A differerential pressure anemometer

A differerential pressure anemometer S. Kazadi, J. Lee, T.Lee, A. Bose June 6, 2015 Abstract There are a number of different applications where omnidirectional anemometers are needed rather than directional anemometers. An example is sampling of wind speed in a VAWT application. Omnidirectional anemometers are typically expensive or inaccurate. In this paper, we demonstrate the design and implementation of a three-dimensional envelope which, when combined with a hot wire directional anemometer, yields accurate wind speed measurements independent of direction. We examine the accuracy and find that wind speed, when measured with the combined system, is accurate to 1% when compared with an identical, properly oriented anemometer. 1 Introduction The performance of directional anemometers of any type is affected by the shifting of the direction of the wind. Wind direction shifting requires subsequent directional measurements and readjustment of the direction of the anemometer. Anemometers that do not require this constant realignment [5, 7] may be preferable in a variety of cases. Anemometers that are unidirectional can be relatively complex, requiring emitters and sensors as well as computational elements to determine wind direction[6, 1, 4]. Alternatively, they are prone to mechanical limitations due to their moving parts and to friction[3, 2]. As a result, it is advantageous to envision an anemometer that is relatively cheap and accomplishes the measurement goal passively. It is well understood that in wind flowing over a hemisphere in a direction parallel to its flat side (Fig. 1), the airflow over the curved side moves faster than that moving parallel to the flat side. As a result, the air pressure on the curved side is smaller than that on the flat side. The difference in pressures makes possible a vertical jet internal to the hemisphere in the event that a hole is drilled through the hemisphere. As the hemisphere is rotationally symmetric, it is posited that not only will this airflow have no dependence on the wind direction, but the internal wind speed will be a predictor of the overall wind speed. This paper empirically examines the design and implementation of a differential pressure anemometer based on the pressure differential using a hot wire anemometer placed within the hemisphere as an internal wind measurement device. Section 2 gives the basic design of the anemometer. Section 3 describes the wind tunnel and experimental conditions used to test the anemometer. Section 4 presents the data collected in wind tunnel tests. Section 5 concludes. 2 A differential-pressure anemometer A hemisphere in a low speed wind flow is schematically shown in Figure 2.1, ignoring turbulence at the downwind side of the hemisphere. Notably, the air speed at the top of the hemisphere is faster than that at the bottom. As a result, the air pressure at the top is lower than that at the bottom. Figure 2.1: Wind flowing over a hemisphere. In order to measure the wind speed in a way that is not affected by the wind direction, a hole was bored along the rotational axis of the hemisphere. This provides a pathway for the higher pressure air along the bottom of the hemisphere to move toward the lower pressure region at the top. By embedding a directional anemometer in 1

this hole, the wind speed through the hole can be measured. The system is depicted in Figure 2.2. Figure 2.2: A diagram and picture of the differential pressure anemometer using an embedded anemometer. The anemometer is embedded in the vertical hole, providing wind measurements that are independent of wind direction. An anemometer fully embedded within the hemisphere does not affect airflow, and so the entire system is rotationally invariant. In our experiments, we approximated this with an anemometer with a long thin boom, presenting a minimal aspect to the wind. As a result, the measured airspeed within the hole is independent of the wind direction. We will demonstrate that the induced wind in the hole enables the measurment of wind speed with high fidelity. Figure 3.1: Our suction type wind tunnel with flow straightening door and viewing window. The differential pressure anemometers were mounted on a flat platform held up by four long, thin legs. The platform doubled as the bottom of the sensor, and the axial hole was bored completely through the platform. Into the sensor we embedded removable cores with an outer diameter equalling that of the bored hole and through which holes of various sizes were bored (Figure 3.2). (a) 3 Experimental procedure We constructed three different windbenders so as to examine the performance sensitivity of the design with respect to device size. These wind benders had diameters of 6, 8, and 10 inches. In order to determine the performance of our sensor, we constructed a very small suction-type wind tunnel (Figure 3.1). The wind tunnel was characterized as to its wind speed as a function of position in the wind tunnel in three dimensions. Once characterized, it was possible to find regions within the three-dimensional space in which the wind speed was nearly constant. These regions of airflow were used to carry out all experiments. The wind tunnel was equipped with an open-loop differential speed controller in order to examine performance at different wind speeds. Figure 3.2: The removable core with an internal hole (3D printed). Replacing the cores enabled the testing of different hole sizes. Differing hole sizes produce different functionality, enabling the testing of function sensitivity to the size of the bored hole. A second identical anemometer was placed below the platform to measure the wind speed at the platform level. This provided the external windspeed while the internal anemometer provided the internal wind speed. The experimental setup is depicted in Figure 3.3. 2

(a) (a) Figure 3.3: The experimental setup of the windtunnel experiments. Data was recorded twenty times each for each plug hole size and wind speed combination on both sensors. The wind speeds ranged from a minimum of one mph to eight mph. Each wind speed measurement was temporally separated by at least ten seconds, enabling different measurements to become independent. This procedure was repeated for each windbender. (c) Figure 4.1: Vertical internal wind speed s dependance on external wind speed. Data is graphed for 6 inch diameter (a), 8 inch diameter, and 10 inch diameter (c) windbenders. 4 Data and analysis We took data as indicated in Section 3. These data are graphed in Figure 4.1. Each graph displays data for a specific windbender size. Each graph includes data from multiple hole sizes. Each of the data sets for each hole size was fitted using a least square method to a third order polynomial over the data set and this fitted curve represents the performance curve for the sensor over the data set. A sensitivity analysis was performed and it was found that exclusion of small amounts of data from the data set resulted in a change in the fitted curve of less than 10 3. We averaged data from independent measurements of the same wind speed. These are time-averaged values of the vertical measurement that would result from a non-instantaneous wind speed measurement. Each of these were then used with the fitted performance curve to determine the ambient wind speed. This data is graphed in Figure 4.2. Clearly, the smaller the vertical hole, the more accurate the sensor. 3

(a) directional anemometer is incorporated. The resulting device has been shown to have a very good performance with a measured accuracy of at least 1% in comparison to the test anemometer at wind speeds between three and eight mph. Moreover, this good performance was robust across different windbender sizes and hole sizes. Extensions to faster wind are straightforward and will be undertaken in a future study. The present study used an anemometer that was partially external; future studies will use completely embedded anemometers. Finally, performance comparisons will be performed with heterogeneous groups of wind speed sensors and yaw mechanisms. References [1] T. Karalar. An acoustic digital anemometer. PhD Dissertation. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, 2002. [2] S. Pindado, J. Cubas, and F. Sorribes- Palmer. The cup anemometer, a fundamental meteorological instrument for the wind energy industry. Research at the IDR/UPM Institute. Sensors, 14, pp. 21418-21452, 2014. (c) Table 4.2: Time-averaged ambient speed measurements graphed with the fitted performance curves of the sensors. Data is graphed for 6 inch diameter (a), 8 inch diameter, and 10 inch diameter (c) windbenders. Notably, the ratio of the error in the wind speed estimation to the actual wind speed is small, indicating a very good performance. It is also clear that the error is smaller for smaller windbenders and smaller hole sizes. Such a performance affector will need to be taken into account when the design is used in practice. 5 Summary and conclusions We presented a simple solid external device which can be built around a standard directional anemometer which enables the acquisition of wind speed independent of the direction of the wind using existing directional anemometers. This device comprises a simple hemisphere with a hole bored through the center into which the [3] I. Walker. Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research. Geomorphology, 68(1), pp. 57-76, 2005. [4] F. Durst, B. Howe, and G. Richter. Laser- Doppler measurement of crosswind velocity." Applied Optics, 21(14) pp. 2596-2607, 1982. [5] V. Kakate et. al. Study of Measurement and Control Aspects of Wind Tunnel. International Journal of Innovative Research In Electrical, Electronics, Instrumentation, and Control Engineering, 2(3), pp. 1291-1294, 2014. [6] F. Jiang. Silicon-micromachined flow sensors. PhD Dissertation, California Institute of Technology, 1998. [7] S. Shkundin and V. Stuchilin. New Acoustic Anemometers for the Mining Industry. Proceedings of the 17th International Mining Confress and Exibition of Turkey, 4

Ankarat, Turkey, June19-22, pp. 787-792, 2001. 5