Freestream

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

Laminar Flow Control on a Swept Wing with Distributed Roughness

Abstract: The work cumulated in a series of laminar-turbulent transition flight-test experiments on a swept wing with the goal of validating the spanwise-periodic distributed roughness elements (DRE) technology in a Reynolds number range applicable to SensorCraft technology. Phase I of the program measured freestream turbulence levels that were nominally 0.05% to 0.06% of the freestream speed and thus established the suitability of the flight environment for the laminarization flights. Phase II of the program did the baseline transition measurements on the airfoil i.e. with and without DRE technology. The region of laminar flow was extended from 30% to 60% chord at a chord Reynolds number of Rec = 8 x10 6 and sweep angle, Λ = 30°. Establishing the origins of turbulent flow and transition from laminar to turbulent flow remains an important challenge of fluid mechanics. The common thread connecting aerodynamic applications is the fact that they deal with bounded shear flows (boundary layers) in open systems (with different upstream or initial amplitude conditions). It is well known that the stability, transition, and turbulent characteristics of bounded shear layers are fundamentally different from those of free shear layers. Likewise, open systems are fundamentally different from those of closed systems. The distinctions are trenchant and thus form separate areas of study. For the classic open system, no mathematical model exists that can predict the transition Reynolds number on a simple flat plate because the influences of freestream turbulence, sound, and surface roughness are incompletely understood. With the maturation of linear stability methods and the conclusions that breakdown mechanisms are initial-condition dependent, more emphasis is now placed on the understanding of the source of initial disturbances than on the details of the later stages of transition.

Experimental Investigation of Laminar Near Wakes Behind 20 Wedges M=6

Abstract: An experimental investigation at Mm = 6 has been conducted to determine mean-flow properties in near wakes behind several 20° included- angle wedges at zero angle of attack. One cold-wall (H = 0.3 in., TW/TQ = 0.19) and two adiabatic-wall (H = 0.15 in., H = 0.3 in.) configurations were tested. Freestream Reynolds numbers were varied from 0.5 X 10 5 to 2 X 105 per in. for each model. Flowfield mappings and flow-property profiles were obtained in the base region for the wedge of 0.3-in. base height with and without cooling by combining Pitot- pressure data with total temperature and mass flux results from hot-wire measurements. The variation of total pressure along streamlines was initially negligible during the shearlayer turning process. Downstream boundaries of these isentropic turns corresponded to viscous-layer edges that were positioned in the outer portions of the shear layers, indicating that wake shocks originated from within viscous regions of the shear layer.

Characteristics of shallow turbulent near wakes at low Reynolds numbers

Abstract: The present study deals with the noninvasive measurement of both velocity and concentration in the near region of shallow turbulent wakes using a laser-Doppler anemometer and a video-imaging technique. A 40 mm wide flat plate placed normal to the flow is used as the wake generator. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. Tests were conducted at two depths of flow (h =20 and 40 mm) and the boundary layer thickness of the approaching flow is comparable to the depth. The Reynolds number of the flow based on the approaching freestream boundary layer momentum thickness varies from 180 to 400, while, the Reynolds number based on the test body width was maintained nearly constant (4000). Measurements were carried out at three axial stations (2.5, 5, and 10 plate widths) downstream of the bluff body. At each axial station, the velocity measurements were carried at distances of h/4, h/2, and 3h/4 from the channel bottom and spanning the cross section of the wake. Appropriate length and velocity scales are identified to characterize the wake