Walter R. Lempert

Bio: Walter R. Lempert is an academic researcher from Ohio State University. The author has contributed to research in topics: Laser & Mach number. The author has an hindex of 38, co-authored 218 publications receiving 4231 citations.

Transverse velocity increments in turbulent flow using the RELIEF technique

Abstract: Non-intrusive measurements of the streamwise velocity in turbulent round jets in air are performed by recording short-time displacements and distorsions of very thin tagging lines written spanwise into the flow. The lines are written by Raman-exciting oxygen molecules and are interrogated by laser-induced electronic fluorescence (relief). This gives access to the spatial structure of transverse velocity increments without recourse to the Taylor hypothesis. The resolution is around 25 μm, less than twice the Kolmogorov scale η for the experiments performed (with Rλ≈360–600).The technique is validated by comparison with results obtained from other techniques for longitudinal or transverse structure functions up to order 8. The agreement is consistent with the estimated errors – a few percent on exponents determined by extended-self-similarity – and indicates significant departures from Kolmogorov (1941) scaling.Probability distribution functions of transverse velocity increments Δu over separations down to 1:8η are reported for the first time. Violent events, with Δu comparable to the r.m.s. turbulent velocity fluctuation, are found to take place with statistically significant probabilities. The shapes of the corresponding lines suggest the effect of intense slender vortex filaments.

Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses

Abstract: The paper discusses experimental characterization of a surface dielectric barrier discharge plasma sustained by repetitive, high-voltage, nanosecond duration pulses. The discharge pulse energy is controlled primarily by the pulse peak voltage and scales approximately linearly with the length of the electrodes. Images of the plasma generated during the discharge pulse, taken by a nanosecond gate ICCD camera, show that the plasma remains fairly uniform in the initial phase of the discharge and becomes filamentary at a later stage. The temperature rise in the discharge, operated in both continuous mode and in burst mode, is inferred from UV/visible emission spectra. Phase-locked schlieren images are used to measure the speed of the compression waves generated by the nanosecond pulse discharge and the density gradient in the wave. The density gradient is inferred from the schlieren images using absolute calibration by a pair of wedged windows. The results demonstrate that discharge filaments generate compression waves with higher amplitude and higher speed compared with waves produced in a diffuse discharge. The density gradient in the compression waves is compared with numerical modeling of propagating compression waves produced by short-pulse localized heating, and shows satisfactory agreement between the model and the experimental results.

Ignition of premixed hydrocarbon–air flows by repetitively pulsed, nanosecond pulse duration plasma

Abstract: The paper presents results of plasma assisted combustion experiments in premixed hydrocarbon–air flows excited by a low-temperature transverse repetitively pulsed discharge plasma. The experiments have been conducted in methane–air and ethylene–air flows in a wide range of equivalence ratios, flow velocities, and pressures. The plasma was generated by a sequence of high-voltage (∼10 kV), short pulse duration (∼50 ns), high repetition rate (up to 50 kHz) pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation. On the other hand, the extremely low duty cycle of the repetitively pulsed discharge, ∼1/500, greatly improves the discharge stability and helps sustaining diffuse and uniform nonequilibrium plasma. Generating this repetitively pulsed plasma in premixed hydrocarbon–air flows results in ignition and flameholding, occurring at low plasma temperatures, 140–300 °C, inferred from the nitrogen second positive band system spectra. At these conditions, the reacted fuel fraction, measured by the FTIR absorption spectroscopy, is up to 80%. The experiments demonstrate significant methane and ethylene conversion into CO, CO2, and H2O even at the conditions when there is no flame detected in the test section. At these conditions, fuel oxidation occurs due to plasma chemical reactions, without ignition. This provides additional evidence for the nonthermal fuel oxidation triggered by plasma-generated radicals.

Quantitative flow visualization in unseeded flows

Abstract: ▪ Abstract The various tools for flow visualization have been significantly expanded over the past several years through the use of molecular scattering and molecular laser-induced fluorescence. These approaches have added the capability of sampling individual small volume elements within a flow, and by using cameras for detection, they are easily extended to sample lines and cross-sectional planes. This localized measurement capability means that these approaches can be made quantitative even in complex and/or unsteady flow fields. If the molecular species is naturally occurring, such as oxygen or nitrogen in air, then no seeding is required. Furthermore, in these applications, images of the flow can be frozen in time by using a short pulse laser for illumination. The distribution of the molecules reflects the true physics of the flow, so even raw images taken in this manner give an immediate understanding of flow field properties. With proper calibration, the images can be further analyzed to yield quan...

Review of ultra-high repetition rate laser diagnostics for fluid dynamic measurements

Abstract: Recent advances in ultra-high repetition rate (100?kHz and above) laser diagnostics for fluid dynamic measurements are reviewed. The development of the pulse burst laser system, which enabled several of these advances, is described. The pulse burst laser system produces high repetition rate output by slicing the output of a low power continuous wave laser and passing the resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systems have been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generated at between 50 and 1000?kHz. Combined with the capabilities of several types of commercially available high-speed cameras, these systems have been used to make a wide variety of high repetition rate and 3D flow measurements. Several examples of various high repetition rate laser diagnostics are described, including flow visualization, filtered Rayleigh scattering, planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence, molecular tagging velocimetry and 3D flow visualization.

Interstellar Turbulence I: Observations and Processes

Abstract: ▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

The phenomenology of small-scale turbulence

Abstract: Small-scale turbulence has been an area of especially active research in the recent past, and several useful research directions have been pursued. Here, we selectively review this work. The emphasis is on scaling phenomenology and kinematics of small-scale structure. After providing a brief introduction to the classical notions of universality due to Kolmogorov and others, we survey the existing work on intermittency, refined similarity hypotheses, anomalous scaling exponents, derivative statistics, intermittency models, and the structure and kinematics of small-scale structure—the latter aspect coming largely from the direct numerical simulation of homogeneous turbulence in a periodic box.

Plasma-liquid interactions: A review and roadmap

Abstract: Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

The Spectrum of Turbulence

Abstract: The clustered distribution of interstellar matter raised C.F. von Weizsacker’s interest in turbulent flow with its nested vortex structures, its intermittent distribution of strongly active, dissipative turbulent bursts amidst more quiet regions, all this strongly fluctuating in time. Evidently many time scales are present, the smaller vortex structures circulating faster, being advected by the slower, larger ones. Also the spatial structures of the turbulent vortices or eddies display many scales. The prevailing impression of a turbulent flow field is its structural similarity on the various scales: zoomed smaller parts of the flow field look like larger ones, in a statistical sense. Such systems are properly described by power or scaling laws of the physical quantities of interest.

Plasma assisted ignition and combustion

Abstract: In recent decades particular interest in applications of nonequilibrium plasma for the problems of plasma-assisted ignition and plasma-assisted combustion has been observed. A great amount of experimental data has been accumulated during this period which provided the grounds for using low temperature plasma of nonequilibrium gas discharges for a number of applications at conditions of high speed flows and also at conditions similar to automotive engines. The paper is aimed at reviewing the data obtained and discusses their treatment. Basic possibilities of low temperature plasma to ignite gas mixtures are evaluated and historical references highlighting pioneering works in the area are presented. The first part of the review discusses plasmas applied to plasma-assisted ignition and combustion. The paper pays special attention to experimental and theoretical analysis of some plasma parameters, such as reduced electric field, electron density and energy branching for different gas discharges. Streamers, pulsed nanosecond discharges, dielectric barrier discharges, radio frequency discharges and atmospheric pressure glow discharges are considered. The second part depicts applications of discharges to reduce the ignition delay time of combustible mixtures, to ignite transonic and supersonic flows, to intensify ignition and to sustain combustion of lean mixtures. The results obtained by different authors are cited, and ways of numerical modelling are discussed. Finally, the paper draws some conclusions on the main achievements and prospects of future investigations in the field.