Low-turbulence Wind Tunnel Research Articles

Aerodynamics of a NACA4412 Airfoil in Ground Effect

The flow characteristics over a NACA4412 airfoil are studied in a low turbulence wind tunnel with moving ground simulation at a Reynolds number of 3.0 x 105 by varying the angle of attack from 0 to 10 deg and ground clearance of the trailing edge from 5% of chord to 100%. The pressure distribution on the airfoil surface was obtained, velocity survey over the surface was performed, wake region was explored, and lift and drag forces were measured. To ensure that the flow is 2-D, particle image velocimetry measurements were performed. A strong suction effect on the lower surface at an angle of attack of 0 deg at the smallest ground clearance caused laminar separation well ahead of the trailing edge. Interestingly, for this airfoil, a loss of upper surface suction was recorded as the airfoil approached the ground for all angles of attack. For angles up to 4 deg, the lift decreased with reducing ground clearance, whereas for higher angles, it increased due to a higher pressure on the lower surface. The drag was higher close to the ground for all angles investigated mainly due to the modification of the lower surface pressure distribution.

Vortex Wakes Of Bird Flight: Old Theory, New Data AndFuture Prospects

Flying birds leave a vortex wake. Fluid dynamic theory in the form of Helmholtz’ theorems dictate the allowable topologies, and Kelvin’s circulation theorem requires that changes in wake circulation are directly proportional to force changes on the wing/aerofoil that generated the wake. Much bird flight research has therefore been focused on the properties of trailing wake vortices behind birds, since an accurate quantitative description of these will reveal also the aerodynamics of bird wings. The first vortex theory of bird flight assumed the periodic shedding of discrete vortex loops, each one generated during a downstroke, while the upstroke was considered aerodynamically functionless. This view received some support from early visualization experiments of take-off flight or very low speeds, while experiments at a higher speed (U = 7 m/s) in one species showed undulating wing-tip vortices of similar circulation on both down and upstroke. The necessary force asymmetry between downstroke and upstroke was obtained by wing flexing during the upstroke. Then followed an almost 20-year drought, with no further quantitative experiments, until recently when digital particle imaging velocimetry (DPIV) was successfully deployed in a low-turbulence wind tunnel, and where the same small (30 g) bird could be studied across a large range of flight speeds (4–11 m/s). These new experiments revealed a much more complicated wake pattern than previous data suggested, mainly due to the improved experimental resolution. The bird generated structures most closely resembling vortex loops at slow speeds, which gradually transformed into something similar to a constant circulation wake at the highest speeds. However, the wakes were never as clean as the idealized cartoon models of the vortex theory of bird flight, and previous paradoxical results were shown to be attributable to the resulting difficulty in accounting for all wake components. New DPIV data on other species indicate that these findings are quite general.

Effects of transverse air flow on burning rates of rectangular methanol pool fires

The effects of a transverse air flow on the burning rate of square and rectangular methanol pools have been studied experimentally. The variables altered were the size and shape of the pool, and the air speed. The responding variable measured was a characteristic burning rate. Direct images of flame luminosity were collected to help interpret the burning rate data. The square pools ranged in size from 7.5 × 7.5 to 30 × 30 cm . The rectangular pools tested were the same area as a 15 × 15 cm square pool, but with aspect ratios of 4:1. The air flow ranged in speed from quiescent to 5.5 m/s and was provided by a low-turbulence wind tunnel. The pools were held outside the tunnel boundary layers, so they would experience near-uniform upstream air flow conditions, and oriented perpendicular to the pool's leading edge. The results of this study were that the quiescent burning rate per unit area was essentially constant for all pools tested. As the transverse air flow was introduced a variety of responses in burning rate were observed for different sizes and shapes of the pool. Depending on the size of the square pool, the burning rate showed a 2.5 times monotonic increase, invariance, or nonmonotonic response to increasing air speeds. The nonmonotonic burning rate response corresponded to the flame separating from the trailing edge of the pool. The use of pools of the same area, but different shape showed that the response of burning rate to air speed does not correlate to pool area, but supported an analysis based on the ratio of momentum to buoyancy forces that suggested that the characteristic dimension for combustion was the stream-wise length of the pool.

Active control of boundary-layer instabilities

Active control of spatially evolving three-dimensional instability waves in the boundary layer of a flat plate with zero pressure gradient has been investigated both numerically and experimentally. The boundary layer was artificially excited by various computer-generated perturbations in a low-turbulence wind tunnel so as to create a three-dimensional field of instability waves. Sensors were used to detect the on-coming disturbances and appropriate control signals sent to the downstream actuators to generate counteracting disturbances. The key element of this type of active control is the determination of the transfer function of the control system connecting the sensors with the actuators. Modelling by linear theory showed that the amplitudes of the disturbances downstream were significantly suppressed when the causal transfer function was applied. To simplify the control problem different strategies using approximations to the transfer function were used. It was shown that with a very simple transfer function almost the same result could be achieved as with the full causal transfer function. The modelling was validated experimentally, first by an off-line control using different control modes, i.e. different transfer functions, and then by a real-time hardware implementation using one of the modes. Good agreement between the experimental measurements and numerical predictions indicated that a simple control strategy could be developed to inhibit the growth of the three-dimensional instability waves over extensive regions of the flow.

Metabolic costs of avian flight in relation to flight velocity: a study in Rose Coloured Starlings (Sturnus roseus, Linnaeus)

The metabolic costs of flight at a natural range of speeds were investigated in Rose Coloured Starlings (Sturnus roseus, Linnaeus) using doubly labelled water. Eight birds flew repeatedly and unrestrained for bouts of 6 h at speeds from 9 to 14 m s(-1) in a low-turbulence wind tunnel, corresponding to travel distances between 200 and 300 km, respectively. This represents the widest speed range where we could obtain voluntarily sustained flights. From a subset of these flights, data on the wing beat frequency (WBF) and intermittent flight behaviour were obtained. Over the range of speeds that were tested, flight costs did not change with velocity and were on an average 8.17+/-0.64 W or 114 W kg(-1). Body mass was the only parameter with a significant (positive) effect on flight costs, which can be described as EE(f)=0.741 M(0.554). WBF changed slightly with speed, but correlated better with body mass. Birds showed both types of intermittent flight, undulating and bounding, but their frequencies did not systematically change with flight speed.

Colours in a complex fluid flow

The addition of colours to flow visualization allows a third dimension to be added to the intrinsically two-dimensional information rendered by an image. In the present work, the velocity domain and the wall heat transfer in the near field of a jet in cross-flow is experimentally investigated by making extensive use of coloured images. Tests are performed in a low turbulence wind tunnel at a jet Reynolds number equal to 8000 and for velocity ratios ranging from 1 to 5. Data are obtained with Particle Image Velocimetry (PIV) and with infrared thermography applied to the steady state heated-thin-foil technique.

Quantitative studies of the wakes of freely flying birds in a low-turbulence wind tunnel

A novel application of DPIV methods is presented for measuring velocity and vorticity distributions in vertical cross sections through the wake of a freely flying bird (thrush nightingale) in a wind tunnel. A dual-camera system is used, and successive cross-correlation operations remove lens/camera distortions, and then the undisturbed background flow, so that the final operation simply examines the disturbance effect of the bird alone. The concentration and tuning of processing methods to the disturbance quantities allows full exploitation of the correlation calculation and estimation algorithms. Since the ultimate objective is to deduce forces and power requirements on the bird itself from the wake structure, the analytical procedure is followed through an example on a fixed airfoil, before sample results from extensive bird flight tests are described. The wake structure of the thrush nightingale in slow (5-m/s) flight is qualitatively quite similar to those previously described in the literature, but certain quantitative details are different in important respects.

Wing in Ground Effect Characteristics of Circular-Arc Aerofoils for Aero-Train

In this paper, the aerodynamic characteristics of a symmetrical ground effect wing are measured in a low turbulence wind tunnel. To change the directions of an Aero-Train in practical use, it must shuttle back and forth. Circular-arc aerofoil seems to be suitable for the Aero-train, while it can run back and forth by changing only the attack angle of wings. Lift and drag characteristics of NACA 6412-modified, circular-arc aerofoils with thickness ratio 9% and 12% were clarified by the 6-component balance. Suction and moving-belt system were used to simulate the ground. Characteristics of two-dimensional aerofoils and three-dimensional wings were compared, and lift-to-drag rations of three aerofoils were compared. Separations on the upper surface of the circular-arc aerofoils were visualized by tuft and oil spot method. It was found that increase of drag in ground effect condition was caused by increase of the induced drag and by enlargement of the separation area on upper surface of circular-arc aerofoils. A circular-arc aerofoil with thickness ratio 9% at the angle of attack 2 degrees had enough lift-to-drag ratio for Aero-Train.

Quantitative studies of the wakes of freely flying birds in a low-turbulence wind tunnel

A novel application of DPIV methods is presented for measuring velocity and vorticity distributions in vertical cross sections through the wake of a freely flying bird (thrush nightingale) in a wind tunnel. A dual-camera system is used, and successive cross-correlation operations remove lens/camera distortions, and then the undisturbed background flow, so that the final operation simply examines the disturbance effect of the bird alone. The concentration and tuning of processing methods to the disturbance quantities allows full exploitation of the correlation calculation and estimation algorithms. Since the ultimate objective is to deduce forces and power requirements on the bird itself from the wake structure, the analytical procedure is followed through an example on a fixed airfoil, before sample results from extensive bird flight tests are described. The wake structure of the thrush nightingale in slow (5-m/s) flight is qualitatively quite similar to those previously described in the literature, but certain quantitative details are different in important respects.

Bubble Bursting and Laminar Separation Unsteadiness on a Multi-Element High Lift Configuration

The purpose of this work is to gain insight into the problem of the bursting of a laminar separation bubble in the nose region of a slotted flap on a multi-element airfoil configuration. The interest in bubble bursting, which, especially on multi-element configurations has not been thoroughly investigated yet, was raised by the experimental results obtained while testing a computer designed flap in the low speed low Turbulence wind tunnel of the Delft University of Technology. A sudden and dangerous stall was found to occur on the flap surface due to the bursting of a bubble in its nose region. CFD calculations, both laminar and RANS, were first performed on a single element configuration, namely the Eppler E387 airfoil (Re = 105) and then on the multi-element configuration tested in the wind tunnel (Re = 2 ·106). The results show that a strong vortex shedding can potentially originate from the separated shear layer, both on the flap and on the single element airfoil, suggesting a close connection between the two cases. The effect of transition on the structure of the bubble was investigated by imposing and then varying the transition point. The computational results suggest that the unsteady behavior of the laminar bubble has to be regarded as a major issue in order to find an effective bursting predictor method for airfoil design.

Wing Rock Suppression Using Recessed Angle Spanwise Blowing

Introduction O NE of the limitationsto combat effectivenessof Ž ghter aircraft is wing rock—a high angle-of-attack dynamic motion manifested primarily in a limit-cycle roll oscillationsphenomenon. It is an aerodynamicphenomenonthat causeslossof stabilityand control in the lateral/directional mode. The leading-edge vortices emanating from the high-performance Ž ghter aircraft are one of the main causes of wing rock at high angles of attack. An effective means of suppressing wing rock is through the direct control and manipulation of leading-edge vortices at these  ight regimes. The technique of “blowing”2i4 has been of greatest interest because it is found to be highly effective in controlling the vortex orientation. This Note involves the experimental implementation of one such blowing technique—recessed angle spanwise blowing (RASB), using a simple rule base controller to a slender delta wing model in a lowturbulence wind tunnel. Through interpretation of the observations and results obtained from the experiments,an analysis is conducted to assess the effectiveness of RASB in suppressing wing rock. The results are substantiated by the  ow visualization experiments and force balance experiments. The excellent correlationsof the results and observations demonstrate the effectiveness of the RASB technique in its stipulated role of wing rock suppression.

Design and Testing of a Winglet Airfoil for Low-Speed Aircraft

The PSU 94-097 airfoil has been designed for use on winglets of high-performance sailplanes. The design problem is difficult because the airfoil must operate over a wide range of Reynolds numbers, and this range includes values that are relatively low. To validate the design tools, as well as the design itself, the airfoil was tested in the Penn State Low-Speed, Low-Turbulence Wind Tunnel from Reynolds numbers of 2.4 × 10 5 to 1.0 × 10 6 . In addition to transition-free measurements, potential drag reductions using artificial turbulators were explored, although the benefits were found to be limited for this application. Finally, performance predictions from two well-known computer codes are compared to the data obtained experimentally, and both are found to generate results that are in good agreement with the wind-tunnel measurements. Nomenclature CP pressure coefficient, (pl - p∞ )/q∞ L. lower surface R Reynolds number based on free-stream conditions and airfoil chord S. boundary-layer separation location, xS/c T. boundary-layer transition location, xT/c U. upper surface c airfoil chord cd profile-drag coefficient cl section lift coefficient cm section pitching-moment coefficient about the quarter-chord point p static pressure, Pa (lbf/ft 2

On the Viscous Drag of a Plate with Spherical Recesses

The drag of flat plates with spherical recesses was measured using the direct balance method. The experiments were run in a low-turbulence wind tunnel with a cross-section measuring 1000 × 1000 mm and a length of 400 mm. Three surfaces with recesses 7.0, 3.9, and 1.3 mm in diameter and 0.5, 0.3, and 0.2 mm deep, respectively, were tested. It is shown that on the Reynolds and Mach number ranges Re= (3–9)· 106 and M ≤ 0.3 the spherical recesses add to the drag of a flat plate in turbulent flow. The recesses have almost no effect on the location of laminar-turbulent transition, which occurs at Ret ∼ 3· 106.

Experimental study of flow past a square cylinder at high Reynolds numbers

Measurements of two-components of velocity in the wake of a square cylinder using a hot-wire anemometer are reported. Two Reynolds numbers, namely 8700 and 17,625, have been considered. The measurements were carried out in a low-speed, low-turbulence wind tunnel. Benchmark experiments at much lower Reynolds numbers show good agreement between the present experiments and published results. At higher Reynolds numbers, the experimental data reveal anticipated trends in terms of wake recovery and turbulence decay. Both velocity and velocity fluctuations show symmetry about the wake axis. The experimental data have been compared with the large eddy simulation (LES) calculation reported by Wang et al. [University of Illinois at Urbana – Champaign (1996) Report CFD 96-03] and LDV measurements of Lyn et al. [J Fluid Mech (1995) 304: 285–319]. The agreement among the three sets is generally acceptable in terms of the time-averaged velocity components, but not the velocity fluctuations. The turbulence fluctuations in the present experiments are seen to be lower than in the referred work. The differences have been traced to factors such as the aspect ratio, blockage ratio and upstream turbulence. Experiments with increased upstream turbulence did show a reduction in the discrepancy between the present experiments and the published data. An assessment of the experimental data in terms of physical mechanisms revealed that (a) streamwise normal stresses were correlated with the vortex centers, and (b) the turbulence kinetic energy profiles are similar to the turbulence shear stress. Spectral analysis of the velocity signals was carried out in the present work. Energy transfer from the mean flow to the streamwise velocity fluctuation was confirmed in the near wake. A redistribution of the kinetic energy between the streamwise and transverse components of velocity over a longer distance downstream was subsequently observed.

Ballistic seed projection in two herbaceous species

We found that the majority of ballistic seeds tested [N = 74 of 78 in Vicia sativa ssp. nigra (Fabaceae); N = 25 of 27 in Croton capitatus var. capitatus (Euphorbiaceae)] were projected at angles that would yield a greater distance than the average of seeds with the same initial speed projected at random angles. In addition, the median of fractional distance error (maximum distance - seed distance)/(maximum distance), of the seeds were 0.11 and 0.04 for V. sativa and C. capitatus, respectively. Seed projection distance was modeled by using initial projection angle, initial speed, and measured drag, along with other seed data. We improved upon previous such studies by using dual-angle high-speed stroboscopic photography to determine initial projection angle and speed. We also measured seed drag in a low-turbulence wind tunnel. Seed projection positions on the plant, which also affect seed projection distance, were found to be primarily from the top of the plant, with 98 of 137 and 407 of 407 fruits in the upper half of the plant for V. sativa and C. capitatus, respectively. Our findings are significant because they suggest that in addition to the ballistic projection mechanism itself, the species studied have additional adaptations that result in enhanced seed projection distance from the parent plant.

Transition detection for laminar flow aircraft using microphones beneath the surface of laser drilled suction panels

The possibility of detecting transition through the very small laser drilled perforations in panels representing the suction surface of a hybrid laminar flow aircraft is examined. The method uses miniature microphones to detect changes to the noise received from the boundary layer. Tests using a flat plate rig in a low-turbulence wind tunnel at Reynolds numbers up to 3.8 million per metre, demonstrate that the boundary layer state can be defined in this manner, most simply through measurement of the root mean square (r.m.s.) of the microphone signal. It is shown that the r.m.s. reaches a peak in the transition zone and that when the boundary layer is fully turbulent the value is still significantly higher than it was before transition. Porosity in the range 0.8-6.4 percent was examined, with nominal hole diameters of 0.06 and 0.10 mm in 0.9 mm thick laser drilled suction surface specimens. Suction flow through the surface was found not adversely to affect the operation of the system. The experiment was limited to low Reynolds numbers because the high background noise in the wind tunnel made detection of the boundary layer element of the signal increasingly difficult to define as speed increased. It is considered that test in flight will be needed to prove fully the validity of the method. A preliminary design of an installation for this purpose is suggested that allows the suction flow to be maintained over the measuring region.

Boundary-layer instability noise on aerofoils

An experimental and theoretical investigation has been carried out to understand the tonal noise generation mechanism on aerofoils at moderate Reynolds number. Experiments were conducted on a NACA0012 aerofoil section in a low-turbulence closed working section wind tunnel. Narrow band acoustic tones were observed up to 40 dB above background noise. The ladder structure of these tones was eliminated by modifying the tunnel to approximate to anechoic conditions. High-resolution flow velocity measurements have been made with a three-component laser-Doppler anemometer (LDA) which have revealed the presence of strongly amplified boundary-layer instabilities in a region of separated shear flow just upstream of the pressure surface trailing edge, which match the frequency of the acoustic tones. Flow visualization experiments have shown these instabilities to roll up to form a regular Kármán-type vortex street.A new mechanism for tonal noise generation has been proposed, based on the growth of Tollmien–Schlichting (T–S) instability waves strongly amplified by inflectional profiles in the separating laminar shear layer on the pressure surface of the aerofoil. The growth of fixed frequency, spatially growing boundary-layer instability waves propagating over the aerofoil pressure surface has been calculated using experimentally obtained boundary-layer characteristics. The effect of boundary-layer separation has been incorporated into the model. Frequency selection and prediction of T–S waves are in remarkably good agreement with experimental data.

回流式風洞における煙を使用した可視化実験装置

An apparatus for flow visualization around oscillating wing was designed and assembled. To validate the modern CFD. the experimental data not only about the pressure distributions on the wing surfaces but also the flow patterns around the wing are needed. The present system aims to offer the experimental information on the flow patterns around oscillating 2-D wings. In this report, the outlines of the driving mechanism for the 2-D wing and the smoke generator are presented together with the related other devices. It is shown that the system works well in the low-turbulence wind tunnel at NAL and expected that it can be used for the research of low speed unsteady aerodynamics and aeroelasticity.

A Study on the Wing Structure and Flapping Behavior of a Dragonfly.

This study is concerned with the wing structure and aerodynamic characteristics of a dragonfly in flight. The structural properties of dragonfly wings were studied through measurements of certain morphological parameters. The scanning electron microscopic observation showed the morphological characteristics of the dragonfly wing. Dragonflies were examined in a small low-turbulence wind tunnel. In the experiment on the measurements of wing flapping, an optical displacement detector was used to measure the displacement of the dragonfly wing. In the experiment on the measurements of the velocity fluctuation, a hot-wire anemometer was used to measure the velocity field. The spectrum of dragonfly flight was revealed by the measurements of velocity fluctuation.

Measurement and Analysis of the Flowfields Induced by Suction Perforations

The flow physics of laminar flow control suction surfaces is revealed by performing a detailed fundamental experimental investigation of an isolated suction perforation. A unique series of nonintrusive, high-resolution measurements are obtained using a three-component laser Doppler velocimetry system, and experiments are conducted in a low-speed, low-turbulence wind tunnel. The suction perforation flowfields are mapped for a range of sub- and supercritical suction rates and are found to be highly three dimensional. A rich variety of flowfield features is observed, including a pair of counter-rotating longitudinal vortices, multiple corotating longitudinal vortices, spanwise variations of the mean flow, and inherently unstable boundary-layer profiles. Critical suction limits, over a range of freestream speeds, are determined, and a new design criterion for critical suction is established. It is also shown that for sufficiently small perforations, irrespective of suction flow, boundary-layer transition does not occur. Further analyses of the measurements explore the possibility of interaction between the crossflow vortices and the suction-induced longitudinal vortices