Engineering topics
Luo, H.
Publications and source records attributed to Luo, H..
The Compressive Behavior of Isocyanate-crosslinked Silica Aerogel at High Strain Rates
Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114-4386/s. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young's moduli (or 0.2% offset compressive yield strengths) at a strain rate approx.350/s were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551,0.628 and 0.731 g/cu cm, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g/cu cm ), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of approx.17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.
Development and Demonstration of Active Noise Control Concepts
This report details design methods for and feasibility of an Active Noise Control (ANC) system using flush-wall-mounted sensors and actuators to reduce turbofan engine rotor-stator interaction noise. ANC concepts capable of suppressing discrete-tone spinning modes containing several cut-on radial mode were identified, developed analytically, and evaluated. Separate ANC systems that suppressed at least three radial modes in a cylindrical inlet duct and three radial modes in an exhaust annulus were developed. These designs resulted in inlet duct and exhaust duct tests that were performed at NASA on the 4-ft ANC Fan in the NASA Glenn AAPL facility. Effective suppression of 2-BPF spinning mode m = 2 tone noise was achieved over a range of fan speeds 1800 to 2450 rpm, where up to 4 radials were present. In the inlet duct, up to 12 dB reduction was obtained for 3 radial modes, and up to 4 dB was obtained with 4 radial modes. In the exhaust duct, up to 15 dB PWL reduction was obtained with either two or three radial modes present. Thus, the ability to suppress multiple radial modes for tones in both the inlet and exhaust ducts has been successfully demonstrated. Implications of ANC system design requirements on installation and system integration issues for ANC systems capable of suppressing higher order radial mode content when applied to a 767 using twin CF6 engines were evaluated analytically. The analytical results indicated an ANC system must be part of an integrated design to be effective.