The conventional hot-wire static calibration procedure for the measurement of absolute turbulence intensities with constant-temperature hot-wire anemometers is investigated and serious errors are found. An alternative calibration procedure is developed which involves shaking the wire at low frequencies in a uniform flow. A series of tests indicate that this dynamic calibration method is more accurate and consistent than the conventional procedure.A method for verifying various calibration procedures is demonstrated. This method involves the measurement of velocity perturbations in a series of Karman vortex streets. The velocity perturbation amplitude is held fixed, but the frequency varies from one vortex street to another. This method also acts as a direct check of the hot-wire system frequency response.
The conventional ‘bridge-feedback amplifier’ constant-temperature hot-wire anemometer is analysed to determine its static and dynamic response. The effects of moderate feedback amplifier gain, bridge imbalance, stray bridge reactance, amplifier offset voltage, lack of common mode rejection, amplifier frequency response and departure from constant transconducture are included. The root loci of the system are mapped out and the consequences of the analysis are discussed from the viewpoint of both the operator and the designer.
SummaryMany investigators have studied the aerodynamics of axial flow turbomachinery but none has produced a complete map of the three-dimensional flow behind a rotor row. This is of considerable interest to the aero-acoustician. A system is described which uses a constant temperature hot-wire anemometer to analyse the flow behind such a rotor. Although much information may be extracted by using the technique, its interpretation depends to a large extent on its form of presentation. An analysis of the flow behind a research fan is used as a means of discussing various forms of visual presentation.
This study proposes a unique approach to convert a voltage signal obtained from a hot-wire anemometry to flow velocity data by making a slight modification to existing temperature-correction methods. The approach was a simplified calibration method for the constant-temperature mode of hot-wire anemometry without knowing exact wire temperature. The necessary data are the freestream temperature and a set of known velocity data which gives reference velocities in addition to the hot-wire signal. The proposed method was applied to various boundary layer velocity profiles with large temperature variations while the wire temperature was unknown. The target flow velocity was ranged between 20 and 80 m/s. By using a best-fit approach between the velocities in the boundary layer obtained by hot-wire anemometry and by the pitot-tube measurement, which provides reference data, the unknown wire temperature was sought. Results showed that the proposed simplified calibration approach was applicable to a velocity range between 20 and 80 m/s and with temperature variations up to 15 °C with an uncertainty level of 2.6% at most for the current datasets.
Calibration of hot-wire probes operated in a constant-temperature mode in water at low velocities is discussed. Operation under circumstances where natural convection effects are important is considered. A method of calibrating a constant-temperature hot-wire probe for variations in fluid temperature is presented. The method consists of varying wire overheat during calibration at a constant fluid temperature. A relation is derived analytically relating anemometer output with a variable overheat resistance to anemometer output with fluid temperature variations. An experimental study to verify the analysis is presented.
Simplified Calibration Method for Constant-Temperature Hot-Wire Anemometry