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

A boundary layer is a thin flow region immediately adjacent to the surface of a solid body in a flow field. The region of flow outside of the boundary layer, called the free stream region has a constant velocity. However, within the boundary layer there is a velocity gradient due to friction at the surface. The boundary layer typically undergoes several stages.

First the laminar boundary state, followed by the transition state and finally, the turbulent boundary layer state, which involves irregular flow and fluctuations, like mixing or eddying. The boundary layer is the basis for the calculation of skin friction drag on aircraft.

Skin friction drag is created within the boundary layer and is due to the viscous shear stress exerted on the surface. Skin friction drag is proportional to fluid dynamic viscosity, mu, and the local stream wise velocity shear strain rate, which is the gradient of the streamwise velocity in the normal direction. So it becomes significant for large areas, such as an airplane wing. Additionally, skin friction drag is higher in turbulent flow, since the fluid particles interact with the surface at high momentum.

One way to measure turbulent boundary layer properties is using hot wire anemometry, which is based on two principles related to the cooling effect of flow on a heated wire. According to the first principle, when a fluid flows over a hot surface, the convective heat coefficient changes, which results in changes in the surface temperature.

The second principle is Joule's law, which states that an electrical conductors heat dissipation, Q, is proportional to the square of the electric current, I, applied to the conductor. We can use the two principles to determine the velocity of fluid flow surrounding a heated metallic wire probe, by measuring the electrical potential E, that has to be applied to maintain a constant temperature of the wire.

A commonly used hot wire technique is Constant Temperature Anemometry or CTA. CTA consists of a very thin metallic wire, called the probe, which is connected to the arm of a Wheatstone bridge. The Wheatstone bridge controls the electrical potential and adjusts it as needed in order to maintain a constant temperature across the wire. Any cooling is caused by fluid flow around the wire. Thus, the change in the potential is a function of the heat transfer coefficient and by extension is a function of velocity.

In this experiment, we will demonstrate the use of a Constant Temperature Anemometry setup to measure the turbulent boundary layer over a flat plate.

First, we will learn how the Constant Temperature Anemometer, or CTA, system responds to flow signal changes using a wind tunnel. To begin, secure the hot wire probe of the CTA system inside of the wind tunnel using a support shaft.

Then, set up a DC power supply, signal generator, and oscilloscope. The components are connected as shown. To begin, turn on the hot wire power supply, the signal generator and the oscilloscope. Set the signal generator to supply a square wave input to the Wheatstone bridge with a 150 mV amplitude and a 10 kHz frequency.

Observe the output signal in the oscilloscope to make sure that the frequency and amplitude are correct. Now close the test section, plug in the serial cable, turn on the wind tunnel and set the wind speed to 40 mph. Once the airflow stabilizes, measure the width of the signal overshoot, tau, observed on the oscilloscope. Use the measured value of tau to calculate the cut-off frequency for the hot wire system using this equation. Then turn off the wind tunnel airflow.

Next we will establish the correlation between wind speed and the electrical potential of the Wheatstone bridge. To begin, raise the CTA probe in the vertical direction so that it is in the free stream region. Start the wind tunnel control software and then start the virtual instrument software. Set the sampling rate to 10 kHz and the number of samples to 100,000.

Now, with the wind tunnel airspeed set to 0 mph, record the voltage on the Wheatstone bridge. Then, increase the wind tunnel airspeed at increments of 3 mph up to 15 mph, measuring the voltage at each increment. Be sure to allow the air flow to stabilize before recording the voltage measurement.

Next, increase the wind tunnel air speed up to 60 mph in 5-mph increments, measuring the voltage at each increment. When all measurements are complete, reduce the airspeed to 30 mph and then turn off the wind tunnel airflow.

Using the same setup as before, lower the CTA probe slowly until it touches the test section floor, which will act as the flat plate. Set the airflow to 40 mph. Keep the sampling frequency at 10 kilohertz and the number of samples at 100,000. Record the voltage at the lowest vertical setting, which is next to the flat plate and in the boundary layer.

Now, move the probe vertically in steps of 0.05 mm up to a height of 0. 5 mm, recording the voltage reading at each position. Then, increase the probe height in increments of 0.1 mm up to a height of 1. 5 mm. And then in steps of 0.25 mm up to a final height of 4 mm, while recording the voltage at each increment.

When all of the measurements have been made, reduce the wind speed to 20 mph and then turn off the airflow. Then shut off the power supply, signal generator, and oscilloscope.

The first step in analyzing the data is to use the data acquired during the calibration step of the experiment, to determine the correlation between the hot wire voltage and air speed. There are several different methods to do this, which involve fitting the data to known heat transfer relationships, and it's covered in detail in the appendix of this video.

Once the mathematical relationship has been determined, use the voltage measurements to calculate velocity at each vertical height. After adjusting the nominal height to account for any artifacts from an overbent probe, plot the velocity profile u(y), which can then be used to determine the boundary layer displacement thickness.

This value represents the distance that the plate would have to be moved vertically in order to obtain the same flow rate that occurs between the surface and the fluid. We can also calculate the momentum thickness, defined as shown, which is the distance the plate would have to be moved vertically in order to have the same momentum that exists between the fluid and plate.

From these two parameters, we can calculate the shape factor, H. The shape factor is used to determine the nature of the flow, where a shape factor of about 1.3 indicates fully turbulent flow and about 2.6 for laminar flow. Between these values is transitional flow. In the case of this experiment, the shape factor was calculated as 1.9, indicating transitional flow.

In summary, we learned about boundary layer flow development, and then used a Constant Temperature Anemometry setup to analyze the turbulent boundary layer over a flat plate and observe low behavior.