Vortex Detection Research Articles

A Variational Method for Detecting and Characterizing Convective Vortices in Cartesian Wind Fields

Abstract Vortex detection algorithms are required for both research and operational applications in which data volume precludes timely subjective examination of model or analysis fields. Unfortunately, objective detection of convective vortices is often hindered by the strength and complexity of the flow in which they are embedded. To address this problem, a variational vortex-fitting algorithm previously developed to detect and characterize vortices observed by Doppler radar has been modified to operate on gridded horizontal wind data. The latter are fit to a simple analytical model of a vortex and its proximate environment, allowing the retrieval of important vortex characteristics. This permits the development of detection criteria tied directly to vortex properties (e.g., maximum tangential wind), rather than to more general kinematical properties that may poorly represent the vortex itself (e.g., vertical vorticity) when the background flow is strongly sheared. Thus, the vortex characteristic estimates provided by the technique may permit more effective detection criteria while providing useful information about vortex size, intensity, and trends therein. In tests with two simulated supercells, the technique proficiently detects and characterizes vortices, even in the presence of complex flow. Sensitivity tests suggest the algorithm would work well for a variety of vortex sizes without additional tuning. Possible applications of the technique include investigating relationships between mesocyclone and tornado characteristics, and detecting tornadoes, mesocyclones, and mesovortices in real-time ensemble forecasts.

Inter-blade flow analysis of a propeller turbine runner using stereoscopic PIV

The paper presents the averaged velocity field inside the inter-blade channel of a propeller turbine runner measured using a stereoscopic particle image velocimetry (SPIV) technique. In this manner measurements have been performed without any modification of the flow patterns, with the averaged three-dimensional velocity fields reconstructed from phase-averaged acquisition data based on the blade azimuth. Main and secondary flows were analyzed for nine operating conditions, ranging from part to full load. The radial velocities and gradients play major roles in the inter-blade flow development. Using the λ2-definition for vortex detection, leading edge vortices were detected and identified under part load conditions.

Experimental study on the effect of pulsating inflow to an enclosure for improved mixing

Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ=0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.

Numerical simulation of shock–vortex interaction in Schardin’s problem

Shock diffraction over a two-dimensional wedge and subsequent shock–vortex interaction have been numerically simulated using the AUSM\(+\) scheme. After the passage of the incident shock over the wedge, the generated tip vortex interacts with a reflected shock. The resulting shock pattern has been captured well. It matches the existing experimental and numerical results reported in the literature. We solve the Navier–Stokes equations using high accuracy schemes and extend the existing results by focussing on the Kelvin–Helmholtz instability generated vortices which follow a spiral path to the vortex core and on their way interact with shock waves embedded within the vortex. Vortex detection algorithms have been used to visualize the spiral structure of the initial vortex and its final breakdown into a turbulent state. Plotting the dilatation field we notice a new source of diverging acoustic waves and a lambda shock at the wedge tip.

Sidelobe-modulated optical vortices for free-space communication

We propose and experimentally demonstrate a new method for free-space optical (FSO) communication, where the transmitter encodes data into a composite computer-generated hologram and the receiver decodes through a retrieved array of sidelobe-modulated optical vortices (SMOVs). By employing the SMOV generation and detection technique, the usual stringent alignment and phase-matching requirement of the detection of optical vortices is released. In transmitting a gray-scale picture with 180×180 pixels, a bit error rate as low as 3.01×10(-3) has been achieved. Due to the orbital angular momentum multiplexing and spatial paralleling, this FSO communication method possesses the ability to greatly increase the capacity of data transmission.

Improved Doppler Velocity Dealiasing for Radar Data Assimilation and Storm-Scale Vortex Detection

The Doppler velocity dealiasing technique based on alias-robust VAD and variational (AR-Var) analyses developed at the National Severe Storms Laboratory for radar data quality control and assimilation is further improved in its two-step procedures: the reference check in the first step and the continuity check in the second step. In the first step, the alias-robust variational analysis is modified adaptively and used in place of the alias-robust velocity-azimuth display (VAD) analysis for all scan modes (rather than solely the WSR-88D volume coverage pattern 31 with the Nyquist velocityvNreduced below 12 m s−1and the TDWR Mod80 withvNreduced below 15 m s−1), so more raw data can pass the stringent threshold conditions used by the reference check in the first step. This improves the dealiased data coverage without false dealiasing to better satisfy the high data quality standard required by radar data assimilation. In the second step, new procedures are designed and added to the continuity check to increase the dealiased data coverage over storm-scale areas threatened by intense mesocyclones and their generated tornados. The performances of the improved dealiasing technique versus the existing techniques are exemplified by the results obtained for tornadic storms scanned by the operational KTLX radar in Oklahoma.

Ентропійна модель розподілу енергії турбулентних вихорів

Empirical data of frequency distribution of atmospheric turbulent fluctuations from the degree of intensity indicate non-Gaussian nature of such dependence. The so-called "heavy tail" distribution is observed. Such distributions, as opposed to Gaussian, tend to maintain high probability of fluctuations, which could be very intense. This could be a very important factor of the deterioration of the security of aircraft flight. Nowadays, for the statistical description of such distributions it is often used either normal distribution law or Rayleigh distribution. These Gaussian hypotheses could be applied in the case of independent outcomes that could not be considered valid for the turbulent velocity fluctuations, since the turbulent vortices exercise mutual continuous energy exchange. The article suggests the ratio, named by the authors as extreme hyperbolic distribution law for the detection of turbulent vortices of various intensities proposed. The dependences were offered according to its parameters. This law is derived from the maximum entropy method of distribution of a limited set of "resources" (here - energy) on the set of a finite number of "carriers" (here - vortices). The authors believe that this dependence corresponds to the most economical distribution of the kinetic energy among the turbulent vortices

Time-of-Flight Measurements of Vortices Emitted from Quantum Turbulence in Superfluid 4He

An oscillating obstacle generates quantum turbulence in superfluids, when vortices remained attached to obstacle surfaces or vortex rings collided with it during oscillation. Turbulence provides a source of vortices; however, the characteristics of these vortices are not clear. In the present work, we report the flight of vortices emitted from quantum turbulence in superfluid 4He at low temperatures, using vibrating wires as a generator and a detector of vortices. A vortex-free vibrating wire can detect only the first colliding vortex ring, though it will be refreshed after low vibration and be able to detect a vortex ring again. By measuring a period from the start of turbulence generation to the vortex detection repeatedly, we find an exponential distribution of time-of-flights with a non-detection period t0 and a mean detection period t1, suggesting a Poisson process. Both periods t0 and t1 increase with increasing distance between a generator and a detector. A vortex flight velocity estimated from period t0 suggests that the sizes of the emitted vortex rings distribute to a range smaller than a generator thickness or a generator vibration amplitude. Vortices are emitted radially from a turbulence region, at least in the direction of oscillator vibration.

Detection of Planorbis planorbis and Anisus vortex as first intermediate hosts of Alaria alata (Goeze, 1792) in natural conditions in France: Molecular evidence

Alaria alata (Goeze, 1792), a trematode that parasitizes canids, usually needs two intermediate hosts to complete its life cycle: an aquatic freshwater snail and an amphibian. Although many studies have been undertaken on the wild boar's role as paratenic host, owing to the potential threat to human health, few have sought to identify the snails that act as first intermediate hosts in natural conditions. Adopting a molecular approach, with specific markers for a portion of the second internal transcribed spacer (ITS-2), we detected haplotypes of A. alata furcocercariae in two snail species (Planorbis planorbis and Anisus vortex), identified by molecular analysis (ribosomal 18S, mitochondrial 16S and COI). This study provides the first description of snails naturally emitting A. alata furcocercaria in Western Europe.

PIV measurement of turbulent flow within a street canyon: Detection of coherent motion

Turbulent flow inside a street canyon was investigated in a wind-channel. Velocity measurements were performed in vertical planes by means of particle image velocimetry (PIV) at the repetition rate 500Hz. Two geometries were used for comparison purposes: buildings with pitched roofs and with flat roofs. Both induce different dynamic regimes in the street, so that the generated turbulent flows are of different categories. Velocity data were analysed by proper orthogonal decomposition (POD). POD identifies the most dominant modes with high coherency in the flow and also evaluates the relative contributions of each mode to the overall kinetic energy of turbulence. Rigorous analysis of correctness of the physical interpretation of such decomposition was carried out. POD reconstruction of the original vector field was performed and the accuracy of the method was evaluated. Wavelet analysis was applied to the time-series of the POD expansion coefficients in order to reveal the dynamics of the modes. Vorticity, calculated from the original velocity data, was decomposed by POD as well. Finally, the correlation between the vorticity and other methods for vortex detection was assessed.

A statistical test to detect vortices in the current fields of bodies of water

Vortices could play an important role in the occurrence of certain biological phenomena, such as the massive proliferation of harmful algae in bodies of water. Many measures exist to detect vortices in fluids, but little is known about the stochastic behavior of these quantities with data that contain statistical noise. Consequently they do not provide control over the probability of false positives and give little information about the risk of false negatives. Obtaining such control requires a statistical testing procedure. In this paper, we develop a test for vortices in random current fields using only the directions of the current observed at points on a regular grid. We construct a change-point test for spatially ordered angular data to detect the presence of a local vortex. A global vortex detection procedure based on this test is developed and applied to a data set from a lagoon located in the south of France. It is shown that this procedure can detect the presence of multiple vortices with good accuracy.

Comparison of optical vortex detection methods for use with a Shack-Hartmann wavefront sensor

In this paper we compare experimentally two methods of detecting optical vortices from Shack-Hartmann wavefront sensor (SHWFS) data, the vortex potential and the contour sum methods. The experimental setup uses a spatial light modulator (SLM) to generate turbulent fields with vortices. In the experiment, many fields are generated and detected by a SHWFS, and data is analysed by the two vortex detection methods. We conclude that the vortex potential method is more successful in locating vortices in these fields.

Towards wake vortex safety and capacity increase: the integrated fusion approach and its demands on prediction models and detection sensors

Wake vortices and the prevention of wake vortex encounters are both an issue of safety and capacity in today’s air transportation system. Current wake vortex separations are safe but also very conservative and thus have an adverse effect on capacity of airports. This article deals with the concept of fused wake vortex prediction and detection with the objective to deliver wake vortex strength and position with a high level of accuracy and reliability. The collaboration approach aims at fusion of models and sensors available for forecast and detection of hazardous wake turbulence in order to improve the overall system performance using the complementary capabilities of the single components. Different methods of coupling models with measurements are introduced and resulting the aspect of requirements for the prediction model and measurement sensor is presented. The implementation of an error-state system is presented and compared to sole prediction and sole sensor results. The results indicate that the fusion approach delivers benefits like reduced uncertainty of prediction and increased availability of detection and thus has the ability to increase airport and air space capacity while maintaining or even improving current wake vortex safety.

Influence of Flow Coefficient and Flow Structure on Rotational Cavitation in Inducer

Cavitation instability is a major vibration source in turbopump inducers, and its prevention is a critical design problem in rocket-engine development. As reported by Kang et al., (2009, “Cause of Cavitation Instabilities in Three Dimensional Inducer,” Int. J. Fluid Mach. Syst., 2(3), pp. 206–214), the flow coefficient plays an important role in the onset of cavitation instabilities such as rotating and asymmetric cavitation. At high flow rates, various cavitation instabilities occur; on the other hand, as the flow coefficient is reduced, these cavitation instabilities either become absent or may change in character. The purpose of the present study is to investigate the relationship between rotating cavitation and flow coefficient through numerical simulations using the Combustion Research Unstructured Navier-stokes solver with CHemistry (CRUNCH) computational fluid dynamics (CFD) code (Ahuja et al., 2001, “Simulations of Cavitating Flows Using Hybrid Unstructured Meshes,” J. Fluids Eng.Trans ASME, 123(2), pp. 331–340), and to investigate the internal flow. As a first step, the interaction between the tip vortex and inducer blade was investigated through steady-state simulations. The tip vortex was identified by a vortex detection variable, i.e., the Q-function, a second invariant of the velocity tensor, and the distance between the blade and Q-function peak was measured. For a better understanding of cavitation instabilities, unsteady simulations were also performed for two different flow coefficients. The internal flow was carefully investigated, and the relation between cavity collapse/growth and the change in angle of attack was evaluated. The tip-vortex interaction is not a primary cause of unsteady cavitation, but the negative flow divergence caused by cavity collapse has a great influence on the flow angle. Moreover, changes in flow angle also introduce backflow from the tip clearance; these two factors are primary causes of cavitation instability. When the flow coefficient is large, the backflow is weak, and the interaction with the cavity collapse is strong. In contrast, as the flow coefficient decreases, stronger backflow occurs, and the interaction between backflow, cavity collapse, and flow angle weakens.

Descriptive analysis of a mode transition of the flow over an open cavity

Incompressible flow computations have been performed on a cavity with a length to depth ratio of 2. Depending on the Reynolds number, three qualitatively different unsteady flow regimes have been identified. In the first (regular) regime the second Rossiter mode dominates the spectrum, in the second (transitional) regime two incommensurable frequencies compete with each other via mode switching, and finally, in the third (suppressed) regime a lower frequency dominates. The various forms of appearance of these regimes in the flow field are identified and discussed. Among others, the relative importance of the upstream and downstream vortices, the spatial concentration of certain frequency components, and the amplitude of the instability waves all change radically with the change of regime. In search of the hydrodynamic causes for the qualitative changes in the flow field it has become necessary to identify vortices more precisely than usual. We critically evaluate the conditions of applicability of the Truesdell–Okubo–Weiss vortex detection criterion. We propose a new vortex identification scheme by including terms describing both the local and the convective acceleration. This scheme is more accurate than that of Hua and Klein by being truly first order. The results justify the additional effort; vortical structures are more clearly identifiable with the new criterion.

Comparison of wavelet analysis with velocity derivatives for detection of shear layer and vortices inside a turbulent boundary layer

Detection of highly energetic structure in the turbulent flow can be easily done by wavelet analysis, especially with Morlet function as a mother wavelet for detection of repetitious low-frequency structures and Mexican hat for detection of vortices. Since wavelet analysis can be simply applied on one-point measurement, the better understanding of its meaning is welcome. In this project, turbulent boundary layer was investigated with focus on intermittent dynamics. PIV snapshot were used for comparison with Wavelet analysis results, since PIV was performed with high sample frequency (500 Hz). Instantaneous snapshots of velocity and vorticity were studied also by POD and spectra of particular expansion coefficients were computed. Vorticity from planar velocity vectors was used as a suitable methods for recognition of strongly deformed streamlines. Nevertheless, to identify a trajectory of vortex and estimate the true advective velocity of vortex core in the flow, swirling strength or the second invariant could have been used as a proper tool.

Detection of near-wall streamwise vortices by measurable information at wall

The relationship between near-wall streamwise vortices and flow quantities that can be measured at wall, i.e., streamwise and spanwise wall shear stress τwx, τwz and fluctuating wall pressure p′w, is studied using the DNS databases of fully developed turbulent channel flow. It is found that all the three quantities τwx, τwz and p′w are closely related with near-wall streamwise vortices. τwx corresponds to the sweep and ejection motion induced by the downstream streamwise vortex. τwz is the spanwise footprint of the overhead vortical structure. Compared with the wall shear stresses, the relationship between p′w and the near-wall streamwise vortex is more complex. In the upstream of the detecting point, the high pressure region (p′w > 0) corresponds to the sweep motion on the down-wash side of the streamwise vortex, while the low pressure region (p′w < 0) corresponds to the ejection on the up-wash side of the structure; at the detecting point, a low pressure region is formed just underneath the streamwise vortex; in the downstream, the correspondence between p′w and the sweep/ejection motion is in opposite to that in the upstream. Considering the practical requirement of turbulence control, a random blowing/suction at the wall is introduced to examine the robustness of the relationship. The results show that τwx is greatly contaminated, while p′w and τwz still exhibit an excellent correspondence with near-wall streamwise vortices.

Vortex Visualization in Ultra Low Reynolds Number Insect Flight

We present the visual analysis of a biologically inspired CFD simulation of the deformable flapping wings of a dragonfly as it takes off and begins to maneuver, using vortex detection and integration-based flow lines. The additional seed placement and perceptual challenges introduced by having multiple dynamically deforming objects in the highly unsteady 3D flow domain are addressed. A brief overview of the high speed photogrammetry setup used to capture the dragonfly takeoff, parametric surfaces used for wing reconstruction, CFD solver and underlying flapping flight theory is presented to clarify the importance of several unsteady flight mechanisms, such as the leading edge vortex, that are captured visually. A novel interactive seed placement method is used to simplify the generation of seed curves that stay in the vicinity of relevant flow phenomena as they move with the flapping wings. This method allows a user to define and evaluate the quality of a seed's trajectory over time while working with a single time step. The seed curves are then used to place particles, streamlines and generalized streak lines. The novel concept of flowing seeds is also introduced in order to add visual context about the instantaneous vector fields surrounding smoothly animate streak lines. Tests show this method to be particularly effective at visually capturing vortices that move quickly or that exist for a very brief period of time. In addition, an automatic camera animation method is used to address occlusion issues caused when animating the immersed wing boundaries alongside many geometric flow lines. Each visualization method is presented at multiple time steps during the up-stroke and down-stroke to highlight the formation, attachment and shedding of the leading edge vortices in pairs of wings. Also, the visualizations show evidence of wake capture at stroke reversal which suggests the existence of previously unknown unsteady lift generation mechanisms that are unique to quad wing insects.

Adaptive Extraction and Quantification of Geophysical Vortices

We consider the problem of extracting discrete two-dimensional vortices from a turbulent flow. In our approach we use a reference model describing the expected physics and geometry of an idealized vortex. The model allows us to derive a novel correlation between the size of the vortex and its strength, measured as the square of its strain minus the square of its vorticity. For vortex detection in real models we use the strength parameter to locate potential vortex cores, then measure the similarity of our ideal analytical vortex and the real vortex core for different strength thresholds. This approach provides a metric for how well a vortex core is modeled by an ideal vortex. Moreover, this provides insight into the problem of choosing the thresholds that identify a vortex. By selecting a target coefficient of determination (i.e., statistical confidence), we determine on a per-vortex basis what threshold of the strength parameter would be required to extract that vortex at the chosen confidence. We validate our approach on real data from a global ocean simulation and derive from it a map of expected vortex strengths over the global ocean.

Temporal evolution of the RCS of aircraft wake vortices

Knowledge of the radar scattering characteristics is one of the key issues for the development of wake vortex detection technology. This paper studies the temporal evolution of the RCS (radar cross-section) of wake vortices. The RCS–time plot is observed to increase as a whole and step at a certain time. These properties could provide help to the design of wake vortex detection radar and the optimization of radar station layout. The special spiral structures within the wake, together with the Bragg scattering theory, are used to well explain these phenomena, and some representative radar experiments are also included to verify them.