A crossed hot-wire technique for complex turbulent flows

This paper describes a crossed hot-wire technique for the measurement of all components of mean velocity, Reynolds stresses, and triple products in a complex turbulent flow. The accuracy of various assumptions usually implicit in the use of crossed hot-wire anemometers is examined. It is shown that significant errors can result in flow with gradients in mean velocity or Reynolds stress, but that a first order correction for these errors can be made using available data. It is also shown how corrections can be made for high turbulence levels using available data.     

Calibration of hot-wire probes using non-uniform mean velocity profiles

A simple iterative procedure is illustrated here to overcome two problems frequently encountered in hot-wire anemometry: (1) the calibration of hot wires at low velocity and (2) to account for the wall-proximity correction in the actual measurements. The low-velocity calibration method used an iterative correction procedure based on laminar pipe velocity profiles. It is shown that the use of this approach, together with a simple wall proximity correction procedure, provides a simple and accurate calibration of both single and cross hot wires, and gives reliable measurements.     

4D Magnetic resonance velocimetry for mean velocity measurements in complex turbulent flows

An adaptation of a medical magnetic resonance imaging system to the noninvasive measurement of three-component mean velocity fields in complex turbulent engineering flows is described. The aim of this paper is to evaluate the capabilities of the technique with respect to its accuracy, time efficiency and applicability as a design tool for complex turbulent internal geometries. The technique, called 4D magnetic resonance velocimetry (4D-MRV), is used to measure the mean flow in fully developed low-Reynolds number turbulent pipe flow, Re=6400 based on bulk mean velocity and diameter, and in a model of a gas turbine blade internal cooling geometry with four serpentine passages, Re=10,000 and 15,000 based on bulk mean velocity and hydraulic diameter. 4D-MRV is capable of completing full-field measurements in three-dimensional volumes with sizes on the order of the magnet bore diameter in less than one hour. Such measurements can include over 2 million independent mean velocity vectors. Velocities measured in round pipe flow agreed with previous experimental results to within 10%. In the turbulent cooling passage flow, the average flow rates calculated from the 4D-MRV velocity profiles agreed with ultrasonic flowmeter measurements to within 7%. The measurements lend excellent qualitative insight into flow structures even in the highly complex 180° bends. Accurate quantitative measurements were obtained throughout the Re=10,000 flow and in the Re=15,000 flow except in the most complex regions, areas just downstream of high-speed bends, where velocities and velocity fluctuations exceeded MRV capabilities for the chosen set of scan parameters. General guidelines for choosing scanning parameters and suggestions for future development are presented.     

A hot-wire probe configuration and data reduction method to minimize velocity gradient errors for simultaneous measurement of three velocity components in turbulent flows

A highly resolved turbulent channel flow direct numerical simulation (DNS) with Re _?? ?=?200 has been used to investigate the influence of the velocity gradients on the measurement accuracy of a hot-wire probe capable of measuring all three velocity components simultaneously. A new proposed sensor arrangement has been tested. First, the effective cooling velocity was determined for each sensor of the idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, velocity component statistics were calculated, neglecting the velocity gradients over the probe sensing area, and they were compared to the DNS database values. It has been shown that the influence of the velocity gradients on the new proposed arrangement is minimized. Its accuracy was compared to existing three- and four-sensor configurations as well as to two-sensor X- and V-array probes.     

A swinging arm calibration method for low velocity hot-wire probe calibration

This paper describes a swinging arm calibration facility for the calibration of hot-wire probes at low velocities. During the motion of the probe the anemometer signal is recorded digitally using pre-set angle positions for the start and stop of the data acquisition. The evaluation method described in this paper is based on an integral procedure over a fixed, known probe path length. Tests in the velocity range 1 to 6 m/s demonstrated a high degree of repeatability of the calibration results. Furthermore, the test facilities can easily be adjusted for other low velocity ranges.     

An improved diffuse interface method for three-dimensional multiphase flows with complex interface deformation

An improved diffuse interface (DI) method is proposed for accurately capturing complex interface deformation in simulations of three-dimensional (3D) multiphase flows. In original DI methods, the unphysical phenomenon of interface thickening or blurring can affect the accuracy of numerical simulations, especially for flows with large density ratio and high Reynolds number. To remove this drawback, in this article, an interface-compression term is introduced into the Cahn-Hilliard equation to suppress the interface dispersion. The additional term only takes effect in the interface region and works normal to the interface. The difference of the current method from the previous work is that the compression rate can be adjusted synchronously according to the magnitude of local vorticity, which is strongly correlated to the interface dispersion and changes with the computational time and interface position. Numerical validations of the proposed method are implemented by simulating problems of Laplace law, Rayleigh-Taylor instability, bubble rising in a channel, and binary droplet collision. The obtained results agree well with the analytical solutions and published data. The numerical results show that the phenomenon of interface dispersion is suppressed effectively and the tiny interfacial structures in flow field can be captured accurately.     

A control volume finite element method for three-dimensional three-phase flows

A novel control volume finite element method with adaptive anisotropic unstructured meshes is presented for three-dimensional three-phase flows with interfacial tension. The numerical framework consists of a mixed control volume and finite element formulation with a new P₁DG-P₂ elements (linear discontinuous velocity between elements and quadratic continuous pressure between elements). A “volume of fluid” type method is used for the interface capturing, which is based on compressive control volume advection and second-order finite element methods. A force-balanced continuum surface force model is employed for the interfacial tension on unstructured meshes. The interfacial tension coefficient decomposition method is also used to deal with interfacial tension pairings between different phases. Numerical examples of benchmark tests and the dynamics of three-dimensional three-phase rising bubble, and droplet impact are presented. The results are compared with the analytical solutions and previously published experimental data, demonstrating the capability of the present method.     

A new method of visualizing liquid flows and measuring velocity fields

A method of visualizing and measuring the velocity field of a liquid flow proposed at the Institute of Mechanics at the Moscow State University in 1967 [3] is discussed. It consists of creating and measuring the size of an artificial cavity behind an extended cavity forming body with small transverse dimension (cavitation probe) placed across the flow at a given position. Because the transverse dimension of the probe is small, the flow deformation is slight.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 101–108, March–April, 1982.     

A method of determining complex moduli of viscoelastic materials

A method is introduced whereby the complex moduli of viscoelastic materials may be determined in a relatively simple and accurate manner by means of calibration of the measuring system using a specimen of known properties. The appropriate data-reduction equations are presented and use of the method is demonstrated for determination of complex moduli for bovine bones over a four-octave frequency range.     

A simple method for determining principal-stress directions in embedded-polariscope models

A disadvantage of the embedded-polariscope method is the inability to observe isoclinics because the embedded Polaroid sheets are fixed within the model. Perforating the embedded active layer with an array of small-diameter holes allows observation of stress trajectories in the layer from the isochromatic patterns. Further, it is shown that the full-field fringes are little disturbed by the presence of the small perforations. The techniques of model fabrication are described, as well as an extension of the method to reveal bending-stress trajectories in plates loaded out of plane.     

A simple method for simulating viscoelastic fluid flows in slit channel

A low-cost semi-analysis finite element technique, named the finite piece method (FPM) is presented in this article. It aims to solve three-dimensional (3D) viscoelastic slit flows. The viscoelastic stress of the fluid is modelled using an K-BKZ integral constitutive equation of the Wagner type. Picard iteration is used to solve non-linear equations. The FPM is tested on flow problems in both planar and contraction channels. The accuracy of the method is assessed by comparing flow distributions and pressure with results obtained by 3D finite element method (FEM). It shows that the solution accuracy is excellent and a substantial amount of computing time and memory requirement can be saved.     

A simple method of evaluating the complex moduli of polystyrene blends

In the search for a workable mixing rule, use was made of experimental data for complex moduli of melts of narrow molar mass distribution polystyrenes and their homogeneous blends. In the course of this work two basic observations were made as to the nature of the relaxation time spectra of these blends:

1. The relaxation strength (a product of the weight fraction and the plateau modulus) of a component of large molecules is reduced by the presence of shorter molecules, the latter molecules acting like ordinary diluent molecules even if their molar masses are larger thenM _c .
2. The relaxation time of a molecule (known from measurements on the respective monodisperse component) is considerably changed by the blending. The width of the distribution of relaxation times, as expected from the known composition of the blend, is significantly reduced.

For both processes approximate empirical equations could be found. It turned out that, after the application of the required modifications, the complex moduli of the components could successfully be added in order to obtain the complex moduli of the blend at circular frequencies characteristic for the flow and rubber transition regions. On the basis of these results one may expect that for the melt of any linear polymer the linear viscoelastic properties can be evaluated with reasonable accuracy from the knowledge of the molar mass distribution.     

A method to estimate turbulence intensity and transverse Taylor microscale in turbulent flows from spatially averaged hot-wire data

A new approach to evaluate turbulence intensity and transverse Taylor microscale in turbulent flows is presented. The method is based on a correction scheme that compensates for probe resolution effects and is applied by combining the response of two single hot-wire sensors with different wire lengths. Even though the technique, when compared to other correction schemes, requires two independent measurements, it provides, for the same data, an estimate of the spanwise Taylor microscale. The method is here applied to streamwise turbulence intensity distributions of turbulent boundary layer flows but it is applicable generally in any turbulent flow. The technique has been firstly validated against spatially averaged DNS data of a zero pressure-gradient turbulent boundary layer showing a good capacity to reconstruct the actual profiles and to predict a qualitatively correct and quantitatively agreeing transverse Taylor microscale over the entire height of the boundary layer. Finally, the proposed method has been applied to available higher Reynolds number data from recent boundary layer experiments where an estimation of the turbulence intensity and of the Taylor microscale has been performed.     

Use of hot-wire anemometry for measuring the nanopowder flow velocity

A new technique for measuring the flow velocity of nano-scale powders is used. The hot-wire anemometry method widely used in gas flows is employed for investigating nanopowder flows. By way of illustration, the flows of nanopowders of aluminum oxide C and silicon dioxide aerosil A-90 and A-380 in a vertical channel are studied. The results obtained show that nanoscale powder flow investigation by means of the hot-wire anemometry is promising.     

On the accuracy of simultaneously measuring velocity component statistics in turbulent wall flows with arrays of three or four hot-wire sensors

A highly resolved turbulent channel flow direct numerical simulation (DNS) with Re _τ = 200 has been used to investigate the ability of probes made up of arrays of three or four hot-wire sensors to simultaneously and accurately measure statistics of all three velocity components in turbulent wall flows. Various virtual sensor arrangements have been tested in order to study the effects of position, number of sensors and spatial resolution on the measurements. First, the effective cooling velocity was determined for each sensor of an idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, simulating the response of the virtual probes to obtain the effective velocities cooling the sensors, velocity component statistics have been calculated neglecting the velocity gradients over the probe sensing area. A strong influence of both mean and fluctuation velocity gradients on measurement accuracy was found. A new three-sensor array configuration designed to minimize the influence of the velocity gradients is proposed, and its accuracy is compared to two-sensor X- and V-array configurations.     

A simple pendulum technique for the calibration of hot-wire anemometers over low-velocity ranges

A method for low velocity calibration of hot-wire anemometers, in which the probe is mounted on a pendulum arm, is presented. The calibration constants are determined from recorded traces of the anemometer signal obtained in the forward swing of the pendulum with the probe mounted at two different radii along the arm. Typical calibration results are presented and the use of a modified King's law in the low Reynolds number range is discussed.List of Symbols A, B, n calibration constants in King's law - E anemometer voltage output - E ₀ anemometer voltage output for zero flow velocity - Re Reynolds number - r radius along the pendulum arm - T period of oscillation of the pendulum arm - t time - U velocity - y ⁺ non-dimensional distance from the wall Greek Symbols root mean square of the velocity difference in half the forward swing - angular position of pendulum - angular velocity of pendulum arm Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and by the Atomic Energy of Canada Limited.     

An oscillating hot-wire technique for resolving the magnitude and direction of velocity measurements using single hot-wire sensors

Thermal anemometry is a classic flow-velocity measurement technique that is known to suffer from the inability to discern the flow direction. The current paper describes an innovative approach whereby an oscillating hot wire is used to extract velocity direction and magnitude information from single hot-wire measurements. It is shown that the new sensor operates in one of two modes depending on the velocity amplitude of the wire oscillation. Furthermore, results from applying the technique to measure the phase-averaged velocity in an oscillating pipe flow experiment are presented. The results from the hot-wire measurements show good agreement with those from laser Doppler velocimetery measurements in the same facility.     

Comment on the Clauser chart method for determining the friction velocity

A known difficulty with using the Clauser chart method to determine the friction velocity in wall bounded flows is that it assumes, a priori, a logarithmic law for the mean velocity profile. Using both experimental and DNS data in the literature, this note explicitly shows how friction velocities obtained using the Clauser chart method can potentially mask subtle Reynolds-number-dependent behavior.

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简支梁中截面挠度计算的一种简易方法

介绍了一种首先利用简支梁的对称性找出等价悬臂梁, 然后利用叠加法求出等效悬臂梁自由端的挠度, 从而得到一般载荷作用下简支梁中间截面挠度的方法.