Narayanan Komerath

Pressure‐sensitive paint as a distributed optical microphone array

Pressure-sensitive paint is presented and evaluated in this article as a quantitative technique for measurement of acoustic pressure fluctuations. This work is the culmination of advances in paint technology which enable unsteady measurements of fluctuations over 10 kHz at pressure levels as low as 125 dB. Pressure-sensitive paint may be thought of as a nano-scale array of optical microphones with a spatial resolution limited primarily by the resolution of the imaging device. Thus, pressure-sensitive paint is a powerful tool for making high-amplitude sound pressure measurements. In this work, the paint was used to record ensemble-averaged, time-resolved, quantitative measurements of two-dimensional mode shapes in an acoustic resonance cavity. A wall-mounted speaker generated nonlinear, standing acoustic waves in a rigid enclosure measuring 216 mm wide, 169 mm high, and 102 mm deep. The paint recorded the acoustic surface pressures of the (1,1,0) mode shape at approximately 1.3 kHz and a sound pressure level of 145.4 dB. Results from the paint are compared with data from a Kulite pressure transducer, and with linear acoustic theory. The paint may be used as a diagnostic technique for ultrasonic tests where high spatial resolution is essential, or in nonlinear acoustic applications such as shock tubes.

Results from laser sheet visualization of a periodic rotor wake

A laser sheet has been applied to the visualization of a periodic rotor wake produced by a model rotorcraft in forward flight. A stiff, two-bladed teetering rotor is mounted independently of an idealized airframe consisting of an instrumented circular cylinder with a hemispherical nose. The model was installed in a wind tunnel, and tests were carried out at three different advance ratios (forward flight speeds) at one value of rotor collective pitch angle. The laser sheet technique enables detailed flow visualization to be carried out. Quantitative data regarding the location of the rotor tip vortex can be obtained. Results from the flow visualization confirm that the rotor wake is a complex flowfield not amenable to simple analysis. However, the details of the tip vortex geometry are both periodic and repeatable. The motion of the rotor wake in close proximity to the airframe is examined. The behavior of the tip vortex is strongly affected by the presence of the airframe. In particular, the interaction of the rotor tip vortex with the airframe results in the destruction of the vortex core; apparently the vorticity is diffused, and the vortex loses its definition. No vortex filaments appear below the airframe as identifiable flow features. Examples of airframe surface pressure data are presented and are seen to correlate well with the results from the flow visualization.

Correlations of Rotor Wake/Airframe Interaction Measurements with Flow Visualization Data

Narayanan M. Komerath Associate Professor School of Aerospace Engineering Georgia Institute of Technology Atlanta, Georgia 30332 revised version of a paper included in the Proceedings of the 46th Annual Forum of the AHS. ABSTRACT Interaction between the aerodynamics of the rotor and the airframe causes large unsteady pressure fluctuations on rotorcraft airframes. A two-bladed rotor and a hemispherecylinder airframe model are used to study these pressure fluctuations in a wind tunnel, simulating low-speed forward flight conditions. Controlled displacement of the airframe is used to obtain finelyspaced pressure data. These are correlated with azimuth-resolved, quantitative laser sheet visualization of the dynamics of the tip vortices and the inboard vortex sheets from the rotor, along the top of the airframe. Three prominent periodic interactions are observed on the airframe surface. These are the effects of blade motion, the approach and impingement of the tip vortex, and of the vortex sheet. These interactions determine, to a large degree, the overall pressure distribution on the airframe surface, and hence the airframe unsteady airloads. The pressure signatures from the blade passage and the vortex impingement are quite different, and are explained using 2-dimensional models. NOMENCLATURE Cp Pressure coefficient based on freestream dynamic pressure CPmean (Pmean -P∞)/q∞ CPinst (Pinst -P∞)/q∞ CPsteady CP(V∞/Vsteady)2 CPunsteady Punsteady/q∞ CT Rotor thrust coefficient, T/(ρπΩ2R4) h Vertical distance between 2-D vortex core and surface H Vertical spacing between rotor hub center and airframe centerline Pmean Mean static pressure P∞ Freestream static pressure Pinst Instantaneous static pressure Punsteady Unsteady component of static pressure (Pinst Pmean) q∞ Freestream dynamic pressure R Rotor radius T Rotor Thrust U Velocity at surface beneath 2-D vortex Vblade Velocity of blade section Vsteady Far field velocity in a steady reference frame. *: This work was part of the first authors Ph.D. dissertation, performed at the School of Aerospace Engineering, Georgia Institute of Technology. This is a V∞ Tunnel free-stream speed X Streamwise distance from hub center XL Local horizontal coordinate Revised from paper at the AHS Forum, 1990. XN X co-ordinate of airframe nose

Inverse Methods for Deblurring Pressure-Sensitive Paint Images of Rotating Surfaces

Image blurring is a problem encountered when pressure-sensitive paint is applied to rotating surfaces such as rotorcraft blades. The issue is particularly problematic near the leading and trailing edges of the blade; these are the regions where the impact of blurring is the most significant, yet they also contain the most valuable pressure information. Recent work has developed image-deblurring techniques based on deconvolution of the image with a point-spread function based on the known lifetime decay of pressure-sensitive paint and the rotation speed of the blade. This deblurring technique is effective in recovering information at the blade edges when the amount of blurring is not too high. However, the existing deblurring algorithm assumes rectilinear motion and uniform distribution of luminophore lifetime (i.e., constant pressure distribution). The objective of this work is to relax the rectilinear assumption by allowing for rotational blur and to assess the impact of strong pressure gradients on the ...

An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall

The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.

Retail Beamed Power for a Micro Renewable Energy Architecture: Survey

Retail delivery of electric power through millimeter waves is relevant in developing areas where the market for micro devices outpaces the power grid infrastructure. It is also a critical component of an evolutionary path towards terrestrial and space-based renewable power generation. Narrow-band power can be delivered as focused beams to receivers near end-users, from central power plants, rural distribution points, UAVs, stratospheric airship platforms or space satellites. The paper surveys the available knowledge base on millimeter wave beamed power delivery. It then considers design requirements for a retail beamed power architecture, in the context of rural India where power delivery is lagging behind the rapid rise in demand for micro-drives and connectivity.

Near‐Millimeter Wave Issues for a Space Power Grid

This paper reports continuing work on an evolutionary revenue‐generating approach to Space Solar Power. The 220 GHz atmospheric transmission window is chosen, leaving open the option of using millimeter wave or laser wavelengths. The progression from frequency to system business case is laid out, seeking the performance figures needed for a self‐sustaining system and to open up Space Solar Power in 15 to 17 years from first launch. An overall transmission efficiency in excess of 30 percent is required, from DC to beamed power and back to DC or high‐voltage AC, to meet a delivered free‐market price target of 30 cents per KWH, or 20 percent if a price of 45 cents per KWH. Climate data show that rain obscuration is a non‐issue for many of the renewable‐power sites that comprise the market. The technology of direct solar conversion to DC and to beamed power would satisfy the needed efficiencies but requires advances in nano‐scale fabrication with dielectrics.

Unsteady Aerodynamics of Rotorcraft in Ground Effect

This paper brings together experimental and computational work done in the study of low speed ground effect of rotorcraft. The paper recaps previous work done experimentally to study the unsteadiness in the rotorcraft flowfield using flow visualization and hot wire measurements. These experimental results are compared with computational results obtained using a classical vortex method coupled with an image !system to simulate the ground. The comparisons are adequate; however, the computations use a lifting-line approach leading to some discrepancies in time traces of velocities in the wake. The paper also presents experimental results obtained in measuring fuselage loads in ground effect. I. Introduction The flowfield around a rotorcraft flying close to the ground presents a fluid dynamics problem where large fluctuations are observed under nominallj quasi-steady flight conditions, separated by seemingly-random intervals. In ground effect (IGE) conditions, the wake i)f a helicopter rotor interacts with the ground which causes significant perturbation to the flow near the rotor blades, is well as the rest of the craft. Interactions between the main rotor wake and the ground have been associated with :he formation and passage of a ground vortex in transitional flight. The basic question that arises I rom the reported flight test results, is whether 1. The unsteadiness arises from the craft interacting with different regions of an otherwise quasi-steady flowfield, due to changes in wind direc tion, ground clearance or aircraft speed; 2. Or whether long-period fluc tuations are generated in an otherwise periodic flowfield under fixed flight conditions. Detailed experimental studies of the rotor wake and ground vortex were performed with an isolated model rotor above a static ground plane at low advance ratio, and various ground heights. This study aims to quantify time scaies of unsteadiness by bringing together results from pulsed laser sheet flow visualization, hotwire anemometry, and fuselage force measurement. In pl evious work with this experimental set-up, it was shown that the wake was steady enough in the absence of ground effect, to enable clear quantification of the unsteadiness caused by ground effect. This unsteadiness was quantified using laser sheet imaging of vortex dynamics. It was then shown that large transient velocity fluctuations occurred with long intervals, in the ground-vortex and in the rotor inflow regions. The present paper investigates a decision point in the investigation of unsteady ground effect. It is argued that transients could occur due to two basically different situations, or a combination of these situations

Implications of inter-satellite power beaming using a Space Power Grid

This paper considers the conditions, opportunities and constraints under which beamed power might be attractive for space-based enterprises. First, an evolutionary, revenue-generating architecture for beaming electric power through space to terrestrial and space-based customers is presented, to understand the likely environment where full-scale Space Solar Power will become a reality. 12The Space Power Grid approach trades the large risks inherent in the launch cost reductions required for GEO-based microwave SSP concepts, against the nearer-term prospects of advances in millimeter wave and infrared laser conversion, beaming and reception. Space is envisioned as a venue for real-time power exchange between terrestrial power plants. With a constellation of 100 millimeter wave relay spacecraft in 2000km sun synchronous orbits trading with 200 ground stations, the system is shown to break even inside 17 years with fairly realistic parameters, facilitating a revenue-generating scale-up to full SSP. Conditions needed to break even by 50 years from project inception at a level of 3.4 Terawatts are presented. The logic of beaming to space-based customers is explored, concluding that this becomes viable mostly where high power levels are needed, implying either power-intensive commercial enterprises in orbit, or beamed propulsion.

Design of a millimeter waveguide satellite for Space Power Grid

A central element in the use of Space as a power grid for near-real-time power exchange, is a constellation of satellites, capable of receiving and relaying beamed power at multi-megawatt level with extremely high efficiency and low thermal losses. The conceptual design of such a satellite is considered in this paper. Preliminary calculations indicate that power delivery and transmission will use millimeter waveguides inside the satellite to guide the power between antennae, and as feeds for the antennae. The paper seeks solutions for the milimeter waveguides, antennae and thermal control systems in order to refine the mass and efficiency estimates. While the standardized design is for 2000-kilometer orbits, an option to use fewer satellites between as few as two participating nations at 5500 km is also considered at the startup of the system. The NASA system design process is followed wherever practical. However, some innovations are required beyond to move into the regime of 60MW power beams with millimeter waveguides. Corrugated waveguides are identified as a solution for 220 GHz beam redirection, directly coupled to antenna elements.