Helen L. Reed

Bio: Helen L. Reed is an academic researcher from Texas A&M University. The author has contributed to research in topics: Boundary layer & Swept wing. The author has an hindex of 30, co-authored 193 publications receiving 4602 citations. Previous affiliations of Helen L. Reed include Stanford University & Arizona State University.

Stability and transition of three-dimensional boundary layers

Abstract: ▪ Abstract The recent progress in three-dimensional boundary-layer stability and transition is reviewed. The material focuses on the crossflow instability that leads to transition on swept wings and rotating disks. Following a brief overview of instability mechanisms and the crossflow problem, a summary of the important findings of the 1990s is given.

Boundary-layer receptivity to freestream disturbances

Abstract: The current understanding of boundary-layer receptivity to external acoustic and vortical disturbances is reviewed. Recent advances in theoretical modeling, numerical simulations, and experiments are discussed. It is shown that aspects of the theory have been validated and that the mechanisms by which freestream disturbances provide the initial conditions for unstable waves are better understood. Challenges remain, however, particularly with respect to freestream turbulence

Linear Stability Theory Applied to Boundary Layers

Abstract: The objective of this review is to provide a critical evaluation of linear stability theory for wall-bounded shear flows, with an emphasis on results, not techniques. The results deal with applications to different flowfields related to aircraft systems. Because the flight condition is usually a low-disturbance environment, stability plays an important role in the transition process. Linear stability theory is not a new topic, so emphasis is placed on the most recent accomplishments.

Stability of three-dimensional boundary layers

Abstract: The computational modeling of the transition process characteristic of flows over swept wings is discussed. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full three-dimensional Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that regular stability theory is insufficient to explain the discrepancies between experiments and between theory and experiment. The leading edge region of a swept wing will be considered in a three-dimensional spatial simulation with random disturbances as the initial conditions.

Simulation of swept-wing vortices using nonlinear parabolized stability equations

Abstract: The nonlinear development of stationary crossflow vortices over a 45° swept NLF(2)-0415 airfoil is studied. Previous investigations indicate that the linear stability theory (LST) is unable to accurately describe the unstable flow over crossflow-dominated configurations. In recent years the development of nonlinear parabolized stability equations (NPSE) has opened new pathways toward understanding unstable boundary-layer flows. This is because the elegant inclusion of nonlinear and non-parallel effects in the NPSE allows accurate stability analyses to be performed without the difficulties and overhead associated with direct numerical simulations (DNS). NPSE results are presented here and compared with experimental results obtained at the Arizona State University Unsteady Wind Tunnel. The comparison shows that the saturation of crossflow disturbances is responsible for the discrepancy between LST and experimental results for cases with strong favourable pressure gradient. However, for cases with a weak favourable pressure gradient the stationary crossflow disturbances are damped and nonlinearity is unimportant. The results presented here clearly show that for the latter case curvature and non-parallel effects are responsible for the previously observed discrepancies between LST and experiment. The comparison of NPSE and experimental results shows excellent agreement for both configurations.Through this work, a detailed quantitative comparison and validation of NPSE with a careful experiment has now been provided for three-dimensional boundary layers. Moreover, the results validate the experiments of Reibert et al. (1996), and Radeztsky et al. (1993, 1994) suggesting that their databases can be used by future researchers to verify theories and numerical schemes. The results show the inadequacy of linear theories for modelling these flows for significant crossflow amplitude and demonstrate the effects of weak curvature to be more significant than slight changes in basic state, especially near neutral-stability locations.

Local and global instabilities in spatially developing flows

Abstract: The goal of this survey is to review recent developments in the hydro­ dynamic stability theory of spatially developing flows pertaining to absolute/convective and local/global instability concepts. We wish to dem­ onstrate how these notions can be used effectively to obtain a qualitative and quantitative description of the spatio-temporal dynamics of open shear flows, such as mixing layers, jets, wakes, boundary layers, plane Poiseuille flow, etc. In this review, we only consider open flows where fluid particles do not remain within the physical domain of interest but are advected through downstream flow boundaries. Thus, for the most part, flows in "boxes" (Rayleigh-Benard convection in finite-size cells, Taylor-Couette flow between concentric rotating cylinders, etc.) are not discussed. Further­ more, the implications of local/global and absolute/convective instability concepts for geophysical flows are only alluded to briefly. In many of the flows of interest here, the mean-velocity profile is non-

Recent Developments in Rayleigh-Bénard Convection

Abstract: ▪ Abstract This review summarizes results for Rayleigh-Benard convection that have been obtained over the past decade or so. It concentrates on convection in compressed gases and gas mixtures with Prandtl numbers near one and smaller. In addition to the classical problem of a horizontal stationary fluid layer heated from below, it also briefly covers convection in such a layer with rotation about a vertical axis, with inclination, and with modulation of the vertical acceleration.

Secondary Instability of Boundary Layers

Abstract: The problem of transition from laminar to turbulent flow in viscous bound­ ary layers is of great practical interest. The low skin-friction coefficient of laminar boundary-layer flow is very attractive to those who lay out the engines or pay the fuel for high-speed vehicles such as airplanes. However, the low mixing of fluid properties such as chemical species, heat, or momen­ tum may be intolerable for others who design these engines or cope with the danger of separation in adverse pressure gradients; they may clearly prefer a turbulent state of the flow. Therefore, it would be highly desirable to at least predict, if not to control, whether the flow under consideration is laminar or turbulent. The tremendous efforts of decades of intense research, however, have been to little avail (Reshotko 1976). The empirical en-criterion is still the standard tool in engineering practice, although it is known to ignore essential ingredients of the physics of transition and therefore may dangerously mislead if used beyond the supporting data base. Numerical transition simulations have gained reliability in repro­ ducing the transition process in sufficient detail to extract information unobtainable from laboratory experiments. However, the inherent assumptions of stream wise periodicity and temporal growth of the bound­ ary layer, in addition to the uncertainty of initial conditions, prevent predicting transition in practice. Hence, theory still holds an important place in identifying inherent mechanisms and structures of the transition process and in explaining otherwise unintelligible observations. The past decade saw some important progress in stability theory, slow or fast, depending on the reader's judgment ..

Parabolized stability equations

Abstract: Parabolized stability equations (PSE) have opened new avenues to the analysis of the streamwise growth of linear and nonlinear disturbances in slowly varying shear flows such as boundary layers, jets, and far wakes. Growth mechanisms include both algebraic transient growth and exponential growth through primary and higher instabilities. In contrast to the eigensolutions of traditional linear stability equations, PSE solutions incorporate inhomogeneous initial and boundary conditions as do numerical solutions of the Navier-Stokes equations, but they can be obtained at modest computational expense. PSE codes have developed into a convenient tool to analyze basic mechanisms in boundary-layer flows. The most important area of application, however, is the use of the PSE approach for transition analysis in aerodynamic design. Together with the adjoint linear problem, PSE methods promise improved design capabilities for laminar flow control systems.