Vortices shed by accelerating flat plates

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, May 2017Flow around flat plates that were uniformly accelerated from rest with acceleration of 13g is analysed with overset mesh from Star CCM+ commercial CFD software. The particular interest is more on the vortices shed from the plate edges. Three 8mm thick plates of the same cross-sectional areas (108mm length equilateral triangular, 71mm length square and 80mm diameter circular) were simulated. The validation of the numerical method was achieved by using laser vapor sheet method to visualize the flow profiles of accelerating circular plate and comparing the CFD and experimental results. The CFD and experimental results were consistent with each other. It was found that when a plate accelerated in air, it displaced air particles out of its way. The shear layers of air separated from the front edges of the plate and rolled around a vortex core forming a primary vortex ring in the plate wake. The size of the primary vortex increased with Reynolds number (Re) that was increasing with time. This was because as Re increased, more fluid particles were displaced from the front face of the plate at a time. More displacement of the fluid particles led to shear layers separating from the plate edges with stronger momentum resulting in larger vortex ring. The shape of the primary vortex depended on the shape of the accelerating plate. For the circular plate, all the points on the front edge being equidistant from the plate centroid, fluid particles were evenly displaced from that separation edge. The result was an axis-symmetric ring of primary vortex around a circular vortex core. The asymmetric plates (triangular and square) did not evenly displace air particles from their edges of separation. The result was an asymmetric vortex ring. More air particles separated from the plate at separation points closest to the plate centroid and led to the largest vortical structure there. That is; the primary vortex ring was largest at the midpoints of the plate edges because they were the closest points of separation from the plate centroid. The size of the primary vortex continuously reduced from the mid-points of the plate edges to the corners. The corners had the smallest primary vortical structure due to being furthest points of separation from the plate centroid. The parts of the vortex ring from the two edges of the plate interacted at the corner connecting those edges.MT 201

Study on the Flow Characteristics of a Bluff Body Cut From a Square Cylinder

The objective of this project is to study on the flow characteristics of a bluff body from a square cylinder using numerical method. Flow characteristics for each cutting and rotating angle of the bluff body of the cylinder will be compared and studied using numerical method which is GAMBIT and FLUENT software. A lot of tall buildings are square in horizontal cross sectional shapes. Due to dominant, wind around their area; they will experience dominant wind buildings in certain directions

LARGE EDDY SIMULATION OF THE FLOW AROUND A FINITE SQUARE PRISM MOUNTED ON A GROUND PLANE

Dynamic subgrid-scale models for large eddy simulation (LES) offer the promise of being able to dynamically calibrate the residual stress field to the local flow conditions. The first part of thesis reports on the application of two different dynamic subgrid-scale (SGS) models to predict the turbulent wake of a finite-height square prism mounted vertically on a ground plane. The prism aspect ratio was AR = 3, and the Reynolds number, based on the prism width and freestream velocity, was Re = 500. The approach flow was laminar with a thin boundary layer; the thickness at the location of the prism was approximately 0.2D. The flow over the top of the prism interacts with the flow along the ground plane and the vertical shear layers from the sidewalls to create a complicated wake structure. Both a linear dynamic Smagorinsky model (DSM) and dynamic nonlinear model (DNM) were implemented and tested for their ability to resolve the complex wake structure. Investigation of the dissipation of turbulence kinetic energy reveals that the DSM has a much larger SGS dissipation, whereas the DNM has a greater resolved-scale dissipation. The backscatter associated with the DSM is much more pervasive than that predicted by the DNM, which accounts for the numerical instability of the DSM. Overall, specific differences are observed in the wake predicted by the two SGS models, including some features of the mean velocity field. The second part of the thesis explores the phase-averaged structure of the wake based on the prediction of the DNM. Phase-averaging based on the Strouhal number reveals a wake structure with quasi-periodic features that is much different from the mean vorticity field, which is characterised by two pairs of counter-rotating streamwise vortex tubes. The phase-averaged near-wake structure is dominated by vertical vortex cores formed by the shear layers being shed from the two sides of the prism. These tubes re-orient and interact as they detach from the prism and move downstream, giving evidence of the half-loop structures documented in previous studies. Each half-loop consists of a short vertical core near the ground plane, and a connector strand that tilts back upstream toward the prism. The shedding process is almost symmetric just downstream of the prism, but develops an asymmetric pattern farther downstream, characterised by the alternate development of half-loop structures on opposite sides of the wake. Further downstream in the wake, the phase-averaged flow is dominated by approximately streamwise vortex tubes associated with the connector strands

New Advances in Fluid Structure Interaction

Fluid–structure interactions (FSIs) play a crucial role in the design, construction, service and maintenance of many engineering applications, e.g., aircraft, towers, pipes, offshore platforms and long-span bridges. The old Tacoma Narrows Bridge (1940) is probably one of the most infamous examples of serious accidents due to the action of FSIs. Aircraft wings and wind-turbine blades can be broken because of FSI-induced oscillations. To alleviate or eliminate these unfavorable effects, FSIs must be dealt with in ocean, coastal, offshore and marine engineering to design safe and sustainable engineering structures. In addition, the wind effects on plants and the resultant wind-induced motions are examples of FSIs in nature. To meet the objectives of progress and innovation in FSIs in various scenarios of engineering applications and control schemes, this book includes 15 research studies and collects the most recent and cutting-edge developments on these relevant issues. The topics cover different areas associated with FSIs, including wind loads, flow control, energy harvesting, buffeting and flutter, complex flow characteristics, train–bridge interactions and the application of neural networks in related fields. In summary, these complementary contributions in this publication provide a volume of recent knowledge in the growing field of FSIs

THE EFFECT OF ASPECT RATIO ON THE AERODYNAMIC FORCES AND FREE END PRESSURE DISTRIBUTION FOR A SURFACE-MOUNTED FINITE HEIGHT CYLINDER

The surface-mounted finite-height cylinder is a fundamental engineering shape and can be found in a multitude of industrial applications. As a result, the local flow field is of great importance in the design of cylindrical components such as heat exchangers or buildings. While two-dimensional (2-D or “infinite”) cylinders are well-understood, the effects of the ground plane and the cylinder free end are significant and require further study. Of particular interest in this thesis is the pressure distribution on the free end of the cylinder, and a mean normal force that develops from it. A vast majority of studies on this topic have focused on short cylinders with a small aspect ratio (AR = height/diameter). The work in this thesis is an attempt to characterize how the pressure distribution and mean aerodynamic forces are influenced by the aspect ratio of the cylinder and the boundary layer thickness of the flow. The little-researched mean normal force, the mean drag force and its resultant mean bending moment, and the associated vortex shedding in the wake are investigated, along with the mean surface pressures and pressure fluctuations for the cylinder free end. A cylinder was designed for use in measuring these parameters for 22 evenly spaced aspect ratios in a range from 0.5 ≤ AR ≤ 11, and an additional cylinder and boundary layer were used to generate data for four different values of relative boundary layer thickness in the range 0.60 ≤ δ/D ≤ 2.86. The results of this research fit in well with published data, and reveal that the flow regimes appear to be marked by two critical aspect ratios, located approximately at AR = 2.5 and AR = 6. Below the lower critical AR, the boundary layer and ground plane effects are dominant, and the Strouhal number and the mean drag and mean normal force coefficients are drastically reduced. The mean bending moment coefficient is high at low AR, possibly owing to the high point of action of the drag force caused by the velocity distribution in the boundary layer. Between the two critical aspect ratios, the mean force coefficients and Strouhal number are relatively insensitive to AR. Above the upper critical AR, the mean drag coefficient increases towards the value for a 2-D cylinder, while the mean normal force coefficient reduces, and is expected to approach a small, constant value. A vertical wall shear force that acts in the opposite direction of the free end pressures may account for the difference between the mean normal force results obtained from integration and those obtained from direct measurements. For high AR, the bending moment coefficient and point of action are relatively unchanged. The free end pressure distributions reveal similar features to previously published data, including “eye-like” enclosed regions of minimum pressure on the upstream half of the cylinder face, and an enclosed region of maximum pressure on the downstream half of the cylinder face. The eye-like structures disappear above the upper critical AR, and are replaced with a band of minimum pressure, upon which the pressure distribution is no longer influenced by aspect ratio. These two critical AR, along with the free end surface pressures and aerodynamic forces, are influenced by the boundary layer thickness, such that a thicker boundary layer creates higher critical aspect ratios. This work is among the first to use consistent flow conditions in showing the effect of two critical aspect ratios on multiple fluid forces and flow structures over a large range of AR. This includes the mean normal force and bending moment. The range of values for the critical aspect ratios is narrowed by the use of small incremental changes in the cylinder aspect ratio. The pressure distributions, and the pressure fluctuations, on the cylinder free end were established in greater detail than earlier published studies as well, and the effects of a change in aspect ratio and boundary layer thickness can be clearly seen. It is hoped that the work contained herein will be an aid to the design and optimization of finite cylinders in future engineering applications

Aerodynamic Optimization and Wind Load Evaluation Framework for Tall Buildings

Wind is the governing load case for majority of tall buildings, thus requiring a wind responsive design approach to control and assess wind-induced loads and responses. The building shape is one of the main parameters that affects the aerodynamics that creates a unique opportunity to control the wind load and consequently building cost without affecting the structural elements. Therefore, aerodynamic mitigation has triggered many researchers to investigate various building shapes that can be categorized into local (e.g. corners) and global mitigations (e.g. twisting). Majority of the previous studies compare different types of mitigations based on a single set of dimensions for each mitigation types. However, each mitigation can produce a wide range of aerodynamic performances by changing the dimensions. Thus, the first millstone of this thesis is developing an aerodynamic optimization procedure (AOP) to reduce the wind load by coupling Genetic Algorithm, Computational Fluid Dynamics (CFD) and an Artificial Neural Network surrogate model. The proposed procedure is adopted to optimize building corners (i.e. local) using three-dimensional CFD simulations of a two-dimensional turbulent flow. The AOP is then extended to examine global mitigations (i.e. twisting and opening) by conducting CFD simulations of three dimensional turbulent wind flow. The procedure is examined in single- and multi-objective optimization problems by comparing the aerodynamic performance of optimal shapes to less optimal ones. The second milestone is to develop accurate numerical wind load evaluation model to validate the performance of the optimized shapes. This is primary achieved through the development of a robust inflow generation technique, called the Consistent Discrete Random Flow Generation (CDRFG). The technique is capable of generating a flow field that matches the target velocity and turbulence profiles in addition to, maintaining the coherency and the continuity of the flow. The technique is validated for a standalone building and for a building located at a city center by comparing the wind pressure distributions and building responses with experimental results (wind tunnel tests). In general, the research accomplished in this thesis provides an advancement in numerical climate responsive design techniques, which enhances the resiliency and sustainability of the urban built environment

Flow control for road vehicle drag reduction

This thesis covers topics that span bluff-body aerodynamics, hybrid RANS-LES CFD methods, flow control and model-order reduction. These topics arise from investigating the flow past three geometries: the bullet shaped D-body, the canonical squareback Ahmed body and the commerical Nissan NDP. The study on the D-body was aimed at transitioning the research group from the restrictive block-structured formulated StreamLES solver to the more flexible OpenFOAM code that can use unstructured meshes. Linear feedback control for base pressure increase was applied as was done in the work by Dalla Longa et al. (2017). Identification of the plant, G(s), that represents the wake's response to forcing was completed and correlated well with the results from Dalla Longa et al. (2017). The same can also be said of the sensitivity based designed feedback control law, K(s). When applied in simulation, an attenuation of the base pressure fluctuations was, as desired, achieved, although the base pressure increased by 24.5% as opposed to the 38% achieved by Dalla Longa et al. (2017). In the study on the squareback Ahmed body, wall-resolving (WRLES) and wall-modelled (WMLES) large eddy simulation were successfully applied. First, a simulation setup that is both able to resolve wake bimodality, while remaining reasonable in computational resource use, was created. Subsequently, variants of this setup were used to identify a flow feature that plays a critical role in forcing wake bimodality events. More specifically, a heavily under-resolved WMLES simulation in which both the near-wall and part of the outer-region of the turbulent boundary layer are Reynolds-averaged did not capture the front recirculation bubble near the Ahmed body nose; neither did it resolve a bimodal wake switching event. Meanwhile, the simulations with a more refined near-wall mesh did capture the front separation bubble as well as bimodal switching events of the wake. This front separation bubble sends out powerful hairpin vortices that interact with the rear wake. Specifically, these vortices go on to produce significant amounts of TKE, which, upon convection to the rear of the Ahmed body, ultimately help trigger a bimodal event. The Ahmed body study also involved the application of linear feedback control for drag reduction as was done in the D-body study. In the short term, mean blowing did lead to a base pressure increase, but as the zero-net-mass-flux (ZNMF) jet settled, it oscillated around zero making its effects indiscernible. The final geometry analyzed was the Nissan NDP. This was done by performing benchmark wall-resolving LES (WRLES). First, the benefit of appending a rear cavity to an otherwise "squareback" geometry was assessed. It was concluded that the cavity allows the wake to move more freely about the rear base. Specifically, the wake is freed from its more restricted motion that is present with the "squareback" Nissan NDP. In doing so, the drag reduction achieved with the cavity appendage is about 13.6%. Work on the Nissan NDP also involved an assessment of a moving ground in the simulation. It was concluded that, in the stationary ground simulation, flow detachment at the ground where the flow exits from the underbody has an adverse drag effect. In other words, although moving ground simulations better replicate the real-world conditions, the stationary ground variant is in this case more conservative, as it returns slightly higher drag values.Open Acces

The effect of turbulence and shear on the flow around three dimensional square cylinders

PhDThe effect of turbulence and shear on the flow around square cylinders has been investigated (i) for models without a free end at incidence a in the range 00<a< 45 0 for models with and without end plates and, (ii) for models with a free end at a=00 and a= 45 0 for model height H to width D ratio H/D in the range 2 :5 H/D 5 11. The Reynolds number, based on 4 the model width was 4.8 x 10 It has been found that end plates are necessary to simulate a two dimensional flow condition in uniform smooth flow provided the wall boundary layer is greater than about 10% of the model span. In other uniform flow conditions, away from the wall boundary layer affected region, which is about the physical thickness of the boundary layer, two dimensionality could be assumed. In linear shear flow., end plates reduced the base pressure in the low velocity region. Two dimensional model results agree well with the published results. Finite square cross-section cylinders exhibit a free end region of length ZF and a root region. Three regimes, Low H/D, Middle H/D and High H/D are found in all flow conditions. The drag on the finite cylinder in smooth flow is lower than that in turbulent flows. Provided the flow is turbulent, increasing turbulence decreases the drag. For a particular flow condition, increasing H/D ratio increases the drag. It appears that in the high H/D regime there is a shedding of the free end eddy. The shedding frequency is lower than the shedding frequency in the root region of the cylinder. The effect of shear can be ascertained by correcting the flow results for the local velocity and thus shear need not be considered a critical flow simulation parameter

LARGE EDDY SIMULATION OF FLOW AROUND A FINITE SQUARE CYLINDER

The main objective of this research is to develop, document and study numerically the flow around finite-height square cylinders mounted on a ground plane, particularly in the near-wake region, under various geometrical conditions. Both the time-averaged and instantaneous flow fields are studied. This thesis consists of three main parts: a comprehensive study of flow over an aspect ratio AR = 5 square cylinder, the effect of sub-grid scale (SGS) models on the numerical simulation and the effect of aspect ratio on the flow structure. The first part of the thesis presents the time-averaged and instantaneous flow fields for flow over a wall-mounted finite-height square cylinder of aspect ratio of AR = 5 at a Reynolds number of Re = 500. The time-averaged flow field results are shown to be in good agreement with experiments. Comparison of the time-averaged results with the velocity field for a square cylinder immersed in a thicker boundary layer, suggests that the boundary layer thickness especially affects the upwash flow (Wang et al., 2009). The instantaneous velocity fields provide an in-depth view of the unsteady nature of the flow field. For the flow over a square cylinder of AR = 5, the instantaneous velocity fields are symmetric near the free end. However, antisymmetric patterns observed downstream may be an indication of the presence of periodic von-Karman type vortices. Since the wake regions are characterized by large-scale unsteady motions, turbulent flow over bluff bodies is well suited to large eddy simulation in which the large energy-containing scales of motion, which are responsible for most of the momentum transport, are resolved whereas the small-scale turbulent fluctuations are modeled. In the second part of the thesis, the performance of the three SGS models, the Smagorinsky model (SM), dynamic Smagorinsky model (DSM) and dynamic non-linear model (DNM) are studied for two grid sets of lower and higher resolution. The results indicated that in case of the DSM insufficient grid resolution leads to erroneous predictions, whereas the DNM is a major improvement as the predictions are similar on both the coarse and fine grids. In the third and final part of the thesis, the effect of aspect ratio on the flow over a wall-mounted finite-height square cylinder is numerically investigated. The wake of a finite square cylinder is studied for three aspect ratios of AR = 3, 5 and 7. The time-averaged vorticity was shown to vary with aspect ratio, e.g. as the aspect ratio increases, the vortex structures in a horizontal plane at mid-height became shorter and rounder in shape. The flow field of the finite cylinder is known to be strongly affected by the aspect ratio (Adaramola et al., 2006). For cylinders with relatively small aspect ratios, the two ends affect the flow patterns and significantly alter the flow structure

Preliminary Test Predictions for Scale Ram-Air Parachute Testing

The present thesis proposes a preliminary analysis to predict the aerodynamic performance for experimental tests of ram-air parachutes in a wind tunnel. A scaled experimental test setup is developed for determining the aerodynamic coefficients of lift () and drag () conducted in a wind tunnel. Additionally, a CFD approach where a steady-state parachute shape defined based on experiments, photographs, and literature, is presented. The accuracy of the simulation depends considerably on the ability to resolve the canopy geometry. Therefore, a CAD geometry generation is implemented for flexible control of the canopy structure by implementing design parameters, e.g., chord, span and planform shape. Distortions caused by inflation and suspension line tensions on the canopy structure are simulated by the manipulation of the surfaces in the CAD design. The numerical results compared with experimental data from the literature under similar flow conditions showed good agreement for the values of and a relative constant offset for the values of for the range of angles of attack analyzed. The difference for the values of was attributed mainly to effects of the geometry deformation and suspension lines drag during the experimental tests. Additionally, simulations with a domain size equal to the dimensions of a wind tunnel test section showed an increase of 26% in the lift curve slope and strong wing tip vortices compared to the baseline model because of wall interaction effects. Finally, experimental tests using correction factors to compensate lift and drag measurements are recommended to directly validate the numerical results