The subject of this paper is the experimental determination of the flow characteristics of three-dimensional wall jets. The jets were produced from a diffuser with a rectangular outlet. The diffuser outlet size and flow rate were varied to produce both low and high outlet aspect ratios and Reynolds numbers. Velocity profile measurements were made to determine the centerline velocity decay and the extent of the lateral and vertical spread of the jet.

In this paper, new requirements for the characteristics of anemometers used for low-velocity measurements indoors, as well as requirements for the signal processing, are presented The static calibration, dynamic response, and temperature compensation of the anemometers, as well as the directional sensitivity and the design of the velocity transducer, are considered, together with the period and the sampling rate of the measurements.

A study was made of the impact of well-documented random velocity fluctuations and periodic temperature fluctuations with different amplitudes and frequencies on the accuracy of the mean velocity and the standard deviation of the velocity measured by three low-velocity anemometers with omnidirectional probes. The anemometers were tested in an airflow at 225 combinations of mean velocity, amplitude of the velocity, and temperature fluctuations, as well as frequency of the temperature fluctuations, as identified during field measurements.

Thermal anemometers with heated velocity sensors are mostly used for low-velocity measurements in rooms. The heated velocity sensor generates an upward, free convection flow that interacts with the airflow where measurements are to be performed and, thus, has an impact on the accuracy of the velocity measurements. Tests were performed with four anemometers available on the market to identify this impact in an airflow with a constant velocity and in an airflow with a periodically fluctuating velocity.

A comprehensive investigation was made of the dynamic behavior of five low-velocity thermal anemometers with omnidirectional sensors. Both the shape of the dynamic response curves of the instruments and their dynamic response were different. The dynamic response of the anemometers was mainly influenced by the frequency of the velocity fluctuations and only slightly by the mean velocity of the airflow and the amplitude of the velocity fluctuations.

Thermal anemometers with omnidirectional sensors are recommended in the standards to be used for low-velocity measurements indoors. Requirements for the directional sensitivity of the velocity sensor are prescribed. However, a method for testing the directional sensitivity of low-velocity anemometers does not exist. A simple test procedure is used in practice to identify the so-called "yaw" and "roll" directional sensitivity of an omnidirectional velocity sensor.