Next Generation Scanning LIDAR Systems for Optimizing Wake Turbulence Separation Minima

Numerous studies have been performed to better understand the behavior of wake vortices with regards to aircraft characteristics and weather conditionsover the pastten years. These studies have led to the development of the aircraft RECATegorization(RECAT) programs in Europe and in USA. Its phase one focused on redefining distance separation matrix with six static aircraft wake turbulence categories instead of three with the current International Civil Aviation Organization(ICAO) regulations. In Europe, the RECAT-EU regulation is now entering under operational implementation atseveral key airports. As proven by several research projects in the past, LIght Detection And Ranging(LIDAR) sensors are considered as the ground truth wake vortex measurements for assessing the safety impact of a new wake turbulence regulation at an airport in quantifying the risks given the local specificities. LIDAR's can also be used to perform risk monitoring after the implementation. In this paper, the principle to measure wake vortices with scanning coherent Doppler LIDARs is described as well as its dedicated post-processing. Finally the use of WINDCUBELIDAR based solution for supporting the implementation of new wake turbulenceregulation is described along with satisfyingresults that have permitted the monitoring of the wake vortex encounter risk after the implementation of a new wake turbulence regulation.

Experimental investigation of the effect of Reynolds number on flow structures in the wake of a circular parachute canopy

In the present study, an experimental study was conducted to characterize the effect of Reynolds number on flow structures in the turbulent wake of a circular parachute canopy by utilizing stereoscopic particle image velocime- try （Stereo-PIV） technique. The parachute model tested in the present study was attached by 28 nylon suspension lines and placed horizontally at the test section center of the wind tunnel. The obtained results showed that with the in- crease of Reynolds number, the intensities of the vortices near the downstream region of the canopy skirt were found to increase accordingly. However, the increase of Reynolds number did not result in a significant change in ensemble- averaged normalized x-component of the velocity, ensembleaveraged normalized vorticity, normalized Reynolds stress, and normalized turbulent kinetic energy distributions in the turbulent wake of the circular parachute canopy. The obtained results are very useful to further our understanding about the unsteady aerodynamics in the wake of flexible circular parachute canopies and to constitute a reference for CFD computation.

SIMULATION OF SHIP GENERATED TURBULENT AND VORTICAL WAKE IMAGING BY SAR

Synthetic Aperture Radar (SAR) imaging of ocean surface features is studied. The simulation of the turbulent and vortical features generated by a moving ship and SAR imaging of these wakes is carried out. The turbulent wake damping the ocean surface capillary waves may be partially responsible for the suppression of surface waves near the ship track. The vortex pair generating a change in the lateral flow field behind the ship may be partially responsible for an enhancement of the waves near the edges of the smooth area. These hydrodynamic phenomena as well as the changes of radar backscatter generated by turbulence and vortex are simulated.An SAR imaging model is then used on such ocean surface features to provide SAR images.Comparison of two ships' simulated SAR images shows that the wake features are different for various ship parameters.

Shear Flows in the Near-Turbulent Wake Region of High Speed Trains

Two flow cases for scaled high speed train models with different length are numerically analyzed in the framework of the improved delayed detachededdy simulation model.Specific attention is paid to the shear flows and related mechanisms in the near turbulent wake created by these moving models.In particular,a comparative analysis is made on the distributions of turbulent kinetic energy(TKE)and turbulence production(TP)in planes perpendicular to the streamwise direction.The numerical results suggest that,in the wake region very close to the tail,significant TKE and TP can be ascribed to the dynamic interaction between powerful eddies and strong shear,which explain why these quantities are sensitive to the shear strength.The shear flows are essentially governed by the boundary layers developing along the streamwise direction on the train surfaces,especially from the under-body region and the side walls.For other positions located in the downstream direction away from the tail,the interaction of vortices with the non-slip ground serves as a mechanism to promote transfer of energy from weak eddies to turbulence through the shear present in planes parallel to the ground.

THE NEW METHOD OF SOLVING VELOCITY CORRELATION FUNCTIONS AND APPLICATION TO THE PLANE TURBULENT WAKE

The results of plane turbulent wake given by Chou Peiyuan are considered as the first order ap- proximation and put into the equations of turbulent fluctuation.The equations are solved numerically within the range of micro-scale by means of spectrum method.The double,triple and quadruple fluctuating velocity correlations are obtained by computation.They are in good agreement with experimental results.

On helical behavior of turbulence in the ship wake

Turbulent ship wake conservation at a long distance is among unsolved problems at present. It is well known that far wakes have a vortical structure and slowly expand with distance. As was obtained by Dubrovin et al., slow expansion of the wake may be related to the distribution of turbulent viscosity in it. In our work we study the effect of helicity in the wake on the behavior of turbulent viscosity. Taking into account the helical nature of the wake, we can clarify the difference between turbulence inside and outside the wake on the one hand and slowing down of its expansion with time on the other hand.

Experimental Study of Turbulent Wake Behind a Sine Shaped Island in a Shallow-Water Layer

A series of experiments is conducted to study shallow-water flow in the wake of a sine shaped island. Digital particle imaging velocimetry (DPIV) is used to measure velocities in the turbulent wake behind a sine shaped island for different characteristic coefficients S. Flow streamlines are given for the wake flows. The measured results show that the characteristic coefficient S is uniquely related to the flow pattern around a sine shaped island in a shallow water layer. An S value of approximately 0.20 is the critical value for transition from a vortex street to unsteady flow and a value of approximately 0.40 is the critical value for transition from unsteady flow to steady flow.

Interaction between the atmospheric boundary layer and a standalone wind turbine in Gansu—Part I： Field measurement

Experiments and numerical simulations of the wake field behind a horizontal-axis wind turbine are carried out to investigate the interaction between the atmospheric boundary layer and a stand-alone wind turbine. The tested wind turbine（33 k W） has a rotor diameter of 14.8 m and hub height of 15.4 m. An anti-icing digital Sonic wind meter, an atmospheric pressure sensor, and a temperature and humidity sensor are installed in the upstream wind measurement mast. Wake velocity is measured by three US CSAT3 ultrasonic anemometers. To reflect the characteristics of the whole flow field, numerical simulations are performed through large eddy simulation（LES） and with the actuator line model. The experimental results show that the axial velocity deficit rate ranges from 32.18% to 63.22% at the three measuring points. Meanwhile, the time-frequency characteristics of the axial velocities at the left and right measuring points are different. Moreover, the average axial and lateral velocity deficit of the right measuring point is greater than that of the left measuring point. The turbulent kinetic energy（TKE） at the middle and right measuring points exhibit a periodic variation, and the vortex sheet-pass frequency is mostly similar to the rotational frequency of the rotor. However, this feature is not obvious for the left measuring point. Meanwhile, the power spectra of the vertical velocity fluctuation show the slope of-1, and those of lateral and axial velocity fluctuations show slopes of-1 and-5/3, respectively.However, the inertial subranges of axial velocity fluctuation at the left, middle, and right measuring points occur at 4, 7, and7 Hz, respectively. The above conclusion fully illustrates the asymmetry of the left and right measuring points. The experimental data and numerical simulation results collectively indicate that the wake is deflected to the right under the influence of lateral force. Therefore, wake asymmetry can be mainly attributed to the lateral force exerted by the wind turbine on the fluid.

Unsteady RANS simulation of wind flow around a building shape obstacle

This work aims to find the origin and connection of the surface,near-wake,and far-wake structures in the flow encompassing a high-rise building for a high Reynolds number.The origin and interconnection of the stream-wise tip vortices,with the other components of the wake,is analysed in this study for the current scenario.The Unsteady Reynolds Averaged Navier-Stokes equations(URANS)together with the realizable k-εturbulence model have been used in this investigation to study the turbulent wake flow following a ground-surface-attached square shape building.A moderately big obstacle aspect ratio of 4,a Reynolds number of 12,000,and a thin evolving boundary layer thickness have been used in the flow modeling.The designed flow addresses the reversed-flows at the outlet during computation to improve the accuracy of the realizable k-e model.The Reynolds stress components are retrieved using the Boussinesq approach.The wake’s principal compositions,including span-wise-side eddies and area of high stream-wise vorticity in the uppermost portion of the wake,are illustrated by both three-dimensional(3D)representations and planner projections of the mean flow distributions.A braided vortex formation,composed of asymmetric hairpin vortexes,is witnessed in the far-wake area.The association of the near-wake vortex structures with the far-wake and near-wall flow,which is associated with the flow strengths,is also discussed.In this investigation,few areas of large stream-wise vorticity magnitude,like tip vortexes,are correlated to the 3D curving of the fluid motion,and tip vortices did not continuously reach to the free end part of the building.The 3D fluid motion interpretation,which combined several measurements of the flow distribution encompassing the cylinder,shows that the time-averaged near-wake structures are formed of two segments of distinct source and section of dominance.Furthermore,addressing reversed-flow during computation shows notable improvement in the results.

Particle Image Velocimetry Measurement of Flow Across Tube Bundle in Waste Heat Boiler

In the present paper the experimental investigations of flow across staggered tube bundles in waste heat boiler are conducted by mans of PIV (Particle Image Velocimetry) system. Flow visualization and velocity distribution of the wake between different cylinders are measured in detail. It is concluded that there are still Von Karman vortices in the wake and the phenomena of vortex shedding, pairing, merging are observed in the flow. The Von Karman vortices can’t fully developed because of the existence of the downstream cylinder There is interaction between main streams and vortices, and the development of the vortex is enhanced by this interaction. Meanwhile some statistical results are performed. The distribution of correlated variables of the velocity fluctuations u’ u’, u’ v’, v’ v’ and the space correlation coefficients are obtained.