Flow separation

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

On Turbulent Lubrication

Abstract: The paper concerns the hydrodynamic turbulent motion in the lubricant layer. Proceeding from the Reynolds equations and introducing the approximations currently used in lubrication problems, owing to the lubricant film thickness, the general motion equations for turbulent lubrication are written.Using the Prandtl mixing length hypothesis, exact and approximate solutions are obtained for the velocity distribution into the lubricant layer. The results are discussed by pointing out the pressure gradient and the Reynolds number influence on the velocity distributions, as well as the differences with respect to the laminar flow.In order to obtain simple formulae, the exact dependence of the rate of flow on the pressure gradient into a dimensionless form is replaced by a linear relation, the slope of which depends on the Reynolds number. This approximation allows the obtainment of the pressure differential equation under a simple form. The pressure equation is integrated in case of journal bearings, by assuming...

Flow over periodic hills: an experimental study

Abstract: Two-dimensional flow over periodically arranged hills was investigated experimentally in a water channel. Two-dimensional particle image velocimetry (PIV) and one-dimensional laser Doppler anemometry (LDA) measurements were undertaken at four Reynolds numbers ( $$\text{5,600} \le Re \le \text{37,000}$$ ). Two-dimensional PIV field measurements were thoroughly validated by means of point-by-point 1D LDA measurements at certain positions of the flow. A detailed study of the periodicity and the homogeneity was undertaken, which demonstrates that the flow can be regarded as two-dimensional and periodic for $$Re \ge \text{10,000}$$ . We found a decreasing reattachment length with increasing Reynolds number. This is connected to a higher momentum in the near-wall zone close to flow separation which comes from the velocity speed up above the obstacle. This leads to a velocity overshoot directly above the hill crest which increases with Reynolds number as the inner layer depth decreases. The flow speed up above that layer is independent of the Reynolds number which supports the assumption of inviscid flow disturbance in the outer layer usually made in asymptotic theory for flow over small hills.

A Critical Commentary on Mean Flow Data for Two-Dimensional Compressible Turbulent Boundary Layers,

Abstract: : This volume presents a discussion of mean flow profile data for compressible boundary layers. In AGARDograph 223, (Fernholz and Finley, 1977) the predecessor of this volume, data were presented for nominally two-dimensional flows for which mean flow profile data were available in tabular form. Data for 59 experimental boundary layer studies were given, with a brief introduction. Comment and discussion were reserved for this volume, AGARDograph 253. Suitable cross-references are given to relate the discussion in this volume to the earlier publication. The commentary in this volume includes discussion of the theroetical basis for interpretation of measurements, concepts from low-speed studies, interpretation of mean flow data with and without pressure gradient. The causes of normal pressure gradients are described, so as to allow an estimation of their magnitude, and their influence on boundary layer scale lengths. A third volume is planned in this series, which will include additional data, some of which is discussed here. This work was sponsored by the Fluid Dynamics Panel of AGARD. (Author)

Effect of concentrated wall suction on a turbulent boundary layer

Abstract: The effect of suction, applied through a short porous wall strip, on a low Reynolds number self‐preserving turbulent boundary layer has been quantified by measuring the local wall shear stress and the main Reynolds stresses downstream of the strip. When the suction rate is sufficiently high, pseudo‐relaminarization occurs almost immediately downstream of the strip. Farther downstream, transition occurs followed by a slow return to a fully turbulent self‐preserving state. During relaminarization, the measured skin friction coefficient cf falls below the level corresponding to the no suction value, reaching a minimum where transition begins. An empirical cf distribution is proposed that groups together results obtained at different streamwise stations and different suction rates. Of all the measured Reynolds stresses, the longitudinal turbulence intensity recovers relatively quickly from the change in boundary conditions while the wall‐normal turbulence intensity and the Reynolds shear stress are significan...

Compressible cavity flow oscillation due to shear layer instabilities and pressure feedback

Abstract: A computational analysis was performed on a compressible flow oscillation due to shear layer instabilities over a cavity and pressure feedback in a cavity of length-to-depth ratio 3 at Mach 1.5 and 2.5. The mass-averaged NavierStokes equations were solved. Turbulence closure was achieved using a k-u> model with compressibility corrections. Self-sustained oscillations were produced. Negative form drag coefficient was observed within an oscillatory cycle due to mass ejection from the cavity near the trailing edge and vortex production near the leading edge. The shock wave-expansion wave interaction patterns, modes of the oscillation, sound pressure level, and time-averaged surface pressure were compared with experimental results of previous investigations and good agreement was achieved, particularly the time-averaged pressure. The prediction showed a marked improvement over earlier analysis.