Elsevier

Measurement

Volume 104, July 2017, Pages 117-122
Measurement

A drag force anemometer using finite length cylinder with direction measurement capability

https://doi.org/10.1016/j.measurement.2017.02.006Get rights and content

Highlights

  • A new wind speed and direction measurement technique by using drag force.

  • New anemometer satisfies the requirements of WMO.

  • Competitive cost.

Abstract

This study proposes a new design idea to measure wind speed and direction using drag force. For this aim, load cells which are get aligned with the geographical directions were used. A prototype was built and tested in a wind tunnel located at WEAR Laboratory in Erciyes University. The tests were performed for the direction of 225° between 5 m/s and 25.2 m/s and the outputs were compared with reference velocities. The results were shown that the mean measurement errors of the new design were less than 3% and the mean direction errors were less than 3° respectively, which provided WMO accuracy requirements.

Introduction

Utilization of renewable energy resources in the short and long term electricity generation plans of the countries is getting higher importance. In particular, the installation of power plants based on wind energy is increasing day by day, and the wind is leading the countries' energy policies around the world. According to a report issued by the Global Wind Energy Council (GWEC), a worldwide wind power plant of about 63 GW installed in 2015 and the cumulative energy production based on wind energy has reached 432 GW. Based on estimates covering the period 2016–2020, it is expected to reach 792 GW by the end of 2020 with the installed power of the worldwide. Referring to the projection covering these years, an average of approximately 72 GW wind turbines will be needed every year [1]. Hence, it is of great importance that new wind farms are planned to be built and that the planned wind speed measurements are made correctly and reliably. The performance parameters of the present wind instruments, a new wind instrument which has accurate real-time three-dimensional data for wind speed, wind direction, and turbulence is required [2].

There are various types of wind speed and direction measurement techniques. Each of techniques has its pros and cons, and researchers have conducted several studies to analyze them. Measuring wind speed and direction is done at different ways: cup anemometer with a vane, Pitot tube, propeller, hot-wire anemometer, with ultrasonic, remote wind measuring techniques with sound (SODAR), light (LIDAR), electromagnetic waves (RADAR), or with laser-based devices such as Laser Doppler anemometer.

Cup and propeller anemometers and wind mills which are capable of measuring direction if they are equipped with suitable tails [3]. On the other hand, they are fit for outdoor conditions. For having this advantageous the cup anemometers are not only used in meteorological measurement but they also widely preferred in determining wind turbine performance evaluation. With the advantage of having the lowest uncertainty of measurement of atmospheric winds this type of anemometers can withstand outdoor condition as well [3]. The main problem for measuring wind speed using Pitot tube has been reported as misalignment of the tube axis and velocity vector. In order to solve this problem adding extra taps on the tube have been proposed. In this way, velocity measurements have been reported insensitive to angle of attack [4]. Pitot tube anemometers are very sensitive to environmental conditions. Like pitot tube anemometers, hot wire anemometers have been reported to be sensitive at the environmental conditions. Incapability of direction measurements are drawbacks for both anemometer types. Moreover, other measurement systems above mentioned (such as SODAR, LIDAR and RADAR) are complex and expensive.

Specifications of anemometers which are going to be used in determination of wind energy are determined by International Electro technical Committee. On the other hand, specifications of anemometers which are going to be used in to collect meteorological data were determined by World Meteorology Organization (WMO).

The aim of this study is to develop a simple, cheaper and improvable wind measurement system using drag force of a circular cylinder within the accuracy limits of WMO requirement. Because the proposed anemometer will measure wind speed and direction simultaneously, a cylindrical object was selected to get advantageous of symmetry. In this study, our focus is the region of Reynolds number where the drag coefficient remained constant.

The flow over a circular cylinder is a function of Reynolds number (Re). Variation of drag coefficient has been studied well by many researchers [5]. Mallick and Kumar [6] conducted a research about cylindrical bodies with various cylinder diameter and air velocity. They concluded that increasing both velocity and diameter caused to increase in drag force (FD). Their study confirmed that drag coefficient was not a constant value but it was a function of speed, diameter, density and fluid viscosity. One of the main pillars of this study is to take advantage of drag force concept. Since the focus of this study is to get constant drag coefficient region, literature has been reviewed. It was revealed from the review study that there was no consensus to figure out the limits of constant drag region. Anderson [7] stated that the constant drag coefficient was in the range of 103 < Re < 3 × 105 for an infinitely long cylinder. However, Zdrakovich [8] conveyed that upper limit of subcritical flow started around 2 × 105. The variation of drag coefficient (CD) for a circular cylinder for different diameters over the Reynolds number range of 10−1–106 was collected by Schlichting. The diameters used to graph of CD variation were in the range of 0.05 mm to 300 mm and constant drag region was presented from 104 to 105 [9]. Cook [10] defined a relatively broader limits for constant drag coefficient region with respect to Schlichting, that was around 5 × 103 < Re < 2 × 105. In Fig. 1, the constant drag regions above mentioned were demonstrated. Firstly, the proposed anemometer design criteria were set to experimental data obtained during studies had performed in WEAR. It was seen from Fig. 1 that the experimental data of this current study closes to the results of Uematsu &Yamada and Schlichting.

Zuo [11] measured the drag coefficient of a cylinder for the Reynolds numbers up to 2.61 × 105. Air density required for the determination of the drag force was calculated based on temperature, relative humidity and barometric pressure. Kinematic viscosity was assumed as 1.5 × 10−5 m2/s. He has indicated that drag coefficient has dropped much earlier, namely at a point where 1.12 × 105. He has noticed that the early drop, caused by turbulence. The methodology for determining air density used by Zuo has been introduced in [12].

In order to measure wind speed using drag force, one of the main element that need to be determined is aspect ratio (AR). AR influence on finite length circular cylinder was reviewed by Fox and West [13]. They presented a research report about cantilevered cylinders immersed in a low-turbulence flow at Re = 4.4 × 104 for various aspect ratios in the range of 4–30. According to their report, the distribution of drag along the cantilever was unique for the aspect ratios less than 13. Again reference to their report the maximum drag was observed at the free end of the cylinder as expected. Considering these conclusions and taking into consideration of dimensions of WEAR wind tunnel, the proposed anemometer aspect ratio has been selected as 4.87.

After setting aspect ratio, drag coefficient of the circular cylinder used for sensing wind speed was analyzed. In order to make accurate assumption for determining drag coefficient Uematsu and Yamada [14] suggestion was presented. Their measurements were based on the Reynolds number range of 3.8 × 104–1.4 × 105. The diameters of finite cylinders were recorded as 6 cm and 11.3 cm. They obtained an empirical equation (Eq. (1)) showing the relevance of the mean drag coefficient in the subcritical regime as a function of diameter and height. They concluded that the drag coefficient was less sensitive to H/D ratio for H/D < 5, where H was the height and D was the diameter of the cylinder. This conclusion is in line with report provided in [13]. Uematsu and Yamada equation [14] is given below.CD=1.2·(0.58+0.17logHD1.6Using Eq. (1), drag coefficient of the proposed anemometer was calculated as 0.808. But, the drag coefficients were obtained from the mean pressure distribution and the influence of the skin friction and profile drag was not considered in this study, and all the results of the drag coefficient are only pressure drag coefficients in the Uematsu and Yamada’s experiments [15], [16]. Thus, the drag coefficients obtained by Uematsu and Yamada are smaller than other experiments. In addition, for the blockage effect on the pressure distribution any correction was no applied to the results in this study.

Section snippets

Materials and method

An object immersed in a flow is subjected to a drag force, FD which is given by Eq. (2).FD=0.5·CD·ρ·A·v2In Eq. (2); CD, ρ, v, A represent drag coefficient, density, velocity and cross sectional area of circular cylinder, respectively. Another topic which is needed to be taken into account is determination of air density in terms of drag force calculation. The methods for estimating air density are given in European Association of National Metrology Institutes (EURAMET) guideline. According to

Results and discussion

The results were compared to the World Meteorology Recommendations (WMO) in terms of performance comparison. The wind speed measurements below 10 m/s of the anemometer were better than the accuracy requirements of WMO which is 0.5 m/s. Direction measurements were recorded as less than ±3° error whereas WMO requires ±5° for the range of 0–360°.

Fig. 9 shows that the percentage velocity error of the proposed anemometer with respect to reference is less than 3% at all measurement points. The

Comparison of the proposed anemometer with basic anemometers

A table of comparisons of anemometers was given in Table 1. As is shown from the table, the proposed anemometer, which is patent-pending, has high degree of resolution and accuracy. In addition, the proposed anemometer is competitive in terms of cost, and its cost is less than 200 $ even it is in prototype phase, and it is expected that it will be less than 100 $ in the mass production. Moreover, the mass production cost of the proposed anemometer can be reduced to less than 85 $ using 3 load

Conclusion

In this study, a new design idea to measure wind speed and direction using drag force was presented. A measurement range for wind speed was set as the first step of realization of the proposed new anemometer. Then, a set of proper sensors, namely load cells, were utilized. An electronic circuit with a power supply, a set of amplifiers, and a microcontroller was designed. The proposed technique is making enable to measure wind speed and direction using drag force. It was deduced that the

Acknowledgments

The authors would like to acknowledge funding from the Scientific Research Projects Unit of Erciyes University under the contract no: FDK-2013-4744. In addition, the authors would also like to thank personally staffs of WEAR Laboratory for helping to wind tunnel experiments.

References (22)

  • M.M. Zdrakovich
    (1997)
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