As discussed in Tennekes and Lumley , planar mixing layers are self-preserving so that mean velocity and turbulent stress profiles become invariant with respect to a horizontal distance . Hence, such profiles can be expressed in terms of local length and velocity scales. The self-preservation hypothesis is consistent with numerous observations that turbulent mixing close to the canopy–atmosphere interface exhibits a number of universal characteristics. Among these universal characteristics are the emergence of friction velocity and canopy height as representative velocity and length scales . In addition to exerting drag on the airflow aloft, canopy foliage acts as a source for many passive, active, and reactive scalar entities such as heat , water vapor , ozone , and isoprene resulting in a ‘‘scalar mixing layer.’’ A scalar mixing layer is a generalization of a thermal mixing layer, which is defined as a plane layer formed between two coflowing streams of same longitudinal velocity but different temperatures . Unfortunately, theoretical and laboratory investigations demonstrate that pure thermal mixing layers are not self-preserving . In fact, laboratory measurements of grid-generated turbulence in a thermal mixing layer by LaRue and Libby and Ma and Warhaft illustrate that computations utilizing self-preservation assumptions overestimate the maximum measured heat flux by more than 25%. Hence, instabilities responsible for momentum vertical transport may not transport scalars in an analogous manner. Given the complexities of mean scalar concentration profiles within forested systems , the dissimilarity in scalar and momentum sources and sinks, grow tray the large atmospheric stability variation close to the canopy–atmosphere interface, and the complex foliage distribution with height all suggest a need to further investigate the applicability of Raupach et al.’s ML analogy to scalar mass transport.

The objective of this study is to investigate whether the characteristics of active turbulence, typically identified from vertical velocity statistics, can be extended to mass transport at the canopy–atmosphere interface. In Raupach et al. , particular attention was devoted to eddy sizes responsible for the generation of coherency and spectral peaks in the vertical velocity. Here, eddy sizes responsible for cospectral peaks of scalar fluxes and their relationship to active turbulence are considered. Active turbulence is identified from orthogonal wavelet decomposition that concentrates much of the vertical velocity energy in few wavelet coefficients. The remaining wavelet coefficients, associated with ‘‘inactive,’’ ‘‘wake,’’ and ‘‘fine scale turbulence’’ are thresholded using a Lorentz wavelet approach advanced by Vidakovic and Katul and Vidakovic . Wavelet spectra and cospectra are also used to investigate the characteristics of active turbulence for the two stands and for a wide range of atmospheric stability conditions. Much of the flow statistics derived by Raupach et al. in the time domain are also extended to the wavelet domain. Since canopy sublayer turbulence is intermittent in the time domain with defined spectral properties in the Fourier domain, orthonormal wavelet decompositions permit a simultaneous time–frequency investigation of both flow characteristics.The use of utility All-Terrain Vehicles as working machines adds a heavy burden to the American public health system . According to data from the 2019 National Electronic Injury Surveillance System, over 95,000 emergency department visits were due to an ATV-related incident. Around 36.8 % of those ED visits involved youth younger than 18 years old, and 15.3 % of the incidents happened on farms or ranches . Indeed, using utility ATVs in the farm setting is extremely dangerous for youth; ATVs are one of the most frequently cited causes of incidents among farm youth .

ATVs have three or four low-pressure tires, narrow wheelbase, and high center of gravity . Due to safety concerns, the production of three-wheelers ceased in the United States in 1987 . Three-wheelers were known to be even more prone to rollovers than four-wheeled ATVs . Utility ATVs and sport models have several design differences. Utility models have higher ground clearance, stronger torque for hauling and towing, rear and front racks for carrying loads or mounting equipment, a hitch to pull implements, and heavyer weights . Accordingly, utility ATVs are more suitable and more commonly used for tasks in agricultural settings. Therefore, in this study, agricultural ATVs are defined as utility ATVs used on farms and ranches. Agricultural ATVs have heavy weights and fast speeds that require complex maneuvering. Youth’s physical capabilities may not be sufficient to perform those complex maneuvers correctly. In fact, many studies have shown that youth are more vulnerable to injuries than adults because of their less developed physical capabilities and psychological and behavioral characteristics , which likely affect their ability to safely operate agricultural vehicles . Furthermore, previous studies have shown that ATV-rider misfit is another important risk factor . Despite compelling evidence showing that utility ATVs are unsuitable for youth, the most popular guidelines for ATV-youth fit disregard the rider’s physical capabilities. Instead, those recommendations are based on the rider’s age , vehicle’s maximum speed , vehicle’s engine size , or farm machinery training certificate . For instance, youth as young as 14 can operate utility ATVs while employed on non-family-owned farms if they receive training through an accredited farm machinery safety program, such as the National Safe Tractor and Machinery Operation Program . The NSTMOP training includes tractor and ATV education, where students must pass a written knowledge exam and a functional skills test to receive a certificate .

Nevertheless, programs such as the NSTMOP lack appropriate coverage of specific ATV-related subjects, such as active riding and physical matches of ATVs and youth. If the ATV is not fit to the rider, they will likely be unable to properly operate the ATV’s controls, which increases their chance of incidents and consequently may lead to injuries and fatalities. In addition, the traditional guidelines adopted to fit ATVs for youth are inconsistent in evaluating their preparedness to ride. The suggested fitting criteria are subject to variances in state law and lack scientifically-based evidence. While some recommendations based upon the riders’ physical capabilities exist , the adoption of these recommendations has not gained attention because they are not comprehensive and lack quantitative and systematic data. Recommendations based on riders’ physical capabilities appear to provide a better foundation to determine if the machine is suitable for the rider . Therefore, there is a need to evaluate youth-ATV fit based on the riders’ physical capabilities . Since 95 % of all ATV-related fatalities involving youth between 1985 and 2009 included agricultural ATVs , the purpose of this study is to evaluate the mismatches between the operational requirements of utility ATVs and the anthropometric characteristics of youth. It has been hypothesized that youth are mainly involved in ATV incidents because they ride vehicles unfit for them. This study evaluated ergonomic inconsistencies between youth’s anthropometric measures and utility ATVs’ operational requirements. The ability of youth to safely operate ATVs was evaluated through computer simulations that comprised 11 fit criteria and male-and-female youth of varying ages and height percentiles operating 17 utility ATV models.Youth-ATV fit was analyzed through virtual simulations and was carried out in five steps. First, 11 guidelines were identified for the fit of youth and ATVs. The second step consisted of identifying a database containing anthropometric measures of youth of various ages , genders , and height percentiles . The third step consisted of collecting the dimensions of 17 ATV models to create a three dimensional representation of them. The fourth step consisted of using SAMMIE CAD and Matlab to evaluate if the youth’s anthropometric measures conform to the guidelines identified in step one. Lastly, hydroponic trays the results of the virtual simulations were validated in field tests with actual riders and ATVs.The fit criteria provide movement-restraint thresholds that check if the rider can safely reach all controls and perform active riding, which requires the operator to shift their center of gravity to maintain the vehicle’s stability, especially when turning or traveling on slopes . Maintaining a correct posture is essential because, otherwise, the rider’s ability to control the vehicle is compromised, which puts them and potential bystanders at risk. The reach criteria considered in this study were selected based on the recommendations of the following institutions: National 4-H Council , U.S. Consumer Product Safety Commission , Intermountain Primary Children’s Hospital , and Farm and Ranch eXtension in Safety and Health Community of Practice . Disregarding overlaps, these guidelines consisted of 11 anthropometric measures of fit, which are presented in Table 1.Human mockups were developed in SAMMIE CAD. This computer program allows users to create customized virtual humans based on eight anthropometric dimensions, as shown in Fig. 1a. In total, 54 youth mockups were created, a combination of two genders, nine ages , and three body size percentiles in height . The age range was selected because most youth start operating farm machinery at 8 years old , and most ATV-related crashes occur with riders younger than 16 years old . Two adult mockups were also created to establish a baseline for comparisons.

The anthropometric measures used as input to SAMMIE CAD were retrieved from the database of Snyder et al. , which includes measurements from 3,900 subjects from 2 to 18 years of age for both genders. The adopted anthropometric measures were based on the mean values of groups of subjects with the same age, gender, and height. One of the required inputs was not available in the database used for this study. Therefore, the missing input was computed using the available data. The seated shoulder height was calculated by subtracting the head and neck length from the seated height .In total, 17 utility ATV models were evaluated. Selected models consisted of vehicles of varying engine sizes from the most common ATV manufacturers on U.S. farms . General descriptive variables such as manufacturer, model, series, engine capacity , drive terrain , transmission, and suspension type were recorded. ATV mockups were developed based on the spatial coordinates of selected ATV features . An original attempt to record spatial coordinates of ATV features consisted of using Photogrammetry, a technique in which several pictures of an object are taken from various angles and then processed to create a 3-D model. Nevertheless, this technique proved inefficient, as initial trials were time-consuming, and the results had unsatisfactory accuracy. A second attempt consisted of using a virtual reality tracking system. This alternative proved fast to implement with excellent accuracy ; hence, this technique was selected and presented in the following section.The VR tracking system utilized in this experiment consisted of two controllers and two infrared laser emitter units . The system allows the user to move in 3-D space and use motion-tracked handheld controllers to interact with the environment. The system uses the lighthouses to shoot horizontal and vertical infrared laser sweeps that are detected by photodiodes positioned in the surrounding of the controller’s surface . The position and orientation of the controllers are calculated by the difference in time at which each photodiode is hit by the laser . By placing the controller over selected vertices of ATV features, it was possible to record their spatial coordinates, which allowed the development of the 3-D ATV mockups. A custom program was developed to calibrate the system, log, and manipulate data. This program was initially retrieved from Kreylos and then modified to meet the specific needs of the present study. Examples of these functionalities are a 3-D grid, which allows for real-time visualization of labeled points, and a measuring tool . A probe was custom-manufactured and attached to the controllers to ease the calibration process and data collection. The probe was made of metal and had a rounded tip, which made it wear-resistant and prevented it from damaging the ATVs. The measurements were collected inside a tent covered by a white rooftop that reduces the interference of solar rays in the communication between the lighthouses and the photodiodes in the controllers. In total, 38 points were collected per ATV. The points were selected aiming to get an efficient representation of all selected ATV controls and additional features that were used to assist the virtual simulations, such as the seat and the footrests. After data filtering, the data were processed in SAMMIE CAD for a 3-D representation of the evaluated vehicle, as shown in Fig. 2.ATV-rider fit was evaluated through SAMMIE CAD and Matlab. Fit criteria 4, 5, 6, 7, 8, 9, and 10 were evaluated in SAMMIE CAD because their assessment involved complex interactions between riders and ATVs, such as measuring the angle of the rider’s knee while riding.