Justin Jaworski

The noise generating and suppressing characteristics of bio-inspired rough surfaces

It is hypothesized that the structure of the down covering the flight feathers of larger species of owls contributes to their ability to fly almost silently at frequencies above 1.6kHz. Microscope photographs of the down show that it consists of hairs that form a structure similar to that of a forest. The hairs initially rise almost perpendicular to the feather surface but then bend over in the flow direction to form a canopy with an open area ratio of about 70%. Experiments have been performed to examine the noise radiated by vertical filaments and by a large open area ratio canopy suspended above a surface. The canopy is found to dramatically reduce pressure fluctuations on the underlying surface, in a manner that is found to be consistent with the theory of flows over and through vegetation. While the canopy can produce its own sound, particularly at high frequencies, the reduction in surface pressure fluctuations can reduce the noise scattered from an underlying rough surface at lower frequencies. Theoretical studies are also being performed to look at the noise radiating/suppressing characteristics associated with the flexibility of the hairs, and to examine the extent to which the aeroacoustics of the down can be modeled by treating it as a porous layer.

Experimental and Theoretical Analysis of Bio-Inspired Trailing Edge Noise Control Devices

© American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This work focuses on new experimental investigations into the function of bio-inspired trailing edge noise reduction treatments, and complementary theoretical analysis. Aeroacoustic wind tunnel tests of an instrumented DU96-W180 airfoil model at high (~2.5 million) Reynolds number reveal various effects of these treatments, termed “finlets.” Data collected include far-field noise, measured using a phased array microphone system, unsteady surface pressure near the trailing edge, lift and mean surface pressure distribution over the entire airfoil, and drag measured in its wake. Downstream effects of the finlets are investigated by positioning them at various distances upstream of the trailing edge. Off-design conditions are investigated by skewing the finlets to simulate the presence of spanwise flow. The results demonstrate the robustness of the present design and reveal additional insight into the effects of finlets on the structure of the boundary layer turbulence approaching the trailing edge. A simple mathematical model is introduced which demonstrates the noise-reducing capability of finlets through the displacement of noise-producing vortices. The influence of shape and position is investigated through the model, and results are qualitatively similar to the experimental findings.

Thrust Augmentation of Flapping Airfoils in Low Reynolds Number Flow Using a Flexible Membrane

Abstract The unsteady aerodynamic thrust and aeroelastic response of a two-dimensional membrane airfoil under prescribed harmonic motion are investigated computationally with a high-order Navier–Stokes solver coupled to a nonlinear membrane structural model. The effects of membrane prestress and elasticity are examined parametrically for selected plunge and pitch–plunge motions at a chord-based Reynolds number of 2500. The importance of inertial membrane loads resulting from the prescribed flapping is also assessed for pure plunging motions. This study compares the period-averaged aerodynamic loads of flexible versus rigid membrane airfoils and highlights the vortex structures and salient fluid–membrane interactions that enable more efficient flapping thrust production in low Reynolds number flows.

The steady aerodynamics of aerofoils with porosity gradients

This theoretical study determines the aerodynamic loads on an aerofoil with a prescribed porosity distribution in a steady incompressible flow. A Darcy porosity condition on the aerofoil surface furnishes a Fredholm integral equation for the pressure distribution, which is solved exactly and generally as a Riemann–Hilbert problem provided that the porosity distribution is Hölder-continuous. The Hölder condition includes as a subset any continuously differentiable porosity distributions that may be of practical interest. This formal restriction on the analysis is examined by a class of differentiable porosity distributions that approach a piecewise, discontinuous function in a certain parametric limit. The Hölder-continuous solution is verified in this limit against analytical results for partially porous aerofoils in the literature. Finally, a comparison made between the new theoretical predictions and experimental measurements of SD7003 aerofoils presented in the literature. Results from this analysis may be integrated into a theoretical framework to optimize turbulence noise suppression with minimal impact to aerodynamic performance.

Acoustic emission from one-dimensional vibrating porous panels in a single-sided flow

The acoustic far-field pressure is determined for one-dimensional finite-chord panels with uniform porosity in a single-sided uniform flow. The unsteady, non-circulatory pressure on the panel is computed using a previously established analysis method. The acoustic field is computed using the Green’s method. Results from this acoustic analysis identify the sensitivity of the far-field pressure magnitude and directivity to changes in flow Mach number, the reduced frequency of the panel vibration, and the panel porosity level characterized by a Darcy-type porosity boundary condition.

Accelerated Acoustic Boundary Element Method and the Noise Generation of an Idealized School of Fish

A transient, two-dimensional acoustic boundary element solver is developed using double-layer potentials accelerated by the fast multiple method for application to multibody, external field problems. The formulation is validated numerically against canonical radiation and scattering configurations of single and multiple bodies, and special attention is given to assessing model error. The acoustic framework is applied to model the vortex sound generation of schooling fish encountering 2S and 2P classes of vortex streets. Vortex streets of fixed identity are moved rectilinearly in a quiescent fluid past representative schools of two-dimensional fish, which are composed of four stationary NACA0012 airfoils arranged in a diamond pattern. The induced velocity on the fish-like bodies determines the time-dependent input boundary condition for the acoustic method to compute the sound observed in the acoustic far field. The resulting vortex noise is examined as a function of Strouhal number, where a maximum acoustic intensity is found for (St approx 0.2), and an acoustic intensity plateau is observed for swimmers in the range of (0.3< St < 0.4). In the absence of background mean flow effects, numerical results further suggest that the value of Strouhal number can shift the acoustic directivity of an idealized school in a vortex wake to radiate noise in either upstream or downstream directions, which may have implications for the the study of predator-prey acoustic field interactions and the design of quiet bio-inspired underwater devices.