Investigation of the Effect of Vegetation on Flow Structures and Turbulence Anisotropy around Semi-Elliptical Abutment

In the present experimental study, the effect of vegetation on flow structure and scour profile around a bridge abutment has been investigated. The vegetation in the channel bed significantly impacted the turbulent statistics and turbulence anisotropy. Interestingly, compared to the channel without vegetation, the presence of vegetation in the channel bed dramatically reduced the primary vortex, but less impacts the wake vortex. Moreover, the tangential and radial velocities decreased with the vegetation in the channel bed, while the vertical velocity (azimuthal angle > 90°) had large positive values near the scour hole bed. Results showed that the presence of the vegetation in the channel bed caused a noticeable decrease in the Reynolds shear stress. Analysis of the Reynolds stress anisotropy indicated that the flow had more tendency to be isotropic for the vegetated bed. Results have shown that the anisotropy profile changes from pancake-shaped to cigar-shaped in the un-vegetated channel. In contrast, it had the opposite reaction for the vegetated bed.

Stationary solutions of incompressible viscous flow in a wall-driven semi-circular cavity

Stationary numerical solutions of incompressible viscous flow inside a wall-driven semicircular cavity are presented. After a conformal mapping of the geometry, using a body-fitted mesh, the Navier-Stokes equations are solved numerically. The stationary solutions of the flow in a wall-driven semi-circular cavity are computed up to Re = 24000. The present results are in good agreement with the published results found in the literature. Our results show that as the Reynolds number increases, the sizes of the secondary and tertiary vortices increase, whereas the size of the primary vortex decreases. At large Reynolds numbers, the vorticity at the primary vortex centre increases almost linearly stating that Batchelor’s mean-square law is not valid for wall-driven semi-circular cavity flow. Detailed results are presented and also tabulated for future references and benchmark purposes.

Investigation of the Normal Blowing Approach to Controlling Wingtip Vortex Using LES

The characteristics and control of a wingtip vortex are of great significance when considering drag reduction and flight safety of transportation aircrafts. The associated aerodynamic phenomenon resulting from rolling up of a wingtip vortex includes boundary layer flow, shear layer separation, and vortex breakdown, while the interaction of a wingtip vortex with the airframe causes induced drag, wingtip noise, etc. This paper studies a normal blowing method utilized to control the wingtip vortex. Large eddy simulation (LES) technique applied to a straight NACA0012 wing having a chord length ( c ) of 0.4 m is adopted for this study. The Reynolds number based on the chord length is 1.6 × 10 6 and the angle of attack is 12°. The computational approach utilized the dynamic Smagorinsky-Lilly subgrid model for 3D simulations. Normal blowing from a high aspect ratio jet from the wingtip lower surface was used to control the wingtip vortex. From 0.05c to 0.30c, the blowing slit width was 1 mm, with the slit exit treated as a velocity inlet boundary condition which supplied the blowing jet with a momentum coefficient of 0.28%. Results of axial velocity and span-wise pressure distribution of the clean airfoil presented good agreement with known experimental data. LES results indicate that normal blowing suppresses the primary vortex strength, while the vortex core radius, maximum induced velocity, axial vorticity flux, and pressure peak of the primary vortex are reduced by 25%, 28%, 46%, and 52%, respectively. Flow field structures before and after blowing show that blowing suppresses the shedding, coiling, and convergence of the free vortex layers near the primary vortex. This study also shows that normal blowing generates a jet-induced vortex at the location of the secondary vortex, while backflow, volume expansion, and spiral burst can be observed in the jet-induced vortex. The bursting jet-induced vortex destroys the jet-like flow structure of the primary vortex at the trailing edge.

Large-Eddy Simulations of Pollutant Removal Enhancement from Urban Canyons

Techniques for improving the removal of pollution from urban canyons are crucial for air quality control in cities. The removal mainly occurs at the building roof level, where it is supported by turbulent mixing and hampered by roof shear, which tends to isolate the internal canyon region from the atmospheric flow. Here, a modification of roof infrastructures is proposed with the aim of increasing the former and reducing the latter, overall enhancing the removal mechanisms. The topic is investigated by numerical experiment, using large-eddy simulation to study the paradigmatic case of a periodic square urban canyon at $$ Re=2 \times 10^4$$ R e = 2 × 10 4 . Two geometries are analyzed: one with a smooth building roof, the other having a series of solid obstacles atop the upwind building roof. The pollutant is released at the street level. The simulations are successfully validated against laboratory and numerical datasets, and the primary vortex displacement detected in some laboratory experiments is discussed. The turbulence triggered by the obstacles destroys the sharp shear layer that separates the canyon and the surrounding flow, increasing the mixing. Greater vertical turbulent mass fluxes and more frequent ejection events near the upwind building (where pollution accumulates) are detected. Overall, the obstacles lead to a reduction in the pollution concentration within the canyon of about $$34\%$$ 34 % .

Influence of Active Flow Control on Blunt-Edged VFE-2 Delta Wing model

This paper highlights the flow topology above blunt-edged delta wing of VFE-2 configuration when an active flow control technique called ‘blower’ is applied in the leading edge of the wing. The flow topology above blunt-edged delta wing is very complex, disorganised and unresolved compared to sharp-edged wing. For the sharp leading-edged wing, the onset of the primary vortex is fixed at the apex of the wing and develops along the entire wing towards the trailing edge. However, the onset of the primary vortex is no longer fixed at the apex of the wing for the blunt-edged case. The onset of the primary vortex develops at a certain chord-wise position and it moved upstream or downstream depending on Reynolds number, angle of attack, Mach number and the leading-edge bluntness. An active flow control namely ‘blower’ technique has been applied in the leading edge of the wing in order to investigate the upstream/downstream progression of the primary vortex. This research has been carried out in order to determine either the flow on blunt-edged delta wing would behave as the flow above sharp-edged delta wing if any active flow control is applied. The experiments were performed at Reynolds number of 0.5×106, 1.0×106 and 2.0×106 corresponding to 9 m/s, 18 m/s and 36 m/s in UTM Low Speed wind Tunnel based on the mean aerodynamic chord of the wing. The results obtained from this research have shown that the blower technique has significant effects on the flow topology above blunt-edged delta wing. The main observation from this study was that the primary vortex has been shifted 20% upstream when the blower technique is applied. Another main observation was the ability of this flow control to delay the formation of the vortex breakdown.

Accuracy of the Cell-Set Model on a Single Vane-Type Vortex Generator in Negligible Streamwise Pressure Gradient Flow with RANS and LES

Passive flow control devices are included in the design of wind turbine blades in order to obtain better performance and reduce loads without consuming any external energy. Vortex Generators are one of the most popular flow control devices, whose main objective is to delay the flow separation and increase the maximum lift coefficient. Computational Fluid Dynamics (CFD) simulations of a Vortex Generator (VG) on a flat plate in negligible streamwise pressure gradient conditions with the fully-resolved mesh model and the cell-set model using Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) were carried out, with the objective of evaluating the accuracy of the cell-set model taking the fully-resolved mesh model as benchmark. The implementation of the cell-set model entailed a considerable reduction of the number of cells, which entailed saving simulation time and resources. The coherent structures, vortex path, wall shear stress and size, strength and velocity profiles of the primary vortex have been analyzed. The results show good agreements between the fully-resolved mesh model and the cell-set mode with RANS in all the analyzed parameters. With LES, acceptable results were obtained in terms of coherent structures, vortex path and wall shear stress, but slight differences between models are visible in the size, strength and velocity profiles of the primary vortex. As this is considered the first application of the cell-set model on VGs, further research is proposed, since the implementation of the cell-set model can represent an advantage over the fully-resolved mesh model.

Propeller locations study of a generic delta wing UAV model

Purpose This study aims to investigate the effects of propeller locations on the aerodynamic characteristics of a generic 55° swept angle sharp-edged delta wing unmanned aerial vehicle (UAV) model. Design/methodology/approach A generic delta-winged UAV model has been designed and fabricated to investigate the aerodynamic properties of the model when the propeller is placed at three different locations. In this research, the propeller has been placed at three different positions on the wing, namely, front, middle and rear. The experiments were conducted in a closed-circuit low-speed wind tunnel at speeds of 20 and 25 m/s corresponding to 0.6 × 106 and 0.8 × 106 Reynolds numbers, respectively. The propeller speed was set at constant 6,000 RPM and the angles of attack were varied from 0° to 20° for all cases. During the experiment, two measurement techniques were used on the wing, which were the steady balance measurement and surface pressure measurement. Findings The results show that the locations of the propeller have significant influence on the lift, drag and pitching moment of the UAV. Another important observation obtained from this study is that the location of the propeller can affect the development of the vortex and vortex breakdown. The results also show that the propeller advance ratio can also influence the characteristics of the primary vortex developed on the wing. Another main observation was that the size of the primary vortex decreases if the propeller advance ratio is increased. Practical implications There are various forms of UAVs, one of them is in the delta-shaped planform. The data obtained from this experiment can be used to understand the aerodynamic properties and best propeller locations for the similar UAV aircrafts. Originality/value To the best of the author’s knowledge, the surface pressure data available for a non-slender delta-shaped UAV model is limited. The data presented in this paper would provide a better insight into the flow characteristics of generic delta winged UAV at three different propeller locations.

Diversion of Pulsatile Flow Over a Rectangular Sidewall Cavity Using Superhydrophobic Mesh

Pulsatile flow over open cavity represents one type of physiological phenomenon related to a few common cardiovascular diseases, such as cerebral sidewall aneurysm and arrhythmia-induced thromboembolism in the left atrium appendage (LAA). In recent years, endovascular treatments using mesh-based implants have become increasingly popular. In this paper, we study the characteristics of pulsatile flow over a simplified sidewall cavity under two Reynolds/Womersley number conditions using Particle Image Velocimetry. The impacts of a regular mesh and a superhydrobobically-coated mesh on the cavity flow are investigated. Our results quantify the phase-to-phase changes of the flow fields and reveal the formation and the transport of the primary vortex over the ostium of the rectangular cavity. Results suggest the meshes diverted the main flow away from the cavity and prohibited the development of the primary vortex. A penetrated jet flow was formed near the front side of the cavity due to the presence of the mesh. The superhydrophobic mesh dramatically reduced the kinetic energy of the penetrated jet into the cavity. It indicates the mesh flow diversion is effective because of the destruction of the shear-induced vortex dynamics that causes flow stagnation on the rear cavity wall. Our results also indicate the superhydrophobic coating is potentially beneficial in terms of reducing the hemodynamic loading inside the cavity.

Numerical Study on Effect of Secondary Surface on Rectangular Vortex Generator

One way of achieving higher efficiency in electro-mechanical is by inducing vortices over the heated surface with the help of a vortex generator (VG). The strength of these vortices is proportionate to the amount of heat transported. In this paper, the evolution and propagation of the produced primary vortex behind a VG with the attached secondary surface (SS) are studied experimentally and numerically. The addition of SS is found to augment heat transfer significantly with an additional drag. The obtained experimental results complement the numerical predictions for the modified VG. Linear regression analysis is performed to optimize the geometry of SS for a higher heat extraction rate and lower drag. The SS placed at an optimum location increases the Nusselt number on the heated plate by 8.9%, with a decrement in the drag by 3.2%, compared to the reference case. The addition of SS produces a vortex of higher strength and propagates downstream at a slower rate. Moreover, it exposes the vortex to higher shear in the flow, which in turn enhances the heat transfer rate.

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.