Property and Velocity Measurements in a Supersonic/Hypersonic Flow
Abstract : This work presents a non-intrusive laser-based diagnostics technique to measure instantaneous thermodynamic properties and velocity in a compressible fluid flow. The fluid is irradiated with a pulsed narrow-bandwidth Nd:YAG laser at 532 nm. Light scattering from a point in the flow field is collected over a solid angle of about + or - 20 degrees and focused into a narrow line on an intensified CCD camera using an anamorphic optical system. A molecular iodine filter absorbs the radiation at different degrees with angles, thus producing different intensity levels on each pixel along the line. A portion of the light is split and focused on the same CCD chip unfiltered, thus providing an intensity reference from which the fluid density is found. The scattering spectrum and Doppler frequency shift are dependent on the observation angle and fluid properties, and so the values of pressure, temperature and streamwise velocity at every laser pulse using a well-known scattering model.
A molecular filter based technique for simultaneous measurements of velocity and thermodynamic properties
A molecular filter based flow diagnostic technique termed Filtered Angularly-Resolved Raleigh Scattering (FARRS) is used to measure velocity, density, and temperature simultaneously. The technique uses an anamorphic optical system to record the Rayleigh scattered signal viewed at different angles and focused onto an intensified CCD camera. The experimentally obtained intensity versus viewing angle profiles are then combined with a computational model to solve for the velocity, density, and temperature. The technique was used in a supersonic axisymmetric jet operated at a variety of flow conditions from Mach 1 to Mach 2.3. Three different FARRS optical arrangements were investigated The first arrangement was used to make average measurements in the core of the jet where the velocity and thermodynamic conditions are well known. The measured values are within 11% for the velocity and 7% for the other thermodynamic properties. The second optical arrangement was used to measure three components of velocity in an isothermal supersonic jet where the temperature is approximately constant across the mixing layer. Both average velocity and turbulence intensities were determined. The final optical system measures the instantaneous streamwise velocity, density and temperature simultaneously. For all the FARRS cases, the optical configurations are described and methods of backing out the velocity, density, and temperature from the experimental intensity profiles are discussed. f Currently Assistant Professor, Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ, 08855, Member AIAA * Professor, Associate Fellow AIAA NOMENCLATURE A; = transmission b = thickness of shear layer f = frequency fo = laser frequency I = intensity Io = reference intensity k = Boltzmann's constant ko = incident unit light wave vector k, = observed unit light wave vector K = magnitude of the scattered wave vector Me = equivalent Mach number M = molecular weight P = pressure P0 = stagnation pressure r = Rayleigh scattering profile in terms of dimensional quantities R(x,y) = nondimensional Rayleigh scattering profile T = temperature T0 = stagnation temperature U = Average streamwise velocity U0 = Average streamwise velocity at jet exit core u,v, w = Instantaneous velocities Y = velocity vector V0 = most probable molecular velocity x = dimensionless frequency x, = streamwise direction y = y-parameter, ratio of the collision frequency to the acoustic spatial frequency y, = lateral direction y0 = lateral location of U/U0 = 0.5 velocity in shear layer z, = spanwise direction a = viewing angle AfD = Doppler shift A = wavelength of laser 0 = angle between the incident and scattered wave vectors p = density
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths