Vibration Vortex shedding

VoHex shedding The vortex-shedding frequency of the fluid in cross-flow over the tubes may coincide with a natural frequency of the tubes and excite large resonant vibration amplitudes. 

The vibration induced by the fluid flowing over the tube bundle is caused principally by vortex shedding and turbulent buffeting. As fluid flows over a tube vortices are shed from the down-stream side which cause disturbances in the flow pattern and pressure distribution round the tube. Turbulent buffeting of tubes occurs at high flow-rates due to the intense turbulence at high Reynolds numbers. 

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The concept of cross-flow velocity is quite important in understanding how heat exchangers work. This concept is related to a flow phenomenon called vortex shedding. Perhaps you have seen a wire quivering in the wind. What causes the wire to vibrate with such energy ... 

Preheater vibration. Air preheaters or any type of waste-heat recovery device designed for horizontal flow across vertical tubes, may be subject to vibration produced by the velocity of gas across the tube banks. The velocity produces a vortex-shedding wave pattern that could correspond to the natural harmonic frequency of the tube bank. If the natural harmonic frequency is reached, excessive vibration of the tubes will occur. Redesign of the internal baffle system by inserting dummy baffles can stop the vibration. 

Figure 48-6 The effect of vortex shedding on a stack subjected to a steady wind is oscillation of the cantilevered cylinder in a direction transverse to that of the wind (left). Theory says that vortices are shed intermittently from each side of the stack, causing the motion. Studies of such dynamic wind effects show only the first vibration mode to be significant...

Tube vibrations in a tube bundle are caused by oscillatory phenomena induced by fluid (gas or liquid) flow. The dominant mechanism involved in tube vibrations is the fluidelastic instability or fluidelastic whirling when the structure elements (i.e., tubes) are shifted elastically from their equilibrium positions due to the interaction with the fluid flow. The less dominant mechanisms are vortex shedding and turbulent buffeting. 

Now we will briefly describe two additional mechanisms vortex shedding and turbulent buffeting. It should be noted that these mechanisms could cause tube vibrations, but their influence on a tube bundle is less critical compared to the fluidelastic instabilities described earlier. 

Tube Bundle Natural Frequency. Elastic structures vibrate at different natural frequencies. The lowest (fundamental) natural frequency is the most important. If the vortex shedding or turbulent buffeting frequency is lower than the tube fundamental natural frequency, it will not create the resonant condition and the tube vibration problem. Hence, the knowledge of the fundamental natural frequency is sufficient in most situations if / is found to be higher than / or fb. Higher than the third harmonic is generally not important for flow-... 

When a fluid flows past a bluff body, the wake downstream will form rows of vortices that shed continuously from each side of the body as illustated in Figure 4.16. These repeating patterns of swirling vorticies are referred to as Karman vortex streets named after the fluid dynamicist Theodore von Karman. Vortex shedding is a common flow phenomenon that causes car antennas to vibrate at certain wind speeds and also lead to the collapse of the famous Tacoma Narrows Bridge in 1940. Each time a vortex is shed from the bluff body it creates a sideways force causing the body to vibrate. The frequency of vibration is linearly proportional to the velocity of the approching fluid stream and is independent of the fluid density. 

The frequency of vortex shedding or vibration of the bluff body is determined as follows ... 

Vessels that qualify as flexible may or may not be required to be checked for dynamic response. This could include a dynamic analysis, which is a check of elastic instability, or a vibration analysis for vibration amplification due to vortex shedding. See procedure 4-8 Vibration of Tall Towers and Stacks, for additional information. 

The deflections resulting from vortex shedding are perpendicular to the direction of wind flow and occur at relatively low wind velocities. When the natural period of vibration of a stack or column coincides with the frequency of vortex shedding, the amplitude of vibration is greatly magnified. The frequency of vortex shedding is related to wind velocity and vessel diameter. The wind velocity at which the frequency of vortex shedding matches the natural period of vibration is called the critical wind velocity. 

Once a vessel has been designed statically, it is necessary to determine if the vessel is susceptible to wind-induced vibration. Historically, the rule of thumb was to do a dynamic wind check only if the vessel L/D ratio exceeded 15 and the POV was greater than 0.4 seconds. This criterion has proven to be unconservative for a number of applications. In addition, if the critical wind velocity, V,., is greater than 50 mph, then no further investigation is required. Wind speeds in excess of 50 mph always contain gusts that will disrupt uniform vortex shedding. 

Critical mnd velocity The velocity at which the frequency of vortex shedding matches one of the normal modes of vibration. 

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