Showing papers on "Turbulence published in 1998"

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Abstract: Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

The formation and evolution of synthetic jets

Abstract: A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet i...

Analysis of transport phenomena

Abstract: Preface List of Symbols CHAPTER 1. DIFFUSIVE FLUXES AND MATERIAL PROPERTIES 1.1 INTRODUCTION 1.2 BASIC CONSTITUTIVE EQUATIONS 1.3 DIFFUSIVITIES FOR ENERGY, SPECIES, AND MOMENTUM 1.4 MAGNITUDES OF TRANSPORT COEFFICIENTS 1.5 MOLECULAR INTERPRETATION OF TRANSPORT COEFFICIENTS 1.6 LIMITATIONS ON LENGTH AND TIME SCALES References Problems CHAPTER 2. FUNDAMENTALS OF HEAT AND MASS TRANSFER 2.1 INTRODUCTION 2.2 GENERAL FORMS OF CONSERVATION EQUATIONS 2.3 CONSERVATION OF MASS 2.4 CONSERVATION OF ENERGY: THERMAL EFFECTS 2.5 HEAT TRANSFER AT INTERFACES 2.6 CONSERVATION OF CHEMICAL SPECIES 2.7 MASS TRANSFER AT INTERFACES 2.8 MOLECULAR VIEW OF SPECIES CONSERVATION References Problems CHAPTER 3. FORMULATION AND APPROXIMATION 3.1 INTRODUCTION 3.2 ONE-DIMENSIONAL EXAMPLES 3.3 ORDER-OF-MAGNITUDE ESTIMATION AND SCALING 3.4 " IN MODELING 3.5 TIME SCALES IN MODELING References Problems CHAPTER 4. SOLUTION METHODS BASED ON SCALING CONCEPTS 4.1 INTRODUCTION 4.2 SIMILARITY METHOD 4.3 REGULAR PERTURBATION ANALYSIS 4.4 SINGULAR PERTURBATION ANALYSIS References Problems CHAPTER 5. SOLUTION METHODS FOR LINEAR PROBLEMS 5.1 INTRODUCTION 5.2 PROPERTIES OF LINEAR BOUNDARY-VALUE PROBLEMS 5.3 FINITE FOURIER TRANSFORM METHOD 5.4 BASIS FUNCTIONS 5.5 FOURIER SERIES 5.6 FFT SOLUTIONS FOR RECTANGULAR GEOMETRIES 5.7 FFT SOLUTIONS FOR CYLINDRICAL GEOMETRIES 5.8 FFT SOLUTIONS FOR SPHERICAL GEOMETRIES 5.9 POINT-SOURCE SOLUTIONS 5.10 MORE ON SELF-ADJOINT EIGENVALUE PROBLEMS AND FFT SOLUTIONS References Problems CHAPTER 6. FUNDAMENTALS OF FLUID MECHANICS 6.1 INTRODUCTION 6.2 CONSERVATION OF MOMENTUM 6.3 TOTAL STRESS, PRESSURE, AND VISCOUS STRESS 6.4 FLUID KINEMATICS 6.5 CONSTITUTIVE EQUATIONS FOR VISCOUS STRESS 6.6 FLUID MECHANICS AT INTERFACES 6.7 FORCE CALCULATIONS 6.8 STREAM FUNCTION 6.9 DIMENSIONLESS GROUPS AND FLOW REGIMES References Problems CHAPTER 7. UNIDIRECTIONAL AND NEARLY UNIDIRECTIONAL FLOW 7.1 INTRODUCTION 7.2 STEADY FLOW WITH A PRESSURE GRADIENT 7.3 STEADY FLOW WITH A MOVING SURFACE 7.4 TIME-DEPENDENT FLOW 7.5 LIMITATIONS OF EXACT SOLUTIONS 7.6 NEARLY UNIDIRECTIONAL FLOW References Problems CHAPTER 8. CREEPING FLOW 8.1 INTRODUCTION 8.2 GENERAL FEATURES OF LOW REYNOLDS NUMBER FLOW 8.3 UNIDIRECTIONAL AND NEARLY UNIDIRECTIONAL SOLUTIONS 8.4 STREAM-FUNCTION SOLUTIONS 8.5 POINT-FORCE SOLUTIONS 8.6 PARTICLES AND SUSPENSIONS 8.7 CORRECTIONS TO STOKES' LAW References Problems CHAPTER 9. LAMINAR FLOW AT HIGH REYNOLDS NUMBER 9.1 INTRODUCTION 9.2 GENERAL FEATURES OF HIGH REYNOLDS NUMBER FLOW 9.3 IRROTATIONAL FLOW 9.4 BOUNDARY LAYERS AT SOLID SURFACES 9.5 INTERNAL BOUNDARY LAYERS References Problems CHAPTER 10. FORCED-CONVECTION HEAT AND MASS TRANSFER IN CONFINED LAMINAR FLOWS 10.1 INTRODUCTION 10.2 PECLET NUMBER 10.3 NUSSELT AND SHERWOOD NUMBERS 10.4 ENTRANCE REGION 10.5 FULLY DEVELOPED REGION 10.6 CONSERVATION OF ENERGY: MECHANICAL EFFECTS 10.7 TAYLOR DISPERSION References Problems CHAPTER 11. FORCED-CONVECTION HEAT AND MASS TRANSFER IN UNCONFINED LAMINAR FLOWS 11.1 INTRODUCTION 11.2 HEAT AND MASS TRANSFER IN CREEPING FLOW 11.3 HEAT AND MASS TRANSFER IN LAMINAR BOUNDARY LAYERS 11.4 SCALING LAWS FOR NUSSELT AND SHERWOOD NUMBERS References Problems CHAPTER 12. TRANSPORT IN BUOYANCY-DRIVEN FLOW 12.1 INTRODUCTION 12.2 BUOYANCY AND THE BOUSSINESQ APPROXIMATION 12.3 CONFINED FLOWS 12.4 DIMENSIONAL ANALYSIS AND BOUNDARY-LAYER EQUATIONS 12.5 UNCONFINED FLOWS References Problems CHAPTER 13. TRANSPORT IN TURBULENT FLOW 13.1 INTRODUCTION 13.2 BASIC FEATURES OF TURBULENCE 13.3 TIME-SMOOTHED EQUATIONS 13.4 EDDY DIFFUSIVITY MODELS 13.5 OTHER APPROACHES FOR TURBULENT-FLOW CALCULATIONS References Problems CHAPTER 14. SIMULTANEOUS ENERGY AND MASS TRANSFER AND MULTICOMPONENT SYSTEMS 14.1 INTRODUCTION 14.2 CONSERVATION OF ENERGY: MULTICOMPONENT SYSTEMS 14.3 SIMULTANEOUS HEAT AND MASS TRANSFER 14.4 INTRODUCTION TO COUPLED FLUXES 14.5 STEFAN-MAXWELL EQUATIONS 14.6 GENERALIZED DIFFUSION IN DILUTE MIXTURES 14.7 GENERALIZED STEFAN-MAXWELL EQUATIONS References Problems CHAPTER 15. TRANSPORT IN ELECTROLYTE SOLUTIONS 15.1 INTRODUCTION 15.2 FORMULATION OF MACROSCOPIC PROBLEMS 15.3 MACROSCOPIC EXAMPLES 15.4 EQUILIBRIUM DOUBLE LAYERS 15.5 ELECTROKINETIC PHENOMENA References Problems APPENDIX A. VECTORS AND TENSORS A.1 INTRODUCTION A.2 REPRESENTATION OF VECTORS AND TENSORS A.3 VECTOR AND TENSOR PRODUCTS A.4 VECTOR-DIFFERENTIAL OPERATORS A.5 INTEGRAL TRANSFORMATIONS A.6 POSITION VECTORS A.7 ORTHOGONAL CURVILINEAR COORDINATES A.8 SURFACE GEOMETRY References APPENDIX B. ORDINARY DIFFERENTIAL EQUATIONS AND SPECIAL FUNCTIONS B.1 INTRODUCTION B.2 FIRST-ORDER EQUATIONS B.3 EQUATIONS WITH CONSTANT COEFFICIENTS B.4 BESSEL AND SPHERICAL BESSEL EQUATIONS B.5 OTHER EQUATIONS WITH VARIABLE COEFFICIENTS References Index

Turbulent transport reduction by zonal flows : Massively parallel simulations

Abstract: Three-dimensional gyrokinetic simulations of microturbulence in magnetically confined toroidal plasmas with massively parallel computers showed that, with linear flow damping, an asymptotic residual flow develops in agreement with analytic calculations. Nonlinear global simulations of instabilities driven by temperature gradients in the ion component of the plasma support the view that turbulence-driven fluctuating zonal flows can substantially reduce turbulent transport. Finally, the outstanding differences in the flow dynamics observed in global and local simulations are found to be due to profile variations.

Lectures on Geophysical Fluid Dynamics

Abstract: 1. Fundamentals 2. Introduction to Geophysical Fluid Dyunamics 3. Non-inertial Theory of Ocean Circulation 4. Vorticity and Turbulence 5. Statistical Fluid Dynamics 6. Geostropic Turbulence 7. Hamiltonian Fluid Dynamics

A numerical study of breaking waves in the surf zone

Abstract: This paper describes the development of a numerical model for studying the evolution of a wave train, shoaling and breaking in the surf zone. The model solves the Reynolds equations for the mean (ensemble average) flow field and the k–e equations for the turbulent kinetic energy, k, and the turbulence dissipation rate, e. A nonlinear Reynolds stress model (Shih, Zhu & Lumley 1996) is employed to relate the Reynolds stresses and the strain rates of the mean flow. To track free-surface movements, the volume of fluid (VOF) method is employed. To ensure the accuracy of each component of the numerical model, several steps have been taken to verify numerical solutions with either analytical solutions or experimental data. For non-breaking waves, very accurate results are obtained for a solitary wave propagating over a long distance in a constant depth. Good agreement between numerical results and experimental data has also been observed for shoaling and breaking cnoidal waves on a sloping beach in terms of free-surface profiles, mean velocities, and turbulent kinetic energy. Based on the numerical results, turbulence transport mechanisms under breaking waves are discussed.

Mean-flow scaling of turbulent pipe flow

Abstract: Measurements of the mean velocity profile and pressure drop were performed in a fully developed, smooth pipe flow for Reynolds numbers from 31×103 to 35×106. Analysis of the mean velocity profiles indicates two overlap regions: a power law for 60 9×103). Von Karman's constant was shown to be 0.436 which is consistent with the friction factor data and the mean velocity profiles for 600 5%) than those predicted by Prandtl's relation. A new friction factor relation is proposed which is within ±1.2% of the data for Reynolds numbers between 10×103 and 35×106, and includes a term to account for the near-wall velocity profile.

Poloidal Flow Driven by Ion-Temperature-Gradient Turbulence in Tokamaks

Abstract: We show that linear collisionless processes do not damp poloidal flows driven by ion-temperature-gradient (ITG) turbulence. Since these flows play an important role in saturating the level of the turbulence, this level, as well as the transport caused by ITG modes, may be overestimated by gyrofluid simulations, which employ linear collisionless rotation damping.

Dissipation in compressible magnetohydrodynamic turbulence

Abstract: We report results of a three-dimensional, high resolution (up to 5123) numerical investigation of supersonic compressible magnetohydrodynamic turbulence. We consider both forced and decaying turbulence. The model parameters are appropriate to conditions found in Galactic molecular clouds. We find that the dissipation time of turbulence is of the order of the flow crossing time or smaller, even in the presence of strong magnetic fields. About half of the dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by the turbulence, while strong fields remain well ordered.

Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements

Abstract: Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (“ground truth”) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (〈u′2〉, 〈w′2〉) and covariance (〈u′w′〉) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measure...

Dissipation in Compressible MHD Turbulence

Abstract: We report results of a three dimensional, high resolution (up to 512^3) numerical investigation of supersonic compressible magnetohydrodynamic turbulence. We consider both forced and decaying turbulence. The model parameters are appropriate to conditions found in Galactic molecular clouds. We find that the dissipation time of turbulence is of order the flow crossing time or smaller, even in the presence of strong magnetic fields. About half the dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by the turbulence, while strong fields remain well ordered.

Kinetic Energy Decay Rates of Supersonic and Super-Alfvénic Turbulence in Star-Forming Clouds

Abstract: We present numerical studies of compressible, decaying turbulence, with and without magnetic fields, with initial rms Alfven and Mach numbers ranging up to five, and apply the results to the question of the support of star-forming in- terstellar clouds of molecular gas. We find that, in 1D, magnetized turbulence actually decays faster than unmagnetized turbulence. In all the regimes that we have studied 3D turbulence—super-Alfvenic, supersonic, sub-Alfvenic, and subsonic—the kinetic energy decays as (t t0) � , with 0.85 < � < 1.2. We compared results from two entirely different algorithms in the unmagnetized case, and have performed extensive resolution studies in all cases, reaching resolutions of 256 3 zones or 350,000 particles. We conclude that the observed long lifetimes and supersonic motions in molecular clouds must be due to external driving, as undriven turbulence decays far too fast to explain the observations.

Turbulence modeling for time-dependent RANS and VLES : a review

Abstract: Reynolds stress models and traditional large-eddy simulations are reexamined with a view toward developing a combined methodology for the computation of complex turbulent flows. More specifically, an entirely new approach to time-dependent Reynolds-averaged Navier-Stokes (RANS) computations and very large-eddy simulations (VLES) is presented in which subgrid scale models are proposed that allow a direct numerical simulation (DNS) to go continuously to a RANS computation in the coarse mesh/infinite Reynolds number limit. In between these two limits, we have a large eddy simulation (LES) or VLES, depending on the level of resolution. The Reynolds stress model that is ultimately recovered in the coarse mesh/infinite Reynolds number limit has built in nonequilibrium features that make it suitable for time-dependent RANS. The fundamental technical issues associated with this new approach, which has the capability of bridging the gap between DNS, LES and RANS, are discussed in detail. Illustrative calculations are presented along with a discussion of the future implications of these results for the simulation of the turbulent flows of technological importance.

Direct numerical simulation analysis of flame surface density concept for large eddy simulation of turbulent premixed combustion

Abstract: Large eddy simulation (LES) is a promising tool for numerical simulations of reacting flows, especially when combustion instabilities are encountered. Nevertheless, a difficulty occurs in developing subgrid scale models for premixed combustion, because the flame front is generally too thin to be resolved on the computational grid. An approach based on the filtering of the balance equation for the progress variable c using a LES filter larger than the mesh size is proposed and investigated. Despite its similarities with the field description based on the G-equation, the advantage of this approach is that c and related quantities such as subgrid-scale flame surface density are physically well defined and may be easily extracted from direct numerical simulations (DNS) or experimental data to analyze and validate models. A three-dimensional DNS database where a laminar premixed flame interacts with a homogeneous and isotropic turbulent flowfield is used to investigate unresolved turbulent scalar transport and filtered reaction rate. The unresolved transport is found to exhibit a gradient or a countergradient feature, depending on the heat release parameter and the turbulence level. This finding is in aggreement with previous observations of the turbulent transport in Reynolds-averaged Navier-Stokes (RANS) equations. Nevertheless, the unresolved convective flux is lower than the resolved one. Accordingly, model uncertainties will have probably less dramatic consequences than in RANS because a part of the countergradient phenomenon will be incorporated into the motion of the resolved flow structures. The filtered reaction rate is closed, introducing a subgrid-scale flame surface density, Σ, modeled, in a first step, with an algebraic expression similar to the Bray-Moss-Libby (BML) formulation widely used in RANS context. This concept is very attractive because it could be refined using, for example, a dynamic algebraic formulation or a balance equation for Σ.

Direct numerical simulation of turbulence modulation by particles in isotropic turbulence

Abstract: The modulation of isotropic turbulence by particles has been investigated using direct numerical simulation (DNS). The particular focus of the present work is on the class of dilute flows in which particle volume fractions and inter-particle collisions are negligible. Gravitational settling is also neglected and particle motion is assumed to be governed by drag with particle relaxation times ranging from the Kolmogorov scale to the Eulerian time scale of the turbulence and particle mass loadings up to 1. The velocity field was made statistically stationary by forcing the low wavenumbers of the flow. The calculations were performed using 963 collocation points and the Taylor-scale Reynolds number for the stationary flow was 62. The effect of particles on the turbulence was included in the Navier–Stokes equations using the point-force approximation in which 963 particles were used in the calculations. DNS results show that particles increasingly dissipate fluid kinetic energy with increased loading, with the reduction in kinetic energy being relatively independent of the particle relaxation time. Viscous dissipation in the fluid decreases with increased loading and is larger for particles with smaller relaxation times. Fluid energy spectra show that there is a non-uniform distortion of the turbulence with a relative increase in small-scale energy. The non-uniform distortion significantly affects the transport of the dissipation rate, with the production and destruction of dissipation exhibiting completely different behaviours. The spectrum of the fluid–particle energy exchange rate shows that the fluid drags particles at low wavenumbers while the converse is true at high wavenumbers for small particles. A spectral analysis shows that the increase of the high-wavenumber portion of the fluid energy spectrum can be attributed to transfer of the fluid–particle covariance by the fluid turbulence. This in turn explains the relative increase of small-scale energy caused by small particles observed in the present simulations as well as those of Squires & Eaton (1990) and Elghobashi & Truesdell (1993).

Experimental study of vortex breakdown in swirling jets

Abstract: The goal of this study is to characterize the various breakdown states taking place in a swirling water jet as the swirl ratio S and Reynolds number Re are varied. A pressure-driven water jet discharges into a large tank, swirl being imparted by means of a motor which sets into rotation a honeycomb within a settling chamber. The experiments are conducted for two distinct jet diameters by varying the swirl ratio S while maintaining the Reynolds number Re fixed in the range 300

Turbulent flow over hills and waves

Abstract: This is a review of the mechanisms that control neutrally stable turbulent boundary-layer flow over hills and waves, their relative magnitudes, and how they exert their greatest effects in different regions of the flow. We compare calculations based on various analytical and computational models with each other and with relevant experimental data. We discuss practical applications of these studies.

Large eddy simulation of the subcritical flow past a circular cylinder: numerical and modeling aspects

Abstract: SUMMARY The turbulent flow past a circular cylinder (Re=3900) was computed by large eddy simulation (LES). The objective was not to investigate the physical phenomena of this flow in detail but to study numerical and modeling aspects which influence the quality of LES solutions. Concerning the numerical method, the most important component is the discretization of the non-linear convective fluxes. Five different schemes were investigated. Also, the influence of different grid resolutions was examined. Two aspects play an important role on the modeling side, namely the near-wall model and the subgrid scale model. Owing to the restriction to low Reynolds numbers in this study, no-slip boundary conditions were used at solid walls. Therefore, only the second aspect was taken into account. Two different subgrid scale models were applied. Additionally, LES computations without any subgrid scale modeling were carried out in order to prove the performance of the models. The results were evaluated by comparison with available experimental data. © 1998 John Wiley & Sons, Ltd.

The Energy Dissipation Rate of Supersonic, Magnetohydrodynamic Turbulence in Molecular Clouds

Abstract: Molecular clouds have broad linewidths suggesting turbulent supersonic motions in the clouds. These motions are usually invoked to explain why molecular clouds take much longer than a free-fall time to form stars. It has classically been thought that supersonic hydrodynamical turbulence would dissipate its energy quickly, but that the introduction of strong magnetic fields could maintain these motions. In a previous paper it has been shown, however, that isothermal, compressible, MHD and hydrodynamical turbulence decay at virtually the same rate, requiring that constant driving occur to maintain the observed turbulence. In this paper direct numerical computations of uniformly driven turbulence with the ZEUS astrophysical MHD code are used to derive the absolute value of energy dissipation as a function of the driving wavelength and amplitude. The ratio of the formal decay time of turbulence E_{kin}/\dot{E}_{kin} to the free-fall time of the gas can then be derived as a function of the ratio of driving wavelength to Jeans wavelength and rms Mach number, and shown to be most likely far less than unity, again showing that turbulence in molecular clouds must be constantly and strongly driven. (abridged)

Break-up and atomization of a round water jet by a high-speed annular air jet

Abstract: The near- and far-field break-up and atomization of a water jet by a high-speed annular air jet are examined by means of high-speed flow visualizations and phase Doppler particle sizing techniques. Visualization of the jet's near field and measurements of the frequencies associated with the gas–liquid interfacial instabilities are used to study the underlying physical mechanisms involved in the primary break-up of the water jet. This process is shown to consist of the stripping of water sheets, or ligaments, which subsequently break into smaller lumps or drops. An entrainment model of the near-field stripping of the liquid is proposed, and shown to describe the measured liquid shedding frequencies. This simplified model explains qualitatively the dependence of the shedding frequency on the air/water momentum ratio in both initially laminar and turbulent water jets. The role of the secondary liquid break-up in the far-field atomization of the water jet is also investigated, and an attempt is made to apply the classical concepts of local isotropy to explain qualitatively the measurement of the far-field droplet size distribution and its dependence on the water to air mass and momentum ratios. Models accounting for the effect of the local turbulent dissipation rate in the gas on both the break-up and coalescence of the droplets are developed and compared with the measurements of the variation of the droplet size along the jet's centreline. The total flux of kinetic energy supplied by the gas per unit total mass of the spray jet was found to be the primary parameter determining the secondary break-up and coalescence of the droplets in the far field.

A numerical investigation on the effect of the inflow conditions on the self-similar region of a round jet

Abstract: In this paper we consider the direct numerical simulation (DNS) of a spatially developing free round jet at low Reynolds numbers. Simulation of a spatially evolving flow such as the jet requires boundary conditions, which allow entrainment into the turbulent flow across the lateral boundaries of the computational domain. The boundary conditions which satisfy this requirement are so-called traction free boundary conditions. After showing that these boundary conditions lead to a correct behavior of the velocity near the lateral boundary of the jet, we will consider the DNS of the jet flow at a Reynolds number of 2.4×103 and compare the results with experimental data obtained by Hussein et al. [J. Fluid Mech. 258, 31 (1994)] and by Panchapakesan and Lumley [J. Fluid Mech. 246, 197 (1993)]. The results of our numerical simulations agree very well with the experimental data. Next we use the DNS to investigate the influence of the shape of the velocity profile at the jet orifice on the self-similarity scaling f...

Post-stall flow control on an airfoil by local unsteady forcing

Abstract: By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of post-stall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Karman street.

Passive scalar statistics in high-Péclet-number grid turbulence

Abstract: The statistics of a turbulent passive scalar (temperature) and their Reynolds number dependence are studied in decaying grid turbulence for the Taylor-microscale Reynolds number, Rλ, varying from 30 to 731 (21[les ]Peλ[les ]512). A principal objective is, using a single (and simple) flow, to bridge the gap between the existing passive grid-generated low-Peclet-number laboratory experiments and those done at high Peclet number in the atmosphere and oceans. The turbulence is generated by means of an active grid and the passive temperature fluctuations are generated by a mean transverse temperature gradient, formed at the entrance to the wind tunnel plenum chamber by an array of differentially heated elements. A well-defined inertial–convective scaling range for the scalar with a slope, nθ, close to the Obukhov–Corrsin value of 5/3, is observed for all Reynolds numbers. This is in sharp contrast with the velocity field, in which a 5/3 slope is only approached at high Rλ. The Obukhov–Corrsin constant, Cθ, is estimated to be 0.45–0.55. Unlike the velocity spectrum, a bump occurs in the spectrum of the scalar at the dissipation scales, with increasing prominence as the Reynolds number is increased. A scaling range for the heat flux cospectrum was also observed, but with a slope around 2, less than the 7/3 expected from scaling theory. Transverse structure functions of temperature exist at the third and fifth orders, and, as for even-order structure functions, the width of their inertial subranges dilates with Reynolds number in a systematic way. As previously shown for shear flows, the existence of these odd-order structure functions is a violation of local isotropy for the scalar differences, as is the existence of non-zero values of the transverse temperature derivative skewness (of order unity) and hyperskewness (of order 100). The ratio of the temperature derivative standard deviation along and normal to the gradient is 1.2±0.1, and is independent of Reynolds number. The refined similarity hypothesis for the passive scalar was found to hold for all Rλ, which was not the case for the velocity field. The intermittency exponent for the scalar, μθ, was found to be 0.25±0.05 with a possible weak Rλ dependence, unlike the velocity field, where μ was a strong function of Reynolds number. New, higher-Reynolds-number results for the velocity field, which smoothly follow the trends of Mydlarski & Warhaft (1996), are also presented.

Local investigation of superfluid turbulence

Abstract: We investigate flows of helium IV driven by two counter-rotating disks, in a range of temperatures varying between 1.4 and 2.3 K. The local pressure fluctuations obtained on a small total-head tube are analyzed. Above Tλ, the sensor allows to measure the local velocity fluctuations, and below Tλ, it determines the local fluctuations of a linear combination of the normal and superfluid flow components. Above and below Tλ, Kolmogorov spectra are clearly obtained, with similar Kolmogorov constants. Evidence for persistence of inertial range intermittency in the superfluid region is presented. At all temperatures below Tλ, the structure function exponents are found indistinguishable from those currently observed in normal fluid turbulence.

An update on the energy dissipation rate in isotropic turbulence

Abstract: Direct numerical simulations of homogeneous turbulence in a periodic box are examined here to support the traditional expectation that the dissipation rate at high Reynolds numbers is independent of fluid viscosity, and is a constant of order unity when scaled on the integral scale and root-mean-square velocity. However, the numerical value of the constant appears to depend on details of forcing at low wavenumbers, or, perhaps, the structure of the large scale itself.

Three-dimensional coherent states in plane shear flows

Abstract: Three-dimensional steady states in plane Couette flow and traveling-wave solutions in plane Poiseuille flow are calculated for stress boundary conditions. The procedure is tied to a self-sustaining mechanism associated with the coherent structures that have been observed in turbulent shear flows. The exact states in both types of flow are remarkably similar to each other and to the coherent structures. They survive down to Reynolds numbers below the critical value for turbulence onset. These results suggest that the underlying process is generic and fundamental to both transitional and developed turbulence.