Con J. Doolan

Linear-Parameter-Varying Control of an Improved Three-Degree-of-Freedom Aeroelastic Model

Aeroelasticity is a broad term that describes the often complex interactions between aerodynamics and structural mechanics. The active control of aeroelastic phenomena is of particular research interest as it can lead to a reduction in weight and an increase in performance of an airframe. For more background on the analysis and control of aeroelastic systems, the reader is referred to the article by Mukhopadhyay [1]. A two degree-of-freedom wing section that is allowed to pitch and plunge due to supporting translational and torsional springs has often been used as a testbed for novel aeroelastic control methodologies. Two such apparatus are commonly used for the experimental validation of these methodologies; the Benchmark Active Control Technologies (BACT) wing [1, 2], and the Nonlinear Aeroelastic Test Apparatus (NATA) [3, 4]. This work focusses on the NATA platform, and presents an improved dynamic model along with a dynamic-less state-feedback Linear Parameter Vary-

Low Reynolds number flow over a square cylinder with a splitter plate

Flows over a square cylinder of side length D with and without a splitter plate are numerically investigated at a Reynolds number of 150. The length of the splitter plate is varied systematically from L=0.5D to L=6D so the sensitivity of the flow structure to the inclusion of the splitter plate can be inspected. It is found that the splitter plate introduces a strong hydrodynamic interaction to the near wake of the cylinder and the length of the plate affects significantly the flow structure. The behavior of the flow can be grouped into three regimes. For short plate lengths (0≲L≲D), the free shear layers are convected further downstream before rolling up when the plate length is increased. For intermediate plate lengths (1.25D≲L≲4.75D), a secondary vortex is clearly visible around the trailing edge of the splitter plate and the shear layers begin to roll up closer to the trailing edge. For long plate lengths (L≳5D), a regime is observed in which the free shear layers reattach to the splitter plate. The s...

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

Flat-Plate Interaction with the Near Wake of a Square Cylinder

B LUFF bodies are integral components of aircraft, high-speed trains, automobiles, and many forms of industrial equipment. The minimization of mean and fluctuating force levels generated by bluff bodieswhen placed in afluid streamhas the benefits of reducing drag, vibration, and radiated sound. It is therefore very important that a good understanding of the flowfield about bluff bodies be obtained to achieve these aims. This Note explores how lowReynolds number bluff-body wake interference can affect the generation of unsteady flowand force. The case chosen for study is the interaction of a square cylinder and an infinitely thin flat plate. A rigid square cylinder is one of the most basic forms of a bluff body and when placed in a uniform fluid stream has been shown to exhibit strong vortex shedding, resulting in fluctuating forces and the radiation of sound in the form of an aeolian tone. Previous fluid dynamic experimental studies [1,2] exhibit this strong vortex shedding over a wide range of Reynolds number (10–10). Numerical studies investigating the fluid dynamics [3–6] also confirm this behavior. Despite the wealth of experimental data available for circular cylinders, experimental or numerical data for square cylinders are rare. Available studies include the work of Inoue [7], who performs a numerical simulation of compressible flow about a square cylinder at Re 150. Zhou et al. [8] use an upstream plate to suppress fluctuating lift on a square cylinder. They find that there is an optimal position and size for the upstream plate for effective lift suppression. Studies concerning the control of noise from a circular cylinder are more common. Recent numerical studies [9,10] investigate the use of a splitter plate attached to the base of a circular cylinder. These studies show that vortex shedding can disappear once the splitter plate achieves a certain length. Numerical solutions of the compressible unsteady Navier–Stokes equations for tandem square cylinders [11] show that significant force and noise reduction can occur if the cylinders are placed at a critical spacing in a region in which vortex shedding from the upstream cylinder is suppressed. If this separation distance is increased, the force and noise increase rapidly, due to the reestablishment of upstream vortex shedding and the associated vortex impingement on the downstream cylinder. Other work includes the experimental time-resolved force measurements of tandem bluff bodies by Sakamoto et al. [12] and Alam and Zhou [13], which were used to investigate the phase difference between the aerodynamic forces of each body in the vortex-shedding regime (i.e., the interbody spacing was such that vortex shedding was allowed to occur from each body). In each case, the phase varied linearly with separation distance. Further, Alam and Zhou developed a theoretical model based on the convective properties of isolated bluff-body wakes that agrees reasonably with experimental results. This Note will consider the case of an infinitely thin flat plate placed in the near wake of a square cylinder at a Reynolds number of Re 150. To the author’s knowledge, there have been no previous investigations of the interaction of a thin flat plate with the near wake of a square cylinder. The Note is structured as follows. After outlining the numerical approach (Sec. II), a solution of the incompressible Navier–Stokes equations for the single square cylinder case is presented and validated against available experimental and numerical results (Sec. III). A second solution is then presented that includes the downstream plate (Sec. IV), and the flow and force results are analyzed. The Note concludes with a summary of the key results.

Flow-Induced Sound of Wall-Mounted Finite Length Cylinders

This paper presents the results of an experimental investigation of the sound produced by flow interaction with a wall-mounted finite-length cylinder of circular or square cross section. Acoustic measurements have been taken in an anechoic wind tunnel at a range of flow speeds and for a wide variety of aspect ratios (cylinder length-to-diameter ratio). Unsteady velocity data have also been measured in the cylinder wake using hot-wire anemometry, and these data are related to far-field noise measurements to determine the flow mechanisms responsible for noise generation. The cylinder aspect ratio was found to be an important parameter that controls vortex shedding behavior and hence tonal noise generation. Multiple peaks in the noise spectra can be attributed to different vortex cells in the near wake, the number and strength of which are controlled by aspect ratio influencing flow over the tip or cylinder–wall junction.

On the aeroacoustic tonal noise generation mechanism of a sharp-edged plate

This letter presents an experimental study on the tonal noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number. Flow and far-field noise data reveal that, in this particular case, the tonal noise appears to be governed by vortex shedding processes. Also related to the existence of the tonal noise is a region of separated flow slightly upstream of the trailing edge. Hydrodynamic fluctuations at selected vortex shedding frequencies are strongly amplified by the inflectional mean velocity profile in the separated shear layer. The amplified hydrodynamic fluctuations are diffracted by the trailing edge, producing strong tonal noise.

Reduction of Flow Induced Tonal Noise through Leading Edge Tubercle Modifications

A sinusoidal modification to the leading edge of an airfoil (tubercles) has led to the elimination of tonal noise for a NACA 0021 airfoil at a Reynolds number, Re ~ 120,000. It has also been found that the overall broadband noise is reduced for a considerable range of frequencies surrounding the peak in tonal noise. Investigations have also revealed that changing the amplitude and spacing between the tubercles has an effect on noise reduction. The mechanism of noise reduction is believed to be strongly related to the formation of streamwise vortices which are generated by tubercles. These vortices most likely have an effect on the stability characteristics of the boundary layer, hence influencing the velocity fluctuations of the shear layer near the trailing edge. In addition, spanwise variations in separation location are thought to affect the vortex shedding process, which could influence the feedback mechanism.

Flow and Noise Simulation of the NASA Tandem Cylinder Experiment using OpenFOAM

ow results compare well with published experimental data. The major discrepancies between numerical and experimental ow results can be attributed to neglecting the spanwise velocity component during simulation. Acoustic computations were made using two-dimensional ow data and a compact form of Curle’s theory with spanwise and temporal statistical models that introduced random perturbations into the time-domain signals. The upper and lower frequency limits of the acoustic simulation method were selected using arguments based on acoustic compactness and an estimate of near-eld acoustic eects. Acoustic simulation results compare well with experiment about the main tone. Further improvements are necessary to broaden tones at the harmonics.

Multiple line arrays for the characterization of aeroacoustic sources using a time-reversal method

This letter investigates the use of multiple line arrays (LAs) in a Time-Reversal Mirror for localizing and characterizing multipole aeroacoustic sources in a uniform subsonic mean flow using a numerical Time-Reversal (TR) method. Regardless of the original source characteristics, accuracy of predicting the source location can be significantly improved using at least two LAs. Furthermore, it is impossible to determine the source characteristics using a single LA, rather a minimum of two are required to establish either the monopole or dipole source nature, while four LAs (fully surrounding the source) are required for characterizing a lateral quadrupole source.

A Review of Wind Turbine Noise Perception, Annoyance and Low Frequency Emission

Current literature concerning the perception, annoyance and emission of low-frequency noise from wind turbines are reviewed. Wind turbine noise has been shown to be annoying to people with annoyance related to noise load. Other factors, such as those related to visual, economic and psychological effects, were also shown to affect a persons annoyance of wind turbine noise. Published infrasound (noise at frequencies less than 20 Hz) measurements show that levels at typical residential set-back distances are too low to be directly audible, but may be perceived via window rattling. On the other hand, low-frequency noise levels, in the frequency range of 20–200 Hz may exceed audibility thresholds and it is postulated they may be correlated with annoyance. A review of general low-frequency noise annoyance studies is presented and highlights the similarities of many wind turbine noise complaints with those due to low-frequency noise. The paper concludes with a suggestion to develop a new methodology that can simultaneously acquire annoyance and noise data at the time a person believes they are annoyed by wind turbine noise.