Effects of Heaving Motion on the Aerodynamic Performance of a Double-Element Wing in Ground Effect

The broad implication of the paper is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of multi-element wings,currently considered as a promising aspect for the improvement of the aerodynamic correlation between CFD,wind tunnel and track testing in race car applications.The relationship between the varying aerodynamic forces,the vortex shedding,and the unsteady pressure field of a heaving double-element wing is investigated for a range of mean ride heights,frequencies,and amplitudes,using a two-dimensional(2D)unsteady Reynolds-averaged Navier-Stokes(URANS)approach and an overset mesh method for modelling the moving wing.The analysis of the results shows that at high frequencies,i.e.,k≥5.94 and amplitudes a/c≥0.05 the interaction of the shear vorticity between the two elements results in the generation of cohering leading and trailing edge vortices on the flap,associated to the rapid variation of thrust and downforce enhancement.Both the occurrence and intensity of these vortices are dependent upon the frequency,amplitude,and mean ride height of the heaving wing.The addition of the flap significantly alters the frequency of the shed vortices in the wake and maintains the generation of downforce for longer time in ground proximity.The comparison with the static wing provides evidence that the dynamic motion of a race car wing can be beneficial in terms of performance,or detrimental in terms of aerodynamic correlation.

Effects of site conditions on earthquake ground motion and their applications in seismic design in loess region

The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake(Ms8.0) and the 2013 Minxian-Zhangxian earthquake(Ms6.6). The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, but also amplification effects of site conditions, topography and thickness of loess deposit, on ground motion. In this paper, we chose Dazhai Village and Majiagou Village as the typical loess site affected by the two earthquakes for intensity evaluation, borehole exploration, temporary strong motion array, micro tremor survey, and numerical analysis. The aim is to explore the relations between amplification factors and site conditions in terms of topography and thickness of loess deposit. We also developed site amplification factors of ground motion for engineering design consideration at loess sites. The results showed that the amplification effects are more predominant with increase in thickness of loess deposit and slope height. The amplification mayincrease seismic intensity by 1 degree, PGA and predominant period by 2 times, respectively.

Influence of Anteroposterior Symmetrical Aero-Wings on the Aerodynamic Performance of High-Speed Train

The running stability of high-speed train is largely constrained by the wheel-rail coupling relationship,and the continuous wear between the wheel and rail surfaces will profoundly affect the dynamic performance of the train.In recent years,under the background of increasing train speed,some scientific researchers have proposed a new idea of using the lift force generated by the aerodynamic wings(aero-wing)installed on the roof to reduce the sprung load of the carriage in order to alleviate the wear and tear of the wheel and rail.Based on the bidirectional running characteristics of high-speed train,this paper proposes a scheme to apply aero-wings with anteroposterior symmetrical cross-sections on the roof of the train.After the verification of the wind tunnel experimental data,the relatively better airfoil section and extension formof anteroposterior symmetrical aero-wing is selected respectively in this paper,and the aero-wings are fixedly connected to the roof of the train through the mounting column to conduct aerodynamic simulation analysis.The research shows that:compared with the circular-arc and oval crosssections,this paper believes that the crescent cross-section can form greater aerodynamic lift force in a limited space.Considering factors such as aerodynamic parameters,ground effect,and manufacturing process,this paper proposes to adopt aero-wings with arc type extension form and connect them to the roof of the train through mounting columns with shuttle cross-section.When the roof of the train is covered with aero-wings and runs at high speed,the sprung load of the carriages can be effectively reduced.However,there are certain hidden dangers in the tail carriage due to the large amount of lift force,so,the intervention of the aero-wing lifting mechanism is required.At the same time,it is necessary to optimize the overall aerodynamic drag force reduction in the followup work.

Unsteady flow structure of an airfoil in ground effect

Particle image velocimetry （PIV） experimental results of wake flow structure of a NACA0012 airfoil with small attack angle mounted above water surface are introduced.The experiment was carried out in a small-scale wind-wave tunnel.The diameter of wind-wave tunnel test section is 1.7 m （long） × 0.4 m （width） × 0.4 m （height）.The flow fields around the airfoil were measured under four diffierent conditions by varying the distance between the airfoil and the water surface.The attack angle of the airfoil was kept 10- during the experiment.For each experimental condition,the time series of particle images was captured to calculate continuous evolution of the velocity fields.The velocity fields were ensemble averaged to get the statistic parameters such as mean velocity and vorticity.Typical instantaneous velocity fields for each case are introduced to show the basic flow structure of wind surface flow separation.The aerodynamic loads acting on the airfoil are analyzed qualitatively according to the mean vorticity distribution in the flow field based on the theory of vorticity aerodynamics.The results indicate that the flow structures and drag/lift force of the airfoil alter remarkably with the changing distance between the airfoil and water surface.

Experimental investigation on tip vortices and aerodynamics of a wing with ground effect

The tip vortices and aerodynamics of a NACA0012 wing in the vicinity of the ground were studied in a wind tunnel.The wing tip vortex structures and lift/drag forces were measured by a seven-hole probe and a force balance,respectively.The evolution of the flow structures and aerodynamics with a ground height were analyzed.The vorticity of tip vortices was found to reduce with the decreasing of the ground height,and the position of vortex-core moved gradually to the outboard of the wing tip.Therefore,the down-wash flow induced by the tip vortices was weakened. However,vortex breakdown occurred as the wing lowered to the ground.From the experimental results of aerodynamics,the maximum lift-to-drag ratio was observed when the angle of attack was 2.5°and the ground clearance was 0.2.

Flapping dynamics of a flexible propulsor near ground

The flapping motion of a flexible propulsor near the ground was simulated using the immersed boundary method. The hydrodynamic benefits of the propulsor near the ground were explored by varying the heaving frequency (St) of the leading edge of the flexible propulsor. Propulsion near the ground had some advantages in generating thrust and propelling faster than propulsion away from the ground. The mode analysis and flapping amplitude along the Lagrangian coordinate were examined to analyze the kinematics as a function of the ground proximity (d) and St. The trailing edge amplitude (\(a_\mathrm{tail}\)) and the net thrust (\(\overline{{F}}_x\)) were influenced by St of the flexible propulsor. The vortical structures in the wake were analyzed for different flapping conditions.

On the unsteady motion and stability of a heaving airfoil in ground effect

This study explores the fluid mechanics and force generation capabilities of an inverted heaving airfoil placed close to a moving ground using a URANS solver with the Spalart-Allmaras turbulence model. By varying the mean ground clearance and motion frequency of the airfoil, it was possible to construct a frequency-height diagram of the various forces acting on the airfoil. The ground was found to enhance the downforce and reduce the drag with respect to freestream. The unsteady motion induces hysteresis in the forces’ behaviour. At moderate ground clearance, the hysteresis increases with frequency and the airfoil loses energy to the flow, resulting in a stabilizing motion. By analogy with a pitching motion, the airfoil stalls in close proximity to the ground. At low frequencies, the motion is unstable and could lead to stall flutter. A stall flutter analysis was undertaken. At higher frequencies, inviscid effects overcome the large separation and the motion becomes stable. Forced trailing edge vortex shedding appears at high frequencies. The shedding mechanism seems to be independent of ground proximity. However, the wake is altered at low heights as a result of an interaction between the vortices and the ground.

Zeeman Effect of the Rovibronic Ground State of I35Cl at Hyperfine Level

Zeeman effect at the hyperfine level of the rovibronic ground state of I35Cl are determined on the basis of ｜I1JF1I2FMF｜ via an effective Hamiltonian matrix diagonalization method. Perturbations of the Zeeman sub- levels are observed and the perturbation selection rules are summarized as well. Several potential applications of such Zeeman effect are suggested.

Numerical simulation of the influence of ground effect on the performance of multi section wings

the establishment of multi-element airfoil in steady and unsteady ground effect N-S equation turbulence model, the S-A model of multi element airfoils during takeoff and landing high attack angle change numerical simulation analysis, the calculation results show that the lower altitude, lift and drag wing angle decreased; the greater the ground the effect is more obvious, the greater the loss of lift. The simulation results show that the lift coefficient is slightly less than that of unsteady numerical simulation, and the drag coefficient is slightly less than that of unsteady numerical simulation. The ground disturbance to the wing not only affects the steady state flow field, but also is closely related to the unsteady aerodynamic performance. The results of this study can provide a reference for the design and flight control of large aircraft wings.

Dynamic Stall on High-Lift Airfoil 30P30N in Ground Proximity

Computational prediction of stall aerodynamics in free air and in close proximity to the ground considering the 30P30N three-element high-lift configuration is carried out based on CFD simulations using the OpenFOAM code and Fluent software. Both the attached and separated flow regimes are simulated using the Reynolds Averaged Navier-Stokes (RANS) equations closed with the Spalart-Allamaras (SA) turbulence model for static conditions and pitch oscillations at Reynolds number, Re = 5 x 106 and Mach number, M = 0.2. The effects of closeness to the ground and dynamic stall are investigated and the reduction in the lift force in close proximity to the ground is discussed.

Analysis of the Adjacent Terrain Effect on the Properties of Ground Motion

The explicit finite element analysis method combined with the artificial transmitting boundary theory is performed to evaluate the adjacent terrain effects on ground motion,and the influence of the distance between adjacent terrains on the topographical amplification effects on ground motion is studied. The results show that:( 1) Compared to the case of a single hill,the presence of adjacent hills has little effect on the shape of the spectral ratio curve,but has a significant effect on the value of spectral ratio,which is dependent on the locations of observation points.( 2) The presence of adjacent hills has a greater effect on high-frequency ground motion,and with the increase of the distance between adjacent hills,such an effect weakens gradually,and the effect of the composite topography combined with multiple hills on ground motion gradually approaches that of a single hill.

Aerodynamic performance of hovering micro revolving wings in ground and ceiling effects at low Reynolds number

Numerous investigations have been conducted to understand the wall effects on rotors.The purpose of this study is to further investigate the aerodynamic performance of revolving wings,especially when it is very close to the ground and ceiling(i.e.,less than half the wingspan)at low Reynolds numbers.Hence,the ground and ceiling effect for hovering micro revolving wings at low Reynolds numbers are investigated by improving the theoretical models.The theoretical model for the ground effect is established based on the wall-jet assumption,and that for the ceiling effect is improved by considering the uneven spanwise distribution of induced velocity.These two models are validated by comparing the results of experiments and CFD simulations with the Lattice-Boltzmann Method(LBM).Both ground and ceiling effects are found helpful to enhance the thrust,especially with small wing-wall distances,by making a difference to the induced velocity and the pressure distribution.By comparing the thrust generation and aerodynamic efficiency between the ground and ceiling effects,the former is found more helpful to the thrust augmentation,and the latter is more beneficial for the aerodynamic efficiency promotion.

Hovering rotor aerodynamics in extreme ground effect

This paper presents an experimental and numerical study of the aerodynamics of a moderate-scale rotor hovering in the Extreme Ground Effect(EGE)where rotor height-offground is below half the rotor radius.The tip vortex field was visualized by using the PIV technique.The aerodynamic performance,tip vortex trajectory,wall jet characteristics,surface pressure and velocity fields were measured and analyzed.To explore more deeply the flow mechanisms of the extreme ground effect,Detached Eddy Simulation(DES)was conducted on completely structured meshes.The results showed significant deviations of the rotor performance in EGE from that in Regular Ground Effect(RGE)with the rotor heights of more than half the rotor radius.Moreover,the flow structures of the rotor in EGE are considerably complex,such as the wall jet and groundwash flow separation.The rotor wake flow and tip vortices impact the ground more frequently,resulting in distinctive characteristics of the surface pressure and velocity fields in EGE.

The future and technique challenges of high-speed ground effect vehicle enrolled in maritime transportation

This paper presents a high-speed ground effect vehicle(HS-GEV)used specifically for maritime transportation.Given the limitations of current vessels,including various types of watercraft and high-speed boats,in fulfilling of needs in different maritime transportation scenarios,the HS-GEV emerges as a promising solution to address unmet requirements.To efficiently accomplish maritime transportation missions with quickness and safety,several critical features are emphasized,including short take-off on water,flight maneuverability and flight stability.The key techniques required to achieve these features,as well as recent progress highlights,are introduced.Following and promoting these crucial techniques is also suggested as a future step to improve HS-GEV performance.With its predominant features,the HS-GEV holds immense application value in enhancing a high-speed maritime transportation system that aligns with the evolving needs of the real world.

Computational Simulation of Turbulent Flow Around Tractor-Trailers

A method to evaluate the properties of turbulent flow in proximity to the vehicle and close to the ground surface has been elaborated.Numerical simulations have been performed on the basis of the Unsteady Reynolds-averaged Navier-Stokes equations(URANS)written with respect to an arbitrary curvilinear coordinate system.These equations have been solved using the Spalart-Allmaras differential one-parametric turbulence model.The method of artificial compressibility has been used to improve the coupling of pressure and velocity in the framework of a finite volume approach.Time-averaged distributions of pressure fields,velocity components,streamlines in the entire area and near the tractor-trailer,as well as integral and distributed characteristic parameters(such as coefficients of pressure,friction and drag force)are presented.According to our results,the turbulent flow accelerates in the area of the tractor cabin and in the gap between surfaces.Above the driver’s cabin,a pressure drop occurs due to a sharp acceleration of flow in this area.Downstream,pressure is restored and becomes almost constant in proximity to the edge of the trailer.The dimensions of the separation area exceed the length of the transport system several times.Though agreement with experimental results is relatively limited due to the two-dimensional nature of the numerical simulations,the present approach succeeds in identifying the main physical effects involved in the considered dynamics.It might be used in future studies for initial approximate assessments of the influence of the vehicle shape on its aerodynamic characteristics.

Shock-Wave/Rail-Fasteners Interaction for Two Rocket Sleds in the Supersonic Flow Regime

Rocket sleds belong to a category of large-scale test platforms running on the ground.The applications can be found in many fields,such as aerospace engineering,conventional weapons,and civil high-tech products.In the present work,shock-wave/rail-fasteners interaction is investigated numerically when the rocket sled is in supersonic flow conditions.Two typical rocket sled models are considered,i.e.,an anti-D shaped version of the rocket sled and an axisymmetric slender-body variant.The dynamics for Mach number 2 have been simulated in the framework of a dynamic mesh method.The emerging shock waves can be categorized as head-shock,tailing-shock and reflected-shock.An unsteady large-scale vortex and related shock dynamics have been found for the anti-D shaped rocket sled.However,a quasi-steady flow state exists for the slender-body shaped rocket sled.It indicates that the axisymmetric geometry is more suitable for the effective production of rocket sleds.With the help of power spectral density analysis,we have also determined the characteristic frequencies related to shock-wave/rail-fasteners interaction.Furthermore,a harmonic phenomenon has been revealed,which is intimately related to a shock wave reflection mechanism.

Numerical investigation on unsteady characteristics of ducted fans in ground effect

Ducted fans are widely used in various applications of Unmanned Aerial Vehicles(UAVs)due to the high efficiency,low noise and high safety.The unsteady characteristics of ducted fans flying near the ground are significant,which may bring stability problems.In this paper,the sliding mesh technology is applied and the Unsteady Reynolds Averaged Navier-Stokes(URANS)method is adopted to evaluate the influence of ground on the aerodynamic performance of ducted fans.The time-averaged results show that the ground leads to the decrease of duct thrust,the increase of rotor thrust and the decrease of total thrust.The transient results show that there exist small-scale stall cells with circumferential movements in ground effect.The stall cells start to appear at the blade root when the height is 0.8 rotor radius distance,and arise at both the blade root and tip when the height drops to 0.2.It is found that the unsteady cells rotate between blade passages with an approximate relative speed of 30%-80%of the fan speed,and lead to thrust fluctuations up to 37%of the total thrust.The results are essential to the flight control design of the ducted fan flying vehicle,to ensure its stability in ground effect.

The influence of the leading-edge angle of subgrade on the aerodynamic loads of a high-speed train in a wind tunnel

The purpose of this study is to establish the correlation between the boundary layer over the subgrade and the aerodynamic loads act-ing on the train model in conventional wind tunnel tests.Firstly,flow characteristics around the subgrade with different leading-edge angles(15°,30°and 45°)are investigated through the particle image velocimetry(PIV)experimental test method.Then,wind tunnel tests of the aerodynamic performance of a high-speed train are carried out.The results are compared with previous experimental data obtained by moving model tests.Results show that,due to the presence of a boundary layer,the pressure acting on the lower part of the train head decreases,while other locations are not significantly affected.This is the reason for the reduction of the aerodynamic drag and lift on the train.In addition,the reduction effects become more obvious when the thickness of the boundary layer increases.The experimental results obtained could serve as a calibration of aerodynamic forces for wind tunnel tests on high-speed trains.

Complex Flow for Wing-in-ground Effect Craft with Power Augmented Ram Engine in Cruise

Power augmented ram （PAR）engine is a popular equipment to reduce the requirement of power for takeoff and improve aerodynamic performance. To provide detailed insight into the aerodynamic characteristics of wing-in-ground effect （WIG）craft with PAR engine, numerical simulations are carried out on WIG craft models in cruise. Simplified engine models are applied to the simulations. Two cruise modes for PAR engine are considered. The aerodynamic characteristics of the WIG craft and other features are studied. Comparisons with WIG craft model without PAR show that shutoff of PAR engine results in an increase in drag and less change in lift. Accordingly for the work of PAR engine, the air flow blown from the engine accelerates the flow around the upper surface and a high-speed attached flow near the trailing edge is recorded. With the schemed PAR flow, more suction force is realized and the flow features over the wing vary noticeably. It is also shown that the Coanda effect,provided with an attached flow, introduces an appropriate and practical flow mode for WIG craft with PAR engine in cruise. The results refresh our understanding on aerodynamic characteristics of WIG craft.

Fluid Dynamics of Flapping Insect Wing in Ground Effect

The fluid dynamics of flapping insect wing in ground effect is investigated numerically in this study. To model the insect wing cross-section in forward-flight mode, the laminar flow over a NACA0012 airfoil animated by a combination of harmonic plunge and pitch rotation is considered. To implement the simulation, the proposed immersed boundary-lattice Boltzmann method is employed. By fixing the Reynolds number and the amplitude of motion, we systematically examine the influences of the distance between the foil and the ground and the flapping frequency on the flow behaviors. As compared to the situation out of ground effect, the forces for foil placed in close proximity to the ground show some differences. The mean drag coefficient is increased at low frequency and decreased at high frequency. Meanwhile, the mean lift coefficient is increased at both low and high frequencies and decreased at middle frequency. Moreover, an interesting phenomenon with oblate vortices due to vortex interaction with the ground is observed.