Showing papers on "Flow separation published in 1990"

The structure of two-dimensional separation

Abstract: The separation of a two-dimensional laminar boundary layer under the influence of a suddenly imposed external adverse pressure gradient was studied by time-accurate numerical solutions of the Navier–Stokes equations. It was found that a strong adverse pressure gradient created periodic vortex shedding from the separation. The general features of the time-averaged results were similar to experimental results for laminar separation bubbles. Comparisons were made with the ‘steady’ separation experiments of Gaster (1966). It was found that his ‘bursting’ occurs under the same conditions as our periodic shedding, suggesting that bursting is actually periodic shedding which has been time-averaged. The Strouhal number based on the shedding frequency, local free-stream velocity, and boundary-layer momentum thickness at separation was independent of the Reynolds number and the pressure gradient. A criterion for onset of shedding was established. The shedding frequency was the same as that predicted for the most amplified linear inviscid instability of the separated shear layer.

Graphical visualization of vortical flows by means of helicity

Abstract: Helicity density and normalized helicity are introduced as important tools for the graphical representation of three-dimensional flowfields that contain concentrated vortices. The use of these two quantities filters out the flowfield regions of low vorticity, as well as regions of high vorticity but low speed where the angle between the velocity and vorticity vectors is large (such as in the boundary layer). Their use permits the researcher to identify and accentuate the concentrated vortices, differentiate between primary and secondary vortices, and mark their separation and reattachment lines. The method also allows locating singular points in the flowfield and tracing the vortex-core streamlines that emanate from them. Nomenclature H = helicity Hd = helicity density Hn — normalized helicity MOO = freestream Mach number ReD = Reynolds number V = velocity a = angle of attack co = vorticity

Viscous drag reduction in boundary layers

Abstract: The present volume discusses the development status of stability theory for laminar flow control design, applied aspects of laminar-flow technology, transition delays using compliant walls, the application of CFD to skin friction drag-reduction, active-wave control of boundary-layer transitions, and such passive turbulent-drag reduction methods as outer-layer manipulators and complex-curvature concepts. Also treated are such active turbulent drag-reduction technique applications as those pertinent to MHD flow drag reduction, as well as drag reduction in liquid boundary layers by gas injection, drag reduction by means of polymers and surfactants, drag reduction by particle addition, viscous drag reduction via surface mass injection, and interactive wall-turbulence control.

Vortex Generator Jets—Means for Flow Separation Control

Abstract: Stalled regions (zones of detached or separated flow sometimes followed by reattachment) in a turbulent boundary layer may be eliminated by a technique called the vortex-generator-jet (VGJ) method. The method employs spanwise arrays of small, skewed, and pitched jets from holes in the surface. Low-speed air-flow experiments are described which 1) demonstrate that the VGJ method creates longitudinal (streamwise) vortices in the boundary layer downstream of the jet holes similar to the vortices behind solid vortex generators and 2) show that the cross-stream mixing associated with these vortices is effective in reduction and elimination of stalled regions.

Airfoil design and data

Abstract: This book reflects the author's experience in the development of a computer program for the application of potential flow and boundary layer theories to the design and analysis of subsonic airfoils, including the evaluation of total lift, drag and moment coefficients. The design allows readers to compute the airfoil shape by means of a simple mathematical method from the properties of their velocity distribution, which can be specified in such a way that the boundary layer flow fulfills some given requirements. The most significant boundary layer phenomena such as laminar turbulent transition, laminar separation bubbles and boundary layer separation are considered. The first five chapters of this book describe a procedure for the design and analysis of subsonic airfoils. The data section contains 116 new airfoils for a wide range of Reynolds numbers and application requirements, including general aviation aircraft, tailless airplanes, gliders and propellers. The design features are explained and the input data for computer codes are given for all airfoils.

Controlling Mechanism of Local Scouring

Abstract: Experimental study of clear water scouring around a circular cylinder shows that the scour mechanism is coupled to the three-dimensional separation of the upstream boundary layer and the periodic vortex shedding in the wake of the cylinder. The first scour appears in the wake of the cylinder. The main scouring agent in the upstream region is a system of horseshoe vortices. The vortices have a periodical character that causes a triple-scour profile to develop in the upstream region. During scouring, the number and periods of horseshoe vortex shedding undergo no appreciable change. Despite the clear water stage, the transport phenomenon is periodical. Transport of sediment takes place through turbulent scales of comparable size to macro-length scales. The size of horseshoe vortices are representative for the macroscale. Wake scouring is caused by the primary wake vortices and the accelerated side flow. The process is characterized by a strong periodical transport and the formation of ripples. The periodicity is controlled by the shedding frequency of the wake vortices. Collars attached to the cylinder cannot prevent the formation of the vortices.

Flow of non-newtonian fluids in a pipe

Abstract: Measurements of mean axial velocity and of the three normal stresses have been obtained in fully developed pipe-flow with four concentrations of a polymer (sodium carboxymethyl cellulose) in aqueous solution and with water and viscous Newtonian fluids encompassing a range of Reynolds numbers from 240 to 111,000. The results quantify the delay in transition from laminar to turbulent flow caused by shear-thinning, the suppression of turbulent fluctuations particularly in the radial and tangential components of normal stress, and the drag reduction at the higher Reynolds numbers. They also confirm that the maximum drag reduction asymptote is appropriate to these shear-thinning solutions.

Control of wall-separated flow by internal acoustic excitation

Abstract: We explore the control of wall-separated flow on a NACA 63 3 -018 airfoil and a circular cylinder by using the internal acoustic excitation technique. Experimental study of the characteristics of the flow under internally emanating acoustic waves is performed in an open-type, suction wind tunnel. Tests are carried out at the Reynolds number ranging from 6.3 × 10 3 to 5.0 × 10 5 based on the relevant characteristic lengths, the airfoil chord, and the cylinder diameter

Control of low-speed airfoil aerodynamics

Abstract: Nomenclature CD = drag coefficient ( = 2D/pUlc) Cf = local skin-friction coefficient ( = 2rw/pUl) CL = lift coefficient ( = 2L/pU^c) Cq = suction coefficient ( = I vw I / UQ) c = airfoil's chord D = drag force per unit span L = lift force per unit span P = instantaneous hydrostatic pressure PO = pressure outside boundary layer P = mean pressure R = wall's radius of curvature /?6* = displacement thickness Reynolds number (= Uod*/v) RO = momentum thickness Reynolds number ( = U0de/v) Re = Reynolds number based on distance from leading edge (or chord) and freestream velocity T = instantaneous temperature T = mean temperature J7, = instantaneous velocity component Uj = mean velocity component U0 = velocity outside the boundary layer t/oo = freestream velocity HI = fluctuating velocity component u* = friction velocity ( = Vr^/p) vw = normal velocity of fluid injected or withdrawn through the wall X-, = Cartesian coordinates x = streamwise distance from leading edge y = normal distance from the wall y = normal distance in wall units ( = yu*/v) Z = spanwise coordinate a. = angle of attack 6 = boundary-layer thickness 60 = momentum thickness 6* = displacement thickness /x = dynamic coefficient of viscosity v = kinematic viscosity v/u * = viscous length scale (wall unit) p = density — p~uv = tangential Reynolds stress T.W = shear stress at the wall ( = pu *) [AJo = instantaneous spanwise vorticity at the wall _ [fijo = rnean spanwise vorticity at the wall (= — [dU/dy]Q)

Measurements of base pressure in the wake of a cylinder at low Reynolds numbers

Abstract: Measurements have been made of the base pressure coefficients of long circular cylinders at low Reynolds numbers from 35 to 1100 in a cross flow. The variation of "base suction" (negative base pressure coefficient or - C_(pb)) with Reynolds number shows a remarkably good correspondence with the variation of the Strouhal number. It is also possible to relate the variation in suction with the physical modes of wake formation found in previous studies. It is shown that the level of base suction is affected in only a minor fashion by whether parallel or oblique shedding is present, and it is found that the data are independent of the base pressure hole sizes and cylinder diameters that were used. The base suction is essentially constant along the span of a cylinder (outside of regions extending about 20 diameters in from each end), and the results arc independent of cylinder aspect ratio (L/D) provided that one exceeds a critical value of L/D.

An experimental investigation of an incompressible turbulent boundary layer in the vicinity of separation

Abstract: The following is an investigation into the effects of small changes in the static pressure distribution on the development of an axisymmetric, incompressible, turbulent boundary layer with incipient separation. The pressure distribution was closely controlled to study three cases, in which the skin friction was either approximately zero, slightly negative, or slightly positive along a fixed length. Mean flow and turbulence structure in air were measured using pulsed-wire and hot-wire anemometry.These measurements show characteristic properties of steady turbulent boundary layers both on the verge of separation and with a long, shallow separation bubble. There is an asymptotic velocity defect law near separation. A linear relationship between χW, the wall value of the reverse-flow parameter, and the form parameter H12 suggests the importance of χW in characterizing the boundary layer. The occurrence of the first reverse-flow events coincides with the vanishing of the logarithmic law, the asymptotic mean velocity profile, and a sudden drop in the values of the skewness SW and the flatness FW of the skin friction. This implies that the presence of instantaneous reverse flow is associated with a complete change in the nature of the near-wall flow, well upstream of mean separation. As the three cases were investigated in a single test section under closely controlled conditions with the same experimental techniques, this data set is well suited to a sensitivity study. It is possible to show the effect of small changes in the upstream pressure gradient on the separation region and to distinguish the effect of mean reverse flow from that of the adverse pressure gradient. This effect of the reverse flow is displayed most clearly in a plateau in near the wall and in unusual behaviour of the skewness and the flatness profiles over the inner half of the boundary layer.

Impingement of a low Reynolds number turbulent circular jet onto a flat plate at normal incidence

Abstract: The velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV. The velocity field has been recorded at several instants in time, producing thousands of simultaneous two-dimensional velocity measurements for each realization. The instantaneous velocity, vorticity and rate-of-strain fields reveal the interaction of vortices near the impinging wall within the radial wall jet downstream from the stagnation point. An ensemble average of the instantaneous fields produces a mean velocity field of the jet flow, which reveals many of the processes leading to boundary layer separation and vortex breakaway within the wall jet. The PIV system extracts the velocity measurements using a two-dimensional autocorrelation method, and can obtain thousands of highly accurate velocity measurements within a few minutes. The structure found in these experiments may be similar to the ground level structure of atmospheric microburst phenomena.

Control of flow separation by acoustic excitation

Abstract: To control the leading-edge flow separation on an airfoil by means of acoustic excitation, the response to the incident sound and the resulting flow instability are studied experimentally and therically on the basis of the linear stability theory, for a flat-plate airfoil, at a chord Reynolds number R c =4 x 10 4 .

Turbulent Shear Flow Over Surface Mounted Obstacles

Abstract: The mean flow field surrounding obstacles attached to a wall under a turbulent boundary layer is analysed. The analysis concentrates on how major features of the flow are influenced by model geometry and the incident shear flow. Experimental data are analysed in terms of non-dimensionalised variables chosen on the basis that their effect on major flow features can be simply appreciated. The data are restricted to high Reynolds number shear layers thicker than the attached obstacle. The work shows that data from a wide range of flows can be collapsed if appropriate non-dimensional scales are used.

Investigation of several passive and active methods for turbulent flow separation control

Abstract: Relative performance of several passive and active methods for controlling two-dimensional turbulent separated flow associated with a curved backward-facing ramp were investigated at low speeds. Surface static pressure measurement and oil flow visualization results indicate that submerged vortex generators, vortex generator jets, elongated arches at +-alpha, and large-eddy breakup devices at +-alpha placed near the baseline separation location reduce flow separation and increase pressure recovery. Spanwise cylinders reduce flow separation but decrease pressure recovery downstream. Arches with alpha = 0 deg, Helmholtz resonators, and Viets' fluidic flappers examined so far have no significant effect in reducing separation. Wall cooling computation indicates that separation delay on a partially cooled ramp is nearly the same as on a fully-cooled ramp while minimizing the frictional drag increase associated with the wall cooling process.

Sand ripples under sea waves Part 2. Finite-amplitude development

Abstract: In the present paper the authors formulate a theory to predict the time development of sand ripples characterised by small but finite amplitude under the action of surface gravity waves. The theory is based on a weakly nonlinear stability analysis of a flat sandy bottom subject to viscous oscillatory flow. The parameters of the problem (namely the Reynolds number of the flow and the Reynolds and Froude numbers of sediments) are assumed to fall within a neighbourhood of the critical conditions determined in Blondeaux (1990). The analysis can predict the actual ripple height wavelength and profile when flow separation is absent, i.e. for the case of rolling grain ripples. Assuming Sleath's (1984) criterion for separation, the values of the relevant parameters at which transition from rolling-grain ripples to vortex ripples occurs are predicted. A comparison between theoretical findings and experimental data supports the validity of the present theory.

Similarity solutions of the two-dimensional unsteady boundary-layer equations

Abstract: The method of Lie group transformations is used to derive all group-invariant similarity solutions of the unsteady two-dimensional laminar boundary-layer equations. A new method of nonlinear superposition is then used to generate further similarity solutions from a group-invariant solution. Our results are shown to include all the existing solutions as special cases. A detailed analysis is given to several classes of solutions which are also solutions to the full Navier–Stokes equations and which exhibit flow separation.

A Review of Vortex Structures and Associated Coherent Motions in Turbulent Boundary Layers

Abstract: The experimental and computational evidence for the existence and role of vortices in turbulent boundary layers is briefly reviewed. Quasi-streamwise and transverse vortices are considered, and various published conceptual models for horseshoe-like vortical structures are compared. The causes for upright and inverted horseshoe-shaped vorticity lines are discussed, and the distinction between vorticity lines and vortices is demonstrated. Finally, results from a numerically-simulated turbulent boundary layer are used to compute distributions of diameter, height, and strength for quasi-streamwise and spanwise vortices. These results confirm that quasi-streamwise vortices are clustered near the wall, while spanwise vortices are distributed throughout the layer. The variation of spanwise vortex core diameter with distance from the wall is found to be consistent with the mixing-length distribution for a boundary layer.

Separation and reattachment of non-newtonian fluid flows in a sudden expansion pipe

Abstract: In the current flow visualization studies, the role of non-Newtonian characteristics (such as shear-rate-dependent viscosity and viscoelasticity) on flow behavior across the sudden expansion step in a circular pipe is investigated over a wide range of Reynolds numbers including the turbulent flow. The expansion ratios tested are 2.000 and 2.667 and the range of the Reynolds number covered in the current flow visualization tests are 10–35 000 based on the inlet diameter. The reattachment lengths for the viscoelastic fluids in the laminar flow regime are found to be much shorter than those for the Newtonian fluid. In addition they decrease significantly with increasingly concentration of viscoelastic fluid at the same Reynolds number. However, in the turbulent flow regime, the reattachment length for the viscoelastic fluids is two or three times longer than those for water, and gradually increases with increasing concentration of viscoelastic solutions, resulting in 25 and 28 step-height distances for 500 ppm and 1000 ppm polyacrylamide solutions respectively. This may be because the elasticity in polyacrylamide solutions suppresses the eddy motion and controls separation and reattachment behavior in the sudden expansion pipe flow. The reattachment lengths for the purely viscous non-Newtonian fluids are found to be almost the same as those for water.

Small submerged vortex generators for turbulent flow separation control

Abstract: The performance of low-profile submerged vortex generators for controlling moderate two-dimensional turbulent flow separation has been investigated experimentally. Surface static pressure measurements, as well as surface oil flow visualizations, have been used to explore the effect of these vortex generators on separation and reattachment locations and downstream pressure recovery. Drag measurements have also been used to evaluate the device (or parasitic) drag of these vortex generators. All of the vortex generators investigated have been shown to reduce the reattachment distance and increase pressure recovery. A small submerged (Wheeler-type) vortex generator with a device height of only 10% of the boundary-layer thickness is shown to perform as well as a conventional vane-type vortex generator with a device height (and device drag) an order-of-magnitude higher.

Near-wall turbulence

Abstract: Topics presented include the similarity of organized structures in turbulent shear flows, a conceptual model of the viscous wall region, the effects of surface curvature on the law of the wall, and the role of inflectional velocity profiles in wall bounded flows. Also presented are a review of knowledge on pressure fluctuations, stochastic estimation of coherent structures in turbulent boundary layers, the effect of Reynolds number on the organized motion in a turbulent boundary layer, and the structure of turbulence in heat transfer. Also discussed are the effects of high free-stream turbulence on heat transfer in turbulent boundary layer, wave interaction and stability problems, the modeling of unsteady boundary layers, and the transport of enstrophy in a turbulent boundary layer.

Measurements in a separation bubble on an airfoil using laser velocimetry

Abstract: An experimental investigation was conducted to measure the reverse flow within the transitional separation bubble that forms on an airfoil at low Reynolds numbers. Measurements were used to determine the effect of the reverse flow on integrated boundary-layer parameters often used to model the bubble. Velocity profile data were obtained on an NACA 663-018 airfoil at angle of attack of 12 deg and a chord Reynolds number of 140,000 using laser Doppler and single-sensor hot-wire anemometry. A new correlation is proposed based on zero velocity position, since the Schmidt (1986) correlations fail in the turbulent portion of the bubble.

A supersonic turbulent boundary layer in an adverse pressure gradient

Abstract: This investigation describes the effects of an adverse pressure gradient on a flat plate supersonic turbulent boundary layer (Mf ≈ 2.9, βx ≈ 5.8, Reθ, ref ≈ 75600). Single normal hot wires and crossed wires were used to study the Reynolds stress behaviour, and the features of the large-scale structures in the boundary layer were investigated by measuring space–time correlations in the normal and spanwise directions. Both the mean flow and the turbulence were strongly affected by the pressure gradient. However, the turbulent stress ratios showed much less variation than the stresses, and the essential nature of the large-scale structures was unaffected by the pressure gradient. The wall pressure distribution in the current experiment was designed to match the pressure distribution on a previously studied curved-wall model where streamline curvature acted in combination with bulk compression. The addition of streamline curvature affects the turbulence strongly, although its influence on the mean velocity field is less pronounced and the modifications to the skin-friction distribution seem to follow the empirical correlations developed by Bradshaw (1974) reasonably well.