The vortex persistence theory predicts that the addition of a sufficiently strong, stationary vortex near a wall will reduce the wall fluxes in turbulent flows. To test the theory flow visualization and hot-wire experiments in a water tunnel at moderate Reynolds number reveal that a persistent vortex will relaminarize the boundary layer along wavy even in the presence of freestream turbulence. This result is consistent with Dawson [7] who found similar boundary layer behavior using a von Karman separatrix shaped wavy wall, and Balle [2], who's results showed laminar wall heat fluxes under stationary vortices using the von Karman wavy wall. Based on Dawson's flow visualization, and following Balle's analogy, the separatrix of a Kelvin-Stuart Cat's Eyes flow pattern was replaced with a solid wavy wall to stabilize the vortices shed by an array of half-delta vortex generators just upstream of the leading edge of the wavy wall. The correct angle of the array allows for persistent vortices to traverse along the grooves of the wavy wall, and displacing the array even slightly yields different turbulent fluxes. The present flow visualization and hot-wire anemometry results suggest that using the cat's eyes wavy wall gives better relaminarization properties than the von Karman shaped wavy wall, which can have many real world applications.
Analysis of Turbulent Boundary Layers focuses on turbulent flows meeting the requirements for the boundary-layer or thin-shear-layer approximations. Its approach is devising relatively fundamental, and often subtle, empirical engineering correlations, which are then introduced into various forms of describing equations for final solution. After introducing the topic on turbulence, the book examines the conservation equations for compressible turbulent flows, boundary-layer equations, and general behavior of turbulent boundary layers. The latter chapters describe the CS method for calculating two-dimensional and axisymmetric laminar and turbulent boundary layers. This book will be useful to readers who have advanced knowledge in fluid mechanics, especially to engineers who study the important problems of design.
An analysis of time-mean-turbulent boundary layer velocity profiles measured in a rapidly accelerating flow suggests that the outer region of the velocity profiles consists of essentially inviscid, rotational flow. The extent of this inviscid outer region was observed in some cases to exceed 90 percent of what is ordinarily thought of as the turbulent boundary layer thickness. On the other hand, the inner frictional region of these velocity profiles appears to have turbulent characteristics similar to those of more conventional turbulent boundary layers. Hence, the outer edge boundary condition for this inner region is more properly the external rotational flow region than the free stream.
Wall-bounded tip vortices appear in a variety of aerodynamic applications, such as, aircraft engines, inlet S-ducts, turbomachinery tip leakage, and vortex generators used for flow separation control. Studies show that vortex flows with high swirl that are exposed to a sufficient adverse pressure gradient will trigger the onset of vortex instability. However, it is not known whether the addition of freestream turbulence or the presence of a wall will stabilize a vortex due to momentum entrainment or trigger early bursting via amplification of an instability pathway. In this study, a wall-bounded vortex is analyzed in a low-speed wind tunnel to evaluate the effects that adverse pressure gradient and freestream turbulence have on vortex stability boundaries. Experimental techniques such as high-speed stereoscopic particle image velocity is used to capture three-dimensional flow fields of the wall bounded vortex. In addition to the PIV measurements, point measurements are taken using Laser Doppler Velocimetry to obtain turbulent characterization of the freestream and boundary layer flow. The results in this study predict an increase in vortex stability with freestream turbulence. Near a wall, further increases in vortex stability is depicted due to increased diffusion and reduction in rotational momentum. Furthermore, turbulent kinetic energy in a vortex core is decreased in high freestream turbulence when the vortex is near a wall due to a reduction in roll up and entrainment of high turbulence that exists in turbulent boundary layers near the wall. Turbulent boundary layers exposed to high freestream turbulence and adverse pressure gradients appear in a variety of aerodynamic application such as the suction side of airfoils within gas turbine engines and on aircraft wings. Non-equilibrium turbulent boundary layers exposed to a non-constant adverse pressure gradient have been shown to influence the outer region of the mean boundary layer profile due to the resilience of larger scale superstructures. However, it is not known whether the addition of freestream turbulence will increase the effect of history in a turbulent boundary layer due to increased entrainment of larger scales or decrease in effects from breakdown of these superstructures in the freestream. In this study, an experimental technique known as laser Doppler velocimetry is applied to obtain 2-dimensional, high-speed velocity measurements used for capturing mean profiles, turbulent statistics, and turbulent characterization of the freestream flow. The results in this study predict that pressure gradient history effects can have influence on Reynolds stress distributions past the outer region and into the log-region of the boundary layer due to diffusion of turbulence towards the wall. Additionally, pressure gradient effects, and thus the effects of a boundary layer's history, are significantly reduced when subjected to freestream turbulence due to dominant influence of turbulence within the wake region. Lastly, turbulent statistics show that a straight contoured wall, containing a smaller local pressure gradient but sustained over longer distance, has a greater adverse pressure gradient effect on a boundary relative to concave or convex contouring.
This experiment studied the effect of streamwise vortices on unsteady turbulent boundary-layer separation. The objectives were to document the flow field, to characterize the time response of the boundary layer, and to understand the actual mechanisms by which the streamwise vortices modify boundary-layer behavior. A new configuration for non-obtrusive three-component Laser Doppler Anemometry (LDA) determined the phase averaged velocity and Reynolds stress components, in an unsteady water tunnel, at a momentum thickness Reynolds number of 1840. The streamwise vortices were created by three pairs of half-delta wing vortex generators, while the boundary-layer separation was controlled through impulsively initiated opposite-wall suction, which created a strong adverse pressure gradient. The time response of the freestream velocity demonstrates that convection is the primary mechanism by which vortex generators modify the response of the boundary layer. There is an initial fast response throughout the boundary layer which is unaffected by the presence of vortex generators, followed by a slow or convective response, the magnitude of which is substantially modified by the presence of the vortex generators. Flow control, Unsteady turbulent layers. Aircraft, Vortex generators. (jes).
The recent accomplishments are reviewed for a research program employing combined analytical-experiments techniques to study the three dimensional characteristics and behavior of vortex motions associated with the turbulence production process in turbulent boundary layers. Progress is described in the development of a new image processing technique which allows the derivation of quantitative data from flow visualization images. The method is used to search for the role of hairpin vortices in the turbulence production process. In the analytical portion of the study, calculations have been carried out to compute the evolution of a hairpin vortex in a shear flow; the interaction of a pair of hairpins has been examined as well as the viscous response at a wall due to the motion of a hairpin vortex. Comparison of these computer simulations with the experimental studies is very encouraging. Computations for the evolving flow between wall layer streaks during a typical cycle in the wall layer of a turbulent boundary layer have also been carried out; these studies show two possible routes to breakdown of the wall layer flow leading to the production process. Keywords: Turbulent boundary layers; Hairpin vortices; Vortex motion.
