Vortex Velocity Profiles Research Articles

Thickness Effect on the Thrust Generation of Heaving Elliptic Airfoils

The influence of the Reynolds number on the propulsive characteristics of insect wings is investigated by focusing on the aerodynamic transition from drag to thrust. The effect of both the thickness ratio and the Reynolds number on the thrust generation of elliptic airfoils is investigated using a lattice Boltzmann method. Three Reynolds numbers (Re = 50,100, and 185) and two Strouhal numbers (Sr = 0.2 and 0.4) are treated, while the thickness ratio is varied from 0.05 to 1.0. For all investigated Reynolds numbers, it is found that the airfoils oscillating at Sr = 0.2 do not produce thrust. For Sr = 0.4, it is found that thrust is produced at Re = 185. It is found that an airfoil of approximately 10% thickness produces maximum thrust. Thus, it can be said that, for thrust generation, there exists critical Reynolds and Strouhal numbers, and the thickness ratio is also a crucial parameter. By investigating the vortex pattern, velocity profiles, and vorticity intensities of the vortex cores behind the oscillating airfoils, it is found that 1) mushroom vortex patterns indicative of thrust do not completely guarantee the thrust generation, and 2) a phase lag between the thrust-force indicating vortex pattern and the resulting thrust production is observed. The present investigation shows that thrust-producing airfoils should produce strong vortices, enough to overcome the momentum deficit due to the boundary layer. Present results are obtained within the limitation of the laminar flow assumption.

Measurements of the velocity field of a wing-tip vortex, wandering in grid turbulence

Velocity measurements were performed in a wing-tip vortex wandering in free-stream turbulence using two four-wire hot-wire probes. Vortex wandering was well represented by a bi-normal probability density with increasing free-stream turbulence resulting in increased amplitude of wandering. The most dominant wavelength of wandering was found to remain unaffected by free-stream conditions. Two-point velocity measurements were used to reconstruct the vortex velocity profile in a frame of reference wandering with the vortex. Increasing turbulence intensity was found to increase the rate of decay of the vortex peak circumferential velocity while the radial location of this peak velocity remained unchanged. These results are consistent with several possible vortex decay mechanisms, including the stripping of vorticity by azimuthally aligned vortical structures, transfer of angular momentum from the vortex to these structures during their formation and the deformation and breakup of the vortex by strong free-stream eddies.

High Resolution Wake Capturing Methodology for Hovering Rotors

A high-resolution Reynolds-averaged Navier‐Stokes (RANS) solver is applied to study the evolution of tip vortices from rotary blades. The numerical error is reduced by using high-order accurate schemes on appropriately refined meshes. To better resolve the vortex evolution, the equations were solved on multiple overset grids that ensured adequate resolution in an efficient manner. For the RANS closure, a one equation wall-based turbulence model was used with a correction to the production term to account for the stabilizing effects of rotation in the core of the tip vortex. While experimental comparison of the computed vortex structure beyond a few chord lengths downstream of the trailing edge is lacking in the literature, reasonable validation of the vortex velocity profiles is demonstrated up to a distance of 50 chord lengths of evolution for a single-bladed rotor. For the two-bladed rotor case, the tip vortex could be tracked up to two rotor revolutions with minimal diffusion. The accuracy of the computed blade pressures and vortex trajectories confirm that the inflow distribution and blade-vortex interaction are represented correctly. The accuracy achieved in the validation studies establishes the viability of the methodology as a reliable tool that can be used to predict vortex evolution and the aerodynamic performance of hovering rotors.

Critical wake vortex encounter scenarios

A simulation-based method was developed to investigate the severity of Wake Vortex Encounters (WVEs). This paper describes an important part of this method: the determination of worst-case WVE conditions, which is referred to as Worst-Case Search (WCS). The WCS results permit to reduce time and costs of WVE severity related piloted simulator tests and allow the comparison of the hazard that vortices of different generator aircraft exert on a follower aircraft. The WCS is based on a high fidelity, offline simulation of the follower aircraft that includes the interacting wake vortex, a hazard criterion that rates the severity of each WVE, and a pilot model. It can be formulated as an optimisation problem that is solved with the optimisation tool MOPS (Multi Objective Parameter Synthesis). MOPS varies the encounter geometry until the simulation yields maximum values of the WVE hazard criterion. Worst-case encounter conditions for different parameters were investigated and sensitivity studies performed. The influence of the height above ground, the core radius, and the models for the wake vortex velocity profiles on the severity of a WVE was examined. To show the capability of the method, a comparison of the severity of a WVE behind two different heavy transport aircraft for a 20 t aircraft was made. The WCS method demonstrated its applicability and delivered the worst-case encounter geometry.

Encounters with Aircraft Vortex Wakes: The Impact on Helicopter Handling Qualities

A study concerning the risks associated with locating a helicopter final approach and takeoff area alongside an active runway at a busy international airport is documented. Specifically, by the use of desktop simulation, the problem is addressed of an in-flight helicopter encountering the shed wing tip vortex from a large transport aircraft. Measurements are reviewed of vortices taken using coherent laser radar equipment located under the final approach path to London Heathrow Airport to establish a model of the vortex velocity profile. The simulations have shown the helicopter can experience transient pitch attitude changes of up to 40 deg and accumulated descent rates of 1500 ft/min (457 m/min) during the encounter, assuming that no pilot intervention is made. Established handling qualities metrics have been adapted and suggest that these responses may produce handling difficulties. The simulations were also broadened to consider a range of helicopter designs and the effects of shed vortices from a number of large transport aircraft

Liquid recirculation and bubble breakup beneath ventilated gas cavities in downward pipe flow

The dispersion of bubbles into down-flowing liquids is often encountered in a number of industrial applications involving pipe flow, bubble columns and loop reactors. Usually a gas horizontal sparging device is used to generate bubbles that are carried downward with the bulk liquid flow. At low gas flowrates discrete bubbles are formed. However, at higher gas flowrates a ventilated cavity attached to the sparger is formed. For downward pipe flow the liquid forms an annular jet, which entrains gas into the recirculation region immediately beneath the ventilated cavity. The rate of gas entrainment and the size of the bubbles produced is determined by the pipe diameter, liquid and gas volumetric flowrates and the strength of the recirculation region below the base of the ventilated cavity. In this study a model was developed to predict the liquid velocity field and bubble breakup in the recirculation region. The velocity profile was modelled using the potential flow solution of the Hill's vortex, where the strength of the vortex was assumed to be directly proportional to the velocity of the annular wall jet. The proportionality constant was found to be 0.38, based on predictions obtained using the commercial code CFX. The CFX velocity profile predictions for the central part of the recirculation region were very similar to the Hill's vortex velocity profile. Bubble breakup was modelled using a critical Weber number concept, based on the predicted velocity profile within the recirculation region. It was found that the prediction of bubble size was in general agreement with experimental observations when a critical Weber number of 4.7 was assumed. A digital high-speed video was used to observe the liquid and bubble motion at the base of the ventilated cavity. The video was used to obtain estimates of the recirculating liquid flow velocity, which compared reasonably well with predictions based on the Hill's vortex model. The video evidence also highlighted the unsteady nature of the flow, particularly the actual gas entrainment process, and possible reasons for this behaviour are presented.

Spatially Resolved Dynamic Correlation in the Vortex State of High Temperature Superconductors

We report transport measurements in the mixed state of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals using a contact geometry which enables control of the gradient of the Lorentz force acting on the vortices. The spatial dependence of the vortex velocity profile in the liquid and solid vortex phases is resolved providing information about the transverse dynamic correlation of vortices. While in the liquid phase the vortices respond locally to the driving force, in the solid regime they show long range velocity correlation over macroscopic distances. At high driving force gradients, vortex-vortex shear stresses exceed the elastic limit and the dynamics is characterized by plastic motion.

Coherent Laser Radar and the Problem of Aircraft Wake Vortices

Abstract Aircraft wake vortices have been studied with a 10·6 μm coherent c.w. lidar. Good agreement is found between the form of experimentally observed Doppler spectra and theoretical calculations and modelling. Detailed measurements of spectra have yielded core radii, tangent velocities and vortex velocity profiles which are in reasonable accord with theoretical formulations. During a two week trial over 130 aircraft passages were recorded with the lidar equipment. Even in low signal to noise conditions vortex spectra were recorded on most occasions at a level above expectation. The characteristic cusp-like features of such spectral records are illustrated with a novel colour display.

A general formulation for the decay of swirling motion along a straight pipe

This paper describes an analytical approach, based on the conservation of angular momentum flux, to predict the decay of swirling motion due to frictional effects along a straight duct. A combined forced vortex and shear-type velocity profiles were used and the analysis could be applicable to any types of swirling flow. Good agreement with measurements in a straight pipe with different level of inlet swirl intensities (Kitoh O.; J. Fluid Mechanics, 1991) can be obtained with the approximate solution. The present formulation can also demonstrate that the swirling motion along a straight pipe does not always decay exponentially but depending on inlet conditions and the variation of friction coefficient with downstream distance.

Flow visualization of compressible vortex structures using density gradient techniques

Mathematical results are derived for the schlieren and shadowgraph contrast variation due to the refraction of light rays passing through two-dimensional compressible vortices with viscous cores. Both standard and small-disturbance solutions are obtained. It is shown that schlieren and shadowgraph produce substantially different contrast profiles. Further, the shadowgraph contrast variation is shown to be very sensitive to the vortex velocity profile and is also dependent on the location of the peak peripheral velocity (viscous core radius). The computed results are compared to actual contrast measurements made for rotor tip vortices using the shadowgraph flow visualization technique. The work helps to clarify the relationships between the observed contrast and the structure of vortical structures in density gradient based flow visualization experiments.

Laboratory simulation of rotating atmospheric boundary layer flows over obstacles

The present study fits in the frame of a research program concerning in general the dynamics of airflow in the atmospheric boundary layer and in particular the influence of terrestrial rotation on the movements of air masses interacting with natural extended obstacles (mountains). The experiment has been performed by the method of hydraulic simulation, using schematic models at reduced scale in a channel placed on a rotating platform. We only considered the case of a neutral atmosphere and studied the wake of an obstacle with semi-circular section and the reciprocal interaction of two obstacles of this kind placed perpendicularly to the flow. In this last case we investigated the influence of the distance between the obstacles on their wake length and on the vorticity conditions inside the wakes. Among the various results we obtained, the modifications of the velocity profiles over the reliefs and their dependence on the rotation velocity are particularly interesting. We discuss here our results from two different points of view, namely the purely hydraulic one (which includes the effects of different rotation velocities) and the atmospheric one, according to which the model simulates—with given reduction scales—an actual situation characterized by a fixed value of the Coriolis parameter. As to the first approach to the problem, we found that: 1) The roll with horizontal axis, which is observed behind an obstacle, becomes narrower and narrower as the rotation velocity of the platform increases, while its stability in time and its definition in space increase. In general, it may be said that rotation plays a stabilizing role for vortex dimensions and velocity profiles. 2) The transversal velocity behind the obstacles may attain values about twice the mean longitudinal velocity of the flow. 3) When rotating, the roll is thicker at its left edge than on the right one, due to the transversal flux which provides fluid supply. 4) When the thickness of the boundary layer is increased by making the channel bottom rough, the above-mentioned phenomena are emphasized; moreover, with a second obstacle placed in the flow (and not too far from the first) the transversal velocity components increase, due to a canalization effect. 5) The accelerations of the low layers over the obstacles are strongly amplified by rotation, due to the action of Coriolis' force. As for the second approach, we checked the extent to which the simulation procedures adopted for our laboratory flows, and for their boundary conditions, can be representative of the features of atmospheric phenomena. In order to do that, we compared the dynamical structure of the low flow layers over the obstacle with the analogous structure observed in the field and in wind tunnel by other authors, as well as with the predictions of a few theoretical models. Inside the lowest part of the planetary boundary layer, where the overshooting due to the relief is confined, a good consistency was found among all these results, in particular for what concerns the maximum of overflow velocity.

Measured Wake-Vortex Characteristics of Aircraft in Ground Effect

In support of the NASA wake vortex alleviation program, measurements were made of the influences of a ground plane on vortex trajectories and velocity profiles within lift-generated wakes. The wakes were generated by towing 0. 61-m (2-ft) span models of a B-747 and DC-10-30 under water in a ship model basin. The models were configured with landing flaps and flight spoilers to investigate the wake characteristics of these aircraft in ground effect at simulated full-scale distances of 19-116 m (62-380 ft) above the ground. The ground plane caused modifications in the vortex trajectories but did not alter vortex interactions and merging patterns in these multiple vortex wakes. Some distortions in vortex vertical (tangential) velocity profiles were recorded as a result of vortex lateral motions and vortex interactions with the viscous boundary layer on the ground plane; however, maximum tangential velocities remained unchanged.

A laboratory model of dust devil vortices

Abstract The results of a laboratory investigation of the dust devil vortex are presented. Vortex velocity and temperature profiles were measured for various initial conditiions of boundary heat input and outer flow swirl. Three different categories of core structure were observed depending on the amount of axial flow force and circulation strength produced in the experiment and dust devil models are constructed based on the measured mean behavior of each core category.

Vortex Measurements Behind a Swept Wing Transport Model

A backscatter laser Doppler velocimeter which simultaneously senses the axial and the tangential components of the velocity has been used to measure the velocity distributions in the near wake of a swept wing semispan transport model in a wind tunnel. The model configuration included nacelles, pylons, antishock bodies, and wing flaps which could be deflected 27 deg. Typical wake vortex velocity profiles are presented for the flaps-retracted and the flaps-deployed 27 deg configurations, respectively.

Evaluation of Pipe Elbows as Flow Meters

A number of flanged, 90○ commercial elbows of several sizes, ranging from 3 in. to 12 in. in diameter are calibrated as elbow flow meters. The accuracy to which a calibration equation can be predicted using only the nominal elbow size and the average calibration results of several similar elbows is determined. This accuracy can be improved if precise measurements of the elbow diameter and radius of bend are obtained and incorporated into the calibration equation. A relatively simple and inexpensive method for accurately determining the radius of bend with casting plaster is presented. The effects of installing the elbow flow meters in the proximity of other piping components are investigated and a modification of the classical vortex velocity profile analysis for elbow flow meters is presented.