Large Scale Structures in Elevated Jet Normal to Crossflow

Transverse jet from elevated source is found in various environmental and industrial field which include smoke exhausting from stack into atmosphere and sewage water disposal in deep-ocean. The experiment is carried out in water tunnel using flow visualization and Laser Doppler Velocimetry. Analysis has been performed for axisymmetric round jet of aspect ratio of 9.0 with the velocity ratio varying up to 2.5 at Reynolds number (based on free stream crossflow velocity and jet external diameter) of 1000. Result shows the formation of different jet shear layer vortices with varying velocity ratio are: (i) clockwise-downwash vortices (velocity ratio less than 0.3), (ii) delayed-regular-clockwise vortices (between 0.3 to 0.7), (iii) regular-clockwise vortices (between 0.7 to 1.4) at the lee side of the jet shear layer, (iv) irregular-anticlockwise vortex at the upstream along with clockwise vortices at the lee side of the jet shear layer that together forms mushroom vortices (between 1.4 to 1.9) and (v) regular-mushroom vortices (above 1.9). The other vortices found are stack-end vortex (less than 0.9) in the wake near free end, upright vortices (above 0.9) in jet-wake and hairpin vortices (between 0.3 to 0.6) in downstream which is the stretched part of evolving shear layer.

Exact Coherent States and the Nonlinear Dynamics of Wall-Bounded Turbulent Flows

Wall-bounded turbulence exhibits patterns that persist in time and space: coherent structures. These are important for transport processes and form a conceptual framework for important theoretical approaches. Key observed structures include quasi-streamwise and hairpin vortices, as well as the localized spots and puffs of turbulence observed during transition. This review describes recent research on so-called exact coherent states (ECS) in wall-bounded parallel flows at Reynolds numbers Re [Formula: see text] 104; these are nonturbulent, nonlinear solutions to the Navier–Stokes equations that in many cases resemble coherent structures in turbulence. That is, idealized versions of many of these structures exist as distinct, self-sustaining entities. ECS are saddle points in state space and form, at least in part, the state space skeleton of the turbulent dynamics. While most work on ECS focuses on Newtonian flow, some advances have been made on the role of ECS in turbulent drag reduction in polymer solutions. Emerging directions include applications to control and connections to large-scale structures and the attached eddy model.

Nonlinear Destructive Interaction between Wind and Wave Loads Acting on the Substructure of the Offshore Wind Energy Converter: A Numerical Study

Even though the offshore wind industry’s growth potential is immense, the offshore wind industry is still suffering from problems, such as the large initial capital requirements. Many factors are involved, and among these, the extra costs incurred by the conservative design of offshore wind energy converters can be quickly addressed at the design stage by accounting for the nonlinear destructive interaction between wind and wave loads. Even when waves approach offshore wind energy converters collinearly with the wind, waves and wind do not always make the offshore wind energy converter’s substructure deformed. These environmental loads can intermittently exert a force of resistance against deformation due to the nonlinear destructive interaction between wind and wave loads. Hence, the nonlinear destructive interaction between wave and wind loads deserves much more attention. Otherwise, a very conservative design of offshore wind energy converters will hamper the offshore wind energy industry’s development, which is already suffering from enormous initial capital expenditures. In this rationale, this study numerically simulates a 5 MW offshore wind energy converter’s structural behavior subject to wind and random waves using the dynamic structural model developed to examine the nonlinear destructive interaction between wind and wave loads. Numerical results show that the randomly fluctuating water surface as the wind blows would restrict the offshore wind energy converter’s substructure’s deflection. Nonuniform growth of the atmospheric boundary layer due to the wavy motions at the water surface as the wind blows results in a series of hairpin vortices, which lead to the development of a large eddy out of hairpin vortices swirling in the direction opposite to the incoming wind near the atmospheric boundary layer. As a result, the vertical profile of the longitudinal wind velocity is modified; the subsequent energy loss drastically weakens the wind velocity, which consequently leads to the smaller deflection of the substructure of the offshore wind energy converter by 50% when compared with that in the case of wind with gusts over a calm sea.

An Experimental Investigation of Coherent Structures and Induced Noise Characteristics of the Partial Cavitating Flow on a Two-Dimensional Hydrofoil

In many practical submerged objects, various types of cavitation such as bubble, sheet, and cloud cavitation occur according to flow conditions. In spite of numerous theoretical, numerical, and experimental studies, there are still many problems to be solved such as induced noise and damage potential due to cavitation. In this paper, an experimental investigation on coherent structures and induced noise characteristics of partial cavitation on a two-dimensional hydrofoil is presented. Experiments that focused on the dynamics of cavitation clouds were conducted in a cavitation tunnel. Using high-speed visualization, the series process consisting of inception, growth, and desinence of the partial cavity was investigated. The noise generated during the process was also measured, and the correlation with the cavity pattern was examined. The results show that the periodic behavior of cavitation clouds is directly reflected in the noise characteristics. In addition, the visualization of coherent structures within the sheet and cloud cavity provides a qualitative understanding of hairpin vortices and their packets, which play a dominant role in turbulent cavitating flows.

Effect of the In-Hole Vortical Structures on the Cylindrical-Hole Film-Cooling Effectiveness

Understanding the flow from a cooling hole is very important to be able to properly control film cooling of turbine blades. For this purpose, large eddy simulation (LES) investigation of the flow inside a cylindrical film cooling hole is presented in this paper. Two different geometries, with different hole metering lengths, are investigated at a blowing ratio of 0.5. The main flow structure in the hole are the hairpin vortices that originate from a shear layer formed due to flow separation near the hole entry. The comparison of these hairpin vortices in the two cases with different hole metering length is presented in detail. The results show that in case of the hole with longer length the hairpin vortices dissociate within the hole itself. In such a case a uniform flow is seen at the hole exit. However, when the hole length is significantly decreased, it is shown that these vortices exit the hole and effect the vortex structures outside the hole, thereby accounting for the reduction in film cooling effectiveness. Overall, these results bring forth one other major reason for the reduction in film cooling effectiveness with reduction in hole length, i.e. the exit of in-hole hairpin vortices into the crossflow.

Laminar Vortex Shedding in the Wake of a Cube

Flow around a cube is numerically studied in the laminar vortex shedding regime at Re = 276. The objective is to examine the three dimensional vortex shedding mechanism and understand the temporal behaviour of the wake. Hairpin vortices were identified using λ2 criterion. The wake of the cube sheds paired hairpin vortices which moves in the streamwise direction and attains a constant shape with time. The analysis of separation distance and angular orientation of hairpin vortices for flow around a cube are presented here for the first time in the literature. The separation (d) between the paired hairpin vortices scales as t−1/2. The orientation of hairpin vortices change with time and attain a near-normal orientation with respect to the axial direction. A quasi-periodic nature of the flow has been revealed by the phase plots. The drag and side forces generated due to the flow are studied with pressure force mostly contributing to the drag. One of the side force coefficients dominates owing to the asymmetry of the wake in one plane and symmetry in the other orthogonal streamwise plane. These results clearly bring out the asymmetric nature of flow in the shedding regime.

Large-Eddy Simulation of Shaped Hole Film Cooling with the Influence of Cross Flow

In the highly-loaded turbine blade passage, cross flow is driven by the lateral gradient. It strongly influences the cooling performances in the endwall region. In this research, the effect of cross flow on the shaped film cooling hole is studied by Large Eddy Simulation (LES); modal analysis is conducted with an incremental POD (iPOD) approach, which makes the analysis of the large data sets from LES feasible. It is shown that the symmetry of the counter rotating vortex pair (CRVP) is destroyed. The large-scale vortex induced by end-wall cross flow plays an important role in both shape and convection of hairpin vortices and horseshoe vortices, which influences the coolant distribution. This study suggests that the effects of cross flow should be considered for the design of end-wall film cooling. It also indicates the high efficiency of the iPOD approach, which can be used to analyze large amounts of high-dimensional data.