Relevance of zero lift drag coefficient and lift coefficient to Mach number for large aspect ratio winged rigid body

Synthetic analysis is conducted to the wind tunnel experiment results of zero lift drag coefficient and lift coefficient for large aspect ratio winged rigid body.By means of wind tunnel experiment data,the dynamics model of the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body is amended.The research indicates that the change trends of zero lift drag coefficient and lift coefficient to Mach number are similar.The calculation result and wind tunnel experiment data all verify the validity of the amended dynamics model by which to estimate the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body,and thus providing some technical reference to aerodynamics character analysis of the same types of winged rigid body.

Influence of Autonomous Sailboat Dual-Wing Sail Interaction on Lift Coefficients

To analyze the influence of the chord length ratio and angle of attack on lift coefficients and explore the interaction mechanism between the two,we established a calculation model of the pressure distribution coefficient on the airfoil surface and lift coefficient of a dual-wing sail on the basis of the vortex panel method.Computational fluid dynamics was used in auxiliary calculation and analysis.Results revealed a reciprocal interference between the front-wing and rear-wing sails.The total lift coefficient of the dual-sail increased with an increase in the front sail chord length.The lift coefficient of the rear sail decreased with an increase in the front sail chord length or angle of attack.The front sail wake affected the pressure distribution on the upper and lower surfaces of the rear sail leading edge.

Analysis of Influencing Factors on Lift Coefficients of Autonomous Sailboat Double Sail Propulsion System Based on Vortex Panel Method

Sail is the core part of autonomous sailboat and wing sail is a new type of sail. Wing sail generates not only propulsion but also lateral force and heeling moment. The latter two will affect the navigation status and bring resistance. Double sail can effectively reduce the center of wind pressure and heeling moment. In order to study the effect of distance between two sails, airfoil and attack angle on the total lift coefficient of double sail propulsion system, pressure coefficient distribution and lift coefficient calculation model have been established based on vortex panel method. By using the basic finite solution, the fluid dynamic forces on the two-dimensional sails are computed.The results show that, the distance in the range of 0 to 1 time chord length, when using the same airfoil in the fore and aft sail, the total lift coefficient of the double sail increases with the increase of distance, finally reaches a stable value in the range of one to three times chord length. Lift coefficients of thicker airfoils are more sensitive to the change of distance. The thicker the airfoil, the longer distance is required of the total lift coefficient toward stable.When different airfoils are adopted in fore and aft sail, the total lift coefficient increases with the increase of the thickness of aft sail. The smaller the thickness difference is, the more sensitive to the distance change the lift coefficient is. The thinner the fore sail is, the lower the influence will be on the lift coefficient of aft sail.

Influences of Lorentz force on the hydrofoil lift

In this paper, Lorentz forces are proved to be able to suppress separation in flows over hydrofoils. Furthermore, a differential equation of pressure distributions on the hydrofoil surface is derived, from which it is found that BVF （boundary vortex flux） σ is a suitable criterion for describing the lift coefficient variations during the electromagnetic control process. According to our numerical results, the periodic variations of lift for a hydrofoil at an attack angle of 17 ° are analyzed and its inherent mechanism is discussed in detail with the concept of BVE On the other hand, the effects of Lorentz force on the hydrofoil＇s lift are investigated both experimentally and numerically for different magnitudes and locations.

Swirling-strength based large eddy simulation of turbulent flow around single square cylinder at low Reynolds numbers

In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation （LES） is considered as a better simulation model. However, the sub-grid scale （SGS） models reported so far have not ascertained under what flow conditions the LES can lapse into the direct nu-merical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteris-tics of zero-incident incompressible turbulent flows around a single square cylinder （SC） at a low Reynolds number range Re ∈ [103, 104]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re=2 500 and 104, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square （RMS） values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls. The instantaneous factor of swirling strength intermittency （FSI） exhibits some laminated structure involved with vortex shedding.

Fluid Flow Past a Circular Cylinder with Tandem Rod and Staggered Rod at Low Reynolds Number

The flow past a primary cylinder with one tandem control rod and one staggered control rod is simulated in this paper through solving the Navier-Stokes equations. Two examples are simulated to validate the model, and the results matched well with those of previous researches. The Reynolds number based on the diameter of the primary cylinder is 500. The diameter ratio between the control rod and the primary cylinder （d/D） is 0.25. It was found that the effect of the combination of one upstream tandem control rod and one staggered control rod on the hydrodynamics of the primary cylinder is a linear superposition of the effect of a corresponding single control rod, and the effect of the upstream tandem control rod is dominant at larger spacing ratios such as G/D=2. For the combination of a downstream tandem control rod and a staggered control rod, the effect of the control rods is different from that of the corresponding single control rod in the region of 0.2〈G/D〈0.5 ＆ 30°〈a〈120° and 0.9〈G/D〈1.4 ＆ 30°〈a〈50°, where the additional effect is obvious. In this case, the effect of the downstream tandem control rod is dominant at small spacing ratios （such as G/D=0.1）. At moderate spacing ratios such as G/D=0.4, the effects of the tandem control rod and the staggered control rod are comparable in both cases.

Performance Investigation of 2D Lattice Boltzmann Simulation of Forces on a Circular Cylinder

The lattice Boltzmann method (LBM) is employed to simulate the uniform flow past a circular cylinder. The performance of the two-dimensional LBM model on the prediction of force coefficients and vortex shedding frequency is investigated. The local grid refinement technique and second-order boundary condition for curved walls are applied in the calculations. It is found that the calculated vortex shedding frequency, drag coefficient and lift coefficient are consistent with experimental results at Reynolds nu...

CFD-Based Numerical Analysis of a Variable Cross-Section Cylinder

Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration(VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation(LES) is conducted for studying the fluid-structure interaction. The vortex shedding in the wake, the motion trajectories of a cylinder, the variation of drag and lift forces on the cylinder are analyzed. The results show that the vortices of variable cross-section cylinder are chaotic and are varying along the cylinder. In places where cross-sections are changing significantly, the vortices are more irregular. The motion trail of the cylinder is almost the same but irregular. The drag and lift coefficients of the cylinder are varying with the changes of diameters.

AIRCRAFT LIFT AND MANEUVERABILITY ENHANCEMENT BY GENETIC GEOMETRY OPTIMIZATION OF LEX CONFIGURATION

To enhance the maneuverability of the selected aircraft model, a standard genetic algorithm （GA） is used as an optimization method for the preliminary design of the leading-edge extension （LEX） layout. The aerodynamic loads and the maximum lift coefficient of the complete aircraft configuration （fuselage＋wing＋tail） are computed by using the modified three-dimensional low-order panel method in conjunction with the semi-empirical formulas of DATCOM. Results show that the lift coefficient increases approximately 20.5%- 15.3% for Mach number 0. 4-0.8 and 6.8% for Mach number 1.2, and its maximum value approximately 9.5% -15.0% for Machnumber 0.2-0.95when LEXis installed. A 6.6%-8.0 % gain at altitudes of 1-5 km on the turn rate maneuverability and the corner speed have been achieved in the subsonic regime.

LARGE EDDY SIMULATION AND SPECTRUM ANALYSIS OF FLOW AROUND TWO SQUARE CYLINDERS ARRANGED SIDE BY SIDE

Large eddy simulation cooperated with the second order full extension ETG(Euler-Taylor-Galerkin) finite element method was applied to simulate the flow around two square cylinders arranged side by side at a spacing ratio of (1.5.) The second order full extension ETG finite element method was developed by Wang and He. By means of Taylor expansion of terms containing time derivative, time derivative is replaced by space derivative. The function of it is equal to introducing an artificial viscosity term. The streamlines of the flow at different moments were obtained. The time history of drag coefficient, lift coefficient and the streamwise velocity on the symmetrical points were presented. Furthermore, the symmetrical problem of the frequency spectrum of flow around two square cylinders arranged side by side were studied by using the spectral analysis technology. The data obtained at the initial stage are excluded in order to avoid the influence of initial condition on the results. The power spectrums of drag coefficient, lift coefficient, the streamwise velocity on the symmetrical points were analyzed respectively. The results show that although the time domain process of dynamic parameters is non-symmetrical, the frequency domain process of them is symmetrical under the symmetrical boundary conditions.

APPLICATION OF WEIGHTED NONOSCILLATORY AND NON-FREE-PARAMETER DISSIPATION DIFFERENCE SCHEME IN CALCULATING THE FLOW OF VIBRATING FLAT CASCADE

A dual-time method is introduced to calculate the unsteady flow in a certain vibrating flat cascade. An implicit lower-upper symmetric-gauss-seidel scheme（LU-SGS） is applied for time stepping in pseudo time domains, and the convection items are discretized with the spatial three-order weighted non-oscillatory and non-free-parameter dissipation difference （WNND） scheme. The turbulence model adopts q-co low-Reynolds-number model. The frequency specmuns of lift coefficients and the unsteady pressure-difference coefficients at different spanwise heights as well as the entropy contours at blade tips on different vibrating instants, are obtained. By the analysis of frequency specmuns of lift coefficients at three spanwise heights, it is considered that there exist obvious non-linear perturbations in the flow induced by the vibrating, and the perturbation frequencies are higher than the basic frequency. The entropy contours at blade tips at different times display an intensively unsteady attribute of the flow under large amplitudes.

Drag and Lift Force Acting on a Rotational Spherical Particle in a Logarithmic Boundary Flow

The drag and lift forces acting on a rotational spherical particle in a logarithmic boundary flow are numerically studied. The effects of the drag velocity and rotational speed of the sphere on the drag force are examined for the particle Reynolds number from 50 to 300 and for the dimensionless rotational angular speed of 0≤Ω≤1.0. The influence of dimensionless roughness height Z0 of the wall is also evaluated for z0 ≤ 10. The results show that the drag forces on a sphere both in a logarithmic flow and in a uniform unsheared flow increase with the increase of the drag velocity. For 50≤Rep≤300, -↑CD increases with decreased roughness height z0. The time-averaged drag coefficient is also significantly affected by rotational speed of the sphere and roughness height zo. The lift coefficient -↑CL increases with increased rotational speed and decreases with increased roughness height.

Instability analysis and drag coefficient prediction on a swept RAE2822 wing with constant lift coefficient

Swept wing is widely used in civil aircraft,whose airfoil is chosen,designed and optimized to increase the cruise speed and decrease the drag coefficient.The parameters of swept wing,such as sweep angle and angle of attack,are determined according to the cruise lift coefficient requirement,and the drag coefficient is expected to be predicted accurately,which involves the instability characteristics and transition position of the flow.The pressure coefficient of the RAE2822 wing with given constant lift coefficient is obtained by solving the three-dimensional Navier-Stokes equation numerically,and then the mean flow is calculated by solving the boundary layer（BL） equation with spectral method.The cross-flow instability characteristic of boundary layer of swept wing in the windward and leeward is analyzed by linear stability theory（LST）,and the transition position is predicted by eNmethod.The drag coefficient is numerically predicted by introducing a laminar/turbulent indicator.A simple approach to calculate the lift coefficient of swept wing is proposed.It is found that there is a quantitative relationship between the angle of attack and sweep angle when the lift coefficient keeps constant;when the angle of attack is small,the flow on the leeward of the wing is stable.when the angle of attack is larger than 3°,the flow becomes unstable quickly;with the increase of sweep angle or angle of attack the disturbance on the windward becomes more unstable,leading to the moving forward of the transition position to the leading edge of the wing;the drag coefficient has two significant jumping growth due to the successive occurrence of transition in the windward and the leeward;the optimal range of sweep angle for civil aircraft is suggested.

A multi-objective and CFD based optimization of roof-flap geometry and position for simultaneous drag and lift reduction

As the transport sector is responsible for the consumption of a vast proportion of the oil produced,it is mandatory to research feasible solutions to tackle this issue.The appli-cation of aerodynamic attachments for passiveflow control and reducing resisting aerodynamic forces such as drag and lift is one of the most practicable ways to minimize vehicle energy con-sumption.Theflaps are one of the most innovative aerodynamic attachments that can enhance theflow motion in the boundary layer at the trailing edge of the wings.In the present paper,theflap is designed and modeled for controlling the airflow at the roof-end of a 2D Ahmed body model,inspired by the schematic of theflap at the trailing edge of the wing.As a result,theflap’s geometry and position from the roof-end of the car model are parameterized,which leads to having four design variables.The objective functions of the present study are the vehicle’s drag coefficient and lift coefficient.25 Design of Experiment(DOE)points are considered enabling the Box-Behnken method.Then,each DOE point is modeled in the computational domain,and theflow-field around the model is simulated using Ansys Fluent software.The results obtained for the DOE points are employed by different regressors,and the relation between design variables and objective functions is extracted using GMDH-ANN.The GMDH-ANN is then coupled with three types of optimization algorithms,among which the Genetic algorithm proves to have the most ideal coupling process for optimization.Finally,af-ter analyzing the variations in the geometry and position of the roofflap from the car roof-end,the roof-flap with specifications of L=0.1726 m,a=5.0875°,H=0.0188 m,and d=0.241 m can optimize the car drag and lift coefficients by 21.27% and 19.91%,respec-tively.The present research discusses the opportunities and challenges of optimal design roof-flap geometry and its influence on car aerodynamic performance.

Hydrodynamics of the interceptor on a 2-D flat plate by CFD and experiments

Nowadays, the use of interceptor by both partial and total dynamic lift crafts is quite common. In this article, a lot of evidence is given regarding the effectiveness of interceptor. The interceptor, when placed at the stern region, changes the pressure distribution around the craft. Its presence affects drag force, lifting force and the position of pressure＇s center leading to a new trim. This study focuses on hydrodynamic effects of interceptors on a 2-D fiat plate based on both computational fluid dynamic （CFD） and experimental approaches. The Reynolds average Navier-Stokes （RANS） equations are used to model the flow around a fixed flat plate with an interceptor at different heights and attack angles. Based on finite volume method and SIMPLE algorithm which uses static structures, this model can be analyzed and the RANS results can be compared with the experimental data obtained in the current channel of the laboratory of waves and current of COPPE/UFRJ （LOC in Portuguese acronym）. According to the results, the increase of pressure at the end of the flat plate was proportional to the interceptor height. In addition, the existence of interceptors can significantly increase the lift force coefficient at high angles of attack also proportional to the interceptor height. The presence of interceptor at the end of the fiat plate increased both the lift coefficient and the drag coefficient but hydrodynamic drag did not grow as fast as the lift coefficient did. The lift coefficient increased much more. Furthermore, the results showed that the interceptor effectiveness is proportional to the boundary layer thickness at the end of the fiat plate. As the interceptor was inside the boundary layer alterations of flow speed led to changes in boundary layer thickness, directly affecting interceptor＇s efficiency. Optimum choice of interceptor height had a great effect on its efficiency, and in choosing it the flow speed and length of the boat must be taken into consideration.

Experimental and Numerical Investigation of the Effects of Passive Vortex Generators on Aludra UAV Performance

A study of the effects of passive vortex generators （VGs） on Aludra unmanned aerial vehicle （UAV） aerodynamic characteristics is presented. Both experimental and numerical works are carried out where an array of VGs is attached on Aludra UAV＇s wing. The flow measurements are made at various angles of attack by using 3-axis component balance system. In the numerical investigation, the Reynolds-averaged Navier-Stokes （RANS） code FLUENT 6.3TM is used in the simulations with fully structured mesh with Spalart-Allmaras （S-A） turbulence model and standard wall function. The comparison between the experimental and numerical results reveals a satisfactory agreement. The parametric study shows that higher maximum lift coefficient is achieved when the VGs are placed nearer to the separation point. In addition to this, shorter spanwise distance between the VGs also increases the maximum lift coefficient, rectangular and curve-edge VG performs better than triangular VG.

Effect of Streamwise Fences on Secondary Flows and Losses in a Twodimensional Turbine Rotor Cascade

To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fences were made out of 0.7 mm thick brass sheet and the heights of the fences were 14 mm, 18 mm respectively. The curvature of the fences was the same as that of the blade camber line. The fences were fixed normal to the end wall and at half pitch away from the blades. The experimental program consists of total pressure, static pressure measurements at the inlet and outlet of the cascade, by using five-hole probe. In addition, static pressure on the blade suction surface and pressure surface was also obtained. Fences are effective in preventing the movement of the pressure side leg of the horseshoe vortex. Consequently the accumulation of low energy fluid on the suction surface is minimised. End wall losses are reduced by the fences due to weakening of the end wall cross flow.

Studies on a low Reynolds number airfoil for small wind turbine applications

In contrast to large horizontal axis wind turbines (HAWTs) that are located in areas dictated by optimum wind conditions, small wind turbines are required for producing power without necessarily the best wind conditions. A low Reynolds number airfoil was designed after testing a number of low Reynolds number airfoils and then making one of our own; it was tested for use in small HAWTs. Studies using XFOIL and wind tunnel experiments were performed on the new airfoil at various Reynolds numbers. The pressure distribution, C p , the lift and drag coefficients, C L and C D , were studied for varying angles of attack, α. It is found that the airfoil can achieve very good aerodynamic characteristics at different Reynolds numbers and can be used as an efficient airfoil in small HAWTs.

Aerodynamic Performance of the Locust Wing in Gliding Mode at Low Reynolds Number

Gliding is an important flight mode for insects because it saves energy during long distance flight without wing flapping. In this study, we investigated the influence of locust wing corrugation on the aerodynamic performance in gliding mode at low Reynolds number. Numerical simulations using two-dimensional Navier-Stokes equations are applied to study the gliding flight, which reveals the interaction between forewing and hindwing. The lift of the corrugated airfoil in a locust wing decreases from the wing root to the tip. Simulation results show that the pressure drags on the forewing and hindwing increase with an increase in wing thickness; while the lift-drag ratio of the airfoil is marginally affected by the corrugation on the airfoil. Geometric parameters analysis of the locust wing is also carried out, which includes the corrugation height, the corrugation placement and the shapes of leading and trailing edges.

Precise drag prediction of airfoil flows by a new algebraic model

We report the results of accurate prediction of lift(C L)and drag(C D)coefficients of two typical airfoil flows(NACA0012 and RAE2822)by a new algebraic turbulence model,in which the eddy viscosity is specified by a stress length(SL)function predicted by structural ensemble dynamics(SED)theory.Unprecedented accuracy of the prediction of C D with error of a few counts(one count is 10−4)and of C L with error under 1%-2%are uniformly obtained for varying angles of attack(AoA),indicating an order of magnitude improvement of drag prediction accuracy compared to currently used models(typically around 20 to 30 counts).More interestingly,the SED-SL model is distinguished with fewer parameters of clear physical meaning,which quantify underlying turbulent boundary layer(TBL)with a universal multi-layer structure,and is thus promising to be more easily generalizable to complex TBL.The use of the new model for the calibration of flow condition in experiment and the extraction of flow physics from numerical simulation data of aeronautic flows are discussed.