Wind Tunnel Velocity Research Articles

Aerodynamic performances and near wake of an Ahmed body under unsteady flow conditions

This paper experimentally characterizes unsteady effects and flow fields around the Ahmed Body, by analyzing global forces and detailed wake effects. The results are compared to those obtained under steady conditions, with varying wind tunnel velocities and different yaw angles between the model and the free stream. Unsteady fields are generated by means of oscillating blades positioned at the inlet of the test section, whose amplitudes and frequencies can be easily controlled. Specifically, low frequencies, around a few Hertz, as those in the typical range generating load oscillations on vehicles, are considered. The results in terms of force coefficients, obtained by a dynamometric balance, and velocity fields, obtained by Particle Image Velocimetry, are processed in order to derive time-average statistics and also phase-average statistics, as related to forcing blade instantaneous positioning. This type of analysis can be performed thanks to the high temporal resolution of measurement systems, around 100 Hz for the force measurements and around 4000 Hz for the velocity measurements. Results in steady conditions well compare with previous results in references, both as functions of wind tunnel velocity and yaw angles. In unsteady conditions, whatever amplitude is considered, time-average drag and lift coefficients and their dependence on yaw angle are consistently lower compared to the steady case. Phase-averaged coefficients in unsteady conditions can oscillate by around 20 % in comparison to time-average values and these fluctuations are strongly dependent on yaw angle and amplitude of oscillations, thus suggesting that they both contribute to instantaneous loads. Present investigations are related to improvements in set-up of control systems in assisted-driving (self-driving) vehicles.

A multivariate statistical analysis of the noise emitted by an installed propeller

Novel-aircraft concepts consider the possibility of placing the propulsor very close to the fuselage to ingest the incoming airframe boundary layer. In this configuration, the engine takes the inflow at a reduced velocity, consuming less fuel in the combustion process. However, this induces a series of noise consequences that alter tonal and broadband noise components. The present work reports an experimental investigation to analyse the sound emitted by a propeller ingesting a turbulent boundary layer. Experiments have been performed in the anechoic wind tunnel at the University of Bristol and the set-up consisted of a two-bladed propeller close to a tangential flat plate to simulate the installed effects. A tripping device was placed 1 m upstream of the propeller and was used to generate a turbulent boundary layer at the propeller location. The wind tunnel velocity was fixed at 33 m/s keeping the advance ratio at J = 0.65. Far-field noise has been acquired using a microphone array positioned parallel to the plate, directly overhead of the propeller. The data were analysed in the frequency domain, providing a characterisation of spectral quantities. Furthermore, wavelet analysis was performed to investigate the time evolution of the identified pressure features. Results show evident haystacking humps close to higher BPF harmonics, due to the ingestion of the boundary layer. Moreover, the wavelet analysis revealed the intermittent nature of the haystacking humps, clearly visible at higher BPF harmonics with low-frequency energy content.

Terminal Velocity of Wheat Stem Nodes versus Internodes for Similar Particle Dimensions

Highlights Terminal velocities were measured for wheat stem nodes and internodes for similar particle dimensions to investigate the feasibility of aerodynamic separation. Mean measures of terminal velocities for wheat stem nodes and internodes were 4.91 and 3.35 m s-1, respectively, that coincided with values of 4.92 and 3.37 m s-1 calculated for spherical particles (Mohsenin, 1970). Wheat stem particle mass ranged from 0.015 (internode) to 0.041 g (node) that significantly correlated with terminal velocity ranging from 3.13 to 5.14 m s-1, respectively. Wheat stem particle density ranged from 112 to 297 kg m-3 that significantly correlated with terminal velocity ranging from 3.12 to 5.11 m s-1, respectively. Abstract. Efficient separation of physiological plant components potentially improved the targeting of components to best uses. The terminal velocity property used an opposing air velocity to equilibrate particle weight with the sum of the drag and buoyancy forces. This study used particles of similar dimensions to ascertain the effect of particle mass and density on experimental measures of terminal velocity in a wind tunnel and as calculated by reliable equations. Similar particle diameters, lengths, and volumes of wheat stems ranged from 0.362 to 0.376 cm, 1.25 to 1.28 cm, and 0.131 to 0.141 cm3, respectively. Moisture content was 12% wet basis. Wheat stem internodes had individual particle mass and density ranging from 0.015 to 0.019 g and 113 to 144 kg m-3, respectively, and mean Terminal Velocity Wind Tunnel (TVWT) terminal velocities for wheat stem internodes that ranged from 3.13 to 3.58 m s-1. Nodes had individual particle mass and density ranging from 0.031 to 0.041 g and 236 to 297 kg m-3, respectively, and mean TVWT terminal velocities for wheat stem nodes that ranged from 4.62 to 5.14 m s-1. Thus, no overlap in values was observed for particle mass, particle density, and terminal velocity between wheat stem internode and wheat stem node. This observation supports the potential of using terminal velocity to separate node from internode for similar-sized wheat stems at a given moisture content. Keywords: Aerodynamic separation, Anatomical component, Biomass property, Physical experiment, Sorting, Terminal velocity, Vertical wind tunnel, Wheat stem particles.

Single-pass wind tunnel testing for recirculating virus aerosol control technologies

A number of recirculating flow aerosol control technologies have been commercialized to mitigate aerosol-transmitted virus infections. Many of these technologies incorporate filters for particle collection and some may also incorporate technologies for virus inactivation. Given the wide variety of commercially available aerosol control technologies to consumers, it is extremely important to develop standardized methods to characterize their performance in bioaerosol removal and inactivation, such that technologies can be compared on an “equivalent-test” basis. However, no standard procedures have been established to evaluate the effectiveness of bioaerosol removal and inactivation in recirculating aerosol control technologies. We propose the use of a single-pass tunnel to assess the performance of bioaerosol control technologies, as single-pass wind tunnels can be sealed with well-controlled velocity and particle concentration profiles. Here, we specifically describe the construction of a single-pass wind tunnel and apply it to three recirculating aerosol control technologies, incorporating UV-C sources, filters, and electrostatic precipitators, respectively. We utilize a porcine respiratory coronavirus (PRCV) challenge aerosol, generated via pneumatic nebulization of a high titer (∼107 TCID50 mL−1) viral suspension. Following guidelines similar to those used in the ANSI/ASHRAE Standard 52-2 test procedure for HVAC filters, in single-pass wind tunnel tests, velocity uniformity and particle uniformity are first monitored across the cross-section of the tunnel. The size distribution of viable particles is additionally determined in advance of tests by the collection of particles in the wind tunnel using a cascade impactor, with both RT-qPCR and titration used to quantify viruses collected on each impaction stage. We show that the viable particle size distribution follows the volumetric size distribution of the nebulized virus-laden suspension, and that this distribution can be tuned to be similar in shape to the observed distribution of aerosol from human respiratory activities. Following tunnel and virus aerosol characterization, for each tested technology, using triplicate tests, the single-pass log reduction based on RT-qPCR and viable virus titration is determined by simultaneously collecting virus aerosol particles upstream and downstream of the control technology. The tested technologies in this study have titration-based single-pass log reductions in the 1.5–4.0 range.Overall, design and testing suggest that the single-pass wind tunnel approach is a tractable method to examine the efficacy of aerosol control technologies in removing and inactivating viruses in aerosols, and suggest that such technologies should be described by their single-pass log reduction and operating flow rate, with the test virus size distribution reported alongside test results. In addition, we examine the limits of detection in single-pass wind tunnel tests in comparison to chamber tests, and in doing so find that for most control technologies, the wind tunnel test will yield higher concentrations downstream or during sampling, and hence clearer results for the log reduction.

Application of oil and dye flow visualization in incompressible turbomachinery flows

Flow visualization is one of many available tools in experimental fluid mechanics and is used from the primary stages of fluid mechanics research in order to identify the physical sizes and locations of the flow features under consideration. Most of the fluids used in engineering applications are transparent (water, air, etc) and flow visualization techniques are used in order to make their flow patterns visible. Simple flow visualization experiments are relatively inexpensive and they can be easily implemented providing with a first feeling of the characteristics of the flow domain. Subsequently, flow visualization techniques are of great applicability in complex flow fields and especially in turbomachinery applications where the flow is characterized by three dimensional and secondary flow patterns. In general, fluid motion can be visualized by surface flow visualization, by particle tracer methods or by optical methods. The former flow visualization technique reveals the streamlines of fluid flows around a solid surface. In this paper flow visualization techniques applied in two different cases of experimental testing (fans in crossflow and cascade experiment) are presented. In both cases, the mixture of paint was prepared using a highly volatile light mineral or heavy machine oil of viscosities of approximately100cPand200cP, respectively, together with very fine pigments of Titanium Dioxide (TiO2) or fluorescein sodium in various colors. After the preparation of the mixture, a homogenous thin film was applied onto the whole plate surface by painting it with a soft brush. The air stream which flows over the surface of the plate, modifies the concentration and the homogeneity of the oil film, according to the flow conditions very close to the wall. The film was dried by the airflow and photographed for further consideration while the time taken for drying depended on the wind tunnel velocity as well as, on the pigmentation of the mixture. Successful and un-successful flow visualization tests are herein presented while each case is respectively commented as far as the mixtures, the proportions used and the application onto the rigs are concerned.

Rotational and translational oscillations of cylinders of small aspect ratio in the air flow

Rotational and translational oscillations of three cylinders differing in the ratio of length to cylinder diameter (elongation) are considered. The cylinders were suspended in the test section of the low velocity wind tunnel on a wire suspension containing steel springs. In the equilibrium position, the axis of the cylinders is directed horizontally and parallel to the velocity vector of the incoming flow. Under the influence of the air flow, the cylinders could make rotational or translational oscillations. Semiconductor strain gauges are attached to the two suspension springs, measuring the periodically changing tension of the springs during oscillations. The analog signal from the strain gauges was transmitted to a PC oscilloscope, which transmitted it in digital form to a computer. After calibration of the device and decoding of the signal, the frequencies and amplitudes of translational oscillations in the vertical direction and rotational oscillations around the horizontal axis passing through the center of the cylinder and perpendicular to the velocity vector of the incoming flow were determined. It turned out that in the studied range of cylinder elongations there is a transition from rotational to translational oscillations. A cylinder with elongation 1.9 performs steady rotational oscillations under the influence of wind, the amplitude of which increases with increasing air flow velocity. The previously proposed mathematical model correctly predicts rotational oscillations. The square of the amplitude of rotational oscillations is a linear function of the Strouhal number if the velocity of the air flow is sufficiently large. Translational oscillations of the cylinder with elongation 1.9 are damped. A decrease in the elongation of the cylinder to 1.5 is accompanied by a decrease in the amplitudes of rotational vibrations. At low air flow velocities, translational oscillations with a small amplitude are registered. Further reduction of the elongation to 1.0 leads to the complete absence of rotational oscillations. The amplitude of translational oscillations is growing. Translational oscillations are realized in a limited range of air flow velocities.

Sensitivity Investigation on the Pressure Coefficients Non-Dimensionalization

The scaling of large structures to investigate their aerodynamics in wind tunnels is a common and robust procedure to estimate important magnitudes, including pressure coefficients. Different aspects can affect the estimation of pressure coefficients; four examples are the non-dimensionalization, blockage, non-stationarity, and non-Gaussianity of the wind tunnel velocity. This paper shows the variability of pressure coefficients due to these four aspects for the case study of a closed box section of a suspended bridge. It was estimated that the pressure coefficients of similar pressure taps vary significantly due to different sets of wind velocity time history used to non-dimensionalize the wind tunnel pressures. In addition, the stationarity of the wind velocity process was not confirmed for all wind velocity sets and the non-Gaussianity of the wind velocity time history was confirmed.

Droplet Size Distribution Characteristics of Aerial Nozzles by Bell206L4 Helicopter under Medium and Low Airflow Velocity Wind Tunnel Conditions and Field Verification Test

To investigate the spray atomization characteristics of aerial nozzles adapted to manned agricultural helicopters under medium-low airflow velocity (0 to 27.8 m/s) conditions, the droplet size test was carried out in both wind tunnel and field tests. In the wind tunnel test, the laser diffraction device (LDD) was used to test the spray droplet size of CP02, CP03, and CP04 aerial nozzles. A Bell206L4 helicopter was used in the field test. The results in the wind tunnel test showed that due to the nozzles had been used for a long time and the cause of wear, the spray stability of individual nozzles was affected during the test. The limitation of droplet size measurement by using LDD was also found. The field test results showed that the main distribution range of the droplet size measured in the field test was consistent with the results of the wind tunnel test, but the droplet size value was significantly higher and the uniformity of droplet size distribution was poorer than the wind tunnel test value due to the influence of the actual environment. However, the results of field test and wind tunnel test can still be used as a reference for each other.

Power coefficient measurements of a novel vertical axis wind turbine

AbstractWind tunnel experiments were performed to evaluate the power coefficients of three vertical axis wind turbines (VAWTs), namely, (a) conventional VAWT (two‐bladed troposkien shape), (b) novel 50% shifted troposkien shape‐vertical axis wind turbine (50% STS‐VAWT), and (c) novel 100% STS‐VAWT. All turbines had the same height, swept area, and NACA0015 airfoil with the same chord length. During the power coefficient measurements for each one of the turbines, the wind tunnel velocity was varied while the turbine rotation was kept constant through rheostatic load adjustment. Two different turbine rotations were used (600 rpm and 700 rpm). The resulting tip speed ratios (TSRs) ranged from approximately 2 to 6 in the current investigation. Both the power coefficients and the tip speed ratios were corrected against wind tunnel blockage effects. The 50% STS‐VAWT model showed an overall better aerodynamic performance (both corrected and uncorrected power coefficients) when compared against the other two configurations, likely due to a combination of (a) power generation increased due to blade‐wake interaction (BWI) reductions and (b) power generation capacity related to the blade length of each turbine.

Experimental study of drag reduction on a golf driver club using discrete surface elements

A low-speed wind tunnel investigation is presented documenting drag reduction to a golf driver club. Geometrical modifications (or elements) were attached to the golf driver. The size, spacing, and location of the elements were varied. Wind tunnel velocities spanned a range from the amateur to professional golfer. Measurements included both force balance and surface pressure accompanied by flow visualization to aid in flow diagnostics. The results indicated that a 40% reduction in drag can be achieved, primarily due to reduced pressure over the forward face of the crown and increased pressure over the crown’s aft extents. This reduction in drag was estimated to yield a 0.54 m/s (1.2 mph) club head speed gain, causing a 3.65 m (4 yds) increase in carry distance.

Atomization characteristics of multi-type aerial nozzles in wind tunnel and low airflow velocity condition in manned agricultural helicopter

In order to explore the atomization characteristics of the LICHENG series nozzle for manned agricultural helicopter under medium and low airflow velocity conditions, droplet size tests on five sizes of nozzles 11002, 11003, 11004, 11006 and 11010 were carried out by laser diffraction device (LDD), based on the high-low speed composite wind tunnel for agricultural aviation designed by the National Center for International Collaboration Research on Precision Agricultural Aviation Pesticides Spraying Technology (NPAAC) of South China Agricultural University (SCAU). The results showed that under the three spray pressures of 30, 40 and 50 psi, the droplet sizes of the five types of nozzles expressed similar trends with the increase of airflow velocity. Among them, the droplet size variation of nozzles 11006 and 11010 with larger orifice size was the most significant, the largest range of D V0.1 was nozzle 11010, and the largest range of D V0.5 and D V0.9 was nozzle 11006. In addition, the changes of spray pressure would directly affect the quality of spray, especially the nozzles 11002, 11003 and 11004 with small orifice sizes were significantly affected. Under the condition of medium and low airflow velocity of 0-27.8 m/s, 89% of spread value (SV) by the five nozzles were in the range of less than 15%, but there were some test nozzles with poor spray stability during the test, which made the measurements of droplet size value larger, resulting in a maximum test deviation of up to 210.9 I¼m. The result also exposed the limitations exist in wind tunnel droplet size testing by using LDD, and an increase in airflow velocity resulted in a larger measurement droplet size. This phenomenon was particularly significant in the D V0.5 test results of various types of nozzles. This study can provide experimental data guidance for the optimization design and parameter selection of aerial nozzle for manned agricultural helicopter. Keywords: manned helicopter, aerial nozzle, wind tunnel, droplet size, agricultural application, spray classification DOI:  10.33440/j.ijpaa.20190201.0031  Citation: Yao W X, Lan Y B, Hoffmann W C,  Guo S, Chen S D, Wen S, et al.  Atomization characteristics of multi-type aerial nozzles in wind tunnel and low airflow velocity condition in manned agricultural helicopter.  Int J Precis Agric Aviat, 2019; 2(1): 9–17.

The role of wing bending deflection in the aerodynamics of flapping micro aerial vehicles in hovering flight

Scientists have been improving the aerodynamic performance of flapping micro aerial vehicles by drawing inspiration from birds and insect flight. In this research study, first, the flapping mechanism of the black-headed gull is designed and then it is constructed in order to investigate the effects of wing bending deflection on the aerodynamic performance. Thrust generation, power consumption and power loading are considered as performance parameters. Three wings representing different underlying structures, namely flexible membrane, rigid membrane and airfoil, are fabricated with the same planform to investigate the roles of flexibility, thickness and camber. Experiments are performed for flapping frequencies ranging from 1.5 Hz to 6 Hz, 10 degrees angle of attack and no wind tunnel velocity (hovering flight). The results indicate that the aerodynamic performance is improved by using the bending deflection mechanism in comparison with the simple flapping mechanism. Moreover, we can conclude that the performance of the airfoil wing is superior to flexible and rigid wings.

Design of a pulsed jet actuator for separation control

A pulsed jet actuator for separation control has been developed for application in a real-scale wind tunnel experiment with focus on separation behind the pylon-wing junction. This paper covers the last steps of hardware design and testing on the way to a successful demonstration of flow control at the pylon-wing junction. The concept of the actuator is based on fluidic technology and applies pulsed blowing without the help of moving parts. Design parameters are given by numerical simulations of the experimental application case and are validated in ground test. Results of these ground tests are presented and parameters for wind tunnel test settings are derived. The further evaluation of ground test results yields a relation of momentum coefficient, supply mass flow and wind tunnel velocities. It is shown that for the given wind tunnel experiment levels of constant momentum coefficient approximately coincide with levels of constant velocity ratio. All requirements with respect to flow control parameters and model geometry can be satisfied with the presented design, paving the way for a successful demonstration of flow control in a large-scale wind tunnel test.

DESIGN AND CONSTRUCTION OF A PROTOTYPE SOLAR UPDRAFT CHIMNEY IN ASWAN/EGYPT

This work is part of a joint project funded by the Science and Technology Development Fund (STDF) of the Arab republic of Egypt and the Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany. Continuation of the use of fossil fuels in electricity production systems causes many problems such as: global warming, other environmental concerns, the depletion of fossil fuels reserves and continuing rise in the price of fuels. One of the most promising paths to solve the energy crisis is utilizing the renewable energy resources. In Egypt, high insolation and more than 90 percent available desert lands are two main factors that encourage the full development of solar power plants for thermal and electrical energy production. With an average temperature of about 40 °C for more than half of the year and average annual sunshine of about 3200 hours, which is close to the theoretical maximum annual sunshine hours, Aswan is one of the hottest and sunniest cities in the world. This climatic condition makes the city an ideal place for implementing solar energy harvesting projects from solar updraft tower. Therefore, a Solar Chimney Power Plant (SCPP) is being installed at Aswan City. The chimney height is 20.0 m, its diameter is 1.0m and the collector is a four-sided pyramid, which has a side length of 28.5 m. A mathematical model is used to predict its performance. The model shows that the plant can produce a maximum theoretical power of 2 kW. Moreover, a CFD code is used to analyse the temperature and velocity distribution inside the collector, turbine and chimney at different operating conditions. Static calculations, including dead weight and wind forces on the solar updraft chimney and its solar collector, have been performed for the prototype. Mechanical loading and ambient impact on highly used industrial structures such as chimneys and masts cause lifetime-related deteriorations. Structural degradations occur not only from rare extreme loading events, but often as a result of the ensemble of load effects during the life-time of the structure. A Structural Health Monitoring (SHM), framework for continuous monitoring, is implemented on the solar tower. For the ongoing case study, the types of impacts, the development of the strategic sensor positioning concept, examples of the initially obtained results and further prospects are discussed. Additional wind tunnel tests have been performed to investigate the flow situation underneath the solar collector and inside the transition section. The flow situation in and around the SCPP has been simulated by a combination of the wind tunnel flow and a second flow inside the solar tower. Different wind tunnel velocities and volume flow rates have been measured respectively. Particle Image Velocimetry (PIV) measurements give some indication of the flow situation on the in- and outside of the solar tower and underneath the collector roof. Numerical simulations have been performed with the ANSYS Fluent to validate the experimental tests.

Comprehensive Evaluation of Fast-Response, Reynolds-Averaged Navier–Stokes, and Large-Eddy Simulation Methods Against High-Spatial-Resolution Wind-Tunnel Data in Step-Down Street Canyons

Three computational fluid dynamics (CFD) methods with different levels of flow-physics modelling are comprehensively evaluated against high-spatial-resolution wind-tunnel velocity data from step-down street canyons (i.e., a short building downwind of a tall building). The first method is a semi-empirical fast-response approach using the Quick Urban Industrial Complex (QUIC-URB) model. The second method solves the Reynolds-averaged Navier–Stokes (RANS) equations, and the third one utilizes a fully-coupled fluid-structure interaction large-eddy simulation (LES) model with a grid-turbulence inflow generator. Unlike typical point-by-point evaluation comparisons, here the entire two-dimensional wind-tunnel dataset is used to evaluate the dynamics of dominant flow topological features in the street canyon. Each CFD method is scrutinized for several geometric configurations by varying the downwind-to-upwind building-height ratio ( $$H_\mathrm{d}/H_\mathrm{u}$$ ) and street canyon-width to building-width aspect ratio (S / W) for inflow winds perpendicular to the upwind building front face. Disparities between the numerical results and experimental data are quantified in terms of their ability to capture flow topological features for different geometric configurations. Overall, all three methods qualitatively predict the primary flow topological features, including a saddle point and a primary vortex. However, the secondary flow topological features, namely an in-canyon separation point and secondary vortices, are only well represented by the LES method despite its failure for taller downwind building cases. Misrepresentation of flow-regime transitions, exaggeration of the coherence of recirculation zones and wake fields, and overestimation of downwards vertical velocity into the canyon are the main defects in QUIC-URB, RANS and LES results, respectively. All three methods underestimate the updrafts and, surprisingly, QUIC-URB outperforms RANS for the streamwise velocity component, while RANS is superior to QUIC-URB for the vertical velocity component in the street canyon.

Investigation of the boundary layer characteristics for assessing the DBD plasma actuator control of the separated flow at low Reynolds numbers

The present study intends to investigate the boundary layer characteristics to assess the potentiality of the single dielectric barrier discharge plasma actuators (SDBDPAs) to reattach the separated flow at low Reynolds numbers. The effect of the actuator geometrical parameters and of the Reynolds number on the device control authority was experimentally investigated. For this aim, a curved wall plate, which profile shape was designed to reproduce the suction surface of a low-pressure turbine (LPT) blade, was installed in the test section of a closed loop wind tunnel and a groove was made over it, at the front of the adverse pressure gradient region, for allocating a SDBDPA. Three actuators, characterized by different streamwise width, were manufactured by photolithography technique and they were tested. The velocity flow field, in both presence and absence of external flow, was investigated by particle image velocimetry (PIV) measurements. When the actuator was turned on, a sinusoidal voltage excitation with amplitude of 8kV and frequency of 2kHz was applied and the dissipated power (P‾el) was retrieved by electrical characterization.The effect of the active flow control was firstly estimated by analyzing the plasma induced velocity fields in absence of external flow. Subsequently the wind tunnel inlet free stream velocity (vx,∞in) was set to 1.54m/s. The velocity, turbulence intensity (Tu) and vorticity (ωz) fields together with the boundary layer shape factor (H12) and momentum coefficient (cμ) were evaluated in both presence and absence of actuation. All the aforementioned analyses together with the estimation of the device electrical-to-fluidic energy conversion efficiency (ηfm) allowed identifying the best actuator geometry. Then, that configuration was chosen to investigate the effects of the wind tunnel velocity on the device control authority. The tested vx,∞in values ranged from 1.54m/s up to 3.16m/s. In absence of actuation, a large reverse flow and high turbulence intensity was observed in the separation region. Considering the actuated cases, it was found that at P‾el≈7W, the SDBDPA operation always implied a reduction of the separated region, of the flow angle and of the turbulence intensity. Moreover, the plasma induced jet had a larger impact on the flow at lower velocities and a low flow control effect was noticed at the highest vx,∞in values. The H12 factor evaluation confirmed the flow regimes at the different tested velocities (i.e. cμ values). The whole data set allowed to evaluate the actuator success for separation control and to identify a threshold value of the cμ coefficient delimiting the still detached flow from the reattached one.

Experimental Study of CLARK-Y Smoothed Inverted Wing with Ground Effect

Aerodynamic characteristics of inverted single element airfoil with and without ground effect had been investigated experimentally in Low-Speed Wind Tunnel of test cross section area (0.7x0.7 m) and maximum velocity 55 m/s to quantify the aerodynamic characteristics of inverted CLARK-Y smoothed airfoil. The ground effect was introduced by using a fixed flat board moves vertically to produce the required distance between the airfoil and ground board. The model was tested with and without ground effect and at various wind tunnel velocities (Reynolds numbers), ride heights (ground clearances) and angles of attack. Data obtained in airfoil experiments include sectional forces and surface pressure data. Results are compared with the free-drive case and with published works. Data indicated that the pressure distribution increased at the upper and lower surfaces in ground proximity, at low angles of attack, and increased also with incidence. The negative lift coefficient increased with angle of attack except the angles of attack larger than 10 and with ground effects except at ride heights of less than 0.1c due to the force reduction phenomena. The drag coefficient increased with the ground effect which caused a decrease in the airfoil efficiency. The aerodynamic characteristics remain relatively an affected by the velocity change. The experimental data were compared with published works and showed good results.

Wind Tunnel Analysis of the Airflow through Insect-Proof Screens and Comparison of Their Effect When Installed in a Mediterranean Greenhouse.

The present work studies the effect of three insect-proof screens with different geometrical and aerodynamic characteristics on the air velocity and temperature inside a Mediterranean multi-span greenhouse with three roof vents and without crops, divided into two independent sectors. First, the insect-proof screens were characterised geometrically by analysing digital images and testing in a low velocity wind tunnel. The wind tunnel tests gave screen discharge coefficient values of Cd,φ of 0.207 for screen 1 (10 × 20 threads·cm−2; porosity φ = 35.0%), 0.151 for screen 2 (13 × 30 threads·cm−2; φ = 26.3%) and 0.325 for screen 3 (10 × 20 threads·cm−2; porosity φ = 36.0%), at an air velocity of 0.25 m·s−1. Secondly, when screens were installed in the greenhouse, we observed a statistical proportionality between the discharge coefficient at the openings and the air velocity ui measured in the centre of the greenhouse, ui = 0.856 Cd + 0.062 (R2 = 0.68 and p-value = 0.012). The inside-outside temperature difference ΔTio diminishes when the inside velocity increases following the statistically significant relationship ΔTio = (−135.85 + 57.88/ui)0.5 (R2 = 0.85 and p-value = 0.0011). Different thread diameters and tension affects the screen thickness, and means that similar porosities may well be associated with very different aerodynamic characteristics. Screens must be characterised by a theoretical function Cd,φ = [(2eμ/Kpρ)·(1/us) + (2eY/Kp0.5)]−0.5 that relates the discharge coefficient of the screen Cd,φ with the air velocity us. This relationship depends on the three parameters that define the aerodynamic behaviour of porous medium: permeability Kp, inertial factor Y and screen thickness e (and on air temperature that determine its density ρ and viscosity μ). However, for a determined temperature of air, the pressure drop-velocity relationship can be characterised only with two parameters: ΔP = aus2 + bus.

GPC-Based Gust Response Alleviation for Aircraft Model Adapting to Various Flow Velocities in the Wind Tunnel

A unified autoregressive (AR) model is identified, based on the wind tunnel test data of open-loop gust response for an aircraft model. The identified AR model can be adapted to various flow velocities in the wind tunnel test. Due to the lack of discrete gust input measurement, a second-order polynomial function is used to approximate the gust input amplitude by flow velocity. Afterwards, with the identified online aeroelastic model, the modified generalized predictive control (GPC) theory is applied to alleviate wing tip acceleration induced by sinusoidal gust. Finally, the alleviation effects of gust response at different flow velocities are estimated based on the comparison of simulated closed-loop acceleration with experimental open-loop one. The comparison indicates that, after gust response alleviation, the wing tip acceleration can be reduced up to 20% at the tested velocities ranging from 12 m/s to 24 m/s. Demonstratively, the unified control law can be adapted to varying wind tunnel velocities and gust frequencies. It does not need to be altered at different test conditions, which will save the idle time.

Linear stability analysis of wind turbine wakes performed on wind tunnel measurements

AbstractWind tunnel measurements were performed for the wake produced by a three-bladed wind turbine immersed in uniform flow. These tests show the presence of a vorticity structure in the near-wake region mainly oriented along the streamwise direction, which is denoted as the hub vortex. The hub vortex is characterized by oscillations with frequencies lower than that connected to the rotational velocity of the rotor, which previous works have ascribed to wake meandering. This phenomenon consists of transversal oscillations of the wind turbine wake, which might be excited by the vortex shedding from the rotor disc acting as a bluff body. In this work, temporal and spatial linear stability analyses of a wind turbine wake are performed on a base flow obtained with time-averaged wind tunnel velocity measurements. This study shows that the low-frequency spectral component detected experimentally matches the most amplified frequency of the counter-winding single-helix mode downstream of the wind turbine. Then, simultaneous hot-wire measurements confirm the presence of a helicoidal unstable mode of the hub vortex with a streamwise wavenumber roughly equal to that predicted from the linear stability analysis.