Sectional Drag Research Articles

A Dynamic Urban Canopy Parameterization for Mesoscale Models Based on Computational Fluid Dynamics Reynolds-Averaged Navier–Stokes Microscale Simulations

Urban canopy parameterizations (UCPs) are necessary in mesoscale modelling to take into account the effects of buildings on wind and turbulent structures. This study is focused on the dynamical part of UCPs. The main objective is twofold: first, computing important UCP input parameters (turbulent length scales and the sectional drag coefficient) by means of Reynolds-averaged Navier–Stokes (RANS) simulations of turbulent flow; and second, comparing UCP variables with spatially-averaged properties obtained from RANS simulations for the same configurations. The results show the importance of using a suitable parameterization of the drag force for different packing densities. An urban canopy parameterization that is a compromise between simplicity and accuracy is proposed. This scheme accounts for the variation of drag coefficients with packing densities, and has a parameterization of turbulent length scales. The technique adopted ensures that, at least for the simple configurations studied, the urban canopy parameterization gives values of spatially-averaged variables similar to those computed from a more complex simulation, such as RANS that resolves explicitly the flow around buildings.

Ion wake effects on the Coulomb ion drag in complex dusty plasmas

The ion wake effects on the Coulomb drag force are investigated in complex dusty plasmas. It is shown that the ion wake effects significantly enhance the Coulomb ion drag force. It is also found that the ion wake effects on the Coulomb drag force increase with an increase in the Debye length. In addition, the ion wake effects on the momentum transfer cross section and Coulomb drag force are found to be increased with increasing thermal Mach number, i.e., decreasing plasma temperature. It is also found that the Coulomb ion drag force would be stronger for smaller dust grains.

Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions

Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following plan area densities: λ p = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ p . It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s assumption for both types of arrays and for both small and large λ p . On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant with height except in the case of staggered arrays with high λ p .

Building morphological characteristics and their effect on the wind in Beijing

An urban boundary layer model (UBLM) is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-to-width ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.

Variation of the Sectional Drag Coefficient of a Group of Buildings with Packing Density

Reynolds-averaged Navier–Stokes (RANS) simulations of turbulent flow over groups of buildings with different packing densities are reported. The results for a selected packing density are compared with direct numerical simulations (DNS) previously validated against wind-tunnel data. The present study is focused on average properties of the flow, especially on the drag coefficients, and is a first attempt to provide information on these parameters (their values are not generally known) for a range of packing densities, for a given staggered arrangement of cubes using RANS methods. However, some of the limitations of RANS have come to light. Hence, it is recommended that such simulations are ‘calibrated’ against experimental or DNS data, as is done here.

On linearization of Morison force given by high three-dimensional sea wave groups

In-line loading on slender marine structures may be computed by means of the Morison equation, which includes the inertia term (depending on wave acceleration) and the drag term (depending on square velocity). In the presence of random sea waves (either two- or three-dimensional waves), the Morison equation needs a linearization in the drag term, in order to obtain the force spectrum. In this paper, the Boccotti’s Quasi-Determinism theory is applied for the calculation of the drag force given by high three-dimensional wave groups. It is shown that when a crest-to-trough wave of given height H occurs on a vertical pile, the quotient between maxima of sectional drag Morison force and of force given by linearization (both calculated at a fixed depth z ) is equal to C times H / H s , where H s is the significant wave height. The coefficient C is equal to 1.25 for narrow-band spectra, whatever be the value of z is. For the three-dimensional random wave groups it is obtained that C is equal to 1.25 for z close to 0; the value of C slightly decreases on approaching the bottom. Then, it is shown that the Borgman linearization is not conservative for the calculation of extreme drag forces in three-dimensional waves: for example the maximum drag Morison force given by a wave height H equal to 2 times H s , is close to 2.3 times the maximum force given by linearization. The results are finally validated by the means of Monte Carlo simulations of random sea waves.

Bat wing airfoil and planform structures relating to aerodynamic cleanliness

In this paper we examine 12 species of Western Australian bat for anatomical and morphometric attributes related to wing lift and drag characteristics. We present values for bat wing camber (typically 6.5–9%) and its location, measurements of wing planform and tip shape (typically elliptical but with two different tip designs), dimensions of wing leading-edge flaps (typically 8–10.5% of hand wing chord but with some species having much larger flaps up to 18%) and then discuss several features related to airflow separation control. All species assessed had thin, low-camber airfoil sections, an optimisation appropriate to the range of Reynolds Numbers in which bats fly. Wing relative cleanliness was consistent with, and functionally appropriate to, species foraging strategy. The interceptors had the point of maximum camber well forward and no trailing edge wing fences, optimisations for minimum drag generation. The air-superiority bats had leading-edge fences optimised for maximum lift generation while maintaining low drag. Surface bats were characterised by their low-aspect-ratio wingtips and the absence of optimisations for either low section drag or high lift. The frugivore and the carnivore appear to be discrete optimisations while the emballinurid had a long and broad leading edge flap in combination with a high-aspect-ratio tip. We propose a range of lift and drag coefficient values for use in models of metabolic power output.

Challenges Facing Future Micro-Air-Vehicle Development

Nomenclature Ar = rotor disk area CD = sectional drag coefficient CD0 = zero-lift drag coefficient Clα = lift-curve slope CP = power coefficient CPi = induced power coefficient CP0 = profile power coefficient CT = thrust coefficient c = chord length D = drag force D.L . = disk loading L = lift force m = mass P.L . = power loading SF = separated flow T = rotor thrust V = local wind velocity perceived by flap W = weight W f = final weight Wo = gross takeoff weight α = blade section angle of attack η = efficiency μ = dynamic viscosity ρ = air density σ = rotor solidity = flapping amplitude (peak to peak)

Experimental investigation of the effect of nozzle shape and test section perforation on the stationary and non-stationary characteristics of flow field in the large transonic TsAGI T-128 Wind tunnel

AbstractThe results are presented for a cycle of experimental investigations of flow field characteristics (static pressure distribution, static pressure fluctuations, upwash, boundary-layer parameters) in the perforated test section of the transonic TsAGI T-128 Wind Tunnel. The investigations concern the effect of nozzle shape, wall open-area ratio, Mach and Reynolds numbers on the above-outlined flow characteristics. During the tests, the main Wind-tunnel drive power is measured. Optimal parameters of the nozzle shape and test section perforation are obtained to minimise acoustic perturbations in the test section and their non-uniformity in frequency, static pressure field non-uniformity, nozzle and test section drag and, accordingly, required main Wind-tunnel drive power.

A 3D investigation of the dynamic loads over an array of in-line cylinders at low KC and Re numbers

The oscillating flow around an infinite array of circular cylinders at low Keulegan-Carpenter numbers and Reynolds numbers is investigated numerically using a 3D mathematical model. Three different regimes of the Tatsuno and Bearman (1990) map are investigated; the first one is a 2D symmetric regime, whereas the second and the third are asymmetric and 3D. In each regime, the gap between two neighboring cylinders has been varied in a very wide range, from the case of practically isolated cylinder to the case of very strong interference. As regards the isolated cylinder, the mean value of the inertia coefficient is much more affected by the 3D effects than that of the drag coefficient. Conversely, the variation along the axial direction of the sectional drag coefficient is much stronger than that found for the sectional inertia coefficient. In all cases investigated, the reduction of the gap between cylinders has two main effects: first, it causes the reduction of the Morison coefficients; second, in the 3D regimes, the gap enhances 3D motion in the range of moderate values due to the interaction between vortex structures released by two neighboring cylinders, whereas a further reduction of the gap in the range of small values tends to reduce the three-dimensionality of flow field up to a complete suppression occurring for very small values of the gap. Finally, we propose a very simple parameterization that gives the Morison coefficients as a function of the gap once the values for the case of isolated cylinder are known.

Design and Wind Tunnel Study of a Top-mounted Diverterless Inlet

Combined with a UAV of the shape like Global Hawk, a new inlet is advanced to obtain high performance in both Radar Cross Section(RCS) and aerodynamic drag. Efforts are made to achieve this goal such as adopting a top-mounted inlet configuration, utilizing the diverterless technique and putting forward a new shape of entrance. A design method is brought forward and verified by wind tunnel tests. Results indicate: (1) Despite the negative effect of the front fuselage and the absence of the conventional boundary diverter, the performance of the top mounted diverterless inlet advanced here (Ma: 0.50-0.70, α: − 4°-6°, σ > 0.975) is equivalent to that of conventional S shaped inlet with diverter; (2) The integration of the inlet with the fuselage is realized by the utilization of a special inlet section and the diverterless technique, which disposes the whole inlet in the shield of the head of UAV, improving the drag characteristics and the stealthy performance of the aircraft; (3) The bump which is equal to the local boundary layer thickness in height can divert the boundary layer effectively. As a result, no obvious low total pressure zone is found at the outlet of the inlet; (4) According to the experimental results, negative angle of attack is favorable to the total pressure recovery and positive angle of attack is favorable to the total pressure distortion, while yaw brings bad effects on both; (5) The design of cowl lip is of great importance to the inlet performance at yaw, therefore, further improvement of the inlet performance will rely on the lip shapes of the cowl chosen.

A canopy model of mean winds through urban areas

AbstractAn urban canopy model is developed for spatially averaged mean winds within and above urban areas. The urban roughness elements are represented as a canopy‐element drag carefully formulated in terms of morphological parameters of the building arrays and a mean sectional drag coefficient for a single building. Turbulent stresses are represented using a mixing‐length model, with a mixing length that depends upon the density of the canopy and distance from the ground, which captures processes known to occur in canopies. The urban canopy model is sufficiently simple that it can be implemented in numerical weather‐prediction models.The urban canopy model compares well with wind tunnel measurements of the mean wind profile through a homogeneous canopy of cubical roughness elements and with measurements of the effective roughness length of cubical roughness elements. These comparisons give confidence that the basic approach of a canopy model can be extended from fine‐scale vegetation canopies to the canopies of large‐scale roughness elements that characterize urban areas.The urban canopy model is also used to investigate the adjustment to inhomogeneous canopies. The canonical case of adjustment of a rural boundary layer to a uniform urban canopy shows that the winds within the urban canopy adjust after a distance x0 = 3Lc lnK, where Lc is the canopy drag length‐scale, which characterizes the canopy‐element drag, and ln K depends weakly on canopy parameters and varies between about 0.5 and 2. Thus the density and shape of buildings within a radius x0 only determine the local canopy winds. In this sense x0 gives a dynamical definition of the size of a neighbourhood.The urban canopy model compares well with observations of the deceleration of the wind associated with adjustment of a rural boundary layer to a canopy of cubical roughness elements, but only when the sectional drag coefficient is taken to be somewhat larger than expected. We attribute this discrepancy to displacement of streamlines around the large‐scale urban roughness elements, which yields a stress that decelerates the wind. A challenge for future research is to incorporate this additional ‘dispersive stress’ into the urban canopy model. Copyright © 2004 Royal Meteorological Society

Hover Performance Prediction Using Full-Potential Method and Comparison with Experiments

A computational and experimental effort to investigate the e ow around rotor blades in hover is presented. The experimental analysis of the e owe eld in the immediate vicinity of the blade is supported by laser velocimetry (LV) performed on the three-dimensional velocity e eld around the bladesection and in its nearwake. A specie c LV data processingmethod based onthemomentumequationapplied around a bladesectionhasbeen developed to identify the contribution of the induced-drag component to the total sectional drag coefe cient. From the computational approach, some improvements to a vortex embedding full-potential method are checked by direct comparisons withtestdata. Comparisonsareperformed foroverallrotorairloads (thrustand torquecoefe cients ),localsectional coefe cients (liftanddragcontributions ),ande owe eldcharacteristics,includingthetipvortexpathandthespanwise circulation distribution.

Design and Predictions for High-Altitude (Low Reynolds Number) Aerodynamic Flight Experiment

A sailplane being developed at NASA Dryden Flight Research Center will support a high-altitude flight experiment. The experiment will measure the performance parameters or an airfoil at high altitudes (70,000 - 100,000 ft), low Reynolds numbers (2 x 10(exp 5) - 7 x 10(exp 5)), and high subsonic Mach numbers (0.5 and 0.65). The airfoil section lift and drag are determined from pilot and static pressure measurements. The locations of the separation bubble, Tollmien-Schlichting boundary-layer instability frequencies, and vortex shedding are measured from a hot-film strip. The details of the planned flight experiment are presented as well as several predictions of the airfoil performance.

New absolute pressure standard in the range 1 Pa to 7 kPa

A new standard has been developed from the FRS4 piston gauge that was presented at the Second International Seminar organized by the Consultative Committee for Mass and Related Quantities (CCM), Paris, 1993. This instrument extends the range of absolute pressure normally achieved with pressure balances below the minimum 1.5 kPa. It offers improved uncertainties over series expansion calibration systems down to pressures of a few pascals. It also has a continuously variable output pressure, which provides very rapid calibrations. The vacuum FRS4 has a large area piston attached to an electronic balance and held centrally in the cylinder by flexure hinges. The resolution is 2 × 10-3 Pa from 0 kPa to 2 kPa and 2 × 10-2 Pa to 7 kPa. The reference pressure under the piston is maintained with a hybrid turbomolecular and triple section drag pump. It should be noted that in the FRS4 the reference pressure is under the piston rather than above as it is with normal piston gauges. The gas flowing down the gap between the piston and cylinder is pumped from the gap before it reaches the base of the piston by the intermediate stage of the hybrid pump operating in a split-flow mode. This allows the reference pressure to be held at 1 × 10-3 Pa or better for the lower pressures in the measurement range and not more than 1 × 10-2 Pa above 2 kPa. The instrument is readily transportable and should offer a level of reproducibility of 0.01% of reading or better plus a few millipascals, which, until now, has not been available except in a few highly specialized and definitely non-portable instruments.

A numerical method for the simulation of steady and unsteady cavitating flows

A numerical method for the prediction of cavitating flows around lifting bodies is presented. The algorithm employs a one-fluid Navier–Stokes-enthalpy solver that can handle variable fluid properties, along with properly formulated water–vapor mixture state laws, in order to account for the two-phase flow of water and vapor and the transition from one phase to the other. The method allows for the simulation of steady and unsteady, two-dimensional, cavitating flows for low and moderate Reynolds numbers. Employment of the method for various test cases shows good agreement with available experimental measurements and observations and demonstrates interesting phenomena related to cavitation and its interaction with viscous mechanisms. Simulation of Reynolds number dependent phenomena, like cavitation inception and total section drag force, are only predicted qualitatively in the present study. However, the method shows promise for reliable quantitive predictions.

The aerodynamic performance of paragliders

AbstractAn analysis of paraglider performance has revealed that wing section drag is the most significant contribution to overall drag. Wind tunnel measurements performed on two-dimensional hollow models indicate that intake drag is less significant than previously thought. An experimental investigation into the characteristics of a ‘quasi ’ -two-dimensional flexible model consisting of solid ribs covered with a fabric skin was performed at realistic Reynolds numbers. The main cause of performance deterioration was found to be a significant reduction in section lift coefficient when compared to a similar solid wing section. This is believed to be mainly due to two factors: a large trailing edge separation and the deformation of the wing between ribs. The deformation was measured and it was shown that the deformed shape is less capable of generating high lift coefficients than the design section. It is thought that the extent of the trailing edge separation is increased due to the presence of streamwise grooves caused by the shape deformation of the wing. The shape of the separated region was found to be strongly three-dimensional with the separation point being about half a chord-length further upstream along the ribs. A small separation bubble was also observed immediately behind the lip of the intake, due to the fabric ‘flaring’ open. Based on the observations presented a number of suggestions for improved wings have been made.

Effects of Zero-Mass “Synthetic” Jets on the Aerodynamics of the NACA-0012 Airfoil

A numerical study was conducted to investigate the effect of an array of zero-mass synthetic jets on the aerodynamic characteristics of the NACA-0012 airfoil. Flowfield predictions were made using a modified version of the NASA Ames ARC2D unsteady, two-dimensional, compressible Navier-Stokes flow solver. An unsteady surface transpiration boundary condition was enforced over a user-specified portion of the airfoil's upper or lower surface to emulate the time variation of the mass flux out from and into the airfoil's surface. Here, a sinusoidal function which describes the approximate time behavior of the mass flux across the airfoil's surface was used. Our numerical results have indicated that zero-mass jets can, with the careful selection of their peak amplitude and frequency, enhance the lift characteristics of airfoils (helicopter rotor blades, wings, etc.). Effects of the jet peak suction and blowing velocities, oscillation frequency and, jet placement on the time histories of the sectional lift, drag and moment are presented for two angles of attack and one free stream Mach number. Flowfield results which provide more insight into the mechanics of the interaction between the array of jets and the developing boundary layer over the airfoil are presented.

On the effect of a strake-like junction fillet on the lift and drag of a wing

SummaryAn experimental study of the effects on the low-speed aerodynamic characteristics of a strake-like fillet is described, modelled on one used on an Airbus A320 variant fitted at the leading edge of a swept wing-plate junction. The wing, swept back at 20°, was of NACA 0015 section and chord 500 mm, both normal to its leading edge. A turbulent boundary layer had developed on the plate well ahead of the junction. The tests were conducted at a unit Reynolds number of 1.56 x 106 m-1.Surface pressure distributions were measured on the plate in the neighbourhood of the leading edge junction and also on the aerofoil and fillet at wing incidences of 0°, 3°, 6°, 9° and 12°. These were supplemented by surface oil-flow studies.The mean velocity and turbulence intensity fields around the leading edge were examined for incidences of 0° and 9°, using both a single tube yaw meter developed for the purpose and an X-wire anemometer. The X-wire anemometer was also used downstream of the trailing edge of the swept wing; five of the Reynolds stresses and the mean velocity field were measured.The sectional lift coefficients on the aerofoil were found to diminish as the junction was approached, slightly more so with the fillet than without it. The sectional drag coefficients due to pressure increased as the junction was approached, the fillet moderating this increase to only a small extent.However, the addition of the drooped fillet modified the flow considerably. The horseshoe-like vortex was less well defined than without it. At zero incidence, the peak in the turbulence intensity levels was virtually eliminated on what became effectively the compression side of the wing due to the local camber introduced by the asymmetric fillet. The turbulence levels were also reduced by the addition of the fillet at an incidence of 9°. However, the turbulent activity was spread through a larger proportion of the viscous region. The secondary flows and the turbulence activity in the wake are associated with unrecoverable kinetic energy and will be manifest as drag on the surfaces forming the junction.It is concluded that a carefully designed fillet, optimised for the cruise incidence of an aircraft, can reduce the peak turbulence levels in the junction. It remains unclear whether the total drag associated with the junction flow can be reduced significantly.However because of its effects on the turbulence, there may be other benefits, for example on the efficiency of downstream elements, such as fuselage-mounted engine intakes or the following stages of an axial flow machine. Junction fillets might also be used to control the scouring of river beds around bridge piers.

Testing Three-Dimensional Bluff-Body Models in a Low-Speed Two-Dimensional Adaptive Wall Test Section

Thin, sharp-edged disk models were evaluated in a low-speed two-dimensional adaptive flexible wall test section to determine the optimum adaptive wall testing environment for three-dimensional bluff-body models, by providing model testing recommendations for nominal solid blockage ratio and model span ratio. Drag coefficient measurements obtained under straight wall and adapted wall conditions showed that for a two-dimensional adaptive wall test section, the model span ratio imposes a more severe restriction upon model size than does the nominal solid blockage ratio. Minimum wall interference conditions were achieved with adapted walls for nominal solid blockage ratios less than 3 percent and model span ratios less than 21 percent, independent of the nominal test section aspect ratio, based on favorable comparison with previously-published experimental data. Data obtained under straight wall conditions confirmed that wall interference effects can only be neglected in conventional, straight-walled test sections for solid blockage ratios less than 0.5 percent and model span ratios less than 10 percent. The post-test boundary correction method of Maskell was successfully used to adjust the straight wall test section drag coefficient measurements of the larger models for wall interference effects, but no direct measurements of wall interference are used with this method. The results support the careful use of a two-dimensional wall adjustment strategy for three-dimensional nonlifting flows.