N. Komerath

Measurement of the Interaction Between a Rotor Tip Vortex and a Cylinder

The transient interaction between a cylinder and the trailing vortex from a rotor in forward flight is studied. Phase-averaged laser velocimetry and surface pressure measurements made with flush-mounted microphones are used to study the velocity and pressure variations during such an interaction. Vorticity contours constructed from the velocity measurements exhibit the presence of a secondary structure with vorticity opposite in sense to that of the primary tip vortex. This structure moves rapidly around the tip vortex from upstream to downstream. The pressure variations caused by the tip vortex on the surface of the cylinder are smooth as the vortex core passes by, and no evidence is found of fine structure inside the vortex core region. After vortex interaction, the secondary structure causes large variations in the surface pressure before being dissipated. Calculations using measured vortex strength and speed data indicate that the distortions and deflections of the vortex immediately prior to impingement on the surface differ significantly from those computed using two-dimensional potential flow concepts. Nomenclature Cpu = unsteady pressure coefficient: difference between the instantaneous pressure and the local mean static pressure, normalized by the tunnel dynamic pressure R - rotor radius r = radial position along the rotor radius U = velocity component along the tunnel axis, positive going downstream Um = freestream velocity V = vertical velocity component, positive downward X = distance parallel to the tunnel axis, origin is at the rotor hub center Xb - distance parallel to the tunnel axis, measured from the nose of the cylinder Yb = distance along the lateral direction from the cylinder axis Zb = vertical distance from the cylinder axis, positive

Velocity measurements of airframe effects on a rotor in a low-speed forward flight

Aerodynamic interactions between the rotor and the airframe of a rotorcraft can have severe effects and are difficult to predict analytically. To attack this problem, the velocity field of a two-bladed rotor has been measured in a wind tunnel with and without an airframe model in proximity. The periodic and time-averged velocity fields were measured using a laser velocimeter in planes both parallel to and above and below the rotor tip path plane at an advance ratio of 0.1 and a rotor tip Mach number of 0.29. The data were shown to be free of tunnel wall effects. The effect of including the cylindrical airframe model were measured. For the geometry studied, airframe influence on the rotor flowfield was mostly confined to the front half of the rotor disc. Hub effects were noticeable, even with the minimal-sized hub used. Strong vortex interaction effects were observed using strobed laser sheet flow visualization and measured using laser velocimetry.

Visualization and Measurement of the Tip Vortex Core of a Rotor Blade in Hover

Detailed measurements with a laser Doppler velocimeter (LDV) have been performed in the tip region and in the tip vortex core of a single-bladed model rotor in hover. The testing was conducted at a rotor tip speed of 32 m/s, a Reynolds number of 269,000, and at two values of the rotor thrust coefficient, 0.0022 and 0.0057. Strobed laser sheet flow visualization was used to verify the steadiness of the tip vortex trajectory in the near wake and quantify the vortex trajectory to guide LDV surveys of the vortex core. A remotely aligned off-axis receiving optics system enabled measurement of vortex core velocity profiles at large focal lengths. The core self-induced velocity components extracted from these data are presented. The data exhibit evidence of secondary structure even inside the rotational core of the vortex, the axial velocity profile along the core has been extracted and presented in the wake of a spinning rotor. It is seen that the tip vortex of a rotating blade differs considerably in structure from a fixed-wing vortex.

Flow over a twin-tailed aircraft at angle of attack. II - Temporal characteristics

Revised May 5, 1991 Flow Over a Twin-Tailed Aircraft at Angle of Attack. Part I: Spatial Characteristics N.M. Komerath1, S.G.Liou2, R.J. Schwartz3, J.M. Kim3 School of Aerospace Engineering Georgia Institute of Technology Atlanta, Georgia 30332 ABSTRACT A quantitative study is reported on the low-speed flow environment of scale models of a twin-tailed fighter aircraft at high angles of attack. Laser sheet flow visualization is used to observe the various sources of vortex generation, and the evolution of these vortex flows. Surface tufts are used to observe the nature of flow separation on the vertical tails as angle of attack is varied. Laser Doppler velocimetry is used to quantify the time-averaged three-dimensional velocity field, and histograms of velocity, in selected planes proceeding from the inlets to the vertical tails. No concentrated vortex is observed near the vertical tails, however, the tails are seen to be immersed in a vortex flow of large radius. Flow separation propagates up the outside surfaces of the vertical tails, with increasing angle of attack; however, the flow on the inside surfaces of the tails remains largely attached. The flow angularity at the tails varies widely along the tail span, is sensitive to angle of attack, and fluctuates over a wide range at each location. These results are found to be relatively insensitive to the precise modeling of inlet through-flow and inlet attitude. Contours of the root-mean square velocity fluctuations indicate that the largest fluctuations occur in the separated flow immediately above the wing surfaces. ______________ __________________________________________________________ 1: Associate Professor. Member, AIAA 2. Post-Doctoral Fellow. Member, AIAA 3. Graduate Research Assistant. Student Member, AIAA

Model for rotor tip vortex-airframe interaction. II - Comparison with experiment

The interaction between a vortex and a solid boundary involves the development of a variety of length and time scales, some of which are due to very strong viscous effects. Prediction of these phenomena from first principles would remove a major obstacle in the computation of flows around rotorcraft. Quantitative comparisons are made with experimental results for a rotor tip vortex approaching a cylindrical airframe with a hemispherical leading edge.

Measurements of the Near Wake of a Rotor in Forward Flight

This paper describes initial measurements of the near wake of a 2-bladed teetering, untwisted, squaretipped rotor in forward flight. Issues of periodicity and repeatibility of the core are studied using laser velocimetry and flow-visualization on the front of the rotor wake where cycle to cycle variations are expected. Core passage uncertainty from cycle to cycle is less than 1° of rotor azimuth. Velocity measurements on the advancing blade side show wake-like core-axial velocities higher than the core circumferential velocity for the first 180° of vortex age. Both the axial and circumferential velocities approach 50% of blade tip speed. The inboard vortex sheet has substantial wake-like velocities and rolls up into a concentrated circulatory region of high wake-like axial velocity rotating opposite to the tip-vortex, within 30° from the blade. The fully-developed values of the vortex core circulation, radius and axial velocity remain constant over 180° of age as current measurements indicate. Measurements on the front, rear and retreating blade side of the rotor are in progress.

Turbulent static pressure fluctuations away from flow boundaries

A technique for measuring static pressure fluctuations and the pressure-velocity correlation away from walls in interior and exterior turbulent flows is described. Direct response calibration using Fourier transform techniques enables the use of a microphone pitot probe and a hot-film sensor to derive the static pressure spectrum. Experiments verifying the assumptions made in using this technique are described. Theoretical computations are made that show excellent agreement with the measured values. The pressure fluctuations away from walls are seen to be much larger than those sensed by wall-mounted transducers.

Time Resolved Thermometry by Simultaneous Thermocouple and Rayleigh Scattering Measurements in a Turbulent Flame

Abstract This paper investigates the utility and accuracy of the use of fine-wire thermocouples for time resolved thermometry in turbulent flames. The problem, if course, is that there is no unique time constant in a turbulent flame with widely fluctuating temperature. An analytical formulation is presented which shows that while little error may be expected in determination of r.m.s. values of temperature, there may be a substantial error made in deduction of the mean temperature, under certain circumstances. Experimentally, in a premixed methane-air flame, a comparison is made of a time resolved compensated thermocouple measurement with a simultaneous measurement by molecular Rayleigh scattering, and the results compared with the theoretical predictions. The thermocouple, compensated through a single time constant, behaved admirably, under these conditions.

Tailored Force Fields for Space‐Based Construction

In Space, minor forces exerted over long periods can produce major results. Force fields of various kinds can be used to build large structures, superseding the human‐intensive construction techniques of today. In this paper we consider how several techniques now used in other fields can be generalized and applied to Space‐based construction. Radiation pressure exerted by coherent beams on scattering objects is today used in microscale positioning. Standing‐wave fields offer important advantages ‐ the radiation force in a standing wave field can be 3 orders of magnitude greater than that of the source. The strong analogy between optical/electromagnetic and acoustic radiation is used to extend a microgravity flight result from acoustic standing wave fields to electromagnetic fields. ‐ walls of complex shape can be formed automatically. The interim architecture to bootstrap an economy which will permit large‐scale construction projects is briefly considered.

Scalar-velocity correlations in a turbulent premixed flame

Measurements are described of the correlations between fluctuations in velocity, static pressure and temperature in a premixed turbulent jet flame. Two component laser velocimetry, thermocouples and a cooled microphone Pitot probe are employed along with appropriate response compensation and spectral analysis techniques to obtain the correlations from time-resolved measurements. Axial and radial surveys of the flame are presented. Evidence is found of counter-gradient diffusion in some regions of the flame. The pressure-velocity correlation is seen to be negative, and its values are comparable to those of other quantities entering the turbulent kinetic energy balance.