Measurement of turbulent diffusion field behind a line heat source in a homogeneous shear flow

Turbulent diffusion behind a line heat source immersed in a nearly homogeneous shear flow is investigated. Test section of the wind tunnel is divided into 15 equal width channels equipped with adjustable internal flow resistances for the production of homogeneous shear flow. A combination of X-type hot wire for the measurement of velocity fluctuations and a cold wire for the measurement of temperature fluctuations has been devised to simultaneously obtain the cross correlations between the velocity and temperature fluctuations. The results show that all the distributions of velocity and scalar field are shifted toward the low velocity region of the homogeneous shear flow. The integral scales for velocity and scalar are different from each other in the shape of profiles so that the time scale ratio R does not remain constant. And the distributions of mean and fluctuating temperatures collapsed into a Gaussian curve by normalizing with mean half width and peak temperature.     

Air velocity and flow measurement using a Pitot tube

The accurate measurement of both air velocity and volumetric airflow can be accomplished using a Pitot tube, a differential pressure transducer, and a computer system which includes the necessary hardware and software to convert the raw transducer signals into the proper engineering units. The incorporation of sensors to measure the air temperature, barometric pressure, and relative humidity can further increase the accuracy of the velocity and flow measurements. The Pitot tube measures air velocity directly by means of a pressure transducer which generates an electrical signal which is proportional to the difference between the pressure generated by the total pressure and the still air (static pressure). The volumetric flow is then calculated by measuring the average velocity of an air stream passing through a passage of a known diameter. When measuring volumetric flow, the ‘passage of a known diameter’ must be designed to reduce air turbulence as the air mass flows over the Pitot tube. Also, the placement of the pitot tube in the passage will influence how accurately the measured flow tracks the actual flow through the passage. Calibrating the measurement system in a wind tunnel can further increase the accuracy of the velocity and the flow measurements. This objective of this paper is to provide the field engineer with single, concise source of information on flow measurement using a Pitot tube.     

冷却塔风筒内流速场分布的研究

在分析冷却塔构成的基础上，建立了描述冷却冷却塔风筒内流场分布的数学模型，进而利用MATLAB语言求出了偏微分方程组的数值解，绘出了风筒内的流场。经研究发现，在风筒的中心处，存在一个空气倒吸的区域。最后，提出了消除空气倒吸现象的措施。     

Experimental assessment on the performance of hot wire anemometry in and around a permeable medium by comparison with Particle Image Velocimetry

The air flow through a test section partially obstructed by a permeable array of wires was measured simultaneously by Hot Wire Anemometry (HWA) and Particle Image Velocimetry. The objective of the study was the assessment of the suitability of HWA for the measurement of flow velocities amid and adjacent to groups of small obstacles. In the present case the obstacles are set in a regular array configuring a highly permeable structure. The probe was placed at three characteristic positions: in the free flow close to the wire array, inside the permeable medium, and at the interface between the permeable structure and the free flow. The measurements were performed with the hot wire operating under natural convection and mixed convection heat transfer, and operating the hot wire at different overheat ratios. Natural convection plumes extending over several permeable volume elements were detected when the hot wire was under natural convection, in some cases reaching velocities up to 60 mm/s downstream from the hot wire position. For low velocity flows, natural convection can be regarded as a flow velocity offset, which becomes negligible at local velocities higher than 0.03 m/s. For higher velocities, in the mixed convection regime, the intrusivity of the HWA probe becomes relevant. Furthermore, the flow in the test section used in the study presents a linear instability that produces velocity fluctuations. Availing ourselves of this phenomenon we verified the dynamic response of the HWA at the lowest velocity where the flow shows periodic fluctuations; for a local mean velocity of (0.131 ± 0.012) m/s the HWA showed a satisfactory dynamic response up to 20 Hz.     

Flow and deposition measurement of foam beads in a closed re-circulating wind tunnel for snowdrift modelling

A facility was created for flow and deposition measurement of foam beads (Expanded Polystyrene) in a closed re-circulating wind tunnel to model the snowdrift experimentally. The geometrical and flow similarities between the model (foam beads) and prototype (snow) particles were established for the wind tunnel experiments. The wind tunnel was instrumented with the electronic regulator, anemometer and newly developed flux measuring sensors along with a data logger to carry out the experiments for different simulated terrain conditions. An array of optical sensors for measurement of foam beads flux was calibrated with the known quantity of the material and used to find the flux variation with wind speed inside the tunnel. All three modes of drifting transportation namely creeping, saltation and suspension could be observed through the transparent acrylic wall for data collection. The threshold velocity of foam bead movement was 4.05 m/s in the horizontal section and same was 2.7 m/s in the inclined section at an angle of 30° downward (a critical angle of avalanche starting zone). The process of snow deposition by the snow fence was simulated with the foam beads deposition around the scaled model (1:50) of a snow fence in the wind tunnel. The experiments helped to find a flow measuring technique for light drifting material like foam beads and the modelling of drifting snow.     

三叶片直线翼垂直轴风力机非定常流场速度分析

垂直轴风力机在回转过程中,叶片尾流的相互干涉和叶片攻角变化,使垂直轴风力机周围流场异常复杂。为探明直线翼垂直轴风力机在二维流场中速度分布及风力机叶片迎风角度变化关系,在风洞试验中采用激光多普勒测速仪(Laser Doppler velocimetry,LDV)技术,对所设计的三叶片直线翼垂直轴风力机流场风速进行试验研究,获得了该风力机叶片周围流场的速度分布情况。在建立直线翼垂直轴风力机在不同转速下叶片迎风角度变化的数学模型基础上,应用仿真软件对被测风力机流场进行分析计算。通过数学模型得知,来流风速夹角随回转角的变化情况可用正弦函数近似表示, 并且随着叶尖速比的增大逐渐减小。风洞试验和CFD结果表明,风力机在回转过程中,叶片前缘场域有乱流生成,并且该域风速值偏大;而在叶片旋转内部以及下流区域内会形成一个宽大的低速区域,并且伴随叶尖速比的增加,低速区域具有扩大趋势。     

A detailed experimental airfoil performance investigation using an equipped wind tunnel

An experimental study is conducted to investigate the impact of different parameters on the performance of CFJ0025-065-196 airfoil. For this purpose, an equipped wind tunnel with the motor capacity of 7 kW as well as high-tech measuring instruments are employed, and the impact of attack angle on the average velocity profiles for different jet flow velocities, in addition to the self-similar velocity profiles for different attack angles, and the wake effect are found and discussed comprehensively. The experiments are done for five different values of the attack angle, which are 0, 5, 12, 20, and 25° as well three magnitude for the jet flow velocity, including 0, 19.7, and 32.2 m s⁻¹. According to the results, the attack angle in which the performance of the airfoil decreases has an upward trend when the jet flow velocity increases, and it reaches from 20 to 25° when jet flow velocity changes from 0 to 19.7 m s⁻¹. Moreover, increasing the attack angle from 0 to 12° makes the jet frountier wider whereas a downward trend happens by chaning the attack angle from 12 to 25°. Additionally, the higher the jet velocity is, the weaker wake stream has.     

Measurements of wall shear stress in horizontal air–water bubbly flows

The mean and time-varying fluctuation property of local wall shear stress of horizontal air–water bubbly flows in a circular pipe of 35 mm I.D. is measured using a TSI-1268W hot film probe. Data are collected in both entrance and developed regions of the flows. The variation of wall shear stress with L/D is analyzed, and the entrance length is determined to be 52–65 D at present studies. It is found that the wall shear stress is not uniform around the pipe circumference due to the asymmetrical phase distribution in the flows. The mean shear stress tends to decrease circumferentially from the pipe bottom to top. An increase of air flow rate at a constant water flow rate would further lower the wall shear stress at the upper part of the pipe and at the same time raise the wall shear stress at lower part of the pipe in both entrance and developed regions. An increase of water flow rate at a constant air flow rate would result in an increase of wall shear stress at all circumferential positions. The statistical property of wall shear stress is also discussed.     

Effect of the pipe bend of the morning glory spillway on the cavitation number

Cavitation phenomena always threats the hydraulic structures with high velocity flow and suddenly varying the flow direction. It is assumed that the process of the flow depleting with high ratio through the morning glory spillway accompanied by suddenly varying the flow direction caused by the bend pipe can deteriorate the condition of the cavitation. Introducing the ratio R/D (diameter size of the vertical and horizontal shaft divided with radius of the bend pipe) can be beneficial to propose the novel design plan to avoid the cavitation caused by the bend pipe. Present study carried out a series experimental tests to validate a numerical model of the morning glory spillway with different R/D ratio to show the impacts of the suddenly varying the flow direction on the variation of the cavitation number. The cavitation values of the free, semi-submerged and submerged flow conditions showed that cavitation was mainly occurred on the internal arch of the bend pipe for free flow and the level of the danger for semi-submerged flow was the major danger level for internal arch of the bend pipe and due to submerged flow, the crest level at the entrance of the morning glory spillway was additionally threated by cavitation. To prevent from the cavitation, the values of the cavitation number due to different R/D as 0.55, 0.45, 0.35, and 0.25 were calculated by employing the validated numerical model due to high danger of the cavitation in semi-submerged condition. Results showed that due to D/R > 0.35, the possible cavitation damage is obtained for the internal and external arch of the bend pipe of the morning glory spillway; however, for D/R < 0.25, the bend pipe position is located in safe zone and the danger of the cavitation is negligible.     

PSV and marker detection techniques for investigating the wake of an oscillating cylinder in a large wind tunnel

Although Particle Image Velocimetry techniques are widely used in the experimental investigation of the flow structure around obstacles, the application of these techniques in wind tunnels is frequently limited by the dimension of the areas involved, which make the image acquisition set-up expensive.This paper examines the use of Particle Streak Velocimetry (PSV) and marker detection techniques to measure the flow field around bodies with relatively large dimensions. In spite of its minimal requirements in terms of acquisition set-up, the method permits the investigation of complex phenomena.The tests described in the paper were carried out in air using a rigid cylinder, free to oscillate in a wind tunnel. Coupling PSV and marker detection techniques with the phase average of the measured velocity fields, the evolution of vortex shedding behind a cylinder with a diameter of 200 mm was measured with an acquisition area of about 1000×1200 mm.     

Vortical flows over a delta wing at high angles of attack

The vortex flow characteristics of a sharp-edged delta wing at high angles of attack were studied using a computational technique. Three dimensional, compressible Reynolds-averaged Navier-Stokes equations were solved to understand the effects of the angle of yaw, angle of attack, and free stream velocity on the development and interaction of vortices and the relationship between suction pressure distributions and vortex flow characteristics. The present computations gave qualitatively reasonable predictions of vortical flows over a delta wing, compared with past wind tunnel measurements. With an increase in the angle of yaw, the symmetry of the pair of leading edge vortices was broken and the vortex strength was decreased on both windward and leeward sides. An increase in the free stream velocity resulted in stronger leading edge vortices with an outboard movement.     

直流低速风洞流场特性研究

对一直流低速风洞的试验段流场品质进行实验研究,利用IFA300热膜风速仪测量了风洞试验段的流场,获得试验段两截面中心线上速度和湍流度分布,结果表明,该风洞具有较好的流场均匀性和稳定性,具备了开展横流中的紊动射流实验研究的基本条件。     

Development of an air velocity and flow measurement system by using a novel circular disc

In this paper, development of a sensor using a circular disc for air velocity measurement based on the drag force equation is proposed. The air velocity measurement sensor is basically designed with a load cell in order to determine the drag force. The circular disc is used for creating a drag force, and by using load cell, the drag force that acts on the circular disc is measured. As the circular disc's drag coefficient can be considered constant at Reynolds numbers between 10³ and 10⁶, it can be possible to obtain the explicit equation of drag force. The remaining components of drag force equation are obtained by measurement. The proposed air velocity measurement sensor is characterized by wind tunnel measurements. All measurements were performed in an ISO/IEC 17025 accredited calibration laboratory – the Wind Tunnel of Turkish State Meteorological Service. The characterization measurements were performed at air velocities between 1 m/s and 20 m/s. The correction factors were calculated and a calibration curve for the proposed air velocity measurement sensor was obtained. The calibration curve's linearity was higher than 0.99 and a comparison the results from a Micromanometer with Pitot-Tube shows that for the designed working range, the sensor has an acceptable performance for time-averaged air velocity measurements according to the requirements of the World Meteorological Organization.     

Flow rate measurement in flows with asymmetric velocity profiles by means of distributed thermal anemometry

Flow rate in closed conduits is one of the most frequently measured parameters in industrial processes and in gas and water supply. For an accurate measurement, flow meters typically require a fully developed symmetric flow profile with preferably no radial or tangential velocity components. This is commonly secured by mounting flow meters in a pipe at a sufficiently long distance downstream any change in cross-section or pipe direction. In this paper, we introduce a new approach for flow rate measurement of gases or liquids that employs a novel spatially resolving fluid velocity sensor basing on thermal anemometry. The new principle allows accurate flow rate measurements for non-axisymmetric velocity profiles, even directly after pipe bends, T-junctions or other alterations in the pipe geometry. This is exemplified for air flow in three different pipe bend configurations.     

不同喷射速度下厢体气膜减阻效果的实验研究

研究厢体表面渗透空气流的喷射速度对气膜在空气中的减阻效果的影响。通过气膜发生系统控制有气膜厢式车实验模型的气膜空气喷射速度，在30 m/s的流动空气速度下，于可调速拟层流风力发生装置中开展与无气膜厢式车实验模型的对比测阻实验，比较不同喷射速度下，有气膜厢式车实验模型的受阻情况。实验结果表明，在流速恒定的拟层流空气流体环境下，厢体表面渗透空气流有效地降低了厢体所受的空气阻力，且在一定范围内，减阻率随喷射速度的增大近似呈线性增长。     

三维超声波换能器测风阵列研究

针对三维测风技术中常用的垂直正交测风阵列在消除阴影效应影响方面的缺陷,根据流场特性,设计了一种基于特殊四面体棱边构建的非正交测风阵列。该阵列旨在降低尾迹区湍流对测风路径平均风速的影响,并且对绕流干扰严重的测风路径进行数据补偿。利用GAMBIT软件建立测风阵列模型,通过FLUENT软件改变流场内气体流速和雷诺数等环境参数,仿真两种阵列在低速层流区和高速湍流区的不同性能,获取了速度分布云图和三组换能器所在路径速度折线图,说明了非正交测风阵列能够有效提高三维测风的精度。     

An experimental study on the performance of air/water direct contact air conditioning system

Direct contact air conditioning systems, in which heat and mass are transferred directly between air and water droplets, have many advantages over conventional indirect contact systems. The purpose of this research is to investigate the cooling and heating performances of direct contact air conditioning system for various inlet parameters such as air velocity, air temperature, water flow rate and water temperature. The experimental apparatus comprises a wind tunnel, water spray system, scrubber, demister, heater, refrigerator, flow and temperature controller, and data acquisition system. The inlet and outlet conditions of air and water are measured when the air contacts directly with water droplets as a counter flow in the spray section of the wind tunnel, and the heat and mass transfer rates between air and water are calculated. The droplet size of the water sprays is also measured using a Malvern Particle Analyzer. In the cooling conditions, the outlet air temperature and humidity ratio decrease as the water flow rate increases and as the water temperature, air velocity and temperature decrease. On the contrary, the outlet air temperature and humidity ratio increase in the heating conditions as the water flow rate and temperature increase and as the air velocity decreases.     

Probe and method for simultaneous measurements of 'true' instantaneous temperature and three velocity components in turbulent flow

In the measurement of turbulent flows the need has always existed to obtain correct instantaneous values of temperature and three components of velocity at a particular point. Many proposed lengthy approximate correction methods that attempt to account for 'nonlinear' effects (cross contamination between different quantities) in hot-wire measurements cannot be considered satisfactory. The availability of powerful digital computers for theoretical and experimental studies has placed pressure on experimentalists to develop better probes and methods. This paper attempts to answer these pressures by describing the development of a special four-wire probe and a method of processing the obtained signals. The use of four 0.625-microm-diam sensors makes the probe practically interference free. The processing method is based on the simultaneous solution of four complete nonlinear response equations for the sensors, yielding in principle exact instantaneous values of velocity components and temperature. Additional features of the processing method include: instantaneous full correction for tunnel free stream velocity and temperature fluctuations, first-order correction for dc drifts of the signals during data acquisition, subtraction of all 60-Hz related noise, and correction for streamwise displacement of sensors based on instantaneous streamwise velocity.     

Effects of Schmidt number on near-wall turbulent mass transfer in pipe flow

Large Eddy simulation (LES) of turbulent mass transfer in circular-pipe flow has been performed to investigate the characteristics of turbulent mass transfer in the near-wall region. We consider a fully-developed turbulent pipe flow with a constant wall concentration. The Reynolds number under consideration is Re _τ = 500 based on the friction velocity and the pipe radius, and the selected Schmidt numbers (Sc) are 0.71, 5, 10, 20 and 100. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and turbulent mass fluxes were employed to close the governing equations. The current paper reports a comprehensive characterization of turbulent mass transfer in circular-pipe flow, focusing on its near-wall characteristics and Sc dependency. We start with mean fields by presenting mean velocity and concentration profiles, mean Sherwood numbers and mean mass transfer coefficients for the selected values of the parameters. After that, we present the characteristics of fluctuations including root-mean-square (rms) profiles of velocity, concentration, and mass transfer coefficient fluctuations. Turbulent mass fluxes and correlations between velocity and concentration fluctuations are also discussed. The near-wall behaviour of turbulent diffusivity and turbulent Schmidt number is shown, and other authors’ correlations on their limiting behaviour towards the pipe wall are evaluated based on our LES results. The intermittent characteristics of turbulent mass transfer in pipe flow are depicted by probability density functions (pdf) of velocity and concentration fluctuations; joint pdfs between them are also presented. Instantaneous snapshots of velocity and concentration fluctuations are shown to supplement our discussion on the turbulence statistics. Finally, we report the results of octant analysis and budget calculation of concentration variance to clarify Sc-dependency of the correlation between near-wall turbulence structures and concentration fluctuation in the vicinity of the pipe wall.