Showing papers on "Starting vortex published in 1989"

Airfoil self-noise and prediction

Abstract: A prediction method is developed for the self-generated noise of an airfoil blade encountering smooth flow. The prediction methods for the individual self-noise mechanisms are semiempirical and are based on previous theoretical studies and data obtained from tests of two- and three-dimensional airfoil blade sections. The self-noise mechanisms are due to specific boundary-layer phenomena, that is, the boundary-layer turbulence passing the trailing edge, separated-boundary-layer and stalled flow over an airfoil, vortex shedding due to laminar boundary layer instabilities, vortex shedding from blunt trailing edges, and the turbulent vortex flow existing near the tip of lifting blades. The predictions are compared successfully with published data from three self-noise studies of different airfoil shapes. An application of the prediction method is reported for a large scale-model helicopter rotor, and the predictions compared well with experimental broadband noise measurements. A computer code of the method is given.

Investigation of the flow structure around a rapidly pitching airfoil

Abstract: A numerical study is presented for unsteady laminar flow past a NACA 0015 airfoil that is pitched, at a nominally constant rate, from zero incidence to a very high angle of attack. The flowfield simulation is obtained by solving the full two-dimensional compressible Navier-Stokes equations on a moving grid employing an implicit approximate-factorization algorithm. An evaluation of the accuracy of the computed solutions is presented, and the numerical results are shown to be of sufficient quality to merit physical interpretation. The highly unsteady flowfield structure is described and is found to be in qualitative agreement with available experimental observations. A discussion is provided for the effects of pitch rate and pitch axis location on the induced vortical structures and on the airfoil aerodynamic forces.

Vortex splitting and its consequences in the vortex street wake of cylinders at low Reynolds number

Abstract: Based on the observation of vortex splitting in the laminar wake of thin flat plates placed parallel to the flow, an investigation on the consequences of such events for the von Karman vortex street in the wake of circular cylinders was carried out. It was found that a ‘‘design break line’’ of vortex axes can lead to the decoupling of a wake flow from the always present disturbances deriving from the ends. The decoupling gives rise to parallel vortex shedding of a slightly higher frequency, instead of the oblique or slanted vortex shedding at a lower frequency usually observed.

Structure of Tip Clearance Flow in an Isolated Axial Compressor Rotor

Abstract: Ensemble-averaged and phase-locked flow patterns in various tip clearances of two axial compressor rotors were obtained by aperiodic multisampling technique with a hot wire in the clearance and with a high-response pressure sensor on the casing wall. A leakage flow region distinct from a throughflow region exists at every clearance. In the case of a small tip clearance, the leakage jet flow interacts violently with the throughflow near the leading edge, and a rolling-up leakage vortex decays downstream. As the clearance increases, a stronger leakage vortex comes into existence at a more downstream location, and a reverse flow due to the vortex grows noticeably. A scraping vortex is recognized at the pressure side near the trailing edge only for the small clearance. A horseshoe vortex appears in the upstream half of the through flow region for every tip clearance. The solidity does not affect the flow pattern substantially except for the interaction of the leakage vortex with the adjacent blade and wake.

Existence of steady vortex rings in an ideal fluid

Abstract: We prove the existence of global steady vortex rings in an ideal fluid with given propagation speed W > 0 , flux constant k ⩾ 0 and any bounded, positive, nondecreasing vorticity function.

Phenomenology of Kármán vortex streets in oscillatory flow

Abstract: Vortex wakes of circular cylinders at low Reynolds numbers have been investigated Sound waves are superimposed on the flow in mean flow direction In this configuration the Karman vortices are shed at the sound frequency or at subharmonics of the sound frequency The Karman vortex street is treated as a nonlinear self-excited flow oscillator with forced oscillations Using a flow visualization technique a variety of wake structures has been identified as a function of sound frequency and sound amplitude, but independent of the Reynolds number The superimposed sound influences the distribution of circulation and accordingly the shedding mechanism Primary vortex and secondary vortex are shed simultaneously from one side of the cylinder The alternate vortex shedding is arranged spatially and temporally Structures along the vortex axes are revealed

Vortex ring dynamics at a free surface

Abstract: The results of a flow visualization study of the evolution of vortex rings near a free surface are presented. The vortex rings were formed underwater with their axis parallel to the free surface one ring diameter below the surface. It is shown that vortex lines in the ring open during the interaction with the surface. The resulting flow field consists of vortex lines, beginning and terminating at the free surface. Data are reported on several features of the vortex ring evolution, which show a large reduction of the propagation speed and an oscillation of the vortex eccentricity as it propagates downstream after the interaction.

Convergence of the vortex method for vortex sheets

Abstract: Computation of the evolution of vortex sheets is delicate because of Kelvin–Helmholtz instability and singularity formation (infinite curvature). Convergence of the point vortex method and the vortex blob method is demonstrated for vortex sheets with both spatial and temporal discretization and with simulated roundofl error. The initial data is assumed to be a small analytic perturbation of a flat, uniform sheet. The proof works for a short time interval, certainly less than the first time of singularity formation. The analysis is performed in an analytic function space using the abstract Cauchy–Kowalewski Theorem. A numerical-analytic interpretation of analyticity is given.

On the interaction of vortex rings and pairs with a free surface for varying amounts of surface active agent

Abstract: Observations are reported of the interaction with a free surface of vortex rings and vortex pairs moving normal to the surface when different amounts of surface active agents are present on the surface. At a vortex ring Reynolds number Γ/ν≈3800, the interaction with a contaminated free surface results in the generation of secondary and tertiary vortex rings that limited the outward motion of the vortex ring core. When the experiment was repeated with a cleaner surface the formation of the secondary vortex ring was delayed so that the outward motion and stretching of the vortex ring core was much more than for the contaminated surface. At a Reynolds number Γ/ν≈18 000, the vortex pair was observed to rebound from the free surface contrary to what one would expect for an inviscid flat boundary. When the surface was cleaned by draining away a portion of the contaminated surface water the amount of rebound was reduced. These changes in interaction are believed to be caused by the reduction in concentration of the surface active agent which, in turn, results in a reduced generation of secondary vorticity ahead of the vortex ring or pair before and during the interaction with the surface.

An experimental study of shock wave/vortex interaction

Abstract: The interaction of a supersonic streamwise vortices (of Mach number 2.2, 3.0, and 3.5) with a normal shock wave has been experimentally investigated, and is found to be highly unsteady. Five-hole pressure-probe and temperature measurements ahead of the interaction are used as initial conditions for an axisymmetric Navier-Stokes calculation. The numerical results supports the hypothesis that supersonic vortex breakdown is an important factor in the observed interaction flow pattern.

Interaction of an oscillating vortex with a turbulent boundary layer

Abstract: The effects on mean flow and turbulence caused by meander of a vortex embedded in a two-dimensional boundary layer were investigated experimentally by driving a forced lateral oscillation of the vortex generator. Upstream, the vorticity contours without forcing were found to be round. The forced vortex generator oscillation caused a flattening of the time-averaged vorticity contours and changes in some of the Reynolds stresses. The results indicate that the unforced vortex did not meander significantly upstream, and that the effects of meander can be understood qualitatively from production of Reynolds stresses by the forced motion acting with the exiting three-dimensional velocity field. Farther downstream, the observed differences in the mean vorticity and Reynolds stresses caused by forcing were smaller, mainly because the vortex was substantially diffused at this station, resulting in smaller mean velocity gradients.

Starting flow and structures of the starting vortex behind bluff bodies with sharp edges

Abstract: Experimental investigations were made in a water channel on the starting flow around several bluff bodies with sharp edges: flat plates, circular disks and hollow hemispheres. Details of the flow structures were visualized using the hydrogen bubble technique. Three-fold structures of the starting vortex behind flat plates were observed. The shedding of the vortex sheet from the edge was also studied.

The Low Frequency Oscillation in the Flow Over a NACA0012 Airfoil with an “Iced” Leading Edge

Abstract: The unusually low frequency oscillation in the wake of an airfoil, studied in [1], is explored experimentally as well as computationally for a NACA0012 airfoil with a “glaze ice accretion” at the leading edge. Experimentally, flow oscillations are observed at low frequencies that correspond to a Strouhal number of about 0.02. This occurs in the angle of attack range of 8° to 9°, near the onset of static stall for this airfoil. With a Navier-Stokes computation, “limit-cycle” oscillations in the flow and in the aerodynamic forces are also observed at low Strouhal numbers. However, the occurrence of the oscillation is found to depend on the turbulence model in use as well as the Reynolds number.

Flow visualization study of delta wings in wing-rock motion

Abstract: Water tunnel flow visualizations have been conducted for the wing-rock phenomenon in the cases of models having different leading-edge sweeps and roundness. Wing rock, which is noted to occur in the absence of asymmetric vortex liftoff, vortex breakdown, and static hysteresis, is seen to be initiated by flowfield asymmetries that are induced by flow disturbances and vortex interactions near the apex region; one vortex is thereby strengthened while the other is weakened, inducing a roll moment. Leading-edge roundness decreases vortex strength and reduces vortex spacing, reducing the tendency to wind rock and coupled vortex motions at higher angles-of-attack.

The vortex ring merger problem at infinite reynolds number

Abstract: : The physics of three-dimensional incompressible fluid flow, as is well known, is an extremely difficult and not well understood subject. The mathematical theory of the Navier-Stokes and the Euler equations is incomplete and a detailed qualitative understanding of the dynamics is, for the most part, lacking. One focus of recent research which aims at an understanding of key features of turbulence and of the possible breakdown in regularity of solutions of the fluid equations has been on the motion of vortex filaments. Motivations for the study of vortex filaments are the prevalence of thin vortex tubes in experimentally observed flows, the fact that the Euler equations can be thought of as an evolution equation for a continuum of interacting vortex filaments, and a view of turbulence as being characterized by wildly stretching and dissipating vortex filaments. A vortex flow problem which has been the subject interesting laboratory and numerical experiments is the vortex ring merger problem. In this problem the evolution over a short interval of time of two initially parallel (or slightly inclined) co-rotating vortex rings of the same strength is studied. The rings are observed to come together and reconnect, in the sense that much of the vorticity field becomes composed of vortex lines which join the two initially distinct vortex rings. This merger occurs on a timescale which is much shorter than that expected from a simple dimensional analysis based on the magnitude of viscosity and on the vortex ring radius.(kr)

Interaction of a vortex ring and a laminar flame

Abstract: The present paper reports the results of an experimental study of the interaction of a vortex ring (or more appropriately, a vortex bubble) with a laminar flame. Vortex bubbles of various diameters are generated in a vortex generator driven by a loudspeaker and propagate downwards in a vertical flame tube to interact “head on” with a laminar flame. Different combustible mixtures can be used in the flame and the vortex generator. Successful ignition of a combustible vortex bubble is only possible if the spark is located at the vortex core of the bubble and not elsewhere. This suggests that the tangential velocity gradient causes the flame to be quenched by the flame stretch mechanism as in a stagnation flow. In the penetration of the vortex bubble into the hot products behind the flame, ignition is never observed to occur at the bubble surface. Instead the hot products are entrained into the bubble and enter through the rear stagnation region. Ignition occurs when sufficient hot products have been entrained and mixed with the combustible gases in the bubble. Ignition is observed to originate near the bubble axis and spread radially outward to be entrained into the vortex core. Measurements of the entrainment time (time between the entrance of the bubble into the hot products and ignition) indicate that the entrainment time is very weakly dependent on the vortex circulation in accord with the observation of Maxworthy of non-reating vortex rings. Combustion time (period between ignition and complete burning of thevortex bubble) is found to be independent of the mixture composition and is of about the same, order of magnitude as the entrainment time.

Vortex flowmeter with inertially balanced vortex sensor

Abstract: A vortex shedding flowmeter comprises a vortex generating bluff body disposed across a first cross section of a flow passage and an inertially balanced vortex sensing planar member disposed across a second cross section of the flow passage parallel to the vortex generating bluff body on a plane parallel to the central axis of the flow passage in a pivotable arrangement about a pivot axis generally coinciding with the central axis of the flow passage, wherein one extremity of the vortex sensing planar member is connected to a transducer converting an alternating torque about the pivoting axis generated by vortices shed from the vortex generating bluff body and experienced by the vortex sensing planar member to an alternating electromotive force providing information on the flow rate of the fluid moving through the flow passage.

On leading edge vortex and its control

Abstract: A simple model for the leading edge vortex and a postulation for the existence of a critical vorticity concentration above which a stable leading edge vortex cannot be maintained were proposed. Using the model and postulation, various aspects of vortex control by blowing were discussed. Blowing can be envisioned as a means to control the effective sweep and span of a delta wing by redistributing the vorticity. Mixing between the blowing jet and the vortex has a major effect on the effectiveness of controlling the vortex. Different positioning of one vortex relative to another can either enhance or delay the breakdown of the vortices.

Laboratory Measurements of Axial Pressures in Two-Celled Tornado-like Vortices

Abstract: An experimental study of two-celled vortex flows was conducted in a Ward-type tornado vortex chamber (TVC). Time-averaged, stream-static pressure measurements on the vortex axis and observations of the visualized flow in two-celled vortices are reported. The static pressure measured on the vortex axis increases with height downstream of the vortex breakdown, with the axial pressure gradient tending to zero only well aloft. Visualization indicates that the flow downstream of the breakdown in the TVC is everywhere two-celled, with the strongest axial downflow occurring at middle levels. Analysis of the vertical momentum equation strengthens the argument that the turbulent stresses can play an important role in the axial momentum balance of two-celled vortices by opposing the “filling” of the vortex core with higher stagnation pressure fluid from aloft, therefore helping to maintain low pressure and high velocities near the surface.

Response of a filamentary vortex to sound

Abstract: The motion of a test vortex filament induced by a sound wave of frequency high in comparison with the vortex time scales, at low Mach numbers, is studied. To leading order the filament moves with the local speed of the sound wave. The small but finite compressibility introduces retardation effects that modify this behavior: for example, there is a self‐induction effect for a point vortex. A differential equation, depending on one cutoff parameter that quantifies this assertion both in two and three dimensions, is derived in a localized induction approximation. This is done by use of a variational principle that governs inviscid irrotational adiabatic flow. The velocity and pressure, generated by the vortex, are expressed in terms of a multivalued velocity potential, as convolutions of a Green’s function and a source localized on the filament. Substitution of these expressions into the action, whose extrema give the equations of fluid flow, renders it a functional of the vortex filament history. Its extrema, with respect to variations of this history, determine the equation of vortex evolution. The multivalued velocity potential is used to recast the usual theory of vortex sound. Incompressible vortex dynamics is used as an example to introduce and illustrate these methods.

Experimental Investigation of the Parallel Vortex-Airfoil Interaction at Transonic Speeds

Abstract: Unsteady vortex-airfoil interaction experiments at transonic Mach numbers ranging from 0.7 to 0.85 and airfoil chord Reynolds numbers of 3.5 X 106 to 5.5 x 10 6 were conducted in the University of Texas at Arlington high Reynolds number transonic wind-tunnel facility. The experiments were designed to simulate a two-dimensional blade-vortex interaction problem frequently encountered in rotocraft applications. The interaction experiments involved positioning a two-dimensional vortex generator upstream of a NACA 0012 airfoil section and impulsively pitching the vortex generator airfoil such that the starting vortex interacts with the downstream airfoil. The vortex generator was pitched about its 0.25 chord position with nondimensional pitch rates in the range of 0.004-0.008. Experiments were conducted at several vortex core positions above and below the downstream airfoil. The results indicate a substantial change in the pressure distribution over the leading 30% of the interacting airfoil. Experimental data for Mach numbers representing supercritical flows over the airfoil resulted in a very strong interaction of the vortex and the airfoil shock wave; at close encounters, these interactions resulted in unsteady local flow separation of the leading 40970 of the airfoil chord. Experiments with stronger vortices at supercritical Mach numbers resulted in a forward propagation of the shock wave. The pressure distribution of the downstream airfoil was not sensitive to Reynolds number variations for the range of Reynolds numbers simulated in the test program.

Trapped vortex cavity

Abstract: An airfoil having a trailing edge member pivotally connected to a leading edge member. The trailing edge member having a vortex generating cavity that can be exposed or concealed from a stream of fluid flowing over the outer surface of the members.

Flow visualization studies of the Mach number effects on the dynamicstall of an oscillating airfoil

Abstract: Compressibility effects on dynamic stall of a NACA 0012 airfoil undergoing sinusoidal oscillatory motion were studied using a stroboscopic schlieren system. Schlieren pictures and some quantitative data derived from the are presented and show the influence of freestream Mach number and reduced frequency on the dynamic stall vortex

Computational aerodynamics of ordered vortex flows

Abstract: The physical aspects, the mathematical modeling, the development and application of computational methods for the simulation of vertical flows are described The high-Reynolds number vortex flows considered are assumed to be steady, sub-critical and such that the structure of the flow is well-ordered and consists of thin shear layers and slender vortex cores embedded in otherwise irrotatlonal flow The applications considered are vortex wakes and the flow about three-dimensional (slender) configurations with the flow separating from known separation lines, mostly the sharp leading edges