Constant Temperature Anemometry: A Tool to Study Turbulent Boundary Layer Flow

1. Hot-wire system dynamic response determination

The purpose of this procedure is to understand how fast the anemometer system can respond to flow signal changes. This capability is gauged by measuring the frequency response when the signal turns on and off by applying a square wave.

1. Secure the hot wire probe of the CTA system inside of a wind tunnel using a support shaft.
2. Set-up a DC power supply, signal generator and oscilloscope and connect them as shown in Figure 2(a). The signal generator supplies a square wave input to the Wheatstone bridge, and the output waveform is visualized on the oscilloscope.
3. Turn on the hot-wire power supply, the oscilloscope and the signal generator.
4. Set-up the signal generator to output a square wave with 150 mV amplitude and 10 kHz frequency.
5. Observe the output signal in the oscilloscope to make sure that the output wave form frequency and amplitude are correct.
6. Close the test section and plug in the serial port. Turn on the wind tunnel and set the airspeed to 40 mph.
7. Once the airflow stabilizes, measure the width of the signal overshoot, τ, from the oscilloscope. See Figure 3 for the definition of τ.
8. Use the measured value of τ to obtain the cut-off frequency of the hot-wire system using the equation: f_cut = 1/1.5τ.
9. Turn off the wind tunnel.

2. Hot-wire calibration

The purpose of this procedure is to establish the correlation between the airspeed and the electric potential of the Wheatstone bridge. This allows the flow velocity to be measured.

1. Adjust the hot-wire probe to the vertical position so that it is a far enough distance away from the flat plate, which in this case is the floor of the wind tunnel, so that it is located in the free stream region.
2. Start the wind tunnel control software.
3. Open the virtual instrument software, and set the sampling frequency to 10 kHz and the number of samples to 100,000. These parameters are determined by the flow characteristics of the flow field to be measured and may vary depending on the knowledge of convergence requirement of the targeted statistics.
4. Set the wind tunnel speed to 0 mph, and record the voltage on the Wheatstone bridge.
5. Increase the wind tunnel air speed by increments of 3 mph up to 15 mph. Allow the flow to stabilize at each air speed before measuring the voltage.
6. Increase the wind tunnel air speed by increments of 5 mph up to 60 mph, and measure the voltage at each increment.
7. When all measurements are complete, reduce the air flow to 30 mph, and then turn off the wind tunnel.

Figure 3. Schematic for width of signal overshoot, τ as observed on an oscilloscope during a square wave test.

3. Boundary layer survey

1. Using the same set-up as in the previous experimental section, slowly lower the hot-wire probe until it touches the test section floor, which will act as a flat plate.
2. Turn on the wind tunnel and set the airspeed to 40 mph, the sampling frequency to 10 kHz and the number of samples to 100,000 like previously.
3. Record the voltage reading at the lowest vertical setting, which is next to the flat plate and in the boundary layer.
4. Move the probe vertically, increasing the height in steps of 0.05 mm up to the height of 0.50 mm, and record the voltage reading at vertical position.
5. Increase the probe height in steps of 0.10 mm up to the height of 1.50 mm, and record the voltage reading at vertical position.
6. Increase the probe height in steps of 0.25 mm up to the final height of 4.00 mm, and record the voltage reading at vertical position.
7. When all measurements are complete, reduce the air speed to 20 mph, and then turn off the wind tunnel, CTA, power supply, oscilloscope, and function generator.