Showing papers on "Flow separation published in 1970"

A numerical study of three-dimensional turbulent channel flow at large Reynolds numbers

Abstract: The three-dimensional, primitive equations of motion have been integrated numerically in time for the case of turbulent, plane Poiseuille flow at very large Reynolds numbers. A total of 6720 uniform grid intervals were used, with sub-grid scale effects simulated with eddy coefficients proportional to the local velocity deformation. The agreement of calculated statistics against those measured by Laufer ranges from good to marginal. The eddy shapes are examined, and only the u-component, longitudinal eddies are found to be elongated in the downstream direction. However, the lateral v eddies have distinct downstream tilts. The turbulence energy balance is examined, including the separate effects of vertical diffusion of pressure and local kinetic energy.It is concluded that the numerical approach to the problem of turbulence at large Reynolds numbers is already profitable, with increased accuracy to be expected with modest increase of numerical resolution.

Large-scale motion in the intermittent region of a turbulent boundary layer

Abstract: The outer intermittent region of a fully developed turbulent boundary layer with zero pressure gradient was extensively explored in the hope of shedding some light on the shape and motion of the interface separating the turbulent and non-turbulent regions as well as on the nature of the related large-scale eddies within the turbulent regime. Novel measuring techniques were devised, such as conditional sampling and conditional averaging, and others were turned to new uses, such as reorganizing in map form the space-time auto- and cross-correlation data involving both the U and V velocity components as well as I, the intermittency function. On the basis of the new experimental results, a conceptual model for the development of the interface and for the entrainment of new fluid is proposed.

Turbulent boundary-layer wall-pressure fluctuations on smooth and rough walls

Abstract: Turbulent boundary-layer wall-pressure measurements were made with ‘pinhole’ microphones three times smaller (relative to a boundary-layer displacement thickness) than microphones used in earlier work. The improved high-frequency resolution permitted examination of the influence of high-frequency eddies on smooth-wall pressure statistics. It was found that the space-time decay rate is considerably higher than previously reported. Measurements of cross-spectral density made with 5 Hz bandwidth filters disclosed low phase speeds at low frequency and small separation. Measurements were repeated on rough walls and parallels were drawn from knowledge of a smooth-wall boundary-layer structure to propose a structure for a rough-wall boundary layer. The effect of independently varying roughness height and separation on the large and small-scale turbulence structure was deduced from the measurements. It was found that roughness separation affected the very large-scale structure, whereas the roughness height influenced the medium and very small-scale turbulence.

Entrainment and the structure of turbulent flow

Abstract: Although the entrainment of non-turbulent fluid into a turbulent flow occurs across sharply defined boundaries, its rate is not determined solely by the turbulent motion adjacent to the interface but depends on overall properties of the flow, in particular, on those that control the energy balance. In the first place, attention is directed to the many observations which show that the motion in many turbulent shear flows has a structure closely resembling that produced by a rapid, finite, plane shearing of initially isotropic turbulence. The basic reasons for the similarity are the stability and permanence of turbulent eddies and the finite distortions undergone by fluid parcels in free turbulent flows. Next, the existence of eddy similarity and the condition of overall balance of energy are used to account for the variation of entrainment rates within groups of broadly similar flows, in particular mixing layers between streams of different velocities and wall jets on curved surfaces. For some flows which satisfy the ordinary conditions for self-preserving development, no entrainment rate is consistent with the energy balance and self-preserving development is not possible. Examples are the axisymmetric, small-deficit wake and the distorted wake. Finally, the implications of an entrainment rate controlled by the general motion are discussed. It is concluded that the relatively rapid entrainment in a plane wake depends on an active instability of the interface, not present in a constant-pressure boundary layer whose slow rate of entrainment is from ‘passive’ distortion of the bounding surface by eddies of the main turbulent motion. Available observations tend to support this conclusion.

The turbulent boundary layer over a wall with progressive surface waves

Abstract: Turbulent boundary layer interaction with wavy wall in wind tunnel, discussing wall pressure drag and surface waves interaction

Turbulent flow drag reduction and degradation with dilute polymer solutions

Abstract: Experimental studies of drag reduction and polymer degradation in turbulent pipe flow with dilute water solutions of unfractionated polyethylene oxide are described. Drag reduction results indicate that the magnitude of the reduction cannot be correlated on the basis of weight average molecular weight, rather the phenomenon depends strongly on the concentration of the highest molecular weight species present in the molecular weight distribution. Polymer degradation in turbulent flow is found to be severe for high molecular weight polymers causing appreciable changes in drag reduction and molecular weight with the duration of flow. Data indicates that drag reduction exists in the limit of infinite dilution suggesting that the phenomenon is due to the interaction of individual polymer molecules with the surrounding solvent and that the extent of reduction is relatively independent of pipe diameter when a comparison is carried out at equal solvent wall shear stresses. Consideration of the high viscosity obtained with solutions in an irrotational laminar flow field suggests this is due to polymer molecule deformation and that this phenomenon is central to the mechanism of turbulent flow drag reduction.

Theoretical and experimental studies of the shock wave-boundary layer interaction on compression surfaces in hypersonic flow

Abstract: : A detailed description is given of the theoretical and experimental studies of shock wave-laminar boundary layer interaction on curved compression surfaces in hypersonic flow. Integral forms of the boundary layer equations were used for the conservation of mass, streamwise momentum, normal momentum, moment of streamwise momentum and energy to describe the development of attached and separated boundary layers on a curve surface. In the experimental study, skin friction, heat transfer and pressure measurements were made on a series of flat plate-cylindrical arc-wedge compression surfaces. Pitot and cone pressure measurements were made above the surface in the separation and reattachment regions of separated flows to estimate the static pressure difference across the boundary layer. The radius of curvature of the cylindrical arc, the inclination of the wedge, and the unit Reynolds number of the free-stream were varied to examine their effect on the properties of both attached and separated interaction regions over the models.

Some relations between drag and flow pattern of viscous flow past a sphere and a cylinder at low and intermediate Reynolds numbers

Abstract: The results of a numerical evaluation of the Navier-Stokes equations of motion for the case of a viscous fluid streaming past a sphere are presented in terms of the length of the standing eddy behind the sphere and in terms of the angle of flow separation at the sphere. Emphasis was placed on calculating these quantities at Reynolds numbers between 20 and 40 where no reliable theoretical or experimental values are available. In support of these calculations, it is shown that the values for the drag on a sphere previously calculated by us from the Navier-Stokes equations of motion by the same numerical technique as that used for calculating the eddy length and angle of flow separation agree well with our recent, extensive drag measurements for a wide Reynolds number interval. Our results are used to make a comparison between drag and flow field as predicted by analytical solutions and numerical solutions to the Navier-Stokes equations of motion. Some limitations of the analytical solutions to predict correct values for the drag, and to describe the correct nature of the flow field, are pointed out. It is shown further that a plot of [(D/Ds) – 1] versus log NRe, where D is the actual drag on a sphere, Ds is the Stokes drag, and NRe is the Reynolds number, reveals that the variation of the drag on a sphere with Reynolds number follows well defined regimes, which correlate well with the regimes of the flow field around a sphere. A similar relationship between ‘drag-regime’ and flow field pattern is discussed for the case of viscous flow past a cylinder.

Wall-pressure fluctuations beneath turbulent boundary layers on a flat plate and a cylinder

Abstract: Measurements of the turbulent pressure field on the outer surface of a 3 in. diameter cylinder aligned with the flow were made at a point approximately 24 ft. downstream of the origin of the turbulent boundary layer in an air stream of 145 ft./sec. The boundary-layer thickness was 2·78 in. and the Reynolds number based on momentum thickness was 2·62 × 104. The wall-pressure measurements were made with pressure transducers constructed from 0·06 in. diameter lead–zirconate–titanate disks mounted flush with the wall. The measurements including root-mean-square, power spectrum, and correlations of the wall pressure are compared with the existing experimental results for the turbulent pressure field beneath a plane boundary layer. The streamwise convection speed deduced from longitudinal space-time correlation measurements was almost identical to that obtained in the plane boundary layer. The rate of decay of the maxima of the space-time correlation of the pressure produced by the convected eddies was double that in a plane boundary layer. The longitudinal and transverse scales of the pressure correlation were approximately equal (in a plane boundary layer the transverse scale is larger than longitudinal scale) and were one-half or less than the longitudinal scale in the plane boundary layer. It is concluded that the effect of the transverse curvature of the wall is an overall reduction in size of pressure-producing eddies. The reduction in transverse scale of the larger eddies is greater than that of the smaller eddies. In general, the smaller eddies decay more rapidly and produce greater spectral densities at high frequencies owing to the unchanged convection speed.

Two-Dimensional Transonic Wind Tunnel Tests of Three 15-Percent Thick Circulation Control Airfoils

Abstract: : Two relatively thin Circulation Control (CC) elliptic airfoils were tested subsonically to determine their characteristics as proposed helicopter rotor tip sections. These airfoils, employing tangential trailing edge (Coanda) blowing, had shown very promising transonic characteristics in previous tests. It was the purpose of the subonic retests to determine if these thin sections could generate low speed characteristics which would be equally impressive. Due to its more forward slot location, the 15-percent thick pure elliptic section displayed effective subsonic operation at positive angle of attack, reducing drag while producing lift coefficients up to 3.5. The rounded trailing edge configuration, with further aft slot and better Coanda deflection of the jet, generated lift coefficients up to 4.25 (with a preference for negative incidence), but experienced higher drag levels. As a result of the small nose radii and low test Reynolds number, both sections were limited in performance by leading edge separation. At a fixed momentum coefficient, variation in slot height indicated that better performance was obtained for reduced heights. This was due primarily to higher energy levels in the jet sheet, but the lower bound on slot height was limited by boundary layer building in very small nozzles. Comparison of both CC sections to the more conventional NACA 0012 blade section indicated far greater lift capabilities with circulation control. However, due to blowing power requirements, equivalent efficiency was less at positive incidence, than for the conventional section.

Boundary-layer displacement and leading edge bluntness effects on attached and separated laminar boundary layers in a compression corner. I - Theoretical study

Abstract: This paper describes a theoretical analysis of highly cooled attached and separated regions of shock wave-laminar boundary-layer interaction in the presence of strong streamwise pressure gradients generated by boundary-layer displacement effects at the leading edge. This method is an extension of an earlier analysis by Holden1'12 to conditions where the inviscid flow cannot be described by simple isentropic flow relationships, and where the boundarylayer upstream of the main interaction is subjected to a strong pressure gradient. The analysis is compared with measurements described in Part II of the study. For strong leading edge displacement effects (%z, > 1), the analysis predicts that highly cooled boundary layers in hypersonic flow will be supercritical; a supercritical-subcritical jump is therefore required to join the solution to the subcritical viscous layer at separation. An examination of the experimental measurements indicates that the supercritical-subcritical jump does not reflect a sudden and basic change in the flow mechanics of separation, but is an approximation necessary because the conventional boundary-layer equations cannot adequately describe the viscous interaction process leading to separation. For some high Mach number, low Reynolds number conditions, we were unable to obtain a unique solution, without recourse to experimental data, by locating a critical point in the throat region of the flow. As in the separated region, there is serious question whether the conventional boundary-layer equations can be used to adequately describe the mechanism of boundary-layer reattachment in these flows.

Experimental Investigation of a Laminar Two‐Dimensional Plane Wall Jet

Abstract: An experimental investigation was made of a two‐dimensional wall jet at low Reynolds numbers with small natural disturbances in the jet. The main characteristics of the laminar regime are in agreement with theoretical results. The stability of the flow was studied for the Reynolds number range of 270‐770. Longitudinal velocity fluctuations exhibited two regions of greatest amplitude, one region on each side of the maximum velocity peak. Growth of these fluctuations was found to be in agreement with linear stability theory. The disturbance region farthest from the wall appeared to dominate the stability of the velocity profiles. A characteristic frequency of the disturbance varied directly with the local maximum velocity and inversely with the local boundary layer thickness.

An experimental determination of the turbulent Prandtl number in a developing temperature boundary layer

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Measurements in a three-dimensional turbulent boundary layer induced by a swept, forward-facing step

Abstract: An experiment is reported, in which turbulent shear-stresses as well as mean velocities have been measured in a three-dimensional turbulent boundary layer approaching separation. It is shown that even very close to the wall the stress vector does not align itself with the mean velocity gradient vector, as would be required by a scalar ‘eddy viscosity’ or ‘mixing length’ type assumption. The calculation method of Bradshaw (1969) is tested against the data, and found to give good results, except for the prediction of shear-stress vector direction.

FORTRAN program for calculating compressible laminar and turbulent boundary layers in arbitrary pressure gradients

Abstract: FORTRAN program for calculating compressible laminar and turbulent boundary layers in arbitrary pressure gradients

Optimization of airfoils for maximum lift

Abstract: The pressure distribution which provides the maximum lift without separation for a monoelement airfoil in an incompressible flow is determined using existing boundary-layer theory and the calculus of variations. The airfoil profiles corresponding to these pressure distributions are determined using second-order airfoil theory. The results indicate maximum lift coefficients as high as 2.8 for Reynolds numbers between five and ten million, and the corresponding drag coefficients are on the order of 0.01. Compressibility has not been considered directly, however the form of the optimum pressure distributions suggests that the critical Mach numbers should be on the order of 0.35.

Control of Separation of Flow

Abstract: This chapter presents the control of separation flow. The purpose of separation flow control is to raise the efficiency or to improve the performance of vehicles and fluid machineries involving many engineering applications. The chapter presents basic principles for control of separation with pertinent examples. The separation of flow may be controlled in two ways: (1) by prevention or delay of the onset of separation with elimination or decrease of separated regions and (2) by provoking localized separated flow by utilizing the characteristics of separated flow. As the two governing factors of flow separation are adverse pressure gradient and viscosity, the control of separation can be achieved by changing or maintaining the structure of viscous flow so that these two governing factors prevent or delay the separation. There are two methods of control, depending upon whether power is required or not required to achieve this goal.

On the mass transport induced by oscillatory flow in a turbulent boundary layer

Abstract: Oscillatory flow in a turbulent boundary layer is modelled by using a coefficient of eddy viscosity whose value depends upon distance from a fixed boundary. A general oscillatory flow is prescribed beyond the layer, and the model is used to calculate the mass transport velocity induced by this within the layer. The result is investigated numerically for a representative distribution of eddy viscosity and the conclusions interpreted in terms of the mass transport induced by progressive and standing waves. For progressive waves, the limiting value of the mass transport velocity at the outer edge of the layer is the same as for laminar flow. For standing waves, the limiting value is reduced relative to its laminar value but, within the lowermost 25% of the layer, there is a drift which is reversed relative to the limiting value. This is considerably stronger than its counterpart in the laminar case and, in view of the greater thickness of the turbulent layer, it may make a dominant contribution to the net movement of loose bed material by a standing wave system.

Reflexion of an oblique shock wave by a turbulent boundary layer

Abstract: Measurements are presented of the reflected wave field produced by a plane oblique shock wave impinging on a turbulent boundary layer at an initial Mach number of 2·5. The outgoing waves are either a single shock, with the same deflexion as the incident shock, or a shock of approximately 10° deflexion followed by a region of compression in which is embedded an expansion fan having the same turning as the incident shock. The transition between these two types of wave field was not studied, but it is fairly abrupt and appears to be closely linked to the onset of boundary-layer separation. The observed wave systems broadly agree with the suggestions of a number of previous workers, but not with a recent theoretical treatment. Surface-pressure measurements and oil flow photographs are used to determine the onset of separation, and from these it is found that the overall pressure rise associated with incipient separation is rather smaller than previous work would suggest.

Influence of measured freestream disturbances on hypersonic boundary-layer transition

Abstract: Free stream disturbances influence on hypersonic boundary layer transition Reynolds number in heated and unheated flows

On the turbulent flow over a wavy boundary

Abstract: Two hypotheses concerning the turbulent flow over an infinitesimal-amplitude travelling wave are investigated. One hypothesis, originally made by Miles, is that the wave does not affect the turbulence and therefore the turbulent Reynolds stresses are dependent only on height above the mean wave surface. Alternatively, the proposal that turbulent stresses are primarily dependent on height above the instantaneous wave surface is examined. Numerical solutions of the appropriate equations are compared with Stewart's recent experimental results and with the approximate solutions employed by Miles and others. No definite conclusion can be reached from comparison with experimental results since the predicted flows are quite sensitive to details of the mean velocity profile near the wave surface where no data was taken. It is found that the asymptotic results do not apply for the conditions investigated.

Use of Coles' universal wake function for compressible turbulent boundary layers

Abstract: Compressible turbulent boundary layers velocity profiles, Coles universal wake function, least squares methods, wake-wall representation, etc

Transition from the turbulent to the laminar regime for internal convective flow with large property variations.

Abstract: Transition from turbulent to laminar regime as consequence of high heating rates for internal convective flow, noting roles of Nusselt numbers and friction factors

Acceleration and cooling effects in laminar boundary layers - Subsonic, transonic, and supersonic speeds

Abstract: Subsonic, transonic and supersonic laminar boundary layers acceleration and cooling effects, discussing heat transfer and gas enthalpy