Effect Of Large-scale Motions Research Articles

Spatio-temporal dynamics of flow separation induced by a forward-facing step submerged in a thick turbulent boundary layer

The effects of large-scale motion (LSM) on the spatio-temporal dynamics of separated shear layers induced by a forward-facing step submerged in a thick turbulent boundary layer (TBL) are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and step height was 13 200. The oncoming TBL was developed over a cube-roughened surface and the thickness was 6.5 times the step height. The step height was chosen to coincide with the elevation where the dominant frequency of streamwise fluctuating velocity in the TBL occurred. At this elevation, the local turbulence intensity was 14.5 %. Distinct regions of elevated Reynolds stresses were observed upstream and downstream of the leading edge of the step. The unsteady dynamics of the separation bubbles upstream and downstream of the step was investigated using the reverse flow area. Both separation bubbles exhibit low-frequency flapping motion, and the dominant frequency of the downstream separation bubble is identical to the dominant frequency of the streamwise fluctuating velocity in the oncoming TBL at the step height. As the low-velocity region of LSM passes over the step, the downstream separation bubble is enlarged and subsequently undergoes a high-frequency oscillation. Turbulence motions were partitioned into low-, medium- and high-frequency regimes based on spectral analysis of the Reynolds stresses. The contributions from these partitioned turbulence motions are used to elucidate the effects of LSM on the elevated Reynolds stresses in the shear layers upstream and downstream of the step.

On the large- and small-scale motions in a separated, turbulent-boundary-layer flow

ABSTRACTAdverse-pressure-gradient turbulent boundary layer flow was inspected at Reynolds number based on momentum thickness, , using particle image velocimetry in a refractive-index-matching flume. Proper orthogonal decomposition was used to quantify the effect of large-scale motions on the Reynolds stresses at the onset of separation and within the separated flow. Results show that approximately of the Reynolds shear stress, , is due to large-scale motions containing of the turbulence kinetic energy at the tested Reynolds number. The decomposed velocity field revealed that only the first of the modes is sufficient to recover of the turbulence kinetic energy. In this partition, the large-scale motion contribution to the streamwise component of the Reynolds normal stress, , is about and continues to grow with flow separation. In addition, the large- and small-scale motions equally contributed to the vertical component of the Reynolds normal stress, , and the contribution of the large-scale motions increased as the flow separated. Overall, results emphasise the significant impact of the large-scale motions on the Reynolds stresses in the separated flow, which may impact flow control strategies.

Signature of large-scale motions on turbulent/non-turbulent interface in boundary layers

The effect of large-scale motions (LSMs) on the turbulent/non-turbulent (T/NT) interface is examined in a turbulent boundary layer. Using flow fields from direct numerical simulation, the shape of the interface and near-interface statistics are evaluated conditional on the position of the LSM. The T/NT interface is identified using the vorticity magnitude and a streak detection algorithm is adopted to identify and track the LSMs. Two-point correlation and spectral analysis of variations in the interface height show that the spatial undulation of the interface is longer in streamwise wavelength than the boundary-layer thickness, and grows with the Reynolds number in a similar manner to the LSMs. The average variation in the interface height was evaluated conditional on the position of the LSMs. The result provides statistical evidence that the interface is locally modulated by the LSMs in both the streamwise and spanwise directions. The modulation is different when the coherent structure is high- versus low-speed motion: high-speed structures lead to a wedge-shaped deformation of the T/NT interface, which causes an anti-correlation between the angles of the interface and the internal shear layer. On the other hand, low-speed structures are correlated with crests in the interface. Finally, the sudden changes in turbulence statistics across the interface are in line with the changes in the population of low-speed structures, which consist of slower mean streamwise velocity and stronger turbulence than the high-speed counterparts.

Predicting the response of small-scale near-wall turbulence to large-scale outer motions

A phenomenological model is provided, based on post-processing Direct Numerical Simulation (DNS) data at Reτ = 1020, which permits the near-wall-turbulence statistics to be predicted from a “universal signal,” free from the effects of large-scale motions, in combination with information on the large-scale motions in the outer log-law region. The separation of large-scale and small-scale motions is effected, unusually, by means of the “Empirical Mode Decomposition” method, without explicit wavelength cutoffs. The model first yields the universal field by removing, from a full-volume turbulence field at an arbitrary time level, the effects of large-scale convective displacements (footprints), the modulation of the small-scale motions, caused by the large-scale motions, and distortions arising from sweep-induced splatting. In contrast to other modelling efforts, the present framework extends to all three velocity components, as is demonstrated by reference to joint (u − v) and (u − w) probability-density functions (PDFs). The model is then successfully used to reconstitute the full near-wall statistics by combining the universal field with the outer large-scale motions at any time level other than that for which the universal field was determined.

The IR-resummed Effective Field Theory of Large Scale Structures

We present a new method to resum the effect of large scale motions in the Effective Field Theory of Large Scale Structures. Because the linear power spectrum in ΛCDM is not scale free the effects of the large scale flows are enhanced. Although previous EFT calculations of the equal-time density power spectrum at one and two loops showed a remarkable agreement with numerical results, they also showed a 2% residual which appeared related to the BAO oscillations. We show that this was indeed the case, explain the physical origin and show how a Lagrangian based calculation removes this differences. We propose a simple method to upgrade existing Eulerian calculations to effectively make them Lagrangian and compare the new results with existing fits to numerical simulations. Our new two-loop results agrees with numerical results up to k∼ 0.6 h Mpc−1 to within 1% with no oscillatory residuals. We also compute power spectra involving momentum which is significantly more affected by the large scale flows. We show how keeping track of these velocities significantly enhances the UV reach of the momentum power spectrum in addition to removing the BAO related residuals. We compute predictions for the real space correlation function around the BAO scale and investigate its sensitivity to the EFT parameters and the details of the resummation technique.

The effects of large-scale motion on the regeneration of turbulent dynamo

The kinematic turbulent dynamo equations are studied in the presence of a large-scale velocity field. The two length-scales approach is employed and solutions of the equations are found in the limit of small bulk motion and shear, and for large Reynolds numberR m . The regeneration term is calculated up to second-order in 1/R m using cyclonic convective turbulent velocity field.