Electronic colon cleansing (ECC) aims to segment the colon lumen from a patient abdominal image acquired using an oral contrast agent for colonic material tagging, so that a virtual colon model can be constructed. Virtual colonoscopy (VC) provides fly-through navigation within the colon model, looking for polyps on the inner surface in a manner analogous to that of fiber optic colonoscopy. We have built an ECC pipeline for a commercial VC navigation system. In this paper, we present an improved ECC method. It is based on a partial-volume (PV) image-segmentation framework, which is derived using the well-established statistical expectation-maximization algorithm. The presented ECC method was evaluated by both visual inspection and computer-aided detection of polyps (CADpolyp) within the cleansed colon lumens obtained using 20 patient datasets. Compared to our previous ECC pipeline, which does not sufficiently consider the PV effect, the method presented in this paper demonstrates improved polyp detection by both visual judgment and CADpolyp measure.

The radial relative velocity (RV) between particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence-planar 4-frame particle tracking velocimetry-using routine PIV hardware. It improves particle positioning and pairing accuracy over the 2-frame holographic approach by de Jong et al. (2010) without using high-speed cameras and lasers as in Saw et al (2014). Homogeneous and isotropic turbulent flow ( = 357) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean = 0.09, standard deviation 0.05) or high-Stokes aerosols (mean = 3.46, standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( = 398; = 0.10 and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with at small particle separations, , and decreases with at large , indicating the dominance of "path-history" and "inertial filtering" effects, respectively. However, at small the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of particles and finite light-sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that particle polydispersity increases mean inward RV at small , while finite laser thickness also overestimates mean inward RV at small , This study demonstrates the feasibility of accurately measuring RV using routine hardware and verifies, for the first time, the path-history and inertial filtering effects on particle-pair RV at large particle separations experimentally.2

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Enclosed flow apparatuses with negligible mean flow are emerging as alternatives to wind tunnels for laboratory studies of homogeneous and isotropic turbulence (HIT) with or without aerosol particles, especially in experimental validation of Direct Numerical Simulation (DNS). It is desired that these flow apparatuses generate HIT at high Taylor-microscale Reynolds numbers ( λ R ) and enable accurate measurement of turbulence parameters including kinetic energy dissipation rate and thereby λ R . We have designed an enclosed, fan-driven, highly symmetric truncated-icosahedron 'soccer ball' airflow apparatus that enables particle imaging velocimetry (PIV) and other whole-field flow measurement techniques. To minimize gravity effect on inertial particles and improve isotropy, we chose fans instead of synthetic jets as flow actuators. We developed explicit relations between λ R and physical as well as operational parameters of enclosed HIT chambers. To experimentally characterize turbulence in this nearzero-mean flow chamber, we devised a new two-scale PIV approach utilizing two independent PIV systems to obtain both high resolution and large field of view. Velocity measurement results show that turbulence in the apparatus achieved high homogeneity and isotropy in a large central region (48 mm diameter) of the chamber. From PIV-measured velocity fields, we obtained turbulence dissipation rates and thereby λ R by using the second-order velocity structure function. A maximum λ R of 384 was achieved. Furthermore, experiments confirmed that the root mean square (RMS) velocity increases linearly with fan speed, and λ R increases with the square root of fan speed. Characterizing turbulence in such apparatus paves the way for further investigation of particle dynamics in particle-laden homogeneous and isotropic turbulence.

The effects of Reynolds number ( ) and Stokes number ( ) on particle-pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare well at ≳ 10 , but were higher than both DNS and theory at ≲ 10 . For ≳ 1, 〈 − 〉 from all three matched well except for ≲ 10 , in which experimental values were higher, while 〈 2 〉 from experiment and DNS were much higher than theoretical predictions. We discuss potential causes of these discrepancies. What this study shows is the first experimental validation of and effect on inertial particle-pair 〈 − 〉 in homogeneous and isotropic turbulence.3

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