Wind Tunnel Wall Research Articles

Nonequilibrium turbulence modeling study on light dynamic stall of aNACA0012 airfoil

A computational study on the nonequilibrium turbulence modeling effects for the prediction of the light stall phenomenon has been done for the NACA0012 airfoil. For this, an unsteady thin-layer Navier-Stokes solver was developed that is capable of solving the flowfield around an airfoil undergoing unsteady harmonic motion. In the program, the Baldwin-Lomax and Cebeci-Smith turbulence models were used as baseline models, and the Johnson-King turbulence model was used to study the nonequilibrium effects. It was found that the nonequilibrium effects are important for the prediction of the light stall, and only the Johnson-King model yields light stall hysteresis loop that is similar to the experiment. It was also found that the wind-tunnel wall effects are important, and a mean angle-of-attack increase in the computation was necessary to yield a better agreement with the experiment.

Asymptotic methods for the prediction of transonic wind-tunnel wall interference

Two limiting cases related to transonic wind-tunnel wall interference are considered. The first concerning slender airplanes is briefly treated only as it pertains to the main discussion relating to high aspect ratio configurations. Principal features of the slender airplane case are a three-deck flow structure as well as an area rule for interference. Pressure-specified wall conditions are incorporated by addition of averages and Fourier transforms of the wall pressures that modify the far field that controls the interference. For a high aspect ratio two-deck second case, the interference is driven by the imaging in the Trefftz plane projection of the experimental pressure-specified control surface of the (outer deck) trailing vortex system in the Trefftz plane

Finite element modelling of sheared flow effects on the radiation characteristics of acoustic sources in a circular duct

AbstractThe recent interest in propeller noise generation, stimulated by development of new propeller types for commerical propjets, has generated a need for the ability of measure the noise characteristics of propellers. However, wind tunnel noise measurements are affected by reflections from the wind tunnel walls. Computer codes predicting the free‐field noise of a propeller and its noise field in a circular wind tunnel allow validating the use of wind tunnel measurements to predict free‐field noise characteristics. A wind tunnel contains flow which is uniform in the duct axial direction, but can vary in the radial direction. It can be shown that a third‐order differential equation governs the acoustic pressure field for such a duct containing radially sheared subsonic flow. This third‐order problem is then posed as a coupled pair of equations which are second‐order in terms of acoustic density and first‐order in terms of an artificial variable which represents the effects of the flow being sheared. It is shown that this form of the problem allows a natural extension of the existing numerical solution techniques for non‐sheared flow. The sheared flow problem is presented, and a finite element method is developed to yield a solution for propeller‐type acoustic forces. The finite element code and method are refined with numerical experiments, and results are presented for a specific propeller and duct geometry. Good agreement is shown between this method and an alternate approach to the sheared flow problem using a piecewise constant representation of the velocity in the boundary layer. This validates both the numerical methods.

Wind Tunnel Wall Effects in a Linear Oscillating Cascade

Experiments in a linear oscillating cascade reveal that the wind tunnel walls enclosing the airfoils have, in some cases, a detrimental effect on the oscillating cascade aerodynamics. In a subsonic flow field, biconvex airfoils are driven simultaneously in harmonic, torsion-mode oscillations for a range of interblade phase angle values. It is found that the cascade dynamic periodicity—the airfoil-to-airfoil variation in unsteady surface pressure—is good for some values of interblade phase angle but poor for others. Correlation of the unsteady pressure data with oscillating flat plate cascade predictions is generally good for conditions where the periodicity is good and poor where the periodicity is poor. Calculations based upon linearized unsteady aerodynamic theory indicate that pressure waves reflected from the wind tunnel walls are responsible for the cases where there is poor periodicity and poor correlation with the predictions.

Wind tunnel errors in drag measurements of power conductors

The aerodynamic drag coefficients of electrical power conductors, as measured in wind tunnel experiments, are in general much higher than those inferred from field observations of operating lines. The discrepancy may result from errors in the field measurements or the wind tunnel tests. The purpose of this project was to identify errors in the wind tunnel testing technique. A series of experiments was conducted to investigate the importance of the model aspect ratio and the end conditions present at the junction of the model and the wind tunnel wall. The results show significant errors are possible in the wind tunnel experiments due to these effects but none can account for the large discrepancy between wind tunnel and field data.

Aeroelastic effects of spoiler surfaces on a low-aspect-ratio rectangular wing

An experimental research study to determine the effectiveness of spoiler surfaces in suppressing flutter onset for a low-aspect-ratio, rectangular wing has been conducted in the Langley Transonic Dynamics Tunnel (TDT). The wing model used in this flutter test consisted of a rigid wing mounted to the wind-tunnel wall by a flexible, rectangular beam. The flexible beam was connected to the wing root and cantilever mounted to the wind-tunnel wall. The wing had a 1.5 aspect ratio based on wing semispan and a NACA 64AGIO airfoil shape. The spoiler surfaces consisted of thin, rectangular aluminum plates that were vertically mounted to the wing surface. The spoiler surface geometry and location on the wing surface were varied to determine the effects of these parameters on the classical flutter of the wing model. Subsonically, the experiment showed that spoiler surfaces increased the flutter dynamic pressure with each successive increase in spoiler height or width. This subsonic increase in flutter dynamic pressure was approximately 15% for the maximum height spoiler configuration and for the maximum width spoiler configuration. At transonic Mach numbers, the flutter dynamic pressure conditions were increased even more substantially than at subsonic Mach numbers for some of the smaller spoiler surfaces. But for larger spoiler sizes (in terms of either height or width) the spoilers forced a torsional instability in the transonic regime that was highly Mach number dependent. This detrimental torsional instability was found at dynamic pressures well below the expected flutter conditions. Variations in the span wise location of the spoiler surfaces on the wing showed little effect on flutter. Flutter analysis was conducted for the basic configuration (clean wing with all spoiler surface mass properties included). The analysis correlated well with the clean wing experimental flutter results.

Complex variable boundary element method for external potential flows

We describe a simple,efficient complex variable boundary element method algorithm for calculating potential flow past single and multicomponent airfoils in free air, in ground effect, in an infinite cascade, and in solid, open, and perforated wall wind tunnels. The theoretical development is based on the representation of the complex disturbance velocity by the Cauchy-type integral, formulating the airfoil problem as an exterior (Riemann-)Hilbert problem and accounting for the outer constraints using the concept of Green's function in the complex plane

Free-stream turbulence and tube spacing effects on surface pressure fluctuations for two tubes in an in-line arrangement

An experimental investigation of surface pressure distributions (mean and r.m.s.) on two tubes (circular cylinders) mounted in an in-line or tandem arrangement was carried out in a low-speed, closed-circuit wind tunnel. The measurements were mainly performed at a subcritical Reynolds number, Re = 2·0 × 104, at three different turbulence intensities of the approaching cross-flow (0·1, 1·4 and 3·2%). The spacing between the tubes was in the range of 1·25 ≤ S/D ≤ 5·0. The fluctuating pressures were recorded by a microphone in a pinhole arrangement. End plates were used to minimize the influence of the boundary layers on the wind tunnel walls. The results indicate that the effect of an increased free-stream turbulence intensity is strongly dependent on the tube spacing S/D. It is suggested that this is due to the influence of the free-stream turbulence on the separated boundary layer from the upstream tube. The behaviour of this shear layer determines the type of flow occurring around the tubes and therefore also the magnitude of the forces acting on the tubes. The so-called critical spacing between the tubes was found to decrease as the turbulence intensity was increased. An overall increase in the r.m.s. pressure coefficient was also found as the turbulence intensity was increased.

On the vortex wake of an animal flying in a confined volume

When an animal flies near a boundary, the airflows it generates interact with that boundary. These interactions may have a significant effect on flight performance, as measured by quantities such as the energy rate to sustain flight, or the circulation of the vortices bound on the wing or shed in the wake (or, equivalently, by the lift and induced drag coefficients). The problem of hovering and slow flight within a confined volume is considered by a theoretical model based on helicopter practice, and by flow visualization experiments. The wake takes the form of a strong recirculating flow within the volume, and because of this recirculation the boundaries appear to cause a large reduction in the induced power required for sustained flight, even when their distance from the animal is several times greater than the wingspan. The correction factor relative to ideal momentum jet theory is greater than for the hovering ground effect, forward flight ground effect, or wind tunnel wall interference problems at comparable distances. The flow pattern that develops in the presence of floor and wall interactions in hovering or slow flight includes a large-diameter vortex ring trapped underneath the animal; this vortex ring is conjectured to be analogous to that below a helicopter in slow descending flight in the 'vortex ring state’. Performance measurements for animals in hovering flight within a confined volume may underestimate power for free hovering by a significant margin. Comparable boundary effects may also be important in confined forward flight. Because of speed-related changes in the wake, and the rise in induced power at lower speeds, the appropriate correction to total mechanical power is dependent on air speed, becoming progressively greater as speed reduces. Some wind tunnel measurements of total metabolic power have produced the apparently anomalous result that power is independent of flight speed within measurement error. These observations may be explained - at least in part - by boundary effects caused by interaction between the wake and the walls of the wind tunnel.

Hydrodynamical Forces on a Moving Body in Ground Effect

An experimental technique is developed for a vehicle moving steadily in the vicinity of the ground. The towing tank with a steadily advancing carriage is used and the unfavorable effects of the boundary layer on the wall of wind tunnel is fully removed. Experiments with a box-shaped model and a car model revealed some interesting features of lift, drag and side force at various clearance from the ground. Lift force is more sensitive to the boundary layer and it should not be measured in a wind tunnel. The flow with vortices near the bottom surface of the body gives one of the most important effects on the forces.

Evaporation of polydisperse organicaerosols at ambient conditions

The evaporation of polydisperse ethanol aerosols(10 μm–200 μm) has been investigatedexperimentally and theoretically in a vertical wind tunnel. Droplet evaporation rates were followed by in-situ measurement of size distributions at two downstream locations from the point of injection under varying conditions of gas velocity, temperature and relative humidity. Droplet deposition on to the wind tunnel walls was found to be significant, thus becoming additionally responsible for the large variation in droplet size distributions along the tunnel.

Low‐Frequency Rotational Noise in Closed‐Test‐Section Wind Tunnels

The effects of closed-section wind-tunnel walls on the sound field radiated from a helicopter rotor are investigated by means of numerical simulations, summarizing the findings reported by Mosher (1986). The techniques used to model the rotor and the test section (including geometry, wall absorption, and measurement location) are outlined, and the results are presented in extensive tables and graphs. It is found that first-harmonic acoustic measurements obtained in a hard-walled wind tunnel twice as wide as the rotor diameter do not accurately represent the free-field rotational noise, that the relationship between the sound-pressure levels in the wind tunnel and in the free field is complex, that multiple near-field measurements are needed to characterize the direct acoustic field of the rotor, and that absorptive linings are of little value in enlarging the accurate-measurement zone.

Transonic wind-tunnel wall interference prediction code

A small disturbance transonic wall interference prediction code has been developed that is capable of modeling solid, open, perforated, and slotted walls as well as slotted and solid walls with viscous effects. This code was developed by modifying the outer boundary conditions of an existing aerodynamic wing-body-pod-pylon-winglet analysis code. The boundary conditions are presented in the form of equations which simulate the flow at the wall, as well as finite difference approximations to the equations. Comparisons are presented at transonic flow conditions between computational results and experimental data for a wing alone in a solid wall wind tunnel and wing-body configurations in both slotted and solid wind tunnels.

Effect of Helmholtz resonators on boundary-layer turbulence

HELMHOLTZ resonators were investigated as a passive device for the purpose of modifying a turbulent boundary layer. Resonators, imbedded in a wall, also occur inadvertently when porous walls are used to reduce boundarylayer growth or as acoustic control devices in inlet ducts and combustors. In all cases it is of interest to know the effect of the resonators on the boundary layer. Helmholtz resonators are formed by cavities that are vented by a small orifice (see Fig. 1). They respond to excitation from the shear layer at the orifice as well as from incident acoustic waves. Previous experimental work '~4 has focused on the conditions for resonance in a single resonator. In the present study, a row of 10 resonators was positioned across the span of a wind-tunnel wall. Separate studies5'6 were conducted to determine the response of the resonator row to the boundary layer and the interaction that occurred between adjacent resonators. The relevant results are that peak response was achieved at a freestream velocity of 26.5 m/s, and the frequency and amplitude of the cavity pressure oscillations were 570 Hz and 143 dB (equivalent to 80% of the freestream dynamic pressure). In addition, adjacent resonators showed a strong antiphase locking with an average coherence coefficient of 70% and a phase lag of 150 to 180 deg. The phase locking between resonators was localized, and coherence decreased rapidly with resonators that were further away. Contents Experiments were conducted in a wind tunnel with a test section of 0.71 m x 1.0 m and 2.7 m long (28 in. x 40 in. x 9 ft). Ten resonators were formed with orifices of diameter D = 1.03 cm (0.406 in.) and 0.305 cm (0.120 in.) thickness. They were spaced 3.05 cm (1.20 in) on centers occupying the central 27.5 cm (10.8 in) of the 1.0 m (40 in.) span. At the location of the resonators, with flow parameters chosen to produce a resonant condition, the boundary layer had a thickness of d = 2.8 cm (1.1 in.), friction velocity UT = 1.0 m/s, and freestream speed of 26 m/s (58 mph). The corresponding Ret> was 5500. Measurement locations were chosen at the streamwise (X) positions X/D= -3,-0.5, 0.5, 3, 10,20,40, and 80 together with six spanwise (Z) positions, three directly in line with the resonator orifices and three half-way between the orifices. Laser velocimeter measurements of the streamwise and vertical velocity were obtained and processed to yield values of the

Two-dimensional wind-tunnel wall interference

An examination is conducted of two approximate methods for estimating the correction due to upstream flow inclination effects considered in Kemp's (1979) method; the methods are (1) an image vortex in the contraction, and (2) a rotation of streamlines passing through the test section upstream end. The magnitude of the corrections predicted by the two methods is different. The method based on the rotation of streamlines is noted to be in closer agreement with the results of the more refined calculations.

Residual interference and wind-tunnel wall adaptation

We review a linear theory of residual interference in adaptive wall wind tunnels. One-variable, two-variable, and interface discontinuity methods are analyzed. Their autocorrective properties and connections with some well-known adaptation criteria, such as the single-step convergence formulas and the functional relationships for unconfined flow, are discussed

Analytical study of wind-tunnel acoustic testing of propellers

An analytical study based on finite element models is made of the acoustic testing of propellers in a wind-tunnel environment. Features that are considered are the subsonic mean flow, including the effect of a boundary layer; acoustically absorbing wind-tunnel walls; and a lifting line representation of the propeller. The boundary layer is approximated by several layers of uniform mean flow with interlayer boundary conditions forcing continuity of particle displacement. Results are presented as simulated acoustic measurements made at various locations around the propeller in the wind tunnel. Comparisons are made with similar simulated measurements generated by a free-field, finite element model of the same propeller. In the present study, the tunnel Mach number is limited to M = 0.5, only purely resistive wall treatment is used, and relatively thick boundary layers are considered. It is found that acoustic pressures near the propeller and removed from the walls compare well with pressures in a free-field environment.

Tunnel interference assessment from measurements on two interfaces

An analytic assessment of two-dimensional wind-tunnel wall interference has been formulated. With measured upwash velocity components along two streamwise interfaces in the tunnel, the Prandtl-Glauert equation for the flowfield is solved by the Fourier transform technique. Analytic formulas for the tunnel-induced interference velocity components at the model are presented

The interaction of Helmholtz resonators in a row when excited by a turbulent boundary layer

The interaction between individual Helmholtz resonators when arranged in a row and excited by a turbulent boundary layer was explored experimentally. A wind tunnel wall, which had a turbulent boundary layer, provided the test region for the resonators that were placed across the span. Ten resonator orifices were flush mounted on the wall and the cylindrical resonator cavities were behind the orifice plate. In a series of tests, the effect of resonator spacing upon the response level, frequency, and the coherence and phase between resonators was determined. When two resonators were paired the oscillating cavity pressure level increased by 1 dB (at orifice spacings of one boundary layer thickness). This effect decreased with increased spacing. The same trend occurred for the correlation between resonator signals. When a row of several resonators was tested, the coherence and response level enhancement were less than that for two paired resonators. In both row and paired configurations, the oscillation between adjacent cavities was out of phase: the angle varied from 150 to 180 deg. This antiphase behavior was attributed to a fluid mechanic coupling as contrasted to acoustic coupling. The frequency of a single resonator’s response was seen to be an almost linear function of air speed. This behavior was also observed when interacting with other resonators, although the frequencies were increased by about 1%. Estimates were made of the appropriate outside end-correction factor that would be associated with these frequency changes.

Optimum porosity of wind tunnel walls at low supersonic velocities

The interaction of oblique shocks and expansion waves with perforated wind tunnel walls at\(M_\infty \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{ > } 1\) is theoretically investigated. The conditions in which these perturbations are not reflected from the walls and, consequently, do not distort the flow over the model are found. The results of an experiment which confirmed the theoretical conclusions are presented. A comparison is made with the experimental data of other authors.