The evolution of hairpin conditional eddies is being studied by direct numerical simulation. It is found that eddies of sufficient strength are capable of autogeneration creating replicas of themselves which can eventually form long packets of hairpins that fill the wall layer.

The structures of separated flow are studied using particle-image velocimetry.

A pulsed laser instrument is being used to measure instan- taneous velocity fields in fully developed, low-Reynolds- number, turbulent channel flow. The structure in this flow is being studied as a function of Reynolds number, and the two-point spatial correlation is being measured. Studies are also conducted in a turbulent boundary layer to observe the effects of Reynolds number as the range of length scales increases. High-resolution techniques are being employed to examine the mechanisms that create the logarithmic layer and the outer wake region.

The structure of thermal convection is being studied by particle-image velocimetry techniques in which two-dimensional velocity vector fields in a planar ``slice'' of the flow are measured. Of particular interest are the structure at high Rayleigh number and measurement of two-point spatial correlations in convection between hot and cold horizontal plates, convection under a stably stratified layer, and convection over nonuniformly heated horizontal surfaces.

A pulsed Nd:YAG system has been developed to record holographically sequential images of fine particles in air for the purpose of measuring three-dimensional vector fields in a volume. The system is being applied to studies of the structure of turbulence in pipe flow and in internal combustion.

The particle-image velocimeter (PIV) technique makes simultaneous measurements of fluid velocity vectors at several thousand points in a fluid flow and provides instantaneous flow patterns. New methods of interrogation and image analysis are being investigated. Systems under development include a new interrogation approach that yields superresolution and a stereo method for 3-D vectors.

Nonequilibrium in a turbulent boundary layer is studied by suddenly imposing transverse shear. The change in shear occurs either at the wall, where the small scales are affected first, or at the outer region of the boundary layer, where the large scales are affected first. The response of the structure of the eddies to the change in surface conditions is being observed by particle-image velocimetry.

We are exploring various vortex systems in which the vorticity is concentrated in well-defined ``coherent struc tures'' as models of turbulent flow situations. These include forced shear layers and wakes, both in two and three dimensions. We are also exploring certain families of analytical solutions that may provide benchmarks for computer simulation codes. The hope is that these models will provide insight into turbulent flows not easily obtained by conventional Reynolds stress models.

The suggestion of coupling diffusive separation to a reversible Stokes flow goes back to G. I. Taylor and was elaborated by J. P. Heller, who patented the process in 1967. Numerical experiments by Aref and Jones showed that chaotic advection by the Stokes flow could lead to significant improvements. Preliminary experiments by Chevray and Dutta at Columbia University seem to verify this conclusion. We are now constructing a device to explore the utility and competitiveness of this mechanism in practice for such applications as the separation of biomolecules and isotopes.

The topology and three-dimensional structure of foams are being studied using both experiments and numerical simulations, with the aim of building realistic models of their behavior, e.g., how they react to bulk forces and how their coarsening proceeds in time. Thin-film soap foams are used for experimental examination of the internal structure of several layers of foam cells. Direct observation with video cameras can follow the coarsening and the associated change in topology. Computer simulations of three-dimensional foams present an algorithmically challenging and computationally taxing problem, since the constraints which govern the growth of 2-D foams cannot be used in 3-D.

High-Rayleigh-number turbulent thermal convection is simulated to determine the effects of Rayleigh number, aspect ratio, mean shear, rotation, and boundary conditions on heat transfer and flow characteristics. A pseudospectral method is implemented on a massively parallel computer that allows for large computational domain sizes and high Rayleigh numbers that would not otherwise be feasible on a single-processor machine. These simulations have shown that at high Rayleigh numbers the strong thermal plumes spin about their vertical axes and are thus tornadic in nature. Temporal and spatial characterization of these swirling plumes and better understanding of their origin are sought.

Flow along a corner formed by the intersection of two solid surfaces, such as a wing-body junction, has been experimentally observed to be more unstable than the corresponding flow over a single flat surface. In this study, a self-similar mean flow in the corner region is obtained through the boundary-layer approximation. The stability of this mean flow is investigated through linear stability analysis for both inviscid and viscous modes. The resulting eigenvalue problems are very large, and efficient computational algorithms involving Arnoldi iteration and polynomial filtering are currently under investigation. Nonlinear evolution of these linearly unstable disturbances will be investigated through direct numerical simulations.

Here we focus on the instability mechanisms responsible for the complex nature of flow behind two-dimensional cylinders at moderate Reynolds numbers. Using linear and weakly nonlinear stability analysis, we investigate the Hopf bifurcation and onset of periodic shedding behind circular and elliptic cylinders. A Floquet stability analysis will be used to study the secondary instability and associated onset of three-dimensionality in the wake. Direct numerical simulations of the cylinder wake have provided a detailed picture of the self-sustained autogeneration of three-dimensional streamwise vortices. Subsequent spanwise subharmonic instability and associated period-doubling mechanisms are currently under investigation.

Here we consider two basic mechanisms for enhanced heat transfer in louvered fin geometries: periodic tripping of the viscous and thermal boundary layers at an interrupted fin surface and periodic unsteadiness in the flow due to separation by the fin geometry. The need to account for complex heat-exchanger geometry and at the same time resolve the range of length and time scales associated with unsteady flows poses computational challenges. Here we adopt a novel numerical methodology that will enable computation of unsteady flow in a complex geometry. Simultaneous experimental verification of numerical results is also underway.

Despite the importance of compressibility in engineering applications, compressible turbulence is not as well understood as its incompressible counterpart. In particular, there are uncertainties as to exactly what the effects of compressibility on turbulence are. Direct numerical simulation provides an opportunity to address these uncertainties because it allows us to use diagnostics that are not possible in experiments. The simulations performed here will be used to distinguish between compressibility and variable property (i.e., density and viscosity) effects, including fluctuating properties.

Flow in a round pipe is one of the most commonly occurring fluid flows in technological applications. It is also a good computational and experimental model for wall-bounded turbulent flows. Because of difficulties the geometry imposes on the numerical methods, direct numerical simulation has only rarely been applied to this flow. New numerical methods based on spline expansions are being applied to overcome the numerical difficulties and produce high-quality simulations of this important flow. The simulations will be used to study the physics of turbulence and to provide initial conditions for simulations of round jets.

Plane wakes are one of a class of turbulent free shear flows that display sensitive dependence on initial or inlet conditions. Direct numerical simulation is being used to characterize this dependence on initial conditions. Several wakes with different disturbance characteristics have been simulated and the resulting wakes are vastly different. Spanwise coherent fluctuations appear to play an important role in the dynamics of the flow and in causing the differences. These results have important implications regarding both the modeling and control of plane wakes.

Renormalization group theory and other modeling techniques are developed for stable and unstable stratified turbulence. The goal is to develop reliable and rational turbulence models that can be used for large-eddy simulations of complex stratified turbulence problems.

Effects of roughness elements of arbitrary shape, placed on the boundary of a horizontal layer of fluid, on the flow patterns of thermal convection are studied by analytical and computational methods. Certain conditions are determined under which the preferred flow pattern is controlled and the heat flux is enhanced by the surface corrugation effects.

This research concerns instabilities that exist in wall-bounded turbulent shear flows and their roles and origins in relation to streaks and large structures in such flows. Studies are based on both analytical and computational methods.

Effects of crystallization on nonlinear convection in a normal or high-gravity binary-alloy melt are investigated. Emphasis is given to examination of the mushy layer near the solidification front. Finite-amplitude effects are studied under certain controlling processes by analytical and computational techniques. The models include the basic physical conditions that are of interest in the field of materials processing.

This research involves studies of the fundamentals of the cross-flow instability mechanism and, in particular, the mechanism of flow instability in the leading-edge region of a swept wing. Nonlinear, higher instability, and wave-interaction regimes are studied by both computational and asymptotic techniques.

An ASSERT grant from AFOSR supports an effort on reactive compressible flow. Theoretical studies on the dynamics of the bow shock oscillations observed are being carried out for blunt projectiles fired into explosive mixtures. An effort to revise plane-detonation stability codes for the purposes of large-scale parametric computations has been started. An effort on stochastic modeling of turbulent combustion is continuing.

Simplified models have been developed by a UI/LANL group that describe deflagration-to-detonation transition (DDT) in porous energetic materials subjected to accidental stimulus. Parametric studies have been carried out to develop a realistic constitutive theory in a conservative, two-phase formulation. Multimaterial calculations are being carried out that simulate experiments at Los Alamos. Two-dimensional simulations of DDT are being carried out.

An experimental apparatus is being devised to study the effects of buoyancy forces on the vortical structures in a stably stratified fluid. A stably stratified, three-layer water channel will be constructed and instrumented with video particle-image velocimetry. Laser-induced fluorescence will also be used to study the baroclinic generation of vorticity in two parallel shear layers. The inhibition of mixing will be quantified in relation to the stratification strength and shear rates. A uniformly stratified shear channel will also be constructed, based on a new design, employing a real-time mixing manifold.

A partially filled circular cylinder is rotated about its horizontal axis of symmetry. The fluid is pulled up the sides of the cylinder, as in the rotomolding manufacturing process. A complicated interplay between gravity, inertial, and viscous forces, characterized by the cylinder rotation rate, volume filling ratio, and viscosity of the fluid, leads to a number of different flow regimes, some exhibiting instabilities. Experiments performed over a wide range of parameter space are aimed at mapping out stable and unstable regimes to help with the design of manufacturing protocols leading to a uniform coating on the inside of the cylinder. Special attention is being paid to the hysteresis in the process.

Multipoint measurements in homogeneous turbulence generated in a wind tunnel are being used to study the statistical structure of turbulent quantities. The preferential location of the small-scale dissipative structures in the turbulent field are being sought by conditional averages of large-scale flow features, such as regions of large rotation or streamwise shear fronts. The aim of the research is to develop efficient models to simulate the intermittent nature of the extreme events observed in high-Reynolds-number turbulence.

Voids are processing defects that significantly degrade the strength of polymer composites. Their control and elimination during processing is extremely important. How they nucleate and when they grow during processing are not well known. A transparent test cell that allows observation of void growth during processing is used to investigate the void growth behavior of expoxies during curing. A video record of the cure cycle is made, and an image analyzer is used to obtain void distributions and diameters. A refined void-growth model is correlated to the experimental data.