Previous Wind Tunnel Experiments Research Articles

The Overall Collection Efficiency of Catching‐Type Precipitation Gauges in Windy Conditions

AbstractThe wind‐induced bias of catching‐type precipitation measurement instruments is quantified using Computational Fluid Dynamics with embedded liquid (raindrops) and solid (snowflakes) particle tracking. The performance of six common commercial gauges having different outer geometry is compared under a range of Precipitation Intensity (PI) and wind speed conditions by means of the numerically calculated catch ratios. Validation of the simulated aerodynamic behavior and its effect on water drop trajectories is provided by previous wind tunnel experiments. The functional dependence of the overall collection efficiency on the PI is derived as a quantitative measure of the instrument performance under a wind climatology with a uniform probability density function. Instruments with aerodynamic design produce a less pronounced diversion of hydrometeor trajectories—at any given size—than those having more traditional geometry. For liquid precipitation measurements, chimney‐shaped instruments rank low, while inverted conical and Nipher shielded instruments are quite performant solutions. The investigated quasi‐cylindrical gauges have intermediate behavior depending on their detailed aerodynamic features. For solid precipitation, all instruments rank low at light to moderate PI, except the Nipher shielded gauge. Results allow selecting the appropriate instrument for the local precipitation climatology at the measurement site and can be used to apply suitable adjustments to the measured precipitation.

Impact of bogie fairing configuration on the aerodynamic performance of high-speed train under crosswind

In this paper, the unsteady aerodynamic characteristics of a three-car grouped high-speed train (HST) with different bogie fairing configurations under the crosswind are studied using improved delayed detached eddy simulation (IDDES) at Re = 1.85 × 106. The numerical method results are validated against the previous wind tunnel experiments. There are three cases with original half-fairing configuration (HFC), non-fairing configuration (NFC) and full-fairing configuration (FFC), respectively. The numerical results show that the bogie fairing configuration has significant effects. Compared to the HFC case, the drag force coefficients of the head, middle and tail cars with NFC increase by 53.13%, 14.87% and 27.04%, and corresponding parts of the FFC case decrease by 49.98%, 24.40% and 12.41%, respectively. The FFC case can also optimize the pressure distribution on the windward and leeward sides of the HST, and the flow structure and vertex distribution around the car body are also optimized. In addition, the increasing encirclement of the fairing configurations reduces the maximum lateral flow velocity inside the bogie cavities by 70% for HFC and 50% for FFC compared to the NFC. Overall, the FFC presents better aerodynamic performance than the NFC and HFC cases under crosswind, which is recommended for the design of the HST.

Estimation of dynamic wind forces on a steel lattice tower based on generalized wind force spectra

This article presents a method for estimating dynamic wind forces on a steel lattice tower. The spectra of local wind forces are derived based on generalized wind force spectra, which have considered the coherence between different axial wind forces under the skewed wind. With the established cross-power spectral density (PSD) matrix of wind forces, the fluctuating wind forces can be simulated in the time domain using the harmony superposition method. The wind tunnel experiments on a typical transmission steel lattice tower are introduced. Using the test data, the coherence of wind forces on different body axes is analyzed, and the corresponding empirical function is fitted. With the empirical PSDs of the generalized forces obtained in the previous wind tunnel experiments, the dynamic wind forces on the transmission tower are simulated in target wind environments. The dynamic responses of the tower are calculated under the estimated wind forces and compared with the measurements in the aeroelastic wind tunnel experiments. Results reveal that the proposed approach to estimating distributed dynamic wind forces on a steel lattice tower performs effectively.

Investigation on Forced Convective Heat Transfer at the Building Façade with Louver Blinds under Different Wind Directions

Exterior louver blinds are one of the most common shading devices attached to the building façade, which will not only screen solar radiation but also affect the flow around the building and change the convective heat exchange at the façade. To better understand how different wind directions affect the convective heat properties of louvered building façades, this paper conducted a CFD simulation with the validation of previous wind-tunnel experiments. A total of 63 shading cases and 7 unshaded cases were both investigated by simulations. It was found that with the increase in wind attack angle θ, the surface-averaged convective heat transfer coefficient (CHTC) at the louvered surface will first increase by about 40~50% (θ from 0° to 60°) and then be cut down by 60% (θ from 60° to 90°). In addition, the convective heat exchange rate at the louver slats will gradually decrease with the growth of θ. Finally, modifications for previous correlations were proposed to quantify the effect of wind directions on the CHTCavg at the blinds and façade, of which the maximum deviation for convective heat exchange rate is less than 18%.

Lifting of Tribocharged Grains by Martian Winds

Abstract It is a long-standing open question whether electrification of wind-blown sand due to tribocharging—the generation of electric charges on the surface of sand grains by particle–particle collisions—could affect rates of sand transport occurrence on Mars substantially. While previous wind tunnel experiments and numerical simulations addressed how particle trajectories may be affected by external electric fields, the effect of sand electrification remains uncertain. Here we show, by means of wind tunnel simulations under air pressure of 20 mbar, that the presence of electric charges on the particle surface can reduce the minimal threshold wind shear velocity for the initiation of sand transport, u *ft, significantly. In our experiments, we considered different samples, a model system of glass beads as well as a Martian soil analog, and different scenarios of triboelectrification. Furthermore, we present a model to explain the values of u *ft obtained in the wind tunnel that is based on inhomogeneously distributed surface charges. Our results imply that particle transport that subsides, once the wind shear velocity has fallen below the threshold for sustained transport, can more easily be restarted on Mars than previously thought.

Verifying the Effect of Wingtip Propellers on Drag Through In-Flight Measurements

This work presents the effect of wingtip-mounted propellers on the aircraft drag polar, identified from in-flight measurements. In previous wind-tunnel experiments, significant reductions in drag and an increase in propulsive efficiency through interaction between induced flow by the wingtip-mounted propellers and the flowfield of the wing itself have been claimed. So far, however, these effects have never been verified in actual flight. That gap is closed here by presenting the results of in-flight measurements. A 33.3% scaled version of the manned, electric e-Genius aircraft has been fitted with a wingtip propulsion system and an elaborate measurement system, which allows quantifying the aforementioned effects. The drag polar is deduced through weighted least-squares parameter estimation for different settings and configurations of the wingtip propellers. The findings are compared to results from previous wind-tunnel experiments. The paper further discusses secondary effects that were observed during the flight tests and their influence on the general potential of wingtip propulsion.

Predicting Boundary-Layer Transition over Backward-Facing Steps via Linear Stability Analysis

Laminar–turbulent transition downstream of a backward-facing step (BFS) in a two-dimensional zero-pressure-gradient boundary layer is investigated computationally for step-height–to–local-displacement-thickness ratios of . The nominally sharp BFS in a previous wind tunnel experiment is approximated via a smooth shape with a specified maximum slope. The laminar basic state solutions with the BFS are computed by solving the Navier–Stokes equations over a curvilinear grid. The linear amplification of Tollmien–Schlichting instabilities from the effects of basic state modification due to the BFS is quantified in terms of the logarithmic amplification ratio (or factor) and computed with the harmonic linearized Navier–Stokes equations. The transition locations measured at nearly all test conditions with correlate well with a computed factor of , demonstrating the first successful stability-based transition criterion related to step excrescences. An explanation for this successful correlation is that the net destabilization due to the BFS is the dominant contributor to the resulting shift in the transition onset location. The effect of any additional receptivity near the BFS does not play an important role in determining the movement of the transition front.

The impact of building surface temperature rise on airflow and cross-contamination around high-rise building.

This paper numerically studies the characteristics of flow field around a high-rise building and the cross-contamination when the building surface is heated by the solar radiation. Firstly, the normalized concentration Kc is used to evaluate the dispersion characteristics under different source locations without surface temperature rise. Under iso-thermal condition, the near-wall pollutant dispersion features revealed by the predicted results are similar to our previous wind tunnel experiment. Then, the effect of wall surface temperature rise on the cross-contamination and the flow fields is evaluated based on the near-wall concentration distributions and the wake zone vortex core positions, respectively. When the building surface temperature rises, the location of vortex core obviously changes comparing with that under iso-thermal condition. The correction formula for the vortex core location with the leeward wall surface temperature rise below 15K is developed. The windward wall surface temperature rise brings more serious pollutant accumulation. The near-wall concentrations increase with the rise of temperature when the pollutant is released from the bottom and middle of leeward wall surface, while the top-release scenario exhibited a contrary tendency. For the three interval ranges of generally recognized Richardson number Ri (Ri < 0.1; 0.1 < Ri < 10; Ri > 10), these results indicate that when Ri is less than 0.1, the effect of wall surface temperature rise on near-wall flow and cross-contamination of small-scale model cannot be ignored.

Numerical prediction of aerodynamic performance for a flying fish during gliding flight

Flying fish is a family of unique aerial–aquatic animals, which can both swim in the water and glide over the sea surface. Most previous studies on their aerodynamic characteristics were based on field observations or measurements of their morphometric parameters. In the present study, we consider three different flying fish models, of which the preliminary one mimics the Cypselurus hiraii in the pectoral fin morphology, following a previous wind tunnel experiment (Park and Choi 2010 J. Exp. Biol. 213 3269–79). Their aerodynamic performances are numerically studied by the computational fluid dynamics (CFD) method. The maximum lift force coefficient of 1.03 is reached at the angle of attack , and the maximum lift-to-drag ratio of 4.7 is achieved at . By choosing appropriately the center of gravity, the flying fish model is proved to be longitudinally stable, according to the negative slope of pitching moment profile. Furthermore, we build a three-degrees-of-freedom (3-DOF) dynamic model in the longitudinal plane based on the aerodynamic coefficients obtained in our simulations, to predict its gliding performance. The results show that the flying fish can achieve a distance up to 45.4 m, and reach a height of 13.2 m, indicating an extraordinary gliding performance. Our numerical simulations are consistent with previous experimental results and theoretical prediction, which can be taken as the basis of further research on robotic flying fish.

Outdoor measurement of wall pressure on cubical scale model affected by atmospheric turbulent flow

Most studies on indoor ventilation have utilized wind-tunnel experiments (WTEs) or computational fluid dynamics based approaches under well-controlled flow conditions. However, the effects of urban boundary layer flow with variable wind directions and various turbulence scales on the ventilation driven by the pressure differences between the upwind and downwind sides of a building within a block array is still under discussion. Therefore, we conducted outdoor experiments at comprehensive outdoor scale model (COSMO) experiment sites in an urban climate to clarify the relationships between the building wall pressure differences and the approaching flow. The pressure coefficients for the outdoor site were comparable with those obtained during previous WTEs. Accordingly, temporal variations in the wind speed and pressure coefficient on the target block were investigated in detail using low-pass filtering operations. The relationships between the filtered wind speed and the pressure differences indicate that the slower temporal variations in the wall pressure showed good agreement with the filtered approaching flow. In addition, the correlation coefficient between the filtered wind speed and the wall pressure differences quantified the apparent coherence between the turbulent flow and the ventilation rate. Furthermore, the statistics of the ventilation rate were determined based on the conventional model to clarify the effects of the turbulent flow on the natural ventilation rate. The ratios between the mean and short-term ventilation rates imply that the short-term ventilation rate presented dramatic temporal fluctuations owing to the various scale turbulence generated by the atmospheric flow.

Open-loop and closed-loop flow control based on Van der Pol modeling

A real-time modeling and control architecture for coupled shear layer von Karman instabilities on a wing section wake is developed. Previous wind tunnel experiments highlighted that these wake instabilities give rise to limit cycles on both local and resulting airloads. The two-state Van der Pol system is employed to model the self-sustained oscillations exhibited by the actual flow. Schemes featuring either one or two coupled Van der Pol oscillators in parallel are taken under consideration. Open-loop and closed-loop control laws to manipulate and reduce the self-sustained oscillations are extensively studied. An adaptive control scheme is developed to optimize the performance of the controller, according to the flow conditions. For the open-loop control scheme, the consistency with experimental tests performed on a wind tunnel model is shown.

Impact of Extreme Land Surface Heterogeneity on Micrometeorology over Spring Snow Cover

Abstract The melting mountain snow cover in spring typically changes from a continuous snow cover to a mosaic of patches of snow and bare ground, inducing an extreme heterogeneity of the land surface. A comprehensive measurement campaign, the Dischma experiment, was conducted during three entire ablation seasons. The aim of this study was to experimentally investigate the small-scale boundary layer dynamics over a melting snow cover with a gradually decreasing snow cover fraction and the associated heat exchange at the snow surface. This study presents a unique dataset combining eddy covariance measurements at different atmospheric levels with maps of snow surface temperatures and snow cover fractions. The experiments evidence diurnal mountain wind systems driving the diurnal cycle of turbulent sensible heat fluxes over snow and the formation of katabatic flows over long-lasting snow patches strongly affecting the temporal evolution of snow surface temperature patterns. The snow cover distribution is also shown to be of vital importance for the frequency of stable internal boundary layer development over snow. For situations with a clear evidence of stable internal boundary layer development over snow, the data reveal a very shallow atmospheric layer adjacent to the snow cover decoupled from the warm-air advection above. These measurements confirm previous wind tunnel experiments that also evidenced a decoupling of the air adjacent to the snow cover from the warmer air above, especially within topographical depressions and when ambient wind velocities are low. For these situations, in particular, all tested energy balance models strongly overestimated the turbulent sensible heat flux directed toward the snow cover.

Separation control with fluidic oscillators in water

The present study assesses the applicability of fluidic oscillators for separation control in water. The first part of this work evaluates the properties of the fluidic oscillators including frequency, cavitation effects, and exerted thrust. Derived from the governing internal dynamics, the oscillation frequency is found to scale directly with the jet’s exit velocity and the size of the fluidic oscillator independent of the working fluid. Frequency data from various experiments collapse onto a single curve. The occurrence of cavitation is examined by visual inspection and hydrophone measurements. The oscillation frequency is not affected by cavitation because it does not occur inside the oscillators. The spectral information obtained with the hydrophone provide a reliable indicator for the onset of cavitation at the exit. The performance of the fluidic oscillators for separation control on a bluff body does not seem to be affected by the presence of cavitation. The thrust exerted by an array of fluidic oscillators with water as the working fluid is measured to be even larger than theoretically estimated values. The second part of the presented work compares the performance of fluidic oscillators for separation control in water with previous results in air. The array of fluidic oscillators is installed into the rear end of a bluff body model. The drag improvements based on force balance measurements agree well with previous wind tunnel experiments on the same model. The flow field is examined by pressure measurements and with particle image velocimetry. Similar performance and flow field characteristics are observed in both water and air.

LES simulation of hemispherical dome's aerodynamic characteristics in smooth and turbulence boundary layer flows

Large Eddy Simulation (LES) technique has been adopted in this research to investigate the aerodynamic characteristics of a hemispherical dome immersed in smooth flow and boundary layer flow at a Reynolds number around 2E+06. Accuracy of the numerical simulation result was validated in comparison with previous wind tunnel experiment performed by Cheng and Fu (2010). Results showed that the numerical simulation result agrees fairly well with the experimental one in mean pressure distributions both in smooth and turbulent flows. Regarding the RMS pressure coefficients, the agreement is fairly good for the smooth flow. However, it is rather poor for the turbulent boundary layer. Furthermore, the numerical simulation offers an insight into the characteristics of the flow near the separation point and provides comprehensive understanding for the location of the separation.

The challenging case of the turbulent flow around a thin plate wind deflector, and its numerical prediction by LES and RANS models

The long recirculation bubble found in many industrial applications, mostly involving thin airfoils and thin plate wind deflectors, presents a challenging case for numerical models based on RANS methodology. A deeper understanding of this methodology and its limitations is gained through a series of numerical simulations of the incompressible flow around a thin flat plate of infinite wingspan at small incidences. In this numerically challenging flow, a thin recirculation zone with highly anisotropic turbulent structures is formed close to the leading edge after boundary layer separation. The importance of capturing anisotropy is thoroughly examined and quantitatively assessed in this paper, through a number of simulations employing both large eddy simulations (LES) and Reynolds average Navier–Stokes (RANS) approaches. The former is validated against previous wind tunnel experiment. Since the experimental data does not provide all tensor components to fully assess RANS quality, and LES results can be considered as a reliable source, they are employed in the full characterization of turbulence and in the assessment of the several RANS models predictions presented in this paper. Quantitative results are shown for the errors in capturing the anisotropy part of the Reynolds stress tensor for several RANS models, including Spalart–Almaras, κ−ω SST and transition k−ω SST.

Large Eddy Simulations of Film-Cooling Flows With a Micro-Ramp Vortex Generator

Large eddy simulations were performed to study the effect of a micro-ramp on an inclined turbulent jet interacting with a cross-flow in a film-cooling configuration. The micro-ramp vortex generator is placed downstream of the film-cooling jet. Changes in vortex structure and film-cooling effectiveness are evaluated. Coherent turbulent structures characteristic of a jet in a cross-flow are analyzed and the genesis of the counter-rotating vortex pair in the jet is discussed. Results are reported for two film-cooling configurations, where the primary difference is the way the jet inflow boundary conditions are prescribed. In the first configuration, the jet conditions are prescribed using a precursor simulation and in the second the jet is modeled using a plenum/pipe configuration. The latter configuration was designed based on previous wind tunnel experiments at NASA Glenn Research Center, and the present results are meant to supplement those experiments. It is found that the micro-ramp improves film-cooling effectiveness by generating near-wall counter-rotating vortices which help entrain coolant from the jet and transport it to the surface. The pair of vortices generated by the micro-ramp are of opposite sense to the vortex pair embedded in the jet.

Wind tunnel experiments on natural snow drift

Previous wind tunnel experiments on snow drift usually used artificial snow as a substitute of natural snow, which can not fully reflect the motion characteristics of snow drift in natural environments. In this paper we conducted a series of experiments in wind tunnel to investigate the motion of natural snow (fresh snow and old snow), which was collected out-door without destroying the surface structure. The results indicated that the threshold velocity of fresh snow is less than that of old snow, and that the mass flux rates of the two kinds of snow exponentially decrease with height whereas the snow transport rate increases exponentially with wind velocity. Based on the PIV measuring of the velocities of snow particles over two kinds of terrains (flat ground and roadbed), we found that the particle velocity obeys a Gaussian distribution for flat ground, top surface and leeward slope of the roadbed. However, for the windward slope of roadbed the particle velocity distribution displays poor correlation with a Gaussian function due to the acceleration of snow particles when saltating across the slop. Statistical analysis showed that impacting and liftoff velocities of snow particles also obey Gaussian distributions.

Influence of mass and damping ratios on VIVs of a cylinder with a downstream counterpart in cruciform arrangement

Previous wind tunnel experiments by the present authors showed that two kinds of longitudinal vortex induce cross-flow vibrations (LVIVs) on a circular cylinder accompanied by the vortex-synchronization (lock-in) by setting another cylinder downstream with proper gaps in cruciform arrangement, like the well-known Karman vortex induced vibration (KVIV) of a single cylinder. In this work, experiments on the three types of VIVs are carried out using a water tunnel to investigate influences of the mass ratio M R and the damping ratio ζ on the oscillation response. It is shown that the two LVIVs, i.e. the trailing- and the necklace-VIVs, occur in water flow over respective ranges of flow velocity U in spite of large differences in values of M R and ζ as compared with experiments in air flow. However, their influences on oscillation responses of the two LVIVs are largely different from each other, and also from the KVIV. They are compared by referring to the de-synchronization prediction based on the dimensional analysis and the equation on cylinder motion. The variation of behaviors among the three types of VIV are attributed to the differences among the three vortices in their threshold functions for synchronization and the vortex-synchronization functions which express the feedback effect of the cylinder oscillation on the alternating lift generated by the vortex.

Evaluation of drag coefficients of poplar-tree crowns by a field test method

To estimate the wind force that causes windthrow damage to a tree, the drag coefficients of actual-sized trees were evaluated by a field test method. In this method, wind velocity and stem deflection were monitored simultaneously. The wind force acting on a tree crown was calculated from stem deflection; stem stiffness was evaluated by conducting tree-bending tests. The results of tests conducted on three poplar trees showed that drag coefficient decreased with an increase in wind velocity. Although the variation in the drag coefficient was large at low wind velocity because of the vibrating behavior of the stem subjected to variable wind force, the variation at wind velocities above 10 m/s was small. The average drag coefficient at a wind velocity of 30 m/s was estimated by the curve-fitting of a power function to the wind velocity-drag coefficient relationship to be 0.102, which was smaller than that of actual-sized conifers studied in previous wind tunnel experiments. The drag coefficients of these crown areas in the defoliation season were smaller than those measured in the leafy season.

Aerodynamic Analysis of a Generic Fighter Using Delayed Detached-Eddy Simulation

The modular transonic vortex interaction configuration was developed at the NASA Langley Research Center to investigate the aerodynamic characteristics of a generic fighter incorporating a chined fuselage and delta wing. Previous experiments showed that the fuselage and leading-edge vortex interactions are detrimental to the vehicle's aerodynamic characteristics for angles of attack greater than 23 deg at low angles of sideslip. This is largely due to abrupt asymmetric vortex breakdown, which leads to pronounced pitch-up and significant nonlinearities in lateral stability that could result in roll departure. An improved understanding of the exact origins of this nonlinear behavior would improve future fighter design, and predictive capabilities of such nonlinearities could drastically reduce the cost associated with flight testing new or modified aircraft. The nonlinearities experienced by the modular transonic vortex interaction configuration at a 30 deg angle of attack, Reynolds number of 2.68 x 10 6 , and Mach number of 0.4 are computed using delayed detached-eddy simulation. Computational predictions of rolling moment compare very well with previous wind-tunnel experiments at the same conditions, including the abrupt nonlinear increase in rolling moment as a function of sideslip angle at small sideslip angles. A detailed investigation of the computational fluid dynamic data confirms that this nonlinearity is due to a rapid change in the flowfield structures from symmetric to asymmetric vortex breakdown.