Drag Estimation Research Articles

Permanent and Transient Upstream Effects in Nonlinear Stratified Flow over a Ridge

Abstract The “high drag” state of stratified flow over isolated terrain is still an impediment to theoretical and experimental estimation of topographic wave drag and mean-flow modification. Linear theory misses the transition to the asymmetrical configuration that produces the enhanced drag. Steady-state nonlinear models rely on an ad hoc upstream condition like Long's hypothesis and can, as a result, be inconsistent with the flow established naturally by transients, especially if blocking is involved. Numerical solutions of the stratified initial value problem have left considerable uncertainty about the upstream alteration, especially as regards its permanence. A time-dependent numerical model with open boundaries is used in an effort to distinguish between permanent and transient upstream flow changes and to relate these to developments near the mountain. A nonrotating atmosphere with initially uniform wind and static stability is assumed. It is found that permanent alterations are primarily due to an...

Characterization of mass and density distributions of sized coal fractions

Novel measurement systems are applied to characterize external surface area, volume, and drag coefficient/mass ( C d/m ) ratios of two sized coal fractions and a sample of carbon spheres. Measurements are made on individual particles with nominal diameters ranging from 75 to 150 μm. The measurement system incorporates an electrodynamic balance, to isolate particles, with video-based and high-speed diode array imaging systems for particle characterization. Accurate estimates of the Stoke's drag are obtained for the coal particles by applying a general solution method developed for slightly deformed spheres. Analysis methods are developed using shape and drag information to calculate mass and density distributions for the two coals studied. Results of these measurements and analyses are validated by independent mass measurements using a particle weighing and counting technique. For the coals studied surface area equivalent particle diameters range from 4% to 20% higher than the corresponding volume equivalent particle diameters. Particle densities ranged from 1.01 to 1.34 g/cm 3 for a 23 particle sample set of Pittsburgh seam hva bituminous coal and from 0.81 to 1.38 g/cm 3 for a 30-particle sample set of New Mexico subbituminous coal. From a combustion perspective, the results presented indicate that particle sphere assumptions employed in most combustion models can lead to significant errors (20%–25% on average for the samples studied) in calculated particle volume and associated thermal mass. Particle-to-particle density variations indicate that even if surface area and volume differences are adequately handled in a heat transfer analysis, large uncertainties result in particle temperature response due to particle property variations. The measurement and analysis techniques presented are capable of providing the detailed particle property data required for developing improved combustion models.

Sag-Mediated Modulated Tension in Terebellid Tentacles Exposed to Flow.

The long, compliant feeding tentacles of the terebellid polychaete Eupolymnia heterobranchia not only stretch out over a sandflat substratum but also extend into flow. Tentacles suspended perpendicular to flow responded to increasing velocity by increasing their sag. An analysis of tension in these tentacles, mathematically analogous to that applicable to suspension bridges, shows that sagging permits the tentacles to avoid increases in tension that would otherwise occur as flow increases. Force modulation was achieved by active muscular control rather than by passive material properties. Although these tentacles would certainly break in the experimental flows if they did not sag, the low tension achieved suggests that some other reason, such as limitations on the adherence of cilia and mucus, accounts for the level of tension observed. Because drag is maximum on tentacles oriented perpendicular to flow, reorientation of tentacles, either by sagging or by dangling parallel to flow, additionally reduces tension by reducing drag. Theoretical estimates of drag on tentacles oriented parallel to flow show that they are never in danger of being broken. Drag is sufficient, however, to assist in passive extension of tentacles. While reorientation is a common mode of drag reduction among marine organisms, sagging represents a novel mechanism of mediating structural forces resulting from flow.

Comparison of numerical methods in transonic aerodynamics

Abstract Although they are not always unique, Euler solutions have an advantage over potential flow calculations because of the way vortex sheets can be captured in three dimensions. An improved estimate of the wave drag may be obtained through transformation of the entropy inequality.

Determination of balloon gas mass and revised estimates of drag and virtual mass coefficients

Ascent modeling of scientific balloon missions is particularly sensitive to accurate knowledge of the balloon lifting gas mass. Transient distortions of the balloon body effectively increase drag beyond classical solid body data. Further, experimenters have reported an increase in virtual mass with developing flow relative to the accepted inviscid determination at incipient flow. In support of the National Aeronautics and Space Administration (NASA) Balloon Program, small scale balloons (175 m 3) were flown with varying lifting gas and total system mass. Stow Away Special (SAS) instrument packages were developed to measure and record acceleration and temperature data during these tests. Top fitting and instrument payload accelerations were measured from launch to steady state ascent and through ballast drop transients. The development of the small (5×7.5×15 m 3), light weight (.5 kg.), self-powered SAS is discussed along with mathematical models developed to determine gas mass, drag and virtual mass coefficients.

The Profile Drag of A Hawk’s Wing, Measured by Wake Sampling in A Wind Tunnel

ABSTRACT The distribution of dynamic pressure behind a Harris’ hawk’s wing was sampled using a wake rake consisting of 15 pitot tubes and one static tube. The hawk was holding on to a perch, but at an air speed and gliding angle at which it was capable of gliding. The perch was instrumented, so that the lift developed by the wing was known and the lift coefficient could be calculated. The mean of 92 estimates of profile drag coefficient was 0.0207, with standard deviation 0.0079. Lift coefficients ranged from 0.51 to 1.08. Reynolds numbers were nearly all in the range 143000–194000. The estimates of profile drag coefficient were reconcilable with previous estimates of the wing profile drag of the same bird, obtained by the subtractive method, and also with values predicted by the ‘Airfoil-ii’ program for designing aerofoils, based on a digitized wing profile from the ulnar region of the wing. The thickness of the wake suggested that the boundary layer was mostly or fully turbulent in most observations and separated in some, possibly as an active means of creating drag for control purposes. It appears that the bird could momentarily either increase or decrease the profile drag of specific parts of the wing, by active changes of shape, and it appeared to use the carpo-metacarpal region especially for such control movements. Further investigation in a low-turbulence wind tunnel would help to resolve doubts about the possible influence of airstream turbulence on the behaviour of the boundary layer.

SIMULATION OF NON-NEWTONIAN FLUID FLOW THROUGH FIXED AND FLUIDIZED BEDS OF SPHERICAL PARTICLES

The flow of Newtonian and power law fluids through beds of particles has been investigated by solving the equations of continuity and momentum in the framework of the free-surface cell model. The finite-element method has been used for predicting the flow fields in terms of the primitive variables u, p, and v. Theoretical estimates of drag for assemblages of particles are obtained for wide ranges of physical and kinematic conditions: 10−3 ≤ Re ≤ 100, 0.4 ≤ n ≤ /, and 0.3 ≤ e ≤ 0.9. The accuracy of the numerical scheme has been tested using known analytical results for creeping Newtonian flow. The drag results obtained herein can be used successfully to predict the friction factor for flow in fixed and fluidized beds and the minimum ftuidizalion velocity for a given liquid-solid system.

Euler analysis of a High-Speed Civil Transport concept at Mach 3

A marching Euler solver, GEM3D, was used to predict the Mach 3 flowfield for the wing and body of a Highspeed Civil Transport (HSCT) concept. The analysis focused on a typical cruise lift coefficient of 0.1 at a = 3 deg. The Euler solution indicated that embedded shocks formed on the upper surface of the inboard wing panel and at the leading edge of the outboard wing panel caused by its supersonic leading-edge condition. According to a simple static-pressure criterion, the embedded wing upper-surface shocks were sufficiently strong to separate a turbulent boundary layer. Comparison of aerodynamic coefficients from the Euler solver with those from linear theory showed that the linear theory estimates of lift and drag were optimistic, which would lead to optimistic estimates of cruise range.

The space-temporal variations of the upper atmosphere density derived from satellite drag data

To improve our knowledge of the variations of the upper atmosphere density in 1989 experiments were made in the USSR using passive standard satellites of the shape of a sphere with the known weight and size. These satellites were set in pairs onboard spacecrafts “Resourse-F”, COSPAR identification No. 89038001. The above spacecrafts were put into orbit on May 25, 1989 and July 18, 1989. The separation of standard satellites from the first spacecraft took place on June 8 and 9 from the second one — on August 6 and 7. These satellites as well as other 58 space objects chosen for the experiment were observed by radars. Improvement of the orbits was made by the trajectory measurements obtained. The movement of the satellites was under control up to their decay in the atmosphere. For the estimation of a satellite drag in the atmosphere, ballistic coefficient was used. It was determined in the process of improvement of orbital elements as accomodation coefficient of the actual drag and the estimated atmosphere density. Besides the above mentioned satellites two more satellites (No. intern 80037001 and 82121001) having the shape of a smooth sphere were used as standard ones. In the considered altitude and time interval with 3 hours' resolution relative variations of the atmosphere density /1/ are built in the form of normalized declinations of the actual density in the given point from those calculated by the dynamic model /2/ with the permanent level of solar and geomagnetic activity. Dependences of variations on indeces F 10.7 and K p built, confirm a possibility of applications of variations for the improvement of the atmosphere density.

Tethered aerothermodynamic research needs

It is widely recognized that there exist significant gaps in our understanding of the aerothermodynamics of flow in regimes that are both hypersonic and rarefied. We refer here principally to flows of orbital velocity at altitudes of 90 km and above. In this paper, we identify and discuss needs for new information essential to progress on problems of design and computation for flight in these regimes. Measurement requirements implied by research needs are discussed in the context of a tethered research satellite. This vehicle is of the approximate mass and general sophistication of the proposed TSS-2. Recent estimates of drag, and tether and vehicle temperature, provide background materials for the discussion.

A Method of Sizing Multi-Cycle Engines for Hypersonic Aircraft

A method of sizing multi-cycle engines for integration with hypersonic vehicles has been developed. The new procedure independently sizes the inlet, each engine cycle, and the nozzle during the vehicle sizing loop to optimize propulsion/aircraft integration. Using uninstalled engine performance for each cycle of a multi-cycle engine along with inlet and nozzle performance and an estimate of aircraft drag, an iterative procedure is utilized to size each component simultaneously. A propulsion system is defined that meets the aircraft thrust requirements at all mission points. The inlet is sized to provide airflow such that the maximum Mach cruise and/or combat thrust conditions are met. Each cycle is sized independently to meet all thrust requirements while minimizing either inlet drag or engine size. Nozzle sizing must trade off thrust, drag and nozzle weight. This methodology has been incorporated into a computer code entitled “Multi-Cycle Engine Sizing Program,” MCESP.

The Taxonomic Status of the Small Ground-Finch, Geospiza (Aves: Emberizidae) of Genovesa Island, Galápagos, and Its Relevance to Interspecific Competition

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Fluid drag estimation based on analysis of the Kozeny-Carman equation.

Based upon an analysis of the Kozeny-Carman equation, a general expression was developed for the fluid drag on a particle which is different in size from the other particles in the bed. It is expected that the limitations of usage of the general equation derived will be the same as for the Kozeny-Carman equation, except that the porosity dependence on the solid fraction of specified species in the theory should be determined otherwise. An interesting effect of the size ratios on the fluid drag was illustrated by use of past experimental data on packing porosity. The present analysis suggests a way of estimating the fluid drag on a particle in assemblages of solid particles of various sizes.

Transonic wave drag estimation and optimization using the nonlinear area rule

Nonlinear area ruling procedures based on the transonic slender body and lift-dominated theories are described as a means of providing low-cost wave drag estimates and optima for basepoint definition. The computational implementation is capable of accurately predicting drag rise of realistic configurations and shows applicability to moderate supersonic Mach numbers. An analogy between the zero-lift and liftdominated case establishes a basis for sizeable wave drag due to lift reduction through planform and sectional shaping. Results illustrating the potential benefits are shown for a fighter configuration in which a small movement of the maximum thickness location of the equivalent body of revolution with the volume fixed gives a fourfold reduction in zero-lift wave drag. This benefit can be translated into similar reductions in transonic wave drag due to lift.

An estimation of drag in front crawl swimming

Propulsive arm forces of twelve elite male swimmers during a front crawl swimming-like activity were measured. The swimmers pushed off against grips which are attached to a 23 m tube at 0.8 m under the water surface. The tube was fixed to a force transducer. Since at constant speed, mean propulsive force equals mean drag force this method also provides the mean active drag on a moving swimmer. The mean propulsive force at a speed of v = 1.48m s −1 appeared to be 53.2 ± 5.8 N which is two to three times smaller than what is reported by other authors for active drag but which is in agreement with values reported for passive drag on a (towed) swimmer who is not moving. Discrepancies with indirect active drag measurements are discussed.

Dynamic estimation of hydrodynamic damping

Data on the acceleration of an oscillating cylinder in water are analysed by a Kalman extended filter. A hydrodynamical forcing model based upon Morison's equation appears to be reasonable and drag estimation appears simple using simulated data. Estimation based upon real data gives an average drag coefficient decreasing with the oscillation amplitude. An assymetric drag coefficient variation over the oscillation period is estimated.

Comment on 'Convergence characteristics of a vortex-lattice method for nonlinear configuration aerodynamics'

It is proposed that the study of Rusak et al. (1985), which reports numerical modeling sensitivities on longitudinal force/moment properties for a vortex-lattice method incorporating free vortex filaments to represent the leading-edge vortex separation, employs a formula that is strongly affected by the particular points of analysis chosen. This results in a narrowly applicable curve fit, where numerical sensitivities of the theory are inappropriately traded off against physical effects that are not modeled in that theory. Attention is also given to questionable drag estimate computations.

Drag measurements for first‐year sea ice over a shallow sea

For a free‐drift case a method is developed for simultaneously determining air (Ca) and water (Cw) drag coefficients by solving the vertically integrated stress balance equation for sea ice. The method allows the algebraic determination of Ca and Cw from measurements of relative wind and relative current at single reference levels, from estimates of the densities of all three fluids and ice thickness, and from estimates of the accelerations caused by the nonsurface stress terms derived from position measurements. The method treats the ice as a natural drag plate and therefore includes contributions from both skin and form drag. One shortcoming of the technique is that if the relative wind and current are colinear, then only the ratio of Ca and Cw can be determined. Another is that the accelerations caused by other forces need to be independently determined. An experiment was conducted on a single floe in the eastern Bering Sea over the continental shelf during March 1981. The site was about 80 km from the ice edge and was occupied for approximately 3 days following the passage of a storm which had broken the pack into 10–20 m diameter floes. Both profile and slab inversion estimates of drag were made from the current meter and anemometer measurements at the site: Ca at 3 m was 4.55×10−3 by the profile method, compared to 3.63–4.39×10−3 for slab, depending on assumptions; Cw at 1.1 m by profile was 24.2×10−3, compared to 18.3–22.0×10−3 for slab. The two methods gave results which were not statistically separable and which were among the highest drags observed for sea ice. The instantaneous stress balance on the floe included contributions from material acceleration, Coriolis force, the two surface stresses, and sea‐surface tilt relative to mean current and tide.

Jet propulsion of the calycophoran siphonophoresChelophyesandAbylopsis

Jet propulsion in the small siphonophoresChelophyesandAbylopsiswas investigated by measuring chamber pressures and thrust exerted in tethered nectophores, and by cinephotography of free swimming colonies. Although of similar design, the two siphon-ophores swim at different speeds, and with very different chamber pressures in the locomotor nectophores. Estimates of drag incurred during swimming, and of the cost of the process in the two forms are discussed.

On the secular decrease in the semimajor axis of Lageos's orbit

The semimajor axis of the Lageos satellite's orbit is decreasing secularly at the rate of 1.1 mm day−1. Ten possible mechanisms are investigated to discover which one (s), if any, might be causing the orbit to decay. Six of the mechanisms, resonance with the Earth's gravitational field, gravitational radiation, the Poynting-Robertson effect, transfer of spin angular momentum to the orbital angular momentum, drag from near-earth dust, and atmospheric drag by neutral hydrogen are ruled out because they are too small or require unacceptable assumptions to account for the observed rate of decay. Three other mechanisms, the Yarkovsky effect, the Schach effect, and terrestrial radiation pressure give perturbations whose characteristic signatures do not agree with the observed secular decrease (terrestrial radiation pressure appears to be too small in any case); hence they are also ruled out. Charged particle drag with the ions at Lageos's altitude is probably the principal cause of the orbital decay. An estimate of charged particle drag based upon laboratory experiments and satellite measurements of ion number densities accounts for 60 percent of the observed rate of decrease in the semimajor axis, assuming a satellite potential of −1V. This figure is in good agreement with other estimates based on charge drag theory. A satellite potential of −1.5V will explain the entire decay rate. Atmospheric drag from neutral hydrogen appears to be the next largest effect, explaining about 10 percent of the observed orbital decay rate.