Anemometer Signals Research Articles

Evolution of a swirling flow in a draft tube model at transient operation regimes

This work deals with the experimental study of transient flow regimes occurring in the laboratory model of a hydraulic turbine draft tube at a change in the control parameters of setup. The transition from the partial load regime, when a precessing vortex core is formed, to the best efficiency point without a core is considered. To set accurately the speed of runner rotation and the flow rate of the working medium, the setup was controlled by special software. To ensure accurate multiple reproduction of a given transition cycle between two regimes, the control program has been upgraded. Using a specially developed procedure of phase averaging of a laser Doppler anemometer (LDA) signal recorded during multiple repetitions of transient regimes, the velocity profiles were obtained for different phases of changing the setup operating conditions.

A novel FBG velocimeter with wind speed and temperature synchronous measurement

Based on frequency demodulation method, a novel fiber Bragg grating (FBG) velocimeter which can achieve wind speed and temperature synchronous measurement is proposed in this paper. The wind speed and temperature synchronous measurement is realized by cup anemometer (CA) signal modulation and Hilbert-Huang transformation (HHT) signal processing. The working principle of the novel FBG velocimeter is demonstrated and its theory calculation model is also set up by using basic mechanical knowledge and blade element momentum (BEM). Further, calibration experiment is carried out on one prototype of the FBG velocimeter to obtain its measurement performance. HHT is introduced to deal with calibration experiment data. After data analyses, the results show that the novel FBG velocimter can achieve high-precision wind speed measurement of 0.012 m/s with minimum detection limit of 0.41 m/s, and its temperature detection precision is 10.6 pm/°C.

Calibration of a system for measuring low air flow velocity in a wind tunnel

This article presents the calibration of a system for measuring air flow velocity in a wind tunnel with a multiple-hole orifice. The comparative method was applied for the calibration. The method consists in equalising the air flow velocity in a test section of the tunnel with that of the hot-wire anemometer probe which should then read zero value. The hot-wire anemometer probe moves reciprocally in the tunnel test section with a constant velocity, aligned and opposite to the air velocity. Air velocity in the tunnel test section is adjusted so that the minimum values of a periodic hot-wire anemometer signal displayed on an oscilloscope screen reach the lowest position (the minimum method). A sinusoidal component can be superimposed to the probe constant velocity. Then, the air flow velocity in the tunnel test section is adjusted so that, when the probe moves in the direction of air flow, only the second harmonic of the periodically variable velocity superimposed on the constant velocity (second harmonic method) remains at the output of the low-pass filter to which the hot-wire anemometer signal, displayed on the oscilloscope screen, is supplied. The velocity of the uniform motion of the hot-wire anemometer probe is measured with a magnetic linear encoder.The calibration of the system for the measurement of low air velocities in the wind tunnel was performed in the following steps:1. Calibration of the linear encoder for the measurement of the uniform motion velocity of the hot-wire anemometer probe in the test section of the tunnel.2. Calibration of the system for measurement of low air velocities with a multiple-hole orifice for the velocities of 0.1 and 0.25 m s−1:- (a) measurement of the probe movement velocity setting;- (b) measurement of air velocity in the tunnel test section with comparison according to the second harmonic method;- (c) measurement of air velocity in the tunnel with comparison according to the minimum method.The calibration uncertainties were calculated after the measurements.

Laser Doppler anemometer signal processing for blood flow velocity measurements

A new method for analysing the signal in a laser Doppler anemometer based on the differential scheme is proposed, which provides the flow velocity measurement in strongly scattering liquids, particularly, blood. A laser Doppler anemometer intended for measuring the absolute blood flow velocity in animal and human near-surface arterioles and venules is developed. The laser Doppler anemometer signal structure is experimentally studied for measuring the flow velocity in optically inhomogeneous media, such as blood and suspensions of scattering particles. The results of measuring the whole and diluted blood flow velocity in channels with a rectangular cross section are presented.

The cup anemometer, a fundamental meteorological instrument for the wind energy industry. Research at the IDR/UPM Institute.

The results of several research campaigns investigating cup anemometer performance carried out since 2008 at the IDR/UPM Institute are included in the present paper. Several analysis of large series of calibrations were done by studying the effect of the rotor's geometry, climatic conditions during calibration, and anemometers' ageing. More specific testing campaigns were done regarding the cup anemometer rotor aerodynamics, and the anemometer signals. The effect of the rotor's geometry on the cup anemometer transfer function has been investigated experimentally and analytically. The analysis of the anemometer's output signal as a way of monitoring the anemometer status is revealed as a promising procedure for detecting anomalies.

Calibration Procedures and Uncertainty in Wind Power Anemometers

Accurate wind measurements are critical in evaluating wind turbine power performance and site assessment. In a turbine power performance evaluation, wind speed readings are matched with corresponding turbine power measurements to produce a power curve for the turbine. For site assessment, the distribution of measured wind speed is used to determine the predicted annual energy production from the wind. Since wind power is proportional to the cube of the wind speed, a small error in the wind measurement could translate to a much greater error in the predicted wind power, which emphasizes the importance of having accurate wind speed readings. To acquire such precision in wind data, it is recommended that individually calibrated anemometers be employed. With these calibrations, it is also recommended that the uncertainty in the calibration be reported so that it may be used not only in the overall uncertainty for turbine power curves and site assessments, but also in improving the performance of an anemometer. A method of presenting calibration uncertainty is defined in the standard IEC 61400-12-1. However, the standard only refers to the measurement uncertainty of the reference wind speed from the particular test facility. It does not include the uncertainty in the anemometer linear transfer function and the errors directly made by the anemometer signal. This paper will discuss: 1) the details of uncertainty reporting as defined by IEC 61400-12-1, 2) a method of extending the uncertainty to include the errors when using the linear transfer function, and 3) a qualitative description of how to determine the uncertainty in a wind speed measurement in the field.

Sampling Bias on Cup Anemometer Mean Winds

AbstractThe cup anemometer signal can be sampled in several ways to obtain the mean wind speed. Here we discuss the sampling of series of mean wind speeds from consecutive rotor rotations, followed by unweighted and weighted averaging. It is shown that the unweighted averaging creates a positive bias on the long‐term mean wind speed, which is at least one order of magnitude larger than the positive bias from the weighted averaging, also known as the sample‐and‐hold method. For a homogeneous, neutrally stratified flow the first biases are 1%–2%. For comparison the biases due to fluctuations of the three wind velocity components and due to calibration non‐linearity are determined under the same conditions. The largest of these is the v‐bias from direction fluctuations. The calculations pertain to the Risø P2546A model cup anemometer. Copyright © 2003 John Wiley & Sons, Ltd.

Linear Time-Invariant Compensation of Cup Anemometer and Vane Inertia

We propose a method to compensate for the phase lag and the amplitudeattenuation in the cup anemometer signal. These two effects, caused by theinstrument's inertia, are the major flaws of the cup anemometer in additionto over-speeding. Since the instrument's response is invariant in wavenumber (not frequency) representation, we transform the signals to becompensated from the time domain to the spatial domain by using Taylor'shypothesis. In the spatial domain we apply a linear time-invariant filterto eliminate the phase lag and the amplitude attenuation. The proposedprocedure improves instrument performance down to spatial scales equal toor smaller than the distance constant of the anemometer. The method for cupanemometer compensation is presented in detail and later adapted for vanes.

Hot-wire/film anemometry measurements in flows with heat transfer and signal correction

Proper interpretation of a constant-temperature anemometer (CTA) signal in a nonisothermal flow requires some temperature correction of the signal. The object of this paper is to describe the use of a systematic procedure which compensates for ambient temperature drift in constant-temperature anemometer operation. The procedure will be demonstrated for a normal (90°) sensor probe.

On the propagation of random errors of constant-temperature anemometers in nonisothermal flows

The use of constant-temperature anemometry for measurements in flows characterized by temperature drifts and fluctuations is difficult because of the dependence of the anemometer response on the temperature of the medium. Methods of combating this problem generally fall into one of several categories in terms of correction methodology. These methodologies include electronic correction, mathematical correction, manual adjustment of the hot resistance, and direct velocity and temperature calibration. This paper examines the random errors associated with mathematical correction of the anemometer signal and assesses the significance of these errors relative to other errors associated with using a constant-temperature anemometer for velocity and/or turbulence measurements.

A maximum likelihood based autocorrelation processor for a photon-resolved laser Doppler anemometer

Maximum likelihood estimation (MLE) techniques were used in the design of an autocorrelation processor for estimating the period (frequency) of a photon-resolved laser Doppler anemometer (LDA) signal. Implementation of these techniques required the derivation and numerical computation of the covariance matrix for the autocorrelation function of the photodetector signal. The processor is insensitive to the particle number density of the system and will work with photon-resolved and non-photon-resolved signals. The processor provides an accurate estimate of the error in each period estimate. Examples are given for the use of a Fisher matrix, derived as part of the MLE procedure, to predict the optimal processor performance of a LDA system as a function of the processor parameters and the system parameters. For periods of time over which the flow is steady the processor can be configured such that improved accuracy for each subsequent estimate can be achieved by utilizing the results from previous estimates. Computer simulations were used to test the processor design.

Reduction of microphone wind noise using local wind velocity measurements.

A system for quieting microphone wind noise using local wind velocity measurements is desirable. The concept is shown to be feasible, however, optimization of the cancellation system requires understanding the correlation characteristics between microphone signals (pressure) and anemometer signals (velocity). Wind data was recorded using microphones and anemometers with separation distances from 2.5 to 20 centimeters. The measurements were made in a single plane parallel to the airflow. Both the microphone pressure and wind velocity signals displayed an approximate 1/f frequency characteristic. In addition, correlation strength between wind signals (both pressure and velocity) decreased with increasing separation distance. Correlation estimates from microphone to microphone, anemometer to anemometer, and microphone to anemometer are presented. It is demonstrated that adaptive signal processing algorithms can be utilized to reduce microphone wind noise using anemometer signals as a reference.

Wind fluctuations near the ground during rain

Measurements of wind speed were made above short grass during periods of rainfall using hot-film anemometers. A filtering algorithm was used to identify spikes in the anemometer signal time series, induced by raindrop impacts on the hot-film sensors. A relatively small number of such spikes, comprising about 0.1–0.2% of the data points, were identified in signals recorded during rainfall with rates ranging from less than 0.1 to 7.2 mm h −1. After these spikes were removed from the time series by filtering, the resulting values of the first four moments of wind speed were very similar to those found above grass during fair weather. Values σ w u ∗ and σ u u ∗ determined during rain using a 3-D sonic anemometer (1.3 and 2.6, respectively) were also in good agreement with corresponding values found during conditions of fair weather and near-neutral atmospheric stability.

Fast-response Simultaneous Measurement of Concentration and Velocity in Mixing of City Gas and Air.

This paper presents a fast-response simultaneous measurement method of velocity and concentration in the mixing of air and city gas mainly composed of methane. The measurement of city gas concentration and fluctuation utilizes the absorption of a laser light by methane in the mixture. A hot-wire anemometer is used to measure velocity. It is shown that the effect of the change in the thermal conductivity of the mixture on a hot-wire anemometer signal can be compensated by using the measured city gas concentration at the same time and location. This method is applied to the simultaneous measurement of concentration and velocity as well as fluctuation in a coaxial flow of city gas and air.

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.

A normal sensor hot-wire/film probe method for the analysis of three-dimensional flows

The resolution of the three-dimensional character of flows using hot-wire/film techniques can be achieved using either single or multiple sensors. This paper introduces a rotational technique employing normal straight-sensor probes particularly suitable for three-dimensional laminar or turbulent flows. The technique is based on the linearized constant-temperature anemometer signal in the case of isothermal flows and the corrected linearized signal in flows involving heat transfer. The method has been tested in the isothermal flow field of a round jet discharging to a cross flow in a water channel.

Vortex shedding from slender cones at low Reynolds numbers

Wind-tunnel experiments on the flows created by a number of slightly tapered models of circular cross-section have shown the presence of spanwise cells (regions of constant shedding frequency) at Reynolds numbers of the order of 100. The experiments have also shown a number of other interesting features of these flows: the cellular flow configuration is dependent on the base Reynolds number and independent of the tip Reynolds number, the frequency jump between adjacent cells is a function of flow speed, taper angle and kinematic viscosity, but is constant along a cone's span, and the unsteady hot-wire anemometer signal is both amplitude and phase modulated. A mathematical model is proposed based on the complex Landau—Stuart equation with a spanwise diffusive coupling term. Numerical solutions of this equation have shown many of the qualitative features observed in the experiments.

Digital processing of hot wire anemometer signals in intermittently turbulent flows

Fast data acquisition and digital signal processing techniques with a 16-bit microcomputer are applied to the study of intermittently turbulent flow in the transitional boundary layer. The digital methods compare favourably with traditional analogue instrumentation and the potential for futher detailed processing is extensive. The results have highlighted some structural features of the intermittent transitional boundary layer which are masked when using bulk time-averaged techniques.

The measurement of 〈U〉, 〈V〉 and 〈uv〉 inside the turbulent spot using conditionally sampled hot-wire anemometer signals

By means of a tripping wire turbulent spots are generated in a conditionally stable laminar boundary layer along a flat plate in a low-turbulence windtunnel. At several vertical and lateral positions the streamwise (U) and vertical (V) velocity components of the spot are measured using anX-shaped hot wire anemometer. The spot's leading edge (LE) and trailing edge (TE) are determined by a high frequency detection criterion.

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.