Yes / The present study investigates the turbulent hydrodynamics in an open channel flow with an emergent and sparse vegetation patch placed in the middle of the channel. The dimensions of the rigid vegetation patch are 81 cm long and 24 cm wide and it is prepared by a 7× 10 array of uniform acrylic cylinders by maintaining 9 cm and 4 cm spacing between centers of two consecutive cylinders along streamwise and lateral directions respectively. From the leading edge of the patch, the observed nature of time averaged flow velocities along streamwise, lateral and vertical directions is not consistent up to half length of the patch; however the velocity profiles develop a uniform behavior after that length. In the interior of the patch, the magnitude of vertical normal stress is small in comparison to the magnitudes of streamwise and lateral normal stresses. The magnitude of Reynolds shear stress profiles decreases with increasing downstream length from the leading edge of the vegetation patch and the trend continues even in the wake region downstream of the trailing edge. The increased magnitude of turbulent kinetic energy profiles is noticed from leading edge up to a certain length inside the patch; however its value decreases with further increasing downstream distance. A new mathematical model is proposed to predict time averaged streamwise velocity inside the sparse vegetation patch and the proposed model shows good agreement with the experimental data. / Debasish Pal received financial assistance from SRIC Project of IIT Kharagpur (Project code: FVP)

Flow-vegetation interactions are critically important for most hydraulic and sediment processes in streams and rivers and thus need to be accounted for in their management. The central goal of this project therefore was to improve the understanding of flow-vegetation interactions in patchy-vegetated river beds, which are typical in rivers. Based on laboratory experiments covering a range of selected hydraulic and patch mosaic scenarios, the hydraulic resistance mechanisms, turbulence structure, and transport mechanisms were studied. The effects of regular patch mosaic patterns (aligned and staggered) on the bulk hydraulic resistance were investigated first. For the cases in which the relative vegetation coverage BSA in respect to the total flume bed is low (BSA = 0.1), the patches mutual positions do not affect values of the friction factor. When the parameter BSA increases to intermediate values (BSA = 0.3), the spatial distribution of the vegetation patches and their interactions become crucial and lead to a significant increase in the bulk hydraulic resistance. When further increase of the vegetation cover occurs (BSA = 0.6), the effects on hydraulic resistance of patch patterns vanish. To clarify the mechanisms of the revealed patch effects on the overall hydraulic resistance, flow structure was assessed at both scales: individual patch and patch mosaic. The presence of a submerged isolated vegetation patch on the bed introduces a flow diversion which strongly alters the velocity field and turbulence parameters around the patch. Coherent structures, generated at the canopy top due to velocity shear, control the mass and momentum transfer between the layers below and above the vegetation patch. At the patch mosaic scale, a complex three-dimensional flow structure is formed around the patches which depends on the patch spacing and spatial arrangements. For the low surface area blockage factor (BSA = 0.1), the patches are sparsely distributed and the wakes are (nearly) fully developed before they are interrupted by the effects of the downstream patches. At the intermediate surface area blockage factor (BSA = 0.3), significant differences in flow structure between the aligned and staggered patches were observed. For the highest surface area blockage factor investigated (BSA = 0.6) both aligned and staggered patch mosaic configurations showed a similar behaviour. The results on the flow structure are used to provide mechanistic explanation of the observed patch mosaic effects on the bulk hydraulic resistance.

Turbulent structures are known to be important physical processes in gravel-bed rivers. A number of limitations exist that prohibit the advancement and prediction of turbulence structures for optimization of civil infrastructure, biological habitats and sediment transport in gravel-bed rivers. This includes measurement limitations that prohibit characterization of size and strength of turbulent structures in the riverine environment for different case studies as well as traditional numerical modeling limitations that prohibit modeling and prediction of turbulent structure for heterogeneous beds under high Reynolds number flows using the Navier-Stokes equations. While these limitations exist, researchers have developed various theories for the structure of turbulence in boundary layer flows including large eddies in gravel-bed rivers. While these theories have varied in details and applicable conditions, a common hypothesis has been a structural organization in the fluid which links eddies formed at the wall to coherent turbulent structures such as large eddies which may be observed vertically across the entire flow depth in an open channel. Recently physics has also seen the advancement of topological fluid mechanical ideas concerned with the study of vortex structures, braids, links and knots in velocity vector fields. In the present study the structural organization hypothesis is investigated with topological fluid mechanics and experimental results which are used to derive a vortex model for gravel-bed flows. Velocity field measurements in gravel-bed flow conditions in the laboratory were used to characterize temporal and spatial structures which may be attributed to vortex motions and reconnection phenomena. Turbulent velocity time series data were measured with ADV and decomposed using statistical decompositions to measure turbulent length scales. PIV was used to measure spatial velocity vector fields which were decomposed with filtering techniques for flow visualization. Under the specific conditions of a turbulent burst the fluid domain is organized as a braided flow of vortices connected by prime knot patterns of thin-cored flux tubes embedded on an abstract vortex surface itself having topology of a Klein bottle. This model explains observed streamline patterns in the vicinity of a strong turbulent burst in a gravel-bed river as a coherent structure in the turbulent velocity field.

<p>We study the effect of minute perturbations by using blowing jets at upstream and bio-inspired micro denticles on turbulence large-scale motions which are observed to be crucial in controlling heat transfer, noise and drag reduction. This work is divided into two phases. In first phase, we studied the effect of blowing perturbations at upstream on large-scale motions and associated co?herent vortical structures which are crucial in enhancing heat transfer by promoting mixing. The second phase is focused on impact of flow dynamics in preventing the biofouling using micro bioinspired structures and the importance of flow regime in designing the antifouling coating us?ing bioinspired structures is demonstrated, and subsequently, separation bubble dynamics and its characterization is carried out for a transonic channel imposed with pressure gradient to further expand our thesis outcomes to utilize micro bioinspired structures in aerospace applications, noise reduction, and to delay separation.</p> <p><br></p> <p>Extensive studies were focused on the importance of large-scale motions (LSM) and their con?tribution to TKE and turbulence mixing. Although there are studies focusing on the λ2 coherent vortical structures and large-scale motions separately, there are no studies addressing the control?ling using upstream perturbations on the large-scale motions and their associated λ2 vortices. In the first phase of our studies, we used the DNS data of channel flow for Reτ = 394 generated using in-house code. In these simulations, we created blowing perturbations using spanwise jets of low blowing ratio, 0.2, placed at upstream. The spatial large-scale motions are extracted using a a novel 3D adaptive Gaussian filtering technique developed based on Lee and Sung [1] for turbulent pipe flows. POD is used to extract the energetic large-scale motions and coherent vortical structures are extracted using λ2-criterion for its efficiency in educing coherent structures in cross flow jets. The results show that the upstream perturbations enhance streamwise heat flux via energetic LSM and also create a secondary peak of scalar production in the log-layer showing that the perturbations alter LSMs to enhance the heat transfer. Filtered large-scale field from Gaussian filtering technique have an integral length scale greater than 2h (where h is channel half-height) are used to obtain λ2 vortices. The resulted λ2 vortices are of ring-type and have higher signature of temperature than their counterpart. The pre-multiplied spectra shows that the upstream perturbations can excite the large-scale wave-numbers which are in the same order as the jet diameter and spacing between them. Simulations show the presence of secondary peak in the log-layer and increased turbulence production which are eminent of large-scales. Furthermore, our results suggest that jet spacing and diameter are crucial in exciting large-scale field to control turbulent flows.</p> <p><br></p> <p>Evans, Hamed, Gorumlu, et al. [2] modeled the denticles present on Mako shark skin into a diverging micro-pillars. They conducted experimental studies in a water tunnel using these on the back of airfoil exposed to an adverse pressure gradient flow. They observed that presence of these pillars reduced the re-circulation bubble (form drag) by 50%. They proposed a blowing and suction type mechanism by which the micro pillars interact with the boundary layer. However, the details of underlying interfacial mechanism is not completely understood. The unique impact of flow conditions on anti-biofouling and the corresponding mechanisms for the first time is illustrated. We employed commercially available bioinspired structures as micro-diverging pillars making it feasible to apply in real life. We demonstrated the underlying mechanism by which bio?inspired structures are responsible for anti-biofouling. To study the pressure gradient effects on the separation under transonic conditions, we performed direct numerical simulations (DNS) in a non?equilibrium flow created by a sinsuoidal contraction and also, we quantified the separation length,</p> <p>detachment, and attachment points of separation bubble imposed with various pressure gradients and their variation in the transonic and subsonic regimes. We noticed that the resultant shear at the attachement led to the enhancement of coherent structures which are extended into the outer layer under transonic flow which is quite different than the subsonic flow.</p>