Visualization of Flow Past a Bluff Body

1. To produce a continuous sheet of bubbles:

1. Set the equipment according to the electrical diagram shown in Figure 3.
2. Fix the positive electrode in the water at the downstream end of the test section (see Figure 4 for reference).
3. Fix the negative electrode upstream and near the point of interest to release the bubbles into the stream before the flow reaches the object of study (see Figure 4 for reference). The water completes the circuit between the two electrodes.
4. Turn on the flow facility
5. Set the dial of the frequency controller to position 2. This will establish a flow rate of about 9x10^-4 m³/s.
6. Turn on the DC power supply and increase the voltage up to about 25 V, the current will set itself around 190 mA.
7. Set the wave form in the signal generator to Square Wave (symbol: ). This generates a 0 V - 5 V square signal that activates the solid-state relay (closing the circuit) in its high position and opens it in the low position
8. For this particular case, the frequency of the square wave is not important. It just needs to be non-zero.
9. Maximize the DC offset (+5 V) in the signal generator. With this setting, the circuit is always closed and the system generates bubbles continuously.

Figure 3. Connections diagram.

Figure 4. Test section. Flow goes from left to right. The negative electrode generates a layer of hydrogen bubbles that are swept away with the flow. The positive electrode is set at the downstream end of the test section to avoid its disturbances. Please click here to view a larger version of this figure.

2. To produce timelines:

1. Turn on the flow facility
2. Set the dial of the frequency controller to position 2. This will establish a flow rate of about 9x10^-4 m³/s.
3. Turn on the DC power supply and increase the voltage up to about 25 V, the current will set itself around 190 mA.
4. Set the wave form in the signal generator to Square Wave (symbol: ). This generates a 0 V - 5 V square signal that activates the solid-state relay (closing the circuit) in its high position and deactivates it (opening the circuit) in the low position
5. Set the DC offset in the signal generator at +1 V.
6. Set the frequency of the square wave in the signal generator at 10 Hz.
7. Set the symmetry of the square wave slightly negative (-2) to increase the space between timelines while conserving the right frequency.

3. To use flow lines to study Von Kármàn vortex streets:

1. Measure the diameter of the rod,, using a caliper. Use S.I. units for this measurement (m).
2. Fix a cylindrical rod downstream of the negative electrode.
3. Cast the light of the high-intensity discharge lamp on the layer of hydrogen bubbles. Make sure the light is not directly behind of the line of view to prevent oversaturation of the imaging system
4. Align the visualization system with the rod; in a way that only the circular tip is visible in front of the camera.
5. Add a mark in the visualization window and downstream of the rod to use it as the reference to count vortex-shed cycles per unit time.

4. Data analysis for flow past a circular cylinder:

1. Determination of the conversion factor from machine units to real space units:
   1. Measure the width of the shadow cast by the rod on the bubble sheet (see figure 2(A) for reference). Take this measurement right at the rod to avoid distortion with distance. This is the diameter of the rod in machine units, (points or pixels, depending on format)
   2. Use the following equation to determine the conversion factor from machine units to real world units:
      
2. Determination of flow velocity:
   1. Choose a group of undistorted timelines upstream of the bluff body.
   2. Measure the distance between the first and last timeline in machine units, (points or pixels).
   3. Count the number of timelines in the group, .
   4. Take note of the frequency of the square wave signal as produced by the signal generator, .
   5. Determine the approaching flow velocity from the following equation:
      
3. Determination of the Reynolds number:
   1. Find the kinematic viscosity of the working fluid (e.g. water m²/s).
   2. Calculate the Reynolds number using equation (1). For that, considering the diameter of the rod () measured in step 3.1, the approaching velocity () determined with equation (5), and the kinematic viscosity determined in step 4.3.1
4. Determination of the Strouhal number: the vortices in the wake of the rod are moving at a different velocity as the timelines in the free stream. Hence, the frequency of vortex shedding needs to be estimated independently.
   1. Define a fixed reference downstream of the rod. This reference could be a fine string attached to the exterior of the tunnel or a digital line added to a video of the flow process.
   2. Count the number of vortex shedding cycles, , crossing the reference during a defined period of time . A vortex shedding cycle is illustrated in Figure 2(A).
   3. Calculate the shedding frequency from the following equation:
      
   4. Use the results from equations (5) and (6) in equation (2) to calculate the Strouhal number.