Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H25: Flow Control V: Injection/Suction |
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Chair: Michael Amitay, Rensselaer Polytechnic Institute Room: 320 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H25.00001: Aerodynamic Control of a Pitching Airfoil by Distributed Bleed Actuation John Kearney, Ari Glezer The aerodynamic forces and moments on a dynamically pitching 2-D airfoil model are controlled in wind tunnel experiments using distributed active bleed. Bleed flow on the suction surface downstream of the leading edge is driven by pressure differences across the airfoil and is regulated by low-power louver actuators. The bleed interacts with cross flows to effect time-dependent variations of the vorticity flux and thereby alters the local flow attachment, resulting in significant changes in pre- and post-stall lift and pitching moment (over 50{\%} increase in baseline post-stall lift). The flow field over the airfoil is measured using high-speed (2000 fps) PIV, resolving the dynamics and characteristic time-scales of production and advection of vorticity concentrations that are associated with transient variations in the aerodynamic forces and moments. In particular, it is shown that the actuation improves the lift hysteresis and pitch stability during the oscillatory pitching by altering the evolution of the dynamic stall vortex and the ensuing flow attachment during the downstroke. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H25.00002: The effects of local blowing perturbations on thermal turbulent structures Can Liu, Guillermo Araya, Stefano Leonardi, Luciano Castillo Blowing is an active flow control technique with several industrial applications, particularly in film cooling of turbine blades. In the past, the effects of localized blowing have been mostly analyzed on the velocity field and its influence of the flow parameters and turbulence structures (Krogstad and Kourakine, 2000). However, little literature can be found on the effects of blowing on the coherent thermal structures. In the present study, an incompressible turbulent channel flow with given steady blowing at the wall is simulated via DNS by means of five spanwise holes. The Reynolds number based on the friction velocity and half channel height is approximately Re = 394 and the molecular Prandtl number is Pr = 0.71. Temperature is considered a passive scalar with isothermal conditions at the wall. Different blowing amplitudes and perturbing angles (with respect to the streamwise direction) are applied to find out their effects on the turbulent thermal structures by means of a two-point correlation analysis. In addition, local reduction and increase of drag are connected to vorticity. The corresponding influence of perturbing amplitudes and angles on the energy budget of thermal fluctuations and turbulent Prandtl numbers are also shown and discussed. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H25.00003: A Linear Proportional Control of Turbulent Flow in a Planar Asymmetric Diffuser Donggun Son, Haecheon Choi We perform a linear proportional control of turbulent flow in a planar asymmetric diffuser (Obi diffuser) for separation delay and pressure recovery. The Reynolds number based on the half of inlet channel height ($\delta$) and bulk mean velocity ($u_b$) is $Re_b = u_b \delta / \nu = 9000$, which is the same condition as done by previous experimental and numerical studies. An actuation for the control is defined at the diffuser throat ($x / \delta = 0$ to $1$) as a wall-normal blowing and suction. A sensing variable (error) for the control is the difference between the instantaneous wall shear stresses at the upper and lower walls. The linear proportional control successfully suppresses the separation bubble at the lower slant wall and reduces the skin friction at the upper flat wall, resulting in the pressure recovery at the exit of diffuser. At an optimal proportional gain, the present control produces $6.7\%$ increase in the exit pressure with delayed separation. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H25.00004: Wavelet diagnostics of the flow control of unsteady separation on a 2D Wind Turbine Airfoil Zhe Bai, Jacques Lewalle, Guannan Wang, Mark Glauser We investigated the aerodynamic characteristics of a 2D wind turbine airfoil. Unsteadiness was associated with the wake of a cylinder upstream of the airfoil. The experiments were conducted in both the baseline case, and with active closed-loop control on the suction surface of the airfoil. The data consisted of surface pressure time series. Continuous wavelet analysis gave the phase, band-pass filtered signals and envelope of harmonics of the fundamental shedding frequency. Coherence of pairs of signals was also used to map the flow characteristics. For the baseline and controlled case, we will report on the relation between phase of the leading edge fluctuations, unsteady flow separation and lift and drag coefficients. Our goal is to develop a more effective controller. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H25.00005: The Role of Vorticity Injection in Separation Control Kunihiko Taira, Phillip Munday Large eddy simulation is performed to examine the role of vorticity injection in separation control of spanwise periodic flow over a NACA0012 airfoil. The computations are conducted with a high-fidelity LES solver CharLES with sufficient grid resolution to resolve the near-wall turbulence at a moderate Reynolds number of $Re = 23,000$. The actuator input is introduced to the flow field through the velocity boundary condition to specify the desired vorticity flux input. The aim of this investigation is to analyze the influence of the injected vorticity magnitude and direction on the separation physics over the airfoil such that the separation is delayed. The vortical perturbation is added to break apart the large spanwise vortices responsible for causing separation and hence delay stall. The range of the vorticity injected is chosen to match those from commonly used flow control devices for separation control. In this study, particular focus is placed on examining the interaction between the actuator input and the inherent Kelvin-Helmholtz and spanwise instabilities. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H25.00006: Numerical Investigation of Virtual Aeroshaping Due to Pitched Synthetic Jets Jason Li, Onkar Sahni Synthetic jets in a (non-separated) crossflow provide a virtual aeroshaping effect in a time-averaged sense, which alters the local characteristics of the crossflow such as pressure gradient. The ability to manipulate virtual aeroshaping is beneficial, e.g., jets mounted on an aerial vehicle allow for control of aerodynamic behavior. A numerical investigation is conducted to study the effects of a pitched synthetic jet actuator on virtual aeroshaping, where the resulting recirculation zones behind the actuators are analyzed. Both geometric and operational parameters of actuators are varied in simulations based on permutations of one geometric parameter: pitch angle (60$^{\mathrm{o}}$, 75$^{\mathrm{o}}$, 90$^{\mathrm{o}})$, and two operational parameters: blowing ratio (to be in O(1.0-2.0)), and actuation frequency (to be in O(500-1000Hz)). In these simulations, the jet is placed in a laminar crossflow (e.g., Blasius boundary layer over a flat plate). A stabilized finite element method with implicit time integration technique is employed. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H25.00007: Dynamic Stall of Finite Span Blades and its Control Keith Taylor, Chia Leong, Michael Amitay An experimental investigational study into a dynamically pitching s809 airfoil at a Reynolds number of 220,000 was conducted. Particle Image Velocimetry was employed to visualize and quantify the flow field around the airfoil. This investigation compares a 2-D configuration with 3-D configuration (i.e., a finite span blade). The difference in the flow field between these two configurations is explored, as the vibrations present in the 3-D configuration (due to the dynamic stall) may contribute to a different apparent flow field than classical results would suggest. In addition, a comparison between lift and drag coefficients, measured on the 2-D and 3-D configurations, is explored, demonstrating how time varying lift and drag forces oscillate at characteristic frequencies associated with the primary vibrational modes of the model. In addition, flow control is applied through the actuation of an array of synthetic jets located near the leading edge of the model, in order to effect changes in the flow field around the model, demonstrating how dynamic stall can be delayed or eliminated during dynamic conditions. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H25.00008: Active Flow Control of a Transonic Shock over Curved Surfaces Abraham N. Gissen, Bojan Vukasinovic, Ari Glezer, Sivaram P. Gogineni The effects of fluidic actuation on the evolution and dynamics of a transonic shock over a two-dimensional convex surface by controlling the ensuing shock-induced separation are investigated in wind tunnel experiments. Actuation is effected by a spanwise array of high-frequency (nominally 10 kHz) fluidic oscillating jets. The flow field upstream and downstream of the shock is investigated using high-speed Schlieren and PIV (3,000fps), and surface pressure measurements. It is shown that control of the shock-induced separating shear layer by exploiting direct control of small-scale motion can alter the degree of flow attachment and have a profound effect on the shock dynamics. The actuation diminishes shock oscillations near the surface, and leads to streamwise shock displacement that is proportional to the actuation strength (as measured, for example, by the mass flow rate coefficient). The strong correlation between the shock displacement and surface pressure are explored for application of closed-loop control. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H25.00009: 3-D Separation Control using Spatially-Compact, Pulsed Actuation George T.K. Woo, Ari Glezer The dynamics of controlled 3-D transitory attachment of stalled flow over a dynamically pitching 2-D airfoil are investigated in wind tunnel experiments. Pulsed actuation is effected over a spanwise fraction of the separated domain on a time scale that is an order of magnitude shorter than the airfoil's characteristic convective time scale using surface-integrated pulsed, combustion-driven actuator jets. The formation, evolution, and advection of vorticity concentrations over the airfoil and in its near wake are computed from high-resolution, phase-locked PIV measurements of the flow field in multiple cross-stream planes. It is shown that transitory attachment spreads toward the outboard, unactuated flow domains and exceeds the spanwise width of the actuation. The attachment is preceded by the formation of 3-D vortical structures that are advected and shed into the near wake. The effect of the actuation on the variation of the lift and pitching moment during the pitching cycle is altered significantly with its phase delay relative to the airfoil's pitching motion and can significantly mitigate the adverse aerodynamic effects of the dynamic stall. [Preview Abstract] |
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