Ram Balachandar

The present study investigates the influence of geometric confinement on the development and organization of moderately (D/d = 5) and severely (D/d = 3) confined axisymmetric turbulent jets using large eddy simulation, where D is the chamber diameter and d is the nozzle exit diameter. The Reynolds number, based on nozzle exit conditions (Uj, d), is 30 000. To quantify deviations from canonical behavior, the confined cases are compared with a free jet at the same Reynolds number. Confinement is shown to fundamentally alter jet development by suppressing radial expansion, accelerating centerline decay, and promoting confinement-controlled recirculation and momentum redistribution. Azimuthal analysis of the wall shear stress in the post-attachment region reveals that both confined cases are dominated by a first azimuthal mode, indicating a preferred large-scale asymmetry. In the moderately confined case, the first mode manifests as a sustained wall attachment and slow circumferential wandering in the jet–wall interaction region, whereas in the severely confined case, it appears only as weak and intermittent modulation without sustained large-scale organization. Further distinction between the confined and free jets is obtained through analysis of an enstrophy-based identification of the jet-background interface. The free jet exhibits a classical turbulent/non-turbulent interface, whereas under stronger confinement, the interface evolves into a marker of localized, high-intensity turbulent activity embedded within a fully turbulent environment.

The influence of helical strakes on the near-bed junction flow and wake dynamics of a wall-mounted circular cylinder is investigated using improved delayed detached eddy simulation. At a Reynolds number of 7500, the characteristics of flow past a helical straked cylinder with a strake height of 0.2d and a pitch of 10d are examined using a plain cylinder as the reference configuration, where d is the cylinder diameter. The cylinder is vertically positioned at the domain's base, with one of the strakes aligned at 0° to the incoming flow. Results show that the strakes suppress the development of the horseshoe vortex by modifying the near-wall junction flow and promoting asymmetric, three-dimensional vortex structures. The wake becomes increasingly incoherent due to the interaction of the vortical structures from multiple strake-induced separating shear layers. Surface pressure distribution indicates localized flow separation along the edges of the strakes, while wall shear stress is redistributed into localized regions of high and low intensity due to the guided motion of the flow by the strakes. Spectral analysis shows that the strakes shift the spectral energy to a broader range of frequencies. Finally, spectral proper orthogonal decomposition reveals a redistribution of modal energy from the low-order to the higher-order modes, highlighting the role of strakes in disrupting and fragmenting the large-scale coherent wake structures.

Block arrays are widely used as laboratory roughness elements, yet how block size and arrangement affect downstream turbulence and momentum transport remains poorly understood. Particle image velocimetry measurements downstream of five beds in subcritical open-channel flow (Re=3.51×104 and Fr=0.08) compare a no-block (NB) reference with four near-cubic block arrays: small aligned (SA), small staggered (SS), large aligned (LA), and large staggered (LS). The approach flow is conditioned by a square-bar turbulence-generating grid, so the study characterizes how a finite block array reorganizes controlled, grid-generated turbulence. All arrays redistribute momentum vertically, producing near-block velocity deficits with faster flow above; block size dominates, larger blocks generating deeper, more intense deficits. Unlike NB, all rough cases sustain predominantly positive Reynolds shear stress across the domain, indicating continuous downward momentum transport attributed to block-generated shear layers and wakes. Turbulent kinetic energy production increases with block size, by 9.0% (SA), 28.6% (SS), 322.2% (LS), and 375.9% (LA) relative to NB. Integral length scales decrease in all rough cases, and quadrant decomposition confirms a shift toward ejection-sweep (Q2–Q4) dominance with geometry-dependent vertical penetration. Sweeps are more frequent near the blocks, but ejections are more energetic and dominate the Reynolds shear stress, even under hole filtering. Within the present conditioned-inflow configuration, block size governs the magnitude and vertical extent of roughness-induced turbulence, while arrangement modulates its spatial organization, guiding block-array design in laboratory open-channel flows.

Reconstruction of missing regions in velocity fields remains a key challenge in experimental fluid mechanics, particularly in complex turbulent flows where measurements are limited by spatial resolution and optical accessibility. This study investigates a transfer learning framework that leverages large-eddy simulation (LES) and particle image velocimetry (PIV) data to reconstruct incomplete wake velocity fields under identical flow conditions. The flow considered is a horizontal circular cylinder at a bed clearance of 0.5 diameters, generating a strongly inhomogeneous wake with pronounced multi-scale interactions. A linear extended proper orthogonal decomposition model is compared with a non-linear U-Net architecture under controlled masking conditions. A transfer learning strategy is introduced, where the network is pretrained on time-resolved LES data and fine-tuned using non-time-resolved PIV measurements. Results show the non-linear model significantly outperforms the linear baseline, particularly in capturing the spatial flow structures and velocity correlations. Transfer learning model yields consistent, though modest, improvements in reconstruction accuracy and improved agreement with velocity correlations and Reynolds shear stress, although the gains are less consistent than those observed for pointwise reconstruction. However, PIV-based models underpredict high-wavenumber energy and fail to fully recover vorticity intermittency, highlighting limitations associated with the reduced small-scale information content and effective spatial resolution of the experimental dataset. These results suggest that computational fluid dynamics-informed pretraining may improve reconstruction fidelity while emphasizing the critical role of data quality in resolving multi-scale turbulent dynamics.

The present study demonstrates the absence of self-similarity formation in the distribution of mean concentration fields. An examination in both the physical and spectral spaces revealed that the convection−diffusion mechanism and the development of short- and long-range dispersion play a minimal role in influencing the nature of self-similarity. The friction concentration is considered as the key factor in determining the emergence of self-similarity. It is observed that as the rate of change of friction concentration fields tends to zero, the mean concentration fields stabilize into self-similar patterns more effectively.

This study employs improved delayed detached eddy simulation to investigate coherent flow structures and bed shear stresses around unsubmerged straight and T-head spur dikes. Vortex topology identified using the λ2 criterion reveals a strong horseshoe-like vortex (HSV) for the straight spur dike, whereas the T-head spur dike produces a weakened HSV and a dominant upstream circulation that suppresses downflow and near-bed vorticity production. Probability density function (PDF) and joint PDFs of velocity fluctuations reveal three distinct instantaneous flow states for the HSV: back-flow, zero-flow, and intermediate-flow, for both configurations. Proper orthogonal decomposition (POD) shows that for the straight dike, the first mode captures the mean-flow upwash and downwash induced by the HSV, while higher modes represent oscillations between the three flow states. In contrast, for the T-head dike, all dominant POD modes are associated with HSV oscillations. Analysis of the POD temporal coefficients shows that the zero-flow condition is more prevalent for the straight dike, whereas the back-flow condition dominates for the T-head dike. In the wake, both configurations generate shear-layer vortices that shed, merge, and form rib-like multiscale structures. Reduced vortex arching in the T-head dike keeps shear layer vortices more vertically oriented, suppressing near-bed horizontal vorticity and sediment entrainment. The bed shear stress distribution shows that the straight dike produces an order-of-magnitude greater probability of exceeding a critical sediment-mobility threshold than the T-head dike.

A systematic comparison is conducted between flow past a cylinder adjacent to a wall and flow past a cylinder near a free surface to examine how confinement modifies wake dynamics and vortex shedding. Experiments and high-fidelity simulations were performed at Re_d=11000 with the cylinder located at 50% of its diameter from each boundary within a thick, fully-developed boundary layer. The near-wall-cylinder (NWC) exhibited a mildly asymmetric wake driven by gap-flow deflection and local recirculation, whereas the near-free-surface cylinder (NFC) exhibited pronounced asymmetry, a hydraulic jump, and a large-scale recirculation zone. Flow separation was delayed on the boundary-facing side in both cases, with stronger effect in the NFC. Turbulence in the NWC was governed by shear-layer and wall interactions, while persistent wake-surface coupling sustained turbulence farther downstream for the NFC. Spectral analysis revealed a low-frequency gap-flow oscillation for the NWC and higher, multi-frequency interactions for the NFC. Vortex dynamics revealed differing mechanism of Kármán-shedding suppression, with complex vortex merging and diffusion processes in the NFC, and vorticity cancelation process in the NWC. Boundary feedback differed significantly, with free-surface deformation strongly coupled with the NFC wake, while the NWC altered bed shear stress and enchanted localized sediment mobilizing events.