Showing papers on "Starting vortex published in 1980"
TL;DR: In this article, the authors have described the nature of the horseshoe vortex system and measured the pressure distribution under the vortex system, and the variation of the vortex and boundary layer separation position with the flow parameters.
285 citations
TL;DR: In this paper, the change in kinetic energy between the initial and final states of the rollup of an infinite vortex sheet into an array of uniform vortices is calculated, and an upper limit on the pitch-to-diameter ratio of the vortex array is found.
Abstract: The change in kinetic energy between the initial and final states of the rollup of an infinite vortex sheet into an array of uniform vortices is calculated From the result, an upper limit on the pitch‐to‐diameter ratio of the vortex array is found This contrasts with a lower limit found from stability considerations by Moore and Saffman By extending the calculation to the case of elliptical vortices, it is found that a combination of energy and stability considerations is able to predict, qualitatively, a number of the observed features of the free shear layer
195 citations
TL;DR: In this paper, a simple dye-in-water method has been used to visualize the growth of a two-dimensional starting flow vortex formed at a wedge-like sharp edge, and several cases were tested corresponding to different wedge angles and to different values of the time exponent in the velocity-time power law describing the starting flow.
Abstract: A simple dye in water method has been used to visualize the growth of a two-dimensional starting flow vortex formed at a wedge-like sharp edge. Several cases were tested corresponding to different wedge angles and to different values of the time exponent in the velocity–time power law describing the starting flow. Photographic sequences showing the time-wise primary vortex growth are presented from which various secondary-flow details are identified. For the larger wedge angles these include a strong secondary vortex and in some cases a small separation bubble-like flow region immediately adjacent to the wedge apex. For a thin-wedge model the formation of what might be interpreted as small rotation centres along the outer turns of the primary-vortex shear layer is observed but these are not seen as a manifestation of an instability phenomenon in the fluid. Measurements of the trajectories of the primary-vortex centre are compared with the predictions of an inviscid similarity theory of the vortex growth. Although the appropriate Reynolds number in the present experiments was relatively low, comparison between theory and experiments is regarded as reasonable with differences being attributed to viscous effects absent in the similarity theory, and also to apparatus wall effects.
118 citations
TL;DR: The role of large vortex structures in the evolution of a two-dimensional shear layer is studied numerically in this paper, where the motion of up to 4096 vortices is followed on a 256 x 256 grid using the cloud-in-cell algorithm.
Abstract: The role of large vortex structures in the evolution of a two-dimensional shear layer is studied numerically. The motion of up to 4096 vortices is followed on a 256 x 256 grid using the cloud-in-cell algorithm. The scaling predictions of self-preservation theory are confirmed for low-order velocity correlations, although the existence of vortex structures produces large fluctuations even in a simulation of this size. The simple picture of the shear layer as a line of vortex blobs, that merge painvise thus thickening the layer, is not seen. On the contrary, the layer seems to thicken by the scattering of vortex structures of roughly fixed size about the midline. The size of the vortex structures does not scale with the layer thickness. A study of the entrainment of a passive marker shows that flow visualization experiments may have overestimated the size of the vortex structures. It appears that the finite area vortices have time to equilibrate between mergings, and the consequences of applying equilibrium statistical mechanics to their internal structure are explored. A simple model is presented which demonstrates how the size and separation of vortex structures may lock into a fixed ratio. This is precisely the type of mechanism that is needed to produce simple scaling in a flow that has initially several distinct length scales. Anumber of consistency checks on the numerical results are performed. In particular, the evolution of the same vortex configuration on two grids of different size is compared. This test showed that, although errors on subgrid scales do propagate to small wavenumbers, the dominant wavenumber of vorticity cascades back ahead of the peak in the error spectrum.
93 citations
TL;DR: In this paper, the development of a vortex sheet due to an initially sinusoidal disturbance is calculated and the time step was determined by the requirement that a quantity (Hamiltonian) that remains invariant as a result of the equations of motion, should not change more than a certain amount in the numerical integration of the equation of motion.
Abstract: In this paper the development of a vortex sheet due to an initially sinusoidal disturbance is calculated. When determining the induced velocity in points of the vortex sheet, it can be represented by concentrated vortices but it is shown that it is analytically more correct to add an additional term that represents the effect of the immediate neighbourhood of the point considered. The equations of motion were integrated by a Runge-Kutta technique to exclude numerical instabilities. The time step was determined by the requirement that a quantity (Hamiltonian) that remains invariant as a result of the equations of motion, should not change more than a certain amount in the numerical integration of the equations of motion. One difficulty is that if a greater number of concentrated vortices are introduced to represent the vortex sheet, the effect of round-off errors becomes more important. The number of figures retained in the computations limits the number of concentrated vortices. Where the round-off errors have been kept sufficiently small, a process of rolling-up of vorticity clearly occurs. There is no point in pursuing the calculations much beyond this point, first because the representation of the vortex sheet by concentrated vortices becomes more and more inaccurate and secondly because viscosity will have the effect of transforming the rolled-up vortex sheet into a region of vorticity.
53 citations
TL;DR: In this article, the authors investigated the relationship between vortex structure and the intensity of the acoustic signal, and numerical results were presented for the unsteady lift and acoustic signal due to blade/vortex interaction.
Abstract: A potential cause of helicopter impulsive noise, commonly called blade slap, is the unsteady lift fluctuation on a rotor blade due to interaction with the vortex trailed from another blade. The relationship between vortex structure and the intensity of the acoustic signal is investigated. The analysis is based on a theoretical model for blade/vortex interaction. Unsteady lift on the blades due to blade/vortex interaction is calculated using linear unsteady aerodynamic theory, and expressions are derived for the directivity, frequency spectrum, and transient signal of the radiated noise. An inviscid rollup model is used to calculate the velocity profile in the trailing vortex from the spanwise distribution of blade tip loading. A few cases of tip loading are investigated, and numerical results are presented for the unsteady lift and acoustic signal due to blade/vortex interaction. The intensity of the acoustic signal is shown to be quite sensitive to changes in tip vortex structure.
31 citations
TL;DR: In this article, an extensive experimental investigation of the symmetric body vortex wake was conducted, where cone probe measurements were made on the leeside of an ogive nose circular cylinder for three different supersonic freestream conditions.
Abstract: An extensive experimental investigation of the symmetric body vortex wake was conducted. Cone probe measurements were made on the leeside of an ogive nose circular cylinder for three different supersonic freestream conditions. Measurements of total pressure, Mach number, and three orthogonal velocity components were made at four angles of attack of the body at various axial stations. In the present paper these data are processed to infer the position of the primary body vortex and vortex feeding sheet in the cross-flow plane, local circulation distribution in the cross-flow plane, vortex core size, wake height, and total circulation in the cross-flow plane. A detailed discussion of the results and data processing is presented.
21 citations
Patent•
22 Feb 1980
TL;DR: In this paper, a columnar vortex shedding member is detected by a detector whose output electrical signal is compared by a comparator (20) with a slice level signal, to generate a pulse train which is synchronous with the vortex shedding frequency.
Abstract: A Karman vortex type flow measuring apparatus comprises a conduit (2) for a fluid whose flow rate or flow velocity is to be measured, and a columnar vortex shedding member (3) extending within the conduit transversely of the fluid flow direction. Frequency of the vortex shedding is detected by a detector (6) whose output electrical signal is compared by a comparator (20) with a slice level signal, to generate a pulse train which is synchronous with the vortex shedding frequency. The slide level is variable in response to fluctuation in the output electrical signal of the detector (6). A highly accurate measurement can be achieved even when the vortex shedding is unstable.
12 citations
01 Sep 1980
TL;DR: VORLAX program was developed to incorporate direct extension of vortex lattice method into supersonic flow regime, thus providing analyst with full flow range capability.
Abstract: Several variations of vortex lattice method that are currently available have proved practical and versatile theoretical tools for aerodynamic analysis and design of planar and nonplanar configurations. Success of method is due in great part to relative simplicity of numerical technique involved and to accuracy of results obtained; however, most of available procedures are for subsonic flow applications. VORLAX program was developed to incorporate direct extension of vortex lattice method into supersonic flow regime, thus providing analyst with full flow range capability.
7 citations
TL;DR: In this paper, two-dimensional transient and quasi-steady separated flows behind a flat plate and a circular cylinder are simulated by inviscid vortex models using a discrete-vortex approximation for free shear layers.
Abstract: Two-dimensional transient and quasi-steady separated flows behind a flat plate and a circular cylinder are simulated by inviscid vortex models using a discrete-vortex approximation for free shear layers. The features of the models are discussed through the comparisons of the numerical results with experimental data.Light is thrown on the time development of wake in transient flow and on a new method for the calculation of vortex shedding from a circular cylinder. The method is developed through the consideration of the treatment of Kutta condition which determines the circumstances of vortex shedding from body surface, on the assumption of the location of the separation point from which free vortices are shed.The time development of wake not only of a flat plate, but of a circular cylinder in transient flow for short times after their starts can be explained with inviscid vortex models.The method proposed here is shown to predict, for circular cylinder, the vortex patterns, the drag, the side force and the Strouhal number to a good degree of agreement with the experimental results.
TL;DR: In this article, the authors use flow visualization to examine the end-wall boundary layer as it passes through a rotating blade row of an axial-flow compressor; they find that concentrations of vorticity, such as the trailing vortex shed from the blade tip, or the horseshoe vortex from the leading edge, are swamped and rapidly dispersed by the large-scale motions in the turbulent endwall layer.
TL;DR: In this paper, exact analytical expressions of induced velocity components at any field point due to a single vortex ellipse and due to an elliptic cylinder of uniform strength with a finite and a semi-infinite length are presented.
Abstract: Exact Analytic expressions are presented of the induced velocity components at any field point due to a single vortex ellipse and due to a vortex elliptic cylinder of uniform strength with a finite and a semi-infinite length. The elliptic cylinder is assumed to deviate very little from the reference circular cylinder. Comparison is made between the derived expressions and those due to a semi-infinite vortex circular cylinder, and sample calculations are presented. This investigation has a direct application to the case of nonaxisymmetric annular aerofoils of elliptic cross sections.
TL;DR: In this paper, the decay parameter at which the similarity solution ceases to exist is computed by a more direct method, and the reason for the breakdown becomes apparent, which confirms and slightly improves the known value of the parameter.
TL;DR: In this article, the formation of vortex street from free shear layers emanating from two-dimensional bluff bodies placed in uniform shear flow which is a model of a laminar boundary layer along a solid wall is studied.
Abstract: Main features of the formation of vortex street from free shear layers emanating from two-dimensional bluff bodies placed in uniform shear flow which is a model of a laminar boundary layer along a solid wall. This problem is concerned with the mechanism governing transition induced by small bluff bodies suspended in a laminar boundary layer. Calculations show that the background vorticity of shear flow promotes the rolling up of the vortex sheet of the same sign whereas it decelerates that of the vortex sheet of the opposite sign. The steady configuration of the conventional Karman vortex street is not possible in shear flow. Theoretical vortex patterns are experimentally examined by a flow-visualization technique.
TL;DR: In this article, the radial density of a vortex in the compressible flow is evaluated with a simple model and it is shown that the width of a low pressure channel on the axis of the nozzle is comparable to the theoretical values.
Abstract: The effect of a vortex in a gas flow on an air-blast arc is investigated. The radial density of a vortex in the compressible flow is evaluated with a simple model. The experiments show that the width of a low pressure channel on the axis of the nozzle is comparable to the theoretical values. The measured electric field strength profile is strongly influenced by the presence of such a vortex. In addition, the thermal interrupting capability is drastically lowered by vortex superimposed on the axial gas flow.
01 Aug 1980
TL;DR: In this paper, the authors modified the Langley-MCARF program for attached flow to accept the free vortex sheet separation-flow model program (Analytical Methods, Inc.-CLMAX) and incorporated viscous effects into the calculation by representing the boundary layer displacement thickness with an appropriate source distribution.
Abstract: The multi-component airfoil program (Langley-MCARF) for attached flow is modified to accept the free vortex sheet separation-flow model program (Analytical Methods, Inc.-CLMAX). The viscous effects are incorporated into the calculation by representing the boundary layer displacement thickness with an appropriate source distribution. The separation flow model incorporated into MCARF was applied to single component airfoils. Calculated pressure distributions for angles of attack up to the stall are in close agreement with experimental measurements. Even at higher angles of attack beyond the stall, correct trends of separation, decrease in lift coefficients, and increase in pitching moment coefficients are predicted.
TL;DR: In this paper, a theoretical model is examined to determine the influence of a slot in a thin airfoil on the unsteady lift caused by vortices shed into the wake.