Design of small uninhabited aerial vehicles is hampered by the relatively sparse database available for aerodynamic properties of basic shapes at low Reynolds number (Re). As a step towards remedying this situation, an experimental investigation of the influence of aspect ratio on the drag of circular cylinders at low Re has been conducted. The experiment itself faced the difficulty of measuring the small forces associated with low Re. A novel solution to this problem has been developed and applied in the Patterson Hall low speed wind tunnel at Mississippi State University. Using this method the drag characteristics of cylinders with aspect ratios varying from 16 down to 1 over a Reynolds number range from 10^5 down to 10^4 have been measured. This presentation will discuss the existing database, the present experimental method and the results that have been obtained.

International Telemetering Conference Proceedings / October 27-30, 1997 / Riviera Hotel and Convention Center, Las Vegas, Nevada / An edge-based hybrid finite element boundary integral (FE-BI) method using cylindrical shell elements is described for analyzing conformal quarter-wave patches embedded in a circular cylinder. Special care is also taken to deal with weight functions, dyadic Green’s function, and feed model. Some types of the patch arrays embedded in different circular radius have been developed. The tests of their VSWRs and radiation characteristics are in good agreement with the theoretical results.

The study aims to explore flow-induced vibration of shear flow past a circular cylinder. The major parameters in the experiment are the natural frequency of the cylinder, and the velocity and velocity gradient of the shear flow approaching the cylinder. The vibration of the cylinder in a water tunnel were measured by two accelerometers to simultaneously obtain the vibration amplitudes in both the streamwise and cross-stream directions. The experimental results show that in the shear flow the cylinder tends of vibrate the orbits of the cylinder vibration become in the streamwise direction while in uniform flow the cylinder vibrates in all directions in the X-Y plane the tendency is obvious for the cylinder with high natural frequency (13Hz). The orbits of the cylinder with low natural frequency (9Hz) are basically similar in shear flow and in uniform flow.

Two different air-water interfacial flows are studied including plunging wave breaking and flow past a vertical surface-piercing circular cylinder using complementary CFDShip-Iowa version 6 including Cartesian grid solver and orthogonal curvilinear grid solver. The plunging wave-breaking process for impulsive flow over a bump in a shallow water flume has been simulated using the exact experimental initial and boundary conditions. The overall plunging wave breaking process is described with major wave breaking events identified: jet plunge, oblique splash and vertical jet. These major events repeat up to four times before entering the chaotic breaking. The simulations show a similar time line as the experiments consisting of startup, steep wave formation, plunging wave, and chaotic wave breaking swept downstream time phases. Detailed wave breaking processes, including wave profile at maximum height, first plunge, entrapped air bubble trajectories and diameters, kinetic, potential, and total energy, and bottom pressures are discussed along with the experimental results. The simulations show differences and similarities with other experimental and computational studies for wave breaking in deep water and sloping beaches. The geometry and conditions in the present study are relevant to ship hydrodynamics since it includes effects of wave-body interactions and wave breaking direction is opposite to the mean flow. Large-eddy simulation with the Lagrangian dynamic subgrid-scale model has been performed to study the flow past a surface-piercing circular cylinder for Re and Fr effect. The flow features near the air-water interface show significant changes with different Reynolds numbers from sub-critical to critical regime. It is shown that the interface makes the separation point more delayed for all regime of Re. Remarkably reduced separated region below the interface is observed for critical Re regime and it is responsible for much reduced wake and recirculation region behind the cylinder and it recovers in the deep flow. At different Fr, significant changes are shown on the air-water interface structures. At lower Fr, relatively smaller bow waves are observed in front of the cylinder with Kelvin waves behind the cylinder and small amount of free-surface roughness and turbulence are also seen in the wake region. For higher Fr, the bow wave increases remarkably with the larger wake region and deeper depression and it breaks with similar features of plunging breakers. Much more small air-water interface structures including splashes and bubbles are observed behind the cylinder. It is hard to distinguish the Kelvin waves behind the cylinder due to much larger free-surface oscillations and turbulence. As Fr increases, the Kelvin wave angle decreases and deeper and narrower depression region behind the cylinder are observed. The flow features around the cylinder are significantly changed due to this cavity region behind the cylinder.

The objective of the study is to investigate the capability of PANS (Partially Averaged Navier-Stokes Simulation) model over a wide range of Reynolds numbers and flow physics. In this regard, numerical simulations of turbulent flow past a circular cylinder are performed at ReD 140,000 and ReD 3900 using the PANS model. The high Reynolds number PANS results are compared with experimental results from Cantwell and Coles, Large Eddy Simulation results from Breuer, and Detached Eddy Simulation results from Travin et al. Low Reynolds number PANS results are compared with experimental results from Ong and Wallace and Large Eddy Simulation results from Breuer. The effects of the various PANS parameters (fk, fε, σku, σεu) on the ability to capture turbulence physics at various Reynolds numbers are studied. It is confirmed, as previously predicted from theoretical considerations that: (i) for high Reynolds number flow fε = 1 and σku = σk × fk2 / fε are most appropriate; and (ii) for low Reynolds number flow fε = fk and σku = σk are most suitable. These choices for the parameters stem from the fact that there is no clear separation of scales between the energy scales and the dissipation scales at low Reynolds number unlike in the high Reynolds number where there is a clear separation of scales between the energy containing scales and the dissipation scales. Also, in both cases it is found that decreasing fk leads to improved accuracy in predicting the flow statistics.

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In the present study, circular cylinders in the cross-flow or the motions of circular cylinders in a fluid at rest are especially of interest in fields, such as offshore and civil engineering or heat exchanger. For last two decades, the researches of the force caused by the fluid on the cylinder surface are mainly studied by the ways of experiment and numerical methods. A time-independent finite different method is developed to solve the two-dimensional fixed or transversely oscillating cylinder passing by a cross flow. The present study focuses on the cylinder under a cross flow with only two kinds of conditions, which are Re = 100, KC = 5 and Re=200, KC=4. The benchmark tests of the present numerical results are made and validated by the reported numerical simulation and experimental results, for instant, the flow visualization of the vorticity contours and the in-line force for a flow across a moving circular cylinder. The developed numerical method can easily apply on the analyses of interactions between viscous flow and motion cylinder. Besides, we also consider the oscillatory flow passes a circular cylinder connecting with a spring. The spring -linking cylinder is released in the beginning on the position of zero deflection of the spring and stares moving due to the influence of the in-line force acting on the cylinder. We can find that the spring-linking cylinder under a oscillating flow produces restoring force and drag force due to considering the influence of the spring and damping effect, the developed numerical method can easily apply on the analyses of interactions between viscous flow and oscillating cylinder.

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The accuracy of existing methods for estimating the drag based on experimental flow field measurements were assessed for two-dimensional bodies. The effects of control volume boundary placement and inherent simplifying assumptions were also investigated. Wind tunnel experiments were performed on a circular cylinder operating at a Reynolds number of 8,000 and 20,000, and on a NACA 0018 airfoil operating at a chord Reynolds number of 100,000 for three angles of attack (α), specifically, 5◦, 10◦, and 15◦. The circular cylinder experiments fall within the the shear layer transition flow regime. Airfoil investigations span both types of flow development common to low Reynolds number airfoil operation. For α = 5◦ and 10◦, a separation bubble forms on the upper surface of the airfoil, while, for α = 15◦, the flow separates without reattaching, resulting in a stalled airfoil. Wake velocity and pressure measurements were performed at several downstream locations to investigate the impact of control volume boundary placement. Wake profiles were measured between 3 and 40 diameters downstream from the circular cylinder axis and between 1 and 4.5 chord lengths from the trailing edge of the airfoil. In addition to wake profiles, the outer flow velocity variation was quantified to investigate the appropriate location to measure freestream flow characteristics in a test section with streamwise-varying outer flow conditions. The results show that drag estimates are strongly dependent on the streamwise position of the measured wake profile for all methods investigated. Drag estimates improved, and streamwise variation decreased, with increasing streamwise position of the flow measurements. For the pressure based method examined, wake measurements should be taken at least 10 times the projected model height downstream of the model. In the case of the circular cylinder, this is equivalent to 10 diameters and, for the airfoil investigated, it is approximately 1 chord length from the trailing edge. For the methods relying on velocity measurements, acceptable estimates of drag were possible when based on measurements taken at least 30 projected heights downstream, i.e., 30 diameters for the circular cylinder and 3 chord lengths for the airfoil model investigated. The findings highlight the importance of providing a detailed description of the methodology and experimental implementation for drag estimates based on flow field measurements. Finally the study offers guidelines for implementing momentum integral based drag calculations in future investigations.

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