Bearing friction effect on cup anemometer performance modelling

Abstract

In the present work, the effect of the friction forces at bearings on cup anemometer performance is studied. The study is based on the classical analytical approach to cup anemometer performance (2-cup model), used in the analysis by Schrenk (1929) and Wyngaard (1981). The friction torque dependence on temperature was modelled using exponential functions fitted to the experimental results from RISØ report #1348 by Pedersen (2003). Results indicate a logical poorer performance (in terms of a lower rotation speed at the same wind velocity), with an increase of the friction. However, this decrease of the performance is affected by the aerodynamic characteristics of the cups. More precisely, results indicate that the effect of the friction is modified depending on the ratio between the maximum value of the aerodynamic drag coefficient (at 0° yaw angle) and the minimum one (at 180° yaw angle). This reveals as a possible way to increase the efficiency of the cup anemometer rotors. Besides, if the friction torque is included in the equations, a noticeable deviation of the rotation rate (0.5-1% with regard to the expected rotation rate without considering friction) is found for low temperatures.

Measuring Relative Wind Speeds in Stratospheric Balloons with Cup Anemometers: The TASEC-Lab Mission

“…The rotor was coupled to the body of a First Class Vector Instruments A100L2 anemometer, whose characteristics are described in Table 2 . Additionally, a heater was attached to the body of the cup anemometer to prevent loss of performance due to low atmospheric temperatures at high altitude levels over the ground [ 39 ]. Figure 2 shows the cup anemometer composed of the designed rotor, the Vector Instruments A100L2 sensor body, and the heater attached to the latter.…”

On the Variation of Cup Anemometer Performance Due to Changes in the Air Density

“…Therefore, it can be concluded that there is a large uncertainty in terms of the calibration of an anemometer for air velocity measurements at very low-density conditions. Since the aerodynamic torque, QA, depends linearly on the dynamic pres reasonable to assume that the relationship between this variable and the torque by the friction forces, Qf, will be a determining parameter in the performances o mometer at least when faced with small variations in this relationship [11]. If th ature of the cup anemometer bearings is constant, it can be assumed that th torque is constant, although as expressed in Equation ( 2), this frictional torque pends (to a lesser extent [11]) on the rotational speed.…”

“…Since the aerodynamic torque, QA, depends linearly on the dynamic pres reasonable to assume that the relationship between this variable and the torque by the friction forces, Qf, will be a determining parameter in the performances o mometer at least when faced with small variations in this relationship [11]. If th ature of the cup anemometer bearings is constant, it can be assumed that th torque is constant, although as expressed in Equation ( 2), this frictional torque pends (to a lesser extent [11]) on the rotational speed. Thus, in the absence of experimental studies, a preliminary hypothesis suggests that, for very low air Since the aerodynamic torque, Q A , depends linearly on the dynamic pressure, it is reasonable to assume that the relationship between this variable and the torque produced by the friction forces, Q f , will be a determining parameter in the performances of the anemometer at least when faced with small variations in this relationship [11].…”

“…where ω is the angular velocity of rotation of the anemometer's rotor, and I is the moment of inertia of the rotor. On the other hand, QA is the torque produced by aerodynamic forces, and Qf is the torque produced by the friction on the shaft that holds the cup anemometer rotor [11]. The aerodynamic torque, QA, depends on the following:…”

“…The effect of temperature on cup anemometers has been studied with interest by several researchers. However, emphasis has almost always been placed on the effect of friction [11,18,19] and on the accumulation of ice on the cups when this instrument operates in extremely cold conditions [20][21][22][23][24][25][26].…”