The hotwire anemometer accompanied by a feedback circuit is referred to as the constant temperature anemometer(CTA), Figure 3.17 - 3.18. The CTA consists of a Wheatstone bridge and a servo amplifier. One arm of the Wheatstone bridge is the probe sensor. As the flow condition varies, the sensor tends to cool appropriately with a resulting change in resistance.The change in resistance leads to an error voltage .These two voltages form the input to the operational amplifier. The selected amplifier has an output current, which is inversely proportional to the change in the resistance of the hot-wire sensor. Feeding this current back to the top of the bridge will restore the sensor's resistance to its original value. The feedback circuit plays an important role in improving the frequency response of the hotwire, typically from 10-20 Hz to several kHz.

The temporal flow feild has been mapped in the present work using an X-probe for obtaining the u and v components of velocity. In two-dimensional measurements, calculation of velocity components involves solving a pair of non-linear, simultaneous equations. Thus, the accuracy of hotwire measurements is affected by the accuracy of the calibration procedure. In addition, the accuracy of the numerical solution that is used to solve the nonlinear simultaneous equations is relevant. Two approaches were applied for data reduction and are discussed in Chapter 15. Both approaches yielded nearly identical velocities, thus showing that numerical aspect of data reduction was satisfactory.

The X-probe supplied by DANTEC has been used in the present research, which mostly satisfies all the above- stated criteria. Additionally, some of recommended electronic tests by the manufacturer have also been carried out to optimize the response of the anemometer's output voltage.