Control Of Separation For Turbulent Boundary Layers Subjected To Wall Curvature And Streamwise Pressure Gradients

Low Pressure Turbine (LPT) stages are important for modern jet engines, and in particular for engines used in Unmanned Aerial Vehicles (UAV). At low operating Reynolds numbers boundary layers remain laminar over a large downstream extent of LPT blades. Laminar boundary layer separation can cause significant reductions in turbine and overall engine performance. In an experimental research program at the Air Force Research Laboratory (AFRL), the potential advantages of Active Flow Control (AFC) for separation control were systematically explored. In collaboration with the experimental effort, we performed numerical investigations of AFC for LPT separation to investigate the relevant physical mechanisms. Numerical simulations can provide insight into flow details that is not possible in experiments. We chose a two-pronged approach: A computationally less efficient but more versatile finite volume code was employed for simulations of the full geometry. Open-loop control strategies were explored in 2-D calculations. Detailed investigations of the VGJ actuation for a flat plate model problem were performed with a very efficient finite difference code. The high-resolution DNS helped identify some of the key physical mechanisms responsible for the effectiveness of VGJs.

Longitudinal vortices in turbulent boundary layers that are subjected to strong wall curvature (both convex and concave) and strong adverse pressure gradients were investigated numerically. The motivation for this research was to uncover the fundamental mechanisms responsible for the generation of longitudinal vortices and their dynamical interaction with the other dominant coherent structures that arise from the presence of wall curvature and strong adverse pressure gradients. This understanding is essential for future implementation of separation control techniques for practical flows when longitudinal vortices are inherently present, or when forced longitudinal vortices are employed for separation control (i.e., on demand vortex generators). Furthermore, such investigations can shed light on the turbulence generation mechanisms for boundary layers subjected to streamwise curvature and pressure gradients when longitudinal vortices are present. The investigations were carried out using DNS, LES and unsteady RANS. Towards this end, we have employed our new Flow Simulation Methodology (FSM) for calculating turbulent flows. The present numerical investigations were a critical test for FSM and the turbulence model used in FSM as the investigated flows are highly complex with the turbulence in strong non-equilibrium.

Boundary Layer and Flow Control: Its Principles and Application, Volume 2 focuses on the layer of fluid in the immediate area of a bounding surface where the effects of viscosity are substantial. This book is organized into two main topics—boundary layer control for low drag, and shock-induced separation and its prevention by design and boundary layer control. It specifically discusses the nature of transition, effect of two-dimensional and isolated roughness on laminar flow, and progress in the design of low drag aerofoils. The onset of separation effects for aerofoils and wings, shock-induced separation for laminar boundary layers, and shock-induced separation for laminar boundary layers are also deliberated. This volume is recommended to physicists and specialists interested in boundary layer and flow control.

A numerical study has been performed using a finite-difference boundary layer computer program that includes corrections to the equations of motion for longitudinal curvature and allows the static pressure to be specified as a known function of x and y. Calculations with and without the corrections for longitudinal curvature and pressure gradient normal to be the local surface have been compared to experimental measurements of a supersonic turbulent boundary layer flow over a surface with concave longitudinal curvature. Substantially improved agreement with experimental measurements is obtained for the calculations including corrections for wall curvature and pressure gradient normal to the local surface for density profile, skin friction coefficient, and boundary-layer integral properties.

This new edition of the near-legendary textbook by Schlichting and revised by Gersten presents a comprehensive overview of boundary-layer theory and its application to all areas of fluid mechanics, with particular emphasis on the flow past bodies (e.g. aircraft aerodynamics). The new edition features an updated reference list and over 100 additional changes throughout the book, reflecting the latest advances on the subject.

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.