Constant Voltage Anemometer Research Articles

Realization of Constant Voltage Anemometer Using an Alternative Signal Conditioning Circuit

Anemometric method is the most accurate and reliable technique for air velocity measurement. In present work, a voltage follower based circuit is proposed, and a low-cost sensor (tungsten filament) is chosen to perform Constant Voltage Anemometer (CVA) operation for air velocity measurement. For performing the real-time experiment, a wind tunnel featuring a rectangular section (test section I) and a circular section (test section II) with a provision for low-cost sensors and a reference Hot Wire Anemometer (HWA) was fabricated. The measurement is repeated multiple times for various air velocities in the range of 3 m/s to 12 m/s. The estimated ‘pdr’ value (ratio of the power dissipated in the sensor, Pw to the difference between heated sensor resistance, Rw and the sensor resistance at ambient temperature, Ra), considered as new output voltage, is analogous with the output voltage (Vo) of the proposed circuit. The range of ‘pdr’ in test sections I and II is 0.033 mW/Ω to 0.867 mW/Ω and 0.0062 mW/ῼ to 0.1059 mW/Ω, respectively. Similarly, overheat ratio, OHR (ratio of the difference between heated sensor resistance, Rw and the sensor resistance at ambient temperature, Ra to the sensor resistance at ambient temperature, Ra) estimated, is also found to be analogous with the change in temperature (difference between the temperature of the heated sensor Tw and the ambient temperature Ta). The range of OHR in test sections I and II is 0.709 to 0.660 and 0.678 to 0.647, respectively. Uncertainty analysis is carried out for the proposed voltage follower based CVA measurement system and the expanded uncertainty with a 95% confidence level is obtained as 0.54 for the whole range of air velocity measurement.

Experimental Investigation and Uncertainty Analysis of Constant Voltage Anemometer Using Low-Cost Sensor

Hot wire anemometer (HWA) is a widely used instrument to perform experimental measurements in laminar, transitional and turbulent flows. A pure, low-cost tungsten filament from an ordinary light bulb is utilized as a hot wire sensor for the analysis of different configurations of constant voltage anemometer (CVA). Experiments are carried out with the proposed sensor for all the three modes of CVA in a self-designed wind tunnel with the laminar flow; the measured velocities are in the mean range of 0.13 m/s to 0.37 m/s. The low-cost sensor is capable of maintaining a constant voltage in spite of the variation in the sensor current caused by fluctuations in the air velocity, in comparison with any commonly used hot wire sensors. Uncertainty analysis is carried out for the proposed low-cost sensor-based CVA measurement system. The maximum standard uncertainty for the output voltage of CVA is less than 0.229 mV for the whole range of measurement. The uncertainty budget is presented along with the estimating procedure. The expanded uncertainties with a 95% confidence level for the basic, compensated and composite CVA are 0.5092, 0.4028 and 0.5148, respectively. This CVA measurement system with an alternative low-cost sensor in a self-designed wind tunnel for laminar flow is a simple, inexpensive and compact model and is proved to be an equitable standby for the CVA Tao systems.

A detailed procedure for measuring turbulent velocity fluctuations using constant-voltage anemometry

A detailed procedure to use a constant-voltage anemometer (CVA) for the accurate measurement of turbulent flows is proposed. The procedure is based on the usual small-perturbation analysis of hot-wire signals. It consists in three steps: (1) the calibration of internal elements, required to estimate the two main electrical parameters of the CVA circuitry that are needed in the data analysis, (2) a flow calibration to relate the CVA output voltage and the hot-wire time constant to the flow velocity, and (3) a data-processing algorithm to recover the fluctuating flow quantities from the output voltage. The procedure is tested in two classical turbulent flows: a zero-pressure-gradient boundary layer and a round jet. In both cases, the CVA results are shown to be essentially indistinguishable from the results obtained with a research-grade constant-temperature anemometer.

On the use of hot-wire anemometry in pulsating flows. A comment on ‘A critical review on advanced velocity measurement techniques in pulsating flows’

In their recent topical review, Nabavi and Siddiqui (Meas. Sci. Technol. 2010 21 042002) recommended the use of hot-wire anemometry for velocity measurements in pulsating flows, especially at high frequency. This recommendation is misleading. The procedures invoked by these authors are valid only for small-amplitude fluctuations, which are of little interest for pulsating flows. When large-amplitude velocity changes occur without flow reversal, new procedures for the correction of the nonlinearities of the hot wire are required. This case was thoroughly investigated for a constant-voltage anemometer by Berson et al (Rev. Sci. Instrum. 2009 80 045102). In addition, we show that when flow reversal occurs—a case most relevant to pulsating flows—accurate hot-wire velocity measurements are not possible because heat transfer is not well defined when velocity passes through zero and changes direction.

Capture of instantaneous temperature in oscillating flows: Use of constant-voltage anemometry to correct the thermal lag of cold wires operated by constant-current anemometry

A new procedure for the instantaneous correction of the thermal inertia of cold wires operated by a constant-current anemometer is proposed for oscillating flows. The thermal inertia of cold wires depends both on the wire properties and on the instantaneous incident flow velocity. Its correction is challenging in oscillating flows because no relationship between flow velocity and heat transfer around the wire is available near flow reversal. The present correction procedure requires neither calibration data for velocity nor thermophysical or geometrical properties of the wires. The method relies on the splitting of the time lag of cold wires into two factors, which are obtained using a constant-voltage anemometer in the heated mode. The first factor, which is intrinsic to the wire, is deduced from time-constant measurements performed in a low-turbulence flow. The second factor, which depends on the instantaneous flow velocity, is acquired in situ. In oscillating flows, data acquisition can be synchronized with a reference signal so that the same wire is alternatively operated in the cold mode by a constant-current anemometer and in the heated mode by a constant-voltage anemometer. Validation experiments are conducted in an acoustic standing-wave resonator, for which the fluctuating temperature field along the resonator axis is known independently from acoustic pressure measurements, so that comparisons can be made with cold-wire measurements. It is shown that despite the fact that the wire experiences flow reversal, the new procedure recovers accurately the instantaneous temperature of the flow.

A strategy to eliminate all nonlinear effects in constant-voltage hot-wire anemometry

A constant-voltage anemometer is subject to nonlinear effects when the operating hot wire is exposed to large velocity fluctuations in the incident flow. This results in the generation of undesirable higher harmonics, just as in the two classic systems, constant-current and constant-temperature anemometers, for which no attempts are normally made to correct the nonlinearities. The present investigation shows that these undesirable higher harmonics can be suppressed in the case of a constant-voltage anemometer. A new approach to process experimental data is proposed. It is based on three explicit equations established and solved with all terms included, i.e., without linearization. These are (1) the first-order differential equation that describes the electronic circuit of a constant-voltage anemometer-this equation permits to deduce the instantaneous resistance of the hot wire from the output voltage of the anemometer; (2) the first-order differential equation that expresses the thermal lag behavior of the hot wire when used in a constant-voltage mode-this equation permits to restore the instantaneous resistance that an ideal wire would have without thermal inertia in the same flow conditions; and (3) the algebraic relation that expresses the heat-transfer law of an ideal wire, according to King's law, a look-up table, or a polynomial fit-this relation permits to deduce the instantaneous flow velocity from the instantaneous resistance of the ideal wire. The proposed method is easily implemented on a personal computer and permits odd turbulence moments, such as skewness factors, to be obtained satisfactorily.

Lead-resistance effects in a constant voltage anemometer

Two effects of the lead resistances connecting the hot wire to a constant voltage anemometer (CVA) were analyzed and tested: one concerns the change in the sensitivity coefficient relating the anemometer output to velocity or temperature fluctuations, and the other the time constant of the hot wire determined by an in situ square-wave test technique. Small perturbations were assumed in both cases. The CVA output sensitivity was found to be reduced and the time constant increased with the lead resistance. Explicit formulas which involve the lead resistance, the cold wire resistance, and the wire overheat, as well as some characteristics of the CVA circuit, were established to take into account these effects. In the ranges tested, each effect can individually introduce as much as 10% error. Product of the two governs the overall response for the CVA. However, because the two effects change in opposite directions, interestingly, variation in the net response from their product is minimized. This feature may be very useful for many engineering applications of the CVA. Results of experiments conducted with the CVA in a subsonic jet are presented. They confirm the analysis and also establish that accurate measurements can be performed even with a large ratio of lead resistance to hot-wire resistance by applying the correction formulas developed with the analysis. Results from earlier experiments in a supersonic boundary layer also are presented.

Electronic noise in a constant voltage anemometer

The electronic noise and the signal-to-noise ratio in a constant voltage anemometer (CVA) are analyzed in terms of the main constitutive elements of the circuit. It is shown that the output voltage due to electronic noise decreases with the wire resistance, permitting one to know the noise upper limit by using the results of the unheated wire. The noise power spectrum increases at high frequencies as f2, like in other anemometers, because of the need to compensate for the thermal lag of the hot wire, thus leading to a reduced signal-to-noise ratio at high frequencies. Explicit formulas are given in terms of wire, CVA, and flow quantities. Measurements of electronic noise in a CVA prototype confirm the theoretical analysis and illustrate some interesting issues concerning measurements of noise and low levels of flow fluctuations.

Constant voltage anemometer operated hot wire at subsonic speeds over wide overheats in unsteady flows

The constant voltage anemometer (CVA) was used to calibrate a hot wire over a wide overheat range. Instead of the output voltage (E) of the anemometer which is normally used, at each test point a quantity represented by “pdr” equal to the ratio of power dissipated in the hot wire (Pw) and the associated difference in the heated resistance of the hot wire (Rw) and its resistance (Ra) at the ambient fluid temperature is calculated. It is shown that the calibration curves so obtained with pdr=Pw/(Rw−Ra) as the output variable instead of E can be represented by a single calibration equation covering the wide overheat range. Overheat variation is equivalent to allowing an ambient temperature change of the fluid at a given setting. It demonstrates that this approach can be used to cover measurements using the hot wire with fluid temperature drifts without using a second hot wire for temperature corrections and without any temperature calibration. The calibration data was then applied to measure the unsteady flow in the near orifice region of synthetic jets with very good results. The measurements confirm the computational predictions that show that although there is flow reversal, over a cycle of oscillation, the synthetic jet actuator spends most of the cycle ejecting rather than ingesting fluid.

Automated constant voltage anemometer for measurements with fluid temperature drifts

An automated procedure has been developed for a constant voltage anemometer to obtain in situ heated and cold resistances of a hot wire in each measurement cycle. From these parameters mean velocity can be deduced in the presence of ambient flow temperature variations, using a calibration curve based on, e.g., King’s law with the assumption of constant coefficients over a given temperature range. The method requires only one hot wire. This is a definite advantage over previous methods established for constant current or constant temperature anemometers where an additional temperature sensor is needed, with a separate temperature calibration of the hot wire. Results from a typical test are presented to demonstrate the procedure. The prototype tested was a 400 kHz bandwidth unit and the measurement cycle can be changed through software commands to suit each test requirements.

Constant Voltage Anemometer Practice in Supersonic Flows

It is shown that the constant voltage anemometer (CVA) is well adapted for measurements in supersonic flows. Small hot-wire time constant, large bandwidth, high signal-to-noise ratio, variable hot-wire overheat, and independence of the cable capacitance effects of the hot wire were all observed with the CVA. Turbulence data were acquired using two methods. In the first method, signals were acquired with the CVA compensated with a fixed time constant of 0.10 ms at all test points. In situ hot-wire time constant and overheat were also acquired at each test point, which were then used in postprocessing of the turbulence signals. In the second method, full hardware compensation with the actual in situ time constant was set at each test point. The former method shows a larger bandwidth. The noise level was approximately the same in both methods. Theoretical estimates of the relative output levels of CVA, constant current anemometer, and constant temperature anemometer at different overheat ratios show that the CVA has larger output voltage level

Constant voltage anemometer practice in supersonic flows

Constant voltage anemometer practice in supersonic flows

A comparative study of constant-voltage and constant-temperature hot-wire anemometers:

The dynamic response of the constant-voltage anemometer (CVA) system was investigated both analytically and experimentally and compared to that of the CTA. The frequency response functions of the CVA system for a number of different circuit parameters and flow conditions were determined via laser-based radiative heating of the hot-wire sensor. A 2nd-order linear systems model of the CVA was developed to provide insight to the dynamic response and to interpret the experimental results. The qualitative variations in the frequency response function with changes in circuit parameters are in agreement with the model. The experimentally determined frequency-response functions of the CVA systems used in this study were found to have little dependence on the wire overheat ratio and Reynolds number.

Automated constant voltage anemometer with in situ measurements of overheat and time constant of the hot wire

The operational features of an automated constant voltage anemometer (ACVA) designed for flight applications have been presented. Diagnostic voltage outputs are provided in ACVA, which will enable the calculation of in situ hot wire time constant and overheat for postprocessing of the turbulence data from the measured output voltage. TTL outputs provide identification of the output state. Using this automated system in a wind tunnel, time constant values obtained as a function of the hot wire Reynolds numbers and hot wire currents (overheats) are presented for 5 and 10 μm hot wires showing agreement with prior analysis and results

Software corrected hot wire thermal lag for the constant voltage anemometer featuring a constant bandwidth at the selected compensation setting

Software compensation correction for thermal lag of a hot wire in the application of a constant voltage anemometer (CVA) for turbulence measurements in the boundary layer of a supersonic wind tunnel has been demonstrated. The CVA was used with a fixed compensation setting while measuring the in situ thermal lag (time constant) of the hot wire. Using the measured time constant, corrections are applied to the fixed compensation output of the CVA in postprocessing of the data. To demonstrate the flexibility of the approach it was used for two compensation settings at a test point to obtain the same results from both settings. A unique advantage of this approach is shown to be that for a given compensation setting in the CVA the bandwidth of the measurements for the test remains constant for all of the different test conditions and yields higher productivity. The results of the turbulence levels measured with this method agree with earlier research using other anemometers. Spectral plots of the mass flux and temperature and the measured in situ time constant responses under different conditions of the hot wire have been presented.

Transfer function analysis of the constant voltage anemometer

The transfer function for the constant voltage anemometer (CVA) circuit has been derived in terms of circuit and hot-wire parameters and the expressions for the natural frequency and damping ratio have been obtained. Bandwidth in each case was determined from the plot of the normalized transfer function. The theoretical bandwidth behavior calculated from the transfer function plots for the prototype agrees with independent tests of the prototype using laser radiation heating of the hot wire in an air jet. The near constant value of the bandwidth of the CVA with the variation in the hot-wire overheat and its Reynolds number that were observed with the laser tests have been substantiated with the theoretical values from the transfer function. Bandwidth testing with sine wave injection, in situ time constant measurement for proper compensation setting, method to optimize the design to have nearly a constant bandwidth even with different compensation time constants and the operational advantages of CVA are also discussed.

A new approach to enhance the sensitivity of a hot-wire anemometer and static response analysis of a variable-temperature anemometer

A new hot-wire anemometer which has greater sensitivity than that of a conventional constant-temperature anemometer (CTA) was proposed. Sensitivity differences between a recently developed constant-voltage anemometer (CVA) and a CTA provided a direct motivation of the present study. To derive a factor which made the difference in sensitivity, static response analyses of a CTA and CVA were performed first by calculations and experiments. It was found that the major factor which is responsible for the sensitivity of an anemometer is not the working resistance of a hot wire itself, but the change in the wire resistance with velocity. Based on this analysis, a new anemometer circuit, where the wire working resistance increases with velocity – that is, the operating mode of the wire becomes variable-temperature – was proposed and named a variable-temperature anemometer (VTA). The VTA was made by substituting a voltage-controlled variable resistor such as a photoconductive cell or a transistor for one arm of the bridge. By positively feeding back the bridge-top signal to the input side of these electronic components, the wire resistance could be increased with velocity automatically. The enhanced sensitivity of the VTA was validated by comparing experiment with the CTA.

NASA Langley Mach 6 Quiet Wind-Tunnel Performance

The flow in the NASA Langley Mach 6 quiet wind tunnel has been investigated to quantify the effectiveness of laminar-flow control techniques used to delay transition of the nozzle-wall boundary layer. The results of this investigation include an assessment of the mean and unsteady nozzle flow to define the quiet core length, and hence performance, over the operating range of the facility. A large, uniform region of Mach 5.91 flow was documented for a variety of unit Reynolds numbers. By using a prototype constant-voltage anemometer to measure the unsteady flowfield, acoustic radiation patterns from the transitional nozzle-wall boundary layers were mapped. These disturbances originating at the Irregular edge of the transitional nozzle-wall boundary layer were shown to follow Mach lines into the test section of the nozzle, thereby limiting the length of the quiet core. With a virtual origin downstream of the nozzle throat, a Reynolds number dependency was found for the amplitudes of the acoustic radiation.