Wind Tunnel Flow Research Articles

Effect of neck geometry on aeroacoustic excitation of a Helmholtz resonator

The aeroacoustic response of a Helmholtz resonator, when attached to a wind tunnel via junctions with different neck geometries, has been examined. The wind tunnel consists of a 2-inch ID glass tube with variable mean flow at atmospheric pressure connected to a cross-junction with two coaxial 5-liter Helmholtz resonators. During the experiment, the acoustic pressure and corresponding frequency is measured in one of the Helmholtz resonators at different wind tunnel flow velocities. By knowing these quantities, the acoustic velocity of the air at the neck of the resonator can be determined. The ratio of the acoustic velocity to the flow velocity as a function of the Strouhal number can be studied for differing neck geometries. Helmholtz resonator neck geometries studied include 90-deg bends and straight sections in various combinations. [This work was supported by the Arkansas Space Grant Consortium and the University of Central Arkansas University Research Council.]

Pressure-Sensitive Paint measurement of the flow around a simplified car model

In applying Pressure-Sensitive Paint (PSP) to low-speed flow wind tunnel testing, it is important to minimize any measurement uncertainties. There are various error sources such as camera noise, misalignment of images due to model displacement and temperature distribution over the model. Among these factors, the effects of temperature distribution change during tests on pressure measurement accuracies were studied in the present paper. Pressure and temperature distributions over a simplified car model (1/10 scale Ahmed model) were measured using PSP and Temperature-Sensitive Paint (TSP). Sequential images were acquired at the same intervals over the entire test period, including for the conditions before and after the tunnel run. As a result, it was found that the measurement error caused by temperature distribution over the model could be reduced using a single-point temperature measurement. In addition, by measuring surface temperature distributions on the model using TSP, it was proved that the most accurate pressure measurement could be made by rationing the wind-off image acquired immediately after shutting down the tunnel to the wind-on image acquired immediately before shutting down the tunnel. Using the present measurement technique, complicated pressure fields over the Ahmed model were successfully visualized.

On drag and lift analysis of transport aircraft from wind tunnel measurements

The determination of drag and lift of transport aircraft from wind tunnel measurements is discussed for compressible as well as incompressible flow. Effects of model support are neglected. No simplifying assumptions on the flow along the test section walls are made. Based on the assumption of correctable wind tunnel flow, balance measurements and wake survey measurements together indicate a way to derive an angle of attack correction towards the in-flight condition. This will require the same level of (sufficient) accuracy for both types of measurements and for the propulsion mass flux. The blockage correction is assumed to be known. Buoyancy correction is briefly discussed. Propulsion is fully taken into account. Wake survey computational postprocessing is treated in detail. It is shown that wake survey measurements can be limited to the wake cross-sectional area.

Flow and Pollutant Dispersion in Street Canyons using FLUENT and ADMS-Urban

This paper is devoted to the study of flow within a small building arrangement and pollutant dispersion in street canyons starting from the simplest case of dispersion from a simple traffic source. Flow results from the commercial computational fluid dynamics (CFD) code FLUENT are validated against wind tunnel data (CEDVAL). Dispersion results from FLUENT are analysed using the well-validated atmos pheric dispersion model ADMS-Urban. The k − e turbulence model and the advection-diffusion (AD) method are used for the CFD simulations. Sensitivity of dispersion results to wind direction within street canyons of aspect ratio equal to 1 is investigated. The analysis shows that the CFD model well reproduces the wind tunnel flow measurements and compares adequately with ADMS-Urban dispersion predictions for a simple traffic source by using a slightly modified k − e model. It is found that a Schmidt number of 0.4 is the most appropriate number for the simulation of a simple traffic source and in street canyons except for the case when the wind direction is perpendicular to the street canyon axis. For this last case a Schmidt number equal to 0.04 gives the best agreement with ADMS-Urban. Overall the modified k − e turbulence model may be accurate for the simulation of pollutant dispersion in street canyons provided that an appropriate choice for coefficients in the turbulence model and the Schmidt number in the diffusion model are made.

Possible uses of optical methods for determining the structure of nonequilibrium hypersonic flows

Results are given of the investigation of self-luminosity of flow in hypersonic wind tunnels with electric-arc heaters, and the possibilities of using this luminosity for the visualization of flow past bodies are analyzed. It is demonstrated that the luminosity is caused by nonequilibrium physicochemical processes in nozzles and is defined by the excitation of low-potential electron-vibrational levels of molecules and atoms of impurities in the flow. It follows from comparison of flow patterns obtained by the shadow method with photographs of flow in the vicinity of bodies that the use of luminosity for determining the structure of flow past models is not always justified. Results are given of applying the method of induced resonance fluorescence for the visualization of flow past bodies.

An investigation of the flow characteristics in the bootdeck region of a scale model notchback saloon vehicle

The results of an experimental investigation of the bootdeck flow structure of a 36 per cent scale model notchback car are presented together with a general review of advanced laser diagnostic techniques suitable for large-scale wind tunnel flow measurement. The tests were used to characterize the flow behaviour over the Reynolds number range 0.74 × 106-4.93 × 106 in the Cranfield University 2.4 m × 1.8 m wind tunnel. The experiments involved flow visualization, rear bootdeck and backlight three-dimensional stereoscopic particle image velocimetry (PIV) measurements, and mean static and unsteady static pressure measurements. Initial results from the flow visualization suggested flow asymmetries originating in the backlight region that were sensitive to the Reynolds number. The PIV data and static pressure data, however, showed little or no sensitivity of flow to Reynolds number with consistent flow structure and levels of unsteadiness from the backlight to the rear bootdeck region. At this stage no definitive reasons can be given for the discrepancies between the flow visualization and the other data, although the flow visualization data were particularly difficult to interpret near the backlight, and the near-wake structure may be bi-stable in this Reynolds number range. More detailed three-dimensional stereoscopic, time-resolved PIV flow data are now planned to quantify conclusively the correct flow structure and its sensitivities.

Determination of flutter derivatives by a taut strip model

The taut strip model is a kind of aeroelastic model, which can be regarded as an in-between model between the sectional model and the full aeroelastic bridge model. Determination of flutter derivatives by taut strip models has rarely been studied. In this paper, a theory of determination of flutter derivatives using the taut strip model is established based on the eigensystem realization algorithm. Considering the restriction of measuring points, modal coordinate expansion technique is introduced to get the information of unknown measuring points. Generalized inverse matrix theory is employed to obtain the stiffness matrix and damping matrix of taut strip models. Then, the flutter derivatives can be extracted by the subtraction of stiffness and damping matrixes. A method of identifying of flutter derivatives in turbulence flow is also proposed through the random decrement technique. Experimental study of determination of flutter derivatives by taut strip model is carried out based on the theory discussed in the paper. A taut strip model of a thin plate is designed, and it is used in smooth flow to determine flutter derivatives as well as in turbulence flow in TJ-2 wind tunnel of Tongji University. In the end, comparisons with theoretical flutter derivatives of a thin-plate section are undertaken to validate the reliability of the theory.

IMPNS Computations of the X-15 Flight Vehicle: A Validation and Verification

A verification and validation study of the IMPNS space-marching flow solver has been undertaken for a complete hypersonic air-vehicle configuration. A hierarchical approach was adopted in which the vehicle aerodynamics was decomposed and related flow phenomenon studied. Using this hierarchy benchmark solutions and laboratory experiments were identified that provide the basis of the verification and validation exercises. Detailed comparisons of iterative and grid converged IMPNS computations with benchmark solutions and wind tunnel measurements are presented. Computations of the X-15 wind tunnel and flight experiments are described and comparison is made with measured surface and off-surface pressure measurements and wind tunnel flow visualization observations that demonstrate the reliability and capability of the IMPNS flow solver for complex configurations.

Design and Performance of a Pumped Counterflow Virtual Impactor

Counterflow virtual impaction uses a flow of gas in a direction opposite the motion of the particles to separate them from gas and smaller particles. In the past such devices have used aircraft flight or wind tunnel flow to impart momentum to the particles. Here we describe the design and performance of an apparatus, termed a pumped counterflow virtual impactor (PCVI), which uses a vacuum pump to provide the flow. We show that this device is capable of inertial separation with a sub-micrometer cutoff diameter, particle enhancement approaching the ratio of the output to the input flow, and replacement of the ambient gas in the output flow with another gas that may be more suitable for downstream analysis techniques. Rejection of input gas and particles smaller than the cutoff diameter can exceed 99.9%.

Some characteristics of the wind flow in the lower Urban Boundary Layer

The characteristics of the wind flow at low levels in the urban environment (Roughness Sublayer (RS)) are quite different from, and to some extent independent of the characteristics of the flow in the upper part of the Urban Boundary Layer. In the RS, in fact, the flow is influenced more by the local geometry, than by a homogeneous energy transfer between horizontal layers. In this paper, the results of wind tunnel flow measurements in and above a regular urban geometry pattern, with street canyons parallel and orthogonal to the oncoming wind, are reported. The statistics and the spectral characteristics of the flow in the RS are discussed. The influence of the oncoming turbulence on the RS flow is analysed, and the differences between the flow in transversal and longitudinal street canyons are evidenced.

Wind Tunnel Hotwire Measurements, Flow Visualization and Thrust Measurement of a VAWT in Skew

The results of experimental research on the wake and induced flow around a vertical axis wind turbine (VAWT) in skew are presented. The previous research on VAWTs in skew is limited because this operation mode has only recently been found to be significant in the operation of VAWTs in the built environment. These results contain hotwire measurements of the incoming flow and wake of a VAWT in nonskewed and skewed flow. The high sampling rate of the hotwire data allows the effects of blade passing to be identified. Flow visualization of the tip vortices is also presented. Thrust measurements of the rotor were performed to understand the effect of skew on thrust variation and to compare with analytical predictions.

Foliage influences forced convection heat transfer in conifer branches and buds

Conifer foliage structures affect branch and bud temperature by altering the development and convective resistance of the thermal boundary layer. This paper examines foliage effects on forced convection in branches and buds of Picea glauca (Moench) Voss and Pinus contorta Dougl. Ex. Loud., two species that represent the range of variation in foliage structure among conifers. Forced convection is characterized by a power law relating Nusselt (heat transfer) and Reynolds (boundary layer development) numbers. Data were collected in a laminar flow wind tunnel for free stream velocities of 0.16-6.95 m s(-1). Scaling parameters were compared against literature values for silver cast branch replicas, a bed of real foliage, cylinders, and tube banks. Foliage structures reduced Nusselt numbers (heat transfer) relative to cylinders, which are typically used to approximate leafless branches and buds. Significantly different scaling relationships were observed for all foliage structures considered. Forced convection scaling relationships varied with foliage structure. The scaling relationships reported here account for variation within populations of branches and buds and can be used to characterize forced convection in a forest canopy.

Modern developments in hypersonic wind tunnels

AbstractThe development of new methods of producing hypersonic wind-tunnel flows at increasing velocities during the last few decades is reviewed with attention to airbreathing propulsion, hypervelocity aerodynamics and superorbital aerodynamics. The role of chemical reactions in these flows leads to use of a binary scaling simulation parameter, which can be related to the Reynolds number, and which demands that smaller wind tunnels require higher reservoir pressure levels for simulation of flight phenomena. The use of combustion heated vitiated wind tunnels for propulsive research is discussed, as well as the use of reflected shock tunnels for the same purpose. A flight experiment validating shock-tunnel results is described, and relevant developments in shock tunnel instrumentation are outlined. The use of shock tunnels for hypervelocity testing is reviewed, noting the role of driver gas contamination in determining test time, and presenting examples of air dissociation effects on model flows. Extending the hypervelocity testing range into the superorbital regime with useful test times is seen to be possible by use of expansion tube/tunnels with a free piston driver.

Surface flow visualization using thermal tufts produced by an encapsulated phase change material

A new thermal tuft method for flow visualization similar to the laser thermal tuft method is described. The earlier laser thermal tuft method involved heating a spot on a surface with a laser, producing a teardrop temperature distribution on a surface coated with thermochromic liquid crystals. In the present study, thermal tufts are produced by embedding small thermal masses in a low thermal conductivity substrate. When a model is exposed to wind tunnel flow, the embedded thermal masses remain (nearly) isothermal while the thermal tufts appear downstream. The advantage of using embedded thermal masses over previous methods is that no laser or heated base is needed. Multiple tufts can easily be achieved. By using an encapsulated phase change material the phase change between solid and liquid (ice) produces isothermal spots. Images of the surface temperature distributions associated with these thermal tufts were obtained using both liquid crystal and infrared thermography. Flow visualization using these thermal tufts is demonstrated for an impinging jet and on a flat surface in a wind tunnel with crossflow.

Modeling droplet collision and coalescence in an icing wind tunnel and the influence of these processes on droplet size distribution

A theoretical model of a two-phase air/dispersed water spray flow in an icing wind tunnel is presented here. The mutual interactions taking place within the dispersed phase known as binary droplet collisions, as well as gravitational sedimentation are considered. Where large droplets and low air stream velocities are concerned, the effect of gravity on droplet dynamics is considerable. Gravity causes the vertical deflection of droplet trajectories and an increase in liquid water content (LWC) in the bottom half of the wind tunnel. Droplet collision tends to influence the size, trajectory and velocity of droplets thus affecting the characteristics of the flow and, thereby, the formation of ice on the object placed in the wind tunnel. The present model simulates droplet motion and droplet collision in an icing wind tunnel, where it may be observed that bouncing, stable coalescence, or coalescence followed by separation are the possible outcomes of collision. In the theoretical examination, firstly, the effect of gravity on the vertical deflection of droplet trajectories and on the vertical distribution of the LWC near the icing object are taken into account, while droplet collision is disregarded. Then both factors are considered and collision outcome is determined together with the size and velocity of post-collision droplets. The initial droplet size distribution (DSD), as it occurs at the nozzle outlet, is estimated by a curve in accordance with previous experimental results. The DSD is determined theoretically near the icing object, which makes it possible to calculate the median volume diameter and the LWC of the aerosol cloud. The simulation results with regard to the LWC are compared to the experimental results obtained in this research and a satisfactory qualitative coincidence is to be found between them.

Wind Tunnel Aerodynamic Tests of Six Airfoils for Use on Small Wind Turbines

This paper presents detailed wind tunnel tests data taken on six airfoils having application to small wind turbines. In particular, lift, drag and moment measurements were taken at Reynolds numbers of 100,000, 200,000, 350,000 and 500,000 for both clean and rough conditions. In some cases, data were also taken at a Reynolds number of 150,000. The airfoils included the E387, FX 63-137, S822, S834, SD2030, and SH3055. Prior to carrying out the tests, wind tunnel flow quality measurements were taken to document the low Reynolds number test environment. Oil flow visualization data and performance data taken on the E387 compare favorably with measurements taken at NASA Langley in the Low Turbulence Pressure Tunnel. Highlights of the performance characteristics of the other five airfoils are presented.

Observations of Boundary-Layer Wind-Tunnel Flow over Isolated Ridges of Varying Steepness and Roughness

Boundary-layer wind-tunnel flow is measured over isolated ridges of varyingsteepness and roughness. The steepness/roughness parameter space is chosento produce flows that range from fully attached to strongly separated. Measurementsshow that maximum speedup at the hill crest is significantly lower than predictedby linear theory and that recovery in the lee of the hill is much slower for stronglyseparated flow over steep terrain. The measurements also show that behaviour ofthe mean and turbulent components of the flow on the downwind side of the ridgeis fundamentally different between separated and non-separated flows. This suggeststhe dominance of much increased turbulence time and length scales in the lee of thehill in association with a production mechanism that scales with the hill length ratherthan the proximity to the surface as on the windward side of the hill crest.

Drag Reduction From Square Base Afterbodies at High Speeds

Experiments have been carried out in the 0.5-m base flow wind tunnel at high speeds evaluating the drag reduction potential of a family of square base afterbodies including jet flow at the base. Direct afterbody total drag measurements have been made on square bases as well as conventional axisymmetric afterbodies with conical and circular arc boat tails having the same annular base area and jet flow parameters. The results show conclusively that, in the Mach-number range of 0.95‐1.60, the square-base afterbodies have globally minimum drag in the range of jet pressure ratio studied; the total drag reduction observed is about 10‐12% relative to the circular arc afterbodies, which can be of significant value in design applications. Certain broad flow features on square-base afterbodies are discussed based on surface-pressure measurements and surface flow-visualization studies. Nomenclature Ab = annular base area Ab/A f = ratio of annular base area to forebody area, 0.23 A f = forebody cross-sectional area A j = area of jet at exit A j/A f = ratio of jet exit area to forebody area, 0.30 C D = afterbody total drag coefficient; drag force/(q∞∗ A f ) CDb = base-drag coefficient; base force/(q∞∗ A f ) C Dβ = boat-tail profile drag coefficient; boat-tail drag force/(q∞∗ A f ) C p = afterbody surface-pressure coefficient; ( p − p∞)/(0.5γ p∞ M 2 ∞ ) Cpb = base-pressure coefficient; ( pb − p∞)/(0.5γ p∞ M 2 ∞) D = forebody diameter of the model, 127 mm db = base diameter d j = nozzle-exit diameter M∞ = freestream Mach number Poj/ p∞ = jet pressure ratio; ratio of stagnation pressure of jet to the freestream static pressure pb = base pressure p∞ = freestream static pressure q∞ = dynamic pressure (0.5γ p∞ M 2 ∞) β = boat-tail angle, see Fig. 3

Accuracy of Sonic Anemometers: Laminar Wind-Tunnel Calibrations Compared to Atmospheric In Situ Calibrations Against a Reference Instrument

Two Gill Solent Ultrasonic anemometers, models 1012R2 and 1210R3, weretested in field parallel measurements against a windvane based hot-film anemometerwith additional sensors for temperature and wet-bulb temperature, the MIUU (MIUU:Meteorology Institute, Uppsala University) instrument. This instrument was shown toretain its precision from laminar wind-tunnel tests when used in atmospheric turbulentflow. This contrasts strongly to the observed results for the two sonic anemometers,which were first calibrated in laminar wind-tunnel flow. Individual three-dimensionalcalibration matrices were constructed, and were shown to reduce the remaining calibration uncertainty for the wind speed to 0.4–0.8% for all azimuths and for angles of attack within ±40°. In the field intercomparison tests of the sonics against the MIUU instrument, it was found that the precision not only of the mean wind speed but of all second-order moments studied (variances and covariances, with and without temperature) deteriorated by a factor of typically three to four. Most of the scatter appears to be random, but in the case of the wind speed, a clear dependence on wind direction is found as well. It is concluded that the correction for the effect of the vertical supporting rods of the R2 and R3 instruments, which gives nearly perfect agreement for laminar flow, does not work entirely satisfactory in the natural turbulent flow. This, in turn, is likely to be so because of high sensitivity of the wake behind the cylindrical supporting rods to the character of the approach flow.

Signal quality improvements by short-coherence holographic particle image velocimetry

Particle holography has proven to be a useful metrological tool for three-dimensional flowvelocimetry. To cope with the problem of noise from out-of-focus particles the technique oflight-in-flight holography (LiFH) has been introduced that utilizes properties of a lasersource of short coherence. While the feasibility of the method has been shown earlier, amore profound quantitative analysis of its performance was still required. Thepresent paper briefly summarizes some essential knowledge on noise in particleholograms, reviews recent approaches for handling noise in deep-field particleholography and presents first experimental checks of these concepts on short-exposureholographic recordings of particle fields in a wind-tunnel flow. The performanceof ordinary and short-coherence particle holography are compared directly byoperating the same laser in either long-coherence or short-coherence mode. It isestablished experimentally that performance is also related to laser beam quality.