Showing papers on "Flow separation published in 1977"

Some experimental studies of vortex rings

Abstract: A series of experiments designed to reveal the properties of high Reynolds number vortex rings, using flow-visualization and laser-Doppler techniques, has uncovered several interesting and unexpected results. Starting at the beginning of the motion, at a nozzle, and proceeding downstream, these include the following. A formation process that is strongly Reynolds number dependent.The amount of vorticity that appears downstream is very close to that predicted by a simple ‘slug’ model. However flow-visualization studies show that such a model is an oversimplification and that an excess of ring vorticity is probably cancelled by the ingestion of vorticity of opposite sign at the nozzle lip.(iii) A new, bimodal form of vortex-core instability has been observed at moderate but not high Reynolds numbers.Azimuthal inhomogeneities in the breaking of these, and the normal instability waves, create an ‘axial’ flow along the vortex core in the turbulent ring. This axial flow takes the form of a propagating wave that has many characteristics of a solitary wave. It is hypothesized that this axial flow prevents further ring instability.The long-term behaviour of the turbulent ring is marked by dramatic changes in its growth rate, which are probably related to changes in the ‘organization’ of the vortex core. The descriptive turbulent-ring model developed in Maxworthy (1974) is substantially confirmed by these experiments and by observation of ring propagation through a stratified ambient fluid.

Coherent motions in the outer region of turbulent boundary layers

Abstract: The technique of simultaneous hot‐wire anemometry and flow visualization has been used to determine the average characteristics of two important scales of motion in the outer region of turbulent boundary layers: large scale motions (average length 1.6δ), and ’’typical eddy’’ motions (average streamwise length approximately 200 ν/u τ). Results showed that the Reynolds number dependent ’’typical eddies’’ produced most of the Reynolds stress in the outer half of the layer at R ϑ≈1200, and that they are formed on the upstream side of large scale motions at all Reynolds numbers investigated. This phase relationship explains the scaling of the frequency of occurrence of outer layer bursts (which are identified with ’’typical eddies’’) on the free stream velocity and overall boundary layer thickness, although it is found that the lengths of the ’’typical eddies’’ scale on inner layer variables. In the log region, roughly one‐half of the large scale motions sampled had zone‐averaged streamwise velocity defects. Ensemble averaged results showed that they were associated with significant Reynolds stress contributions. A structural model showing the phase relationship of ’’typical eddies’’ and large scale motions is presented.

Analysis of the development of dynamic stall based on oscillating airfoil experiments

Abstract: The effects of dynamic stall on airfoils oscillating in pitch were investigated by experimentally determining the viscous and inviscid characteristics of the airflow on the NACA 0012 airfoil and on several leading-edge modifications. The test parameters included a wide range of frequencies, Reynolds numbers, and amplitudes-of-oscillation. Three distinct types of separation development were observed within the boundary layer, each leading to classical dynamic stall. The NACA 0012 airfoil is shown to stall by the mechanism of abrupt turbulent leading-edge separation. A detailed step-by-step analysis of the events leading to dynamic stall, and of the results of the stall process, is presented for each of these three types of stall. Techniques for flow analysis in the dynamic stall environment are discussed. A method is presented that reduces most of the oscillating airfoil normal force and pitching-moment data to a single curve, independent of frequency or Reynolds number.

Experimental Investigation of Oscillations in Flows Over Shallow Cavities

Abstract: Wind tunnel experiments were carried out with three axisymmetric models to study laminar axisymmetric flows over shallow cavities at low subsonic speeds. The first model had a hemispherical nose, the second an ogive-shape nose, and the third an elliptic nose. Constant-temperature hot-wire anemometry was used in measuring both mean and fluctuating quantities. Flow around the cavity was visualized by heating the first model. To study the effect of mass injection on cavity oscillations, air was injected circumferentially all along the base of the cavity. Major results are that the cavity depth has little effect on oscillations in shallow cavities except when the depth is of the order of the thickness of the cavity shear flow, that downstream corner is the key factor in inducing self-sustained oscillations in the cavity shear layer, and that the presence of a back face results in an integral relation between wave length of the propagating disturbances and the cavity width in each mode of cavity operation.

Laminar flow in a square duct of strong curvature

Abstract: Calculated values of the three velocity components and measured values of the longitudinal component are reported for the flow of water in a 90° bend of 40 x 40mm cross-section; the bend had a mean radius of 92mm and was located downstream of a 1[sdot ]8m and upstream of a 1[sdot ]2m straight section. The experiments were carried out at a Reynolds number, based on the hydraulic diameter and bulk velocity, of 790 (corresponding to a Dean number of 368). Flow visualization was used to identify qualitatively the characteristics of the flow and laser-Doppler anemometry to quantify the velocity field. The results confirm and quantify that the location of maximum velocity moves from the centre of the duct towards the outer wall and, in the 90° plane, is located around 85% of the duct width from the inner wall. Secondary velocities up to 65% of the bulk longitudinal velocity were calculated and small regions of recirculation, close to the outer corners of the duct and in the upstream region, were also observed.The calculated results were obtained by solving the Navier–Stokes equations in cylindrical co-ordinates. They are shown to exhibit the same trends as the experiments and to be in reasonable quantitative agreement even though the number of node points used to discretize the flow for the finite-difference solution of the differential equations was limited by available computer time and storage. The region of recirculation observed experimentally is confirmed by the calculations. The magnitude of the various terms in the equations is examined to determine the extent to which the details of the flow can be represented by reduced forms of the Navier–Stokes equations. The implications of the use of so-called ‘partially parabolic’ equations and of potential- and rotational-flow analysis of an ideal fluid are quantified.

Features of a separating turbulent boundary layer in the vicinity of separation

Abstract: Laser anemometer measurements using a directionally sensitive system were obtained for a nominally two-dimensional separating turbulent boundary layer produced by an adverse pressure gradient. An airfoil-type flow was generated in which the flow was accelerated and then decelerated until separation. The results include the skin friction, mean velocity profiles, turbulent shear stresses and intensities, spectra, dissipation rate, turbulent/non-turbulent interfacial intermittency, and eddy speeds.

Incompressible Boundary-Layer Separation

Abstract: For high-Reynolds-number flow over bodies or in confined channels the effects of viscosity are generally limited to a thin layer, the boundary layer, adjacent to the bounding surface. When the imposed pressure gradient is adverse, however, the thickness of the viscous layer increases as momentum is consumed by both wall shear and pressure gradient, and at some point the viscous layer breaks away from the bounding surface. Downstream of this point (or line) of breakaway the original boundary-layer fluid passes over a region of recirculating flow. The point at which the thin boundary layer breaks away from the surface and which divides the region of downstream-directed flow, in which the viscous effects are quite limited in extent, from the region of recirculating flow is known as the separation point.! Two different types of post-separation behavior are known to exist. In some cases the original boundary layer passes over the region or ' recirculating fluid and reattaches to the body at some point downstream, trapping a bubble of recirculating fluid beneath it. The characteristic length of this separation bubble may be of the same order as the upstream boundary-layer thickness or ma�y times the boundary-layer thickness. In other cases, the original boundary-layer fluid never reattaches to the body but passes downstream, mixing with recirculating fluid, to form a wake. For this wake-type of separation the characteristic dimension of the recirculating region is generally of the same order as the characteristic body dimension. In either case, the recirculating flow alters the effective body shape and hence the inviscid flow about the body.

Grid turbulence near a moving wall

Abstract: Decaying grid turbulence was passed over a wall moving at the stream speed. For the high Reynolds number of the experiment, the field due to the wall constraint on the normal component of the velocity fluctuations is found to extend further into the flow than the influence of the viscous boundary condition on the tangential-component fluctuations. Measurements of the variances, length scales and spectra of the three velocity components of the turbulence are compared with the results of a previous experiment and with the theoretical predictions for an idealization of the flow. A simple model for some departures from the theory is proposed.

Laminar flow control laminarization

Abstract: A practical aerodynamically and structurally reasonably efficient laminar flow control (LFC) suction method, removing the slowest boundary layer particles through many closed spaced fine slots, was developed and subsequently applied to a second F94 LFC wing glove in flight: 100 percent laminar flow was observed up to the F94 test limit. Laminar flow on LFC wings in flight is thus possible at a much higher Reynold's number than even in the best low turbulence tunnels as a result of the negligible influence of the atmospheric microscale turbulence on transition. The F94 LFC glove comparison experiments, with suction starting at 0.03c and 0.4c, verified the theoretically predicted boundary layer stabilization by suction starting at 0.08c, thus maintaining laminar flow at substantially higher C sub L numbers as compared to boundary layer stabilization by flow acceleration; i.e., geometry alone without suction upstream of 0.4c.

Drag reduction in two phase and polymer flows

Abstract: The basic dynamics of turbulent boundary layers of several media is described qualitatively: suspensions of several types and polymer solutions. Despite the considerable differences in these media, it is argued that a number of the flows are affected only in the buffer layer, and drag reduction can result if behavior in the sublayer and buffer layer differ. In polymer solutions, it is argued that molecular expansion is responsible for the difference, and experimental evidence of this expansion is presented, and compared with calculations.

The laminar separation of an incompressible fluid streaming past a smooth surface

Abstract: Sychev's (1972) proposal, that in general the laminar separation and breakaway of an incompressible fluid streaming past a smooth surface (e.g. on a bluff body) takes place through a triple-deck structure around the separation point, is examined numerically in this paper. The proposed pattern for large Reynolds number ($Re$) flows is based on a modification of the classical Kirchhoff (1869) free streamline theory, in which a slight adverse pressure gradient is provoked in the inviscid motion immediately ahead of the breakaway. This pressure gradient is just enough to generate a triple-deck development closer to the separation point. The major task then is to decide whether or not a solution of the basic triple-deck problem exists, and is regular at separation, and if it is unique. The numerical investigation, an iterative calculation of the relevant boundary layer problem, together with the potential flow relation between the unknown pressure and displacement, points fairly firmly to both the existence and uniqueness of a solution. Thus, for the bluff body problem when $Re$ $\gg $ 1, the triple-deck determines exactly how far the separation point lies from the position implied by inviscid (Kirchhoff) theory. Comparisons with separating incompressible fluid motions determined numerically from the Navier-Stokes equations and measured experimentally give some support overall to the triple-deck description. For the flow past a circular cylinder the agreement in the variation of pressure and skin friction near separation is in general very encouraging, for Reynolds numbers as low as 30.

Design of Two-Dimensional Wind Tunnel Contractions

Abstract: Design charts for 2-D wind tunnel contractions have been developed using inviscid flow analysis, in which a one-parameter family of wall shapes, based on two cubic arcs, was investigated in detail. The design chart parameters are the maximum wall pressure coefficients at the inlet (as an indicator of the danger of separation at the inlet end) and at the exit (which is related to the exit velocity non-uniformity as well as to separation). For any choice of these two parameters the charts yield the shape parameter and the nozzle length for this particular family of shapes. The charts may be used to design nozzles with no flow separation and with any desired exit velocity uniformity. When the two pressure coefficients are chosen so that separation at both ends is just avoided, the exit boundary layer thickness should be near its minimum. Simple working forms of Stratford’s separation criteria are suggested for the prediction of separation. The results of this study are compared to those obtained earlier for axisymmetric geometries.

Transition and laminar instability

Abstract: The linear stability theory was applied to the problem of boundary layer transition in incompressible flow. The theory was put into a form suitable for three-dimensional boundary layers; both the temporal and spatial theories were examined; and a generalized Gaster relation for three-dimensional boundary layers was derived. Numerical examples include the stability characteristics of Falkner-Skan boundary layers, the accuracy of the two-dimensional Gaster relation for these boundary layers, and the magnitude and direction of the group velocity for oblique waves in the Blasius boundary layer. Available experiments which bear on the validity of stability theory and its relation to transition are reviewed and the stability theory is applied to transition prediction. The amplitude method is described in which the wide band disturbance amplitude in the boundary layer is estimated from stability theory and an interaction relation for the initial amplitude density of the most unstable frequency.

A two-dimensional boundary layer encountering a three-dimensional hump

Abstract: A shallow three-dimensional hump disturbs the two-dimensional incompressible boundary layer developed on an otherwise flat surface. The steady laminar flow is studied by means of a three-dimensional extension of triple-deck theory, so that there is the prospect of separation in the nonlinear motion. As a first step, however, a linearized analysis valid for certain shallow obstacles gives some insight into the flow properties. The most striking features then are the reversal of the secondary vortex motions and the emergence of a ‘corridor’ in the wake of the hump. The corridor stays of constant width downstream and within it the boundary-layer displacement and skin-friction perturbation are much greater than outside. Extending outside the corridor, there is a zone where the surface fluid is accelerated, in contrast with the deceleration near the centre of the corridor. The downstream decay (e.g. of displacement) here is much slower than in two-dimensional flows.

Transitional boundary layer spot in a fully turbulent environment

Abstract: A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary‐layer thicknesses. The scale of the structure is of the order of 10δ in the streamwise direction becoming 2−3δ in the interface region of the boundary layer and is less than 4δ in the spanwise direction, in spite of the fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9U∞. It exhibits features in detailed agreement with those at the outer region of the turbulent boundary layer (interface region) and is consistent with existing two‐ and three‐point space‐time correlations.

Upstream influence and Long's model in stratified flows

Abstract: This paper describes an experimental study of a stratified fluid which is flowing over a smooth two-dimensional obstacle which induces no flow separation and in which effects of viscosity and diffusion are not important. The results are restricted to fluid of finite depth. Various properties of the flow field, in particular the criterion for the onset of gravitational instability in the lee-wave field, are measured and compared with the theoretical predictions of Long's model. The agreement is found to be generally poor, and the consequent inapplicability of Long's model is explained by the failure of Long's hypothesis of no upstream influence, which is demonstrably invalid when stationary lee waves are possible. The obstacle generates upstream motions with fluid velocities which appear to be of first order in the obstacle height. These motions have some of the character of shear fronts or columnar disturbance modes and have the same vertical structure as the corresponding lee-wave modes generated downstream. They result in a reduced fluid velocity upstream below the level of the top of the obstacle, together with a jet of increased fluid velocity above this level which pours down the lee side of the obstacle. This phenomenon becomes more pronounced as the number of modes is increased.

Theory of viscous transonic flow over airfoils at high Reynolds number

Abstract: This paper considers viscous flows with unseparated turbulent boundary layers over two-dimensional airfoils at transonic speeds. Conventional theoretical methods are based on boundary layer formulations which do not account for the effect of the curved wake and static pressure variations across the boundary layer in the trailing edge region. In this investigation an extended viscous theory is developed that accounts for both effects. The theory is based on a rational analysis of the strong turbulent interaction at airfoil trailing edges. The method of matched asymptotic expansions is employed to develop formal series solutions of the full Reynolds equations in the limit of Reynolds numbers tending to infinity. Procedures are developed for combining the local trailing edge solution with numerical methods for solving the full potential flow and boundary layer equations. Theoretical results indicate that conventional boundary layer methods account for only about 50% of the viscous effect on lift, the remaining contribution arising from wake curvature and normal pressure gradient effects.

Unsteady aerodynamic considerations in the design of a drag- reduction spike

Abstract: The Trident I Missile features an extendible nose spike that reduces the drag of the blunt nose by creating a low dynamic-pressure flow separation Experience with the Apollo-Saturn boosters indicates that the flow separation could contribute negative aerodynamic damping to certain critical free-free bending modes It is essential to determine if the undamping is severe enough to endanger the missile structural integrity The aerodynamic damping analysis is presented, including the effects of discontinuous spike deflection that occurs when thermal expansion, due to aerodynamic heating, loosens the spike joints Fluctuating-pressure wind-tunnel test data are also included since the fluctuating pressures produce the most severe lateral aerodynamic load on the "aerospike"

Dependence of turbulent burning velocity on turbulent reynolds number and ratio of flaminar burning velocity to R.M.S. turbulent velocity

Abstract: Measurements are reported of premixed hydrogen-air turbulent burning velocities, made by the double kernel method during explosions. Turbulence was created by four high speed fans within the explosion vessel. The method is described for calibrating the system, which is capable of giving high values of turbulent Reynolds numbers. The values obtained are compared with those of many other workers, over a wide range of burning conditions, mixtures and turbulent parameters. The ratio of turbulent to laminar burning velocity correlates well with both the turbulent Reynolds number and the ratio of laminar burning velocity to r.m.s. turbulent velocity. The use of hydrogen-air mixtures has extended the data on premixed turbulent combustion to regimes with higher values of the last dimensionless ratio. At high values of the ratio there is evidence of a wrinkled laminar flame structure, but at lower values a small scale eddy structure seems to be dominant. There is discussion on these findings, which accord with theoretical expectations.

Experimental study of the stability of heated laminar boundary layers in water

Abstract: Linear sinusoidal disturbances are introduced into the boundary layer over a heated flat plate of uniform surface temperature using a vibrating ribbon. The experiment is performed in a low turbulence water tunnel with free-stream turbulence intensities of 0[sdot ]1–0[sdot ]2%. Measurements are made using temperature-compensated hot-film anemometry. Neutral-stability characteristics obtained for the unheated case agree favourably with previous results obtained both in water and in air. Neutral-stability and spatial disturbance-growth-rate characteristics measured for wall temperatures up to 8°F above the free-stream temperature verify trends established by parallel-flow solutions of the disturbance momentum and energy equations. Disturbance growth rates and the band of amplified disturbance frequencies both decrease as wall heating is increased. The experimentally observed increase in the minimum critical Reynolds number Re c min with increased wall heating agrees with the trend predicted by theory. However, effects of non-parallel flow act to reduce the measured values of Re c min by about 120 units compared with predicted parallel-flow values independent of the level of wall heating.

Reynolds Number Effects on Shock-Wave Turbulent Boundary-Layer Interactions

Abstract: An experiment is described that tests and guides computations of a shock-wave turbulent boundary-layer interaction flow over a 20° compression corner at Mach 2.85. Numerical solutions of the time-averaged NavierStokes equations for the entire flow field, employing various turbulence models, are compared with the data. Each model is evaluated critically by comparisons with the details of the experimental data. Experimental results for the extent of upstream pressure influence and separation location are compared with numerical predictions for a wide range of Reynolds numbers and shock-wave strengths.

Dynamics of boundary layer turbulence and the mechanism of drag reduction

Abstract: The dynamics of the fluctuating velocity field in a turbulent boundary layer is discussed on the basis of a simplified analysis of localized unsteady perturbations in a parallel shear flow. Three classes of such disturbances may be distinguished, namely: (i) large‐scale propagating disturbances which may be represented by a superposition of shear waves; (ii) convected disturbances which decay only slowly through the action of viscosity; and (iii) small‐scale disturbances resulting from the secondary instability of the large‐scale motion. Coupling between large and small scales is incorporated in a two‐scale model in which the large scales are considered driven by the Reynolds stresses produced by the small scales. Shearing by the mean flow is shown to cause intensification of internal shear layers in the flow and could lead to local inflection in the instantaneous velocity profile, thus making secondary instability possible. Drag reducing additives, as modeled by some simple constitutive relations are found to be able to inhibit secondary instability, and would hence lower turbulent stress production according to the model employed.

Separation ahead of Protuberances in Supersonic Turbulent Boundary Layers

Abstract: The data were obtained using the optical-surface indicator technique of visualizing the flow; its accuracy and reproducibility are discussed. The proturberances are immersed in the boundary layer on the wall of a supersonic wind tunnel. The relative importance of various nondimensional groups is evaluated. The variation of primary separation distance is presented as a function of obstacle dimensions, Mach number, and Reynolds number, the last being the least significant. These results do not support some scaling laws found in the literature. An alternative correlation is proposed which applies to both small and large cylindrical protuberances.

Streamline Curvature Effects on Turbulent Boundary Layers

Abstract: A theoretical tool has been developed for predicting, in a nonempirical manner, effects of streamline curvature and coordinate-system rotation on turbulent boundary layers. The second-order closure scheme developed by Wilcox and Traci has been generalized for curved streamline flow and for flow in a rotating coordinate system. A physically based straightforward argument shows that curvature/rotation primarily affects the turbulent mixing energy; the argument yields suitable curvature/rotation terms which are added to the mixing-energy equation. Singular-perturbation solutions valid in the wall layer of a curved-wall boundary layer and a fully developed rotating channel flow demonstrate that, with the curvature/rotation terms, the model predicts the curved-wall and the rotating coordinate system laws of the wall. Results of numerical computations of curved-wall boundary layers and of rotating channel flow show that curvature/rotation effects can be computed accurately with second-order closure.

Approximate Nonlinear Slender Wing Aerodynamics

Abstract: On present-day high-performance aircraft, a large portion of the lift at moderate to high angles of attack is produced by leading-edge vortices, generated by flow separation off the highly swept leading edges of the lifting surfaces employed. It has been shown in an earlier paper how the vortex effects can be superimposed on a modified slender wing theory to give the unsteady longitudinal characteristics of sharp-edged delta wings up to very high angles of attack. The present paper extends the previous analysis to include the effects of leading-edge roundness and trailing-edge sweep on the aerodynamic characteristics. The paper also derives analytic means for prediction of the yaw stability of slender wings and the first-order effects of Mach number. Universal scaling laws are defined for rapid preliminary design estimates of the slender wing lift and rolling moment.