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NASA NTRS1990

An experimental investigation of a low Reynolds number turbulent boundary layer subject to an adverse pressure gradient

The evolution of a low Re(sub theta) turbulent boundary layer in an adverse pressure gradient (APG) is being studied for comparison with direct numerical simulations by Spalart. A short region of favorable pressure gradient (FPG) is applied first to establish a self preserving layer with Re(sub theta) equals approx. 600, which is a suitable initial condition for simulations. The APG is then applied rapidly such that beta equals approx. 2 at Re(sub theta) equals approx. 1500. The streamwise extent of the measurements exceeds the current capabilities of direct simulation so that the results should also serve as a useful data base for Reynolds averaged boundary layer prediction methods and in the future for direct simulation schemes as computer technology evolves.

Watmuff, J. H.↗

NASA NTRSDate not supplied

Effect of the Reynolds Number on the Freestream Disturbance Environment in a Mach 6 Nozzle

To understand the impact of unit Reynolds number on the acoustic disturbance field inside a high-speed wind tunnel, we use Direct Numerical Simulations (DNS) to model the turbulent boundary layers along the walls of a quasi-two-dimensional nozzle configuration. Intended as a stepping stone to fully three-dimensional simulations of freestream noise inside the NASA 20-Inch Mach 6 Wind Tunnel, the present simulations are based on periodic boundary conditions across the spanwise width of the computational domain that corresponds to about one third of the actual tunnel width. These simulations are performed at four different unit Reynolds numbers, ranging from 3.56e6 to 14.0e6 per meter. The predominantly hydrodynamic fluctuations inside the boundary layer are shown to be nearly unaffected by the presence of freestream forcing associated with the impinging acoustic radiation from the opposite wall. Thus, the Reynolds number trends associated with boundary-layer quantities are consistent with previously published DNS of flat-plate boundary layers at similar Mach numbers and wall temperature ratios. The unsteady disturbance environment within the nozzle core region is found to be approximately spatially homogeneous and purely acoustic in nature. The numerical results are used to make comparisons with tunnel noise measurements by Chou et al. [1,2]. Unlike previous comparisons involving static-pressure fluctuations based on the DNS and pitot-pressure fluctuations measured in the wind tunnel, direct comparisons involving the fluctuations in the same physical quantity, namely, the streamwise mass flux, have been reported for the first time. The predicted decrease in the root-mean-square fluctuations in pressure and mass flux with an increasing unit Reynolds number is in agreement with the measurements in the NASA 20-Inch Mach 6 Wind Tunnel. Additional details of the acoustic radiation field are quantified and should be useful toward a digital synthesis of the tunnel disturbance environment that would enable realistic simulations of the natural transition process.

Turbulence↗

NASA NTRSDate not supplied

Effect of the Reynolds Number on the Freestream Disturbance Environment in a Mach 6 Nozzle

Acoustics↗

NASA NTRS1994

Large-eddy simulation of a plane wake

Previously the theoretical development leading to the dynamic localization model (DLM) for large-eddy simulation (LES) was presented. The method has been successfully applied to isotropic turbulence, channel flow, and the flow over a backward-facing step. Here we apply the model to the computation of the temporally developing place wake. The two main objectives of this project are: (1) Use the model to perform an LES of a time developing plane wake and compare the results with direction numerical simulation (DNS) data to see if important statistical measures can be readily predicted, and to provide a relative evaluation of the several versions of the model in terms of predictive capability and cost; and (2) If the tests in (1) show that the model generates reliable predictions, then use the LES to study various aspects of the physics of turbulent wakes and mixing layers.

Ghosal, Sandip↗

NASA NTRS1992

Low-Reynolds-number effects in a fully developed turbulent channel flow

Low-Reynolds-number effects are observed in the inner region of a fully developed turbulent channel flow, using data obtained either from experiments or by direct numerical simulations. The Reynolds-number influence is observed on the turbulence intensities and to a lesser degree on the average production and dissipation of the turbulent energy. In the near-wall region, the data confirm Wei and Willmarth's (1989) conclusion that the Reynolds stresses do not scale on wall variables. One of the reasons proposed to account for this behavior, namely, the 'geometry' effect or direct interaction between inner regions on opposite walls, was investigated in some detail by introducing temperature at one of the walls, both in experiment and simulation. Although the extent of penetration of thermal excursions into the opposite side of the channel can be significant at low Reynolds numbers, the contribution these excursions make to the Reynolds shear stress and the spanwise vorticity in the opposite wall region is negligible. In the inner region, spectra and cospectra of the velocity fluctuations u and v change rapidly with the Reynolds number, the variations being mainly confined to low wavenumbers in the u spectrum.

Antonia, R. A.↗

NASA NTRS2004

Scalable High Performance Computing: Direct and Large-Eddy Turbulent Flow Simulations Using Massively Parallel Computers

This final report contains reports of research related to the tasks "Scalable High Performance Computing: Direct and Lark-Eddy Turbulent FLow Simulations Using Massively Parallel Computers" and "Devleop High-Performance Time-Domain Computational Electromagnetics Capability for RCS Prediction, Wave Propagation in Dispersive Media, and Dual-Use Applications. The discussion of Scalable High Performance Computing reports on three objectives: validate, access scalability, and apply two parallel flow solvers for three-dimensional Navier-Stokes flows; develop and validate a high-order parallel solver for Direct Numerical Simulations (DNS) and Large Eddy Simulation (LES) problems; and Investigate and develop a high-order Reynolds averaged Navier-Stokes turbulence model. The discussion of High-Performance Time-Domain Computational Electromagnetics reports on five objectives: enhancement of an electromagnetics code (CHARGE) to be able to effectively model antenna problems; utilize lessons learned in high-order/spectral solution of swirling 3D jets to apply to solving electromagnetics project; transition a high-order fluids code, FDL3DI, to be able to solve Maxwell's Equations using compact-differencing; develop and demonstrate improved radiation absorbing boundary conditions for high-order CEM; and extend high-order CEM solver to address variable material properties. The report also contains a review of work done by the systems engineer.

Morgan, Philip E.↗

NASA NTRS1994

Decay of Isotropic Turbulence at Low Reynolds Number

Decay of isotropic turbulence is computed using direct numerical simulations. Comparisons with experimental spectra at moderate and low Reynolds numbers (R(sub lambda) less than 70) show good agreement. At moderate to high Reynolds numbers (R(sub lambda) greater 50), the spectra are found to collapse with Kolmogorov scaling at high wave numbers. However, at low Reynolds numbers (R(sub lambda) less than 50) the shape of the spectra at the Kolmogorov length scales is Reynolds number dependent. Direct simulation data from flowfields of decaying isotropic turbulence are used to compute the terms in the equation for the dissipation rate of the turbulent kinetic energy. The development of the skewness and the net destruction of the turbulence dissipation rate in the limit of low Reynolds numbers are presented. The nonlinear terms are found to remain active at surprisingly low Reynolds numbers.

Mansour, N. N.↗

NASA NTRS2003

Large-Eddy Simulation of Conductive Flows at Low Magnetic Reynolds Number

In this paper we study the LES method with dynamic procedure in the context of conductive flows subject to an applied external magnetic field at low magnetic Reynolds number R(sub m). These kind of flows are encountered in many industrial applications. For example, in the steel industry, applied magnetic fields can be used to damp turbulence in the casting process. In nuclear fusion devices (Tokamaks), liquid-lithium flows are used as coolant blankets and interact with the surrounding magnetic field that drives and confines the fusion plasma. Also, in experimental facilities investigating the dynamo effect, the flow consists of liquid-sodium for which the Prandtl number and, as a consequence, the magnetic Reynolds number is low. Our attention is focused here on the case of homogeneous (initially isotropic) decaying turbulence. The numerical simulations performed mimic the thought experiment described in Moffatt in which an initially homogeneous isotropic conductive flow is suddenly subjected to an applied magnetic field and freely decays without any forcing. Note that this flow was first studied numerically by Schumann. It is well known that in that case, extra damping of turbulence occurs due to the Joule effect and that the flow tends to become progressively independent of the coordinate along the direction of the magnetic field. Our comparison of filtered direct numerical simulation (DNS) predictions and LES predictions show that the dynamic Smagorinsky model enables one to capture successfully the flow with LES, and that it automatically incorporates the effect of the magnetic field on the turbulence. Our paper is organized as follows. In the next section we summarize the LES approach in the case of MHD turbulence at low R(sub m) and recall the definition of the dynamic Smagorinsky model. In Sec. 3 we describe the parameters of the numerical experiments performed and the code used. Section 4 is devoted to the comparison of filtered DNS results and LES results. Conclusions are presented in Sec. 5.

Knaepen, B.↗

NASA NTRS1991

The minimal flow unit in near-wall turbulence

Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent, and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer of spanwise vorticity is lifted away from the wall.

Jimeez, Javier↗

NASA NTRS1993

A normal stress subgrid-scale eddy viscosity model in large eddy simulation

The Smagorinsky subgrid-scale eddy viscosity model (SGS-EVM) is commonly used in large eddy simulations (LES) to represent the effects of the unresolved scales on the resolved scales. This model is known to be limited because its constant must be optimized in different flows, and it must be modified with a damping function to account for near-wall effects. The recent dynamic model is designed to overcome these limitations but is compositionally intensive as compared to the traditional SGS-EVM. In a recent study using direct numerical simulation data, Horiuti has shown that these drawbacks are due mainly to the use of an improper velocity scale in the SGS-EVM. He also proposed the use of the subgrid-scale normal stress as a new velocity scale that was inspired by a high-order anisotropic representation model. The testing of Horiuti, however, was conducted using DNS data from a low Reynolds number channel flow simulation. It was felt that further testing at higher Reynolds numbers and also using different flows (other than wall-bounded shear flows) were necessary steps needed to establish the validity of the new model. This is the primary motivation of the present study. The objective is to test the new model using DNS databases of high Reynolds number channel and fully developed turbulent mixing layer flows. The use of both channel (wall-bounded) and mixing layer flows is important for the development of accurate LES models because these two flows encompass many characteristic features of complex turbulent flows.

Horiuti, K.↗

NASA NTRS2010

Heat Entrapment Effects Within Liquid Acquisition Devices

We introduce a model problem to address heat entrapment effects or the local accumulation of thermal energy within liquid acquisition devices. We show that the parametric space consists of six parameters, namely the Rayleigh and Prandtl numbers, the aspect ratio, and heat flux ratios for the bottom, side, and top boundaries of the enclosure. For the range of Ra considered 1 to 10(sup 9), beyond Ra on the order of 10(sup 5), convective instability is the dominant mode of convection in comparison to natural convection. The flow field transitions to asymmetric modes at Ra on the order of 10(sup 7). Direct numerical simulation of a large geometric length scale prototype for Ra on the order of 10(sup 9) shows that the flow field evolves from small wavelength instability which gives rise to nonlinear growth of thermals, propagation of the instability occurs via growth of secondary and tertiary modes, and a travelling wave mode occurs prior to asymmetry. The effect of a large aspect ratio is to increase the number of modes in the vertical direction. Due to the slow diffusion of heat in the prototype, asymptotic states are not readily attained, we show that dynamical similarity can be used for a model which allows the attainment of asymptotic states and that transition to a chaotic state occurs for Ra on the order of 10(sup 9) via a broadband power spectrum. These dynamical events show that for the baseline condition in which heat is absorbed from background laboratory environment, higher heat flux is absorbed at the top and bottom boundaries of the enclosure than a nominal value of 34.9 ergs per square centimeter -second.

Duval, W. M. B.↗

NASA NTRS1992

Direct simulation of the stably stratified turbulent Ekman layer

The Navier-Stokes equations and the Boussinesq approximation were used to compute a 3D time-dependent turbulent flow in the stably stratified Ekman layer over a smooth surface. The simulation data are found to be in very good agreement with atmospheric measurements when nondimensionalized according to Nieuwstadt's local scaling scheme. Results suggest that, when Reynolds number effects are taken into account, the 'constant Froud number' stable layer model (Brost and Wyngaard, 1978) and the 'shearing length' stable layer model (Hunt, 1985) for the dissipitation rate of turbulent kinetic energy are both valid. It is concluded that there is good agreement between the direct numerical simulation results and large-eddy simulation results obtained by Mason and Derbyshire (1990).

Coleman, G. N.↗

NASA NTRS1992

Study of turbulence structure through numerical simulations: The perspective of drag and reduction

The first part of this note concerns organized structures in the near-wall region of a turbulent boundary layer. A particular emphasis is given to those organized motions believed to be responsible for the turbulence production process in the wall region, and hence, indirectly responsible for the viscous drag. Examples are selected to illustrate how the analyses of numerically generated databases have contributed to improve our understanding of the organized structures of turbulent boundary layers. In the second part, results from an exploratory study based on the direct numerical simulation of the concepts for active control of turbulent boundary layers are presented. A significant drag reduction is achieved when the surface boundary condition is modified such that it could suppress the organized motion in the wall region. This drag reduction is accompanied by a significant reduction in the intensity of the organized structures and in the magnitude of Reynolds stresses throughout the flow.

Kim, J. J.↗

NASA NTRS2002

Ray Traces Through Unsteady Jet Turbulence

Results of an ongoing effort to quantify the role turbulence in scattering sound in jets are reported. Using a direct numerical simulation database to provide the flow data, ray paths traced through the mean flow are compared with those traced through the actual time evolving turbulent flow. Significant scattering by the turbulence is observed. The most notable effect is that upstream traveling waves that are trapped in the potential core by the mean flow, which acts as a wave guide, easily escape in the turbulent flow. A crude statistical estimate based on ray number density suggests that directivity is modified by the turbulence, but no rigorous treatment of non-uniformities in the high-frequency approximation is attempted.

Freund, J. B.↗

NASA NTRS1991

Some characteristics of small-scale turbulence in a turbulent duct flow

The fine-scale structure of turbulence in a fully developed turbulent duct flow is examined by considering the 3D velocity derivative field obtained from direct numerical simulations at two relatively small Reynolds numbers. The magnitudes of all mean-square derivatives (normalized by wall variables) increase with the Reynolds number, the increase being largest at the wall. These magnitudes are not consistent with the assumption of local isotropy except perhaps near the duct center-line. When the assumption of local isotropy is relaxed to one of local axisymmetry, or invariance with respect to rotation about a coordinate axis (here chosen in the streamwise direction), satisfactory agreement is indicated by the data outside the wall region. Support for axisymmetry is demonstrated by anisotropy invariant maps of the dissipation and vorticity tensors.

Antonia, R. A.↗

NASA NTRS2002

Computation of High Speed Jet Noise

The objective of this work was to use direct numerical simulation (DNS) techniques to study the physics of noise generation by a high-speed turbulent jet. A Mach 0.9, Reynolds number 3,600 jet was selected because of available experimental data. New numerical methods for generating disturbances at the nozzle and computing far-field sound were developed and reported in the course of this work. Over 25 million mesh points were used in the simulations which ran for over 50,000 timesteps and required over 50,000 processor hours on state-of-the-art parallel computer systems to complete. Figures show a visualization of the jet and sound field, a comparison of the mean flow development with the experiment, a directivity comparison with the experiment, and time spectrum comparison with the experiment. Agreement is seen to be excellent. These are fully document in the attached references. Full details of the work, detailed achievements and conclusions are discussed in appendices, which are copies of publications that resulted from this work. We have studied noise mechanisms in supersonic jets, the refraction of sound by turbulence in subsonic jets, and noise sources in conjunction with a DNS of a Mach 0.9 jet.

Freund, Jonathan B.↗

NASA NTRS2000

A Study of Wake Development and Structure in Constant Pressure Gradients

Motivated by the application to high-lift aerodynamics for commercial transport aircraft, a systematic investigation into the response of symmetric/asymmetric planar turbulent wake development to constant adverse, zero, and favorable pressure gradients has been conducted. The experiments are performed at a Reynolds number of 2.4 million based on the chord of the wake generator. A unique feature of this wake study is that the pressure gradients imposed on the wake flow field are held constant. The experimental measurements involve both conventional LDV and hot wire flow field surveys of mean and turbulent quantities including the turbulent kinetic energy budget. In addition, similarity analysis and numerical simulation have also been conducted for this wake study. A focus of the research has been to isolate the effects of both pressure gradient and initial wake asymmetry on the wake development. Experimental results reveal that the pressure gradient has a tremendous influence on the wake development, despite the relatively modest pressure gradients imposed. For a given pressure gradient, the development of an initially asymmetric wake is different from the initially symmetric wake. An explicit similarity solution for the shape parameters of the symmetric wake is obtained and agrees with the experimental results. The turbulent kinetic energy budget measurements of the symmetric wake demonstrate that except for the convection term, the imposed pressure gradient does not change the fundamental flow physics of turbulent kinetic energy transport. Based on the turbulent kinetic energy budget measurements, an approach to correct the bias error associated with the notoriously difficult dissipation estimate is proposed and validated through the comparison of the experimental estimate with a direct numerical simulation result.

Thomas, Flint O.↗

NASA NTRS1990

A perspective on coherent structures and conceptual models for turbulent boundary layer physics

Direct numerical simulations of turbulent boundary layers have been analyzed to develop a unified conceptual model for the kinematics of coherent motions in low Reynolds number canonical turbulent boundary layers. All classes of coherent motions are considered in the model, including low-speed streaks, ejections and sweeps, vortical structures, near-wall and outer-region shear layers, sublayer pockets, and large-scale outer-region eddies. The model reflects the conclusions from the study of the simulated boundary layer that vortical structures are directly associated with the production of turbulent shear stresses, entrainment, dissipation of turbulence kinetic energy, and the fluctuating pressure field. These results, when viewed from the perspective of the large body of published work on the subject of coherent motions, confirm that vortical structures may be considered the central dynamic element in the maintenance of turbulence in the canonical boundary layer. Vortical structures serve as a framework on which to construct a unified picture of boundary layer structure, providing a means to relate the many known structural elements in a consistent way.

Robinson, Stephen K.↗