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
Germination ecology of Central Asian Stipa spp: differences among species, seed provenances, and the importance of field studies
Stipa-species are wide-spread in Central Asia, but sexual reproduction in the dry steppes is rare. To facilitate conservation and restoration of these important rangelands, we studied germination characteristics of three common Mongolian Stipa-species under field- and lab conditions. Seeds of Stipa krylovii, Stipa gobica and Stipa glareosa were sown at the study site in Southern Mongolia over two consecutive years during which period tests were carried out to ascertain whether competition or herbivory are the main constraints of seedling establishment. In addition, we tested germination and seed viability in the laboratory under two different temperature regimes (20/10°C and 8/4°C), as well as the effect of cold-stratification. The lab experiments also included S. krylovii seeds originating from three climatically different provenances. None of the three Stipa-species seedlings emerged during the first 2 years of the field study. However, after an unusually intense rain event in the third year, 3% of S. krylovii, 0.6% of S. glareosa and 0.1% of S. gobica seeds germinated in the study plots. The factors ‘sowing-year’ and ‘vegetation’ significantly affected seedling emergence, whereas grazing had no effect at all. Under laboratory conditions a high percentage of viable seeds of S. gobica and S. glareosa germinated at both incubation temperatures, and cold-stratification had no effect on germination or viability. Germination of S. krylovii seeds required warmer temperatures and cold-stratification had a positive effect. Such evidence for dormancy was more pronounced in seeds from the moister, northern provenances. Germination of Stipa-species in the field is rare and only possible under exceptionally moist conditions. Conservation should thus concentrate on steppe conservation rather than on restoration. Where artificial reseeding is necessary, differences among species and also among different seed provenances should be taken into account.