Reduction In Drag Coefficient Research Articles

Tip vortex/airfoil interaction for a low Reynolds number canard/wingconfiguration

The effects of the vortical wake shed by a finite span canard on a low Reynolds number airfoil were examined. Aerodynamic performance was evaluated through direct measurements of lift, drag, and 1/4-chord pitching moment. Spanwise static pressure and surface film visualization data were also acquired. A reduction in the down-stream airfoil drag coefficient and an increase in its lift/drag were noted in the presence of the canard for a wide range of configurations

Monthly Mean Wind Stress and Sverdrup Transports in the North Atlantic: A Comparison of the Hellerman–Rosenstein and Isemer–Hasse Climatologies

The monthly mean wind stress climatology of Hellerman and Rosenstein (HR) is compared with the climatology of Isemer and Hasse (IH), which represents a version of the Bunker atlas (BU) for the North Atlantic based on revised parameterizations. The drag coefficients adopted by IH are 21% smaller than the values of BU and HR, and the calculation of wind speed from marine estimates of Beaufort force (Bft) is based on a revised Beaufort equivalent scale similar to the scientific scale recommended by WMO. The latter choice significantly increases wind speed below Bft 8, and effectively counteracts the reduction of the drag coefficients. Comparing the IH stresses with HR reveals substantially enhanced magnitudes in the trade wind region throughout the year. At 15°N the mean easterly stress increases from about 0.9 (HR) to about 1.2 dyn cm−1 (IH). Annual mean differences are smaller in the region of the westerlies. In winter, the effect due to the reduced drag coefficient dominates and leads to smaller stress values in IH; during summer season the revision of the Beaufort equivalents is more effective and leads to increased stresses. Implications of the different wind stress climatologies for forcing the large-scale ocean circulation are discussed by means of the Sverdrup transport streamfunction (ψs): Throughout the subtropical gyre a significant intensification of ψs takes place with IH. At 27°N, differences of more than 10 Sv (1 Sv ≡ 106 m3 s−1) are found near the western boundary. Differences in the seasonality of ψs are more pronounced in near-equatorial regions where IH increase the amplitude of the annual cycle by about 50%. An eddy-resolving model of the North Atlantic circulation is used to examine the effect of the different wind stresses on the seasonal cycle of the Florida Current. The transport predicted by the numerical model is in much better agreement with observations when the circulation is forced by IH than by HR, regarding both the annual mean (29.1 Sv vs 23.2 Sv) and the seasonal range (6.3 Sv vs 3.4 Sv).

The influence of polyvinylacetate additive in water on turbulent velocity field and drag reduction

The effect of polymer concentration on drag reduction was studied experimentally with diluted water solutions of polyvinylacetate in a 2.4 cm I. D. pipe. The instantaneous local velocities of the velocity fields were measured by a one-channel differential laser-Doppler anemometer DISA Mark II, with forward scattering. Concentrations of water-polyvinylacetate over the range from 10 to 2,000 ppm were used. The drag reduction coefficient is proportional to the concentration and hydrolysis degree of the saponificated polyvinylacetate (PVAC) employed. A mechanical degradation in the turbulent shear flow was not observed.

Effect of a triathlon wet suit on drag during swimming

The effect of a triathlon wet suit on drag was studied in 12 subjects (eight male, four female) swimming at different velocities (1.10, 1.25 and 1.50 m.s-1). The active drag force was directly measured during front crawl swimming using a system of underwater push off pads instrumented with a force transducer (M.A.D. system: 6). Measurements were made when swimming over the system with and without a wet suit. A 14% reduction in drag (from 48.7 to 41.8 Newtons) is found at a swimming velocity of 1.25 m.s-1, which is a typical swimming speed for triathlon distances. At 1.50 m.s-1 a reduction in drag of 12% was observed, which suggests that the wearing of such a suit might be beneficial in conventional swimming events. The reduction in drag can explain the higher swimming velocities observed in triathletes using a wet suit. The effect of the reduction is probably largely due to an increased buoyancy inducing less frontal resistance. However, since the effect of the suit on the lighter female swimmers was not different from the effect on the heavier male swimmers, a reduction in friction drag and drag coefficient may also be significant.

Computation of high Reynolds number flow around a circular cylinder with surface roughness

Incompressible high-Reynolds-number flows around a circular cylinder are analyzed by direct integration of the Navier-Stokes equations using finite-difference method. A generalized coordinate system is used so that a sufficient number of grid points are distributed in the boundary layer and the wake. A numerical scheme which suppresses non-linear instability for calculations of high-Reynolds-number flows is developed. The computation of an impulsively started flow at Re = 1200 is compared with corresponding experimental observations, and excellent agreements are obtained.A series of computations are carried out on the flow around a circular cylinder with surface roughness. The height of the roughness in these computations is 0.5% of the diameter. The range of Reynolds numbers is from 103 to 105; no turbulence model is employed. Sharp reduction of drag coefficient is observed near Re = 2 × 104, which indicates that the critical Reynolds number is captured in the present computation.

Commercial vehicle fuel economy — The correlation between aerodynamic drag and fuel consumption of a typical truck

In a programme designed to demonstrate the benefits of reducing the aerodynamic drag of commercial vehicles a number of tests were made in the MIRA 7.9 m × 4.4 m wind tunnel using a full-scale truck fitted with a number of proprietary drag-reducing devices. Then, to establish a relationship between the indicated drag reduction and fuel savings, steady-speed fuel consumption measurements were made on a test track using the vehicle fitted with many of the devices tested earlier in the wind tunnel. The highest measured reduction in drag coefficient, 36%, which was achieved with two devices used together, resulted in a fuel saving of 16% at a steady speed of 80 km h −1 and of 13% at a constant 50 km h −1. Further measurements, made on a 365 km road test route, and using one of the more efficient single devices, indicated a fuel saving of 13.2% under normal road conditions. A good correlation was established between the wind-tunnel measurements of aerodynamic drag and the steady-speed fuel consumption measurements.

Mass Injection and Jet Flow Simulation Effects on Transonic Afterbody Drag

Abstract : An experimental investigation has been performed to determine the effects of boattail injection and jet flow simulation on the afterbody drag of a slender body of revolution in the transonic regime at zero angle of attack, such as engine nacelles and boattailed afterbodies with isolated engines. A correlation between sting and jet diameter has been established. The jet plume and the nozzle pressure ratio simulators have been found appropriate and useful as a testing technique. Boattail mass injection usually produces a drag coefficient reduction and is more effective at high nozzle pressure ratios. Boattail injection is more effective if used in regions of separated flow. (Author)

Effect of Base Cavities on the Aerodynamic Drag of an Axisymmetric Cylinder

SummaryBase cavities (hollow extensions mounted behind the rear end of a bluff body) are known to reduce substantially the aerodynamic drag of two-dimensional bodies. For axisymmetric bodies their effect is not well established, and the objective of this work was to assess their drag-reducing potential. Experiments were performed using three different types of base cavities, one solid walled and two ventilated, each with six different depths ranging from 0.1 to 0.9 body diameters. All three types of cavities reduced the body drag for small cavity depths, with a maximum drag coefficient reduction of 0.027 being obtained with a ventilated cavity 0.2 diameters deep. The reductions are an order of magnitude lower than those reported for two-dimensional bodies, but are achieved with a much shorter cavity depth. Although the ventilated cavities had a beneficial effect for small depths, at large depths (greater than 0.5 diameters) they had an opposite effect – a very large drag increase. The cause of this increase is as yet not understood. Hot-wire investigation revealed that base cavities suppress wake periodicity. For cavities of small depth this correlated with a reduction in drag, but for large cavity depths the trends of the intensity of the periodic motion and of drag were not always the same.

Feasibility of active boundary-layer-control methods for reducing aerodynamic drag on tractor trailer trucks

A wind-tunnel investigation was conducted to determine the effectiveness of boundary-layer control on the cab of a tractor to reduce the aerodynamic drag on a tractor-trailer truck. Tests were performed with suction alone, blowing alone, and the combination of suction and blowing. Significant drag reductions were found, even after accounting for the power required to operate the boundary-layer-control system. Feasibility studies were performed which assumed use of engine cooling airflow as a blowing source and of a separate system with combined suction and blowing. The maximum reduction in the net wind-averaged drag coefficient was found to be 21.9% for the situation involving use of engine cooling airflow with the fan-on. Though substantial, the power savings afforded by boundary-layer control were not significantly different from those afforded by passive means designed to produce similar effects on the flowfield or from declutching the fan instead of using it as a blowing power source.

A Wind-Tunnel Study of the Effect of Gap Flow and Gap Seals on the Aerodynamic Drag of Tractor-Trailer Trucks

The effect of gap width on the aerodynamic drag of a cab-over-engine tractor-trailer combination has been investigated for full-scale gap widths ranging from 0.61 m (24 in) to 1.83 m (72 in.) over a yaw angle range of 0 to 20 deg. The average drag on the vehicle was found to increase by 16 percent as the gap width increased from 0.61 m to 1.83 m. Drag reductions were found when a vertical seal was placed along the vehicle center line between the tractor and the trailer. Generally, the drag reduction increased as the percentage of gap width that was sealed increased, and as the yaw angle increased. The average drag coefficient reduction provided by a full gap seal increased from 0.02 to 0.05 as the gap width increased from 0.61 m to 1.4 m and then decreased slightly for gap widths up to 1.83 m. The effect of vehicle configuration on gap seal effectiveness was evaluated for a gap width of 1.3 m (51 in.) using models of six different tractors and two different trailers. The average drag coefficient reductions that were found ranged from 0.04 to 0.08 with 83 percent of the data being either 0.04 or 0.05. It is shown that the use of gap seals on the nearly half-million vehicles which comprise the nation’s long-haul trucking fleet can result in the conservation of about 1.4 × 109 liters (0.37 × 109 gal) of motor fuel each year.

Negative Ocean–Air Feedback Systems Over the North Pacific in the Transition from Warm to Cold Seasons

Abstract Many short-period climatic variations over North America can he associated with abnormal behavior of the Aleutian low and its upper level wind pattern. Hence, if one could foresee these North Pacific patterns, he would be in a good position to predict some North American short–period climatic variations. This report attempts to show dependence of the strength of the Aleutian low in fall on the sea–surface temperatures in that area developed during the antecedent summer. The physical reasoning considers the sensitivity of the general circulation in this domain against the background of seasonal forcing, i.e., when the thermal roles of the continents and oceans are reversing from warm to cold season. Thus, as progressively colder air masses emanate from northern land areas out to sea, the variable diabatic heating and latent heat exchange play important roles in cyclogenesis (or cyclolysis). Indeed, anticyclogenesis often occurs over cold water because of reduced drag coefficient and less “leakage”...

Nonuniform Concentration—A Mechanism for Drag Reduction

A large reduction in drag coefficient has been observed in certain external flows of aqueous solutions with high molecular weight polymer additives. A change in the near wake configuration is phenomenologically responsible for the drag reduction, but the underlying mechanism is presently unknown. An analogy to known phenomena in particulate suspensions is drawn which suggests nonuniform concentration of the polymer additive as an explanation. An analysis of the boundary layer on a sphere with varying viscosity was made to investigate the effect. The results indicate early transition to turbulence for concentration variations whose length scale is small compared with the momentum boundary layer thickness. Stabilization and delayed transition are indicated for thicker concentration layers. Observations are suggested for the thin concentration layers.

Drag of a Circular Cylinder with Vortex Generators

The following is a report of preliminary tests to determine the effect of vortex generators on the drag of a circular cylinder. It was thought that with suitable placement of the vortex generators on the cylinder, the laminar separation of the boundary layer at subcritical Reynolds numbers might be delayed, and a reduction in drag coefficient obtained.