocene. South China Sea continental margin have developed more than 10 large oil-gas bearing basins and a number of medium-small sized basins. These basins contain abundant mineral resources such as oil & gas. The marginal deepwater area in the north part of South China Sea has become our country’s strategic energy prospecting frontier. The deepwater area of Zhujiangkou and Qiongdongnan basins is the research target in this thesis. The thesis studied deep structure and the earth dynamics of the north part of South China Sea margin, and these researches provide scientific basis for oil-gas resources strategic investigation and valuation in deepwater sea area of north part slope of South China Sea. In order to develop the research of rebuilding velocities and density architecture of earth shell in region of interest, in marginal deepwater area in the north part of South China, we adopted 14 long-cable seismic reflection profile data of 3556.41 kilometers in total, the gravity measurement data along profiles (3851.44 kilometers in total), the magnetic observation along profiles (3838.4 kilometers in total) and depth measurement along profile, the logging data of 11 wells in project, the interpreted fault parameter and preexisting geologic and geophysical research achievement. This thesis has carried out concretely studying research as follows: 1. Overlay-velocity data sampling and analysis, interval velocity calculation, time-depth conversion, model building of earth shell velocity and layering character of earth shell are studied on 14 deep sections. Velocity structure in region of interest has revealed: Changchang is the sag with thinnest crust in Qiongdongnan basin; the sedimentary thickness lowers gradually from north to south, and the thickness change from west to east is milder. The sags’ sedimentary velocities in Qiongdongnan basin have obvious demarcation. The velocity of the 8000 meters sedimentary rocks is 4700 m/s in Shunde sag and Baiyun sag, and is the lowest; at that depth, the velocity very different in Liwan sag and Baiyun sag, which is about 800m/s. 2. Extracting gravity data and building of initial crust density model along the section; With Bouguer gravity anomaly data as constraint, revising density distributes of initial model, and building the crust density model. 3. With crust velocity and density as constraint, correcting the effect of thermobaric field and constructing constitution structure of rock in region of interest. By this research, we known that rocks in Zhujiangkou upper crustal layer are chiefly granite-gneiss, quartzite, granodiorite and basalt, however, rocks in Qiongdongnan basin upper earth shell are chiefly composed of granite-gneiss, quartzite, granodiorite, diorite and basalt. 4. Synthetically crust velocity and density structure, gaining expanding factor on crust and entire crust along section. The result is indicated: the expanding factor in every sag rises from northwest to southeast, which have reflected thinning characteristic of crust from continent to ocean. Intra-crustal deformation degree in Changchang and Ledong-Lingshui sag is bigger than that in Songnan-Baodao sag. Entire crust extension factor in Changchang and Songnan-Baodao sag is greater than that in Ledong-Lingshui sag, which can make an explanation of frequently event and longer heating process in middle-east of Qiongdongnan basin. 5. Synthesize multidisciplinary information to discuss the earth dynamics significance of discordogenic seismic profile in deepwater area of Zhujiangkou and Qiongdongnan basins.

典型人类活动对边坡变形及稳定性的影响研究

China is a mountainous country in which geological hazards occurred frequently, especially in the east of China. Except the geology, topography and extreme climate, the large scale human activities have become a major factor to landslides. Typical human activities which induced landslides are fill, cut and underground mining. On the topic of the deformation mechanism and slope stability, taking three different man-made slopes as examples, deformation mechanism and slope stability were studied by several methods, such as field work, numerical modeling and monitor. The details are as following: (1) The numerical modeling approach advantages over other conventional methods such as limit methods, so the numerical modeling is the major tool in this thesis. So far, there is no uniform failure criterion for numerical simulation. The failure criterion were summarized and analyzed firstly, subsequently the appropriate criterion was determinated. (2) Taking 220kV Yanjin transformation substation fill slope as example, the deformable characteristic, unstable mode and laboratory tests were studied systematically. The results show: the slope deformation was probably caused by a combination effect of unfavorable topographic, geological and hydro geological conditions, and external loading due to filling. It was concluded that the creep deformation of the slope was triggered by external loading applied at the back of the slope. In order to define the calculating parameters, a set of consolidated drained (CD) tests, consolidated undrained (CU) tests, repeated direct shear tests and UCS tests were carried out. The stability of the slope before and after reinforcement was assessed using 3D numerical modeling and shear strength reduction technique. The numerical modeling results showed: the factor of safety (FOS) of the slope was 1.10 in the natural state, and reduced to 1.03 after fill, which was close to the critical state and it caused creeping slip or deformation under rainfall. The failure surface in the slope is in active shear failure, whereas tensile failure occurs at the slope crest. After the site was reinforced with piles, the FOS was 1.27. Therefore, the slope is stable after reinforcement measures were taken. (3) The cut slope stability is a complex problem. Taking the left cut slope of Xiangjiaba as example in this thesis, the deformation and slope stability were studied systematically by numerical modeling and monitor methods. The numerical results show: the displacement is gradually increasing along with the cutting, and the largest displacement is 27.5mm which located at the bench between the elevation 340 and 380. Some failure state units distribute near the undermining part and there is no linked failure state occurred from crest to bottom during cutting. After cutting, some failure units appeared at the ground surface between elevation 340 and 360. The increasing tense stress made the disturbed rock failed. The slope is stable after cutting by the monitor method, such as surface monitor, multipoint displacement meter, inclinometer and anchor cable tensometer. (4) The interaction between underground mining and slope stability is a common situation in mountainous. The slope deformation mechanism induced by underground mining may contributed significantly to slope destabilization. The Mabukan slope in xiangjiaba was analyzed to illustrate this. Failure mechanism and the slope stability were presented by numerical modeling and residual deformation monitor. The results show: the roof deformed to the free face and the floor uplift lightly to the free face. The subsidence basin is formed, but the subsidence and the horizontal movement is small, and there is no failure zone occurred. When the underground mining is going on, the roof deformation, subsidence and the horizontal movements begin increasing. The rock deformation near the free face is larger than the ground surface, and the interaction between these coal seams appeared. There are some tensile failures and shear failures occurred on the roof and floor, and a majority of failure is tensile failure. The roof deformation, subsidence and the horizontal movements increased obviously along with the underground mining. The failure characteristic is shear failure which means the tensile stress transformed to the compressive stress. So the underground mining will induced tensile stress first which lead to structure crack, subsequently the compressive stress appeared which result in slippage. The crest was subjected to horizontal tension which made the rock crack along with the joint. The long term residual deformation monitor demonstrates that the slope is stable after the underground mining stopped.

预应力结构抗滑桩设计计算方法及其在滑坡治理中的应用研究

Landslide is a kind of serious geological hazards and its damage is very great. In recent years, landslides become more and more frequent along with increase of scale of engineering constructions and cause greater loss. Consequently, how to protect landslides has become important research subject in the engineering field. This paper improves the method how to compute landslide thrust and solves the irrational problem in the design of piles because of the irrational landslide thrust according to the theory and technology of existed anti-slide piles and pre-stressed cable anti-slide piles. Modern pre-stressing technology has been introduced and load balancing method has been used to improve the stressing behavior of anti-slide piles. Anchor cables, anti-slide piles and modern pre-stressing technology have been used to prevention complicated landslide. It is an important base to select values for the landslide thrust. An improved method to calculate design thrust of anti-slide piles has been presented in this paper on the base of residual thrust method by comparing existing methods to select values of landslide thrust in the design of anti-slide piles. In the method, residual landslide thrust behind the anti-slide piles and residual skid resistance before the piles has been analyzed, equitable distribution of residual landslide thrust behind the piles has been realized, and the method to select value of design thrust becomes more reasonable. The pre-stressed cable anti-slide piles are developed from the common anti-slide piles and are common method to prevent landslide. Their principle is that internal force of anti-slide piles is adjusted and size of section is diminished by changing constraint conditions of anti-slide piles. For landslides with deep slip surface and large scale of slopes, limitation of the method appears. Such landslides are in need of long piles and anchor cables which are not only non-economic but also can generate larger deformation and leave potential danger after prevention. For solving the problem, a new kind of anti-slide piles, inner pre-stressing force anti-slide piles, is presented in this paper, and its principle is that an additional force, which is generated in the inner anti-slide piles by arranging pre-stressed reinforcement or tight wire in a certain form in interior of anti-slide piles and stretching the steel reinforcement or tight wire, may balance out the internal force induced by landslide thrust whole or partly (load balancing method). The method will change bending moment which anti-slide piles are not good at bearing into compressive stress which piles are good at bearing, improve stressing performance of anti-slide piles greatly, diminish size of section, and make anti-slide piles not fissured in the natural service or postpone appearance of the fissures, and improve viability of anti-slide piles. Pre-stressed cable anti-slide piles and inner pre-stressing force anti-slide piles go by the general name of pre-stressed structure anti-slide piles in the paper, and their design and calculation method is also analyzed. A new calculation method is provided in the paper for design of anti-slide piles. For pre-stressed structure anti-slide piles, a new computation mode is firstly presented in the paper on the foundation of cantilever piles. In the mode, constraint form of load-bearing section of the anti-slide piles should be confirmed according to reservoir conditions in order to figure out amount of pre-stress of the anchor cables, and internal force should be analyzed for the load-bearing section of pre-stressed structure anti-slide piles so as to confirm anchorage section of anti-slide piles. Pre-stressed cables of the pre-stressed cable anti-slide piles can be arranged as required. This paper analyzes the load-bearing section of single-row and double-row pre-stressed cable anti-slide piles and provides a calculation method for design of the pre-stressed cable anti-slide piles. Inner pre-stressing force anti-slide piles are a new kind of structural style. Their load-bearing section is divided into four computation modes according to whether pre-stressed cables are applied for exterior of the anti-slide piles, and whether single-row or double-row exterior pre-stressed cables are applied. The load balancing method is used to analyze the computation modes for providing a method to design the inner pre-stressing force anti-slide piles rationally. Pre-stressed cable anti-slide piles and inner pre-stressing force anti-slide piles are applied to research on Mahe landfall in Yalong Lenggu hydropower station by the improved method to select value of design thrust of anti-slide piles. A good effect is obtained in the analysis.

高山峡谷地区大跨度桥梁岸坡稳定性分析、趋势预测及应用研究

In order to realize fast development of the national economy in a healthy way and coordinate progress with whole society, the country has implemented the strategy of development of the western region. An important action of finishing this strategic task is to accelerate the highway construction in the western region, join the western region and places along the coast, the river, the border with goods and materials, technology, and personnel interchanges, and then drive development of the local economy.The western region was influenced by the Himalaya Tectonization in Cenozoic, and the crust rose and became the plateau. In the course of rising, rivers cut down sharply to form a lot of high mountains and gorges.Because of topography and geomorphology, bridges in the traffic construction in the alpine gorge area are needed. Rivers have characteristics of large flow, fast velocity and high and steep river valley, so building a pier in the river is not only very difficult, but also making the cost increase. At the same time, the impact that the pier is corroded and the bridge base that is drawn to be empty by flow are apt to cause destruction of the pier. For those reasons, suspending bridge and cable-stay bridge are usually adopted with the single and large span. For the large span bridge, the pier foundation could receive ten thousand and more vertical strength, bending moment and near kiloton horizontal thrust.Because bank slope in the alpine gorge district is cut deeply and unsettled big, natural stability is worse under endogenic and exogenic force. When bank slope bears heavy vertical strength, bending moment and horizontal thrust facing the river, it will inevitably make the balance state of rock and soil mass change, bridge bank slope deform, and even destroyed. So the key problem at the time of the large span's bridge construction in the alpine gorge area is how to make it stable.So based on the spot investigation, the Engineering Geology Analysis Method is very important to grasp the bank slope stability. It can provide the bank slope stability macroscopic ally and qualitatively, and reference to the indoor calculation. The Engineering Geology Analysis Method is that by way of analyzing and investigating terms of bank slope instability, stability development trend, the ancient rock slide and devolution in the site, stability comprehensive evaluation primarily, current and future stability of bank slope is gotten, realizing the intention to serving the concrete engineering.After the Engineering Geology Analysis Method is applied to project instances of BeiPan River Bridge and BaLin River Bridge, results are accord with bank slope actual conditions, which proves sites are suited to building bridges from site stability.we often meet bank slope stability issues in the traffic construction in the alpine gorge areao Before the evaluation of the bank slope stability, the engineering geological condition is investigated first. After that, the next exploration target and geology measures are decided. So, the Engineering Geology Analysis Method that the investigation of the engineering geological condition is the main content is quite important in practice. The other evaluations of the bank slope stability are based on it. Because foundation receives very heavy load, for the big span's bridge in the alpine gorge area, a long pile of the large diameter (D^0.8m) is usually selected. In order to reflect rock mass's deformation properties under rock-socketed pile function, the author has used the FLAG30 software for rock and soil mass and done many numerical simulations. By them, the author launches the further investigation on deformation properties of bank slope under different slope angle, pile length, diameter, elastic modulus, load, bank slope's structure, etc. Some conclusion meaningful to the design and produce are obtained.

The Effect of Transmission Design on Force-Controlled Manipulator Performance

Previous research in force control has focused on the choice of appropriate servo implementation without corresponding regard to the choice of mechanical hardware. This report analyzes the effect of mechanical properties such as contact compliance, actuator-to-joint compliance, torque ripple, and highly nonlinear dry friction in the transmission mechanisms of a manipulator. A set of requisites for high performance then guides the development of mechanical-design and servo strategies for improved performance. A single-degree-of-freedom transmission testbed was constructed that confirms the predicted effect of Coulomb friction on robustness; design and construction of a cable-driven, four-degree-of- freedom, "whole-arm" manipulator illustrates the recommended design strategies.

Comparar medios

Por qué compramos un periódico y no otro? ¿Cuál es la razón de que veamos más esta cadena de televisión que la otra? ¿Y por qué tenemos presintonizadas algunas emisoras de radio? Seguro que todos tenemos alguna respuesta a esas preguntas, aunque lo más fácil es que sean gené- ricas en la mayoría de los casos y en bastantes tengan que ver con algu- na opción política. Pero puede ser que nuestros alumnos y alumnas ten- gan unas opciones «heredadas» de la familia (hasta que toman posesión del mando a distancia al menos) o no sean capaces de cuantificarlas de ninguna manera.

Publicidad y matemáticas

No estoy seguro que la importancia del sistema educativo aumente con el paso del tiempo por más que se nos diga a los profesores por parte de las autoridades de turno en ocasiones destacadas y con voz engolada aquello de que vuestra misión es muy importante porque en vuestras manos esta nuestro jóvenes el futuro y la sociedad, pero hay que cerrar los ojos para no ver el despegue fulgurante y la influencia creciente de los medios de comunicación social encabezado por la televisión, un miembro destacado de las nuevas familias, en las que el poder lo tiene quien maneja el mando a distancia, pero seguidos de cerca por la radio, con todos sus principales, la prensa que da carta de existencia a los hechos que recogen y ahora la tela araña que nos invade por internet de una importancia tal que se ha despajado del conjunto de medios.

Intelligent design assistant (IDA): a case base reasoning system for material and design

This paper describes the approach to the modelling of experiential knowledge in an industrial application of Case-Based Reasoning (CBR). The CBR involves retrieval techniques in conjunction with a relational database. The database is especially designed as a repository of experiential knowledge, and includes qualitative search indices. The system is intended to help design engineers and material engineers in the submarine cable industry. It consists of three parts: a materials database; a database of experiential knowledge; and a CBR system used to retrieve similar past designs based upon component and material qualitative descriptions. The system is currently undergoing user testing at the Alcatel Submarine Networks site in Greenwich.

Mathematical modelling: a laser soldering process for an optoelectronics butterfly package

For sensitive optoelectronic components, traditional soldering techniques cannot be used because of their inherent sensitivity to thermal stresses. One such component is the Optoelectronic Butterfly Package which houses a laser diode chip aligned to a fibre-optic cable. Even sub-micron misalignment of the fibre optic and laser diode chip can significantly reduce the performance of the device. The high cost of each unit requires that the number of damaged components, via the laser soldering process, are kept to a minimum. Mathematical modelling is undertaken to better understand the laser soldering process and to optimize operational parameters such as solder paste volume, copper pad dimensions, laser solder times for each joint, laser intensity and absorption coefficient. Validation of the model against experimental data will be completed, and will lead to an optimization of the assembly process, through an iterative modelling cycle. This will ultimately reduce costs, improve the process development time and increase consistency in the laser soldering process.

High Level Environmental Screening Study for Offshore Wind Farm Developments – Marine Habitats and Species Project

High level environmental screening study for offshore wind farm developments – marine habitats and species This report provides an awareness of the environmental issues related to marine habitats and species for developers and regulators of offshore wind farms. The information is also relevant to other offshore renewable energy developments. The marine habitats and species considered are those associated with the seabed, seabirds, and sea mammals. The report concludes that the following key ecological issues should be considered in the environmental assessment of offshore wind farms developments: • likely changes in benthic communities within the affected area and resultant indirect impacts on fish, populations and their predators such as seabirds and sea mammals; • potential changes to the hydrography and wave climate over a wide area, and potential changes to coastal processes and the ecology of the region; • likely effects on spawning or nursery areas of commercially important fish and shellfish species; • likely effects on mating and social behaviour in sea mammals, including migration routes; • likely effects on feeding water birds, seal pupping sites and damage of sensitive or important intertidal sites where cables come onshore; • potential displacement of fish, seabird and sea mammals from preferred habitats; • potential effects on species and habitats of marine natural heritage importance; • potential cumulative effects on seabirds, due to displacement of flight paths, and any mortality from bird strike, especially in sensitive rare or scarce species; • possible effects of electromagnetic fields on feeding behaviour and migration, especially in sharks and rays, and • potential marine conservation and biodiversity benefits of offshore wind farm developments as artificial reefs and 'no-take' zones. The report provides an especially detailed assessment of likely sensitivity of seabed species and habitats in the proposed development areas. Although sensitive to some of the factors created by wind farm developments, they mainly have a high recovery potential. The way in which survey data can be linked to Marine Life Information Network (MarLIN) sensitivity assessments to produce maps of sensitivity to factors is demonstrated. Assessing change to marine habitats and species as a result of wind farm developments has to take account of the natural variability of marine habitats, which might be high especially in shallow sediment biotopes. There are several reasons for such changes but physical disturbance of habitats and short-term climatic variability are likely to be especially important. Wind farm structures themselves will attract marine species including those that are attached to the towers and scour protection, fish that associate with offshore structures, and sea birds (especially sea duck) that may find food and shelter there. Nature conservation designations especially relevant to areas where wind farm might be developed are described and the larger areas are mapped. There are few designated sites that extend offshore to where wind farms are likely to be developed. However, cable routes and landfalls may especially impinge on designated sites. The criteria that have been developed to assess the likely marine natural heritage importance of a location or of the habitats and species that occur there can be applied to survey information to assess whether or not there is anything of particular marine natural heritage importance in a development area. A decision tree is presented that can be used to apply ‘duty of care’ principles to any proposed development. The potential ‘gains’ for the local environment are explored. Wind farms will enhance the biodiversity of areas, could act as refugia for fish, and could be developed in a way that encourages enhancement of fish stocks including shellfish.

Sign language in Spanish television. Study on reception

The present article analyses the preferences of the deaf who use sign language and are users of the TV interpretation service to sign language, as well as the characteristics with which TV channels provide that service in television in Spain. The objective is to establish whether the way in which the aforementioned accessibility service is provided matches the preferences of users or differ from them. The analysis presents the opinion on this service of the deaf that use the Spanish sign language as their first language for communication. A study has also been conducted on the programmes broadcast with sign language during week 10-16/03/2014. The main data collected reveal that the deaf are dissatisfied with broadcasting times. They ask for news programmes with sign language, they would rather have the interpretation carried out by deaf people who use sign language and they prefer that the interpreter is the main image on screen. Concerning the analysis of the programmes broadcast, the study shows that the majority of programmes with sign language are broadcast at night, they are entertainment programmes, the interpretation is carried out by hearing people who use sign language and that their image is displayed in a corner of the screen.

The emotional structure of the advertising message in radio

Radio advertising is suffering from a remarkable crisis of creativity as it has yet not found its role in a radio model based on voice locution and information genres. This article suggests the need for implementing a peripheral or heuristic strategy to attract and hold listeners’ attention. Within this framework, the narration and scene representation are proposed as suitable persuasion techniques. The objective is to design a useful conceptual tool for an efficient creative conception of narration at the service of certain commercial strategy. First, the concept of narrative persuasion is grounded according to the possibilities of the sound code. Second, the keys of scene representation and commercial strategy (brand, product, advantage, benefit and target) within the sound message are presented. And third, these keys are articulated in a model. This model is pre-tested by means of analyzing eight different case-radio ads.

Aplicación de modelos semióticos al análisis de poesía narrativa popular ecuatoriana difundida en audiovisuales

Se expone una fundamentación teórica, lógica y metodológica de tres modelos semióticos: los elementos y funciones de la comunicación de Jakobson; el modelo actancial de Greimas; y los mundos: real, referencia y posible de Eco, aplicados al análisis de poesía narrativa popular difundida en distintos formatos audiovisuales.

Analysis of audiovisual production and music scene in Extremadura through YouTube channels

A study on Extremenian music and audiovisual production has been carried out through YouTube media. The purpose of this study is to analyze all audiovisual products and also assessing what broadcasting processes are conducted by creators to publicize their musical and audiovisual works. A quantitative and qualitative study of audiovisual materials of Extremadura bands was done to find out which processes are most suitable to increase the number of viewers for videos.