層流條件下無黏性均勻顆粒起動(dòng)規(guī)律研究
發(fā)布時(shí)間：2018-09-12 11:30
【摘要】：顆粒起動(dòng)是顆粒運(yùn)動(dòng)的基本問題之一,掌握層流條件下的顆粒起動(dòng)規(guī)律對(duì)研究層流邊界層及高黏度流體中的顆粒運(yùn)動(dòng)有重要的意義,同時(shí)也可豐富層流區(qū)顆粒起動(dòng)實(shí)測(cè)資料。以起動(dòng)拖曳力曲線(Shields曲線)的離散分布為切入點(diǎn),綜合利用試驗(yàn)觀測(cè)、理論分析及數(shù)值模擬相結(jié)合的方法,對(duì)無黏性均勻顆粒的起動(dòng)規(guī)律進(jìn)行了研究。其中試驗(yàn)觀測(cè)采用PIV(Particle Image Velocimetry,粒子圖像測(cè)速)技術(shù)、RIM(Rrefractive Index Matching,折射率匹配)技術(shù)及CCD(Coupled Charged Device,電荷耦合組件)攝影技術(shù)對(duì)無黏性均勻顆粒床的表層過渡段流場(chǎng)進(jìn)行了研究,并分別在矩形有壓管及錐-板環(huán)形水槽中對(duì)無黏性均勻顆粒的起動(dòng)進(jìn)行了系統(tǒng)觀測(cè)。理論分析上,根據(jù)觀測(cè)結(jié)果,綜合考慮顆粒床的表面結(jié)構(gòu)特性,采用滾動(dòng)起動(dòng)力學(xué)模型對(duì)無黏性均勻顆粒的起動(dòng)拖曳力公式進(jìn)行了推求。數(shù)值模型方面,采用Fluent對(duì)不同簡(jiǎn)化床面結(jié)構(gòu)下的顆粒受力進(jìn)行了模擬。主要研究結(jié)果如下:(1)顆粒床和主流之間的過渡段垂線流速分布服從指數(shù)分布規(guī)律。顆粒床對(duì)邊界流場(chǎng)的影響主要集中在表層顆粒的影響上,隨著主流雷諾數(shù)增大過渡段表層的滑移流速增大,但過渡段厚度不隨雷諾數(shù)改變且和表層顆粒粒徑有關(guān),厚度約和表層顆粒粒徑相等,表層顆粒以下流動(dòng)微弱仍然服從Darcy滲流規(guī)律。(2)在矩形有壓管中,采用顆粒床表面沖刷停止的臨界狀態(tài)作為顆粒起動(dòng)的臨界狀態(tài),觀測(cè)發(fā)現(xiàn)層流區(qū)范圍內(nèi)希爾茲數(shù)呈規(guī)則的帶狀分布,流體作用引起顆粒床表面粗化并導(dǎo)致床面顆粒突起減小,使顆粒起動(dòng)拖曳力增大1倍以上。同時(shí),通過改變顆粒形狀及底坡,均能夠觀測(cè)到床面的粗化現(xiàn)象。進(jìn)一步采用粒徑不同的兩種均勻球形顆粒按照不同體積混合比混合進(jìn)行試驗(yàn),觀測(cè)發(fā)現(xiàn)當(dāng)其中粗顆粒的混合體積比大于70%時(shí),臨界狀態(tài)下顆粒床表面的形態(tài)變化主要是由于該顆粒的自身結(jié)構(gòu)調(diào)整引起。綜合表明起動(dòng)拖曳力的離散分布是由無黏性均勻顆粒床的自身結(jié)構(gòu)調(diào)整引起,對(duì)于某些級(jí)配不良的顆粒床在考慮級(jí)配引起的粗化同時(shí),也應(yīng)當(dāng)考慮顆粒床表面結(jié)構(gòu)自身調(diào)整引起的粗化。(3)錐-板環(huán)形水槽中的起動(dòng)觀測(cè)試驗(yàn)表明長(zhǎng)期剪切作用下顆粒的起動(dòng)為連續(xù)的起動(dòng)過程,存在2個(gè)不同的臨界狀態(tài):初始臨界狀態(tài),顆粒從靜止轉(zhuǎn)為運(yùn)動(dòng),但運(yùn)動(dòng)狀態(tài)不穩(wěn)定,一段時(shí)間后顆粒運(yùn)動(dòng)停止,初始臨界狀態(tài)下的起動(dòng)拖曳力和矩形有壓管起動(dòng)試驗(yàn)觀測(cè)結(jié)果基本一致;穩(wěn)定臨界狀態(tài)時(shí),顆粒運(yùn)動(dòng)并開始形成穩(wěn)定的顆粒流動(dòng),穩(wěn)定臨界狀態(tài)下的起動(dòng)拖曳力高于矩形有壓管中的試驗(yàn)觀測(cè)結(jié)果。但整體曲線的分布形式和矩形有壓管中試驗(yàn)結(jié)果分布形式一致,呈現(xiàn)規(guī)則的帶狀分布,因此綜合表明流體剪切力作用下顆粒床表面結(jié)構(gòu)存在自身調(diào)整的特性,且在剪切力長(zhǎng)期作用下顆粒的起動(dòng)拖曳力增大顯著,該現(xiàn)象和矩形有壓管中觀測(cè)到的床面粗化現(xiàn)象一致且直接對(duì)顆粒的起動(dòng)過程進(jìn)行了描述。(4)在矩形有壓管試驗(yàn)及錐-板環(huán)形水槽試驗(yàn)的基礎(chǔ)上,結(jié)合過渡段的流場(chǎng)特性及顆粒床表層的結(jié)構(gòu)特征,建立了顆粒的滾動(dòng)起動(dòng)力學(xué)模型。模型計(jì)算表明顆粒突起不同時(shí),無量綱起動(dòng)拖曳力不同,起動(dòng)拖曳力曲線的形式也不同。實(shí)際起動(dòng)過程中,由于床面粗化,顆粒突起在不斷減小,多數(shù)試驗(yàn)數(shù)據(jù)分布在顆粒突起從0.2至1的范圍內(nèi),當(dāng)顆粒突起小于0.2時(shí),已不能觀測(cè)到顆粒的起動(dòng)。以往的研究中并沒有注意到床面結(jié)構(gòu)的影響問題,純粹建立在試驗(yàn)數(shù)據(jù)基礎(chǔ)上的經(jīng)驗(yàn)公式不能從本質(zhì)上揭示層流區(qū)無黏性均勻顆粒的起動(dòng)規(guī)律。(5)在試驗(yàn)結(jié)果及理論模型的基礎(chǔ)上,采用Fluent數(shù)值模擬,考慮不同床面結(jié)構(gòu)對(duì)目標(biāo)顆粒的影響,對(duì)目標(biāo)顆粒受到的拖曳力及升力進(jìn)行了分析。分析得到由于周圍顆粒的遮擋,目標(biāo)顆粒的迎流面減小,從而使得拖曳力減小,且同一床面結(jié)構(gòu),迎流面的方向改變也會(huì)使得目標(biāo)顆粒受到的拖曳力改變。但對(duì)于升力而言,在本文的模擬條件下,周圍顆粒的遮擋反而會(huì)使得升力增大。床面結(jié)構(gòu)直接影響到顆粒的受力大小,在起動(dòng)臨界狀態(tài),當(dāng)床面結(jié)構(gòu)發(fā)生粗化時(shí),目標(biāo)顆粒所需起動(dòng)拖曳力將發(fā)生變化。該結(jié)果與試驗(yàn)觀測(cè)及理論模型計(jì)算一致,表明床面結(jié)構(gòu)的改變是導(dǎo)致無黏性均勻顆粒起動(dòng)拖曳力離散分布的主要原因。以上研究綜合表明顆粒床表面結(jié)構(gòu)性狀直接影響著顆粒起動(dòng)拖曳力的大小,由于顆粒床表面粗化,在層流區(qū)無量綱起動(dòng)拖曳力曲線(Shields曲線)具有帶狀分布特性。
[Abstract]:Particle start-up is one of the basic problems of particle motion. It is very important to understand the law of particle start-up in laminar boundary layer and high viscosity fluid. It can also enrich the measured data of particle start-up in laminar flow region. The starting law of non-cohesive uniform particles is studied by means of experimental observation, theoretical analysis and numerical simulation. PIV (Particle Image Velocimetry), RIM (Refractive Index Matching) and CCD (Coupled Charged Device) are used in the experimental observation. (2) The flow field in the surface transition zone of the non-cohesive uniform granular bed was studied by photography, and the incipient motion of the non-cohesive uniform granular bed was systematically observed in the rectangular pressurized tube and the cone-plate annular flume respectively. In the numerical model, Fluent was used to simulate the forces acting on the particles under different simplified bed structures. The main results are as follows: (1) The vertical velocity distribution between the particle bed and the main stream obeys the exponential distribution law. Concentrating on the influence of surface particles, the slip velocity increases with the increase of Reynolds number, but the thickness of the transition section does not change with Reynolds number and is related to the particle size of the surface layer. The thickness of the transition section is about the same as the particle size of the surface layer. The flow below the surface particles still obeys Darcy seepage law. (2) In a rectangular pressurized tube, particles are used. The critical state of the particle bed surface scouring stops is regarded as the critical state of the particle start-up. It is found that the Hiltz number in the laminar flow region distributes regularly in a band. Fluid action causes the surface of the particle bed to coarsen and the particle protrusion on the bed surface to decrease, which makes the particle start-up drag force increase more than one time. The coarsening of the bed surface can be observed. Two uniform spherical particles with different particle sizes are mixed according to different volume mixing ratios. It is found that the morphological changes of the bed surface in critical state are mainly caused by the structural adjustment of the particles when the volume ratio of coarse particles is greater than 70%. It is concluded that the discrete distribution of the starting drag force is caused by the self-structural adjustment of the non-cohesive uniform granular bed, and the coarsening caused by the self-structural adjustment of the granular bed surface should also be taken into account when considering the coarsening caused by the gradation of some non-cohesive uniform granular beds. (3) The start-up test in a cone-plate annular flume shows that the long-term shearing is necessary. There are two different critical states: the initial critical state, in which the particles change from static state to motion, but the motion state is unstable. After a period of time, the particle motion stops, and the initial critical drag force and the rectangular pressurized tube start-up test results are basically consistent. The starting drag force in the stable critical state is higher than that in the rectangular pressurized tube, but the distribution of the whole curve is consistent with that of the experimental results in the rectangular pressurized tube, showing a regular band-like distribution, so it shows that the granular bed under the action of the fluid shear force. The surface structure has the characteristic of self-adjusting, and the starting drag force of particles increases remarkably under the long-term shear force. This phenomenon is consistent with the bed coarsening observed in rectangular pressurized tube and directly describes the starting process of particles. (4) Based on the rectangular pressurized tube test and cone-plate annular flume test, the starting drag force of particles increases remarkably. The mechanical model of particle rolling starting is established based on the characteristics of flow field and the structure of particle bed surface in the crossing section.The results show that when the particle protrusion is different,the dimensionless starting drag force is different,and the form of starting drag force curve is different.In the actual starting process,the particle protrusion is decreasing because of the coarsening of bed surface,and most test data are divided into two parts. In the range from 0.2 to 1, the starting of particles can not be observed when the particle protrusion is less than 0.2. Previous studies have not paid attention to the influence of bed structure. Empirical formulas based solely on experimental data can not reveal the starting law of non-viscous uniform particles in laminar flow region. Based on the experimental results and the theoretical model, the drag force and lift of the target particles are analyzed by using Fluent numerical simulation considering the influence of different bed structures on the target particles. But for the lift, the shielding of the surrounding particles will increase the lift. The bed structure directly affects the force on the particles. In the critical state of starting, when the bed structure coarsens, the target particles need to start the drag force. The results are in agreement with experimental observations and theoretical model calculations. It is shown that the change of bed structure is the main reason for the dispersion of the starting drag force of non-cohesive uniform particles. The dimensionless starting drag force curve (Shields curve) in the flow area has a zonal distribution characteristic.
【學(xué)位授予單位】：西北農(nóng)林科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TV149
本文編號(hào)：2238869
[Abstract]:Particle start-up is one of the basic problems of particle motion. It is very important to understand the law of particle start-up in laminar boundary layer and high viscosity fluid. It can also enrich the measured data of particle start-up in laminar flow region. The starting law of non-cohesive uniform particles is studied by means of experimental observation, theoretical analysis and numerical simulation. PIV (Particle Image Velocimetry), RIM (Refractive Index Matching) and CCD (Coupled Charged Device) are used in the experimental observation. (2) The flow field in the surface transition zone of the non-cohesive uniform granular bed was studied by photography, and the incipient motion of the non-cohesive uniform granular bed was systematically observed in the rectangular pressurized tube and the cone-plate annular flume respectively. In the numerical model, Fluent was used to simulate the forces acting on the particles under different simplified bed structures. The main results are as follows: (1) The vertical velocity distribution between the particle bed and the main stream obeys the exponential distribution law. Concentrating on the influence of surface particles, the slip velocity increases with the increase of Reynolds number, but the thickness of the transition section does not change with Reynolds number and is related to the particle size of the surface layer. The thickness of the transition section is about the same as the particle size of the surface layer. The flow below the surface particles still obeys Darcy seepage law. (2) In a rectangular pressurized tube, particles are used. The critical state of the particle bed surface scouring stops is regarded as the critical state of the particle start-up. It is found that the Hiltz number in the laminar flow region distributes regularly in a band. Fluid action causes the surface of the particle bed to coarsen and the particle protrusion on the bed surface to decrease, which makes the particle start-up drag force increase more than one time. The coarsening of the bed surface can be observed. Two uniform spherical particles with different particle sizes are mixed according to different volume mixing ratios. It is found that the morphological changes of the bed surface in critical state are mainly caused by the structural adjustment of the particles when the volume ratio of coarse particles is greater than 70%. It is concluded that the discrete distribution of the starting drag force is caused by the self-structural adjustment of the non-cohesive uniform granular bed, and the coarsening caused by the self-structural adjustment of the granular bed surface should also be taken into account when considering the coarsening caused by the gradation of some non-cohesive uniform granular beds. (3) The start-up test in a cone-plate annular flume shows that the long-term shearing is necessary. There are two different critical states: the initial critical state, in which the particles change from static state to motion, but the motion state is unstable. After a period of time, the particle motion stops, and the initial critical drag force and the rectangular pressurized tube start-up test results are basically consistent. The starting drag force in the stable critical state is higher than that in the rectangular pressurized tube, but the distribution of the whole curve is consistent with that of the experimental results in the rectangular pressurized tube, showing a regular band-like distribution, so it shows that the granular bed under the action of the fluid shear force. The surface structure has the characteristic of self-adjusting, and the starting drag force of particles increases remarkably under the long-term shear force. This phenomenon is consistent with the bed coarsening observed in rectangular pressurized tube and directly describes the starting process of particles. (4) Based on the rectangular pressurized tube test and cone-plate annular flume test, the starting drag force of particles increases remarkably. The mechanical model of particle rolling starting is established based on the characteristics of flow field and the structure of particle bed surface in the crossing section.The results show that when the particle protrusion is different,the dimensionless starting drag force is different,and the form of starting drag force curve is different.In the actual starting process,the particle protrusion is decreasing because of the coarsening of bed surface,and most test data are divided into two parts. In the range from 0.2 to 1, the starting of particles can not be observed when the particle protrusion is less than 0.2. Previous studies have not paid attention to the influence of bed structure. Empirical formulas based solely on experimental data can not reveal the starting law of non-viscous uniform particles in laminar flow region. Based on the experimental results and the theoretical model, the drag force and lift of the target particles are analyzed by using Fluent numerical simulation considering the influence of different bed structures on the target particles. But for the lift, the shielding of the surrounding particles will increase the lift. The bed structure directly affects the force on the particles. In the critical state of starting, when the bed structure coarsens, the target particles need to start the drag force. The results are in agreement with experimental observations and theoretical model calculations. It is shown that the change of bed structure is the main reason for the dispersion of the starting drag force of non-cohesive uniform particles. The dimensionless starting drag force curve (Shields curve) in the flow area has a zonal distribution characteristic.
【學(xué)位授予單位】：西北農(nóng)林科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TV149
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