Experimental studies of the formation of zones resulted from air flow around a system of model buildings

This paper presents the results of working out the methodology for conducting experimental studies of the flow around the objects modeling the urban environmental conditions. The experiments were conducted in the wind tunnel of the Siberian Federal University. Two objects of different heights imitating buildings were considered the models. Special attention was paid to the study of the flow pattern at the tandem arrangement of model buildings. Visualizing the flow, the low-velocity and high-velocity zones, as well as recirculation areas were identified. At that, these zones had their peculiarities in terms of the direction of flow twisting behind each object. The study allowed revealing that the vortices separating from the edges of the studied objects play a special role in the flow formation.

Numerical Simulation of Local Scour around Three Cylindrical Piles in a Tandem Arrangement

Scouring is one of the most common potential causes of bridge pile foundation failure, with loss of life, economic and environmental impacts. Comprehensive studies on the numerical simulation of local scour around pile groups are still limited. This paper presents a numerical simulation using Flow-3D software to calculate the maximum sediment scour depth and investigate the mechanism around the groups of three cylinders in a tandem arrangement. A validation using the experimental study was carried out to confirm the reliability of the present numerical model. By using the Van Rijn transport rate equation and RNG k-ε turbulence model, the results of time evolution of scour depth and bed elevation contour show good agreement with the experimental study. The numerical simulation of three cylinders in a tandem arrangement were conducted with pile spacing ratios, G/D of 2 and 3. The local scour is affected by the horseshoe vortex from the downflow driven by the downward pressure gradient and rotates in front of the pile and the high bed shear stress, triggered by flow acceleration. The deepest maximum local scour depth is always obtained by the front pile as a shield pile, followed by the piles behind. The trend of the maximum local scour depth in a tandem arrangement is in accordance with the experimental studies and has a better agreement than previous numerical studies with the same model setup. This means that the numerical model used to simulate pile groups is accurate and capable of calculating the depth of sediment scour.

Surface Roughness Effects on Flows Past Two Circular Cylinders in Tandem Arrangement at Co-Shedding Regime

This paper contributes by investigating surface roughness effects on temporal history of aerodynamic loads and vortex shedding frequency of two circular cylinders in tandem arrangement. The pair of cylinders is immovable; of equal outer diameter, D; and its geometry is defined by the dimensionless center-to-center pitch ratio, L/D. Thus, a distance of L/D = 4.5 is chosen to characterize the co-shedding regime, where the two shear layers of opposite signals, originated from each cylinder surface, interact generating counter-rotating vortical structures. A subcritical Reynolds number of Re = 6.5 × 104 is chosen for the test cases, which allows some comparisons with experimental results without roughness effects available in the literature. Two relative roughness heights are adopted, nominally ε/D = 0.001 and 0.007, aiming to capture the sensitivity of the applied numerical approach. Recent numerical results published in the literature have reported that the present two-dimensional model of surface roughness effects is able to capture both drag reduction and full cessation of vortex shedding for an immovable cylinder near a moving ground. That roughness model was successfully blended with a Lagrangian vortex method using sub-grid turbulence modeling. Overall, the effects of relative roughness heights on flows past two cylinders reveal changing of behavior of the vorticity dynamics, in which drag reduction, intermittence of vortex shedding, and wake destruction are identified under certain roughness effects. This kind of study is very useful for engineering conservative designs. The work is also motivated by scarcity of results previous discussing flows past cylinders in cross flow with surface roughness effects.

Wind tunnel tests of aerodynamic interference effects on two iced vertical circular cylinders in a tandem arrangement

The aerodynamic characteristics of the two iced vertical circular cylinders in a tandem arrangement, due to the shape change by icing, are complex, and lack systematical investigation. Therefore, a set of wind tunnel tests were carried out to study the aerodynamic characteristics of the leeward vertical cylinder, with ice shape, icing thickness, cylinder spacing, and icing relative position of cylinders varied in the subcritical Reynolds number range in this study. Results show that the icing thicknesses had a greater impact on the lift coefficients of D-shaped ice leeward cylinder at the same angle of attack. The aerodynamic characteristics of the iced leeward cylinder were stable when the ratio (L/D) of cylinder spacing was within the range of 4.8 to 6.2. The change of flow field should be considered in the stability analysis of two circular vertical cylinders. The drag coefficients of the iced leeward cylinder varied significantly due to the shielding effects, especially within the range of 9° attack angle and L < 6.2D. The results of this work can provide an experimental reference for future research on wind resistance of two circular cylindrical structures in engineering practice.

A Numerical Investigation of Turbulent Flow Over Single and Tandem Square Cylinders

This paper presents the three-dimensional flow investigations over the square cylinders placed in the tandem arrangement. Two different flow configurations were investigated in detail; one comprising of a single square cylinder and the other comprising of three square cylinders placed in a tandem arrangement with the spacing of six times the width (w) of each square cylinder. The Reynolds number based on the width of the square cylinder (w) and free stream velocity (Uo) is 22,000. The problem was solved numerically using an Unsteady Reynolds-Averaged Navier-Stokes (URANS) based model and Large Eddy Simulation (LES) based model. Strouhal Number, lift, and drag coefficients were computed for each configuration. By comparing both the models using contour plots of pressure, velocity and vorticity it is found that the LES model is more accurate to capture the turbulence around single and tandem square cylinders. In the tandem arrangement, complex periodic vortex shedding was observed in the wake of each square cylinder. The production of turbulent kinetic energy was also investigated to understand the roles of stresses during flow over the cylinders. The analysis showed that the production of turbulence by normal stresses is higher than that of shear stresses. Furthermore, it was observed that the flow over the first cylinder arranged in tandem is quite identical to that of the single square cylinder. Moreover, the upstream cylinder experienced a higher lift in comparison to the downstream cylinders.

Numerical Investigation on Flow-Induced Vibrations of Circular Sections in Tandem Arrangement Varying Relative Distance

The flow around a circular section has many applications in engineering, specifically in the offshore area, such as platforms, risers, pipelines, etc. These structures, which are submerged in water, are susceptible to flow-induced vibrations (FIV) and might oscillate at unwanted large amplitudes due to currents and waves. The Computational Fluid Dynamics (CFD) is a tool that allows the investigation of different parameters related to the flow and/or to the body. A CFD analysis is important in the initial design stage of riser arrangements, being a way to predict the best relative distance, since the flow around the upstream cylinder may strongly influence the downstream cylinder forces and movements responses, characteristic of wake-induced vibrations (WIV). In this context, this work carried out simulations to initiate numerical investigations about the flow around tandem arrangements and then properly identify characteristics of the fluid model that generate the results obtained in experiments. Two similar rigid circular cylinders in a tandem arrangement were analysed, in the stationary case and in the case allowed to move in two degrees of freedom (2DOF) — transverse and in-flow directions, with mass ratio m* = 10 and no structural damping, ζ = 0%. The computations were performed using OpenFOAM, a CFD open source software, in a two-dimensional flow and, numerical uncertainties studies were conducted to well define the results, presenting the value plus its uncertainty. There were variations of the tandem distances (T = 2D, 3D, 4D, 5.5D and 6D), at Reynolds number Re = 100. These two-dimensional flow simulations at low Re are important at the initial stage of a project, in order to filter the best results and save time for more advanced analysis. A single cylinder in a turbulent flow was also analysed, with m* = 2.52, varying reduced velocity. Analysis were done comparing forces coefficients and amplitudes with the literature, for both upstream and downstream cylinders. The vorticity was investigated, properly identifying vortex shedding pattern. For stationary cylinders, a critical distance of 4D was found, where the forces coefficients increase. For the 2DOF cylinders in laminar flow, the highest amplitude was 1.04D at VR = 7 for the downstream cylinder. When the cylinders are free to move, the initial tandem distance does not interfere the amplitude results of the upstream cylinder when VR &gt; 2, results also found in experiments at high Re, and, that justify the two-dimensional studies in low Reynolds as being the basis of a project.

The aerodynamics of flying snake airfoils in tandem configuration

Flying snakes flatten their body to form a roughly triangular cross-sectional shape, enabling lift production and horizontal acceleration. While gliding, they also assume an S-shaped posture, which could promote aerodynamic interactions between the fore and the aft body. Such interactions have been studied experimentally; however, very coarse models of the snake's cross-sectional shape were used, and the effects were measured only for the downstream model. In this study, the aerodynamic interactions resulting from the snake's posture were approximated using two-dimensional anatomically accurate airfoils positioned in tandem to mimic the snake's geometry during flight. Load cells were used to measure the lift and drag forces, and flow field data were obtained using digital particle image velocimetry (PIV). The results showed a strong dependence of the aerodynamic performance on the tandem arrangement, with the lift coefficients being generally more influenced than the drag coefficients. Flow field data revealed that the tandem arrangement modified the separated flow and the wake size, and enhanced the lift in cases in which the wake vortices formed closer to the models, producing suction on the dorsal surface. The downforce created by the flow separation from the ventral surface of the models at 0° angle of attack was another significant factor contributing to lift production. A number of cases showing large variations of aerodynamic performance included configurations close to the most probable posture of airborne flying snakes, suggesting that small postural variations could be used to control the glide trajectory.