Possibility Of Drag Reduction Research Articles

Determination of groove shape with strong destabilization and low hydraulic drag

Flow through a channel equipped with plane, longitudinal grooves is investigated. We focus on determining changes to the flow dynamics due to applied wall manipulations, especially the possibility of drag-reduction, potential for hydrodynamic destabilization and onset of secondary, nonlinear flow solutions. Considered patterns of geometrical manipulation consist of plane walled grooves of triangular, variations of trapezoidal and up to rectangular shapes and are compared with existing results obtained for channels with sinusoidal corrugations. We show that there exists a strong connection in the response of the flow system to the applied pattern of grooves with the leading Fourier component of the wall pattern. The analysis starts with undisturbed, two-dimensional flows investigated with the focus on hydraulic resistance for wide range of geometric parameters. Secondly, critical conditions for the onset of travelling wave instability are determined and compared with existing results. Thirdly, nonlinear flow solutions, obtained for flows through the geometry with lowest destabilization threshold are analyzed at supercritical conditions. Finally quantification of the diffusive transport intensification due to the kinematics of the nonlinear flow solution is attempted. It is shown that, irrespective of the groove shape, longitudinal wall patterns result in flow destabilization due to traveling wave mode already at very low values of the Reynolds number (<102). At the same time such configurations can be energy efficient since overall drag is marginally increased (or in fact reduced) and at the same time diffusive processes are intensified leading to improved mixing. Implications of this study might help in development of small-scale flow devices operating at low and moderate ranges of Reynolds number with the purpose of intensifying mixing, heat transfer or reaction of chemical or biological compounds.

910 一様水流中の柔軟な弾性膜に発生するフラッタリング(GS-2 バイオエンジニアリング(2))

The fish's thrust results from actions of swimming fish that the fish's body is made to generate traveling waves by bending itself and transmits them downward. We think that fluttering has the possibility of drag reduction because it generates some thrust and assists main power. So we focus on fluttering in fluid. Study of fluttering was done using various kinds of elastic sheets having high and low stiffness. Our experimental models consisted of a flexible rear made of a very thin silicone rubber sheet which was easily deformed and a rigid front made of stainless steel. It was placed in a water tunnel. A laser light sheet method was used to visualize the deformation of the flexible sheet and the flow field around the model. From the visualization experiments, we measured the wave velocity, wavelength, frequency and amplitude of traveling wave on fluttering of a flexible sheet. We calculated fluttering of a flexible sheet with the model that the wave form was assumed and the principle of the energy conservation.

Transonic Profile Flow with Heat Addition

We give a closed form solution for transonic profile flow with internal heat addition using the parabolic method. The equivalence of mass- and heat addition and the possibility of drag reduction is generally discussed. Comparison with earlier CFD-method results and similarity considerations are possible.

The possibility of drag reduction by outer layer manipulators in turbulent boundary layers

Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU’s) were used in single and tandem configurations on a large flat plate. The total drag was measured directly by a force gauge, and the geometrical configuration parameters, as well as the chord Reynolds number, were varied over wide ranges. The highest chord Reynolds number tested was 260 000, which is within the range of interest for practical applications. Also, the device drag was measured directly in flight, which enabled evaluation of the performance of the manipulators, as well as determination of the total skin friction reduction. Despite a substantial skin friction reduction for certain configurations (7% averaged over the part downstream from the manipulators) as well as a low device drag, no drag reduction was found. The present results, and a critical evaluation of previously reported data, make any substantial net drag reduction by use of LEBU’s at high Reynolds numbers seem implausible.

Possibilities of drag reduction by the use of flexible skin.

A flexible aerodynamic surface is considered as a possible means of delaying transition and reducing skin-friction drag. The results of an extensive analytical study on boundary-layer instability and transition in incompressible Blasius-flow over a flexible surface are presented. A simple flexible skin model consisting of a taut membrane and an elastic base has been considered. The mechanical behavior of such a skin is controlled by the mass, the wave propagation velocity, the stiffness, and the damping. The analysis shows that with proper selection of the surface characteristics, the transition can be significantly delayed through reduced amplification rates, even though the critical Reynolds number for instability is only slightly increased. The extent of transition delay and the required surface properties are delineated. The theoretically possible drag reduction is most significant within the Reynolds number range of 3 to 50 X 10 6. Hence, potential fields of application of flexible skins would be in sailplanes, helicopters, small and medium subsonic airplanes, hydrofoils, torpedoes, speed boats, and miniature submarines. Nomenclature a = amplification factor, imaginary part of the complex wave velocity cr = phase velocity, real part of the complex wave velocity Com = wave propagation velocity in the membrane, (T/M)l/<2 d = damping coefficient, Eq. (21) D = damping constant, lb-sec/ft3 KM = mass parameter, Eq. (13) Ks = stiffness parameter, Eq. (15) KD = damping parameter, Eq. (17) m = mass coefficient, Eq. (18) M = membrane mass per unit area, Ib-sec2/ft3 Rx = Reynolds number based on length, Um x/v Rd = Reynolds number based on boundary-layer thickness S = compression stiff ness of base material, Ib/ft3 T = membrane tension, Ib/ft t/oo = freestream velocity, fps x = distance from leading edge along the plate, ft a. = dimensionless wave number, kd /3r = disturbance frequency, a.cr d = boundary-layer thickness, ft d* = boundary-layer displacement thickness, ft v — kinematic viscosity of fluid, ft2/sec p = density of fluid, Ib-sec2/ft3 coo = dimensionless cutoff frequency, (S/M)ll2d/Uo3 w* = circular frequency of disturbance, sec1 ior = dimensionless circular frequency, (3r/Rd = co^v/U m2