落地四坡房屋表面风致雪漂移效应分析

为了获得落地四坡房屋表面积雪分布规律,根据风雪运动机理选取适当的雪粒粒径、积雪密度、沉降速度等条件,考虑积雪侵蚀沉积等影响因素,基于Euler-Euler多相流理论,使用FLUENT软件Mixture多相流模型模拟了立方体周围积雪及高低屋盖模型表面积雪分布,并与实测进行了比较,确定了湍流物理方程、数值风洞尺寸、细部网格及数量、壁面条件等各计算参数设置。以风速、风向角为参数,模拟落地四坡房屋屋面积雪分布得出:随着风速增加,屋面积雪量不断减少,15m/s风速下屋面积雪呈完全侵蚀状态,低风速下屋面积雪更多;屋面各区域积雪漂移随着风向角的改变不断改变,总体表现为侵蚀状态沉积区域较少;在5m/s风速下落地四坡房屋的迎风面各区域积雪分布系数在0.5以下,迎风屋顶各区域积雪分布系数基本为0,背风屋顶各区域积雪分布系数变化幅度高达0.8,背风面各区域积雪分布系数整体保持在0.9附近。得到了5m/s风速下区域积雪分布系数表,可为该类房屋的设计使用提供理论依据。     

攒尖四坡屋面风致雪漂移的数值模拟

为获得攒尖四坡屋面的风致雪漂移规律，基于欧拉-欧拉方法和风雪单向耦合假定，运用计算流体动力学软件，选取Mixture模型分别对立方体周边和高低屋面上的风致雪漂移运动进行数值模拟，将模拟结果与两者的实地观测数据对比，探讨分析数值风洞的关键技术和参数设置，验证数值模拟方法的合理性与可靠性。依据攒尖四坡房屋的使用功能要求，设计分析模型与分析工况，在试算的基础上对屋面进行分区。以风速5 m/s，7 m/s，9 m/s，11 m/s，13 m/s和15 m/s，风向角0°，15°，30°和45°以及屋面坡度25°，30°，34°，40°和45°为分析参数，对攒尖四坡房屋的120种工况进行数值模拟，得到屋面各分区侵蚀沉积的基本规律，提出可用于抗雪设计的屋面积雪分布系数。研究表明，风向角的改变会使屋面积雪分布状态发生较大程度的变化，风速和屋面坡度的变化对屋面整体积雪量有较大影响。     

一种磁力滑动式翼型颤振能量俘获器

风致振动是自然界中普遍存在的一种现象,并且蕴藏着巨大的可利用能源.如何充分利用风致振动引起的结构大幅值响应进行能量俘获,为微电子器件供电是能量俘获领域的一个难题.为了高效俘获风致振动能量,文章提出了一种磁力滑动式翼型颤振能量俘获器.基于半经验非线性空气动力学模型并考虑与磁铁位置相关的机电耦合系数,建立了该能量俘获器的动力学模型,搭建了风洞实验平台,制作了实验样机.通过增加风速和降低风速的方式为能量俘获器提供两种不同的初始状态,发现其具有两个临界风速(5.2 m/s和8.3 m/s),降风速实验中在8.3 m/s风速下出现突跳现象.在数值仿真中,在6.8 m/s和8.2 m/s风速下出现了两个突跳点,和一段多解区域.分析了沉浮位移和电压响应,发现沉浮位移以正弦形式响应,输出电压以非正弦形式响应,并出现明显的偶次谐波.仿真的沉浮位移和电压输出波形与实验波形吻合较好,验证了模型的准确性.能量俘获器的均方根电压随电阻的增加而增加,平均功率随电阻增加呈现先增加后降低的趋势.分析了负载电阻对能量俘获性能的影响,在8.6 m/s风速下,实验中能量俘获器的负载电阻接近线圈内阻值时平均功率达到最大值7....     

沙粒起动风速研究

在风沙流实验中,发展了设陷井捕集蠕移沙粒的方法来测量各种来流条件下的蠕移沙量,再外推得到流体起动风速,试验表明粗糙度不同的沙平面的流体起动风速相同,中还给出实验段轴线风速与野外来流摩阻速度之间的对应关系。     

两种不同弦长螺旋桨的风洞试验及分析

本文介绍了两种不同弦长螺旋桨的风洞试验结果。试验在西北工业大学NF-3风洞的三元试验段进行,试验风速分别为V=20m/s、30m/s和40m/s,每个风速下,螺旋桨旋转速度为:900r/min、1200r/min、1500r/min、1800r/min、2100r/min、2400r/min、2700r/min、3000r/min、3300r/min、3600和3900r/min。风洞试验结果表明:当两种螺旋桨的翼型相同、桨叶角沿径向分布相同,但弦长沿径向的分布不同时,它们的拉力、扭矩、功率、效率以及前进比会有显著区别。其中,1#螺旋的弦长大于2#螺旋桨,在相同的试验风速和螺旋桨转速下,1#螺旋桨的拉力、扭矩和功率高于2#螺旋桨,但效率低于2#螺旋桨。     

风致折叠网壳结构表面积雪分布CFD模拟

为研究风致折叠网壳结构表面积雪的分布规律,基于Euler-Euler方法和空气相与雪相单向耦合的基本假设,运用通用计算流体动力学软件ANSYS Fluent的Mixture多相流模型理论,并考虑壁面上积雪的侵蚀与沉积,建立风致雪漂的数值模型。首先,模拟立方体周围积雪分布并与实测结果对比,探讨与分析数值风洞的关键技术与参数,证实三方程k-kl-ω湍流模型能更好地对风雪两相流进行模拟。在此基础上,以风速和风向角为分析参数,模拟折叠网壳结构表面积雪分布。结果表明,10 m/s以下较低风速的持续作用对积雪分布尤为不利,受风向角变化影响,结构表面积雪的侵蚀与沉积发生在不同分区,其中迎风面被大面积侵蚀、背风面局部沉积,在不同的风向角下同一分区的积雪分布系数相对改变量最高达1.28。模拟获得结构表面在全风向角下的最不利积雪分布系数,为近似体型结构的抗风雪设计理论提供参考依据。     

风机叶片覆冰机理与预测分析

高原和寒冷地区风能资源十分丰富,风机在此地区运行时叶片表面极易出现覆冰现象.针对寒区风机叶 片覆冰问题,本文首先明确覆冰机理以及主要的影响参数;基于数值模拟分析在冻雨条件下,环境因素和叶片几何参数对叶片覆冰的影响.结果表明,当温度不变,风速从2 m/s增加到4 m/s时,叶片表面的最大覆冰厚度增幅高达117％;当风速从4 m/s到10 m/s时,最大冰厚增幅逐渐减小到0.9％,覆冰质量随着风速的增加呈线性增长.当风速不变,温度从-1℃降到-2℃时,叶片表面的覆冰质量和最大冰厚分别增加71％和24％;当温度从-2℃到-5℃时,覆冰质量和最大冰厚增幅减小到1.7％和0.9％.当温度和风速在某一固定值时,覆冰质量和最大厚度随着叶片相对厚度和攻角的增加而增大.     

油润滑条件下弹性金属塑料复合材料的摩擦磨损特性研究

在MPA－2000型盘销式摩擦磨损试验机上评价了油润滑条件下弹性金属塑料复合材料与钢对摩时的摩擦学特性,用扫描电子显微镜观察试样磨损表面形貌并分析其摩损机理,并在试验基础上建立了弹性金属塑料材料与钢对摩时的等摩损率图。结果表明：在低载荷条件下摩擦系数较高,随着载荷数升高摩擦系数降低;当滑动速度小于3．52m/s时,摩擦系数基于稳定在0．030;弹性金属塑料材料的磨损率随滑动速度和载葆的升高而增加,结合等磨损率图分析发现,当载荷小于1515N而滑动速度小于3．52m/s时,弹性金属塑料复合材料的磨损率相对较低;当滑动速度泪地3．52m/s时,弹性金属材料的磨损机理以微切削、挤压变形和犁沟磨损为主,在摩擦副两表面形成转移－依附物;当滑动速度为5．24m/s时,弹性金属塑料材料的磨损以表层软化和熔融为主要特征,所建立的等磨损率图对弹性金属塑料材料的使用有一定的指导作用。     

风致折叠网壳结构表面积雪分布CFD模拟

为研究风致折叠网壳结构表面积雪的分布规律,基于Euler-Euler方法和空气相与雪相单向耦合的基本假设,运用通用计算流体动力学软件ANSYS Fluent的Mixture多相流模型理论,并考虑壁面上积雪的侵蚀与沉积,建立风致雪漂的数值模型。首先,模拟立方体周围积雪分布并与实测结果对比,探讨与分析数值风洞的关键技术与参数,证实三方程k-kl-ω湍流模型能更好地对风雪两相流进行模拟。在此基础上,以风速和风向角为分析参数,模拟折叠网壳结构表面积雪分布。结果表明,10 m/s以下较低风速的持续作用对积雪分布尤为不利,受风向角变化影响,结构表面积雪的侵蚀与沉积发生在不同分区,其中迎风面被大面积侵蚀、背风面局部沉积,在不同的风向角下同一分区的积雪分布系数相对改变量最高达1.28。模拟获得结构表面在全风向角下的最不利积雪分布系数,为近似体型结构的抗风雪设计理论提供参考依据。     

环境模拟实验室中近地层风速廓线的形成研究

在环境模拟实验室中利用阻尼网对实验区形成的近地层风速廓线进行了数值模拟和实验研究.阻尼网的布置方案以变孔隙率阻尼网作为指导,选用30目和16目定孔隙率阻尼网4种不同高度的混合式布置方式.数值模拟采用Fluent软件,将结构复杂的阻尼网简化为具有一定厚度的多孔介质模型,湍流模型为标准k-\varepsilon模型.结果表明可将阻尼网作为多孔介质处理,定孔隙率阻尼网的组合可在短实验段内形成所需近地层的风速廓线,模拟值与实验值吻合良好.     

Measurement of turbulent wall velocity gradients using cylindrical waves of laser light

A new optical instrument has been developed for direct measurement of instantaneous velocity gradients at the bounding wall. Light emerging from two tiny optical slits in the surface is used to form a fan of fringes in the region very near the wall. Doppler frequency of the light scattered by the seed particles is directly proportional to the velocity gradient. The system has been used to measure the statistics of the streamwise and spanwise velocity gradients in a turbulent boundary layer. The streamwise and spanwise rms fluctuations were found to be 38% and 11% of the mean streamwise value respectively. The latter result is subject to a large uncertainty.List of symbols a slit width - B transfer function of the instrument - B ^* normalized transfer function - path-averaged value of the normalized transfer function - c constant in logarithmic velocity profile - C _f skin friction coefficient - d _f fringe spacing - f ₁,f₂ frequencies at the downstream and upstream slits resp. - f _d heterodyne Doppler frequency of the signal - g(t) instantaneous wall velocity gradient - G Clauser shape factor - mean wall velocity gradient - g rms value of the wall velocity gradient - H boundary layer shape factor - i, j, k unit vectors along x, y, z axes - wavenumber of laser light - L major axis of the elliptic cross-section of the laser sheet at the slit - l length of each slit - N number of cycles in a signal - N ₀ number of cycles without frequency-shifting - n difference of the unit vectors u ₁and u ₂ - P power transmitted through a slit - P _o power incident on a slit - Re ₁ Reynolds number based on displacement thickness and free-stream velocity - Re ₂ Reynolds number based on momentum thickness and free-stream velocity - S spacing between the slits - S ^* normalized spacing between the slits - u streamwise velocity - u ₁,u₂ unit vectors along the local directions of propagation of the two cylindrical waves - u _l linear term in the streamwise velocity profile - u _nl nonlinear terms in the streamwise velocity - u _nl ^* normalized value of nonlinear streamwise velocity - u _nl ^* mean streamwise velocity - u friction velocity - u⁺ mean velocity normalized with friction velocity - v velocity component normal to the wall - v ^* normal velocity normalized with streamwise velocity - V velocity vector - w spanwise component of velocity - W minor axis of the elliptic cross-section of the laser sheet at the slit - x streamwise distance - ± x _m limiting values of streamwise distance for a signal - x ^* normalized streamwise distance - x ^* normalized value of x _m - y normal distance - y ⁺ normal distance normalized with friction length scale - z spanwise distance - z ⁺ spanwise distance normalized with friction length scale - half-spreading angle of the cylindrical waves - boundary layer thickness in Coles' profile - ₁ displacement thickness of the boundary layer - ₂ momentum thickness of the boundary layer - ₃ energy thickness of the boundary layer - constant in logarithmic velocity profile - wavelength of laser light - kinematic viscosity - coefficient of wake function in Coles' profile Currently at LSTM, Universitat Erlangen-Nürnberg, Cauerstraße 4, W-8520 Erlangen, BRD     

The effect of initial conditions on the exit flow from a fluidic precessing jet nozzle

The fluidic precessing jet (FPJ) is a member of a family of self-excited oscillating jet flows that has found application in reducing oxides of nitrogen (NO_x) from combustion systems in the high-temperature process industries. Its flow field is highly three-dimensional and unsteady, and many aspects of it remain unresolved. Velocity data, measured close to the exit plane, are presented for a variety of FPJ nozzles with three different inlet conditions, namely, a long pipe, a smooth contraction and an orifice. The results indicate that jet inlets that are known to have nonsymmetrically shedding initial boundary layers, namely those from the orifice or long pipe, cause jet precession to be induced more easily than the smooth contraction inlet, which is known to have a symmetrically shedding initial boundary layer. The nature of the exit flow is dominated by the degree to which a given configuration generates precession. Nevertheless, the three different inlet conditions also produce subtle differences in the exit profiles of mean velocity and turbulence intensity when the flow does precess reliably. Roman symbols d diameter of inlet (m) - D₁ diameter of FPJ chamber (m) - D₂ diameter of FPJ chamber exit lip (m) - E expansion ratio D₁/d - f frequency (Hz) - f_p precession frequency (Hz) - h step height (D₁-d)/2 (m) - n power law index to describe pipe inlet jet (dimensionless) or nth sample passing through LDA probe volume - N total number of bursts sampled (dimensionless) - r radial distance from FPJ chamber axis (m) - rms root-mean-square or fluctuating velocity component, (m/s) - R₁ radius of FPJ chamber (m) - R₂ radius of exit lip (m) - Re Reynolds number u_id/ (dimensionless) - S(f) arbitrary power spectrum (m²/s) - St Strouhal number, f_ph/u_i (dimensionless) - t_n residence (or transit) time of a particle moving through the LDA probe volume (s) - u axial component of mean velocity (m/s) - u_cl axial component of mean centreline velocity (m/s) - u_i bulk inlet velocity near the inlet plane (m/s) - u_n velocity of the nth particle through the LDA probe volume (m/s) - u_vc axial component of mean velocity in the region of the vena contracta (m/s) - u axial component of rms velocity (m/s) - v radial component of mean velocity (m/s) - v radial component of rms velocity (m/s) - w tangential component of mean velocity (m/s) - w tangential component of rms velocity (m/s) - x axial distance from FPJ chamber inlet plane (m) - x axial distance from FPJ chamber exit plane (m)Greek symbols kinematic viscosity of air at 21°C, 14.7×10^-6 m²/s     

The influence of polyvinylacetate additive in water on turbulent velocity field and drag reduction

The effect of polymer concentration on drag reduction was studied experimentally with diluted water solutions of polyvinylacetate in a 2.4 cm I. D. pipe. The instantaneous local velocities of the velocity fields were measured by a one-channel differential laser-Doppler anemometer DISA Mark II, with forward scattering. Concentrations of water-polyvinylacetate over the range from 10 to 2,000 ppm were used. The drag reduction coefficient is proportional to the concentration and hydrolysis degree of the saponificated polyvinylacetate (PVAC) employed. A mechanical degradation in the turbulent shear flow was not observed.List of Symbols a ₁ coefficient in Eq. (3) - a ₂ coefficient in Eq. (3) - D pipe diameter - k coefficient in modified Blasius equation for friction factor - K consistency parameter given by (1 b) - K _i coefficients in Eq. (5) - m coefficient in Eq. (3) - n flow index Eq. (1a), coefficient in Eq. (3) - n ⁺ dimensionless position parameter defined by Eq. (4) - N ⁺ position parameter defined by Eq. (7) - r radial distance from the pipe center - R pipe radius - Re Reynolds number - Re _g generalized Reynolds number, Eq. (9) - t temperature - u ⁺ dimensionless local velocity, /u ^* - u ^* dynamic friction velocity, w(/8) ^0,5 - U ⁺ dimensionless local mean velocity defined by Eq. (6) - time-averaged local velocity - _m time-averaged local velocity at the pipe center - w average velocity over the cross-section of the pipe - X concentration of polymer in water, w · ppm - y distance from the pipe wall - y ⁺ dimensionless distance from the pipe wall, y u ^* / or as in Eq. (8) - friction factor in drag reduction flow - ₀ friction factor of pure water - degree of drag reduction - viscosity - standard deviation A version of this paper was presented at the 9th National Symposium on the measurement of turbulence with laser Doppler and other anemometers, Bratislava, CSSR, 1986     

Pore structures and transport properties of sandstone

To quantitatively analyze the macroscopic properties of the flow in porous media by means of the continuum approach, detailed information (velocity and pressure fields) on the microscopic scale is necessary. In this paper, the numerical solution for incompressible, Newtonian flow in a diverging-converging representative unit cell (RUC) is presented. A new solution procedure for the problem is introduced. A review of the accuracy of the computational method is given.Nomenclature A _ff ^* area of entrance and exit of RUC - A _fs ^* interfacial area between the fluid and solid phases - d throat diameter of RUC (m) - D pore diameter of RUC (m) - i, j unit vector for RUC - L ^* wave length of a unit cell - L _p pore length of RUC (m) - L _t throat length of RUC (m) - n unit outwardly directed vector for the fluid phase - p ^* fluid pressure - ^* cross-sectional mean pressure - _en ^* entrance cross-sectional mean pressure - Re _d Reynolds number - x ^*, r^* cylindrical coordinates - u ^*, v^* velocity - u _cl ^* centerline velocity - _d mean velocity at the throat of RUC (m/s) - _D mean velocity at the large segment of RUC (m/s) Greek viscosity coefficient (Ns/m²) - _p excess momentum loss factor defined in (4.1) - fluid density (kg/m³) - ^* stream function - ^* vorticity - dimensionless circulation defined in (2.7) Symbols - the mean value - * dimensionless quantities     

Velocity measurements in a plane turbulent air jet at moderate Reynolds numbers

An experimental investigation of the moderate Reynolds number plane air jets was undertaken and the effect of the jet Reynolds number on the turbulent flow structure was determined. The Reynolds number, which was defined by the jet exit conditions, was varied between 1000 and 7000. Other initial conditions, such as the initial turbulence intensity, were kept constant throughout the experiments. Both hot-wire and laser Doppler anemometry were used for the velocity measurements. In the moderate Reynolds number regime, the turbulent flow structure is in transition. The average size and the number of the large scale of turbulence (per unit length of jet) was unaffected by the Reynolds number. A broadening of the turbulent spectra with increasing Reynolds number was observed. This indicated that there is a decrease in the strength of the large eddies resulting from a reduction of the relative energy available to them. This diminished the jet mixing with the ambient as the Reynolds number increased. Higher Reynolds numbers led to lower jet dilution and spread rates. On the other hand, at higher Reynolds numbers the dependence of jet mixing on Reynolds number became less significant as the turbulent flow structure developed into a self-preserving state.List of symbols b _u velocity half-width of the jet - C _u, C _u,0 constants defining the velocity decay rate - D nozzle width - E _u one dimensional power spectrum of velocity fluctuations - f frequency - K _u, K _u,0 constants defining the jet spread rate - k wavenumber (2f/U) - L longitudinal integral scale - R ₁₁ correlation function - r separation distance - Re jet Reynolds number (U ₀ D/v) - St Strouhal number (fD/U ₀) - t time - U axial component of the mean velocity - U _m mean velocity on the jet axis - U ₀ mean velocity at the jet exit - u the rms of u - u fluctuating component of the axial velocity - V lateral component of the mean velocity - fluctuating component of the lateral velocity - x axial distance from the nozzle exit - y lateral distance from the jet axis - z spanwise distance from the jet axis - v kinematic viscosity - time lag A version of this paper was presented as paper no. 86-0038 at the AIAA 24th Aerospace Sciences Meeting, Reno NV, USA, January 1986     

Flow of viscoelastic fluids through an abrupt contraction

Summary Developing and fully developed velocity profiles were measured for viscoelastic fluids flowing through an abrupt 2 to 1 glass-contraction. An R 16Weissenberg-Rheogoniometer was used to measure the rheological properties of the viscoelastic fluids in the shear rates range of interest in the contraction. The measured entry lengths for the viscoelastic fluids were significantly less than predictions and experimental values for inelastic fluids with the same power-law parameters. Deviations from inelastic entry behaviour ranged from 11.6–100%, were independent ofReynolds number, but were strongly dependent on the ratio of the friction velocity to the shear wave velocity. Increasing the friction velocity relative to the shear wave velocity resulted in an increased deviation from inelastic behaviour. When the friction velocity was of the same order as the shear wave velocity a zero entry length and a fully developed entry velocity profile were observed. Further increase in the friction velocity relative to the shear wave velocity resulted in anomalous entry behaviour accompanied by unusual flow patterns upstream of the contraction.

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Zusammenfassung Es wurden sich bildende sowie voll ausgebildete Geschwindigkeitsprofile viskoelastischer Flüssigkeiten in einer scharfkantigen Rohrverengung von 2 zu 1 gemessen. Ein Weissenbergsches Rheogoniometer R 16 diente zur Charakterisierung der viskoelastischen Flüssigkeiten im betreffenden Deformationsgeschwindigkeitsbereich.Meßergebnisse für die Einlauflänge viskoelastischer Flüssigkeiten weichen bedeutend von den Voraussagen sowie von Meßergebnissen für unelastische Flüssigkeiten ab, die, nach demOstwald- de Waeleschen Modell berechnet, die gleichen Kenngrößen aufzeigen.Die Abweichung vom viskosen Einlaufverhalten beträgt 11,6 bis 100%. Sie ist unabhängig von der Reynoldschen Zahl, hängt aber sehr stark ab von dem Verhältnis zwischen zwei Geschwindigkeiten u^*=Schubspannungsgeschwindigkeit undu=Scherwellengeschwindigkeit.Eine Erhöhung vonu ^* gegenüberu verursacht eine erhöhte Abweichung vom unelastischen Verhalten. Wenn die zwei Geschwindigkeitenu ^* undu von der gleichen Größenordnung sind, verschwindet die Einlaufsentwicklung und ein vollausgebildetes Geschwindigkeitsprofil tritt schon am Eingang auf. Ein weiteres Erhöhen vonu ^* überu verursacht anomales Einlaufverhalten mit ungewöhnlichem Strömungsbild oberhalb der Verengung.

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On Sabbatical Leave: Dept. of Chemical Engineering, University of Toronto, Toronto 181, Ontario, Canada.     

Convective fluid inertia forces in steady laminar lubrication: Solution of the inverse problem

Summary The present work deals with the problem of determining the influence of the inertial terms solving the inverse problem in the case of a plane slider bearing. The method determines the film geometry under a given pressure distribution when inertial terms are taken into account. The Volterra integral equation — which gives the velocity distribution in every section — was solved in a strictly numerical way. Our results showed that inertial terms determine an increase of the load capacity and a decrease of the flow rate. The friction coefficient proved scarcely influenced by inertial effects. The present method enables us to obtain the results of the linear theory as an asymptotic solution.

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Sommario Il presente lavoro tratta il problema della determinazione dell'influenza dei termini inerziali, mediante la risoluzione del problema inverso, nel caso di un accoppiamento prismatico. Il metodo determina lo spessore del meato per una assegnata distribuzione di pressione. L'equazione integrale di Volterra, che permette di determinare la distribuzione di velocitá in ogni sezione, é stata risolta numericamente. I risultati dimostrano che i termini inerziali determinano un incremento della capacitá di carico ed un decrementa della portata. Il coefficiente di attrito ne é invece debolmente influenzato. I risultati della teoria lineare sono ottenuti come soluzione asintotica.

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Nomenclature a ·L/h _e - h ₀ Q/V ₀=typical film thickness - h _e film thickness at trailing edge - L length of bearing pad - p overpressure - P ^* P/(·V ₀ ² )=u ₁ ^*2 /2 - Q flow rate - u, v velocity components - u ^* u/V ₀ - x, y axial and vertical coordinate - Re (V ₀·L/v)=Reynolds number - V ₀ pad velocity - z _* u ^*2(x ^*, ^*)+u ₁ ^*2 (x ^*) - Z ^* z ^*(x ^*+x ^*, ^*) - inclination angle of the pad - (L/h ₀)²·x ^*/Re - v kinematic viscosity - density - stream function - ^* /(h ₀ V ₀)=non dimensional stream function This research was funded both by the Italian Ministry for Education and the National Research Council (C.N.R.) of Italy     

Prediction of stagnation point heat transfer for a slot jet impinging on a concave semicylindrical surface

A systematic procedure has been laid out for assessment of fluid flow and heat transfer parameters for a slot jet impinging on a concave semicylindrical surface. Based on Walz's modifications of the Karman-Pohlhausen integral method, expressions have been derived for evaluation of the momentum thickness, boundary layer thickness and the displacement thickness at the stagnation point. The work then has been extended for the estimation of thermal boundary layer thickness and local heat transfer coefficients. A correlation has been presented for the Nusselt number at the stagnation point as a function of the Reynolds number for different non-dimensional distances from the exit plane of the jet to the impingement surface.

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Berechnung des Wärmeübergangs im Staupunkt eines Strahles, der aus einer rechteckigen öffnung auf eine konkave halbzylindrische Fläche auftrifft

Zusammenfassung Es wurde eine systematische Prozedur für die Abschätzung von Strömungs- und Wärmeübergangsparametern für einen Strahl, der auf eine konkave halbzylindrische Fläche auftrifft, aufgestellt. Basierend auf Walz's Modifikationen der Karman-Pohlhausen Integral-Methode, wurden Ausdrücke für die Berechnung der Impulsdicke, der Grenzschichtdicke und die Versetzungsdicke am Staupunkt abgeleitet. Die Arbeit wurde dann auf die Abschätzung der thermischen Grenzschichtdicke und der lokalen Wärmeübertragungskoeffizienten ausgedehnt. Es wird eine Beziehung für die Nusselt-Zahl am Staupunkt als eine Funktion der Reynolds-Zahl für verschiedene dimensionslose Abstände von der Austrittsfläche des Schlitzes bis zur Aufprallfläche aufgestellt.

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Nomenclature c _p specific heat at constant pressure - h ₀ heat transfer coefficient at the stagnation point - H distance from the exit plane of the jet to the impingement surface - k thermal conductivity - Nu _.5 Nusselt number based on impinging jet quantities =h _0.50/k - Nu _.5,0 stagnation point Nusselt number =h _{0 0.50}/k - p pressure - p _a ambient pressure - p ₀ maximum pressure or stagnation pressure - p(x) static pressure at a distancex from the stagnation point - p(x*) static pressure at nondimensional distancex* from the stagnation point - Re _J jet Reynolds number =U _J W/ - Re _0.5 Reynolds number based on impinging jet quantities =u _{m0 0.50}/ - T temperature - T* nondimensional temperature =(T–T _W)/(T _J–T _W) - T _a room temperature - T _J jet temperature - T _W wall temperature - u velocity component inx andx directions - u _m jet centerline (or maximum) free jet velocity: external (or maximum) boundary layer velocity aty = _m - u _m0 arrival velocity defined as the maximum velocity the free jet would have at the plane of impingement if the plane were not there - U _J jet exit velocity - W jet nozzle width - x* nondimensional coordinate starting at the stagnation point =x/2 _0.50 - x, y rectangular cartesian coordinates - y coordinate normal to the wall and starting at the wall - ratio of thermal to velocity boundary layer thickness = _T/ _m - ₀ ratio of thermal to velocity boundary layer thickness at the stagnation point - * inner layer displacement thickness - _.50 jet half width at the plane of impingement if the plate were not there - d_.5 free jet (half width) thickness whereu=u _m/2 - _m inner boundary layer thickness atu =u _m - _T thermal boundary layer thickness - nondimensional coordinate normal to wall =y/ _m - _T nondimensional coordinate normal to wall =y/ _T - Pohlhausen's form parameter - dynamic viscosity - kinematic viscosity = / - fluid density - momentum thickness - ₀ momentum thickness at the stagnation point     

Streamwise pseudo-vortical structures and associated vorticity in the near-wall region of a wall-bounded turbulent shear flow

Streamwise pseudo-vortical motions near the wall in a fully-developed two-dimensional turbulent channel flow are clearly visualized in the plane perpendicular to the flow direction by a sophisticated hydrogen-bubble technique. This technique utilizes partially insulated fine wires, which generate hydrogen-bubble clusters at several distances from the wall. These flow visualizations also supply quantitative data on two instantaneous velocity components, and w, as well as the streamwise vorticity, _x . The vorticity field thus obtained shows quasi-periodicity in the spanwise direction and also a double-layer structure near the wall, both of which are qualitatively in good agreement with a pseudo-vortical motion model of the viscous wall-region.List of symbols C _i ,c _i ,d _i constants in Eqs. (2), (3) and (4) - H channel width (m) - Re _H Reynolds number (= U _c H/) - Re Reynolds number (= U _c /) - T period (s) - t time (s) - U mean streamwise velocity (m/s) - U _c center-line velocity (m/s) - u friction velocity (m/s) - u, , w velocity fluctuations (m/s) - x, y, z coordinates (m) - ^* displacement thickness (m) - momentum thickness (m) - mean low-speed streak spacing (m) - kinematic viscosity (m²/s) - phase difference - _x streamwise vorticity fluctuation (1/s) - ( )⁺ normalized by u and - (^–) root mean square value - (^–) statistical average This paper was presented at the Ninth Symposium on Turbulence, University of Missouri-Rolla, October 1–3, 1984     

New local measurements of hydrodynamics in porous media

In this paper, a measurement system is used to carry out local hydrodynamic measurements at the pore scale of a fixed-bed reactor. It consists of four microelectrodes placed on the inner wall of four spheres mounted in tetrahedral form, thus constituting a pore of the fixed bed. Three flow regimes (laminar, inertial and turbulent-like) are identified by the analysis of the signal fluctuations (velocity gradient). The flow structures are characterised by means of the correlation (auto- and cross-) function analysis, and a closure equation required in modelling and simulation is suggested.List of symbols A_e effective area of electrode, m² - C_L concentration, mol/m³ - C_xx auto-correlation function of signal x - d_ip distance between two probes, m - d_m average structure dimension, m - d_M maximum structure size, m - D diffusion coefficient, m²/s - d_e electrode diameter, m - f frequency, s^–1 - F Faraday constant, 96,500 C/equi - F_D(L–S) liquid–solid momentum exchange term, kg/m²/s² - g gravity, 9.81 m/s² - H transfer function - I limiting current, A - L liquid flow rate, kg/m²/s - P static pressure, Pa - P_xx Power spectral density of signal x - Re_p , particle Reynolds number - S velocity gradient, s^–1 - t time, s - T_c integral coherence time, s - u_L liquid phase velocity, m/s - u_L fluctuation of liquid phase velocity, m/sGreek symbols _L fluid volume fraction - porosity - liquid kinematic viscosity, Pa s - _e number of electrons involved in the electrochemical reaction - _L liquid phase specific gravity, kg/m³ - ₀ local liquid phase specific gravity, kg/m³ - time lag (in correlation functions), s - _c coherence time, s - tortuosity - dimensionless frequency