Boundary-layer Tunnel Research Articles

Atmospheric wind pressure distribution on an isolated agricultural silo / Distribuição da pressão atmosférica do vento em um silo agrícola isolado

Storage silos installations are very common in the agricultural areas of Argentina and Brazil. Accidents caused by the action of the wind on the structural components of silos are usually reported. For this reason, the UTN - Regional Rafaela and the Laboratorio de Aerodinámica of the UNNE began to develop experimental studies in a wind tunnel with the objective of analyzing the external pressure distribution on isolated silos and complementing the procedures established in the Argentine Wind Code CIRSOC 102 and in the Brazilian Code NBR 6123. The tests were carried out in the UNNE “Jacek P. Gorecki” wind tunnel, which is a boundary layer tunnel of the Eiffel-type. The total length of the tunnel is 39.65 m, the test section is 2.4 m wide × 1.8 m high × 22.8 m long and an axial fan generates a maximum speed of 25 m/s. Turbulence generators and surface roughness elements were used to simulate atmospheric wind and then a 1:100 scale model of the silo was tested. In this work, results of mean and fluctuating pressure values obtained by wind tunnel tests are presented. The turbulent wind actions defined by the fluctuating pressures are evaluated through the distributions of RMS and peak values.

Analysis of Thermal and Roughness Effects on the Turbulent Characteristics of Experimentally Simulated Boundary Layers in a Wind Tunnel.

The aim of this paper is to analyse the thermal effects in a wind tunnel experiment to simulate the planetary boundary layer (PBL). Experiments were performed in the wind tunnel of the Laboratory of Constructions Aerodynamics at the Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul State, Brazil. This wind tunnel is a closed return low-speed wind tunnel specifically designed for dynamic and static studies on civil construction models. As a novelty, one of the experimental sections of the wind tunnel was equipped with a metal sheet with Peltier elements coupled to it. In other words, thermal effects generating new flow patterns become feasible and open pathways to compare wind tunnel simulations to those in the PBL. Furthermore, measurements obtained with the smooth floor of the wind tunnel were repeated under the same conditions with the addition of the roughness in the floor, and the mechanical turbulence generated by the surface roughness significantly amplified the exchange of momentum and heat between the regions located in vertical direction of the wind tunnel boundary layer. In the presence of turbulent heat flux near the surface, thermal effects contribute to the increase of the turbulence intensity. Turbulent energy spectra for flow velocities and different heights were obtained using the Hilbert–Huang transform method, and the observed convective turbulence energy spectra behavior reproduced those measured in an unstable surface PBL.

Do coherent structures organize scalar mixing in a turbulent boundary layer?

A scalar emanating from a point source in a turbulent boundary layer does not mix homogeneously, but is organized in large regions with little variation of the concentration: uniform concentration zones. We measure scalar concentration using laser-induced fluorescence and, simultaneously, the three-dimensional velocity field using tomographic particle image velocimetry in a water tunnel boundary layer. We identify uniform concentration zones using both a simple histogram technique, and more advanced cluster analysis. From the complete information on the turbulent velocity field, we compute two candidate velocity structures that may form the boundaries between two uniform concentration zones. One of these structures is related to the rate of point separation along Lagrangian trajectories and the other one involves the magnitude of strong shear in snapshots of the velocity field. Therefore, the first method allows for the history of the flow field to be monitored, while the second method only looks at a snapshot. The separation of fluid parcels in time was measured in two ways: the exponential growth of the separation as time progresses (related to finite-time Lyapunov exponents and unstable manifolds in the theory of dynamical systems), and the exponential growth as time moves backward (stable manifolds). Of these two, a correlation with the edges of uniform concentration zones was found for the past Lyapunov field but not with the time-forward future field. The magnitude of the correlation is comparable to that of the regions of strong shear in the instantaneous velocity field.

Heat transfer measurements of a nanoscale hot-wire in supersonic flow

Upon its development and initial characterization, the supersonic variant of the nanoscale thermal anemometry probe (S-NSTAP) was deployed in a supersonic wind tunnel facility, where both freestream and boundary layer measurements were obtained at $$M_\infty =2$$ . The low operating stagnation pressures generated reliable data, where the effects of Reynolds number, Mach number and overheat ratio on the sensor’s heat transfer were investigated in detail. The performance of the S-NSTAP was also compared to that of a conventional cylindrical hot-wire and the S-NSTAP was shown to exhibit unparalleled temporal resolution ( $$\sim $$ 300 kHz). The mass flux sensitivity coefficient of both hot-wires was further computed and appeared to vary between probes, yielding a coefficient twice as large for the conventional probe than for the S-NSTAP. The experimental data obtained from both hot-wires were also compared, via spectral analysis and turbulence statistics, to the results of a numerically modelled turbulent boundary layer.

Wind Turbulence Statistics of the Atmospheric Inertial Sublayer under Near-Neutral Conditions

The inertial sublayer comprises a considerable and critical portion of the turbulent atmospheric boundary layer. The mean windward velocity profile is described comprehensively by the Monin–Obukhov similarity theory, which is equivalent to the logarithmic law of the wall in the wind tunnel boundary layer. Similar logarithmic relations have been recently proposed to correlate turbulent velocity variances with height based on Townsend’s attached-eddy theory. The theory is particularly valid for high Reynolds-number flows, for example, atmospheric flow. However, the correlations have not been thoroughly examined, and a well-established model cannot be reached for all turbulent variances similar to the law of the wall of the mean-velocity. Moreover, the effect of atmospheric thermal condition on Townsend’s model has not been determined. In this research, we examined a dataset of free wind flow under a near-neutral range of atmospheric stability conditions. The results of the mean velocity reproduce the law of the wall with a slope of 2.45 and intercept of −13.5. The turbulent velocity variances were fitted by logarithmic profiles consistent with those in the literature. The windward and crosswind velocity variances obtained the average slopes of −1.3 and −1.7, respectively. The slopes and intercepts generally increased away from the neutral state. Meanwhile, the vertical velocity and temperature variances reached the ground-level values of 1.6 and 7.8, respectively, under the neutral condition. The authors expect this article to be a groundwork for a general model on the vertical profiles of turbulent statistics under all atmospheric stability conditions.

A smoke exhausting method through a baffle-coupled shaft during tunnel fires

In natural ventilation of road tunnel shaft, the plug-holing and boundary layer separation will decrease the efficiency. A new concept (baffle-coupled shaft) is put in place to solve the difficulty. Install baffle on both sides of the shaft (when the fire occurs upstream of the shaft, the baffle upstream of the shaft closes and the baffle downstream of the shaft opens), in the early stage of the fire, it can function as a smoke barrier. Numerical simulation was performed using the Fire Dynamic Simulation (Version 5.5.3). We use the mass flow of carbon monoxide discharged through the shaft as a basis for evaluating the efficiency of exhausting smoke. We could have determined that with the height of the baffle increasing, so does the efficiency of exhaust. In the investigation, the height of urban road tunnels is generally about 5 meters. So, it is appropriate to set the baffle height to 2m. Comprehensive car capacity, economy and cold air distributed, we can get a better He, that is in 1.0-1.5.

Semi-span wind tunnel testing without conventional peniche

Low-speed wind tunnel tests of a flexible wing semi-span model have been implemented in the 9times 7 ft de Havilland wind tunnel at the University of Glasgow. The main objective of this investigation is to quantify the effect of removing the traditional peniche boundary layer spacer utilised in this type of testing. Removal of the peniche results in a stand-off gap between the wind tunnel wall and the model’s symmetry plane. This offers the advantage of preventing the development of a horseshoe vortex in front of the model, at the peniche/wall juncture. The formation of the horseshoe vortex is known to influence the flow structures around the entire model and thus alters the model’s aerodynamic behaviours. To determine the influence of the stand-off gap, several gap heights have been tested for a range of angles of attack at Re=1.5times 10^6, based on the wing mean aerodynamic chord (MAC). Force platform data have been used to evaluate aerodynamic coefficients, and how they vary with stand-off heights. Stereoscopic Particle Imaging Velocimetry (sPIV) was used to examine the interaction between the tunnel boundary layer and model’s respective stand-off gap. In addition, clay and tuft surface visualisation enhanced the understanding of how local flow structures over the length of the fuselage vary with stand-off height and angle of attack. The presented results show that a stand-off gap of four-to-five times the displacement thickness of the tunnel wall boundary layer is capable of achieving a flow field around the model fuselage that is representative of what would be expected for an equivalent full-span model in free-air—this cannot be achieved with the application of a peniche.

CFD prediction of steady and unsteady upwind sail aerodynamics

In this paper, RANS CFD simulations of steady and unsteady upwind sail aerodynamics are verified and validated against wind tunnel experiments previously performed at Politecnico di Milano. Experimental data is presented, and the experimental results are analysed to estimate repeatability. Blockage effects caused by tunnel boundary layers and measurement equipment are estimated using detailed CFD simulations, allowing such effects to be accounted for. A verification procedure is performed, resulting in very low numerical uncertainties. The force predictions are then validated for both steady and unsteady conditions. The steady results show very good agreement, with a maximum error of 3.1%. For the unsteady conditions, good agreement is found for the force mean values, whilst the prediction of force amplitudes is less satisfactory.

Evaluation of high-order concentration statistics in a dispersing plume

In this paper we analyze the concentration higher-order moments of a dispersing plume in a neutral wind tunnel boundary layer. They are evaluated through a fluctuating plume model in which the plume barycenter is directly calculated from the measured mean concentration field. The reliability of the model is firstly verified by comparing the simulated and measured moments up to the fourth order. Then the relationships between higher and lower-order moments are investigated seeking for a proper scaling of the fourth, fifth and sixth-order normalized moments. We found that a general behavior appears when they are expressed as a function of the third-order moments. In this case each order calculated or measured at different distances collapses on the same curve.

Artificial thickening and thinning of cavitation tunnel boundary layers

Measurements of natural, thickened and thinned boundary layer mean velocity profiles on the ceiling of a cavitation tunnel test section are presented. The method of thickening investigated is via an array of transverse injected jets and for thinning via ingestion of the natural boundary layer fluid through a perforated plate. Several jet arrays of different geometric configuration and open area were tested over a range of jet to freestream velocity ratios, Reynolds numbers and cavitation numbers. The thickened and thinned velocity profiles are compared with the laws of the wall and wake using parameters derived from the natural boundary layer profiles. The most significant parameter controlling the degree of thickening is the open area, as predicted by one-dimensional mass and momentum conservation, with improvements achievable depending on the jet array configuration. Of the configurations tested an array of intermediately spaced jets was found more effective for thickening than a single row or either sparsely or closely spaced arrays. The profiles of all configurations were found to compare favourably with the laws of the wall and wake to varying degrees, depending upon the geometry, jet velocity and the streamwise length in terms of the number of boundary layer thicknesses for profile development. The results showed that boundary layers could be artificially thickened from momentum Reynolds numbers of about 30,000 to values of about 100,000, or friction Reynolds numbers from about 10,000 to 35,000. Jet velocity was shown to have a significant effect on cavitation inception and generated noise, demonstrating this must be minimised to optimise cavitation limits. Overall the results suggest that a jet array of large open area, to increase thickness and minimise the jet to freestream velocity ratio, with jets distributed with sufficient spacing to promote mixing provides an idealised configuration.

Ensayos de viento para el Puente de la Constitución de 1812 sobre la Bahía de Cádiz

The 1812 Constitution Bridge over the Cádiz Bay has been subjected to several wind studies and tests using the most advanced methodology and techniques that the current state of the art allows. Full aeroelasticity tests in a boundary layer tunnel have been carried out, taking into account the whole configuration of the bridge during the in-service state, and also considering the most critical stage during the construction. No risk of aerodynamic instability has been found for the design wind speed. Furthermore, static section model tests for determination of static load coefficients for drag, lift and moment, and dynamic section model tests for simulation of the vertical and torsional vibrations of the bridge induced by vortex shedding and buffeting were also carried out. An exhaustive tests program was also developed to investigate the wind action on reference vehicles, evaluating the good performance of the windshield.

Determination of the Diameter of a Cylinder to Insertion in a Low-Speed Wind Tunnel Based on EFD and CFD

The objective of this study was to determine the ideal diameter of a circular cylinder for insertion into the test section of a small wind tunnel, so that there was no flow blockage and influence of the developing wind tunnel boundary layer in the flow around the cylinder. The approaches used were the experimental fluid dynamics (EFD) by the use of Pitot tube and hot wire anemometer (HWA) and the computational fluid dynamics (CFD) simulations, performed with ANSYS CFX software. For this, it was first determined the boundary layer thickness along the test section without the cylinder, through computer simulations. Based on this information, several simulations were performed with cylinders of different diameters to quantify their influence on the rotational flow. From the four studied cylinder diameters, it was found that only the one with 30 mm did not cause bending of the streamlines at the intersection with the boundary layer developing in the tunnel. Moreover, this was the minimum diameter of the cylinder so that the tests with Pitot probe could be used without disturbances caused by its dimensions in the flow to be characterized. After analyzing these results, the cylinder of 30 mm was built and the experimental tests were performed, validating the numerical results.

Mean loads on vaulted canopy roofs

The vaulted canopy roof (VCR) is a widespread structure used in vast areas of South America, especially in parts of Argentina, Brazil and Paraguay. Information about the aerodynamics of VCRs is scarce in the literature, and it is restricted to mean load coefficients, which must be used in combination with the quasi-steady approach. Most of the existing data have been produced during the last decade in South America, but these data are not readily accessible because they were published in Spanish. This paper presents the results of mean wind load coefficients on VCRs obtained in boundary layer tunnel tests, and the aerodynamics of VCRs are discussed by taking into account the data produced during the last decade.

Turbulent Flow Separation Control over a Two-Dimensional Ramp Using Synthetic Jets

An experiment study of flow control using an array of three circular synthetic jets has been carried out in a separated turbulent flow over a two-dimensional ramp in a boundary-layer tunnel. The freestream velocity is 6:5 m=s and the Reynolds number based on the momentum thickness at a reference location upstream of the synthetic jet array is Re 1150. The synthetic array is operated at mean jet-to-freestream velocity ratios ranging from 0.2 to 0.5 and a reduced frequency based on the ramp height of F 0:2 and 0.6. Two-dimensional particle image velocimetry is employed to evaluate the control effect of synthetic jets at different actuator operating conditions in both timeand phase-averaged senses. Stereo particle image velocimetry is used to reveal the vortical structures thatwere responsible for theflow separation delay. It is found that at both reduced frequencies an increase in velocity ratio results in a further reduction in the height of the reverse-flow region, and somedifferences exist in the way inwhich the reverseflowresponds to jet actuation of the twodifferent reduced frequencies. It is also found that at the conditions tested in the present study, the interaction of synthetic jets with the turbulent boundary layer results in formation of hairpin vortices, which induce a further pair of streamwise vortices of the opposite sign of rotation directly underneath. It is believed that it is the streamwise vortices that produce a downwash bringing highmomentum fluid toward the wall, hence reducing the flow separation.

High Reynolds number turbulent wind tunnel boundary layer wall-shear stress sensor

The fluctuating wall-shear stress in a turbulent zero-pressure gradient wind tunnel boundary layer at high Reynolds numbers up to Re Θ = 11,400 is determined by the micro-pillar shear-stress sensor MPS3. The sensor concept has already successfully been applied to liquid fluid flows, where the sensor shows low-pass filter characteristics. The low-pass filter characteristic is favorable especially when turbulent frequencies larger than the damped eigenfrequency of the structure occur. The application in air, however, is critical since the dynamic transfer function of the sensor structures exhibits a strong resonance due to low damping. The current results demonstrate that as long as frequencies of turbulent wall-shear stress fluctuations are below the damped eigenfrequency even a resonant sensor structure can be used to accurately assess the fluctuating wall-shear stress, e.g., in turbulent wind tunnel boundary layers.

Experimental transport-rate budgets in complex 3-D turbulent flow near a wing/body junction

Experimental flow measurements are presented for a wing–body junction flow obtained using laser-Doppler velocimetry. Mean velocity and Reynolds stress data are used to calculate the complete transport-rate budgets of Reynolds stresses and turbulent kinetic energy. The measurements were carried out in the Virginia Tech Boundary Layer Tunnel at a nominal air speed of 27.5 m/s around a NACA 0020 tail and 3:2 elliptical nose wing shape. Data are presented for a two-dimensional turbulent boundary layer (2DTBL), a strongly skewed three-dimensional turbulent boundary layer (3DTBL), a location in the vicinity of a 3-D separation line, and around the center of the vortex in the horse-shoe vortex that forms around the wing. Terms in the transport-rate equations were calculated also using the measured triple order fluctuating velocity products. Results show that the pressure-diffusion approximated by Lumley [Lumley, J.L., 1978. Computation modeling of turbulent flows. Adv. Appl. Mech. 18, 124–176] is an important term in the balance of v 2 ¯ , uv ¯ , and vw ¯ stress budgets; there were distinct differences between the two-dimensional and three-dimensional turbulent boundary layer budgets. Qualitative comparisons of experimental stress-transport-rate budgets to previous DNS results show a better agreement using the anisotropic dissipation rate of Hallbäck et al. [Hallbäck, M., Groth, J., Johansson, A.V., 1990. An algebraic model for nonisotropic turbulent dissipation rate in Reynolds stress closure, october. Phys. Fluids A 2 (10), 1859–1866].

Estimation of the Lagrangian Kolmogorov constant from Eulerian measurements for distinct Reynolds number with application to pollution dispersion model

An expression that allows the determination of the Lagrangian Kolmogorov structure function constant C 0 from the knowledge of the constant C S associated to the second order Eulerian velocity structure function and the γ numerical coefficient relating the Eulerian spectra constant to the Lagrangian one, is suggested. Experimental data from both wind tunnel and atmospheric boundary layer observation were used to determine C S. The study agrees with the universal nature of the Eulerian C S constant for fully developed turbulence. The analysis leads to an estimate of C 0≅5.09±0.93, in agreement with previous studies. This estimated value has been employed in a Lagrangian stochastic dispersion model to simulate the observed concentration obtained from classical diffusion experiments.

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

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

Net pressures on the roof of a low-rise building with wall openings

This paper deals with an investigation of the characteristics of net pressures on two significant roof areas of a low-rise building with two different dominant wall openings. Wind tunnel boundary layer studies were conducted on a corner and a gable-end roof area of a 1:50 geometric scale model of the Texas Tech University (TTU) test building with a corner and a central wall opening. Mean and peak pressure coefficients, RMS values for the pressure coefficient fluctuations about their mean, as well as roof external pressure—internal pressure correlation coefficients were obtained for the entire 360° wind azimuth range. Frequency domain studies were also conducted for a few selected point roof pressure situations from which the frequency-dependent roof external pressure—internal pressure phase difference functions, root coherence functions and the spectral density functions were obtained. The results show that the mean, RMS and peak net pressure coefficients are particularly enhanced relative to the coefficients for the roof external pressure in the ±50° wind range. Zero-time-lag correlation coefficients of up to −0.64 were obtained in agreement with results from past studies, while root coherence values of up to 0.7 were also recorded. It is demonstrated that the provisions of both the Australian/New Zealand wind loading code—the AS/NZS1170.2:2002, and the American wind loading code—the ASCE7-02, are sometimes non-conservative in the prediction of mean and peak net roof pressure coefficients. These are believed to be due to non-conservative internal pressure coefficients allowed for in these codes.

Finite-size scaling of two-point statistics and the turbulent energy cascade generators

Within the framework of random multiplicative energy cascade models of fully developed turbulence, finite-size-scaling expressions for two-point correlators and cumulants are derived, taking into account the observationally unavoidable conversion from an ultrametric to an Euclidean two-point distance. The comparison with two-point statistics of the surrogate energy dissipation, extracted from various wind tunnel and atmospheric boundary layer records, allows an accurate deduction of multiscaling exponents and cumulants, even at moderate Reynolds numbers for which simple power-law fits are not feasible. The extracted exponents serve as input for parametric estimates of the probabilistic cascade generator. Various cascade generators are evaluated.