Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

Plasma Actuator Force Measurements

Abstract: In previous work at the U.S. Air Force Academy, the phenomenology and behavior of the aerodynamic plasma actuator, a dielectric barrier discharge plasma, was investigated. To provide insight into the phenomenology associated with the transfer of momentum to air by a plasma actuator, the velocity distributions upstream and downstream of a plasma actuator with an induced boundary layer were measured using freestream velocities of approximately 4.6 and 6.8 m/s for a range of frequencies (5-20 kHz) and voltages (5-10-kV amplitude). The body forces on the air were calculated using a control volume momentum balance. In a second experiment, time-averaged results were also obtained by measuring the reaction force using a pendulum. A third experiment uses an accelerometer to gain insight into the time-dependent forces or, more specifically, the direction of the forces. The results show that the body force acts within the first 4 mm above the surface of the actuator (within the boundary layer). For a constant peak-to-peak voltage, the body force is proportional to frequency, producing a constant impulse per cycle, and the energy dissipation per cycle and efficiency are independent of frequency. The time-dependent measurements support the theory that the body force of the actuator consists of one large push followed by one small pull during each cycle.

Direct Numerical Simulation Of Hypersonic Boundary-Layer Transition Over A Blunt Cone

Abstract: Direct numerical simulation of transition How over a blunt cone with a freestream Mach number of 6, Reynolds number of 10,000 based on the nose radius, and a 1-deg angle of attack is performed by using a seventh-order weighted essentially nonoscillatory scheme for the convection terms of the Navier-Stokes equations, together with an eighth-order central finite difference scheme for the viscous terms. The wall blow-and-suction perturbations, including random perturbation and multifrequency perturbation, are used to trigger the transition. The maximum amplitude of the wall-normal velocity disturbance is set to 1% of the freestream velocity. The obtained transition locations on the cone surface agree well with each other far both cases. Transition onset is located at about 500 times the nose radius in the leeward section and 750 times the nose radius in the windward section. The frequency spectrum of velocity and pressure fluctuations at different streamwise locations are analyzed and compared with the linear stability theory. The second-mode disturbance wave is deemed to be the dominating disturbance because the growth rate of the second mode is much higher than the first mode. The reason why transition in the leeward section occurs earlier than that in the windward section is analyzed. It is not because of higher local growth rate of disturbance waves in the leeward section, but because the growth start location of the dominating second-mode wave in the leeward section is much earlier than that in the windward section.

Heat Transfer With Very High Free-Stream Turbulence: Part I—Experimental Data

Abstract: Boundary layer heat transfer with very high freestream turbulence is investigated. The problem is studied experimentally by placing a constant-temperature heat transfer surface at various locations in the margin of a turbulent free jet and measuring both the surface heat transfer rate and the turbulence in the freestream. Freestream turbulent fluctuations 20 to 60 percent relative to the mean velocity augment heat transfer 1.8 to 4 times that which would be predicted locally using accepted correlations for turbulent boundary layers at the same Reynolds number. The correlations of Simonich and Bradshaw (1989), Pedisius et al. (1983), and Blair (1983) each fail to describe the present data. For flows over flat surfaces in air with very high freestream turbulence, greater than 0.2, u-prime determines h. A new heat transfer parameter, St-prime, characterizes turbulent boundary layer heat transfer with freestream turbulence on the domain 0-0.65 to within +/- 15 percent for high Reynolds number flows with uniform thermal boundary conditions.

Numerical Solution for Three-Dimensional Inviscid Supersonic Flow

Abstract: An exact finite-difference solution is developed for steady, inviscid, supersonic flow over smooth threedimensional bodies The method is applied to a blunt delta wing having straight leading edges and an elliptical cross section Results are presented for angles of attack up to 30° in both perfect gas and equilibrium air at typical freestream conditions for space shuttle orbiter re-entry The effects of angle of attack on shock layer structure, on surface pressure distributions, and on surface streamline patterns are investigated Marked variations are found in the structure of the entropy layer over different portions of the surface; the implications for boundary-layer heat-transfer analyses are discussed

Large-Eddy Simulation of Transition in a Separation Bubble

Abstract: In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low freestream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is refined to the extent that the simulation qualifies as a "coarse" direct numerical simulation. The simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.