Conventional Wind Tunnel Research Articles

Wind Tunnel Measurement Systems for Unsteady Aerodynamic Forces on Bluff Bodies: Review and New Perspective.

Wind tunnel tests have become one of the most effective ways to evaluate aerodynamics and aeroelasticity in bluff bodies. This paper has firstly overviewed the development of conventional wind tunnel test techniques, including high frequency base balance technique, static synchronous multi-pressure sensing system test technique and aeroelastic test, and summarized their advantages and shortcomings. Subsequently, two advanced test approaches, a forced vibration test technique and hybrid aeroelastic- force balance wind tunnel test technique have been comprehensively reviewed. Then the characteristics and calculation procedure of the conventional and advanced wind tunnel test techniques were discussed and summarized. The results indicated that the conventional wind tunnel test techniques ignored the effect of structural oscillation on the measured aerodynamics as the test model is rigid. A forced vibration test can include that effect. Unfortunately, a test model in a forced vibration test cannot respond like a structure in the real world; it only includes the effect of structural oscillation on the surrounding flow and cannot consider the feedback from the surrounding flow to the oscillation test model. A hybrid aeroelastic-pressure/force balance test technique that can observe unsteady aerodynamics of a test model during its aeroelastic oscillation completely takes the effect of structural oscillation into consideration and is, therefore, effective in evaluation of aerodynamics and aeroelasticity in bluff bodies. This paper has not only advanced our understanding for aerodynamics and aeroelasticity in bluff bodies, but also provided a new perspective for advanced wind tunnel test techniques that can be used for fundamental studies and engineering applications.

Vehicle aerodynamic simulation analysis and research on bumpy road

PurposeIn order to certify the testing of vehicle Cd on the real road can be replaced by the much simplified steady-state simulation process inside wind tunnel.Design/methodology/approachThe paper opted innovative methods to verify the feasibility of using aerodynamic wind tunnel test for vehicle development purpose, including establishing the typical sinusoidal mapping function for road shapes during vehicle driving process to simplify the complexity of the road and choosing three different kinds of road spectrum amplitudes to establish the vehicle motion model, which can accurately predict the vehicle motion characteristics in the road driving process. The new approach simulates the vehicle aerodynamics on the road by taking into account the pitch angle changes during vehicle motion.FindingsBy comparing the transient drag coefficient on road with the steady-state simulated value in the conventional aerodynamic wind tunnel. This paper provides empirical insights about the flow field analysis around the vehicle on road that can verify the feasibility of using aerodynamic wind tunnel test for vehicle development purpose.Research limitations/implicationsBecause of establishing the typical sinusoidal mapping function for road shapes, the study results may lack generalizability. Therefore, researchers are encouraged to test the proposed propositions further.Practical implicationsThis paper includes implications for the flow field analysis around the vehicle on road that can verify the feasibility of using aerodynamic wind tunnel test for vehicle development purpose.Originality/valueThis paper fulfills an identified need to use aerodynamic wind tunnel test for vehicle development purpose.

Screened expanding turning-vane concept

The paper presents the screened expanding corner vane concept, which can turn a channel flow by $$90^\circ$$, while simultaneously increasing the flow area by at least a factor of two and thus halving the mean velocity, all without incurring any significant flow separation. The concept is demonstrated experimentally, and investigated with analytical and computational models. One target application is an ultra-compact closed-circuit wind tunnel, whose overall length is roughly half that of a conventional wind tunnel with the same test section.

Real-Time Aerodynamics Hybrid Simulation: A Novel Wind-Tunnel Model for Flexible Bridges

Despite rapid development in computational fluid dynamics, semiempirical analyses based on parameters identified from spring-mounted sectional models are still widely used to examine wind-induced effects on bridges. In addition, wind tunnel results from full-bridge aeroelastic models, viewed as the most comprehensive representations, are typically a final check for wind design of long-span cable-supported bridges. There are several well-known limitations associated with conventional wind tunnel testing of both sectional and full-bridge models. For example, structural nonlinearities and large deformations are difficult to simulate in sectional models, and only a limited number of modes can be accurately simulated in full-bridge aeroelastic models. To advance aeroelastic modeling of flexible bridges in the wind tunnel, a slightly different version of the real-time hybrid simulation (RTHS) techniques, frequently used in various branches of engineering, is developed here. Specifically, the skeleton of the sectional or full-bridge model, characterizing the dynamic properties (e.g., mass, damping, and stiffness of the structure), is numerically simulated using computational structural dynamics, while its skin, characterizing the aerodynamic and aeroelastic properties, is physically modeled in the wind tunnel. Aerodynamic inputs (gusts) are applied directly on the skin in the wind tunnel, while aeroelastic inputs (motions) are represented by the simulation outputs of the bridge skeleton. On the other hand, the dynamic inputs to the bridge skeleton are acquired from the measured forces (and moments) on the bridge skin. The interactions between the skeleton and skin of the bridge are accomplished through a system consisting of sensors, a network of electromagnetic actuators, and controllers. The time history of the wind-induced bridge responses can be obtained at the end of the proposed real-time aerodynamics hybrid simulation (RTAHS). The feasibility of the RTAHS methodology is demonstrated by a numerical example involving both linear and nonlinear wind-induced forces on the bridge deck.

A Reliable Experimental Methodology for the Study of Wind-Turbine Rotor Blade Aerodynamics

The aerodynamic performance of airfoils and blades designed for modern wind-turbine rotors, which have diameters of the order of hundred meters, must be examined at chord Reynolds numbers matching those of practical applications. In general, such high Reynolds numbers cannot be achieved in conventional wind tunnels. Moreover, knowledge on the boundary-layer transition location is essential to evaluate airfoil and blade performance at these flow conditions. This work presents an experimental methodology that can be applied at flow conditions reproducing those of real wind-turbine rotor blades and simultaneously provides aerodynamic coefficients and transition locations. The experimental methodology consists of: the Temperature-Sensitive Paint (TSP) technique for global, non-intrusive and reliable transition detection; conventional pressure measurements for the determination of the aerodynamic coefficients; and the High Pressure Wind Tunnel Göttingen (DNW-HDG) to run the experiments at Reynolds numbers matching those of real applications. The obtained results can be used to verify airfoil and blade performance and to validate numerical predictions. In the present work, the experimental methodology was applied to systematically investigate the aerodynamic performance of an airfoil designed for the mid-span sections of modern wind-turbine rotor blades. The examined chord Reynolds numbers were as high as 12 million and the angle-of-attack ranged from -14° to +20°. The presented methodology was here demonstrated to be mature for productive testing.

Physical simulations on wind loading characteristics of streamlined bridge decks under tornado-like vortices

Tornado risks for highly important long-span bridges in tornado-prone regions can not be neglected. Rigid-model wind pressure measurements on a streamlined bridge deck were conducted using a tornado vortex simulator, to clarify tornado-induced surface pressure distributions, aerodynamic load coefficients and total force coefficients. We focused on two main parameters: swirl ratio and horizontal distance from tornado center to deck. Obvious discrepancies were observed between tornado-induced wind loading and results from conventional boundary-layer wind tunnels. Strip theory was not applicable since pressure distributions vary for different sections of the deck along the bridge axis. The absolute values of mean pressure coefficients for the deck section near tornado center are largest among all tested sections along the bridge axis. The most unfavorable mean sectional drag force coefficients were found when the bridge model is located at the tornado core radius and largest mean sectional lift force coefficients at the tornado center. With increase in swirl ratio, magnitudes of mean pressure coefficients as well as mean and fluctuating sectional aerodynamic load coefficients become smaller. The unfavorable locations for fluctuating rolling moment coefficients are different from those for mean values, which indicates that the quasi-steady assumption becomes invalid for tornado-induced rolling moment coefficients. Total force coefficients over the entire deck were investigated to evaluate the non-uniform loading characteristics. The findings will be helpful for predicting tornado-induced responses and risks for highly important long-span bridges.

Prototyping of thin shell wind tunnel models to facilitate experimental wind load analysis on curved canopy structures

The topologies of membrane and shell structures are not covered by existing wind load Standards and wind tunnel testing should be used to obtain representative wind loads for these structures. However, accurate scale-models of these organically shaped and often open thin structures are complex, time-consuming and expensive to build. To stimulate experimental research on wind load distributions over these structures, this paper illustrates a prototyping methodology for double curved thin shell wind tunnel models with integrated pressure sensors. The production process is illustrated for a hyperbolic paraboloid roof structure. The obtained wind load distributions are validated with literature for a flat roof and canopy that is made according to the same methodology and for two hyperbolic paraboloid roofs. Results indicate that, compared to conventional wind tunnel models, these thin shell wind tunnel models yield more realistic wind pressure distributions over very thin canopy structures. Finally, Cp-distributions are shown for the hyperbolic paraboloid canopy with the high corner under attack. The production of glass-fibre reinforced composites in a CNC-milled mould is convenient and accurate and facilitates the production of wind tunnel models to be used for wind load measurements on organically shaped thin canopy structures.

Generation and characterization of a non-stationary flow field in a small-scale wind tunnel using a multi-blade flow device

Physical experimentation for non-stationary wind phenomena has been the focal point of much research. Replication of transient, non-synoptic wind events such as thunderstorm downbursts and tornadoes require innovative means that break from traditional boundary-layer wind tunnel experiments. This paper examines the feasibility of a multi-blade flow device capable of reproducing the gust front of a non-stationary outflow. The device is inserted into the test chamber of the small-scale, closed-circuit wind tunnel at Northeastern University. Replication of spatial and temporal features of a non-stationary gust front (slowly-varying flow and turbulence fluctuations) is investigated through velocity measurements at various elevations. The wind velocity record from the Andrews Air Force Base downburst is employed for verification of the experimental procedure; its physical effects on a tall building model are recorded. The results suggest that this device may be used to simulate the main features of simplified non-stationary gust fronts in a conventional wind tunnel without significant modifications to the test chamber.

Study of Aerodynamic Drag of Sports Utility Vehicle by Experimental and Numerical Method

Abstract Aerodynamic styling of the vehicle is one of the promising technologies which can not only improve the fuel efficiency, but also ensure better stability and good handling characteristics of vehicles at higher speed especially on highways. The paper includes assessment of drag force (Fd) and drag coefficient (Cd) by the conventional wind tunnel method. The experimental calculations were performed using subsonic wind tunnel having a test section of 100cm x 30cm x 30 cm. An exact replica of a model of Sports Utility Vehicle (SUV) on reduced scale 1:32 was used to conduct and perform the experiment for calculating Fd and Cd. Three-dimensional (3D) computational analysis was carried out using Gambit as the preprocessing software and Fluent as the solver and post-processor. The comparison of computational approaches with experiment shows that the computed Fd agrees well with the experimental values over the entire range of air velocities.

Downburst related damages in Brazilian buildings: Are they avoidable?

Brazil's territory has shown an enormous potential to generate severe weather and intensive downbursts cases gained attention as a result of the severe destructive patterns not frequently seen before. However, several of the downburst damages on Brazilian buildings and structures follow a similar pattern than those caused by synoptic winds. The significant amount of accidents due to extreme winds in the recent years suggests the need for a review of the Brazilian Wind Code as well as the necessity to reinforce its use in common day buildings design. The new version of the code should incorporate an update of climate data and implement a thunderstorm wind model. This paper brings an overview of the downburst areas with major risk of occurrences, as well as presents an analysis of the damage patterns caused by the downbursts, with special focus on the effects on buildings and structures. It is concluded that, although the flow characteristics of downburst winds and conventional boundary layer simulations of synoptic winds may differ, the use of current conventional wind tunnel simulations and conventional code values are still a valuable tool in the prevention of wind related damages, at least in low and medium-rise buildings.

Characterization of Freestream Disturbances in Conventional Hypersonic Wind Tunnels

Although low-disturbance (“quiet”) hypersonic wind tunnels are believed to provide more reliable extrapolation of boundary-layer transition behavior from ground to flight, the presently available quiet facilities are limited to Mach 6, moderate Reynolds numbers, low freestream enthalpy, and subscale models. As a result, only conventional (“noisy”) wind tunnels can reproduce both Reynolds numbers and enthalpies of hypersonic flight configurations and must therefore be used for flight vehicle test and evaluation involving high Mach number, high enthalpy, and larger models. This paper outlines the recent progress and achievements in the characterization of tunnel noise that have resulted from the coordinated effort within the AVT-240 specialists group on hypersonic boundary-layer transition prediction. The new experimental measurements cover a range of conventional wind tunnels with different sizes and Mach numbers from 6 to 14 and extend the database of freestream fluctuations within the spectral range of boundary-layer instability waves over commonly tested models. New direct numerical simulation datasets elucidate the physics of noise generation inside the turbulent nozzle wall boundary layer, characterize the spatiotemporal structure of the freestream noise, and account for the propagation and transfer of the freestream disturbances to a Pitot-mounted sensor.

Control of stationary cross-flow modes in a Mach 6 boundary layer using patterned roughness

Experiments were performed to investigate passive discrete roughness for transition control on a sharp right-circular cone at an angle of attack at Mach 6.0. A cone angle of attack of $6^{\circ }$ was set to produce a mean cross-flow velocity component in the boundary layer over the cone by which the cross-flow instability was the dominant mechanism of turbulent transition. The approach to transition control is based on exciting less-amplified (subcritical) stationary cross-flow modes through the addition of discrete roughness that suppresses the growth of the more-amplified (critical) cross-flow modes, and thereby delays transition. The passive roughness consisted of indentations (dimples) that were evenly spaced around the cone at an axial location that was just upstream of the first linear stability neutral growth branch for cross-flow modes. The experiments were performed in the air force academy (AFA) Mach 6.0 Ludwieg Tube Facility. The cone model was equipped with a motorized three-dimensional traversing mechanism that mounted on the support sting. The traversing mechanism held a closely spaced pair of fast-response total pressure Pitot probes. The measurements consisted of surface oil flow visualization and off-wall azimuthal profiles of mean and fluctuating total pressure at different axial locations. These documented an 25 % increase in the transition Reynolds number with the subcritical roughness. In addition, the experiments revealed evidence of a nonlinear, sum and difference interaction between stationary and travelling cross-flow modes that might indicate a mechanism of early transition in conventional (noisy) hypersonic wind tunnels.

Design of an experimental wind tunnel system for control applications

In this study, a low-cost experimental wind tunnel system for evaluation of control algorithms and utilization in teaching activities is presented. The system is designed inspired from conventional wind tunnels and has a basic design and low cost as can be realized in every laboratory. The design of mechanical part and electronics hardware and development of computer software are described in detail. The objective of the control system is to ensure the rotation speed of fan, which is placed at the end of the pipe, and track a desired trajectory by controlling air pressure in the pipe. Experimental results are also presented to demonstrate the performance of the system.

Rotor solidity effects on the performance of vertical-axis wind turbines at high Reynolds numbers

Vertical Axis Wind Turbines have yet to see wide-spread use as a means of harvesting the kinetic energy of the wind. This may be due in part to the difficulty in modeling the relatively complex flow field and hence performance of these units. Additionally, similar to Horizontal Axis Wind Turbines, VAWTs are difficult to properly test in a conventional wind tunnel. Typically Reynolds numbers cannot be matched or the turbine geometry must be altered, limiting the applicability of the results. Presented in the following is a set of experiments in a specialized, high-pressure wind tunnel used to achieve high Reynolds numbers with a small-scale model. The performance change of this model is investigated at two different solidities and over nearly a decade of Reynolds numbers (based on diameter: 600, 000 ≤ ReD ≤ 5 × 106). The non- dimensional power coefficient displays behavior consistent with Reynolds number invariance, regardless of solidity. In addition, the change in performance as this limit is approached also shows no direct dependence on the solidity for the VAWT geometry used in this study. The results of this work have direct application for modeling and simulation efforts concerning the performance of new VAWT designs.

Mapped Chebyshev Pseudo-Spectral Method for Dynamic Aero-Elastic Problem of Limit Cycle Oscillation

A mapped Chebyshev pseudo-spectral method is developed as one of the Fourier-spectral approaches and solves nonlinear PDE systems for unsteady flows and dynamic aero-elastic problem in a given time interval, where the flows or elastic motions can be periodic, nonperiodic, or periodic with an unknown frequency. The method uses the Chebyshev polynomials of the first kind for the basis function and redistributes the standard Chebyshev–Gauss–Lobatto collocation points more evenly by a conformal mapping function for improved numerical stability. Contributions of the method are several. It can be an order of magnitude more efficient than the conventional finite difference-based, time-accurate computation, depending on the complexity of solutions and the number of collocation points. The method reformulates the dynamic aero-elastic problem in spectral form for coupled analysis of aerodynamics and structures, which can be effective for design optimization of unsteady and dynamic problems. A limit cycle oscillation (LCO) is chosen for the validation and a new method to determine the LCO frequency is introduced based on the minimization of a second derivative of the aero-elastic formulation. Two examples of the limit cycle oscillation are tested: nonlinear, one degree-of-freedom mass–spring–damper system and two degrees-of-freedom oscillating airfoil under pitch and plunge motions. Results show good agreements with those of the conventional time-accurate simulations and wind tunnel experiments.

Study on Acoustically Transparent Test Section of Aeroacoustic Wind Tunnel

On the purpose of accurate data acquisition for the aeroacoustic testing mostly in open jet test section of aeroacoustic wind tunnel, the large scale anechoic chamber is specifically designed to build the low background noise environment. A newly acoustic test section is presented in this paper, of which the contour is similar as the closed test section, and the wall is fabricated by the fiber fabric, both the characteristics of closed and open jet test section of conventional wind tunnel are combined in it. By thoroughly researching on the acoustics and aerodynamics of this acoustically transparent test section, significant progress in reducing the background noises in test section and improving the ratio of energy of the wind tunnel and some other aspects have been achieved. Acoustically transparent test section behaves better in acoustics and aerodynamics than conventional acoustic test section because of their high definition in detecting the sound sources and great performance in transmitting sounds.

Effects of trips on the oscillatory flow of an axisymmetric hypersonic inlet with downstream throttle

Experimental investigations are conducted on an axisymmetric hypersonic inlet to evaluate the effects of trips on oscillatory flows. The model exit is throttled with a fixed block to generate oscillatory flows at a freestream Mach number of 6 in a conventional wind tunnel and a shock tunnel. Schlieren imaging and pressure measurements are adopted to record unsteady flow features. Results indicate that trips with a 1 mm thickness prominently suppress external separations, shorten oscillatory cycles, and modify pressure magnitudes. Trips can reduce the upstream movement ranges of separated shocks from nose regions to locations axially 142 mm downstream. The oscillatory cycles are shortened from 3.75 ms to 3.25 ms and from 4 ms to 3.13 ms in two facilities. Tripped cases generally exhibit higher pressure magnitudes than those of untripped cases, of which the increment is up to 21 times the freestream static pressure for the farthest downstream transducer in the shock tunnel. The effects of trips are related to the streamwise vortexes in wake flows, in which interactions between external separations modify the separated flow patterns and enhance the sustainment of the forebody boundary layers to backpressure. Flow processes causing increments of oscillatory frequencies and pressure magnitudes are analyzed, while the flow mechanisms dominating the processes still need to be clarified in the future.

UNMANNED ROTORCRAFT IN AGGRESSIVE ENVIRONMENT: AERODYNAMIC FLOW PERFORMANCE AGAINST WIND GUST

Unmanned rotary-wing aircrafts or rotorcrafts are often prone to diverse atmospheric turbulences, and undeniably, abrupt gusts are reckoned to be the most acquainted commotion of them. Time and again, gust turbulence have dictated being the regulating trigger for countless mishaps concerning micro aerial vehicles. Given that the core/main rotor provides principal lift along with governing directional control and dynamic stability for any rotorcraft, the demeanors of thrust or induced air-flow through the gyrating blades largely signify the barebones of its functionality. This paper presents an idiosyncratic approach towards reviewing the impact of wind gust on rotor induced aerodynamic flow performance of an unmanned rotorcraft. Artificial gusts have been generated through the inherent concept of forced pitched oscillation without the use of any conventional wind tunnel. Gust air-speed and the rotor induced air-speed are gauged through the same contraption to provide homogeneous quantitative valuation. Each rotor-halves have been assessed `root-to-tip' across designated span-wise positions against selected strengths of gust. Comparative analysis with normal atmospheric condition indicated gradual loss of cumulative thrust during hover (15-40%) and forward flight (10-30%) within the stipulated gust range and infers the risk of declining altitude. Protuberant imbalance of thrust across the rotor disk during hover indicated the likelihood of lackadaisical half-pitched rolling motion. However, during forward flight, the reduced discrepancy of thrust between the rotor halves signified gradual loss of cruising speed with regards to the increasing gust strength for the specific rotorcraft model. Experimental findings in this study exhibited prospect of appraisal with full-scale rotorcrafts.

Transition due to Roughness on Blunt Capsule: Comparison with Transient Growth Correlation

The effect of uniformly distributed surface roughness on the onset of boundary-layer transition for a capsule geometry was experimentally investigated in a conventional hypersonic blowdown wind tunnel. The thin-walled scale model of the Orion Crew Exploration Vehicle used in this study was mounted at a 28 deg angle of attack in the Adjustable Contour Expansion wind tunnel. A test matrix of 24 runs was developed to experimentally estimate the location of transition and to relate the flow properties at this location and the roughness height to established transient growth scaling. Tests were conducted for four different Reynolds numbers at six quasi-uniform surface roughness height distributions. The tunnel was nominally set to Mach 6.0 and a total temperature of 430 K for each run. Infrared thermometry was used to measure the surface temperature distribution. Heat transfer rates generated through a series of surface temperature maps were processed into normalized Stanton number centerline plots. These plots directly indicated the boundary-layer transition location. Analysis of the results indicated that the experimental data matched the established transient growth scaling.

Aerodynamic Analysis of Grand Prix Cars Operating in Wake Flows

The effect of the upstream wake of a Formula 1 car on a following vehicle has been investigated using experimental and computational methods. Multiple vehicle studies in conventional length wind tunnels pose challenges in achieving a realistic vehicle separation and the use of a short axial length wake generator provides an advantage here. Aerodynamic downforce and drag were seen to reduce, with greater force reductions experienced at shorter axial spacings. With lateral offsets, downforce recovers at a greater rate than drag, returning to the level for a vehicle in isolation for offsets greater than half a car width.The effect of the wake was investigated in CFD using multiple vehicle simulations and non-uniform inlet boundary conditions to recreate the wake. Results closely matched those for a full two-vehicle simulation provided the inlet condition included unsteady components of the onset wake. Creating a nonuniform inlet condition allowed the wake parameters to be modified to test sensitivity to different wake features. Dynamic pressure deficit in the wake is shown to have the greatest impact on the following vehicle, reducing loading on the downforce producing surfaces. Wake up-wash and vortex flows are shown to have a smaller effect on downforce generated by the following car, but have an important role in diverting the dynamic pressure deficit upwards and over the following car.Future regulation changes, aimed at reducing the downforce loss experienced when following another car, should aim to reduce the velocity deficit onset to the following car; either by reducing wheel and underbody wakes, or by extracting the wake using up-wash from the rear wing.