Name: hydrogen bubble flow around a cylinder
Uploaded: 2023-04-30T07:02:00.000Z
Duration: 10 min 32 s
Channel: JoVE

1. To produce a continuous sheet of bubbles:
<ol>
	<li>Set the equipment according to the electrical diagram shown in Figure 3.</li>
	<li>Fix the positive electrode in the water at the downstream end of the test section (see Figure 4 for reference).</li>
	<li>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.</li>
	<li>Turn on the flow facility</li>
	<li>Set the dial of the frequency controller to position 2. This will establish a flow rate of about 9x10-4 m3/s.</li>
	<li>Turn on the DC power supply and increase the voltage up to about 25 V, the current will set itself around 190 mA.</li>
	<li>Set the wave form in the signal generator to Square Wave (symbol: <img src="/files/ftp_upload/10435/10435eq018.jpg" alt="Static equilibrium diagram; ΣFx=0; force balance illustration for educational use." data-altupdatedat="1761830439000">). 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</li>
	<li>For this particular case, the frequency of the square wave is not important. It just needs to be non-zero.</li>
	<li>Maximize the DC offset (+5 V) in the signal generator. With this setting, the circuit is always closed and the system generates bubbles continuously.</li>
</ol>
<img alt="Schematic of DC circuit with signal generator for electrode configuration in electrolysis setup." src="/files/ftp_upload/10435/10435fig3.jpg" data-altupdatedat="1761830439000" data-alt="Figure 3"> 
Figure 3. Connections diagram.
<img alt="Schematic of DC circuit with signal generator for electrode configuration in electrolysis setup." src="/files/ftp_upload/10435/10435fig3.jpg" data-altupdatedat="1761830439000" data-alt="Figure 3"> 
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. <a href="//cloudfront.jove.com/files/ftp_upload/10435/10435fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

2. To produce timelines:
<ol>
	<li>Turn on the flow facility</li>
	<li>Set the dial of the frequency controller to position 2. This will establish a flow rate of about 9x10-4 m3/s.</li>
	<li>Turn on the DC power supply and increase the voltage up to about 25 V, the current will set itself around 190 mA.</li>
	<li>Set the wave form in the signal generator to Square Wave (symbol: <img src="/files/ftp_upload/10435/10435eq018.jpg" alt="Static equilibrium diagram; ΣFx=0; force balance illustration for educational use." data-altupdatedat="1761830439000">). 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</li>
	<li>Set the DC offset in the signal generator at +1 V.</li>
	<li>Set the frequency of the square wave in the signal generator at 10 Hz.</li>
	<li>Set the symmetry of the square wave slightly negative (-2) to increase the space between timelines while conserving the right frequency.</li>
</ol>
3. To use flow lines to study Von Kármàn vortex streets:
<ol>
	<li>Measure the diameter of the rod,<img src="/files/ftp_upload/10435/10435eq019.jpg" alt="Static equilibrium equations ΣFx=0, diagram, illustrating physics principles, force balancing" data-altupdatedat="1761830439000">, using a caliper. Use S.I. units for this measurement (m).</li>
	<li>Fix a cylindrical rod downstream of the negative electrode.</li>
	<li>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</li>
	<li>Align the visualization system with the rod; in a way that only the circular tip is visible in front of the camera.</li>
	<li>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.</li>
</ol>
4. Data analysis for flow past a circular cylinder:
<ol>
	<li>Determination of the conversion factor from machine units to real space units:
	<ol>
		<li>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, <img src="/files/ftp_upload/10435/10435eq020.jpg" alt="static equilibrium equation ΣFx=0 diagram; physics force components analysis" data-altupdatedat="1761830439000"> (points or pixels, depending on format)</li>
		<li>Use the following equation to determine the conversion factor from machine units to real world units: 
		<img src="/files/ftp_upload/10435/10435eq021.jpg" alt="Magnification equation, M=Di/Do, formula image for optics and lens calculations." data-altupdatedat="1761830439000"></li>
	</ol>
	</li>
	<li>Determination of flow velocity:
	<ol>
		<li>Choose a group of undistorted timelines upstream of the bluff body.</li>
		<li>Measure the distance between the first and last timeline in machine units, <img src="/files/ftp_upload/10435/10435eq022.jpg" alt="Static equilibrium equation ΣFx=0, diagram depicting forces at play, educational physics concept." data-altupdatedat="1761830439000"> (points or pixels).</li>
		<li>Count the number of timelines in the group, <img src="/files/ftp_upload/10435/10435eq023.jpg" alt="Static equilibrium diagram, ΣFx=0, with forces and moments, illustrating balance conditions." data-altupdatedat="1761830439000">.</li>
		<li>Take note of the frequency of the square wave signal as produced by the signal generator, <img src="/files/ftp_upload/10435/10435eq024.jpg" alt="Fourier transform equation, diagram of frequency domain conversion, signal processing method." data-altupdatedat="1761830439000">.</li>
		<li>Determine the approaching flow velocity from the following equation: 
		<img src="/files/ftp_upload/10435/10435eq025.jpg" alt="Static equilibrium equation U∞=L·ftl/(Ntl·M) formula; useful for engineering calculations." data-altupdatedat="1761830439000"></li>
	</ol>
	</li>
	<li>Determination of the Reynolds number:
	<ol>
		<li>Find the kinematic viscosity of the working fluid (e.g. water <img src="/files/ftp_upload/10435/10435eq026.jpg" alt="Viscosity formula, ν=1.004×10⁻⁶, calculation diagram, fluid dynamics concept, physics analysis." data-altupdatedat="1761830439000"> m2/s).</li>
		<li>Calculate the Reynolds number using equation (1). For that, considering the diameter of the rod (<img src="/files/ftp_upload/10435/10435eq027.jpg" alt="Static equilibrium ΣFx=0, MA=0 shown in force diagram; vectors, angles for mechanical analysis." data-altupdatedat="1761830439000">) measured in step 3.1, the approaching velocity (<img src="/files/ftp_upload/10435/10435eq001.jpg" alt="U∞ symbol, representing steady-state velocity, used in fluid dynamics equations and calculations." data-altupdatedat="1761830439000">) determined with equation (5), and the kinematic viscosity determined in step 4.3.1</li>
	</ol>
	</li>
	<li>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.
	<ol>
		<li>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.</li>
		<li>Count the number of vortex shedding cycles, <img src="/files/ftp_upload/10435/10435eq028.jpg" alt="Reaction mechanism diagram, kinematic variable N_s, dynamic equation analysis." data-altupdatedat="1761830439000">, crossing the reference during a defined period of time <img src="/files/ftp_upload/10435/10435eq029.jpg" alt="Thermal diffusion diagram; equation for heat transfer; shows energy flow; educational use." data-altupdatedat="1761830439000">. A vortex shedding cycle is illustrated in Figure 2(A).</li>
		<li>Calculate the shedding frequency from the following equation: 
		<img src="/files/ftp_upload/10435/10435eq030.jpg" alt="Frequency calculation formula \( f_s = \frac{N_s}{T} \) for signal processing analysis." data-altupdatedat="1761830439000"></li>
		<li>Use the results from equations (5) and (6) in equation (2) to calculate the Strouhal number.</li>
	</ol>
	</li>
</ol>

In this study, the usage of hydrogen bubbles was demonstrated to extract qualitative and quantitative information from images of flow around a circular cylinder. The quantitative information extracted from these experiments included the free-stream velocity (<img src="/files/ftp_upload/10435/10435eq001.jpg" alt="U∞ symbol, representing steady-state velocity, used in fluid dynamics equations and calculations." data-altupdatedat="1761830439000">), vortex-shedding frequency (<img src="/files/ftp_upload/10435/10435eq009.jpg"...

visualization of flow past a bluff body

Source: Ricardo Mejia-Alvarez, Hussam Hikmat Jabbar and Mahmoud N. Abdullatif, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
Owing to the non-linear nature of its governing laws, fluid motion induces complicated flow patterns. Understanding the nature of these patterns has been the subject of intense scrutiny for centuries. Although personal computers and supercomputers are extensively used to deduce fluid flow patterns, their capabilities are still insufficient to determine the exact flow behavior for complex geometries or highly inertial flows (e.g. when momentum dominates over viscous resistance). With this in mind, a multitude of experimental techniques to make flow patterns evident have been developed that can reach flow regimes and geometries inaccessible to theoretical and computational tools.
This demonstration will investigate fluid flow around a bluff body. A bluff body is an object that, due to its shape, causes separated flow over most of its surface. This is in contrast to a streamlined body, like an airfoil, which is aligned in the stream and causes less flow separation. The purpose of this study is to use hydrogen bubbles as a method of visualizing flow patterns. The hydrogen bubbles are produced via electrolysis using a DC power source by submerging its electrodes in the water. Hydrogen bubbles are formed in the negative electrode, which needs to be a very fine wire to ensure that the bubbles remain small and track fluid motion more effectively. This method is suitable for steady and unsteady laminar flows, and is based on the basic flow lines that describe the nature of the flow around objects. [1-3]
This paper focuses on describing the implementation of the technique, including details about the equipment and its installation. Then, the technique is used to demonstrate the use of two of the basics flow lines to characterize the flow around a circular cylinder. These flow lines are used to estimate some important flow parameters like flow velocity and the Reynolds number, and to determine flow patterns.

Figure 2 shows two representative results of hydrogen bubble visualization of a Von Kármàn vortex street. Figure 2(A) shows an example of a field of streaklines as evidenced by disturbances in the hydrogen bubble sheet. This image is used to extract the diameter of the rod in machine units. Figure 2(B) shows an example of a field of timelines. This image is used to estimate the approaching fluid velo...

Visualization of Flow Past a Bluff Body

Source: Ricardo Mejia-Alvarez, Hussam Hikmat Jabbar and Mahmoud N. Abdullatif, Department of Mechanical Engineering, Michigan State University, East ...

Due to the nonlinear nature of its governing laws, fluid motion induces complicated flow patterns. These flow patterns are influenced by many factors, one of which is flow past an obstacle such as a bluff body. A bluff body is an object that, due to its shape, causes separated flow over most of its surface. Depending on the flow conditions, this flow may become unstable, giving rise to oscillating flow patterns in the wake called vortex shedding. This video will introduce the basics of flow separation and vortex shedding caused by a bluff body and demonstrate a technique used to visualize the resulting flow patterns.First, let's consider the uniform steady flow of water with velocity U infinity called the free stream velocity approaching a circular cylinder. Boundary layer separation on the object's surface leads to the formation of vortices around the body that eventually detach into the wake. When periodic detachment takes place, the vortices generate alternating areas of low pressure behind the body. This process is called the Von Karman vortex street. This repeating pattern occurs at certain ranges of Reynolds number, a dimensionless parameter defined as the ratio of inertial forces to viscous forces. Here, nu is the kinematic viscosity of the fluid, V is the characteristic velocity or U infinity in this case, and D is the cylinder diameter. For example, in the setup in the following demonstration, when the Reynolds number is around five, the flow exhibits two stable counter-rotating vortices behind the cylinder. As the Reynolds number increases, these vortices elongate in the direction of the flow. When the Reynolds number reaches approximately 37, the wake becomes unstable and oscillates sinusoidally as a result of an imbalance between pressure and momentum. The frequency in which vortices are shed off the cylinder is not constant, rather it varies with the value of the Reynolds number. This shedding frequency is characterized by the Strouhal number, which is another dimensionless parameter. The Strouhal number is defined as shown where f is the vortex shedding frequency. Experimental analysis of flow patterns uses four types of flow lines. A path line is the path that a given fluid particle follows as it moves with the flow. A streak line is the continuous locus of all fluid particles whose motion originated from the same location. A streamline is an imaginary line that is instantaneously and locally tangent to the velocity field. Note that path lines, streak lines, and streamlines coincide with each other under steady flow conditions. In the current flow, this corresponds to regions of the flow upstream from the bluff body or far enough from the influence of its wake. On the other hand, path lines, streak lines, and streamlines differ from each other under unsteady flow conditions. In the current flow, this corresponds basically to the wake of the bluff body. Finally, timelines are the continuous locus of fluid particles that were released to the flow at the same instant in time. In the following experiment, we will use a continuous sheet of tiny hydrogen bubbles to analyze flow patterns using timelines and streak lines. Now, let's take a look at how to set up the flow experiment.First, assemble the equipment according to the electrical diagram shown. Fix the positive electrode in the water at the downstream end of the test section. Next, fix the negative electrode upstream. This should be near the point where the bubbles are released into the stream before the flow reaches the object of study. Turn on the flow facility. Then set the dial of the frequency controller to position two in order to establish a mean velocity of about 0.04 meters per second. This velocity corresponds to a flow rate of about 50 to the minus fifth cubed meters per second. Now turn on the DC power supply and increase the voltage to about 25 volts with the current around 190 milliamps. On a signal generator, set the output to a square wave with a zero-volt to five-volt square signal that closes the circuit in its high position and opens it in the low position. Maximize the DC offset to five volts so that the circuit is always closed and the system generates bubbles continuously. To produce timelines, change the DC offset in the signal generator to one volt. Then set the frequency of the square wave to 10 Hertz. Timelines will be produced in the flow. Then set the symmetry of the square wave to minus two in order to increase the space between timelines.First measure the diameter of the rod using a caliper in SI units. Fix the cylindrical rod downstream of the negative electrode. Cast high-intensity light on the layer of hydrogen bubbles, making sure that the light is not directly behind the line of view to prevent over saturation of the imaging system. Align the visualization system with the rod so that only the circular tip is visible in front of the camera. Add a mark in the visualization window and downstream of the rod to use it as the reference point to count vortex shed cycles.First measure the width of the shadow cast by the rod on the bubble sheet. Take the measurement right at the rod to avoid distortion with distance. Use the rod diameter to determine the conversion factor from machine units to real-world units. Next, choose a group of nearly undistorted timelines away from the bluff body and the influence of its wake. Measure the distance L between the first and last timeline in machine units. Count the number of timelines in the group and note the frequency of the square wave. Determine the approaching flow velocity from the following equation. Now using the kinematic viscosity of water, calculate the Reynolds number. Next, determine the Strouhal number by observing the vortices in the wake of the rod. Note that the vortices move at a different velocity as compared to the timelines in the free stream. Using the fixed string as reference, count the number of vortex shedding cycles, NS, crossing the reference point during a defined period of time. Calculate the shedding frequency. Then use the results to calculate the Strouhal number.Now that we have gone over the procedure and analysis, let's take a look at the results. The validity of the result can be determined using a relationship between the Reynolds number and the Strouhal number. The coefficients St* and m depend on the Reynolds number range and can be found in the literature. The Reynolds number in this example is 115. Thus, the values of St* and m can be used to calculate the Strouhal number. The calculated value for the Strouhal number is 0.172, which correlates well to the measured value of 0.169. When this experiment was conducted with varying operational parameters, the calculations of the Reynolds and Strouhal numbers correlated well to the mathematical relationship between the two numbers. This shows how well the bubble method can be used to understand flow patterns around a bluff body.Understanding flow patterns is essential to the design and operation of many types of engineering applications. Pillars of bridges and offshore oil rigs are designed to withstand the turbulence caused by current flow past the structure. Knowing the vortex shedding frequencies at which a given structure will be exposed is critical for its design. In that regard, engineers have to make sure that the natural frequency of the structure is not such that it will resonate with the vortex shedding frequency because this will inevitably lead to catastrophic failure of the structure. It is also essential to study fluid flow around a streamline object such as an air foil or ship hull. By making use of flow lines, engineers can determine parameters such as the angle at which an airplane stalls or even estimate lift characteristics based on flow velocity. You've just watched Jove's video on visualizing flow lines around a bluff body. You should now understand the basics of fluid flow patterns and the Von Karman vortex street, how to set up an experiment to visualize these flow patterns, and how to study the flow behavior. Thanks for watching.

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Hydrogen bubble flow around a circular cylinder shows how fluid moves past a bluff body. A bluff body is an object with a shape that causes separated flow over most of its surface. This is different from a streamlined body, such as an airfoil, which stays aligned with the stream and causes less flow separation.

Fluid motion can form complex patterns because its governing laws are non-linear. Scientists have studied these patterns for centuries. Computers and supercomputers can help predict flow, but they may still fall short for complex shapes or highly inertial flows, where momentum dominates over viscous resistance.

To make these patterns visible, the method uses hydrogen bubbles created by electrolysis. A DC power source is used with electrodes submerged in water. Hydrogen forms at the negative electrode. That electrode must be a very fine wire so the bubbles stay small and can track the fluid motion more effectively.

This technique works for steady and unsteady laminar flows. It uses basic flow lines to show the nature of the flow around objects. The demonstration also uses two basic flow lines to describe the flow around a circular cylinder. These flow lines help estimate flow velocity and the Reynolds number, and they help determine the flow pattern.

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mechanical engineering

This collection introduces a range of concepts that are essential for understanding and designing mechanical systems. Each video examines a specific topic and describes fundamental analytical methods commonly employed to understand physical behaviors.

See how hydrogen bubbles are used to visualize fluid flow past a circular cylinder, estimate velocity and Reynolds number, and reveal flow patterns.

Hydrogen Bubble Flow Around a Cylinder

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