The structure of the vorticity field in turbulent channel flow. II - Study of ensemble-averaged fields (original) (raw)
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Journal of Fluid Mechanics, 1986
An investigation into the existence of hairpin vortices in turbulent channel flow is conducted using a database generated by the large-eddy simulation technique. It is shown that away from the wall the distribution of the inclination angle of vorticity vector gains its maximum at about 45' to the wall. Two-point correlations of velocity and vorticity fluctuations strongly support a flow model consisting of vortical structures inclined at 45' to the wall. The instantaneous vorticity vectors plotted in planes inclined at 45' show that the flow contains an appreciable number of hairpins. Vortex lines are used to display the three-dimensional structure of hairpins, which are shown to be generated from deformation (or roll-up) of sheets of transverse vorticity .
Journal of Fluid Mechanics, 1985
An investigation into the existence of hairpin vortices in turbulent channel flow is conducted using a database generated by the large-eddy simulation technique. It is shown that away from the wall the distribution of the inclination angle of vorticity vector gains its maximum at about 45' to the wall. Two-point correlations of velocity and vorticity fluctuations strongly support a flow model consisting of vortical structures inclined at 45' to the wall. The instantaneous vorticity vectors plotted in planes inclined at 45' show that the flow contains an appreciable number of hairpins. Vortex lines are used to display the three-dimensional structure of hairpins, which are shown to be generated from deformation (or roll-up) of sheets of transverse vorticity .
Population trends of spanwise vortices in wall turbulence
Journal of Fluid Mechanics, 2006
The present effort documents the population trends of prograde and retrograde spanwise vortex cores in wall turbulence outside the buffer layer. Large ensembles of instantaneous velocity fields are acquired by particle-image velocimetry in the streamwise-wall-normal plane of both turbulent channel flow at Re τ ≡ u * δ/ν = 570, 1185 and 1760 and a zero-pressure-gradient turbulent boundary layer at Re τ = 1400, 2350 and 3450. Substantial numbers of prograde spanwise vortices are found to populate the inner boundary of the log layer of both flows and most of these vortices have structural signatures consistent with the heads of hairpin vortices. In contrast, retrograde vortices are most prominent at the outer edge of the log layer, often nesting near clusters of prograde vortices. Appropriate Reynolds-number scalings for outer-and inner-scaled population densities of prograde and retrograde vortices are determined. However, the Re τ = 570 channel-flow case deviates from these scalings, indicating that it suffers from low-Re effects. When the population densities are recast in terms of fractions of resolved prograde and retrograde spanwise vortices, similarity is observed for 100 < y + < 0.8δ + in channel flow and in both flows for 100 < y + < 0.3δ + over the Re τ range studied. The fraction of retrograde vortices increases slightly with Re τ beyond the log layer in both flows, suggesting that they may play an increasingly important role at higher Reynolds numbers. Finally, while the overall prograde and retrograde population trends of channel flow and the boundary layer show little difference for y < 0.45δ, the retrograde populations differ considerably beyond this point, highlighting the influence of the opposing wall in channel flow.
On hairpin vortex generation from near-wall streamwise vortices
Acta Mechanica Sinica, 2015
The generation of a hairpin vortex from near-wall streamwise vortices is studied via the direct numerical simulation (DNS) of the streak transient growth in the minimal channel flow at Re τ = 400. The streak profile is obtained by conditionally averaging the DNS data of the fully developed turbulent channel flow at the same Reynolds number. The near-wall streamwise vortices are produced by the transient growth of the streak which is initially subjected to the sinuous perturbation of the spanwise velocity. It is shown that the arch head of the hairpin vortex first grows from the downstream end of the stronger streamwise vortex and then connects with the weaker, opposite-signed streamwise vortex in their overlap region, forming a complete individual hairpin structure. The vorticity transport along the vortex lines indicates that the strength increase and the spatial expansion of the arch head are due to the stretching and the turning of the vorticity vector, respectively. The hairpin packets could be further produced from the generated individual hairpin vortex following the parent-offspring process.
Hairpin vortex organization in wall turbulencea
Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surfacemounted circular cylinder Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures ͑hairpins or packets͒ into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.
Hairpin vortices in turbulent boundary layers
Journal of Physics: Conference Series, 2014
The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parentoffspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Reτ = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Reτ < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.
A Computational Study of Turbulent Structure Formation
2007
Direct Numerical Simulation of channel flow was utilized to study the evolution of various vortex configurations presented as flow initial conditions. Simulations of longitudinally, laterally and cross-flow oriented vortices suggested that the predominant form of turbulent structure was the half hairpin vortex. This vortical structure was dominant in the simulations seen in this as well as other investigations. In all cases hairpin vortices quickly degenerated to half hairpin or inclined vortical structures. It is hypothesized that these structures function as the predominant momentum transfer mechanism within the boundary layer, entraining fluid into the vortex cores like miniature tornados and transporting this fluid to the top of the boundary layer while simultaneously dragging fluid viscously around the inclined core of the vortex causing mixing of low-speed and high-speed flows.
Vortex organization in the outer region of the turbulent boundary layer
Journal of Fluid Mechanics, 2000
The structure of energy-containing turbulence in the outer region of a zero-pressure- gradient boundary layer has been studied using particle image velocimetry (PIV) to measure the instantaneous velocity fields in a streamwise-wall-normal plane. Experiments performed at three Reynolds numbers in the range 930 < Reθ < 6845 show that the boundary layer is densely populated by velocity fields associated with hairpin vortices. (The term ‘hairpin’ is here taken to represent cane, hairpin, horseshoe, or omega-shaped vortices and deformed versions thereof, recognizing these structures are variations of a common basic flow structure at different stages of evolution and with varying size, age, aspect ratio, and symmetry.) The signature pattern of the hairpin consists of a spanwise vortex core located above a region of strong second-quadrant fluctuations (u < 0 and v > 0) that occur on a locus inclined at 30–60° to the wall.In the outer layer, hairpin vortices occur in streamwise-...
The structure of the vorticity field in homogeneous turbulent flows
Journal of Fluid Mechanics, 1987
The structure of the vorticity fields in homogeneous turbulent shear flow and various irrotational straining flows is examined using results from direct numerical simulations of the unsteady, incompressible Navier-Stokes equations with up to 128 x 128 x 128 grid points. In homogeneous shear flow, the distribution of the inclination angle of the vorticity vectors and contour plots of two-point correlations of both velocity and vorticity are consistent with the existence of persistent vortical structures inclined with respect to the flow direction. Early in the development of these shear flows, the angle of inclination at which most of these structures are found is near 45O; after the flow develops, this angle lies between 35'40". Instantaneous vorticity-vector and vortex-line plots confirm the presence of hairpin vortices in this flow at the two Reynolds numbers simulated. These vortices are formed by the roll-up of sheets of mean spanwise vorticity. The average hairpin leg spacing decreases with increasing Reynolds number but increases relative to the Taylor microscale for developed shear flows. Examination of irrotational axisymmetric contraction, axisymmetric expansion, and plane strain flows shows, as expected, that the vorticity tends to be aligned with the direction of positive strain. For example, the axisymmetric contraction flow is dominated by coherent longitudinal vortices. Without the presence of mean shear, however, hairpin structures do not develop. The simulations strongly indicate that the vorticity occurs in coherent filaments that are stretched and strengthened by the mean strain. When compressed, these filaments appear to buckle rather than to decream in strength.