Fundamental studies of the wake structure for surface-mounted finite-height cylinders and prisms

Surface-mounted finite-height circular cylinders and square prisms can be found in many industrial and engineering applications. The local flow fields around these bluff bodies are not yet well understood due to lack of experimental and numerical data close to the cylinder and prism. The aim of this thesis was therefore to gain an improved physical description of the flow field above the free end surface and around the cylinders and prisms. In the present experimental study, the particle image velocimetry (PIV) technique was used to measure the flow field very close to these bluff bodies in the test section of a low-speed wind tunnel. Four finite circular cylinders and square prisms of aspect ratios AR = 9, 7, 5 and 3 were tested at a Reynolds number of ReD = 4.2×104. At the location of the cylinder or prism, the boundary layer thickness relative to the cylinder diameter or prism width (D) was δ/D = 1.6. PIV velocity field measurements in the near-wake region were made in a vertical plane parallel to the mean flow direction on the flow centreline (the symmetry plane), within 2D upstream and 5D downstream of the cylinder or prism. Additional PIV measurements were carried out in three orthogonal x-z, x-y, and y-z planes above the free end surface of the models. In the near-wake region of the finite circular cylinders, the large recirculation zone contained a vortex immediately behind and below the free end; this vortex was found for all four aspect ratios. A second vortex was found behind the cylinder near the cylinder-wall junction; this vortex was not observed for the cylinder of AR = 3, indicating a distinct wake structure for this cylinder. Similar to the circular cylinder case, in the near-wake region of the square prisms, a vortex was observed immediately behind and below the free end in the recirculation zone. The size and strength of this vortex increased as the aspect ratio of the prism decreased. Also, a second vortex was found near the prism-wall junction downstream of the prisms of AR = 9 and 7, while this vortex was not observed for the prisms of AR = 5 and 3. The PIV results in the near-wake regions of the circular cylinders and square prisms show that the effect of the bluff body shape (circular or square cross-section) is evident in the maximum length of the mean recirculation zone. A considerable difference was seen between the maximum length of the mean recirculation zones of the circular cylinder and square prism of AR = 9, while the shape of the bluff body does not considerably affect the length of the recirculation zones for the bodies of AR = 7, 5, and 3. The present PIV results also provided insight into the separated flow above the free ends, including the effects of AR and body shape. Above the free end of the cylinders, flow separation from the leading edge led to the formation of a mean recirculation zone on the free-end surface. The point of reattachment of the flow onto the free-end surface moved towards the trailing edge as the cylinder aspect ratio was decreased. Large regions of elevated turbulence intensity and Reynolds shear stress were found above the free end. For the finite circular cylinders, the flow pattern above the free end was similar in all three x-z planes for all aspect ratios, consisting of a cross-stream vortex at approximately x/D = 0. According to the PIV results in the x-y planes, one of the main characteristics of the flow over the free end surface of the circular cylinders was a pair of focal points at x/D ≈ 0 and near the edge of the free end. As the cylinder aspect ratio increased, the size and strength of these vortices decreased. Also, the centers of the vortices moved downstream as the aspect ratio increased. For the finite square prism, the large, separated, recirculating flow region extended into the near wake. For the square prism of AR = 3, considerable difference was seen in the free-end flow pattern compared to the more slender prisms of AR = 9, 7 and 5. In particular, a cross-stream vortex formed due to interaction between the separated flow from the leading edge of the prism and the reverse flow over the trailing edge of the free end. This vortex was seen in all three planes at different cross-stream locations for AR = 3 but only in the symmetry plane for AR = 9. Hence, the present PIV results in the x-z planes revealed the effect of the near-wake flow on the flow above the prism free end. The results also showed a considerable effect of the aspect ratio on the mean velocity field as well as the Reynolds stress fields. The results in the x-y planes showed different flow patterns for the prism of AR = 3 including wall-normal vortices close to the free end at the sides of the prism as well as two saddle points close to the corners of the trailing edge and one node downstream of the trailing edge, while for AR = 9, no vortices and node were observed. Two streamwise vortices with opposite sign of rotation were seen in the y-z plane at x/D = 0.2 for all aspect ratios. The present results illustrate in-plane vorticities originating from the vertices of the leading edge of the prism for all aspect ratios

An experimental study of airfoil-spoiler aerodynamics

The steady/unsteady flow field generated by a typical two dimensional airfoil with a statically deflected flap type spoiler was investigated. Subsonic wind tunnel tests were made over a range of parameters: spoiler deflection, angle of attack, and two Reynolds numbers; and comprehensive measurements of the mean and fluctuating surface pressures, velocities in the boundary layer, and velocities in the wake. Schlieren flow visualization of the near wake structure was performed. The mean lift, moment, and surface pressure characteristics are in agreement with previous investigations of spoiler aerodynamics. At large spoiler deflections, boundary layer character affects the static pressure distribution in the spoiler hingeline region; and, the wake mean velocity fields reveals a closed region of reversed flow aft of the spoiler. It is shown that the unsteady flow field characteristics are as follows: (1) the unsteady nature of the wake is characterized by vortex shedding; (2) the character of the vortex shedding changes with spoiler deflection; (3) the vortex shedding characteristics are in agreement with other bluff body investigations; and (4) the vortex shedding frequency component of the fluctuating surface pressure field is of appreciable magnitude at large spoiler deflections. The flow past an airfoil with deflected spoiler is a particular problem in bluff body aerodynamics is considered

Turbulence and Flow–Sediment Interactions in Open-Channel Flows

The main focus of this Special Issue of Water is the state-of-the-art and recent research on turbulence and flow–sediment interactions in open-channel flows. Our knowledge of river hydraulics is deepening, thanks to both laboratory/field experiments related to the characteristics of turbulence and their link to erosion, transport, deposition, and local scouring phenomena. Collaboration among engineers, physicists, and other experts is increasing and furnishing new inter-/multidisciplinary perspectives to the research of river hydraulics and fluid mechanics. At the same time, the development of both sophisticated laboratory instrumentation and computing skills is giving rise to excellent experimental–numerical comparative studies. Thus, this Special Issue, with ten papers by researchers from many institutions around the world, aims at offering a modern panoramic view on all the above aspects to the vast audience of river researchers

Overview of the Applied Aerodynamics Division

A major reorganization of the Aeronautics Directorate of the Langley Research Center occurred in early 1989. As a result of this reorganization, the scope of research in the Applied Aeronautics Division is now quite different than that in the past. An overview of the current organization, mission, and facilities of this division is presented. A summary of current research programs and sample highlights of recent research are also presented. This is intended to provide a general view of the scope and capabilities of the division

DYNAMICS OF A CIRCULAR CYLINDER IN CLOSE PROXIMITY TO A WALL

The effect of wall proximity on the mechanism of flow-induced vibration of a circular cylinder mounted in a wind tunnel and free to vibrate with two degrees-of-freedom near a rigid plane boundary were examined at a Reynolds number of 1.86 x 105. Hot-wire anemometry and cylinder-mounted accelerometers were used to characterize the flow-induced vibration of the cylinder. In the near wall region (gap ratios, G/D\u3c 0.4) the system was found to exhibit symptoms of movement-induced vibration resulting from the push-and-draw effect of the cylinder pushing into and out of the fluid immediately upstream and downstream of the cylinder. The variation of the width of the separated wake region with the cylinder motion produces a force in phase with the cylinder velocity, resulting in excitation of the cylinder motion. For G/D \u3e 1.0, the alternate shedding of vortices (the Kármán vortex street) produces a fluctuating lift on the body. The lift force also has an associated drag fluctuation at twice the vortex shedding frequency For 0.4 \u3e G/D\u3e 1.0, the excitation appears to be due to the combined effects of the movement-induced vibration found in the near wall region with the vortex shedding from the cylinder found for the free cylinder when it is far from the wall.. The system exhibited mechanical coupling of the two degrees-of-freedom, so additional tests will be needed to conclusively confirm the findings presented in this thesis

Wake characteristics of single and tandem emergent cylinders in shallow open channel flow

The present thesis deals with the vertical variability of the wake characteristics behind emergent single and tandem circular cylinders in shallow open channel flow at two Reynolds numbers (ReD = 925, 3000) in order to conceptually investigate the flow past vegetation. The tandem configuration includes two and three slender cylinders arranged in-line with different gap lengths (G) while the cylinders are rigid with a small diameter (D = 6.4 mm). The experiments were conducted in a wide water flume, where a small blockage ratio (~ 0.5%) was achievable. Velocity measurement was performed using particle image velocimetry (PIV) in vertical and horizontal planes in the wake region. The focus of the study is to explore the wake flow characteristics in terms of mean velocity profiles, turbulent parameters and coherent structures along the water depth for all test cases. Proper orthogonal decomposition (POD) was applied to educe information about the coherent structures in the flow in the different horizontal planes

5 European & African Conference on Wind Engineering

The 5th European-African Conference of Wind Engineering is hosted in Florence, Tuscany, the city and the region where, in the early 15th century, pioneers moved the first steps, laying down the foundation stones of Mechanics and Applied Sciences (including fluid mechanics). These origins are well reflected by the astonishing visionary and revolutionary studies of Leonardo Da Vinci, whose kaleidoscopic genius intended the human being to become able to fly even 500 years ago… This is why the Organising Committee has decided to pay tribute to such a Genius by choosing Leonardo's "flying sphere" as the brand of 5th EACWE

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

Performance evaluation of louvered fin compact heat exchangers with vortex generators

Every day large amounts of heat are transferred in many industrial and domestic processes. This heat transfer takes place in a heat exchanger. Any energy savings in heat transfer processes have a significant impact on the fuel consumption and greenhouse gas emissions. More energy efficient heat exchangers help to meet the 20-20-20 climate and energy targets of the European Union. In many applications air is one of the working fluids (e.g. coolers in compressed air systems, heat pumps, air conditioning devices, domestic heating, etc.). When heat is exchanged with air, the main thermal resistance is located at the air side of the heat exchanger. To increase the heat transfer rate, the heat transfer surface area is enlarged by adding fins to the air side of the heat exchanger. When a high compactness is needed, complex interrupted fin surfaces are used. A typical example is the louvered fin design. The main disadvantage of the louvered fins is the high pressure drop. Delta winglets mounted on a heat transfer surface generate vortices which cause an intense mixing of the flow and thin the thermal boundary layers. In contrast to louvered fins, they enhance the heat transfer with a relatively low penalty in pressure drop. The objective of this doctoral work is to evaluate if the thermal hydraulic performance of a louvered fin heat exchanger with round tubes in a staggered layout can be improved by adding delta winglets to the fins. Such compound designs form the next generation of heat exchangers. Both experiments (flow visualizations in a water tunnel and heat transfer and pressure drop measurements in a wind tunnel) and simulations (Computational Fluid Dynamics - CFD) were performed. The louvers affect the main flow, while the delta winglets reduce the wake regions downstream of the tubes. The generated vortices cause three important mechanisms of heat transfer enhancement: a better mixing, a reduction of the thermal boundary layer thickness and a delay of the flow separation from the tube surface. Further, it was found that the vortices do not extend far downstream as they are destroyed by the deflected flow in the downstream louver bank. The compound heat exchanger has a better thermal hydraulic performance than when only vortex generators or only louvers are used. It is shown that for the same pumping power and heat duty, the compound heat exchanger is smaller in volume. Consequently, less space is required, the material cost is lower and (often also) the operational cost is reduced. The combination of louvered fins and vortex generators is mainly interesting for low Reynolds applications, such as HVAC&R applications or in compressed air systems. A well-considered location and geometry of the vortex generators are essential for an improved performance of the heat exchanger

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