Vorticity measurements in turbulent grid flows (original) (raw)
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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 .
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, 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 .
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.
An Experimental Study of Turbulent Mixing in Channel Flow Past a Grid
Processes, 2020
Grid turbulence is considered to be a canonical case of turbulent flow. In the presented paper, the flow structure is analyzed from the point of view of mixing properties, where vortical structures and their properties play a significant role. That is why the effect of various length-scales in turbulence is studied separately. The experimental study uses the Particle Image Velocimetry (PIV) method. The original method for spatial spectrum evaluation is applied. Results on vortex spatial spectrum and isotropy are presented. The scaling of turbulent kinetic energy (TKE) is measured; furthermore, the TKE is decomposed according to the length-scales of the fluctuations. By this method, we found that the decay of TKE associated with the smallest length-scales is more sensitive to the Reynolds number than that at larger length-scales. The TKE at the largest investigated length-scales decays more slowly. The turbulence decay-law is studied for various Reynolds numbers. The second and fourt...
Vorticity in the turbulent flow above variously rough surfaces
EPJ Web of Conferences
Highly turbulent flows above variously rough surfaces were investigated by means of Time-Resolved Particle Image Velocimetry in a wind tunnel. Proper Orthogonal Decomposition was applied to both velocity and vorticity data in order to detect dominant features in the flow based on turbulent kinetic energy and enstrophy, respectively. While both the shape and location of the POD patterns exhibited similarity with other studies, a systematic inconsistency in terms of contribution from the features to the enstrophy between the previously published papers and our results were found.
Vorticity fluxes: A tool for three-dimensional and secondary flows in turbulent shear flows
Journal of Fluids and Structures, 2019
In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers Re h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference w ′ ω ′ y + − v ′ ω ′ z + is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q 1 and Q 4 events, which essentially tilt momentum towards the peak.
Velocity–vorticity correlation structure in turbulent channel flow
A new statistical coherent structure (CS), the velocity-vorticity correlation structure (VVCS), using the two-point cross-correlation coefficient R ij of velocity and vorticity components, u i and ω j (i, j = 1, 2, 3), is proposed as a useful descriptor of CS. For turbulent channel flow with the wall-normal direction y, a VVCS study consists of using u i at a fixed reference location y r , and using |R ij (y r ; x, y, z)| R 0 to define a topologically invariant high-correlation region, called VVCS ij . The method is applied to direct numerical simulation (DNS) data, and it is shown that the VVCS ij qualitatively and quantitatively captures all known geometrical features of near-wall CS, including spanwise spacing, streamwise length and inclination angle of the quasi-streamwise vortices and streaks. A distinct feature of the VVCS is that its geometry continuously varies with y r . A topological change of VVCS 11 from quadrupole (for smaller y r ) to dipole (for larger y r ) occurs at y + r = 110, giving a geometrical interpretation of the multilayer nature of wall-bounded turbulent shear flows. In conclusion, the VVCS provides a new robust method to quantify CS in wall-bounded flows, and is particularly suitable for extracting statistical geometrical measures using two-point simultaneous data from hotwire, particle image velocimetry/laser Doppler anemometry measurements or DNS/large eddy simulation data.
Columnar vortices in isotropic turbulence
Meccanica, 1994
The structure of the intense vorticity regions is studied in numerically simulated homogeneous,isotropic, equilibrium turbulent flow fields at four different Reynolds numbers, in the range Re x = 35-170, and is found to be organized in coherent, cylindrical or ribbon-like, vortices ('worms'). At the Reynolds numbers studied, they are responsible for much of the extreme intermittent tails observed in the statistics of the velocity gradients, but their importance seems to decrease at higher Rex. Their radii scale with the Kolmogomv micmscale and their lengths with the integral scale of the flow, while their circulation increases monotonically with Rea. An explanation is offered for this latter scaling, based in the assumed presence of axial inertial waves along their cores, excited by a random background strain of the order of the root mean square vorticity. This explanation is consistent with the presence of comparable amounts of stretching and compression along the vortex cores. Sommario. La struttura di regioni ad intensa vorticit~t in campi di flusso turbolento omogenei, isotropi ed in equilibrio, simulati numericamente, viene studiata per quattro differenti numeri di Reynolds nell'intervallo Rex = 35 + 170, e si trova che tali regioni si organizzano in vortici coerenti, cilindrici o a forma di nastro ('vermi'). Con rifermento ai numeri di Reynolds studiati, si vede che taft vortici sono responsabili per gran parte delle code estreme ed intermittenti, osservate nelle statistiche dei gradienti di velocith, ma la loro importanza sembra decrescere a pifa alti Rex. I loro raggi scalano con la microscala di Kolmogorov e le loro lunghezze con la scala integrale del flusso, mentre la loro circolazione cresce monotonicamente con Rea. Per quest'ultimo riscalamento viene offerta una spiegazione basata sull'assunzione della presenza di onde inerziali assiali lungo i loro nuclei, eccitate da una deformazione di fondo casuale dell'ordine della radice quadrata della velocit~ media. Questa spiegazione consistente con la presenza di incrementi paragonabili di allungamenti e compressioni lungo i nuclei dei vortici.