Some unique roles of turbulent scales in engineering

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Turbulence: Theory, Types and Simulation

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It is trite to state that flow turbulence plays a critical role in many engineering applications and in nature. Turbulent intensity relative to the prevailing time-averaged velocity is the most familiar parameter used in quantifying the concerned flow turbulence and its influence. In aerodynamics, an increase in turbulent intensity typically advances the occurrence of the drag crisis where the drag coefficient drops sharply. The general notion is that the critical Reynolds number associated with this sudden fall in drag coefficient decreases with increasing intensity of the flow turbulence. Convective heat transfer coefficient is generally enhanced with augmentation of turbulent intensity, though there is significant variation as far as the quantitative increase in Nusselt number with respect to the level of turbulent fluctuation is concern. A few studies have noted that this Nusselt number enhancement by turbulent intensity has an upper limit, beyond which further increase in turbulent intensity can result in a reduction of the Nusselt number. This is but one indication that there may be additional turbulent effects which must be considered other than that of the fluctuation intensity. For the aerodynamic example, a few investigators have observed that particular eddy-bluff body size ratios appear to be more effective in advancing the drag crisis via flow-stream turbulence (turbulent intensity). The ever-complex interactions amongst the underlying scales of turbulence tend to seriously mask the superficially dominating turbulent intensity effects. The differences in the underlying turbulence scales are believed to have caused much discrepancy from one study to another. The discrepancy may be subtle under 'typical' circumstances; however, it can become substantial when some particular conditions are met. This chapter attempts to uncover some of the unique roles played by a couple of key turbulent scales. The scope of this endeavor is limited to the independent influences of Reynolds number, turbulence intensity, and integral length scale on simple bluff body aerodynamics and on convection heat transfer. The office of Taylor microscale is also introduced and briefly explored. © 2012 by Nova Science Publishers, Inc. All rights reserved.



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