Computational Fluid Dynamics Turbulence Model and Experimental Study for a Fontan Cavopulmonary Assist Device.

Head-flow HQ curves for a Fontan cavopulmonary assist device (CPAD) were measured using a blood surrogate in a mock circulatory loop and simulated with various computational fluid dynamics (CFD) models. The tests benchmarked the CFD tools for further enhancement of the CPAD design. Recommended Reynolds-Averaged Navier-Stokes (RANS) CFD approaches for the development of conventional ventricular assist devices (VAD) were found to have shortcomings when applied to the Fontan CPAD, which is designed to neutralize off-condition obstruction risks that could contribute to a major adverse event. The no-obstruction condition is achieved with a von Karman pump, utilizing large clearances and small blade heights, which challenge conventional VAD RANS-based CFD hemodynamic simulations. High-fidelity large eddy simulation (LES) is always recommended; however, this may be cost-inhibitive for optimization studies in commercial settings, thus the reliance on RANS models. This study compares head and power predictions of various RANS turbulence models, employing experimental measurements and LES results as a basis for comparison. The models include standard k-ϵ, re-normalization group k-ϵ, realizable k-ϵ, shear stress transport (SST) k-ω, SST with transitional turbulence, and Generalized k-ω. For the pressure head predictions, it was observed that the standard k-ϵ model provided far better agreement with experiment. For the rotor torque, k-ϵ predictions were 30% lower than LES, while the SST and LES torque values were near identical. For the Fontan CPAD, the findings support using LES for the final design simulations, k-ϵ model for head and general flow simulation, and SST for power, shear stress, hemolysis, and thrombogenicity predictions.

Analysis of 6YO pediatric human body model kinematics and kinetics to determine submarining across naturalistic seating postures.

OBJECTIVES: The aim of this study was to analyze the kinematics and kinetics of a naturalistically seated 6-year-old (6YO) pediatric human body model and evaluate the metrics described by earlier studies for pediatric ATDs to indicate whether different postures and booster seats were more associated with submarining than others in a frontal impact. METHODS: The PIPER 6YO pediatric human body model was restrained on a lowback (LBB) and a highback (HBB) booster child restraint seat (CRS) in four naturalistic seating postures: leaning-forward, leaning-inboard, leaning-outboard, and a pre-submarining posture, and a baseline reference seating position as per the FMVSS No. 213 protocol. A 2012 mid-size sedan finite element (FE) model was used as the vehicle environment. A standard 3-point lap-shoulder belt system was modeled to restrain the child and the CRS in the left-rear vehicle seat. Additionally, a No-CRS condition was modeled in a reference posture and pre-submarining posture in which the occupant's legs bent over the edge of the rear seat. 12 conditions were simulated in LS-DYNA R10.1.0, and kinematics and kinetics were compared to metrics as per prior literature: 1) maximum femur displacement and pelvis rotation, 2) maximum knee-head excursion and maximum change in torso angle, 3) lap belt trajectory relative to pelvis's coordinate frame. RESULTS: The pre-submarining posture on the HBB depicted submarining in all metrics except for the lap belt trajectory. Only the pre-submarining posture in No-CRS depicted submarining through analysis of all metrics. For this pre-submarining No-CRS condition, the mid-abdominal compression was approximately 5 times greater than the average of the mid abdominal compression depths of all other cases and maximum abdominal pressure was at least 22.9 kPa higher than the rest of the conditions. CONCLUSIONS: The results of this study suggest that metrics used to assess submarining for 6YO pediatric occupants in frontal impacts may need to be updated so that they are more accurate for both simulated and physical studies. In addition, the results of this study could be used to design booster seats that discourage postures that could lead to an increased likelihood of submarining-like characteristics in a frontal crash impact.

In-depth analysis of crash contributing factors and potential ADAS interventions among at-risk drivers using the SHRP 2 naturalistic driving study.

OBJECTIVE: Motor vehicle crashes remain a significant problem. Advanced driver assistance systems (ADAS) have the potential to reduce crash incidence and severity, but their optimization requires a comprehensive understanding of driver-specific errors and environmental hazards in real-world crash scenarios. Therefore, the objectives of this study were to quantify contributing factors using the Strategic Highway Research Program 2 (SHRP 2) Naturalistic Driving Study (NDS), identify potential ADAS interventions, and make suggestions to optimize ADAS for real-world crash scenarios. METHODS: A subset of the SHRP 2 NDS consisting of at-fault crashes (n = 369) among teens (16-19 yrs), young adults (20-24 yrs), adults (35-54 yrs) and older adults (70+ yrs) were reviewed to identify contributing factors and potential ADAS interventions. Contributing factors were classified according to National Motor Vehicle Crash Causation Survey pre-crash assessment variable elements. A single critical factor was selected among the contributing factors for each crash. Case reviews with a multidisciplinary panel of industry experts were conducted to develop suggestions for ADAS optimization. Critical factors were compared across at-risk driving groups, gender, and incident type using chi-square statistics and multinomial logistic regression. RESULTS: Driver error was the critical factor in 94% of crashes. Recognition error (56%), including internal distraction and inadequate surveillance, was the most common driver error sub-type. Teens and young adults exhibited greater decision errors compared to older adults (p < 0.01). Older adults exhibited greater performance errors (p < 0.05) compared to teens and young adults. Automatic emergency braking (AEB) had the greatest potential to mitigate crashes (48%), followed by vehicle-to-vehicle communication (38%) and driver monitoring (24%). ADAS suggestions for optimization included (1) implementing adaptive forward collision warning, AEB, high-speed warning, and curve-speed warning to account for road surface conditions (2) ensuring detection of nonstandard road objects, (3) vehicle-to-vehicle communication alerting drivers to cross-traffic, (4) vehicle-to-infrastructure communication alerting drivers to the presence of pedestrians in crosswalks, and (5) optimizing lane keeping assist for end-departures and pedal confusion. CONCLUSIONS: These data provide stakeholders with a comprehensive understanding of critical factors among at-risk drivers as well as suggestions for ADAS improvements based on naturalistic data. Such data can be used to optimize ADAS for driver-specific errors and help develop more robust vehicle test procedures.

Pediatric occupant human body model kinematic and kinetic response variation to changes in seating posture in simulated frontal impacts - with and without automatic emergency braking.

OBJECTIVE: The study quantifies the kinematics of children in booster child restraint systems (CRSs) in various naturalistic seating postures exposed to frontal impacts in a full-vehicle environment, with and without the application of pre-crash automatic emergency braking. METHODS: The PIPER 6YO and 10YO pediatric human body models were positioned in CRSs. The 6YO was restrained on a lowback (LBB) and highback (HBB) booster, while the 10YO was positioned on an LBB and in a NoCRS condition. All simulations used the 3-point seatbelt. The child models were pre-positioned (gravity settled, seatbelt tensioned) in four naturalistic seating postures: leaning-forward, leaning-forward-inward, leaning-forward-outward, and a pre-submarining position, along with a baseline reference seating position. A 2012 Toyota Camry finite element (FE) model was used as the vehicle environment. A standard 3-point lap-shoulder belt system was modeled to restrain the child and CRS in the left-rear vehicle seat. Two vehicle impact cases were considered: with and without a pre-crash AEB. For with-AEB cases, a pre-crash phase was run to incorporate postural changes due to the application of AEB. All seating positions were ultimately subjected to a full-frontal rigid-barrier impact at 35 MPH. A total of 40 conditions were simulated in LS-DYNA. RESULTS: Injury metrics varied widely for both occupants. Shoulder belt slippage was observed for the 6YO leaning-forward-inward on HBB. No head contact was observed for any simulated cases. Forward-leaning and forward-inward-leaning postures generally had greater head excursion across all seating postures. The lap belt rode over the pelvis for pre-submarining postures. Injury metrics for cases with pre-crash AEB were lower compared to their corresponding without-AEB cases. HIC15, head acceleration, upper neck tension force, and upper neck flexion moment were similar or lower for with-AEB scenarios. CONCLUSIONS: Pre-crash AEB reduces the effect of the impact despite the same collision speed as cases without-AEB. This is primarily due to the limited travel distance of the occupant, thus, starting an earlier ride-down during the crash. Moreover, different initial seating postures lead to a wide range of injury exposures. Vehicle and child restraint design should incorporate these seating postures to ensure robust protection of the occupant in a crash.

Responses of the scaled pediatric human body model in the rear- and forward-facing child seats in simulated frontal motor vehicle crashes.

Objective: The study presents the first-ever endeavor at developing 18-, 24-, 30-, 36-, 42-, and 48-month-old pediatric finite element models from the 6-year-old PIPER human body model as a baseline and comparing their responses systematically in rear-facing and forward-facing simulations across similar boundary conditions.Methods: A 6-year-old PIPER model was scaled down to create anthropometric models of the 18-, 24-, 30-, 36-, 42-, and 48-month-old child using the PIPER scaling tool. The models were installed on a convertible car seat (rear-facing and forward-facing configurations) installed with a 3-point lap-shoulder belt in the rear outboard seat of a 2012 Toyota Camry vehicle model finite element model and setup for full-frontal crash simulation (24 G, 120 ms pulse).Results: The forward-facing models showed higher head resultant accelerations for 24-, 36-, 42-, and 48-month-old models (reduction for rear-facing seats ranging from 10% to 32%). For the 18- and 30-month-old models, the maximum head acceleration showed similar values (difference of less than 10%). Upper neck forces and moments were consistently lower for rear-facing models compared to forward-facing. The neck forces were reduced by 83%-90% and the neck moments were reduced by 63%-85% in the rear-facing models compared to their respective forward-facing configurations. The reduction in head injury criterion (HIC36) for rear-facing models ranged from 14% to 51%. The neck injury criterion (Nij) for all forward-facing models was 6 to 9 times the values of their rear-facing counterpart.Conclusions: The study shows the potential benefit of rear-facing orientation compared to forward-facing for children up to 4 years of age in a controlled environment.

Evaluating the response of the PIPER scalable human body model across child restraining seats in simulated frontal crashes.

OBJECTIVE: Booster seats ensure appropriate belt fit for children that a traditional vehicle seat belt cannot offer to small occupants. In this study, the responses of the PIPER 6-year-old human body model are compared to the traditional Q6 anthropomorphic test dummy (ATD). METHODS: Eight frontal impact finite element simulations were run using 4 different child restraining systems on the FMVSS 213 test bench. Kinematics and kinetics were extracted and compared between the 2 child models. RESULTS: The PIPER 6-year-old showed variation by 11.2 ± 14.1% (head resultant acceleration, G), 20.4 ± 50.3% (chest resultant acceleration, G), 272.9 ± 188.4% (chest displacement, mm), 24.8 ± 17.5% (maximum head excursion, mm), -31.5 ± 5.1% (neck force, Fz, N), -73.8 ± 2.8% (neck moment, My, N.m), and -60.4 ± 7.2% (Nij) compared to the Q6. However, the kinematics of both models were nearly similar. CONCLUSIONS: The PIPER model has a flexible neck and shows higher chest displacement compared to the Q6. We hypothesize that this is due to the inherent anatomical and mechanical differences between the human body model and the ATD model. More research is needed to explore these differences systematically.