摘要:

Corona-induced drag reduction was studied numerically over a finite region of a semi-infinite flat plate having small Ohmic surface conductivity for low Reynolds number flow (<100 000, based on the farthest downstream electrode distance). The model simulates a corona discharge along a surface from two parallel wire electrodes of infinite length immersed flush on the surface and oriented perpendicular to the flow. Charge deposition and removal with the conducting surface are included as possible charge transfer mechanisms. The analysis is limited to ions of positive charge. Five coupled partial differential equations govern the numerical model including continuity, momentum, gas phase conservation of charge, Poisson’s equation of electrostatics, and conservation of charge at the solid interface. The governing equations together with empirical breakdown and current–voltage relationships (&Fgr;–Icharacteristic) were evaluated by finite differencing schemes. The calculated results predict “corona thinning” of the boundary layer for a downstream ion flow and a corresponding reduction in drag, in agreement with previous theoretical studies. Various parameters of flow, electricity, and geometry, relating to corona-induced drag, are investigated. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869219

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The unsteady flow of a homogeneous viscous fluid past a straight circular cylinder (radiusl*)confined between two infinite parallel plates (a distanced*apart) relative to a rapidly rotating frame is considered. The cylinder is impulsively started from rest to a uniform velocity. The unsteady form of the boundary-layer equations for a rotating fluid is used to examine the flow Rossby numberRo∼O(E1/2),whereE≪1is the Ekman number. A range of values of the non-dimensional parameterN=lE1/2/Ro(wherel=l*/d*)is considered. For0⩽N<1,the flow pattern resembles that of the non-rotating case(N=0).Initially, the wall shear around the cylinder is positive everywhere. After a time, flow reversal begins at the rear stagnation point and then the position of zero wall shear moves upstream, towards the front stagnation point. The boundary-layer thickness in the region of reversed flow grows with time until a singularity/eruption at a point in the flow occurs. The boundary-layer equations are written in terms of Lagrangian coordinates in order to numerically investigate the finite-time singularity for0⩽N<1.The flow close to the rear stagnation point is also examined in detail for a range of values ofNand results are compared with the large-time asymptotic forms for the growth of the displacement thickness. The analysis suggests the displacement thickness in this region grows exponentially with time, for certain ranges ofN.For0<N<1,the displacement thickness grows exponentially with time in a manner similar to the non-rotating case. ForN>1,the wall shear remains positive for all time. However, for1⩽N<2,the displacement thickness of the boundary layer close to the rear stagnation point again grows exponentially with time. For2<N<3the flow close to the rear stagnation point also grows exponentially with time, although the form of solution differs from that for0⩽N<2.ForN>3,the solution tends to a truly steady limit, consistent with previous studies on the steady problem. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869220

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The instability of an arbitrarily shaped zonal jet on a midlatitude&bgr;-plane is considered within a two-layer quasi-geostrophic model withO(1)linear friction. Depending on the horizontal and vertical shear of the jet, it is susceptible to both barotropic and baroclinic instabilities. The linear stability boundaries are determined numerically for a parameter regime relevant to the Gulfstream. The weakly nonlinear (finite amplitude) evolution of the instabilities is shown to be governed by a Ginzburg-Landau equation and for arbitrary jet shapes the coefficients in this equation are computed numerically. The finite amplitude state is shown to become unstable to Benjamin-Feir sideband instabilities. The mixed baroclinic/barotropic character of the primary instability is crucial to this sideband instability which is shown to lead to complicated spatio-temporal behavior of the jet. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869221

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We discuss the stochasticities of two-triad interactions that occur in two-degree-of-freedom autonomous Hamiltonian systems. The system we study is a two-triad test-wave system consisting of a single internal wave mode (test-wave) interacting with a spectrum of ambient internal wave modes; the ambient modes, however, do not interact among themselves except through a three-wave interaction which includes the test-wave. The present study concerns the effect of nonlinearities on the ocean internal wave field. Our numerical results using the physical parameters appropriate for the deep ocean confirm that the test-wave system is non-integrable. Moreover, there exists a certain separatrix net that fills the phase space and is covered by a thin stochastic layer for a two-triad pure resonant interaction. The stochastic web implies the existence of diffusion of the Arnold type for the minimal dimension of a non-integrable autonomous system. For the non-resonant case, the stochastic layer is formed where the separatrix from KAM theory is disrupted. However, the stochasticity does not increase monotonically with increasing energy. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869222

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Longitudinal vortices with axes in the direction of a mean shear flow can arise due to body force instabilities associated with heating or centrifugal effects. In this paper we are concerned with possible stabilization or destabilization of such instabilities by means of a controlled flow oscillation in the spanwise direction. For the heated case, stabilization occurs up to the values of Rayleigh and Reynolds numbers at which streamwise two-dimensional disturbances become critical. For the centrifugal case, stabilization occurs only for sufficiently large values of the Reynolds number associated with the spanwise oscillation; for smaller values, destabilization occurs. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869213

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Experimental tests were performed on round jets exiting inclined nozzles at a Reynolds number of 9000. Both natural jets and jets forced with single frequencies corresponding toStD=0.25,0.5, 0.75, and 1.0 were examined. In the natural case, the nozzle incline caused a mild increase in the radial spreading in the plane of azimuthal symmetry. The forcing amplified the asymmetric radial spreading by altering the vortex structure. In general, the inclined vortex rings rolled up at an angle slightly smaller than the nozzle incline angle. As the rings moved downstream, they migrated away from the jet centerline and their incline angle increased. Vortex rings generated atStD=0.5did not pair because that Strouhal number was near the “preferred” mode. For nozzles with slight inclines, forcing at larger Strouhal numbers led to pairing nearx/D=2in order to achieve the “preferred” mode. For nozzles with larger inclines, the vortex cores broke down before pairing could occur. Forcing at a lower Strouhal number(StD=0.25)yielded ring formation atStD=0.5and subsequent pairing. Increasing the incline angle moved the pairing location closer to the nozzle lip. Also, the pairing process was found to depend on the nozzle incline angle. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869223

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An experimental investigation of the non-reactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates streamwise vorticity, producing high strain rates which can enhance the mixing of reactants while delaying ignition in a controlled manner. The lobed injectors examined in the present study consist of two corrugated plates between which a fuel surrogate,CO2, is injected into coflowing air. Acetone is seeded in theCO2supply as a fuel marker. Comparison of two alternative lobed injector geometries is made with a straight fuel injector to determine net differences in mixing and strain fields due to streamwise vorticity generation. Planar laser-induced fluorescence (PLIF) of the seeded acetone yields two-dimensional images of the scalar concentration field at various downstream locations, from which local mixing and scalar dissipation rates are computed. It is found that the lobed injector geometry can enhance molecular mixing and create a highly strained flowfield, and that the strain rates generated by scalar energy dissipation can potentially delay ignition in a reacting flowfield. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869224

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The absolute and convective instability characteristics of supersonic wake dominated shear layers, described by a hyperbolic-tangent profile plus a wake component represented by a Gaussian distribution, are investigated. The effects of the Mach number, the freestream temperature ratio and the wake deficit on the boundary of the absolute/convective transition and the branch point parameters (such as frequency, wave number, and the spatial growth rate) are studied using linear stability theory. For supersonic mixing layers, it is found that at a given wake deficit the amount of backflow necessary to cause absolute instability decreases as the convective Mach number increases and that all the branch point parameters vary slowly as functions of the convective Mach number as opposed to subsonic mixing layers. As the wake deficit decreases, the curve separating the absolute and the convective regions moves down to the values of more negative freestream velocity ratio for both subsonic and supersonic mixing layers and the branch point spatial growth rate decreases for a given convective Mach number. The effect of decreasing the wake deficit on the absolute/convective transition boundary is similar to that of increasing the freestream temperature ratio. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869225

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A vertical Bridgman furnace, through which an ampoule containing the melt of a certain dilute binary alloy is pulled at a fixed, and predetermined speed, provides a means of improving certain alloys. Radial segregation in the finished crystalline material can make it unusable. In this paper, we construct an asymptotic theory for the flow and solidification in the ampoule, for large Rayleigh numbers but at a small Biot number. We find large-Rayleigh-number solutions to be, to leading order, completely insensitive to the character of the side-wall boundary condition on vertical velocity. Two-dimensional equivalents of optimization conditions found by Tanveer are recovered for his two limiting cases—large thermal Rayleigh number, and large negative solutal Rayleigh number. Moderate surface tension at the crystal–melt interface is found to have no effect on the optimization conditions for the two limit cases, but it does somewhat reduce the magnitude of the segregation in both limits. In addition, we present new results for the case for which the two Rayleigh numbers are of comparable magnitude and show that there is an optimization possible for this case too. Conversion of the results of this paper to an axisymmetric geometry is shown to be trivial. Keeping careful track of the ordering, we indicate how to proceed to first effects of non-linearity at small Biot and/or Prandtl number. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869226

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Linear and nonlinear Lagrangian equations are derived for stochastic processes that appear as solutions of the averaged hydrodynamic equations, since their moments satisfy the budgets given by these equations. These equations include the potential temperature, so that non-neutral flows can be described. They will be compared with nonlinear and non-Markovian equations that are obtained using concepts of nonequilibrium statistical mechanics. This approach permits the description of turbulent motion and buoyancy, where memory effects and driving forces with arbitrary colored noise may occur. The equations depend on assumptions that concern the dissipation and pressure redistribution. In the approximations of Kolmogorov and Rotta for these terms, the dissipation time scale remains open, which can be determined by the calculation of the production–dissipation ratio of turbulent kinetic energy. The features of these equations are illustrated by the calculation of turbulent states in the space of invariants. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869421

出版商:AIP    

年代:1997

数据来源: AIP