To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Recent progress in flapping wing aerodynamics and aeroelasticity

Entropy Generation Minimization

The topic of this article is a review of the approach to extract pressure fields from flow velocity field data, typically obtained with particle image velocimetry (PIV), by combining the experimental data with the governing equations. Although the basic working principles on which this procedure relies have been known for quite some time, the recent expansion of PIV capabilities has greatly increased its practical potential, up to the extent that nowadays a time-resolved volumetric pressure determination has become feasible. This has led to a novel diagnostic methodology for determining the instantaneous flow field pressure in a non-intrusive way, which is rapidly finding acceptance in an increasing variety of application areas. The current review describes the operating principles, illustrating how the flow-governing equations, in particular the equation of momentum, are employed to compute the pressure from the material acceleration of the flow. Accuracy aspects are discussed in relation to the most dominating experimental influences, notably the accuracy of the velocity source data, the temporal and spatial resolution and the method invoked to estimate the material derivative. In view of its nature of an emerging technique, recently published dedicated validation studies will be given specific attention. Different application areas are addressed, including turbulent flows, aeroacoustics, unsteady wing aerodynamics and other aeronautical applications.

/pdf/piv-based-pressure-measurement-4meoomjt7c.pdf

PIV-based pressure measurement

In this paper, along with the progress of modern wind energy technology, the trends of wind energy technology and potential challenges have been studied thoroughly. It is estimated that within the next 2–3 decades, the Vertical Axis Wind Turbine (VAWT) can dominate the wind-energy technology. The VAWT requires less land space and using the same space; it is capable of producing more wind energy than that of its counterpart. By implying the Fish Schooling Concept effectively and successfully, it is possible to advance the wind-energy technology more. In the last 3–4 decades, the wind turbine capacity has been increased around 30–40 times. With the increase of wind energy capacity, the demand of the energy storage system has been increased significantly. Along with the many energy storage systems, fuel cells and batteries are the two most promising devices to meet the demand in RE systems. The wind-energy technology is established itself but not yet fully mature and hence there are many areas where improvements are required to reduce the cost of wind energy.

http://iran-tarjome.persiangig.com/other/1-s2.0-S1364032113000312-main.pdf

Progress and recent trends of wind energy technology

Wind tunnel blockage corrections: Review and application to Savonius vertical-axis wind turbines

The AIAA Fluid Dynamics Technical Committee’s Low Reynolds Number Discussion Group has introduced several “canonical” pitch motions, with objectives of (1) experimental-numerical comparison, (2) assessment of closed-form models for aerodynamic force coefficient time history, and (3) exploration of the vast and rather amorphous parameter space of the possible kinematics. The baseline geometry is a flat plate of nominally 2.5% thickness and round edges, wall-to-wall in ground test facilities and spanwise-periodic or 2D in computations. Motions are various smoothings of a linear pitch ramp, hold and return, of 40 and 45 amplitude. In an attempt to discern acceleration effects, sinusoidal and linear-ramp motions are compared, where the latter have short runs of high acceleration and thus high noncirculatory lift and pitch. Parameter variations include comparison of the flat plate with an airfoil and ellipse, variation of reduced frequency, pitch pivot point location and comparison of pitch to quasi-steady equivalent plunge. All motions involve strong leading edge vortices, whose growth history depends on pitch pivot point location and reduced frequency, and which can persist over the model suction-side for well after motion completion. Noncirculatory loads were indeed found to be localized to phases of motion where acceleration was large. To the extent discernable so far, closed-form models of lift coefficient on the pitch upstroke are relatively straightforward, but not so on the downstroke, where motion history effects complicate the return from stall. Broad Reynolds number independency, in flowfield evolution and lift coefficient, was found in the 10 to 10 range.

http://www.seas.ucla.edu/sofia/AIAA20101085.pdf

Résumé of the AIAA FDTC Low Reynolds Number Discussion Group's Canonical Cases

Spanwise flow contributes to lift in thin flat-plate zero-pitch-angle flapping wings in quiescent air. It is reasonable to maintain only the kinematics and mechanical complexity absolutely necessary in developing flapping-wing micro air vehicles. This study continues the quantification of the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift. A flapping-wing micro air vehicle with rectangular planform wings fabricated in-house (semispan aspect ratios from 1.5 to 4.0) was used to quantify the contributions to lift from flow along the span of wings at numerous points throughout the flapping cycle under a variety of operating conditions (3-6 Hz and Reynolds numbers of 6000-15,000). These experiments were performed for several aspect ratios for flat-plate and spanwise-cambered wings. The lift force was quantified experimentally using a force transducer and a high-speed camera. Digital particle image velocimetry was used to determine the lift contributions of spanwise flow to the total measured lift. Additionally, the presence of spanwise camber was shown to affect the transient lift behavior.

Lift from Spanwise Flow in Simple Flapping Wings

Trailing vortex structure of low aspect ratio wings was studied in a water tunnel at Reynolds numbers of 8000 and 24,000 using dye injection and digital particle image velocimetry in cross-flow planes in the near wake, for rectangular, semi-elliptical, and delta-wing planforms. The velocity data were used to calculate lift via circulation and effective span, and the results were compared with force balance measurements and classical inviscid theory. The objectives of the study were to assess how low-Reynolds number effects might affect the measurement of lift coefficient from tip-vortex circulation, how well the measurements fit the various theoretical models of lift curve slope for low aspect ratio wings, and the extent to which planform shape affects lift coefficient while aspect ratio and planform area are kept constant. All models were thin flat plates with square edges

Wake Vorticity Measurements for Low Aspect Ratio Wings at Low Reynolds Number

The presence of streamwise vorticity in the vicinity of the wing tip contributes to lift in thin flat plate zero pitch angle flapping wings in quiescent air. In creating flapping wing micro air vehicles it is desirable to maintain only the mechanical and kinematic complexity absolutely necessary to artificially duplicate flapping wing flight. This study quantifies the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift Using a flapping wing micro air vehicle with wings fabricated in-house, streamwise vortices were identified along the span of wings of various aspect ratios and at numerous different points throughout the flapping cycle under a variety of operating conditions. The lift generated by the flapping mechanism was quantified experimentally using a force transducer and a high speed camera. Digital particle image velocimetry was used to determine the contributions of streamwise vorticity to the total measured lift. Further evidence was found of the importance of the relationship between wing span and flapping frequency in the nature of the formation and shedding of vortices.

Aaron Altman

Papers

Wind tunnel blockage corrections: Review and application to Savonius vertical-axis wind turbines

Résumé of the AIAA FDTC Low Reynolds Number Discussion Group's Canonical Cases

Lift from Spanwise Flow in Simple Flapping Wings

Wake Vorticity Measurements for Low Aspect Ratio Wings at Low Reynolds Number

Streamwise Vorticity in Simple Mechanical Flapping Wings