Coherent instability in wall-bounded shear

The mechanism underlying the coherent hairpin process in wall-bounded shear flows is studied. An algorithm for the identification and analysis of flow structures based on morphological operations is presented. The method distils the topology of the flow field into a discrete data set and enables the time-resolved sampling of coherent flow processes across multiple scales. Application to direct simulation data of transitional and turbulent boundary layers at moderate Reynolds number sheds light on the flow physics of the hairpin process. The analysis links the hairpin to an exponential instability which is amplified in the flow distorted by a negative perturbation in the streamwise velocity component. Linear analyses substantiate the connection to an inviscid instability mechanism of varicose type. The formation of packets of hairpins is related to a self-similar process which originates from a single patch of low-speed fluid and describes a recurrence of the dynamics that leads to the formation of an individual hairpin. Analysis of the evolution of several thousand turbulent hairpins provides a statistical characterization of the principal dynamics and yields a time-resolved average of the hairpin process. Comparisons with the transitional hairpin show qualitatively consistent trends and thus support earlier hypotheses of their equivalence. In terms of the causality of events, the results suggest that the hairpin is a manifestation of the varicose instability and as such is a consequence rather than a cause of the primary perturbations of the flow.

Similarity of the streamwise velocity component in very-rough-wall channel flow

The streamwise velocity component is studied in fully developed turbulent channel flow for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a highly non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (±) the standard deviation) are, respectively, about 76(±42) and 145(±150) wall units. The flow is shown to be two-dimensional and fully developed up to the fourth-order moment of velocity. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0.04≤y/h≤0.06, but the profile over the mesh surface does not, even though the mean velocity deficit and higher moments (up to the fourth order) all exhibit outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. The wake strength is shown to increase slightly over the grit surface but decrease over the mesh surface. The latter result is contrary to recent measurements in rough-wall boundary layers. Single- and two-point velocity correlations reveal the presence of large-scale streamwise structures with circulation in the plane orthogonal to the mean velocity. Spanwise correlation length scales are significantly larger than corresponding ones for both internal and external smooth-wall flows.

Large-eddy simulation of a subsonic cavity flow including asymmetric three-dimensional effects

Large-eddy simulations of a cavity configuration yielding a mean flow that exhibits spanwise asymmetry are carried out. Results from the computations reveal that the asymmetry is due to a bifurcation of the whole flow field inside the cavity. It is demonstrated that the bifurcation originates in an inviscid confinement effect induced by the lateral walls. The branch of the bifurcation can be selected by slightly altering the incoming mean flow. Further investigations show that underlying steady spanwise modulations of velocity are amplified under the influence of the lateral walls. The modulation of the streamwise velocity component has the largest energy content and its dominant wavelength contaminates both vertical velocity and pressure. Complementary to these linear interactions, nonlinear energy transfers from streamwise velocity to pressure are also found. A transient analysis highlights the stiff transition from a symmetrical two-structure non-bifurcated flow to a stable unsymmetrical one-and-a-half-structure bifurcated flow. The switch to the bifurcated flow induces an alteration of the Rossiter aero–acoustic loop yielding a change in the dominant Rossiter mode and the appearance of a nonlinear harmonic of the first mode.

Effects of Inclination Angle of Ribs on the Flow Behavior in Rectangular Ducts

The flow behavior in rib-roughened ducts is influenced by the inclination of ribs and the effect is investigated in the present study by Particle Image Velocimetry (PIV). The local flow structures between two adjacent ribs were measured. The Reynolds number was fixed at 5800. The flow field description was based on the PIV results in planes both parallel and perpendicular to the ribbed walls at various locations. The rib angle to the main flow direction was varied as 30 deg, 45 deg, 60 deg and 90 deg. The ribs induce three dimensional flow fields. The flow separation and reattachment between adjacent ribs are clearly observed. In addition, the inclined ribs are found to alter the spanwise distribution of the streamwise velocity component. The streamwise velocity component has its highest values at the upstream end of the ribs, and decreases continuously to its lowest values at the downstream end. Strong secondary flow motion occurs over the entire duct cross section for the inclined ribs. The flow structures between two consecutive ribs show that the fluid flows along the ribs from one end of the ribs to the other end, and then turns back at the transverse center. Downwash and upwash flows are observed at the upstream end and downstream end of the ribs, respectively.

Reattachment of Three-Dimensional Flow Adjacent to Backward-Facing Step

Numerical simulations for incompressible three-dimensional laminar forced convection flow adjacent to backward-facing step in rectangular duct are performed to examine the reattachment region of the separated flow on the stepped wall. The feasibility of utilizing the two-dimensional flow definition and the limiting streamline definition for identifying the reattachment line/region was examined. The downwash and the “jet-like” flow that develops near the sidewall creates significant spanwise flow adjacent to the stepped wall, making it difficult to identify a reattachment line/region both numerically and experimentally. The use of the line/region that identifies the location on a plane adjacent to the stepped wall where the gradient of the mean streamwise velocity component is zero ∂u/∂y|y=0=0 is recommended for code and apparatus validation of three-dimensional separated flow.

Application of the POD to the Similarity Region of an Axisymmetric Turbulent Jet

Multipoint instantaneous measurements of the streamwise velocity component were obtained in the far field region of an axisymmetric turbulent jet from 20 to 69 diameters downstream for jet exit Reynolds numbers ranging from 40,000 to 84,700. The Proper Orthogonal Decomposition (POD) was then applied to a double Fourier transform in time and azimuthal direction of the two-point velocity correlation tensor. The first eigenspectrum, which contains more than 62% of the resolved streamwise energy, has two peaks: one at azimuthal mode-2 at near zero frequency, and another at mode-1 at a local Strouhal number (fx/Uc) of approximately 1. When checked against two-point statistics similarity analysis from Ewing (1995) extended to the POD, the results compare favorably and also give some hints into the resolution necessary to cover the field.

Analyse du sillage proche d'une sphère par une méthode de reconnaissance de formes

A pattern recognition technique has been applied to analyse the near-wake of a sphere for a Reynolds (Re) criteria of 12 900 and 25 900. Measurements of the main components of the down-stream turbulent flow of the dynamic field of a sphere allowed to confirm literature data. Fluctuations of the streamwise velocity component were measured simultaneously by eight hot-wire anemometers arranged in a circular pattern with the radius corresponding to the position of the extrema of the Reynolds stress [Formula: see text]. For streamwise locations xld = 18 (d = 4 cm) and 36 (d = 2 cm) (d, diameter of the sphere; x, longitudinal position relative to the centre of the sphere), in the near-wake of the sphere, structures occur periodically. The Strouhal number characterizing this periodic activity is 0.18. These structures, which also exhibit some spatial periodicity, seem to constitute the counterpart of the Kármán vortex path detected in the wake of a cylinder perpendicular to the main flow. In the wake of a sphere, these structures might appear in the three-dimensional form of continuous crenellated rings for xld = 18, or interrupted in one or several points of their circumference for xld = 36.

Effects of Waves on the Wake of a Surface-Piercing Flat Plate: Experiment and Theory

Results are presented from a towing-tank experiment conducted in order to document the effects of waves on the wake of a surface-piercing body. A unique, simple model geometry is utilized which makes it possible to isolate and identify the most important features of the wave-induced effects. Measurements were made for three wave-steepness conditions: zero, medium, and large. The effects of the waves for the latter two conditions are shown to be significant. In particular, the variations of the external-flow pressure gradients cause acceleration and deceleration phases of the streamwise velocity component and alternating direction of the crossflow, which results in large oscillations of the displacement thickness and wake centerplane velocities as compared to the zero-steepness condition. Remarkably, the wake displays a greater response, that is, a bias with regard to favorable as compared to adverse pressure gradients. The measurements are compared and close agreement is demonstrated with results from Reynolds-averaged Navier-Stokes calculations. Additional calculations are presented, including laminar-flow results, which aid in explicating the characteristics of the near and intermediate wake, the periodic nature of the far wake, and wave-induced separation. Previously, experimental and computational results were presented for the boundary-layer region.

The Flow Over Two-Dimensional Surface-Mounted Obstacles at Low Reynolds Numbers

The flow over a fence and a block mounted in a fully developed channel flow is experimentally investigated as a function of the Reynolds number, blockage ratio and length-to-height ratio using a laser-Doppler-anemometer. The information obtained includes the location and size of the primary and secondary recirculation zones, and profiles of the mean streamwise velocity component. The experiments were carried out in a channel for a Reynolds number in the range 150 < ReH < 4500. Comparisons are drawn between the obstacle flow and the backward-facing step flow.