Showing papers on "Reynolds number published in 1983"
TL;DR: In this paper, the velocity distribution and reattachment length of a single backward-facing step mounted in a two-dimensional channel were measured using laser-Doppler measurements.
Abstract: Laser-Doppler measurements of velocity distribution and reattachment length are reported downstream of a single backward-facing step mounted in a two-dimensional channel. Results are presented for laminar, transitional and turbulent flow of air in a Reynolds-number range of 70 < Re < 8000. The experimental results show that the various flow regimes are characterized by typical variations of the separation length with Reynolds number. The reported laser-Doppler measurements do not only yield the expected primary zone of recirculating flow attached to the backward-facing step but also show additional regions of flow separation downstream of the step and on both sides of the channel test section. These additional separation regions have not been previously reported in the literature.Although the high aspect ratio of the test section (1:36) ensured that the oncoming flow was fully developed and two-dimensional, the experiments showed that the flow downstream of the step only remained two-dimensional at low and high Reynolds numbers.The present study also included numerical predictions of backward-facing step flow. The two-dimensional steady differential equations for conservation of mass and momentum were solved. Results are reported and are compared with experiments for those Reynolds numbers for which the flow maintained its two-dimensionality in the experiments. Under these circumstances, good agreement between experimental and numerical results is obtained.
1,637 citations
TL;DR: In this paper, the development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence, due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers.
Abstract: The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.
863 citations
TL;DR: In this article, a definition of coherent structures in turbulent shear flows is proposed and its implications discussed, and the characteristic coherent structure properties are identified and the analytical and experimental constraints in the eduction of coherent structure are examined.
Abstract: The nature and significance of large‐scale coherent structures in turblent shear flows are addressed. A definition for the coherent structure is proposed and its implications discussed. The characteristic coherent structure properties are identified and the analytical and experimental constraints in the eduction of coherent structures are examined. Following a few comments on coherent motions in wall layers, the accumulated knowledge from a number of recent and ongoing coherent structure investigations in excited and unexcited free shear flows in the author’s laboratory is reviewed. Also briefly addressed are effects of initial conditions, the role of coherent structures in jet noise production and broadband noise amplification, the feedback effect of coherent structures, the use of the Taylor hypothesis in coherent structure description, negative production, turbulence suppression via excitation, validity of the Reynolds number similarity hypothesis, etc. From the detailed quantitative results, a picture of the state of the art in coherent structure studies emerges. While coherent structures are highly interesting characteristic features of (perhaps all) turbulent shear flows, it is argued that their dynamical significance has been overemphasized. These are predominant only in their early stages of formation following instability, or in resonant situations and excited flows, or in regions adjacent to a wall of a turbulent boundary layer. The coherent Reynolds stress, vorticity, and production are comparable to (and not an order of magnitude larger than) the time‐average Reynolds stress, vorticity, and production, respectively, in fully developed states of turbulent shear flows, where incoherent turbulence is also important and cannot be ignored. The concept and importance of coherent structures are here to stay; understanding and modeling of turbulent shear flows will be incomplete without them; but they are not all that matter in turbulent shear flows.
772 citations
688 citations
TL;DR: In this article, the dynamics of inviscid and viscous Taylor-Green (TG) vortex flows are investigated by both direct spectral numerical solution of the Navier-Stokes equations and by power-series analysis in time.
Abstract: The dynamics of both the inviscid and viscous Taylor–Green (TG) three-dimensional vortex flows are investigated. This flow is perhaps the simplest system in which one can study the generation of small scales by three-dimensional vortex stretching and the resulting turbulence. The problem is studied by both direct spectral numerical solution of the Navier–Stokes equations (with up to 256 3 modes) and by power-series analysis in time. The inviscid dynamics are strongly influenced by symmetries which confine the flow to an impermeable box with stress-free boundaries. There is an early stage during which the flow is strongly anisotropic with well-organized (laminar) small-scale excitation in the form of vortex sheets located near the walls of this box. The flow is smooth but has complex-space singularities within a distance $\hat{\delta}(t)$ of real (physical) space which give rise to an exponential tail in the energy spectrum. It is found that $\hat{\delta}(t)$ decreases exponentially in time to the limit of our resolution. Indirect evidence is presented that more violent vortex stretching takes place at later times, possibly leading to a real singularity ( $\hat{\delta}(t) = 0$ ) at a finite time. These direct integration results are consistent with new temporal power-series results that extend the Morf, Orszag & Frisch (1980) analysis from order t 44 to order t 80 . Still, convincing evidence for or against the existence of a real singularity will require even more sophisticated analysis. The viscous dynamics (decay) have been studied for Reynolds numbers R (based on an integral scale) up to 3000 and beyond the time t max at which the maximum energy dissipation is achieved. Early-time, high- R dynamics are essentially inviscid and laminar. The inviscidly formed vortex sheets are observed to roll up and are then subject to instabilities accompanied by reconnection processes which make the flow increasingly chaotic (turbulent) with extended high-vorticity patches appearing away from the impermeable walls. Near t max the small scales of the flow are nearly isotropic provided that R [gsim ] 1000. Various features characteristic of fully developed turbulence are observed near t max when R = 3000 and R λ = 110:
a k − n inertial range in the energy spectrum is obtained with n ≈ 1.6–2.2 (in contrast with a much steeper spectrum at earlier times); th energy dissipation has considerable spatial intermittency; its spectrum has a k −1+μ inertial range with the codimension μ ≈ 0.3−0.7. Skewness and flatness results are also presented.
684 citations
TL;DR: In this article, the steady and unsteady forces were measured by means of a piezobalance, which features a high natural frequency, low interferences and a large dynamic range.
Abstract: Force measurements were conducted in a pressurized wind tunnel from subcritical up to transcritical Reynolds numbers 2.3 × 104 [les ] Re [les ] 7.1 × 106 without changing the experimental arrangement. The steady and unsteady forces were measured by means of a piezobalance, which features a high natural frequency, low interferences and a large dynamic range. In the critical Reynolds-number range, two discontinuous transitions were observed, which can be interpreted as bifurcations at two critical Reynolds numbers. In both cases, these transitions are accompanied by critical fluctuations, symmetry breaking (the occurrence of a steady lift) and hysteresis. In addition, both transitions were coupled with a drop of the CD value and a jump of the Strouhal number. Similar phenomena were observed in the upper transitional region between the super- and the transcritical Reynolds-number ranges. The transcritical range begins at about Re ≈ 5 × 106, where a narrow-band spectrum is formed with Sr(Re = 7.1 × 106) = 0.29.
555 citations
TL;DR: In this paper, an efficient and reliable numerical technique of high-order accuracy is presented for solving problems of steady viscous incompressible flow in the plane, and is used to obtain accurate solutions for the driven cavity.
492 citations
TL;DR: In this article, the bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 103 < U∞/ν < 104, and the non-dimensional frequency was constant independent of Reynolds number.
Abstract: The bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 103 < U∞/ν < 104. When scaled with the variables appropriate for the wall region, the non-dimensional frequency was constant independent of Reynolds number. A strong effect of the sensor size was noted on the measured bursting frequency. Only sensors having a spatial scale less than twenty viscous lengthscales were free from spatial-averaging effects and yielded consistent results. The spatial-resolution problem was apparently the reason for erroneous results reported in the past.
286 citations
TL;DR: Results indicate that flow disturbances of discrete oscillation frequency may be more valuable than turbulence as an indicator of early stages of stenosis development.
273 citations
TL;DR: In this article, the largest Lyapunov exponent and metric entropy of the Couette-Taylor flow data were used to show that motion is restricted to an attractor of dimension 5 for Reynolds numbers.
Abstract: Evidence is presented for low-dimensional strange attractors in Couette-Taylor flow data. Computations of the largest Lyapunov exponent and metric entropy show that the system displays sensitive dependence on initial conditions. Although the phase space is very high dimensional, analysis of experimental data shows that motion is restricted to an attractor of dimension 5 for Reynolds numbers up to 30% above the onset of chaos. The Lyapunov exponent, entropy, and dimension all generally increase with Reynolds number.
263 citations
TL;DR: In this article, a stochastic model of particle dispersion by turbulence, proposed by Gosman and Ioannides (1981), is evaluated, which employs a k-epsilon model to estimate turbulence properties.
Abstract: A stochastic model of particle dispersion by turbulence, proposed by Gosman and Ioannides (1981), is evaluated. The method employs a k-epsilon model to estimate turbulence properties. Dispersion is determined by computing particle motion, with random sampling to obtain instantaneous flow properties for a statistically significant number of particle trajectories. The stochastic model yields good results particularly when eddy lifetimes are evaluated. The method allows the effects of large relative velocities between the particles and the flow, drag properties at Reynolds numbers greater than the Stokes flow regime, and the variations of local turbulence properties to be readily handled, at least for boundary layer flows.
TL;DR: The location of low wall shear stresses, directionally varying stresses, and longer residence times for fluid elements appears to coincide with the localization of early atheromatous plaques in human carotid specimens.
Abstract: Pulsatile flow in an in vitro model of the human carotid bifurcation was studied by flow visualization using hydrogen bubble techniques. A glass model was constructed after determining an average geometry from 57 biplanar angiograms of 22 subjects ranging from 34 to 77 years of age. The flow pulse used was a half-sine wave superimposed upon a mean flow. Maximum and minimum values of the instantaneous Reynolds number were 1200 and 400, respectively, based upon conditions in the common carotid model artery; the frequency parameter was 6.0. The division of flow into the internal external branches was 70:30. Visualization by hydrogen bubbles demonstrated significant deviations from steady flow behavior. Flow separated in the carotid sinus over the entire cycle, but the location and extent of separation varied strongly. The direction of flow near the walls of the model changed sharply during the cycle except for the region near the apex of the bifurcation where the orientation of streaklines was more nearly unidirectional at all times. Bubbles entering the separated flow region tended to remain entrapped there for several cycles. Rapid dispersion of bubbles occurred in the internal branch near the end of systole, suggesting the presence of flow disorder. The location of low wall shear stresses, directionally varying stresses, and longer residence times for fluid elements appears to coincide with the localization of early atheromatous plaques in human carotid specimens.
01 Nov 1983
TL;DR: In this paper, an analysis of the transient response of a packed bed thermal storage unit is presented, in two-spatial dimensions and considering the influence of both axial and radial thermal dispersion for an arbitrary time and radial variation in the inlet fluid temperature.
Abstract: An analysis, with experimental results, for the transient response of a packed bed thermal storage unit is presented. The analysis is in two-spatial dimensions and considers the influence of both axial and radial thermal dispersion for an arbitrary time and radial variation in the inlet fluid temperature. Both charging and recovery modes are included. Spatial variations in void fraction are found to have significant effect on the dynamic response of both fluid and solid temperatures. The influence of radial velocity variations, wall heat capacity and wall thermal losses are considered. The analysis is valid for fluids of various Prandtl numbers. Experimental measurements of temperature distributions in a randomly packed bed of uniform spheres, with air as the working fluid, compare favorably with the analytical results over a broad range of Reynolds numbers. Results are presented for full-sized rock beds which indicate pronounced effects of void distribution in such systems.
TL;DR: In this article, collision efficiencies for gravitation and interception of spherical particles on cylindrical and spherical collectors were calculated using numerical solutions to the complete Navier-Stokes equations available in the literature.
TL;DR: In this paper, the effects of imperfect spatial resolution on hot-film and hot-wire measurements of wall-bounded turbulent shear flows were studied, and the effect of the number of events detected with the variable-interval time-averaging (VITA) technique was found to depend strongly on the sensor length.
Abstract: The effects of imperfect spatial resolution on hot-film and hot-wire measurements of wall-bounded turbulent shear flows were studied. Two hot-film probes of different length were used for measurements of fully developed turbulent channel flow in a water tunnel. In the near-wall region significant effects of spanwise spatial averaging due to finite probe size were found for a probe 32 viscous units long. The maximum turbulence intensity attained a 10% lower value than that for a probe about half as long, and the zero-crossing of the skewness factor was shifted away from the wall. This could be attributed to spatial averaging of narrow low-speed regions. Results for different Reynolds numbers, but with the same sensor length in viscous units, showed that Reynolds-number effects are small, and that much of the reported discrepancies for turbulence measurements in the near-wall region can be ascribed to effects of imperfect spatial resolution. Also the number of events detected with the variable-interval time-averaging (VITA) technique was found to depend strongly on the sensor length, especially for events with short duration.
TL;DR: In view of the relatively high noise levels inherent in noninvasive Doppler ultrasound systems employed clinically, it seems unlikely that detection of stenosis of less than 50% area reduction is feasible unless the Reynolds numbers exceed 1000 or unless pulsatility introduces new unsteady flow features beyond those studied here.
TL;DR: In this paper, a finite-length solution procedure is developed for perturbation equations which are based on Hirs' (1973) turbulent lubrication model and apply to small motions about a centered position.
Abstract: A finite-length solution procedure is developed for perturbation equations which are based on Hirs' (1973) turbulent lubrication model. The equations apply to small motions about a centered position and include the influence of swirl and changes in Reynolds number due to perturbations in clearances. Numerical results are presented for a range of L/D ratios, with and without swirl. For zero swirl, changes in the L/D ratios show results which are similar to those obtained by Black and Jenssen (1970), but when L/D = 1, differences of about 15 percent appear. The results including swirl give physically insupportable results at small L/D ratios, such as a negative cross-coupled stiffness coefficient at L/D = 0.2. This result demonstrates that the complete Hirs turbulence model is not suitable for short seals with significant swirling flow.
TL;DR: The problem of peristaltic transport of a fluid of variable viscosity in a non-uniform tube and channel has been investigated under zero Reynolds number, and long wavelength approximation and the pressure rise is found to be much smaller than the corresponding value in the case of uniform geometry.
TL;DR: In this paper, experimental data on instantaneous velocity and wall shear stress were obtained at a mean Reynolds number of 50000 in a fully developed turbulent pipe flow in which the volumetric flow rate was varied sinusoidally with time around the mean.
Abstract: The present paper is the first part of a two-part report on a detailed investigation of periodic turbulent pipe flow. In this investigation, experimental data on instantaneous velocity and wall shear stress were obtained at a mean Reynolds number of 50000 in a fully developed turbulent pipe flow in which the volumetric flow rate was varied sinusoidally with time around the mean. Two oscillation frequencies at significant levels of flow modulation were studied in detail. The higher of these frequencies was of the order of the turbulent bursting frequency in the flow, and the other can be regarded as an intermediate frequency at which the flow still departed significantly from quasi-steady behaviour. While a few similar experiments have been reported in the recent literature, the present study stands out from the others in respect of the flow regimes investigated, the magnitude of flow modulation, the detailed nature of the measurements and most importantly the identification of a relevant parameter to characterize unsteady shear flows. The present paper contains the main experimental results and comparisons of these results with the results of a numerical calculation procedure which employs a well-known quasi-steady turbulence closure model. The experimental data are used to study the manner in which the time-mean, the ensemble-averaged and the random flow properties are influenced by flow oscillation at moderate to high frequencies. In addition, the data are also used to bring out the capability and limitations of quasi-steady turbulence modelling in the prediction of unsteady shear flows. A further and more detailed analysis of the experimental data, results of some additional experiments and a discussion on the characterization of turbulent shear flows are provided in Part 2 (Ramaprian & Tu 1983).
TL;DR: In this paper, experimentale des proprietes moyennes de l'ecoulement dans des couches limites turbulentes avec des nombres de Reynolds inferieurs a 3000.
Abstract: Etude experimentale des proprietes moyennes de l'ecoulement dans des couches limites turbulentes avec des nombres de Reynolds inferieurs a 3000. Determination des distributions de vitesse et du frottement superficiel
TL;DR: In this article, an investigation of heat transfer from round jets, impinging normally on three instrumented flat plates, for various nozzle-to-target plate distances, with Reynolds numbers ranging from 14 000 to 67 000, and nozzle diameters from 3.18 to 12.7 mm, has been carried out.
TL;DR: The characteristics of the flow around a smooth circular cylinder in the critical Reynolds-number range have been investigated experimentally on the basis of instantaneous mean-pressure-distribution measurements on the cylinder and of hot-wire velocity-fluctuation measurements in the cylinder wake as discussed by the authors.
Abstract: The characteristics of the flow around a smooth circular cylinder in the critical Reynolds-number range have been investigated experimentally on the basis of instantaneous mean-pressure-distribution measurements on the cylinder and of hot-wire velocity-fluctuation measurements in the cylinder wake. Two subregions have been identified in the critical or lower transition; the first characterized by symmetric pressure distributions, intense vortex shedding, and gradual and significant variations in characteristic parameters as the Reynolds number increases, and the second by intense flow oscillations associated with formation and bursting of laminar-separation bubbles on one or both sides of the cylinder, without preference for side. The spectra of the velocity fluctuations in the second, unsteady subrange appear in general with broad band peaks. The spectral peak in the twin-bubble regime which follows this second subrange is sharp but has little energy compared to subcritical peaks.
TL;DR: In this paper, a smooth rectangular channel is studied by dividing the channel into four regions: the inner region of the bed, the outer region of both the bed and the side wall, and the outer regions of both bed and side wall.
Abstract: Velocity distribution in a smooth rectangular channel is studied by dividing the channel into four regions. Region 1 comprises of the inner region of the bed and the outer region of the side wall. Region 2 belongs to the inner regions of both the bed and the side wall. Region 3 consists of the inner region of the side wall and the outer region of the bed. Region 4 forms the outer regions of both the bed and the side wall. The aspect ratio is varied from 2.0 to 8.0 and the Froude number from 0.2 to 0.7. Experiments included the aspect ratio of 1.0 also for Froude numbers of 0.2 and 0.3. The range of Reynolds number defined in terms of the hydraulic radius and the cross sectional mean velocity lies between 10\u4 and 7 x 10\u4.
TL;DR: In this paper, the problem of laminar mixed convection from a horizontal isothermal cylinder is considered and the free stream direction is assumed to be horizontal and perpendicular to the cylinder axis.
01 Jan 1983
TL;DR: In this paper, the Lissaman 7769 airfoil was investigated for the effect of free stream disturbances on the lift and drag performance of the airframe and it was found that the problems associated with obtaining accurate wind tunnel data at low chord Reynolds numbers (i.e., below 200,000) are caused by the extreme sensitivity of the boundary layers to the free stream disturbance environment.
Abstract: The results of an investigation of the influence of free stream disturbances on the lift and drag performance of the Lissaman 7769 airfoil are presented. The wind tunnel disturbance environment is described using hot-wire anemometer and sound pressure level measurements. The disturbance level is increased by the addition of a 'turbulence screen' upstream of the test section and/or the addition of a flow restrictor downstream of the test section. For the Lissaman airfoil it was found that the problems associated with obtaining accurate wind tunnel data at low chord Reynolds numbers (i.e., below 200,000) are com- pounded by the extreme sensitivity of the boundary layers to the free stream disturbance environment. The effect of free stream disturbances varies with magnitude, frequency content, and source of the disturbance.
TL;DR: In this article, a boundary element method for the solution of non-zero Reynolds number incompressible flows in which the non-linear terms are lumped together to form a forcing function is presented.
Abstract: A formulation of the boundary element method for the solution of non-zero Reynolds number incompressible flows in which the non-linear terms are lumped together to form a forcing function is presented. Solutions can be obtained at low to moderate Reynolds numbers. The method was tested using the flow of a fluid in a two-dimensional converging channel (Hamel flow) for which an exact solution is available. An axisymmetric formulation is demonstrated by examining the drag experienced by a sphere held stationary in uniform flow. Performance of the method was satisfactory. New results for an axisymmetric free jet at zero Reynolds number obtained using the boundary element method are also included. The method is ideal for this type of free-surface problem.
TL;DR: In this article, a flow through a straight pipe is passed through a coiled section, and two stabilizing effects come into play: the flow that is turbulent in the straight pipe becomes completely laminar in the coiled part, and the stabilization effect persists to a certain degree even after the flow downstream of the coil has been allowed to develop in a long straight section.
Abstract: When a flow through a straight pipe is passed through a coiled section, two stabilizing effects come into play. First, in a certain Reynolds number range, the flow that is turbulent in the straight pipe becomes completely laminar in the coiled section. Second, the stabilization effect of the coil persists to a certain degree even after the flow downstream of the coil has been allowed to develop in a long straight section. In this paper, we report briefly on aspects related to these two effects.
TL;DR: In this paper, the Batchelor profile of infinite-disk theory is recognized as the limiting flow in water between finite rotating disks, with and without throughflow, in four cases.
Abstract: Laser-Doppler velocity measurements were obtained in water between finite rotating disks, with and without throughflow, in four cases: ω1 = ω2 = 0; ω2/ω1 = −1; ω2/ω1 = 0; ω2/ω1 = 1. The equilibrium flows are unique, and at mid-radius they show a high degree of independence from boundary conditions in r. With one disk rotating and the other stationary, this mid-radius ‘limiting flow’ is recognized as the Batchelor profile of infinite-disk theory. Other profiles, predicted by this theory to coexist with the Batchelor profile, were neither observed experimentally nor were they calculated numerically by the finite-disk solutions, obtained here via a Galerkin, B-spline formulation. Agreement on velocity between numerical results and experimental data is good at large values of the ratio RQ/Re, where RQ = Q/2πνs is the throughflow Reynolds number and Re = R22ω/ν is the rotational Reynolds number.
TL;DR: In this article, the Lissaman 7769 airfoil was investigated for the effect of free stream disturbances on the lift and drag performance of the airframe, and it was found that the problems associated with obtaining accurate wind tunnel data at low chord Reynolds numbers (i.e., below 200,000) are compounded by the extreme sensitivity of the boundary layers to free stream disturbance environment.
Abstract: The results of an investigation of the influence of free stream disturbances on the lift and drag performance of the Lissaman 7769 airfoil are presented. The wind tunnel disturbance environment is described using hot-wire anemometer and sound pressure level measurements. The disturbance level is increased by the addition of a 'turbulence screen' upstream of the test section and/or the addition of a flow restrictor downstream of the test section. For the Lissaman airfoil it was found that the problems associated with obtaining accurate wind tunnel data at low chord Reynolds numbers (i.e., below 200,000) are compounded by the extreme sensitivity of the boundary layers to the free stream disturbance environment. The effect of free stream disturbances varies with magnitude, frequency content, and source of the disturbance.
TL;DR: In this article, numerical solutions for steady viscous flow past a rotating circular cylinder were obtained for Reynolds numbers of 5 and 20 and ratios of the speed of the surface of the cylinder to the fluid speed at infinity from 0 to 0.5.