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Aspects of thermal flow control in hypersonic flight
Abstract
This paper discusses the application of energy deposition for sonic boom mitigation and as actuator device substitute. Classical sonic boom minimization strategies suffer from major shortcomings, such as prohibitively large power requirements or a permanent deterioration of aerodynamic quality - even when sonic boom suppression is not needed. Virtual blunting is suggested here as a combination of classical approaches but with increased flexibility at reduced power consumption. A power estimation based on Rayleigh flow proves to be sufficiently accurate. Numerical simulations are carried out to show that energy deposition is in principle capable of affecting a flow in the same way as a solid spike attachment does. As a second focus, energy deposition is investigated as a mechanism to create forces and torques. As mechanical actuators such as ailerons and rudders are subject to substantial mechanical and thermal loads, it seems desirable to look for alternatives that do not need the weight and complexity of moving parts. It will be shown that forces released on a flow-parallel wall using energy deposition are comparable to forces caused by an inclined flat plate.
This paper discusses some general properties of energy deposition in hypersonics. Extending the flight envelope of passenger aircraft to high Mach numbers comes with problems unsolved before in civil flight, such as sonic boom, or excessive thermal and mechanical loads. Energy deposition is regarded as a possible remedy to a multitude of problems occurring in this flight regime and thus studied here. A new energy source term has been implemented into the DLR TAU CFD code which models a physical distributed heat addition. In a first step, energy deposition in free flow is investigated using this new term. Four flow topologies, dependent on the deposition energy, have been identified, ranging from negligible induced cross flow component, to a massive detached bow shock ahead of the deposition region. In a second step, ramp flow, as a precursor to flow about an airfoil, is studied under the influence of an upstream energy deposition region. An undesired low pressure region could be explained with a simplified model of wave refraction at thermally stratified ramp flow. Key of this model is a flow deflection above the ramp which is needed to balance pressure differences. This model is discussed in detail to fully understand the physical nature of the flow structure. It predicts a way of increasing surface pressure compared to regular ramp flow at reduced total pressure loss. It furthermore predicts the occurrence of premature shock separation and oscillating shock patterns. Analyzing this model, a simple yet accurate functional formula is found for the additional deflection angle due to refraction which simplifies the calculation of the surface pressure distribution as no pressure balance iteration is needed anymore. These studies are currently carried out within the European ATLLAS project which is concerned with the development of hypersonic passenger aircraft. Computations are performed using the DLR TAU code, a finite volume, second order accuracy, compressible flow solver.
A conceptual study is here presented and discusscd on the possibility to transport 200 passengers over a distance of about 7000km in a nominal point-to-point mission through the Atlantic (either London-New York or London-Rio) at a cruise Mach number of 6 and an altitude abont 30km. The aim of the study is not to design a specific airplane but to explore today's state of the art technology limits to realize such kind of concept, i.e. to identify if such a mission could succeed today. Because of the challenge the mission poses, its is being optimised with the major disciplines involved by means of Multi-Disciplinary Optimisation (MDO) tools as a way to realize an optimum integrated airframe/propulsion aircraft. The environmental impact is being analysed in terms of the resulting sonic boom. No experimental data but CFD results by means of independent assessments has been generated. The study indicates that today the available technology provides with sufficient maturity to accomplish with the mission in areas like aerodynamic and thermal resistance materials but in others like sonic boom mitigation it is required a deeper insight in the physics. Finally while the present investigation clear identify that complex designs involving large amount of variables from different disciplines could be only possible via MDO/MDA strategies, today such processes still suffer on lack of robustness of the involved tools.
The present investigation is aimed at evaluating the reliability of the unstructured TAU Code for hypersonic flows. To improve the efficiency, accuracy and robustness, the code has been modified concerning, for example, residual smoothing, limiters and timestep size to handle strong shocks. Validation was done using CFD solutions of different European CFD solvers for well known test cases such as hyperboloid flare. Further investigations have been done for wedge compression corners to study gap flow phenomena. Also, computations for a complex three dimensional configuration including deflected body flaps while considering the gap effect on the thermal loads have been performed. The effect of 3d adaptation has been intensively studied. The present investigation shows that the TAU-code is a reliable and efficient tool for complex cold hypersonic flow solutions, although its performance with respect to computing time should be improved.
Progress is reported in an ongoing numerical study of the application of plasma actuator technology to high-speed flow control. As part of this project, a three-dimensional computer code has been written to solve, simultaneously, the fluid conservation laws, the charged particle continuity equations, and the Poisson equation for the electric potential. In past work, the code was applied to the evaluation of DC glow discharge actuators for hypersonic boundary layer control. Here the numerical model is extended and applied to RF glow discharge devices. Preliminary calculations have reproduced many of the documented features of the a-mode, or low current density RF discharge mode. Ongoing work addresses the behavior of RF discharges in boundary layer flows. An additional study has been carried out of a Mach 14 compression ramp flow using a reduced order model. The effects near reattachment of both steady and unsteady actuation near separation were evaluated. The most beneficial effects of steady actuation were obtained with surface heating and with an upstream-directed body force. With control applied, the shear layer was seen to reattach on the ramp with a slightly shallower angle, leading to reduced velocity and temperature gradients at reattachment, and consequently a reduction in the peak heat flux. Unsteady actuation was seen to introduce a region of hot, slow fluid, similar to a turbulent boundary layer structure, that convected through the reattachment zone, temporarily altering the shear layer in a manner similar to that observed with steady actuation. Future work will address this problem using the high-fidelity model.
Plasma actuators and various forms of volumetric energy deposition have received a good deal of research attention recently as a means of hypersonic flight control. An open question remains as to whether the required power expenditures for such devices can be achieved for practical systems. To address this issue, a numerical study is carried out for Mach 12 flow over a blunt nose elliptic cone to determine the amount of energy deposition necessary for flight control. Energy deposition is simulated by means of a phenomenological dissipative heating model. Validation studies of the flow simulation code in the absence of energy deposition are presented for a Mach 8 elliptic cone flow and a Mach 14 blunt elliptic cone. A parametric study of the effects of energy deposition is carried out for two blunt-nosed elliptic cone configurations: a 3 m long cone at Mach 12.6 and 40 km altitude, and a 0.2 m long cone at Mach 14.2 and roughly 42 km effective altitude. Three different volumetric energy deposition patterns are considered: a spherical pattern, a 'pancake' pattern (oblate spheroid), and a 'bean' pattern (prolate spheroid). The effects of energy deposition are seen to be relatively independent of these patterns. For the Mach 12.6 cone case, the pitching moment generated by energy deposition is 10-30% of that generated by a 0.05 m2 mechanical flap at 2° deflection. For the Mach 14.2 case, the corresponding figure of merit is on the order of 1000%. The effectiveness of volumetric energy deposition for flight control appears to scale strongly on the nondimensional parameter Q/(ρ∞u∞3 L2), and additional computations are warranted to explore this effect.
There is much interest and some controversy concerning the possibility of sonic-boom suppression through air stream alteration by application of force or heat fields. The present paper presents a discussion of the relationship of these more exotic schemes to conventional approaches involving shaping of the aircraft itself, describes the required flowfield alteration for the more promising heat field method and provides a first estimate of power requirements. The results of the study indicate that finite rise-time signatures which offer substantial sonicboom alleviation are theoretically obtainable but that severe, if not unsurmountable, problems of implementation are presented. Nomenclature effective cross-sectional area due to a combination of airplane lift, airplane volume, and the altered airstream initial phantom-body cross-sectional area cross-sectional area of airstream airplane flight altitude or perpendicular distance from model to measuring probe airplane or model reference length phantom-body length Mach number power summation, I %-]dx dx with circulation
The effects of added energy on a compressible flowfield are studied.
One-dimensional flows are explored analytically as diabatic processes,
revealing that in a subsonic regime pressure and density increase and
velocity decreases in front of the diabatic zone, while velocity
increases and density decreases behind the zone. Hugoniot curves are
mentioned, and heat addition in a channel with a variable cross section
is considered. Attention is also given to two-dimensional linear and
nonlinear processes, particularly for heating on a straight front, a
profile with continuously distributed heating, a Prandtl-Meyer
expansion, Broadbent's method, and heating in hypersonic flow. For
hypersonic flows, it is shown that only a small portion of the heat
input is transformed into thrust, the rest being distributed in pressure
and temperature rises in the hypersonic flowfield.
Sonic boom phenomena, discussing supersonic flow near aircraft, atmospheric propagation, distortion, focusing, caustics, turbulence effects and reduction
Numerical investigation of aero-spikes and aero-jet-spikes for Mach 6 SST
- D T Banuti
D.T. Banuti. Numerical investigation of aero-spikes and
aero-jet-spikes for Mach 6 SST. Deliverable D.2.1.5 Contract
No. AST5-CT-2006-030729, ATLLAS, 2008.