Response of a turbulent boundary layer to multiple strain rates
Abstract
A variety of engineering flows are influenced by multiple interacting strain rates. Even though the response in laminar flows to multiple strain rates can be expected to obey simple mechanical laws, the effects of the individual strain rates in the presence of turbulence are most likely not superposable since turbulent motions are affected over a spectrum of scales. Consequently, studies on the response of a turbulent boundary layer exposed to multiple extra strain rates are essential. Detailed measurements of a turbulent boundary layer that was exposed to multiple, extra rates of strain due to streamline curvature and streamwise pressure gradients were taken and analyzed to examine the response of turbulence to multiple strain rates. The two extra strain rates not only interacted with each other, but their interaction was nonlinear. The measurements showed that the outer portion of the boundary layer was more strongly affected by the extra applied strain rates than the inner layer. This behavior was consistent with profiles of the flux Richardson number, where its critical value was exceeded at the same wall-normal location as the upper limit of the log-law region $\rm (y\sp+ \approx 100).$ In the outer portion of the boundary layer an increase of the total strain rate parameter occurred immediately downstream of the onset of convex curvature, which could be attributed to an instant attenuation of the large-eddy breakup cycle. Stabilization of the boundary layer due to convex curvature was augmented by favorable pressure gradients and counteracted by adverse pressure gradient. The turbulence production cycle, characterized by turbulent bursting, was strongest in regions of newly introduced strain rates. It was reduced in regions of locally strong accelerated flow regimes, a trend which was more pronounced for higher pressure gradient ratios, i.e. moderate curvature in combination with strong favorable pressure gradient. This behavior illustrates that both the application rate of the newly introduced strain rate and its magnitude need to be considered for turbulence modeling of complex flows. Consequently, an appropriate modeling framework must account for rapidly changing flow conditions.
Degree
Ph.D.
Advisors
Plesniak, Purdue University.
Subject Area
Industrial engineering|Aerospace materials|Mechanical engineering|Mechanics
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