Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session CX: Aerodynamics II |
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Chair: Serhiy Yarusevych, University of Waterloo Room: Hyatt Regency Long Beach Regency D |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CX.00001: Flow over a Modern Ram-Air Parachute Canopy Mohammad Mohammadi, Hamid Johari The flow field on the central section of a modern ram-air parachute canopy was examined numerically using a finite-volume flow solver coupled with the one equation Spalart-Allmaras turbulence model. Ram-air parachutes are used for guided airdrop applications, and the canopy resembles a wing with an open leading edge for inflation. The canopy surfaces were assumed to be impermeable and rigid. The flow field consisted of a vortex inside the leading edge opening which effectively closed off the canopy and diverted the flow around the leading edge. The flow experienced a rather bluff leading edge in contrast to the smooth leading of an airfoil, leading to a separation bubble on the lower lip of the canopy. The flow inside the canopy was stagnant beyond the halfway point. The section lift coefficient increased linearly with the angle of attack up to 8.5\r{ } and the lift curve slope was about 8{\%} smaller than the baseline airfoil. The leading edge opening had a major effect on the drag prior to stall; the drag is at least twice the baseline airfoil drag. The minimum drag of the section occurs over the angle of attack range of 3\r{ } -- 7\r{ }. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CX.00002: Effect of Leading and Trailing Edge Geometry on the Aeromechanic Characteristics of Membrane Aerofoils Sara Arbos, Zi Pang, Bharathram Ganapathisubramani, Rafael Palacios The geometry of the rigid leading and trailing edges that hold the membrane could affect the aeromechanic performance of membrane wings. In this study the interaction between the supports and a membrane aerofoil is explored. Tests are performed at low Reynolds numbers, $4.3\times10^4$ - $1.1\times10^5$, and incidences of $0^{\circ}$ - $30^{\circ}$. Four different leading and trailing edge geometries have been analysed focusing on the unsteady characteristics of the wake and the structural vibration of the membrane. Results indicate that aeroelastic coupling between vortex shedding and membrane vibration depends upon the type of membrane support. The wake's kinetic energy distribution has been found to be dependent on $\alpha$ for round supports but independent of $\alpha$ for rectangular supports. Finally,correlation between the membrane deflection and the lift generation has been found for all cases studied. Further analysis will be conducted and discussed. [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CX.00003: Characterization of a Three-Dimensional Turret Wake for Active Flow Control Part I: Simulation Christopher Ruscher, Patrick Shea, Ryan Wallace, John Dannenhoffer, III, Mark Glauser The use of airborne optical devices has led to an increased need to study the flow around a turret. Separation around the turret causes density fluctuations that degrade the performance of the optical device. The separation region can be decreased using different methods of flow control, such as suction. A computational fluid dynamics code that employed Reynolds Averaged Navier-Stokes turbulence models was used to estimate the flow field around a turret and was compared with particle image velocimetry data for validation. The k-$\omega $ model performed better than the commonly used k-$\varepsilon $ turbulence model when comparing the separation area and separation strength (integral of the negative streamwise component of velocity) on the center plane of a turret. The k-$\varepsilon $ model predicted the separation area with an error of 74{\%} and the k-$\omega $ model predicted the separation area with an error of 13{\%}. Separation Strength was predicted with an error of 83{\%} and 25{\%} by the k-$\varepsilon $ model and the k-$\omega $ model respectively. The more accurate k-$\omega $ model will be used to guide future flow control experiments. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CX.00004: Characterization of a Three-Dimensional Turret Wake for Active Flow Control Part II: Experimental Study Patrick Shea, Christopher Ruscher, Ryan Wallace, Mark Glauser, John Dannenhoffer, III Experimental measurements have been performed to characterize the wake of a three-dimensional, non-conformal turret. Experiments were performed in a low-speed wind tunnel at Syracuse University using particle image velocimetry, hotwire anemometry and dynamic and static pressure measurements. The objective of the study was to characterize the spatial and temporal nature of the wake region as well as to investigate the importance of the incoming flow field. Computational studies have been performed in conjunction with this work to help guide the experimental study and offer insight into the complex three-dimensional flow field. With a better understanding of the wake and three-dimensional characteristics of the turret flow field, closed-loop, active flow control systems will be developed to help reduce fluctuating loading and aero-optical distortions associated with the turbulent flow field. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CX.00005: Wind turbine performance predictions using propeller vortex lattice methods Brenden Epps, Richard Kimball A major concern in the design and operation of large wind turbines is unsteady blade loads. These can lead to fatigue failure, and hence are a major structural design driver. Prediction and mitigation of peak blade loads is challenging, because the aerodynamic, structural, and controls problems are coupled, often non-linearly. An efficient computational tool is vital to analyze these types of problems early in the design cycle, before structural designs are frozen and full-blown CFD and FEA are performed. The authors present a computational tool for the design and analysis of large wind turbines. The numerical model is based on the propeller vortex lattice lifting line methods utilized by the US Navy as well as commercial propeller designers. The numerical model is implemented in an open-source code suite called OpenProp, which includes turbine optimization routines as well as performance analysis routines. Examples of turbine designs are presented, including actual parts and experimental performance data. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CX.00006: Dynamic Surface and Flow-Field Measurements of a Pitching Wind Turbine Blade John Strike, Manjinder Singh, Michael Hind, Jonathan Naughton Dynamic pitching is used to study the unsteady aerodynamics of wind turbine blade airfoils. The dynamic flow field is characterized in a wind tunnel using surface pressure measurements coupled with Particle Image Velocimetry (PIV). To obtain the unsteady pressure distribution, a 10.16 cm chord DU97W-300 airfoil with 32 pressure ports has been coupled to a pressure transducer module through 1.07 m of 0.86 mm diameter tubing. Pressure data sampled at 500 Hz are used to estimate the unsteady surface pressure utilizing an optimal Wiener deconvolution method. PIV images are systematically acquired at different phases of the airfoil pitching cycle, and Proper Orthogonal Decomposition (POD) is used to reconstruct the unsteady flow field. To compare the current setup with previous studies that use the same airfoil geometry, pressure measurements are taken at a fixed angle of attack. The airfoils are then oscillated about mean angles of attack and amplitudes and frequencies up to 15 Hz that reflect the angle of attack range and reduced frequencies associated with wind turbines in the field. The combined measurements capture the links between flow-field structure and the observed surface pressures. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CX.00007: A Free Wake Numerical Simulation for Darrieus Vertical Axis Wind Turbine Performance Prediction Radian Belu In the last four decades, several aerodynamic prediction models have been formulated for the Darrieus wind turbine performances and characteristics. We can identified two families: stream-tube and vortex. The paper presents a simplified numerical techniques for simulating vertical axis wind turbine flow, based on the lifting line theory and a free vortex wake model, including dynamic stall effects for predicting the performances of a 3-D vertical axis wind turbine. A vortex model is used in which the wake is composed of trailing stream-wise and shedding span-wise vortices, whose strengths are equal to the change in the bound vortex strength as required by the Helmholz and Kelvin theorems. Performance parameters are computed by application of the Biot-Savart law along with the Kutta-Jukowski theorem and a semi-empirical stall model. We tested the developed model with an adaptation of the earlier multiple stream-tube performance prediction model for the Darrieus turbines. Predictions by using our method are shown to compare favorably with existing experimental data and the outputs of other numerical models. The method can predict accurately the local and global performances of a vertical axis wind turbine, and can be used in the design and optimization of wind turbines for built environment applications. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CX.00008: The aerodynamics of jumping rope Jeffrey Aristoff, Howard Stone We present the results of a combined theoretical and experimental investigation of the motion of a rotating string that is held at both ends (i.e. a jump rope). In particular, we determine how the surrounding fluid affects the shape of the string at high Reynolds numbers. We derive a pair of coupled non-linear differential equations that describe the shape, the numerical solution of which compares well with asymptotic approximations and experiments. Implications for successful skipping will be discussed, and a demonstration is possible. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CX.00009: Numerical Investigations of a Wall Jet with Tabs Hamid Rahai, Carlos Orrala, Huy Hoang Numerical investigations of a wall jet with tabs were performed. The tabs were rectangular thin metal plates placed at the jet outlet, either at the top mid boundary, or at the mid-sections of the three unbounded boundaries, perturbing into the jet. The analyses were carried out at a maximum mean velocity of 15 m/sec., which corresponds to an approximate jet Reynolds number based on the vertical jet dimension of 13027. The numerical calculations were performed using the Reynolds-Averaged Navier-Stokes equations with the Wilcox K-$\omega $ turbulence model. Results that include axial and spanwise variations of the mean velocity, velocity vector, turbulent kinetic energy and vorticity at different axial locations show that with a single tab, the spanwise entrainment is enhanced while with the three tabs, both the vertical and spanwise entrainments are increased. The increase in the spanwise entrainment should result in enhanced film cooling applications. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CX.00010: On the skin friction drag reduction in large wind turbines using sharp V-grooved riblets. Application to a 2.5 MW Clipper wind turbine section Roger Arndt, Leonardo Chamorro, Fotis Sotiropoulos Skin friction drag reduction through the use of riblets has been a topic of intensive research during the last decades. Main efforts have been placed on both numerical (mainly DNS) and experimental approaches. In spite of the valuable efforts, the fundamental mechanisms that induce drag reduction are not well established. In this study, wind tunnel experiments were performed to quantify the drag reduction in a wind turbine airfoil using different V-groove riblet structures. A full-scale 2.5MW Clipper wind turbine airfoil section (of 1 meter chord length, typical of the 88{\%} blade span), was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Four different sizes of V-groove riblets were tested at different angles of attack at full scale Reynolds number of Re=2.67$\times $106 (based on the airfoil chord length). Force sensors were used to measure Lift and Drag. A combination of single and cross-wire anemometers were also used to study the turbulent scale-to-scale interaction in the near wall region to better understand the physical mechanisms of drag reduction and flow characteristics in that region. The measurements will be used to develop and test the performance of near-wall boundary conditions in the context of RANS and hybrid RANS/LES models. [Preview Abstract] |
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