Showing papers on "Flow separation published in 1991"

Coherent Motions in the Turbulent Boundary Layer

Abstract: The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.

Rough-Wall Turbulent Boundary Layers

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Transient eddy formation around headlands

Abstract: Eddies with length scales of 1–10 km are commonly observed in coastal waters and play an important role in the dispersion of water-borne materials. The generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models. Using shallow water depth-averaged vorticity dynamics, eddies are shown to form when flow separation occurs near the tip of the headland, causing intense vorticity generated along the headland to be injected into the interior. An analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring (accelerating) to adverse (decelerating), and its occurrence depends principally on three parameters: the aspect ratio [b/a], where b and a are characteristic width and length scales of the headland; [H/CDa], where H is the water depth, CD is the depth-averaged drag coefficient; and [Uo/σa], where Uo and σ are the magnitude and frequency of the far-field tidal flow. Simulations with a depth-averaged numerical model show a wide range of responses to changes in these parameters, including cases where no separation occurs, cases where only one eddy exists at a given time, and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist, interacting strongly with each other. These simulations also demonstrate that in unsteady flow, a strong start-up vortex forms after the flow separates, leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow.

On the dynamics of near-wall turbulence

Abstract: A model of the dynamic physical processes that occur in the near-wall region of a turbulent flow at high Reynolds numbers is described The hairpin vortex is postulated to be the basic flow structure of the turbulent boundary layer It is argued that the central features of the near-wall flow can be explained in terms of how asymmetric hairpin vortices interact with the background shear flow, with each other, and with the surface layer near the wall The physical process that leads to the regeneration of new hairpin vortices near the surface is described, as well as the processes of evolution of such vortices to larger-scale motions farther from the surface The model is supported by recent important developments in the theory of unsteady surface-layer separation and a number of `kernel' experiments which serve to elucidate the basic fluid mechanics phenomena believed to be relevant to the turbulent boundary layer Explanations for the kinematical behaviour observed in direct numerical simulations of low Reynolds number boundary-layer and channel flows are given An important aspect of the model is that it has been formulated to be consistent with accepted rational mechanics concepts that are known to provide a proper mathematical description of high Reynolds number flow

Streamwise vortex production by pitched and skewed jets in a turbulent boundary layer

Abstract: Weak longitudinal (streamwise) vortices produced by the interaction of simple, round wall jets with a two-dimensional flow comprising a turbulent boundary layer were studied experimentally. Like the jets used in the vortex generator jet (VGJ) method of stall control, the jets were pitched up at 45 deg and skewed relative to the freestream as they entered from the wall.

Unsteady wave structure near separation in a Mach 5 compression rampinteraction

Abstract: Fluctuating wall-pressure measurements have been made under the unsteady separation shock and the separated shear layer in a Mach 5 compression ramp-induced turbulent interaction. The freestream unit Reynolds number was 49.6x10 6 m -1 and the turbulent boundary layer developed on the tunnel floor under approximately adiabatic wall-temperature conditions. Conditional sampling and «variable-window» ensemble-averaging techniques have been used to determine ensemble-averaged pressure distributions for different separation shock-wave positions.

The kinematics of turbulent boundary layer structure

Abstract: The long history of research into the internal structure of turbulent boundary layers has not provided a unified picture of the physics responsible for turbulence production and dissipation. The goals of the present research are to: (1) define the current state of boundary layer structure knowledge; and (2) utilize direct numerical simulation results to help close the unresolved issues identified in part A and to unify the fragmented knowledge of various coherent motions into a consistent kinematic model of boundary layer structure. The results of the current study show that all classes of coherent motion in the low Reynolds number turbulent boundary layer may be related to vortical structures, but that no single form of vortex is representative of the wide variety of vortical structures observed. In particular, ejection and sweep motions, as well as entrainment from the free-streem are shown to have strong spatial and temporal relationships with vortical structures. Disturbances of vortex size, location, and intensity show that quasi-streamwise vortices dominate the buffer region, while transverse vortices and vortical arches dominate the wake region. Both types of vortical structure are common in the log region. The interrelationships between the various structures and the population distributions of vortices are combined into a conceptual kinematic model for the boundary layer. Aspects of vortical structure dynamics are also postulated, based on time-sequence animations of the numerically simulated flow.

Vortex-induced boundary-layer separation. Part 1. The unsteady limit problem Re [rightward arrow] [infty infinity]

Abstract: The unsteady boundary-layer flow produced by a two-dimensional vortex in motion above an infinite plane wall in an otherwise stagnant fluid is considered in the limit of infinite Reynolds number. This study is part of a continuing investigation into the nature of the physical processes that occur near the surface in transitional and fully turbulent boundary layers. The adverse pressure gradient due to the vortex leads to the development of a zone of recirculation in the viscous flow near the surface, and the boundary-layer flow then focuses rapidly toward an eruption along a band which is very narrow in the stream wise direction. The evolution of the unsteady boundary layer is posed in Lagrangian coordinates and computed using an efficient, factored ADI numerical method. The boundary-layer solution is found to develop a separation singularity and to evolve toward a terminal stage which is generic in two-dimensional unsteady flows. The computed results are compared with the results of asymptotic theory of two-dimensional boundary-layer separation and the agreement is found to be excellent.

Structure of laminar juncture flows

Abstract: A computational study of both steady and periodic laminar horseshoe vortex flows generated upstream of a cylinder/flat plate juncture is presented. The flowfields are simulated using the full three-dimensional unsteady Navier-Stokes equations and a time-accurate implicit algorithm. A new type of laminar horseshoe vortex topology is identified. For the case of a single primary vortex, this new topology is found to be independent of the computational grid and is also supported by recent experimental flow visualizations. The flat plate skin-friction portraits corresponding to the new and to the standard horseshoe vortex topologies are equivalent, pointing out the nonunique relation between the wall limiting streamline pattern and the three-dimensional flow above the plate. For the new topology, the foremost line of coalescense is an attachment rather than a separation line. This unusual feature illustrates the fact that convergence of skin-friction lines is a necessary but not sufficient condition for separation. As the Reynolds number increases, the flow topology evolves from a single to multiple primary horseshoe vortices, in agreement with experimental observations. At least two different types of triple horseshoe vortex systems are shown to be possible. Above a certain value of the Reynolds number, the juncture flow becomes unsteady and periodic at a frequency that increases with Reynolds number. The unsteady horseshoe vortex process upstream of the cylinder is found in qualitative agreement with experiment. Horseshoe vortices are periodically generated and convected toward the juncture. Vorticity intensification by vortex stretching, and the eruption of vorticity from the plate surface are observed.

Unsteady flow past an airfoil pitching at a constant rate

Abstract: The unsteady flow past a NACA 0012 airfoil that is undertaking a constant-rate pitching up motion is investigated experimentally by the PIDV technique in a water towing tank. The Reynolds number is 5000, based upon the airfoil's chord and the free-stream velocity. The airfoil is pitching impulsively from 0 to 30 deg. with a dimensionless pitch rate alpha of 0.131. Instantaneous velocity and associated vorticity data have been acquired over the entire flow field. The primary vortex dominates the flow behavior after it separates from the leading edge of the airfoil. Complete stall emerges after this vortex detaches from the airfoil and triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.

Vortex-induced boundary-layer separation. part 2. unsteady interacting boundary-layer theory

Abstract: The unsteady boundary layer induced by the motion of a rectilinear vortex above an infinite plane wall is calculated using interacting boundary-layer methods. The boundary-layer solution is computed in Lagrangian variables since it is possible to compute the flow evolution accurately in this formulation even when an eruption starts to evolve. Results are obtained over a range of Reynolds numbers, Re. For the limit problem Re - infinity (studied in Part 1), a singularity develops in the non-interacting boundary-layer solution at finite time. The present results show that the interacting boundary-layer calculations also terminate in a singularity at a time which is earlier than in the limit problem and which decreases with decreasing Reynolds number. The computed results are compared with the length-and timescales predicted by recent asymptotic theories and are found to be in excellent agreement. See also previous abstract.

The effect of small particles on fluid turbulence in a flat‐plate, turbulent boundary layer in air

Abstract: This paper describes the result of both an experimental and an analytical investigation of the response of a two‐dimensional, turbulent boundary layer in air to the presence of particles. Copper shot, 70 μm in diameter, were uniformly introduced into a vertical boundary layer, at a momentum thickness Reynolds number of about 1000. The particle mass flux was set at 20% of the fluid mass flux, and all measurements were made using a single‐component, forward‐scatter laser Doppler anemometer. The measurements clearly demonstrated that the particles damped fluid turbulence, apparently affecting all scales equally. The measurements further showed a strong correlation between the degree of damping and the particle concentration in the log region of the boundary layer.

Correlation of separation shock motion with pressure fluctuations inthe incoming boundary layer

Abstract: Fluctuating wall pressures have been measured simultaneously in the incoming undisturbed turbulent boundary layer and under the unsteady separation shock in a Mach 5 compression ramp interaction Conditional sampling algorithms, a variable-window ensemble averaging technique, and the variable interval time averaging technique have been used to investigate the possibility of a correlation between pressure fluctuations in the incoming flow and the separation shock wave motion

Reynolds shear stress measurements in a separated boundary layer flow

Abstract: Turbulence measurements were obtained for two cases of boundary layer flow with an adverse pressure gradient, one attached and the other separated. A three-component laser Doppler velocimeter system was used to measure three mean velocity components, all six Reynolds stress components, and all ten velocity triple product correlations. Independent measurements of skin-friction obtained with a laser oil-flow interferometer were used to examine the law of the wall in adverse pressure gradient flows where p(+) is less than 0.05. Strong similiarities were seen between the two adverse pressure gradient flows and free shear layer type flows. Eddy viscosities, dissipation rates, and pressure-strain rates were deduced from the data and compared to various turbulence modeling assumptions.

A Numerical Study of Developing Free Convection Between Isothermal Vertical Plates

Abstract: Steady two-dimensional laminar free convection between isothermal vertical plates including entrance flow effects has been numerically investigated. The full elliptic forms of the Navier-Stokes and energy equations are solved using novel inlet flow boundary conditions. Results are presented for Prandtl number Pr = 0.7, Grashof number range 50 {le} Gr{sub b} {le} 5 {times} 10{sup 4}, and channel aspect ratios of L/b = 10, 17, 24. New phenomena, such as inlet flow separation, have been observed. The results cast doubt on the validity of previous elliptic solutions. Comparisons with the approximate boundary-layer results show that a full elliptic solution is necessary to get accurate local quantities near the channel entrance.

Vorticity dynamics in an oscillatory flow over a rippled bed

Abstract: In the present paper we determine the oscillatory flow generated by surface gravity waves near a sea bottom covered with large-amplitude ripples. The vorticity equation and Poisson equation for the stream function are solved by means of a numerical approach based on spectral methods and finite-difference approximations. In order to test the numerical algorithm and in particular the numerical scheme used to generate vorticity along the ripple profile, we also perform an asymptotic analysis, which holds as the time t tends to zero. The main features of the time development of vorticity are analysed and particular attention is paid to the dynamics of the large vortices generated by flow separation at the ripple crests and along the ripple profile. Some of the results obtained by Longuet-Higgins (1981) are recovered; in particular, the present results show a vortex pair shed from the ripple crest every half-cycle. The determination of flow separation along the ripple profile induced by the pressure gradient and the inclusion of viscous effects allows us to obtain accurate quantitative results and detect some important phenomena never observed before.In particular it is shown that: (i) Whenever a vortex structure moves towards the bottom, a secondary vortex is generated near the ripple profile, which interacts with the primary vortex and causes it to move away from the bottom, (ii) Depending on the values of the parameters, the time development of the free shear layer shed from the ripple crest may produce two or even more vortex structures, (iii) Occasionally vortices generated previously may coalesce with the free shear layer shed from the ripple crest, generating a unique vortex structure.

Exploratory study of vortex-generating devices for turbulent flow separation control

Abstract: Flow phenomena and the relative performance associated with several devices for controlling turbulent separated flow were investigated at low speeds. Vortex-generating devices examined included: submerged vortex generators (Wheeler doublet and wishbone types), spanwise cylinders, large-eddy breakup devices (LEBU) at small angle of attack (alpha), and vortex-generator jets (VGJ). Dye flow visualization tests in a water tunnel indicated that wishbone vortex generators in the forward orientation shed horseshoe vortices; wishbone vortex generators oriented in the reverse direction and doublet vortex generators shed streamwise counterrotating vortices; a spanwise cylinder located near the wall and LEBUs at alpha = -10 deg produced eddies which rotated with the same sign as the mean vorticity in a turbulent boundary layer; and the most effective VGJs produced streamwise corotating vortices. Comparative wind tunnel tests conducted on a curved backward-facing ramp indicated that transferring momentum from the outer region of a turbulent boundary layer by embedded streamwise vortices is more effective than by transverse vortices for separation control applications.

Computation of shock wave diffraction at a ninety degrees convex edge

Abstract: This paper presents a numerical study of shock wave diffraction at a sharp ninety degrees edge, using an explicit second-order Godunov-type Euler scheme based upon the solution of a generalized Riemann problem (GRP). The Euler computations produce flow separation very close to the diffraction edge, leading to a realistic development of the separated shear layer and subsequent vortex roll-up. The diffracted shock wave, and the secondary shock wave, are both reproduced well. In addition a pair of vortex shocks are shown to form, extending well into the vortex core.

Flowfield analysis of modern helicopter rotors in hover by Navier-Stokes method

Abstract: The viscous, three-dimensional, flowfields of UH60 and BERP rotors are calculated for lifting hover configurations using a Navier-Stokes computational fluid dynamics method with a view to understand the importance of planform effects on the airloads. In this method, the induced effects of the wake, including the interaction of tip vortices with successive blades, are captured as a part of the overall flowfield solution without prescribing any wake models. Numerical results in the form of surface pressures, hover performance parameters, surface skin friction and tip vortex patterns, and vortex wake trajectory are presented at two thrust conditions for UH60 and BERP rotors. Comparison of results for the UH60 model rotor show good agreement with experiments at moderate thrust conditions. Comparison of results with equivalent rectangular UH60 blade and BERP blade indicates that the BERP blade, with an unconventional planform, gives more thrust at the cost of more power and a reduced figure of merit. The high thrust conditions considered produce severe shock-induced flow separation for UH60 blade, while the BERP blade develops more thrust and minimal separation. The BERP blade produces a tighter tip vortex structure compared with the UH60 blade. These results and the discussion presented bring out the similarities and differences between the two rotors.

Control of laminar separation over airfoils by acoustic excitation

Abstract: The effect of acoustic excitation in reducing laminar separation over two-dimensional airfoils at low angles of attack is investigated experimentally. Airfoils of two different cross sections, each with two different chord lengths, are studied in the chord Reynolds number range of 25,000 is less than R sub c is less than 100,000. While keeping the amplitude of the excitation induced velocity perturbation a constant, it is found that the most effective frequency scales as U (sup 3/2) (sub infinity). The parameter St/R (sup 1/2)(sub c), corresponding to the most effective f sub p for all the cases studied, falls in the range of 0.02 to 0.03, St being the Strouhal number based on the chord.

Turbulent Flow Around a Bluff Rectangular Plate. Part I: Experimental Investigation

Abstract: Measurements are reported for the separted reattaching flow around a long rectangular plate placed at zero incidence in a low-turbulence stream. This laboratory configuration, chosen for its geometric simplicity, exhibits all of the important features of two-dimensional flow separation with reattachment. Conventional hot-wire anemometry, pulsed-wire anemometry and pulsed-wire surface shear stress probes were used to measure the mean and fluctuating flow field at a Reynolds number, based on plate thickness, of 5 × 104 . The separated shear layer appears to behave like a conventional mixing layer over the first half of the separation bubble, where it exhibits an approximately constant growth rate and a linear variation of characteristic frequencies and integral timescales. The characteristics of the shear layer in the second half of the bubble are radically altered by the unsteady reattachment process. Much higher turbulent intensities and lower growth rates are encountered there, and, in agreement with other reattaching flow studies, a low frequency motion can be detected.

On the evolution of a wave packet in a laminar boundary layer

Abstract: The transition process of a small-amplitude wave packet, generated by a controlled short-duration air pulse, to the formation of a turbulent spot is traced experimentally in a laminar boundary layer. The vertical and spanwise structures of the flow field are mapped at several downstream locations. The measurements, which include all three velocity components, show three stages of transition. In the first stage, the wave packet can be treated as a superposition of two- and three-dimensional waves according to linear stability theory, and most of the energy is centred around a mode corresponding to the most amplified wave. In the second stage, most of the energy is transferred to oblique waves which are centred around a wave having half the frequency of the most amplified linear mode. During this stage, the amplitude of the wave packet increases from 0.5 % to 5 % of the free-stream velocity. In the final stage, a turbulent spot develops and the amplitude of the disturbance increases to 27 % of the free-stream velocity.Theoretical aspects of the various stages are considered. The amplitude and phase distributions of various modes of all three velocity components are compared with the solutions provided by linear stability theory. The agreement between the theoretical and measured distributions is very good during the first two stages of transition. Based on linear stability theory, it is shown that the two-dimensional mode of the streamwise velocity component is not necessarily the most energetic wave. While linear stability theory fails to predict the generation of the oblique waves in the second stage of transition, it is demonstrated that this stage appears to be governed by Craik-type subharmonic resonances.

An optimum suppression of fluid forces by controlling a shear layer separated from a square prism

Abstract: This paper deals with the suppression of the fluid forces by controlling a shear layer on one side separated from a square prism. The control of the separated shear layer was established by setting up a small circular cylinder (the control cylinder) in it on one side. Experimental data were collected to examine the effects on the fluid forces and vortex shedding frequency due to variation of the position and diameter of the control cylinder. The results show that (i) the maximum reduction of the time-mean drag and fluctuating lift and drag occurred when the control cylinder was located near what would ordinarily be considered the outer boundary of the shear layer; (ii) the control of the separated shear layer by means of a small cylinder appeared to be effective in suppressing the fluctuating lift and drag rather than the time-mean drag; (iii) in the case of the control cylinder of 6 mm in diameter, the time-mean drag was reduced to about 30 percent, and the fluctuating lift and drag were reduced to approximately 95 and 75 percent, respectively; (iv) the fluid forces and the frequency of vortex shedding of the square prism were mainly dependent on the characteristics of a very thin region near the outer boundary of the shear layer.

Fluid dynamic effects of grooves on circular cylinder surface

Abstract: It is shown that a groove on the surface of a circular cylinder affects movement of the separation point backward and reduces draw even at Reynolds numbers of about a few thousand. Several types of circular-arc cross-section grooves are studied using flow visualisations and numerical simulations. Whether these grooves are effective depends strongly on their positions, and the most effective positions are about 80 deg, measured from the foremost point. When they are effective, cavity flows are developed inside the grooves. This effect corresponds to that of dimples on golf balls and will explain unique characteristics of the drag curve.

Propagating structures in wall-bounded turbulent flows

Abstract: The Karhunen-Loeve (K-L) procedure for generating the empirical eigenfunctions is used to analyze two turbulent channel flow simulations of different geometry and origins. In one instance the Reynolds number ReT based on wall shear velocity, u r, and channel halfwidth, δ, is 80 and in the second instance ReT = 125. The latter case showed a well defined log-layer, while this was absent in the former case. According to accepted convention, the turbulence is said to be continuous when Rer = 80, and fully developed when Rer = 125. In both instances the empirical eigenfunctions reveal the presence of propagating plane wave structures. Thus the contrasts in the two cases is of some importance since they virtually eliminate the possibility that the waves are artifacts.