In-situ swinging arm calibration for hot-film anemometers

A method of calibration for hot-film anemometers is presented. A swinging arm that moves under the influence of gravity serves as both a calibration mechanism and a probe support. The velocity of the probe is found by differentiating the angular position history of the arm and multiplying it with the arm length. Limitations on the quality of calibration data while the arm is accelerating are discussed. The hot film voltage output is then matched to the velocity to find the two constants in King's law. The calibration was tested by taking velocity profile measurements in a laminar boundary layer. The results of these compared well to the Blasius profile.     

Computer-aided calibration and measurements with a quadruple hotwire probe

A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like King's law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988     

In situ calibration of hot wire probes in turbulent flows

A method for in situ calibration of hot-wires in a turbulent flow is presented. The method is particularly convenient (even necessary) for calibrating large probe arrays, like the 143-wire boundary layer rake of the WALLTURB experiment. It is based on polynomial expansion of the velocity statistics in terms of voltage statistics as originally described by George et al. [Exp Ther Fluid Sci 2(2):230–235, 1989]. Application of the method requires knowing reference mean velocity and higher order central moments (with the array in place) of the turbulent velocity at the probe location at only one freestream velocity. These were obtained in our experiment by a stereo PIV plane just upstream of the probe array. Both the procedure for implementing the method and sample results are presented in the article.     

The calibration of (multi-)hot-wire probes. 2. Velocity-calibration

We review a set of velocity calibration methods for one popular configuration of a four-hot-wire probe with the aim of finding a method of estimating with 10% accuracy (based on full scale) the mean flow vector, the rms of the turbulent velocity component and the associated linearized anisotropy invariants. We performed full 3D velocity calibrations of a classic and a sub-miniature probe. With a calibrated King/Jørgensen model and with an interpolation scheme in the real calibrations, we simulated calibrations and (anisotropic) turbulence measurements. Analysis of the idealized and of the realistic simulation data showed that the lookup-table method is the only good tool for processing hot-wire measurements. Polynomial models give large systematic errors. The King/Jørgensen model can only be used for qualitative ends, but it forms a good basis for a simulation-based assessment of the quality of a measured calibration set. Our results indicate that the experimental results reported in the literature, and which were based on hot-wire measurements, may require a reassessment of the calibration method.     

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.     

Multiple hot-film sensor array calibration and skin friction measurement

A method for the calibration of closely spaced multiple hot-film sensor (MHFS) arrays of nickel-on-polyimide-substrate type was presented in this paper. The hot-film sensors were calibrated individually and in situ against the values of the wall-shear stresses determined from boundary-layer velocity profiles (developed on the front portion of a circular cylinder) through a specially designed wall-mounted hot-wire probe. The accuracy and feasibility of the present method was examined through the subsequent measurements of the skin-friction distributions on a circular cylinder (including the removal and bonding of the calibrated MHFS from a 10-cm diameter calibration cylinder and onto the surface of a 8-cm diameter test cylinder) and comparison of their consistency with the published results. The present MHFS calibration method is important in the non-intrusive multipoint measurements of the mean skin friction along the surfaces of two-dimensional aerodynamic objects of interest.     

A calibration technique for multiple-sensor hot-wire probes and its application to vorticity measurements in the wake of a circular cylinder

A calibration technique for multiple-sensor hot-wire probes is presented. The technique, which requires minimal information about the probe geometry, is tested using a four-sensor and a twelve-sensor probe. Two data reduction algorithms are introduced. The first one assumes a uniform velocity over the probe sensing-volume and is applied to the four-sensor probe measurements. The second one assumes a uniform velocity gradient over the sensing volume of the probe. The procedure, when applied to the twelve-sensor probe, is shown to measure the velocity gradient components successfully. In both algorithms, the unknowns (velocity and velocity gradient components) are obtained by solving the resulting systems of nonlinear algebraic equations in a least-squares sense. The performances of the probes and the algorithms are tested with measurements in the wake of a circular cylinder. The statistics and spectra show that the twelve-sensor probe is successful in the simultaneous measurement of all three components of the velocity and all three components of the vorticity vectors.     

Influence of velocity profile on calibration function of Lorentz force flowmeter

A Lorentz force flowmeter is a noncontact electromagnetic flow-measuring device based on exposing a flowing electrically conducting liquid to a magnetic field and measuring the force acting on the magnet system. The measured Lorentz force is proportional to the flow rate via a calibration coefficient which depends on the velocity distribution and magnetic field in liquid. In this paper, the influence of different velocity profiles on the calibration coefficient is investigated by using numerical simulations. The Lorentz forces are computed for laminar flows in closed and open rectangular channels, and the results are compared with the simplified case of a solid conductor moving at a constant velocity. The numerical computations demonstrate that calibration coefficients for solid bodies are always higher than for liquid metals. Moreover, it can be found that for some parameters the solid-body calibration coefficient is almost twice as high as for a liquid metal. These differences are explained by analyzing the patterns of the induced eddy currents and the spatial distributions of the Lorentz force density. The result provides a first step for evaluating the influence of the laminar velocity profiles on the calibration function of a Lorentz force flowmeter.     

七孔探针可压缩流场测量研究

介绍了七孔探针用于亚音速可压缩流的标定方法。作为一种可以同时获得流动速度大小、流动偏角、总压和静压的气动测量装置,七孔探针被广泛应用于各种流动测量,包括可压缩流动。但是它的校准过程周期很长,代价昂贵,影响了探针的推广。本文以数值计算为手段,对七孔探针进行亚音速可压缩流校准与测量的研究。结果表明,其校准拟合精度流动角为2%,内外区的总静压相对标准偏差都没有超过3%,高于相同状态下的实验校准精度。在实际应用中,本方法用于指导传统实验标定方法,可以节约大量的标定时间和成本,使七孔探针在亚音速可压缩流的测量变得简单可行。     

The calibration of (multi-) hot-wire probes. 1. Temperature calibration

We study the performance of the classical relation for the correction for ambient temperature drift of the signal of a hot-wire anemometer and the influence of practical assumptions. It is shown that most methods to estimate the operational temperature via the temperature/resistance coefficient lead to underestimation of the operational temperature and thus to overcorrection of signals for temperature drift. We found that, in the presence of a sensible heat flow, temperature fluctuations cannot be sufficiently removed from the hot-wire signal when one relies on temperature/resistance coefficients from literature. When only slow temperature drift is involved, most literature values give a satisfactory temperature correction, but this depends on the specific combination of a probe and a literature reference. Therefore it is generally advisable to calibrate the value. A method that uses a ratio of (measured) resistances as a function of temperature, which does not require estimation of the operational temperature of the wire, is shown to depend crucially on a parasitic resistance of a few tenths of an ohm. This parameter can be found by optimizing its value using data from a collection of velocity calibrations at different temperatures. This additional calibration alone suffices to estimate the operational temperature of the wire via optimization. A quick calibration procedure (15 min) is proposed and tested.     

Compressible flow hot-wire calibration

 A resistance thermal anemometer operated in compressible flow is sensitive to density, velocity, and total temperature fluctuations. To address these effects, a relatively inexpensive compressible flow calibration facility has been developed and tested. Characteristics of a hot-film anemometer in compressible flow are determined and a correction technique to account for differences in mean density between the calibration facility and the flow under study is proposed. Received: 19 October 1995 / Accepted: 18 October 1996     

Modified calibration technique of a five-hole probe for high flow angles

A new method is described for calibrating a five-hole probe for extending the useful measurement range up to flow angularities of 85°. The calibration method involves adjustment of the calibration coefficients to allow valid calibration at larger flow angles. The extended range calibration curves for flow angularity, total and static pressures are presented. The present range is valid in pitch only when the yaw ports are nulled.     

Cobra probe measurements of mean velocities, Reynolds stresses and higher-order velocity correlations in pipe flow

This paper is concerned with the validation of a four-hole pressure probe, known as a cobra probe, for turbulence measurement. For the first time in the literature, third- and fourth-order velocity correlations measured using a pressure probe are presented. The probe measurements are compared with established data for fully developed pipe flow, and good agreement is found. A new probe calibration methodology and improvements to the data acquisition and processing system are also presented.     

Performance of a combined three-hole conductivity probe for void fraction and velocity measurement in air–water flows

The development of a three-hole pressure probe with back-flushing combined with a conductivity probe, used for measuring simultaneously the magnitude and direction of the velocity vector in complex air–water flows, is described in this paper. The air–water flows envisaged in the current work are typically those occurring around the rotors of impulse hydraulic turbines (like the Pelton and Cross-Flow turbines), where the flow direction is not known prior to the data acquisition. The calibration of both the conductivity and three-hole pressure components of the combined probe in a rig built for the purpose, where the probe was placed in a position similar to that adopted for the flow measurements, will be reported. After concluding the calibration procedure, the probe was utilized in the outside region of a Cross-Flow turbine rotor. The experimental results obtained in the present study illustrate the satisfactory performance of the combined probe, and are encouraging toward its use for characterizing the velocity field of other complex air–water flows.     

Hot-wire calibration over a large temperature range

The hot-wire calibration method as proposed by Cimbala and Park (1990) has been showen to be accurate within a temperature range of 20–45°C. This is a significant extension of the range used by Cimbala and Park (27.5– 34.5°C). The accuracy of the calibration is not affected by the ambient temperature. The calibration curve obtained seems to hold over a long period of time, thus reducing the need for frequent calibrations. Due to contamination the accuracy eventually decreases and the probe has to be re-calibrated.     

惯性测量组件整体标定技术

研究了惯性测量组件中陀螺与加表的标度系数、零偏、安装耦合误差等的标定问题。建立了陀螺与加表的输入输出模型,提出了在三轴摇摆转台上利用六位置试验法对加表标度系数、零偏、耦合误差进行标定的方法;提出了在三轴摇摆转台上利用正负对称的多组速率试验法以及最小二乘法对陀螺标度系数、零偏、耦合误差进行标定的方法,以及利用六位置试验对陀螺与g有关量进行标定的方法。详细介绍了组件输出参数的测量方法并推导误差计算公式。理论分析表明,该方法可在三轴转台上通过位置与速率试验一次性标定出测量元件的相关误差,具有一定的工程应用价值。     

惯性-视觉-磁场传感器组合的低成本标定方法

针对一种融合了惯性-视觉-磁场信息的传感器组合,给出了一套低成本的标定和建模方法.对于传感器模型中的确定性项和非确定性项参数,分别采用了基于输入指令向量模匹配的方法和基于时间序列分析的方法来进行标定和建模,利用所得到的模型补偿之后的传感器输出误差的方差降为原来的1/3以下;对于摄像机坐标系和加速度计坐标系之间的相对旋转矩阵,提出了一种基于重力向量和铅垂线的标定方法,仿真结果表明,标定结果的相对误差小于7%.     

A velocity dependent effective angle method for calibration of X-probes at low velocities

A velocity dependent effective angle (VDEA) method for the calibration of yaw response of hot-wire X-probes at low flow velocities (0.5–6 m/s) is presented. Comparisons with a full velocity vs. yaw-angle method (Österlund 1999) in a smooth wall channel flow indicate that there is only moderate advantage in using the latter method, which is considerably more laborious. Comparisons with direct numerical simulations (DNS) (Moser et al. 1999) and the more common fixed effective angle method (FEA) show that the VDEA method significantly improves estimates of Reynolds stresses compared to the FEA method.     

A calibration technique for a parallel-wire depth probe with conductivity compensation

A new technique of calibrating a parallel-wire depth probe is presented. The technique allows the effects of electric conductivity of liquids to be compensated. The advantage of the probe lies in its simple AC circuitry employed which consists of only a variable resistor together with one AC input and an AC output. The compensation technique employs a two-staging compensation method that allows only one calibration curve to be used for measuring the depth of various liquids with different electric conductivity values.