Turbulent buffeting

Turbulent buffeting is the name given to the fluctuating forces acting on tubes due to extremely turbulent flow of the shellside fluid. The turbulence has a wide spectrum of frequencies distributed around a central dominant frequency which increases as the crossflow velocity increases. This turbulence buffets the tubes which selectively extract energy from the turbulence at their natural frequencies from the spectrum of frequencies present. This is an extremely complex form of excitation. Turbulent buffeting frequencies can be predicted as below  

This method is not recommended for liquids because prediction is based on data for gases. 

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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. 

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. 

Turbulent Buffeting. Turbulent buffeting refers to unsteady forces developed on a body exposed to a highly turbulent flow. The oscillatory phenomenon in turbulent flow on the shell side (when the shellside fluid is gas) is characterized by fluctuating forces with a dominant frequency as follows [123,154] ... 

The correlation was originally proposed for tube-to-diameter ratios of 1.25 and higher. It should be noted that the turbulent buffeting due to the oncoming turbulent flow is important... 

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-... 

Determine the vortex shedding frequency using Eq. 17.157 and turbulent buffeting frequency using Eq. 17.158. Also compute the natural frequency/ of the tubes by using Eq. 17.159 or 17.160. 

Avoid vibration as a result of turbulent buffeting. The following ratio is suggested ... 

The subject of a vertical column or stack responding to wind in dynamic resonance has been addressed in engineering for over 100 years. For the past 50 years, discussion of the subject has grown in engineering publications. The subject is more accurately cdlled fluid-structure interaction. The dynamic resonance is mostly caused by vortex shedding around the column or stack, but where there are two or more stacks, the mechanism of turbulence buffeting can exist. 

The analysis indicated that at the smaller shaft diameter, the expected lateral movement of each skip would not exceed the clearances provided in the design. Recommendations were made regarding management of airflow and potential buffeting of stationary conveyances at ventilation off-takes or at the change in shaft diameter, through tapering of transitions to avoid excessive turbulence. 

The wind velocity is of random nature at a specific time and location, i.e., it is a stochastic process in time and space. The buffeting lift, L(x,t), and moment, Q(x,t), are linear functions of the horizontal and vertical components of the wind turbulence, and they are assumed to be stationary random processes. Hence, the buffeting loads are represented by the cross-spectra of the buffeting lift and moment, which, in turn, are expressed in terms of the cross - spectra of the horizontal and vertical components of the wind velocity [6, 7, 16, 18]. 

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