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Vortex splitting and its consequences in the vortex street wake of cylinders at low Reynolds number Based on the observation of vortex splitting in the laminar wake of thin flat plates placed parallel to the flow, an investigation on the consequences of such events for the von Kármán vortex street in the wake of circular cylinders was carried out. It was found that a ‘‘design break line’’ of vortex axes can lead to the decoupling of a wake flow from the always present disturbances deriving from the ends. The decoupling gives rise to parallel vortex shedding of a slightly higher frequency, instead of the oblique or slanted vortex shedding at a lower frequency usually observed.
AIP Publishing - Tập 1 Số 2 - Trang 189-192 - 1989
Spectral features of wall pressure fluctuations beneath turbulent boundary layers Experimental measurements of the frequency spectra and frequency cross-spectra of the wall pressure fluctuations beneath a turbulent boundary layer were made in a low-noise flow facility. The data, taken over a range of flow speeds, clearly display a dimensionless frequency (ωδ/uτ=50) at which the spectra achieve a maximum and a low-frequency range with an approximately ω2 rolloff. The scaling laws for the low-, mid-, and high-frequency regions of the spectrum are established. The cross-spectral data, obtained over a range of streamwise separations (0.21≤ξ/δ≤16.4), allow for the computations of the decay Γ(ξ,ω) and convection velocity Uc(ξ,ω) of the wall pressure field. These data show the existence of two distinct wave number groups: a high wave number group that scales on the similarity variable k1ξ=ωξ/Uc(ξ,ω) associated with turbulent sources in the log region of the boundary layer, in which eddies decay in proportion to their size, and a low wave number group that defines the cutoff for the large-scale turbulence contributors in the outer region of the boundary layer. The convection velocity data support the conjecture that the major turbulent contributions to the low and high wave number groups come from the outer and inner layers, respectively. These new results, when examined collectively, firmly establish the spectral features of the wall pressure fluctuations, including the low-frequency range, which is highly sensitive to (passive) structures in the outer flow. The locations for the turbulent sources of the wall pressure field are proposed.
AIP Publishing - Tập 3 Số 10 - Trang 2410-2420 - 1991
Local energy transfer and nonlocal interactions in homogeneous, isotropic turbulence Detailed computations were made of energy transfer among the scales of motion in incompressible turbulent fields at low Reynolds numbers generated by direct numerical simulations. It was observed that although the transfer resulted from triad interactions that were nonlocal in k space, the energy always transferred locally. The energy transfer calculated from the eddy-damped quasinormal Markovian (EDQNM) theory of turbulence at low Reynolds numbers is in excellent agreement with the results of the numerical simulations. At high Reynolds numbers the EDQNM theory predicts the same transfer mechanism in the inertial range that is observed at low Reynolds numbers, i.e., predominantly local transfer caused by nonlocal triads. The weaker, nonlocal energy transfer is from large to small scales at high Reynolds numbers and from small to large scales at low Reynolds numbers.
AIP Publishing - Tập 2 Số 3 - Trang 413-426 - 1990
Subgrid-scale backscatter in turbulent and transitional flows Most subgrid-scale (SGS) models for large-eddy simulations (LES) are absolutely dissipative (that is, they remove energy from the large scales at each point in the physical space). The actual SGS stresses, however, may transfer energy to the large scales (backscatter) at a given location. Recent work on the LES of transitional flows [Piomelli et al., Phys. Fluids A 2, 257 (1990)] has shown that failure to account for this phenomenon can cause inaccurate prediction of the growth of the perturbations. Direct numerical simulations of transitional and turbulent channel flow and compressible isotropic turbulence are used to study the backscatter phenomenon. In all flows considered roughly 50% of the grid points were experiencing backscatter when a Fourier cutoff filter was used. The backscatter fraction was less with a Gaussian filter, and intermediate with a box filter in physical space. Moreover, the backscatter and forward scatter contributions to the SGS dissipation were comparable, and each was often much larger than the total SGS dissipation. The SGS dissipation (normalized by total dissipation) increased with filter width almost independently of filter type. The amount of backscatter showed an increasing trend with Reynolds number. In the near-wall region of the channel, events characterized by strong Reynolds shear stress correlated fairly well with areas of high SGS dissipation (both forward and backward). In compressible isotropic turbulence similar results were obtained, independent of fluctuation Mach number.
AIP Publishing - Tập 3 Số 7 - Trang 1766-1771 - 1991
A dynamic mixed subgrid-scale model and its application to turbulent recirculating flows The dynamic subgrid-scale eddy viscosity model of Germano et al. [Phys. Fluids A 3, 1760 (1991)] (DSM) is modified by employing the mixed model of Bardina et al. [Ph.D dissertation, Stanford University (1983)] as the base model. The new dynamic mixed model explicitly calculates the modified Leonard term and only models the cross term and the SGS Reynolds stress. It retains the favorable features of DSM and, at the same time, does not require that the principal axes of the stress tensor be aligned with those of the strain rate tensor. The model coefficient is computed using local flow variables. The new model is incorporated in a finite-volume solution method and large-eddy simulations of flows in a lid-driven cavity at Reynolds numbers of 3200, 7500, and 10 000 show excellent agreement with the experimental data. Better agreement is achieved by using the new model compared to the DSM. The magnitude of the dynamically computed model coefficient of the new model is significantly smaller than that from DSM.
AIP Publishing - Tập 5 Số 12 - Trang 3186-3196 - 1993
On streamwise vortices in turbulent wakes of cylinders Flow-visualization methods were used to explore the structure of streamwise vortices and their interactions with the von Kármán vortices in the immediate wakes of cylinders. The experiments were conducted in a water tunnel at Reynolds numbers from 330–21 000 based on cylinder diameter. Over this entire range of Reynolds number, pairs of counter-rotating streamwise vortices were observed immediately behind the cylinders, with a mean spanwise spacing of approximately one pair per diameter. The streamwise vortices significantly distorted the von Kármán vortices, but only on their upstream-facing sides. The cylinder near-surface-flow topology was found to include a secondary separation line containing ‘‘dominant-foci structures’’ whose spanwise locations correlated with those of the streamwise vortices.
AIP Publishing - Tập 5 Số 2 - Trang 387-392 - 1993
Large eddy simulation of turbulence-driven secondary flow in a square duct The fully developed turbulent flow in a straight duct of square cross section has been simulated using the large eddy simulation (LES) technique. A mixed spectral-finite difference method has been used in conjunction with the Smagorinsky eddy-viscosity model for the subgrid scales. The simulation was performed for a Reynolds number of 360 based on friction velocity (5810 based on bulk velocity) and duct width. The simulation correctly predicted the existence of secondary flows and their effects on the mean flow and turbulence statistics. The results are in good qualitative agreement with the experimental data available at much higher Reynolds numbers. It is observed that both the Reynolds normal and shear stresses equally contribute to the production of mean streamwise vorticity.
AIP Publishing - Tập 3 Số 11 - Trang 2734-2745 - 1991
Global stability of a lid-driven cavity with throughflow: Flow visualization studies Flow visualization studies of a lid-driven cavity (LDC) with a small amount of throughflow reveal multiple steady states at low cavity Reynolds numbers. These results show that the well-known LDC flow, which consists of a primary eddy and secondary corner eddies, is only locally stable, becomes globally unstable, and competes with at least three other steady states before being replaced by a time-periodic flow. The small amount of throughflow present in this system seems to have no qualitative effect on the fluid flow characteristics. These observations suggest that multiple stable steady states may also exist in closed LDC’s. Since stability properties of the closed LDC flows are virtually unexplored, we interpret our flow visualization results by first proposing an expected behavior of an idealized (free-slip end walls) LDC and then treating the problem at hand as a perturbation of the ideal case. The results also suggest that there are nonunique and competing sequences of transitions that lead the flow in a LDC from laminar steady state toward turbulence.
AIP Publishing - Tập 3 Số 9 - Trang 2081-2091 - 1991
Transition to unsteady nonperiodic state in a through-flow lid-driven cavity A flush-mounted hot-film anemometer is used to investigate the transition to unsteady flow in a lid-driven cavity with a small amount of through flow. The quantitative measurements with the hot-film probe confirm the transition from steady to time-periodic state from previous flow visualization studies and show a second transition in the primary state from time-periodic to quasiperiodic flow and a transition to a state with a broad band of frequencies typical of a chaotic state. From these results, it is concluded that the transition to unsteady nonperiodic state in this system is through a torus bifurcation.
AIP Publishing - Tập 4 Số 10 - Trang 2316-2319 - 1992
Compressibility effects in free shear layers High Reynolds number compressible free shear layers were studied experimentally to explore the effects of compressibility on the turbulence field. Previous preliminary results reported by the authors showed that the level and the lateral extent of turbulence fluctuations are reduced as the compressibility, which is characterized by a convective Mach number, is increased. The two convective Mach numbers used in the previous study were relatively close, Mc=0.51 and 0.64, and as a result the conclusions were not concrete. The present results with Mc=0.86 strongly support the earlier results, showing even higher reductions in the level and the lateral extent of Reynolds stresses. The higher-order moments of turbulence fluctuations such as skewness and flatness are reported, which show that the intermittency resulting from the excursion of large-scale structures into the free streams at the edge of shear layers was significantly reduced (both in the level and the extent) because of increased Mc. In the developing region of shear layers, development of mean flow and turbulence fluctuation profiles are reported that have similar trends seen in incompressible shear layers.
AIP Publishing - Tập 2 Số 7 - Trang 1231-1240 - 1990
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