Hirotoshi Kubota

Flat spin of slender bodies at high angles of attack

Low-speed wind tunnel investigations are performed for the flat spin of slender bodies at high angles of attack at Reynolds numbers of (1.2-4.4) xlO5. Effects of the geometrical configuration of the model, freestream Reynolds number, and angle of attack on the flat spin motion are analyzed using spin rate measurements, in situ surface pressure distribution, and static aerodynamic characteristics. It is verified that the flat spin is caused by differences in flow patterns at the critical Reynolds number. The measured side moment can be used to predict the actual flat spin rate. The flat spin can be alleviated by causing early transition of the boundary layer with artificial surface roughness.

Numerical Optimization of Fuselage Geometry to Modify Sonic-Boom Signature

A low-sonic-boom design method is developed by combining a three-dimensional Euler computational e uid dynamics code with a least-squares optimization technique. In this design method, the fuselage geometry of an aircraft is modie ed to minimize the pressure discrepancies between a target low-boom pressure signature and a calculated signature. The aircraft cone gurations that generate three types of low-boom pressure signatures, i.e., e attop type, ramp type, and hybrid type, are successfully designed by this method. It is shown that the sonic-boom intensity of the aircraft designed by linear theory is reduced and the e attop-type ground pressure signature is obtained by this method. The results of the study suggest that this method is a useful tool for low-boom design.

Numerical Simulation of Re-Entry Flow Around the Space Shuttle with Finite-Rate Chemistry

Hypersonic flow around the Space Shuttle is computed for a series of re-entry flight conditions by incorporating a one-temperature model that takes into account 7-species finite-rate chemistry. An implicit higher-order upwind scheme based on a generalized Roes approximate Riemann is used. The aerodynamic characteristics and heating rates are compared with the STS-2 flight data, and good agreement is obtained. These results quantitatively confirm the measured forward shift of the center of pressure and reduction of heating rates caused by real gas effects.

A Fully Implicit High-Resolution Scheme for Chemically Reacting Compressible Flows

The eigenvalues and eigenvectors are analytically derived for general real gas dynamic equations in three-dimensional generalized curvilinear coordinates. In our diagonalizing formulation, the total mass conservation equation is taken into account, and arbitrary nonequilibrium effects, such as chemical reactions or vibrational nonequilibrium, can be treated in the same fashion. Making use of this diagonalization, we construct a fully implicit and high resolution scheme for chemically reacting real gases. Numerical results of two-dimensional shock-induced combustion problem show the efficiency and high resolution of our scheme.

LOW-BOOM DESIGN METHOD BY NUMERICAL OPTIMIZATION

A low sonic-boom design method is newly developed by combining a 3-dimensional Euler CFD code with a least-square optimization technique. This method enables us to analyze the nonlinear and the 3-dimensional effects of aircrafts which cannot be considered by previous low-boom design methods based on linear theory. In this design method, the fuselage geometry of an aircraft is modified in order to minimize the pressure discrepancies between a target low-boom pressure signature and a calculated signature. The aircraft configurations which generate three types of low-boom pressure signatures, flat-top type, ramp type, and hybrid type, are successfully designed by this method. It is shown that the sonic-boom intensity of the aircraft designed by linear theory is greatly reduced and the ground pressure signature of the flat-top type is obtained by this method. The results of the study indicate that this method is a useful tool for low-boom design.

The effect of nose bluntness of a low-boom configuration on sonic-boom

The effect of the nose bluntness of an aircraft on sonic-boom intensities is described. In order to clarify the effect of the nose bluntness, the boom intensity of a low-drag and a low-boom configuration are first predicted. The pressure signatures for near-field are simulated by a three-dimensional Euler finite difference code. The calculated nearfield signatures are extrapolated to far-field by the Thomass waveform parameter method based on a quasi-linear theory. As a result, it is indicated that the boom intensity of a low-boom configuration is about 1.0 [psf] which is about 0.5 [psf] smaller than that of a low-drag configuration in the case of same lift coefficient. A trade off study between the sonicboom intensity and the aircraft drag is next done by using the Dardens boom minimization theory. The fuselage geometry of the low-boom configuration is modified to change the nose bluntness. It is clarified that the parameter yf/L which represents the bluntness of the nose configuration in the Dardens theory has proper values between 0.04 and 0.08.

Computational Studies on Topological Properties of Vortex Core Lines in Separated Flow Field Around High-Angle-of-Attack Cone-Cylinders

The topological properties of a certain type of vortex breakdown, observed in a computed flow field around a high-angle-of- attack cone-cylinder, are investigated. This flow phenomenon, which is defined as the turn-round of its axial velocity, is followed by the formation of the limit cycle on the crossflow plane. In the present study, the vortex breakdown is described as a vortex core line with a pair of focuses on it, and this model provides the simplest structure of the vortex breakdown as far as the topological rules are applied.

Numerical Analysis On Gain Of C6H6-O2-N2 Type GDL

The two-dimensional vibrational non-equilibrium flow field and the nozzle shape effects on CO2 GDL (Gasdynamic Laser) performance are numerically analyzed by a point implicit scheme. It is shown that the Shock Free Minimum Length nozzle (SFML-nozzle) is superior to other nozzles in the viewpoint of GDL characteristics such as the small-signal gain and the maximum power available. Furthermore, the effects of combustion by-products such as O2,CO on the small-signal gain are investigated. It is concluded that such by-products have a negative effect on GDL gain characteristics. The effects of H20 concentration on the small-signal gain are also investigated considering H2O vibrational relaxation. These numerical analyses are performed for the predictions of CO2 GDL.

Characteristics Of C6H6-O2-N2 Type CO2 Gasdynamic Laser

Characteristics of a C6116-02-N2 type combustion-driven CO2 gasdynamic laser, which should be a radiation heating simulator for re-entry of a planetary probe and AOTV, are experimentally and theoretically investigated. Small-signal gain coefficients are obtained for supersonic nozzle expansion ratios of 5, 7.5, 10, 15 and 20. The result shows : 1) qualitative agreement is obtained between the present measurements and the numerical analysis, 2) the small-signal gain grows as the nozzle expansion ratio is increased, and 3) there exists an optimum plenum pressure for small-signal gain coefficient under a certain expansion ratio of supersonic nozzle.Characteristics of a C6116-02-N2 type combustion-driven CO2 gasdynamic laser, which should be a radiation heating simulator for re-entry of a planetary probe and AOTV, are experimentally and theoretically investigated. Small-signal gain coefficients are obtained for supersonic nozzle expansion ratios of 5, 7.5, 10, 15 and 20. The result shows : 1) qualitative agreement is obtained between the present measurements and the numerical analysis, 2) the small-signal gain grows as the nozzle expansion ratio is increased, and 3) there exists an optimum plenum pressure for small-signal gain coefficient under a certain expansion ratio of supersonic nozzle.

Characteristics of Combustion-Driven CO_2 Gasdynamic Laser (III) Characteristics of Laser Output Power

Laser output power characteristics of the combustion-driven CO2 Gasdynamic Laser (CO2GDL) are presented. Laser extraction is carried out with supersonic nozzle of circular-circular (CC) and shock free (SF), which has common area-ratio (A/A*=20) and various expansion length (CC nozzle with expansion length of 30mm and SF nozzle of 45, 33, 20mm). In laser output power measurements, maximum power of 24.5W is obtained at CC 20 nazzle of expansion length of 30mm, which has maximum small-signal gain of 0.5m-1. However, at SF nozzle which has small-signal gain of 0.2-0.3 m-1, laser output power can not be obtained. Investigation of the laser excitation condition suggests that the system requires the threshold value for small-signal gain over 0.23m-1. It is concluded that, with these results a feasible laser excitation system by the CO2GDL with CC nozzle has been established. Increase of small-signal gain, extension of cavity width and selection of most suitable laser light resonator will provide a radiative heating simulator for thermal protection research.