Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.

Study of turbulent structure with hot wires smaller than the viscous length

Abstract: The small-scale structure of the streamwise velocity fluctuations in the wall region of a turbulent boundary layer is examined in a new wind-tunnel facility using hot-wires smaller than any previously constructed (typical dimensions: l = 25 μm, d = 0.5 μm). In the boundary layer in which the measurements were made, the ratio of the hot-wire length to the viscous length is 0.3. The turbulent intensity measured with the small hot wires is larger than that measured with longer wires owing to the better spatial resolution of the small wires. The velocity fluctuations measured by the small hot wires are also analysed to determine the burst frequency at two Reynolds numbers and at various distances from the wall. The dimensionless burst frequency does not depend on the Reynolds number when scaled with wall parameters. However, it increases with Reynolds number when scaled with outer variables. Velocity fluctuations measured by two hot wires, less than two viscous lengths apart, are analysed to reveal the small-scale features present during a burst and in the absence of a burst. The main conclusions are: (1) intermittent small-scale shear layers occur most frequently when bursts are present, less frequently just after a burst, and even less frequently just before a burst; and (2) on occasion the velocity gradient of the small-scale shear layers is as large as the mean-velocity gradient at the wall.

The effects of turbulence on the mean flow past two-dimensional rectangular cylinders

Abstract: There are two main effects of turbulence on the mean flow past rectangular cylinders, just as found earlier for square rods. Small-scale turbulence increases the growth rate of the separated shear layers through increased mixing. Large-scale turbulence weakens regular vortex shedding by reducing spanwise correlation. The consequences of increased mixing and weakened regular vortex shedding depend on the depth-to-height ratio of a rectangular cylinder. In particular, the mean base pressure of a rectangular cylinder can be varied significantly by both the scale and intensity of turbulence.

Influence of a strong adverse pressure gradient on the turbulent structure in a boundary layer

Abstract: A comparison between the turbulent structures found in a zero pressure gradient boundary layer and a boundary layer subjected to a strong adverse pressure gradient is presented. The pressure gradient reverses the direction of the dominant turbulent diffusion, resulting in considerable turbulent transport towards the wall. Two‐point space–time correlations and the invariants show that this reduces the anisotropy in the near wall region and indicate an important reflection of the turbulent motion from the wall back into the outer layer. This is verified by a quadrant analysis [Lu and Willmarth, J. Fluid Mech. 60, 481 (1973)] which demonstrates that the strong events near the wall are totally dominated by motions in the first and fourth quadrants.

Flow dynamics past a simplified wing body junction

Abstract: Junction flows may suffer from secondary flows such as horseshoe vortices and corner separations that can dramatically impair the performances of aircrafts. The present article brings into focus the unsteady aspects of the flow at the intersection of a wing and a flat plate. The simplified junction flow test case is designed according to a literature review to favor the onset of a corner separation. The salient statistical and fluctuating properties of the flow are scrutinized using large eddy simulation and wind tunnel tests, which are carried out at a Reynolds number based on the wing chord c and the free stream velocity U∞ of Rec=2.8×105. As the incoming boundary layer at Reθ=2100 (θ being the boundary layer momentum thickness one-half chord upstream the junction) experiences the adverse pressure gradient created by the wing, a three dimensional separation occurs at the nose of the junction leading to the formation of a horseshoe vortex. The low frequency, large scale bimodal behavior of the horseshoe vortex at the nose of the junction is characterized by multiple frequencies within f.δ/U∞=[0.05−0.1] (where δ is the boundary layer thickness one-half chord upstream the wing). Downstream of the bimodal region, the meandering of the core of the horseshoe vortex legs in the crossflow planes is scrutinized. It is found that the horseshoe vortex oscillates around a mean location over an area covering almost 10% of the wing chord in the tranverse plane at the trailing edge at normalized frequencies around f.δ/U∞=0.2–0.3. This so-called meandering is found to be part of a global dynamics of the horseshoe vortex initiated by the bimodal behavior. Within the corner, no separation is observed and it is shown that a high level of anisotropy (according to Lumley’s formalism) is reached at the intersection of the wing and the flat plate, which makes the investigated test case challenging for numerical methods. The conditions of apparition of a corner separation are eventually discussed and we assume that the vicinity of the horseshoe vortex suction side leg might prevent the corner separation. It is also anticipated that higher Reynolds number junction flows are more likely to suffer from such separations.

Investigation of Supersonic Turbulent Mixing Layer with Zero Pressure Gradient

Abstract: The effect of compressibility on the mixing layer was investigated at Mach number 2.47. Pitot pressure, static pressure, and hot-wire surveys were conducted to investigate the mean flow and the fluctuation quantities. Similarities between supersonic and incompressible mixing layers were observed in normalized velocity profile, normalized power spectral density distribution, and convection velocity distribution. Spreading rate, normalized shear stress, and velocity fluctuation were found to be appreciably smaller than the respective incompressible results; e.g., the momentum thickness growth rates are 0.0073 and 0.035 for supersonic and incompressible flows, respectively. The difference between free and wall-bounded mixing layers is discussed. Development of turbulence structure of mixing layer with increasing Reynolds number was also investigated.