Showing papers on "Turbulence published in 1981"

Organized Motion in Turbulent Flow

Abstract: A review of organized motion in turbulent flow indicates that the transport properties of most shear flows are dominated by large-scale vortex nonrandom motions. The mean velocity profile of a turbulent boundary layer consists of a viscous sublayer, buffer layer, and a logarithmic outer layer; an empirical formula of Coles (1956) applies to various pressure gradients. The boundary layer coherent structure was isolated by the correlation methods of Townsend (1956) and flow visualization by direct observations of complex unsteady turbulent motions. The near-wall studies of Willmart and Wooldridge (1962) used the space-time correlation for pressure fluctuations at the wall under a thick turbulent boundary layer; finally, organized motion in free shear flows and transition-control of mixing demonstrated that the Reynolds number invariance of turbulence shows wide scatter.

Roads to turbulence in dissipative dynamical systems

Abstract: Three scenarios leading to turbulence in theory and experiment are outlined. The respective mathematical theories are explained and compared.

Experiments in nearly homogenous turbulent shear flow with a uniform mean temperature gradient. Part 1

Abstract: A reasonably uniform mean temperature gradient has been superimposed upon a nearly homogeneous turbulent shear flow in a wind tunnel. The overheat is small enough to have negligible effect on the turbulence. Away from the wind-tunnel entrance, the transverse statistical homogeneity is good and the temperature fluctuations and their integral scales grow monotonically like the corresponding velocity fluctuations (Harris, Graham & Corrsin 1977). Measurements of several moments, one- and two-point correlation functions, spectra, integral scales, microscales, probability densities, and joint probability densities of the turbulent velocities, temperature fluctuations, and temperature-velocity products are reported. The heat-transport characteristics are much like those of momentum transport, with the turbulent Prandtl number nearly 1. The temperature fluctuation is better correlated with the streamwise than the transverse velocity component, and the cross-component D12 of the turbulent diffusivity tensor has sign opposite to and about twice the magnitude of the diagonal component D22. Some resemblance of directional properties (relative magnitudes of correlation functions, integral scales, microscales) of the temperature with those of the streamwise velocity is also observed. Comparisons of the present data with measurements in the inner part of a heated boundary layer and a fully turbulent pipe flow (x2/d = 0·25) show comparable magnitudes of temperature-velocity correlation coefficients, turbulent Prandtl numbers and ratios of turbulent diffusivities, and show similar shapes of two-point correlation functions.

Conditional statistics of Reynolds stress in rough-wall and smooth-wall turbulent boundary layers

Abstract: Quadrant analysis has been used to investigate the events contributing to the Reynolds shear stress in zero-pressure-gradient turbulent boundary layers over regularly arrayed rough surfaces of several different densities, and over a smooth surface. By partitioning the stress into ejections, sweeps, and inward and outward interactions, it is shown that sweeps account for most of the stress close to rough surfaces, and that the relative magnitude of the sweep component increases both with surface roughness and with proximity to the surface. The sweep-dominated region delineates a ‘roughness sublayer’ with a depth of up to several roughness element heights, in which the turbulence characteristics depend explicitly on the roughness. In the remainder of the inner (or constant-stress) layer, and in the outer layer, the flow obeys familiar similarity laws with respect to surface roughness.The difference ΔS0 between the fractional contributions of sweeps and ejections to the stress is shown to be well related everywhere to the third moments of the streamwise and normal velocity fluctuations. Experimental proportionalities are established between the third moments and δS0, and are shown to agree with predictions made from cumulant-discard theory.The time scale for the passage of large coherent structures past a fixed point, T, is assumed proportional to the mean time between occurrences in a specified quadrant of an instantaneous stress u'w’ at least H times the local mean stress u'w’, where H is a threshold level. For both the ejection and sweep quadrants and for any choice of H, it is found that T scales with the friction velocity u* and the boundary-layer thickness δ, such that Tu*/δ is invariant with change of surface roughness.

Hydrodynamic instabilities and the transition to turbulence

Abstract: Strange attractors and turbulence.- Hydrodynamic stability and bifurcation.- Chaotic behavior and fluid dynamics.- Transition to turbulence in Rayleigh-Beenard convection.- Instabilities and transition in flow between concentric rotating cylinders.- Shear flow instabilities and transition.- Instabilities in geophysical fluid dynamics.- Instabilities and chaos in nonhydrodynamic systems.- Recent Progress.

Numerical study of small-scale intermittency in three-dimensional turbulence

Abstract: Intermittency effects, in magnitude comparable to the early Batchelor & Townsend (1949) experiments, are studied for stationary, homogeneous, isotropic turbulence by means of a direct spectral simulation on a 643 lattice. The turbulence is kept stationary by a coupling to modes external to the spectral code that model the straining effects of large scales on smaller ones. The rate of energy input and viscosity are free parameters. The interrelations of intermittency and parametrizations of the large scales are discussed. Small-scale universality and a local cascade are necessary if comprehensive models of the large scales are to prove tractable. An iterative method to determine the otherwise arbitrary parameters in such a scheme is proposed but not implemented.The equations for energy and vorticity balance are checked as a function of wave-number. The nonlinear (e.g. vortex stretching) terms in the spectral simulation account for nearly 95% of the vorticity production with the external forcing supplying the rest. The non-dimensionalized one-dimensional energy spectrum agrees well with experiments in the dissipation range at Rλ ∼ 100. The locality of the energy cascade in wavenumber is also examined.First- and second-derivative flatness factors of order 4·5-5·0 and 9·0 respectively are found under stationary conditions with bursts to higher values. Resolution and other systematic errors are explored by extensive runs with a 323 code; de-aliasing all higher-order derivative statistics; and recomputing selected averages after zeroing in succession the highest- and lowest-wavenumber bands. The latter analysis is of some relevance to the experimental problem of gauging how a finite-length hot wire biases a flatness measurement. A host of other higher-order derivative statistics are computed, including the vorticity/rate of strain correlations. Three-dimensional plots of the vorticity reveal persistent and extended tubes, sheets and blobs.

A Numerical Method for Solving the Equations of Compressible Viscous Flow

Abstract: Although much progress has already been made In solving problems in aerodynamic design, many new developments are still needed before the equations for unsteady compressible viscous flow can be solved routinely. This paper describes one such development. A new method for solving these equations has been devised that 1) is second-order accurate in space and time, 2) is unconditionally stable, 3) preserves conservation form, 4) requires no block or scalar tridiagonal inversions, 5) is simple and straightforward to program (estimated 10% modification for the update of many existing programs), 6) is more efficient than present methods, and 7) should easily adapt to current and future computer architectures. Computational results for laminar and turbulent flows at Reynolds numbers from 3 x 10(exp 5) to 3 x 10(exp 7) and at CFL numbers as high as 10(exp 3) are compared with theory and experiment.

Structure in turbulent mixing layers and wakes using a chemical reaction

Abstract: Plane turbulent mixing between two streams of water which contained dilute chemical reactants was studied in a new blow-down water tunnel. In a diffusion-limited reaction, a pH indicator, phenolphthalein, in one stream mixed and reacted with a base, sodium hydroxide, in the other stream to form a visible reaction product. The product was found to exist, as expected, in concentrated regions associated with the large, span-wise-coherent structures of the turbulence. A transition in the mixing was observed in which the aqueous mixing product increased by an order of magnitude. The transition is a consequence of the appearance and development of small-scale three-dimensional motions in the flow. Downstream of the transition, the amount of mixing product was independent of Reynolds number (for an order-of-magnitude increase in the latter) and at most only weakly dependent on Schmidt number.

Parameterization of Small Scales of Three-Dimensional Isotropic Turbulence Utilizing Spectral Closures

Abstract: A spectral equation derived from two-point closures applied to three-dimensional isotropic turbulence is studied from the subgrid-scale modeling point of view, with a cutoff wavenumber kc located in the inertial range of turbulence. Ideas of Kraichnan concerning eddy viscosities are then used to evaluate the parameterized subgrid-scale transfer. This, together with a suitable boundary condition at kc, allows us to predict statistically the large scales (k kc). A k−5/3 energy spectrum extending to kc is recovered without any artificial dissipation range in the neighborhood of kc. This procedure is valid both for forced stationary turbulence and for freely decaying turbulence. The same eddy-viscosity is then introduced in a direct numerical simulation of three-dimensional homogeneous isotropic turbulence without external forcing. Again, the energy spectrum, evaluated by averaging on a spherical shell of radius k, follows the Kolmogorov law u...

On the mixing of a rectangular jet

Abstract: Hot-wire measurements in an incompressible rectangular jet, issuing into a quiet environment at ambient conditions, are presented. A blow-down-type air supply system was used to provide the airflow to a cylindrical settling chamber 1.75 m in length and 0.6 m in diameter. The measurements were made with constant-temperature anemometers in conjunction with linearizers. The two signals from the linearizers were sent through a sum and difference unit which was calibrated from dc to 100 kHz. The distributions of mean velocity and the turbulence shear stresses were measured in the two central planes of the jet stations up to 115 widths downstream of the nozzle exit. Three distinct regions characterized the jet flow field: a potential core origin, a two-dimensional-type region, and an axisymmetric type region. The onset of the second region appeared to be at a location where the shear layers separated by the short dimension of the nozzle meet; and the third region occurred at a downstream location where the two shear layers from the short edges of the nozzle meet. In the central plane, similarity was found both in the mean velocity and shear stress profiles beyond 30 widths downstream of the nozzle exit; profiles of rms velocity showed similarity in the second, but not the third region.

Large-scale flow generation in turbulent convection

Abstract: In a horizontal layer of fluid heated from below and cooled from above, cellular convection with horizontal length scale comparable to the layer depth occurs for small enough values of the Rayleigh number. As the Rayleigh number is increased, cellular flow disappears and is replaced by a random array of transient plumes. Upon further increase, these plumes drift in one direction near the bottom and in the opposite direction near the top of the layer with the axes of plumes tilted in such a way that horizontal momentum is transported upward via the Reynolds stress. With the onset of this large-scale flow, the largest scale of motion has increased from that comparable to the layer depth to a scale comparable to the layer width. The conditions for occurrence and determination of the direction of this large-scale circulation are described.

Helical and Nonhelical Turbulent Dynamos

Abstract: Direct numerical simulations of three-dimensional magnetohydrodynamic turbulence with kinetic and magnetic Reynolds numbers up to 100 are presented. Spatially intermittent magnetic fields are observed in a flow with nonhelical driving. Small-scale helical driving produces strong large-scale nearly force-free magnetic fields.

Turbulent boundary layer at low Reynolds number

Abstract: The results of an experimental investigation of a turbulent boundary layer with zero‐pressure gradient directed toward extending the data base at low Reynolds numbers are presented. The data obtained are concerned primarily with mean‐velocity distributions, skin‐friction coefficients, and distributions of intensity of the longitudinal‐component of the turbulent‐velocity fluctuations for Reynolds numbers based on momentum thickness as low as 465. The validity, at low Reynolds numbers, of the semi‐empirical laws characterizing the inner and outer regions of the boundary layer is examined.

Conditional Sampling in Turbulence Measurement

Abstract: (a) Shear flows which exhibit a turbulent-nonturbulent interface; (b) Shear layers perturbed by interaction with another field of turbu­ lence; (c) Quasi-periodic or periodic flows (these include flows behind pass­ ing blades in rotating machinery, boundary layers over a solid or liquid surface with stationary or progressive periodic waves, and more generally flows subjected to either internal or external peri­ odic perturbations); (d) Coherent structures in different shear flows (these structures are currently receiving close attention by the turbulence community in view of their importance to the flow dynamics).

Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field

Abstract: A strong external dc magnetic field introduces a basic anisotropy into incompressible magnetohydrodynamic turbulence. The modifications that this is likely to produce in the properties of the turbulence are explored for the high Reynolds number case. The conclusion is reached that the turbulent spectrum splits into two parts: an essentially two dimensional spectrum with both the velocity field and magnetic fluctuations perpendicular to the dc magnetic field, and a generally weaker and more nearly isotropic spectrum of Alfven waves. A minimal characterization of the spectral density tensors is given. Similarities to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrotor Tokamak are remarked upon, as are certain implications for the Belcher and Davis measurements of magnetohydrodynamic turbulence in the solar wind.

Turbulence suppression in free shear flows by controlled excitation.

Abstract: Turbulence suppression in the near field of a free shear flow under controlled excitation is investigated in four circular jets, a plane jet, and a plane mixing layer The suppression is a consequence of an excitation-induced modification of the shear layer structure and occurs at the excitation frequency corresponding to the maximally unstable disturbance frequency of the initial free shear layer The most pronounced suppression occurs when the shear layer is excited at a frequency 40% higher than the natural roll-up frequency Excitation at a Strouhal number of about 0017 produces a rapid roll-up and early breakdown of the shear layer, and thus inhibits the formation of the energetic large-scale vortices which otherwise survive farther downstream, grow to larger sizes, and undergo successive pairings in the corresponding unexcited flow

The structure of a separating turbulent boundary layer. Part 1. Mean flow and Reynolds stresses

Abstract: The problem of turbulent-boundary-layer separation due to an adverse pressure gradient is an old but still important problem in many fluid flow devices. Until recent years little quantitative experimental information was available on the flow structure downstream of separation because of the lack of proper instrumentation. The directionally sensitive laser anemometer provides the ability to measure the instantaneous flow direction and magnitude accurately. The experimental results described here are concerned with a nominally two-dimensional, separating turbulent boundary layer for an airfoil-type flow in which the flow was accelerated and then decelerated until separation. Upstream of separation single and cross-wire hot-wire anemometer measurements are also presented. Measurements in the separated zone with a directionally sensitive laser-anemometer system were obtained for U, V , $\overline{u^2}, \overline{v^2}, - \overline{uv}$ , the fraction of time that the flow moves downstream, and the fraction of time that the flow moves away from the wall. In addition to confirming the earlier conclusions of Simpson, Strickland & Barr (1977) regarding a separating airfoil-type turbulent boundary layer, much new information about the separated region has been gathered. (1) The backflow mean velocity profile scales on the maximum negative mean velocity U N and its distance from the wall N . A U + vs. y + law-of-the-wall velocity profile is not consistent with this result. (2) The turbulent velocities are comparable with the mean velocity in the backflow, although low turbulent shearing stresses are present. (3) Mixing length and eddy viscosity models are physically meaningless in the backflow and have reduced values in the outer region of the separated flow. Downstream of fully developed separation, the mean backflow appears to be divided into three layers: a viscous layer nearest the wall that is dominated by the turbulent flow unsteadiness but with little Reynolds shearing stress effects; a rather flat intermediate layer that seems to act as an overlap region between the viscous wall and outer regions; and the outer backflow region that is really part of the large-scaled outer region flow. The Reynolds shearing stress must be modelled by relating it to the turbulence structure and not to local mean velocity gradients. The mean velocities in the backflow are the results of time averaging the large turbulent fluctuations and are not related to the source of the turbulence.

Turbulence Production in Premixed Turbulent Flames

Abstract: —A second order closure theory developed earlier is used to study the processes influencing the turbulent velocity field in a premixed turbulent flame with degrees of heat release of practical interest. The flow field is chosen so that the time-averaged flame structure is one-dimensional and statistically stationary. Earlier work suggests that in the absence of turbulence production due to Reynolds stresses as is the case in a flame orthogonal to the oncoming reactants, the case we consider, dilatation resulting from heat release reduces the level of turbulence. In contrast it is shown here that with sufficient heat release turbulence increases on passage through the flame because of a buoyancy production mechanism arising from the self-induced, mean pressure gradient. This mechanism overwhelms the effects of dilatation at temperature ratios characteristic of combustion. The same buoyancy mechanism also causes counter-gradient diffusion as predicted in an earlier paper and as observed in recent e...

A Study of Barotropic Model Flows: Intermittency, Waves and Predictability

Abstract: The regime flows corresponding to the barotropic nondivergent equation with forcing, drag and subgrid-scale dissipation are studied using spectral model on the plane and on the sphere. The flow regimes obtained exhibit clear evidence of the existence of an enstrophy-cascading inertial range, together with a reverse energy cascade toward small wavenumbers. It is shown, however, that the enstrophy cascade is not associated with the k−3 spectral slope expected from the Kolmogorov-Kraichnan theory of two-dimensional turbulence; the slopes obtained are significantly steeper. This apparent paradox is tentatively resolved by a phenomenological theory of space-time intermittency in two dimensions; it is further shown that such intermittency associated with steeper spectra also restores locality of the nonlinear transfers in wavenumber space. In contrast to the well-known nonlocality typical of two-dimensional non-intermittent turbulent flows. The effect of differential rotation in connection with Rossby ...

On the growth of turbulent regions in laminar boundary layers

Abstract: Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a turbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

Air flow over a two-dimensional hill: studies of velocity speed-up, roughness effects and turbulence

Abstract: Wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough, bell-shaped, two-dimensional hill, with height h and maximum slope 0.26, placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo = 0.02 h. Close agreement is found between the mean velocity and predictions obtained from Taylor's (1977) computational model and Jackson and Hunt's (1975) analytical linearized model, for locations at or upwind of the hill top but not in the wake. Examination of the models shows that the shear stresses are only important in an inner region close to the hill surface, so that, as suggested by Jackson and Hunt (1975), the perturbation to the mean flow outside this region is essentially inviscid. the theory shows that over very rough surfaces, such as wooded or urban terrain, the height of this inner region can be of the same order as the height of the roughness elements (so that in our experiments no measurements could be made in this region). In a second experiment flow over a smooth hill on a rough surface was studied. the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness (in this case rough to smooth). In the lee of a hill, however, the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate. Finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface. Calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field.

Instability and transition in rotating disk flow

Abstract: The stability of three dimensional rotating disk flow and the effects of Coriolis forces and streamline curvature were investigated It was shown that this analysis gives better growth rates than Orr-Sommerfeld equation Results support the numerical prediction that the number of stationary vortices varies directly with the Reynolds number