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Technical Paper

Some Additional Stability and Performance Characteristics of the Scissor/Pivot Wing Configurations

The scissor wing configuration is analyzed for unequal forward/rearward wing area ratios and for different wing sweep schedules of the forward and rearward wings. Clα, CMα, static margin, and sweep schedule results are presented as a function of flight Mach number for various sweep schedules and two wing area ratios. Complete aircraft, lift to drag ratio, and power required results are presented for the configuration that was able to maintain static margin over the largest range of Mach numbers. The potential benefits of the scissor wing configuration are presented and discussed in terms of potential increased performance potential or smaller engine.
Technical Paper

Significance of Thermal Contact Resistance on Performance and Size of Finned Tube Heat Exchangers

The findings of various investigators during the past decade have greatly enhanced the ability to accurately analyze, model, design, and optimize finned tube heat exchangers, including those for automotive air conditioning condensers and evaporators. One parameter quite often neglected is the thermal contact resistance which exists for interference-fitted fin-tube geometries. This paper presents the results of including thermal contact resistance as a parameter in estimating the performance of finned tube heat exchangers.
Technical Paper

Navier-Stokes Computations of Transition to Turbulent Flow Around Airfoils

Numerical solutions of the Reynolds-averaged Navier-Stokes equations were obtained with the two-equation K-ϵ turbulence model. Considering the low-Reynolds-number effect in the closed vicinity of a solid boundary, a stream function and vorticity method was developed to consider both the laminar and turbulent stresses throughout the two-dimensional, incompressible flowfield of any arbitrary geometry. At a low Reynolds number (Re = 30), the initially imposed disturbances around an airfoil are damped out; the flow is laminar. At a moderately high Reynolds number (Re = 1000), instability of laminar flow is obtained by exhibiting cyclic patterns in the stream function and vorticity distributions. Nevertheless, only laminar stress occurs in the entire flowfield. At a higher Reynolds number (Re = 106), turbulent stress, which is about three orders of magnitude larger than the laminar stress, occurs at a certain distance downstream of the leading edge and in the wake region.
Technical Paper

Computational Study of Boundary Layer Control for Improving Airfoil Performance

A computational method was developed for investigating boundary layer control. Solutions of the Reynolds-averaged Navier-Stokes equations were obtained using the two-equation k-∈ turbulence model which includes the low-Reynolds-number effect in the near-wall region. Stream function and vorticity together with the turbulent kinetic energy and its dissipation rate were calculated for the flowfield in a body-fitted coordinate system. By increasing the amount of suction on the upper surface, flow separation could be totally eliminated. Transition from laminar to turbulent flow was delayed. Aerodynamic performance was substantially improved.
Technical Paper

Comparison of Soot Processes Inside Turbulent Acetylene Flames under Atmospheric-Pressure Conditions

Two soot-containing turbulent non-premixed flames burning gaseous acetylene in atmospheric-pressure air were investigated by conducting non-intrusive optical experiments at various flame locations. The differences in burner exit Reynolds numbers of these flames were large enough to examine the influence of flow dynamics on soot formation and evolution processes in heavily-sooting flames. By accounting for the fractal nature of aggregated primary particles (spherules), the proper interpretation of the laser scattering and extinction measurements yielded all the soot parameters of principal interest. With the separation of spherule and aggregate sizes, the axial zones of the prevailing turbulent soot mechanisms were accurately identified. With the high propensity of acetylene fuel to soot, relatively fast particle nucleation process led to high concentrations immediately above the burner exit.
Technical Paper

Analysis of the Effect of Heat Strips on Boundary Layer Development Over a Flat Plate

Two dimensional fourth order boundary layer calculations were made for flows over a flat plate with and without flush mounted surface heating. Constant wall temperature, increasing wall temperature and decreasing wall temperature heating cases were studied for different surface heating lengths. The boundary layer properties; temperature, tangential velocity, normal velocity, vorticity and transition location were studied for these temperature distributions. The boundary layer results indicate that with the proper selection of surface temperature variation and length the transition location can be either increased or decreased. Modified boundary layer properties, due to heating are shown to persist well after heating is stopped, even when the flow is turbulent. The results indicate that this technique may be useful in modifying transition and separation locations over airfoils.
Technical Paper

An Inviscid Aerodynamic Investigation of Wing/Winglet Geometries for RPV Applications

Wings constructed using the Wortmann FX 63-137 low speed airfoil, which operates in a Reynold's number range from 0.28 * 106 to 0.7 * 106, with the addition of winglets are studied to determine the winglet geometry that produces the best increase in wing efficiency. The analysis was done using VSAERO, an inviscid panel code program. All configurations are compared to a wing without winglets to determine the percent increase in efficiency. It is demonstrated that with proper selection of winglet taper ratio, tip setback, height, cant angle, geometric twist angle, and airfoil section induced drag can be significantly reduced. Wings with winglets are shown to be more efficient than wings without winglets for all cases.
Technical Paper

Aerodynamic Investigation of Wing Tip Sails

Wing tip sails were investigated to determine potential aerodynamic improvements for a wing having an aspect ratio of 10 and a taper ratio of 0.43. The airfoil section used for the wing was an NLF- 0215 and the wing tip was rounded. Three tip sails were utilized for all investigations with each tip sail having a root chord that was 20 percent tip chord of the wing. The wing sails were mounted at the tip of the wing along the chord line. Looking along the span towards the wing root the orientation of each sail tip was the same as the wing tip. Initial studies used sails constructed from two Wortman airfoils. A generic cambered tip-sail was also investigated. Individual sail angle of attack as well as sail dihedral and anhedral were investigated. PMARC, an aerodynamic paneling code was used to predict lift, induced drag, and viscous drag with the use of a momentum integral analysis. All viscous predictions were calculated for a Re/foot = 2.19 × 106.
Technical Paper

Aerodynamic Characteristics of Airfoils with Backward-Facing Step Configurations

A computational study of flow developments over airfoils with backward-facing steps is conducted to explore the possibility of enhancing aerodynamic performance of the airfoils by vortex generation. The study focuses on the effects of the separated flow and subsequent vortex formation generated by the step on pressure distributions around two airfoil profiles. Step location and size are varied to determine their effect on lift, drag, and L/D ratio. A discussion of the effects and trends of the various step configurations on airfoil performance is presented along with the results that may serve as a reference for employing a control criteria to optimize airfoil geometries during flight.