Collaborative Research: Integrated investigation of inertial particle pair dynamics in turbulence
Meng, Hui Principal Investigator
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We propose an integrated experimental and numerical investigation of the dynamics of inertial particles in isotropic turbulence. The experimental work will be performed in a new soccer ball turbulence facility that will be built as part of the project. The new facility will be capable of producing isotropic turbulence with a Reynolds number (based on the Taylor microscale) of 500. Into this flow, we will introduce metal coated hollow glass spheres and image those spheres using an advanced version of digital holographic particle image velocimetry (DHPIV). Using a unique optical setup, we will image the particles in single and double exposure modes to obtain position and velocity statistics. Additionally, we will perform direct numerical simulations (DNS) that will be used to: (i) advance the DHPIV technique; and (ii) complement the experimental measurements. Intellectual Merit: The measurements we propose to make in the lab and in silico will allow us to quantify two important aerosol processes: (i) two-particle dispersion of inertial particles; and (ii) inertial particle collision rates, both for the first time. DHPIV will capture position and relative velocity statistics, and through kinematic relationships these data will be used to quantify the dispersion rate and the collision kernel as a function of the particle parameters (Stokes numbers) and Reynolds number. The new facility, combined with a judicious choice of particles, will allow us to isolate the effects of each parameter. A crucial aspect of the velocity measurement is the accurate pairing of particles in the two images. Current algorithms do not work well for inertial particles that don't necessarily follow the flow or remain highly correlated. With the aid of DNS, we will develop a new matching algorithm based on sweeping the time lapse between images. The DNS too will be advanced under this study. Our current algorithm is capable of performing 10243 simulations on our 32-node cluster. However, in order to match the conditions of the proposed experiments, we must increase the resolution. We will modify the data structure of our code so as to take advantage of recent developments in the 3D fast Fourier transform. The new code will be able to run on 100's and even 1000's of processors on the Texas Advanced Computing Center, enabling 20483 simulations and Reynolds numbers of 500. In this way, we will continue our tradition of making quantitative comparisons with the experiments. Additionally, DNS yields more information than the experiments about the flow field, as well as allows us to study Lagrangian statistics. We will perform these studies to test assumptions we have made in the analysis of the experiments, as well as to advance our theoretical understanding of particle dispersion and collision. Broader Impacts: The motion of discrete particles in a turbulent fluid is of great significance to a broad range of engineering flows as well as natural flows. From understanding the competition between growth and oxidation of soot particles in a diesel engine, to quantifying the impact these particles have on the global climate, we are challenged to describe the dispersive and collisional properties of particles in order to get these predictions right. Historically our understanding of turbulence has gone hand-in-hand with our ability to measure the key variables, either experimentally or computationally. The goal of this proposal is to measure the statistical quantities that will allow us to quantify these two important aerosol processes. These results will stimulate an exciting new theoretical understanding, both within our group and elsewhere. Our approach is unconventional in that we intermingle DNS and experiment completely. Indeed, a strength of this work has been our ability to make quantitative comparisons between DNS and experiments. We meet weekly via videoconference to thoroughly discuss all aspects of the work. This provides a rich environment for students, who are exposed, at a high level, to all of the activities. The PIs have been heavily involved with outreach throughout their careers. Recent activities include recruitment and mentoring of women and underrepresented minorities at their respective institutions, outreach within the community, and organization of a series of high profile workshops at the NSF directed towards encouraging underrepresented minorities into the academy.