Effect of chord-wise flexibility in the lift generation of flapping MAV with membrane wing

Purpose The purpose of this study is to experimentally analyse the effect of flexible and stiffened membrane wings in the lift generation of flapping micro air vehicle (MAV). Design/methodology/approach This is analysed by the rectangle wing made up of polyethylene terephthalate sheets of 100 microns. MAV is tested for the free stream velocity of 2 m/s, 4 m/s, 6 m/s and k* of 0, 0.25, 1, 3, 8. This test is repeated for flapping MAV of the free flapping frequency of 2 Hz, 4 Hz, 6 Hz, 10 Hz and 12 Hz. Findings This study shows that the membrane wing with proper stiffeners can give better lift generation capacity than a flexible wing. Research limitations/implications Only a normal force component is measured, which is perpendicular to the longitudinal axis of the model. Practical implications In MAVs, the wing structures are thin and light, so the effect of fluid-structure interactions is important at low Reynold’s numbers. This data are useful for the MAV developments. Originality/value The effect of chord-wise flexibility in lift generation is the study of the effect of a flexible wing and rigid wing in MAV. It is analysed by the rectangle wing. The coefficient of normal force at different free stream conditions was analysed.

On the frequency selection mechanism of the low-Re flow around rectangular cylinders

In the flow past elongated rectangular cylinders at moderate Reynolds numbers, vortices shedding from the leading- and trailing-edge corners are frequency locked by the impinging leading-edge vortex instability. The present work investigates how the chord-based Strouhal number varies with the aspect ratio of the cylinder at a Reynolds number (based on the cylinder thickness and the free-stream velocity) of $Re=400$ , i.e. when locking is strong. Several two-dimensional, nonlinear simulations are run for rectangular and D-shaped cylinders, with the aspect ratio ranging from $1$ to $11$ , and a global linear stability analysis of the flow is performed. The shedding frequency observed in the nonlinear simulations is predicted fairly well by the eigenfrequency of the leading eigenmode. The inspection of the structural sensitivity confirms the central role of the trailing-edge vortex shedding in the frequency locking, as already assumed by other authors. Surprisingly, however, the stepwise increase of the Strouhal number with the aspect ratio reported by several previous works is not fully reproduced. Indeed, with increasing aspect ratio, two distinct flow behaviours are observed, associated with two flow configurations where the interaction between the leading- and trailing-edge vortices is different. These two configurations are fully characterised, and the mechanism of selection of the flow configuration is discussed. Lastly, for aspect ratios close to the jump between two consecutive shedding modes, the Strouhal number is found to present hysteresis, implying the existence of multiple stable configurations. Continuing the lower shedding-mode branch by increasing the aspect ratio, we found that the periodic configuration loses stability via a Neimark–Sacker bifurcation leading to different Arnold tongues. This hysteresis can explain, at least partially, the significant scatter of existing experimental and numerical data.

Ocean atmospheric coupled model to estimate energy and path of cyclone near the coast

;g ns[kk x;k gS fd caxky dh [kkM+h esa vf/kdka’kr% pØokr] gjhdsu vkfn tSlh ok;qeaMyh; ifj?kVuk,¡ viuh xfr ds nk¡bZ vksj c<+rh gSaA ,lh ?kVukvksa dk v/;;u djus ds fy, geus ok;qeaMyh; xfr ;qfXer] egklkxj dh rjy xfrdh; ij fopkj fd;k gSA bl v/;;u esa geus pØokrh; ra= ds dsUnz dks fy;k gS ftlesa rjy xfrdh; lzksr rFkk FkksMh lh nwjh ij rjy xfrdh; flad gksrk gSA bl izdkj fcEc ra= ds rjy xfrdh; f}d ¼MCysV½ fufeZr gksrk gSA rnqijkar fcEc rjax vkSj mlds izfrfcEc rjax ds rjy xfrdh; f}d ¼MCysV½ ij Bksl nhokj ¼;gk¡ ij leqanz dk fdukjk½ ds laca/k esa fopkj fd;k x;kA blesa fcEc ra=] izfrfcEc ra= vkSj /kkjk xfr ls lacaf/kr fefJr fcEc ds rjy xfrdh; lehdj.k ij dk;Z fd;k x;k gSA fcEc ra=] izfrfcEc f}d ¼MCysV½ rFkk /kkjk xfr ds fefJr foHko ij bl 'kks/k i= esa fopkj fd;k x;k gSA xfr lfn’k] QyLo:i nkc dks rjy xfr ds cjukSyh ds lehdj.k dh lgk;rk ls iqu% izkIr fd;k x;kA rnqijkar leqnz ds fdukjs vFkkZr~ nhokj ij U;wure@vf/kdre nkc dh fo’ys"k.kkRed x.kuk dh xbZA vr% ;g ns[kk x;k fd pØokr vFkok gjhdsu dh ekStwnk iou vkSj ÅtkZ dqN izpfyr fLFkfr;ksa ds vk/kkj ij leqnz rV dh vksj vFkok mldh xfr ds nk¡bZ vksj tkrh gSA It is seen that in the Bay of Bengal or in the Gulf, most of the time the atmospheric phenomena, like, cyclone, hurricane etc. move towards right to its motion. To study such occurrences; we have considered fluid dynamics of ocean coupled with atmospheric motion. In the present study we have considered the eye of the cyclonic system that consist of fluid dynamical source and fluid dynamical sink at a small distance apart, and thus, constitute the fluid dynamical doublet of the object system. Then the fluid dynamical doublet of the object system and its image system has been considered with respect to a firm wall (here the sea shore). The fluid dynamical equation of complex potential with respect to the object system, the image system and the stream velocity have been undertaken. The complex potential of the object doublet, image doublet and the stream velocity have been considered. The velocity vector, consequently the pressure has been retrieve with the help of Bernoulli’s equation of fluid motion. Then the minimum /maximum pressure on the wall that is on the sea shore has been calculated analytically. Thus, it is found that on the basis of some prevailing conditions existing wind and energy the cyclone or hurricane move towards the sea coast or to the right of its motion.

AN EXPERIMENTAL STUDY OF THE EFFECT OF VORTEX GENERATOR ON THE FLAT-PLATE BOUNDARY LAYER

This paper is dealing with an experimental study to show the influence of the geometric characteristics of the vortex generators VG son the thickness of the boundary layer (∂) and drag coefficients (CD) of the flat plate. Vortex generators work effectively on medium and high angles of attack, since they are "hidden" under the boundary layer and practically ineffective at low angles. The height of VGs relative to the thickness of the boundary layer enables us to study the efficacy of VGs in delaying boundary layer separation. The distance between two VGs also has an effect on the boundary layer if we take into account the interference between two pairs of VGs. The effect of the changing in (h- the height of vortex generator, d- the average distance between tow vortex generators) on the thickness of the flat plate boundary layer and the drag coefficients has been studied for triangular vortex generator. The measurements of the vortex generator have been changed to determine the optimum boundary layer thickness and the change in drag coefficients. An experiment was done at an average free stream velocity, (U∞,) of 28 m/s. The experiment was conducted in the wind tunnel UTAD-2 University (NAU) Kiev, Ukraine.

Sediment oxygen demand in streams : lab measurements underestimate in situ rates substantially

Global warming is expected to affect stream metabolism significantly; and higher temperatures may lead to higher respiration and thus higher risk of oxygen depletion. It is, therefore, crucial to obtain reliable data on the oxygen dynamics in the different stream compartments. Determination of sediment oxygen demand (SOD) is typically based on lab or field measurement using cores or benthic chamber in which the actual physical conditions in the streams are not possible to mimic perfectly. We compared SOD based on lab core incubations with SOD measured in situ in stream sections where the oxygen exchange between water and air was eliminated artificially. The in situ SOD increased with increasing oxygen concentrations and both the temperature and the oxygen dependency of SOD increased with increasing organic content in the surface sediment. The laboratory rates reached 17 - 83% of the rates obtained in situ. The percentages were especially low at low stream velocity, likely reflecting a pure imitation of the physical conditions near the sediment in the lab when the sediment organic content was high (at low velocity). Therefore, alternative methods, simulating the natural horizontal water flow, are needed to provide reliable information on SOD in streams.

Vortex Trapping Cavity on Airfoil: High-Order Penalized Vortex Method Numerical Simulation and Water Tunnel Experimental Investigation

This paper presents a two-dimensional implementation of the high-order penalized vortex in cell method applied to solve the flow past an airfoil with a vortex trapping cavity operating under moderate Reynolds number. The purpose of this article is to investigate the fundamentals of the vortex trapping cavity. The first part of the paper treats with the numerical implementation of the method and high-order schemes incorporated into the algorithm. Poisson, stream-velocity, advection, and diffusion equations were solved. The derivation, finite difference formulation, Lagrangian particle remeshing procedure, and accuracy tests were shown. Flow past complex geometries was possible through the penalization method. A procedure description for preparing geometry data was included. The entire methodology was tested with flow past impulsively started cylinder for three Reynolds numbers: 550, 3000, 9500. Drag coefficient, streamlines, and vorticity contours were checked against results obtained by other authors. Afterwards, simulations and experimental results are presented for a standard airfoil and those equipped with a trapping vortex cavity. Airfoil with an optimized cavity shape was tested under three angles of attack: 3°, 6°, 9°. The Reynolds number is equal to Re = 2 × 104. Apart from performing flow analysis, drag and lift coefficients for different shapes were measured to assess the effect of vortex trapping cavity on aerodynamic performance. Flow patterns were compared against ultraviolet dye visualizations obtained from the water tunnel experiment.

Design and analysis of smoke flow visualization apparatus for wind tunnel

In the present study, the design and analysis of smoke generator are done for the low-speed wind tunnel. The wind tunnel fan is fitted with the Variable Frequency Drive to produce the wind speed in the range of 3 to 32 m/s with fan speed of 150 to 1500 rpm. The design of smoke generator was done according to Preston Sweeting mist generator principle corresponding to the free stream velocity of 3 m/s. A controlled smoke generator consisting of kerosene reservoir, controlled heater, blower, liquid column height adjustment mechanism, valves etc. was designed and fabricated. The smoke generator produced the smoke at the rate of 154 cm3/s which was close to the design flow rate of 149 cm3/s. To supply the required quantity of smoke in the wind tunnel, the smoke rake of NACA 0010 profile was developed and installed in the rapid contraction section of the wind tunnel to achieve the streamlined flow. The parametric studies were done on the smoke generator at different power inputs and its effects were studied on smoke temperature, smoke discharge and boiling time of the kerosene. The flow visualization was carried out on NACA 0015 airfoil model and the images were captured to examine the flow physics around them under different operating conditions.

Numerical investigations of flow around subsea covers at high Reynolds numbers

Two-dimensional (2D) numerical simulations of flow over wall-mounted rectangular and trapezoidal ribs subjected to a turbulent boundary layer flow with the normalized boundary layer thickness of δ/D = 0.73,1.96,2.52 (D is the height of the ribs) have been carried out by using the Reynolds-averaged Navier-Stokes (RANS) equations combined with the k – ω SST (Shear Stress Transport) turbulence model. The angles of the two side slopes of trapezoidal rib varies from 0° to 60°. The Reynolds number based on the free-stream velocity U ∞ and D are 1 × 106 and 2 × 106. The results obtained from the present numerical simulations are in good agreement with the published experimental data. Furthermore, the effects of the angle of the two side slopes of the trapezoidal ribs, the Reynolds number and the boundary layer thickness on the hydrodynamic quantities are discussed.

An Interval Process Method for Non-Random Uncertain Aeroelastic Analysis

In this paper, we use the Monte Carlo simulation to study aeroelastic behavior caused by non-random uncertain free-stream velocity. For sampling, we use the interval process method. Each family of samples is defined by a correlation function and upper and lower bounds. By using this sampling method, there is no need for constructing precise probability distribution functions; therefore, this method is suitable for practical engineering applications. We studied the aeroelastic behavior of an airfoil and a high aspect-ratio wing.

Evaluation of the global-blockage effect on power performance through simulations and measurements

Blockage effects due to the interaction of five wind turbines in a row are investigated through both Reynolds-averaged Navier-Stokes simulations and site measurements. Since power performance tests are often carried out at sites consisting of several turbines in a row, the objective of this study is to evaluate whether the power performance of the five turbines differs from that of an isolated turbine. A number of simulations are performed, in which we vary the turbine inter-spacing (1.8, 2 and 3 rotor diameters) and the inflow angle between the incoming wind and the orthogonal line to the row (from 0° to 45°). Different values of the free-stream velocity are considered to cover a broad wind speed range of the power curve. Numerical results show consistent power deviations for all the five turbines when compared to the isolated case. The amplitude of these deviations depends on the location of the turbine within the row, the inflow angle, the inter-spacing and the power curve region of operation. We show that the power variations do not cancel out when averaging over a large inflow sector (from −45° to +45°) and find an increase in the power output of up to +1 % when compared to the isolated case. We simulate power performance ‘measurements’ with both a virtual mast and nacelle-mounted lidar and find a combination of power output increase and upstream velocity reduction, which causes an increase of +4 % of the power coefficient. We also use measurements from a real site consisting of a row of five wind turbines to validate the numerical results. From the analysis of the measurements, we also show that the power performance is impacted by the neighboring turbines. Compared to when the inflow is perpendicular to the row, the power output varies of +1.8 % and −1.8 % when the turbine is the most downwind and upwind of the line, respectively.