Particle tracer response across shocks measured by PIV

Abstract

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 s p and length n 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 s p and interrogation window up to n 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 ls, corresponding to values commonly reported in literature, to almost 0.3 ls 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 ls. 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.

Particle imaging through planar shock waves and associated velocimetry errors

“…Over the shock, the flow velocity decreased to 440 m/s and the density increased to 0.86 kg/m 3 . The flow was seeded with 50-nm TiO 2 particles, which are known to form agglomerates of much larger size (Scarano and Van Oudheusden 2003;Ragni et al 2011). The particles were illuminated by a double-pulse Nd:YAG laser (400 mJ per pulse) in a 1-mm-thick light sheet at the center of the test section.…”

Decay of the supersonic turbulent wakes from micro-ramps

“…For these conditions (Mach 2.0 flow), a relaxation time of about 2 µs for the DEHS particles was determined by Ragni et al [20]. The particles were injected into the settling chamber through a seeding probe with multiple orifices.…”

“…The uncertainty due to the flow facility, such as temperature effects is verified to be negligible. The effects of the finite data ensemble size, the image analysis (cross-correlation) algorithm and finite particle tracers' response time [20] are considered here.…”

“…According to the experiment of Ragni et al [20], the response time was estimated to be 1.92µs for DEHS particle. By multiplying the response time and the relevant particle acceleration, which is estimated to be 5×10 6 m/s 2 according to the centrifugal acceleration in the vortical motion, the uncertainty results to be approximately 2%U∞.…”

Determination of mean pressure from PIV in compressible flows using the Reynolds-averaging approach

“…The flow was seeded with Di-Ethyl-Hexyl-Sebacat (DEHS) particles. The particles have a nominal diameter of 1 µm and a response time of about 2 µs (Ragni et al 2010). The particles were introduced into the settling chamber via a seeding rake connected to a PIVTEC atomizing DEHS seeder.…”