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NASA NTRS1931

Reduction of turbulence in wind tunnels

A brief nonmathematical outline is given of modern views as to the nature of the effect of turbulence, and their bearing on the desirability of designing wind tunnels for small or large turbulence. Experiments made on a particular wind tunnel for the purpose of reducing the turbulence are described, to illustrate the influence of certain factors on the magnitude of the turbulence. Moderate changes in the size, shape, and wall thickness of cells of the honeycomb were found to have little effect. The addition of a room honeycomb at the entrance was also of little value in reducing the turbulence. The turbulence decreased with increasing distance between the honeycomb and the measuring station. A further decrease was obtained by using a large area reduction in the entrance cone, with the honeycomb at the extreme entrance end. (author)

Dryden, Hugh L↗

DOE OSTI2024

Quantitative radiography for determining density fluctuations in HED experiments

We have developed a method to extract density fluctuation measurements from x-ray radiographs of high-energy density (HED) instability growth and turbulence experiments. We use this information to calculate density fluctuation statistics for constraining the performance of turbulent mix models in HED systems. The density calculation combines image filtering, removal of systemic effects such as backlighter variation, calculation of transmission across multiple materials, and use of tracer materials to generate an approximate single-material density field. From the density map, we calculate both average density and a variance-like moment b (density-specific-volume covariance), which we compare to our models. We infer both quantities from a single image, which is significantly more information than the historic single scalar mix width measurements. We also develop a method of analyzing simulation outputs that incorporate both the density fluctuation metric from a turbulence model and the bulk material maps from the hydrodynamic code. This analysis helps address the question of how to initialize the simulations for best comparison to data from systems with large separations of scale in the mixing perturbation initial condition. We find that our data analysis method yields 1D average density and b curves with similar morphology and amplitudes as those from preliminary simulation comparisons.

47 OTHER INSTRUMENTATION↗

DOE OSTI2025

Microturbulence Suppression by Alfvén Eigenmodes in the DIII-D Tokamak

Mitigation and suppression of low-𝑘 turbulence are observed during the nonlinear evolution of toroidicity-induced Alfvén eigenmodes (TAEs) in DIII-D experiments. Turbulence mitigation begins when the dominant TAE starts to depart from the typical shear Alfvén wave polarization. TAE activity then evolves into a new state, characterized by more discrete coherent modes, increased total amplitude, and more localized radial structures. During this transition, a narrow shear flow layer forms, driven by an enhanced Reynolds stress force, with a shearing rate that exceeds the local turbulence decorrelation rate, leading to a full turbulence suppression. Furthermore, these observations indicate the imbalance between Reynolds and Maxwell stress forces during the nonlinear evolution of the TAE and their important roles in shear flow generation and turbulence reduction.

Alfvén waves↗

NASA NTRS2009

Turbulent Wing-Leading-Edge Correlation Assessment for the Shuttle Orbiter

This study was conducted in support of the Orbiter damage assessment activity that takes place for each Shuttle mission since STS-107 (STS - Space Transportation System). As part of the damage assessment activity, the state of boundary layer (laminar or turbulent) during reentry needs to be estimated in order to define the aerothermal environment on the Orbiter. Premature turbulence on the wing leading edge (WLE) is possible if a surface irregularity promotes early transition and the resulting turbulent wedge flow contaminates the WLE flow. The objective of this analysis is to develop a criterion to determine if and when the flow along the WLE experiences turbulent heating given an incoming turbulent boundary layer that contaminates the attachment line. The data to be analyzed were all obtained as part of the MH-13 Space Shuttle Orbiter Aerothermodynamic Test conducted on a 1.8%-scale Orbiter model at Calspan/University of Buffalo Research Center in the Large Energy National Shock Tunnels facility. A rational framework was used to develop a means to assess the state of the WLE flow on the Orbiter during reentry given a contaminated attachment-line flow. Evidence of turbulent flow on the WLE has been recently documented for a few STS missions during the Orbiter s flight history, albeit late in the reentry trajectory. The criterion developed herein will be compared to these flight results.

King, Rudolph A.↗

NASA NTRS1975

Subgrid or Reynolds stress-modeling for three-dimensional turbulence computations

A review is given of recent advances in two distinct computational methods for evaluating turbulence fields, namely, statistical Reynolds stress modeling and turbulence simulation, where large eddies are followed in time. It is shown that evaluation of the mean Reynolds stresses, rather than use of a scalar eddy viscosity, permits an explanation of streamline curvature effects found in several experiments. Turbulence simulation, with a new volume averaging technique and third-order accurate finite-difference computing is shown to predict the decay of isotropic turbulence in incompressible flow with rather modest computer storage requirements, even at Reynolds numbers of aerodynamic interest.

Rubesin, M. W.↗

NASA NTRS1987

Unsteady three-dimensional simulation of VTOL upwash fountain turbulence

Numerical simulations of a planar turbulent wall jet and a planar VTOL upwash fountain were performed. These are three dimensional simulations which resolve large scale unsteady motions in the flows. The wall jet simulation shows good agreement with experimental data and is presented to verify the simulation methodology. Simulation of the upwash fountain predicts elevated shear stress and a half velocity width spreading rate of 33% which agrees well with experiment. Turbulence mechanisms which contribute to the enhanced spreading rate are examined.

Childs, Robert E.↗

NASA NTRS1973

Experiments on confined turbulent jets in cross flow.

Results are reported of experiments on the effects of an opposite wall on the characteristics of turbulent jets injected into a cross flow, for unheated and heated jets. Longitudinal and transverse distributions of velocity and temperature are presented for single and multiple circular jets, and trajectories are presented for two-domensional jets. The opposite wall has relatively little effect on a single jet unless the ratio of jet to cross flow momentum flux is large enough for the jet to impinge on the opposite wall. For a row of jets aligned perpendicularly to the cross flow, the opposite wall exerts progressively larger influence as the spacing between jets decreases. Much of the effect of jet and wall proximity can be understood by considering the interaction of the vortex flow which is the major feature of the structure of a single jet in a cross flow. Smoke photographs are shown to elucidate some of the interaction patterns.

Kamotani, Y.↗

NASA NTRS1974

Experiments on confined turbulent jets in cross flow

Results are reported of experiments on the effects of an opposite wall on the characteristics of turbulent jets injected into a cross flow, for unheated and heated jets. Longitudinal and transverse distributions of velocity and temperature are presented for single and multiple circular jets, and trajectories are presented for two-dimensional jets. The opposite wall has relatively little effect on a single jet unless the ratio of jet to cross flow momentum flux is large enough for the jet to impinge on the opposite wall. For a row of jets aligned perpendicularly to the cross flow, the opposite wall exerts progressively larger influence as the spacing between jets decreases. Much of the effect of jet and wall proximity can be understood by considering the interaction of the vortex flow which is the major feature of the structure of a single jet in a cross flow. Smoke photographs are shown to elucidate some of the interaction patterns.

Kamotani, Y.↗

NASA NTRSDate not supplied

Large-Eddy Simulations of a Single-Injector Cooling Flow Using the High-Order Flux Reconstruction Method

A single-injector cooling flow into a heated crossflow was used as a validation case for large eddy simulations (LES) from two high-order CFD codes with comparisons to a state-of-the-art Reynolds averaged Navier-Stokes (RANS) turbulence model. The focus of this paper is on the validation of LES for one of the high-order CFD codes, namely the GFR (Glenn Flux Reconstruction) code that is being developed at NASA Glenn Research Center. Fourth-order LES were performed for the single-injector cooling flow configuration at blowing ratios of both 1.0 and 2.0, and a fifth-order LES was also performed with a blowing ratio of 1.0. The GFR simulations agree very well with the experiment mean quantities and reasonably well with the experiment turbulence quantities. The LES solutions from GFR and the other high-order CFD code, FDL3DI, agreed very closely for nearly every flow quantity examined, despite the fact that these two codes use completely different numerical methods, different grid geometries and domains, and different inflow boundary conditions. Finally, both LES show a significant accuracy improvement over RANS turbulence models for flows with large temperature gradients, where the standard gradient diffusion approximation is insufficient for temperature predictions.

Large Eddy Simulations↗

NASA NTRSDate not supplied

Large Eddy Simulations of a Single-Injector Cooling Flow Using the High-Order Flux Reconstruction Method

Large Eddy Simulation↗

NASA NTRS1995

Microgravity combustion of dust clouds: Quenching distance measurements

The current level of physical understanding of dust combustion phenomena is still in a rudimentary state compared with the understanding of gas combustion processes. The reason for such a lack of fundamental understanding is partially based on the complexity of multiphase combustion and the enormous diversity of chemical-physical properties of heterogeneous combustible mixtures but is largely due to difficulties in the experimental investigation of dust combustion. The influence of gravity on a dust suspension is the main reason. First of all, when particulates (either solid particles or liquid droplets) with a characteristic size of the order of tens of microns are suspended, they rapidly settle in the gravitational field. To maintain a particulate suspension for a time duration adequate to carry out combustion experiments invariably requires continuous convection of particulates at or in excess of the gravitational settling velocity. Of necessity, this makes the experiments turbulent in character and makes it impossible to study laminar dust flames. For particle sizes of the order of microns a stable laminar dust flow can be maintained only for relatively small dust concentrations (e.g., for low fuel equivalence ratios) at normal gravity conditions. High dust loading leads to gravitational instability of the dust cloud and to the formation of recirculation cells in a dust suspension in a confined volume, or to the rapid sedimentation of the dense dust cloud as a whole in an unconfined volume. In addition, many important solid fuels such as low volatile coal, carbon, and boron have low laminar flame speeds (of the order of several centimeters per second). Gravitational convection that occurs in combustion products due to the buoyancy forces disrupts low speed dust flames and, therefore, makes observation of such flames at normal gravity impossible. The only way to carry out 'clean' fundamental experiments in dust combustion over a wide range of dust cloud parameters is in a gravity-free environment. Access to the microgravity environment provided by the use of large-scale drop towers, parabolic flights of aircraft and rockets, and shuttle and space station orbits has permitted now to proceed with a systematic program of dust combustion microgravity research. For example, the NASA-Lewis drop tower and a Lear jet parabolic flight aircraft were used by Ross et al. and by Berlad and Tangirala for experiments with Iycopodium/air mixtures. The Japan Microgravity Center drop shaft (JAMIC) where a microgravity condition of 10(exp -4) g for 10 s is available, was recently used by Kobayashi, Niioka et al. for measuring flame propagation velocities in polymethyl methacrylate dust/air suspensions. Microgravity dust combustion experiments were started at McGill University in the early 90's under the sponsorship of the Canadian Space Agency. Several generations of dust combustion platforms permitting dust combustion microgravity experiments to be carried out on board a parabolic flight aircraft (KC-135, NASA) have been designed and tested. The experimental data and experience gained from this research allowed us to design and build in a current phase of this program the microgravity apparatus for the visual observation of freely propagating constant pressure laminar dust flames. Quenching distances in aluminum dust suspensions have been measured in a wide range of dust cloud parameters in ground-based experiments and in recent microgravity experiments (KC-135 parabolic flights, Houston, February 1995).

Goroshin, S.↗

NASA NTRS1975

Comparison of theory and experiment for homogeneous turbulence with shear

Solutions for uniformly-sheared turbulence, in which the interaction of the turbulence with the mean shear dominates the turbulent self-interaction, are compared with experiment. An anisotropic spectral tensor, which appears general enough to represent the initial experimental turbulence, is used for the initial condition in the calculations. The evolution of one-point turbulence components and microscales, as well as two-point velocity correlations, are considered. In most cases the agreement with experiment is good. The theory correctly predicts the presence of a negative region for two-point longitudinal-velocity correlations only for point separations in the direction normal to the flow and the mean gradient.

Deissler, R. G.↗

DOE OSTI2021

Turbulence theories and statistical closure approaches

When discussing research in physics and in science more generally, it is common to ascribe equal importance to the three components of the scientific trinity: theoretical, experimental, and computational studies. This review will explore the future of modern turbulence theory by tracing its history, which began in earnest with Kolmogorov’s 1941 analysis of turbulence cascade and inertial range [A.N. Kolmogorov, Dokl. Akad. Nauk SSSR, 30, 299, (1941); 32, 19, (1941)]. The 80th Anniversary of Kolmogorov’s landmark study is a welcome opportunity to survey the achievements and evaluate the future of the theoretical approach of turbulence research. Over the years, turbulence theories have been critically important in laying the foundation of our understanding of the nature of turbulent flows. In particular, the Direct Interaction Approximation (DIA) [R.H. Kraichnan, J. Fluid Mech., 5, 497 (1959)] and its subsequent development, known as the statistical closure approach, can be identified as perhaps the most profound single advancement. The remarkable success of the statistical closure has furnished a platform to study such essential concepts as the energy transfer process and interacting scales, and the roles of the straining and sweeping motions. More recently, the quasi-Lagrangian formulation of V. L’vov & I. Procaccia and Kraichnan’s solvable passive scalar model provided powerful ways to explore another fundamental aspect of turbulent flows, the phenomena of intermittency, and the associated anomalous scaling exponents. In the meantime, the theory of fluid equilibria has been developed to describe the large-scale structures that can emerge from turbulent cascades of two-dimensional and geophysical flows at a later time. And yet, despite all these successes, analytical treatments suffer from mathematical complexities. As a result, the utility of theoretical approaches has been limited to relatively idealized flows. On the other hand, in recent decades, computational abilities and experimental facilities have reached an unprecedented scale. Looking beyond the horizon, the imminent deployment of exascale supercomputers will generate complete datasets of the entire flow field of key benchmark flows, allowing researchers to extract additional measurements concerning fully developed, complex turbulent flow fields far beyond those available from the statistical closure theories. Some other developments that could potentially influence the future course of turbulence theories include the advancement of machine learning, artificial intelligence, and data science; likely disruptions arising from the advent of quantum computation; and the increasingly prominent role of turbulence research in providing more accurate climate scientific data. Finally, turbulence theorists can leverage these developments by asking the right questions and developing advanced, sophisticated frameworks that will be able to predict and correlate vast amounts of data from the other two components of the trinity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1980

Summary of all cycle II.5 shear and boundary layer measurements, aerodynamics

The two measurement systems were used to measure mean velocity and velocity, mass flux, and total temperature fluctuations in the turbulent boundary on the fuselage of a KC-135 aircraft. The boundary layer thickness ranged between about 20 and 30 cm for the range of flight Mach numbers from about 0.25 to 0.85 and Reynolds numbers between 3 and 6 x 10 to the 6th power/m. The adaptation of each system for use in airborne applications is discussed. The data obtained from each system are given and compared with each other and they indicate that the two systems represent viable ones for use in future airborne turbulence experiments.

Rose, W. C.↗

NASA NTRS1984

Feasibility of measuring temperature and density fluctuations in air using laser-induced O2 fluorescence

A tunable line-narrowed ArF laser can selectively excite several rotation al lines of the Schumann-Runge band system of O2 in air. The resulting ultraviolet fluorescence can be monitored at 90 deg to the laser beam axis, permitting space and time resolved observation of density and temperature fluctuations in turbulence. Experiments and calculations show that + or - 1 K, + or - 1 percent density, 1 cu mm spatial, and 1 microsecond temporal resolution can be achieved simultaneously under some conditions.

Massey, G. A.↗

NASA NTRS1987

Angular distribution of turbulence in wave space

An alternative to the one-point closure model for turbulence, the large eddy simulation (LES), together with its more exact relative, direct numerical simulation (DNS) are discussed. These methods are beginning to serve as partial substitutes for turbulence experiments. The eddy damped quasi-normal Markovian (EDQNM) theory is reviewed. Angular distribution of the converted data was examined in relationship to EDQNM.

Coleman, G.↗

NASA NTRS1990

On modelling the pressure-strain correlations in wall bounded flows

Turbulence models for the pressure-strain term of the Reynolds-stress equations in the vicinity of a moving wall are evaluated for a high Reynolds number flow using decaying grid turbulence as a model problem. The data of Thomas and Hancock are used as a base for evaluating the different turbulence models. In particular, the Rotta model for return-to-isotropy is evaluated both in its inclusion into the Reynolds-stress equation model and in comparison to a nonlinear model advanced by Sarkar and Speziale. Further, models for the wall correction to the transfer term advanced by Launder et al., Shir, and Shih and Lumley are compared. Initial data using the decaying grid turbulence experiment as a base suggests that the coefficients proposed for these models are high perhaps by as much as an order of magnitude. The Shih and Lumley model which satisfies realizability constraints, in particular, seems to hold promise in adequately modeling the Reynolds stress components of this flow. Extensions of this work are to include testing the homogeneous transfer model by Shih and Lumley and the testing of the wall transfer models using their proposed coefficients and the coefficients chosen from this work in a flow with mean shear component.

Peltier, L. J.↗

NASA NTRS1999

Laminar Dust Flames: A Program of Microgravity and Ground Based Studies at McGill

Fundamental knowledge of heterogeneous combustion mechanisms is required to improve utilization of solid fuels (e.g. coal), safe handling of combustible dusts in industry, and solid propulsion systems. The objective of the McGill University research program on dust combustion is to obtain a reliable set of data on basic combustion parameters for dust suspensions (i.e. laminar burning velocity, flame structure, quenching distance, flammability limits, etc.) over a range of particle sizes, dust concentrations, and types of fuel. This set of data then permits theoretical models to be validated and, when necessary, new models to be developed to describe the detailed reaction mechanisms and transport processes. Microgravity is essential to the generation of a uniform dust suspension of arbitrary particle size and concentration. When particles with a characteristic size on the order of tens of microns are suspended, they rapidly settle in a gravitational field. To maintain a particulate in suspension for time duration adequate to carry out combustion experiments invariably requires continuous convective flow in excess of the gravitational settling velocity (which is comparable with and can even exceed the dust laminar burning velocity). This makes the experiments turbulent in nature and thus renders it impossible to study laminar dust flames. Even for small particle sizes on the order of microns, a stable laminar dust flow can be maintained only for relatively low dust concentrations at normal gravity conditions. High dust loading leads to gravitational instability of the dust cloud and to the formation of recirculation cells in the dust suspension in a confined volume, or to the rapid sedimentation of the dense dust cloud, as a whole, in an unconfined volume. Many important solid fuels such as carbon and boron also have low laminar flame speeds (of the order of several centimeters per second). Convection that occurs in combustion products due to buoyancy disrupts the low speed dust flames and makes observation of such flames at normal gravity difficult.

Goroshin, Sam↗