Experimental equilibrium temperatures and skin friction at the surface of a flat plate cooled by two-dimensional, tangential slot injection are presented for a Mach 6 mainstream. Effects of slot height, slot mass-flow rate, injection gas temperature, and heat conduction to the slot are investigated. Experimental skin friction and effectiveness data in general agree well with predictions from a finite-difference method. Film cooling at Mach 6 was found significantly more effective than at lower speeds, with large reductions in friction drag measured downstream of the slot These results indicate the necessity for a new evaluation of film-cooling systems for hypersonic vehicles.

Film-Cooling Effectiveness and Skin Friction in Hypersonic Turbulent Flow

The current work is an experimental investigation of the dependence of film cooling effectiveness on the injection Mach number, velocity, and mass flux. The freestream Mach number is 2.4, and the injection Mach numbers range from 1.2 to 2.2 for both air and helium injection. The adiabatic wall temperature is measured directly. The injection velocity and mass flux are varied by changing the total temperature and Mach number while maintaining matched pressure conditions between the injected flow and that of the freestream. The total temperature of the freestream is 295 K, and for the injection it ranges from 215-390 K. The results indicate an increase in film cooling effectiveness as the injection rate is increased. With the exception of heated helium runs, larger injection Mach numbers slightly increase the effective cooling length per mass injection rate. The results for helium injection indicate an increase in effectiveness as compared to that for air injection. Heated injection, with the injectant to freestream velocity ratios greater than 1, exhibit a rise in wall temperature downstream of the slot resulting in effectiveness values greater than 1. The experimental results are also compared with earlier studies in the literature.

Influence of injectant Mach number and temperature on supersonic film cooling

A concept for reducing turbulent skin friction drag by altering/controlling the large coherent eddy structures within the turbulent boundary layer is proposed. Results of an ongoing experimental and numerical investigation to develop large-eddy breakup devices (LEBU devices) are presented and indicate that the average skin friction drag downstream of the LEBU devices is reduced by up to 24% compared to 'undisturbed' flat plate levels; device drag requires further reduction before net drag reductions can be realized. Future work is discussed and will focus on reducing device drag by taking advantage of the unsteady 'freestream' ahead of the LEBU devices.

Large-eddy breakup scheme for turbulent viscous drag reduction

A calculation method for turbulent boundary layers and wall jets has been developed in which the effects of large longitudinal surface curvature and the associated normal pressure gradients are included for the first time. This is achieved by using finite difference techniques to represent the equations of mean motion in a system of curvilinear orthogonal coordinates. The Reynolds stress terms in the equations of motion are approximated using an eddy viscosity approach based on the concept of intermittency . Application of the method to wall jet boundary layers is achieved through extension of the basic eddy viscosity model. Comparisons between theory and experiment for boundary layers and wall jets developing over flat or curved surfaces for a wide variety of pressure distributions show encouraging agreement.

Calculation of turbulent boundary layers and wall jets over curved surfaces.

Four main concepts which have significantly reduced skin friction in experimental studies are discussed; suction, gaseous injection, particle additives, and compliant wall. It is considered possible that each of these concepts could be developed and applied in viable skin friction reduction systems for aircraft application. Problem areas with each concept are discussed, and recommendations for future studies are made.

/pdf/a-general-review-of-concepts-for-reducing-skin-friction-3e0jnwfgpu.pdf

A general review of concepts for reducing skin friction, including recommendations for future studies

Finite difference calculation based on eddy diffusivity and mixing length flow theory to characterize supersonic turbulent boundary layer with tangential slot injection

/pdf/calculation-by-a-finite-difference-method-of-supersonic-4lherhyxr2.pdf

Calculation by a finite difference method of supersonic turbulent boundary layers with tangential slot injection

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Calculation by a finite difference method of supersonic turbulent boundary layers with tangential slot injection