Laser Pulse Separation Research Articles

Determination of the flow field inside a Sonolator liquid whistle using PIV and CFD

PIV experiments and CFD simulations were carried out on a mimic of a pilot scale Sonolator Model A inline liquid whistle mixer (Sonic Corp. USA) for water in turbulent flow for Reynolds numbers at the orifice between 17,500 and 77,200. Three different sizes of orifice were used. The results from PIV were compared with the CFD simulations, with both global and local validations being performed. The former focusses on the pressure drop across the Sonolator and the latter was carried out by comparison of local values of velocity magnitude, turbulent kinetic energy and local specific turbulent energy dissipation rate. Velocity magnitude values were found to agree within 10% more than fifteen millimetres downstream of the orifice. A similar level of agreement was found at the orifice for lower flow rates and larger orifices. Factors which precluded this level of agreement for higher flow rates and smaller orifices were the appearance of cavitation and a minimum achievable laser pulse separation, limiting the maximum velocity measureable by the PIV. Agreement between PIV and CFD was also poorer for the turbulent parameters, although the PIV and CFD data had similar trends, the magnitudes were different. The reasons for these discrepancies include the fact that the oscillation period for the orifice jet could not be precisely identified and eliminated from the data and errors inherent in the methods used to estimate the local specific turbulent energy dissipation rate from the PIV data.

Particle tracer response across shocks measured by PIV

The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of the particle response time τ p and length ξ p based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution. Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation time up to τ p and interrogation window up to ξ p still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from 2 to 3 μs, corresponding to values commonly reported in literature, to almost 0.3 μs when particles are properly dehydrated and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with a measured response time lower than 0.4 μs. The same procedure is applied to attempt the measurement of individual particle tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the particle velocity distribution.

Ab initiotime-dependent density-functional-theory study of the frequency comb structure, coherence, and dephasing of multielectron systems in the vuv-xuv regimes via high-order harmonic generation

We present an ab initio nonperturbative investigation of the frequency comb structure and coherence within each order of the high-order harmonic generation (HHG) of rare-gas atoms by means of the time-dependent density-functional theory (TDDFT) with optimized effective potential (OEP) and self-interaction correction (SIC). The $\text{TDDFT}+\text{OEP}\ensuremath{-}\text{SIC}$ equations are solved accurately and efficiently by means of the time-dependent generalized pseudospectral technique. We found that a nested comb structure appears within each order of the harmonics, ranging from the first harmonic all the way to the cutoff region. We explore in detail the temporal coherence and robustness of the comb structure by varying the laser-pulse separation $\ensuremath{\tau}$, the number of pulses $N$, the phase difference between pulses $\ensuremath{\Delta}\ensuremath{\phi}$, and the laser intensity. The frequency comb structure and coherence are preserved in each harmonic regardless of the values of $\ensuremath{\tau}$ and $N$ used for the case of weak and medium strong incident laser-pulse trains. The time-frequency characteristics of the HHG coherence structure are analyzed in details by means of the wavelet transform of the time-dependent induced dipoles. The interference modulation can be attributed to the constant phase relationship of harmonics among successive pulses. However, under superstrong fields, nonuniform and substantial ionization takes place during each pulse, jeopardizing the temporal coherence of the emitted frequency comb modes. Finally, we found that the dynamical electron correlation, which is included in the present $\text{TDDFT}+\text{OEP}\ensuremath{-}\text{SIC}$ treatment but not in the single-active-electron model, is significant for the quantitative exploration of the frequency comb structure and coherence of higher harmonics.

Ab initiotheoretical investigation of the frequency comb structure and coherence in the vuv-xuv regimes via high-order harmonic generation

We present a fully ab initio quantum investigation of the frequency comb structure and coherence within each order of the high-order harmonic generation (HHG) spectrum in the high-frequency vuv-xuv regime. The HHG spectrum driven by a train of equal-spacing short laser pulses is calculated by propagating the time-dependent Schr\odinger equation accurately and efficiently by means of the time-dependent generalized pseudospectral method. We explore the comb structure and coherence by varying the laser pulse separation $\ensuremath{\tau}$, the number of pulses $N$, and the laser intensity. We find that a nested comb structure appears within each of the harmonics, ranging from the first harmonic all the way to the cutoff harmonic, and this global pattern persists regardless of the values of $\ensuremath{\tau}$ and $N$ used. The comb structure of the harmonics originates from quantum interferences among induced dipole pulses. Finally, the frequency comb structure prevails even in the presence of appreciable ionization.

Benchmarking Particle Image Velocimetry with Laser Doppler Velocimetry for Rotor Wake Measurements

An experiment was conducted to identify and measure the sources of uncertainty that are associated with the application of particle image velocimetry to the measurement of the vortical wakes trailing from helicopter rotor blades. Phase-resolved, three-component particle image velocimetry measurements were performed in the wake of a subscale rotor operating in hover, and were compared with high-resolution three-component laser Doppler velocimetry measurements obtained with the same rotor under identical operating conditions. This helped formulate the essential experimental conditions that need to be satisfied for particle image velocimetry to accurately resolve the high-velocity gradient, high streamline curvature flows that are present inside the rotor wake and in the blade tip vortices. Uncertainties associated with the calibration, acquisition, and processing of the particle image velocimetry images were analyzed in detail. It was shown that the optimization of laser pulse separation time is fundamental to reduce the errors associated with acceleration and curvature effects. Similarly, the interrogation window size was shown to play a critical role in determining the velocity gradient bias errors. The correlation between the laser Doppler velocimetry and particle image velocimetry measurements of the tip vortex characteristics, such as core radius and peak swirl velocity, were found to be excellent.

Weak magnetic field behavior of photon echo in a LuLiF4:Er3+ crystal

We report the first observation and study of the photon echo in Er3+:LuLiF4. The energy transition is 4 I 15/2 → 4 F 9/2 (λ = 6536 Å). The density of ErF3 is 0.025 wt %. The operation temperature is 1.9 K. Measurements were made at low (up to 1200 Oe) and even zero external magnetic fields. We studied the behavior of the photon echo intensity vs. the magnetic field magnitude and direction about the crystal axis C and vs. the laser pulse separation t 12 and observed an exponential growth and then, after a certain plateau, an exponential decrease in the photon echo intensity as a function of magnetic field upon increasing the magnetic field from zero. The parameters describing the exponential growth and decrease are independent of the direction of magnetic field. The value of the magnetic field (∼20–200 Oe) at which the echo intensity is maximal and the value of the maximum itself decrease with increasing pulse separation t 12 and the angle Θ between the magnetic field and crystal axis. The echo intensity decreases exponentially with increasing Θ. The parameter describing the exponential decrease is independent of the magnitude of the field. The echo intensity as a function of pulse separation shows exponential decay. The phase relaxation time depends on the magnitude and direction of the magnetic field. T 2 is equal to 202 ± 16 ns at zero magnetic field. A phenomenological formula is suggested, which qualitatively presents the mentioned dependences, and the polarization properties of the backward photon echo in this crystal are studied. Because the ion of trivalent erbium is an optimum data carrier, the above results show that fine control of the multichannel transfer of processed optical information may be achieved by weak magnetic fields.

NUMERICAL MODELLING AND PARTICLE IMAGE VELOCIMETRY MEASUREMENT OF THE LAMINAR FLOW FIELD INDUCED BY AN ENCLOSED ROTATING DISC

The fluid flow field within an enclosed cylindrical chamber with a rotating flat disc was calculated using a finite volume computational fluid dynamics (CFD) model and compared with particle image velocimetry (PIV) measurements. Two particular laminar cases near the Transitional flow regime were investigated : Reynolds number Re = 2.5 x 10 4 , chamber aspect ratio G (h/R d ) = 0.2 and Re = 4.2 x 10 4 , G (h/R d ) = 0.217. This enabled direct comparison with the numerical and experimental results reported by other researchers. The computational details and some major factors that affect the computed accuracy and convergence speed are also discussed in detail. PIV results containing some 4300 velocity vector points in each of seven planes for each case were obtained from the flow field parallel to the rotating disc. It was found that PIV results could be obtained in planes within the boundary layers as well as the core flow by careful use of a thin laser illumination sheet and correct choice of laser pulse separation. There was close agreement between numerical results, the present PIV measurements and other reported experimental and numerical results.

Fast electro-optical system for pulsed ESPI carrier fringe generation

Pulsed Electronic Speckle Pattern Interferometry (ESPI) offers the possibility of analysing fast transient phenomena in experimental mechanics. However, due to the short pulse separation required, addition fringes with very low visibility are generated. Several methods have recently been developed to process these data but they all suffer some limitations. This paper presents an electro-optical system based on the use of a Pockels cell which enables the introduction of carrier fringes between the firing of two consecutive laser pulses. This allows calculation of the wrapped phase distribution from a single fringe pattern using the Spatial Synchronous Detection method and enables the laser pulse separation to be significantly reduced. Preliminary experimental measurements performed using a continuous wave laser are used to illustrate the performance of this approach.

Experimental thermal coupling of laser beams

Energy in a laser beam can be transferred to a solid and increase its thermal energy. In a series of experiments, this thermal coupling has been investigated for pulsed CO2 laser beams. Target material, air pressure, focal spot size, and laser pulse separation time were varied. A feature often seen is an enhancement of the thermal coupling process above a fluence or intensity threshold. A method for the determination of the spatial profile of the heat absorbed by the target using a single on-axis thermocouple is presented. For the focal spot size and other conditions in these experiments, this technique shows a heating profile with a width much larger than the focal spot width.