Vortex heat transfer enhancement by energy-efficient structured plates with zigzag grooves for micro- and macro-scale energy and electronic devices

An energy-efficient flat surface is formed when applying single-row cross-flow zigzag grooves (VVVVVV) in a dense arrangement. Convective heat transfer is considered in turbulent flow around a longitudinal fragment of a plate with a length of 40 and a width of 4 with a package of 14 singlerow inclined backslash(\)-shaped grooves with symmetry conditions at the lateral boundaries. The width of the grooves is 1, the depth is 0.25, the edge rounding radius is 0.2, the angles of inclination are 30°, 45°, 50° and 60°, the pitch is 2.4, the Reynolds number is 104, and the thickness of the incoming boundary layer is 0.175. The phenomenon of anomalous enhancement of the separation flow and heat transfer in zigzag grooves and acceleration of the wall flow discovered in inclined oval-trench dimples has been confirmed.

Accurate PIV Measurement on Slip Boundary using Single-Pixel Algorithm

To accurately measure the near-wall flow by particle image velocimetry (PIV) is a big challenge, especially for the slip boundary condition. Apart from high-precision measurements, an appropriate PIV algorithm is important to resolve the near-wall velocity profile. In our study, single-pixel algorithm is employed to calculate the near-wall flow, which is demonstrated to be capable of accurately resolving the flow velocity near the slip boundary condition. Based on the synthetic particle images, the advantages of the single-pixel algorithm are manifested in comparison with the conventional window correlation algorithm. Specially, the single-pixel algorithm has higher spatial resolution and accuracy, and lower systematic error and random error for the case of slip boundary condition. Furthermore, for experimental verification, micro-PIV measurements are conducted over a liquid-gas interface and the single-pixel algorithm is successfully applied to the calculation of near-wall velocity under the slip boundary condition, especially the negative slip velocity. The current work demonstrates the advantage of the single-pixel algorithm in analyzing the complex flow under the slip boundary condition, such as drag reduction, wall skin friction evaluation and near-wall vortex structure measurement.

Numerical Investigations on Effect of Inflow Parameters on Development of Secondary Flow Field for Linear LP Turbine Cascade

End wall flows contribute the most crucial role in loss generation for axial flow turbine and compressor blades. These losses lead to modify the blade loading and overall performance in terms of stable operating range. Present study aimed to determine the end wall flow streams in a low speed low pressure linear turbine cascade vane using numerical approach. The study includes two sections. The first section includes an attempt to understand different secondary flow streams available at end wall. Location of generation of horseshoe vortex streams and subsequent vortex patterns are identified in the section. The selection of suitable turbulence model among SST (Shear Stress Transport) k–ω and SST γ–θ to identify end wall flow streams is studied in prior in the section. The steady state numerical study is performed using Reynolds Averaged Navier-Stoke’s Equations closed by SST γ–θ turbulence model. The computational results are validated with experimental results available in the literature and are found to be in good agreement. The study is extended for different inflow conditions in later section. The second section includes effect of flow incidence and turbulence intensity on the end wall secondary flow field. Inflow incidences considered for the study are −20°, −10°, 0° (design incidence), +10° and +20°. The inlet turbulence intensities are varied by 1% and 10% for each case. The results revealed different secondary flow patterns at an end wall and found the change in behavior with an inflow conditions. SST γ–θ turbulence model with lower turbulence intensity is more suitable to identify such flow behavior.

Ducted wind turbines in yawed flow: a numerical study

Ducted wind turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform flows are caused by the presence of buildings or other surface discontinuities. For this reason, the aerodynamic performance of DWTs in yawed-flow conditions must be characterized depending upon their geometric parameters and operating conditions. A numerical study to investigate the characteristics of flow around two DWT configurations using a simplified duct-actuator disc (AD) model is carried out. The analysis shows that the aerodynamic performance of a DWT in yawed flow is dependent on the mutual interactions between the duct and the AD, an interaction that changes with duct geometry. For the two configurations studied, the highly cambered variant of duct configuration returns a gain in performance by approximately 11 % up to a specific yaw angle (α= 17.5∘) when compared to the non-yawed case; thereafter any further increase in yaw angle results in a performance drop. In contrast, performance of less cambered variant duct configuration drops for α>0∘. The gain in the aerodynamic performance is attributed to the additional camber of the duct that acts as a flow-conditioning device and delays duct wall flow separation inside of the duct for a broad range of yaw angles.

Investigation of Water Injection Influence on Cloud Cavitating Vortical Flow for a NACA66 (MOD) Hydrofoil

Re-entrant jet causes cloud cavitation shedding, and cavitating vortical flow results in flow field instability. In the present work, a method of water injection is proposed to hinder re-entrant jet and suppress vortex in cloud cavitating flow of a NACA66 (MOD) hydrofoil (Re = 5.1 × 105, σ = 0.83). A combination of filter-based density corrected turbulence model (FBDCM) with the Zwart–Gerber–Belamri cavitation model (ZGB) is adopted to obtain the transient flow characteristics while vortex structures are identified by Q criterion & λ2 criterion. Results demonstrate that the injected water flow reduces the range of the low-pressure zone below 1940 Pa on the suction surface by 54.76%. Vortex structures are observed both inside the attached and shedding cavitation, and the water injection shrinks the vortex region. The water injection successfully blocks the re-entrant jet by generating a favorable pressure gradient (FPG) and effectively weakens the re-entrant jet intensity by 46.98%. The water injection shrinks the vortex distribution area near the hydrofoil suction surface, which makes the flow in the boundary layer more stable. From an energy transfer perspective, the water injection supplies energy to the near-wall flow, and hence keeps the steadiness of the flow field.

Numerical investigation of the wake bi-stability behind a notchback Ahmed body

Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, ${\beta _1} = 17.8\mathrm{^\circ }$ and ${\beta _2} = 21.0\mathrm{^\circ }$ , are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.