Alipour F., Brucker C, Cook D., Gommel A, Kaltenbacher M, Mattheus W, Mongeau L, Nauman E, Schwarze R, Toduka I, Zorner S, (2011) “Mathematical Models and Numerical Schemes for the Simulation of Human Phonation” (Review Paper) Current Bioinformatics, 6(3), pp. 323-343.

An overview about the current state of mathematical models for the human phonation. Mathematical models served as valuable tools for providing insight into the basic mechanisms of phonation and may eventually be of sufficient detail and accuracy to allow surgical planning, diagnostics, and rehabilitation evaluations on an individual basis.

Cook D, George P, and Julias M. 2D/3D Hybrid Structural Model of Vocal Folds, Journal of Biomechanics 45(2), 269 - 274

The spatial dimensionality of the vocal fold vibration is a common challenge in creating parsimonious models of vocal fold vibration. The ideal model is one that is accurate, with the lowest possible computational expense. Inclusion of full 3D flow and structural vibration typically requires massive amounts of computation, whereas reduction of either the flow or the structure to two dimensions eliminates certain aspects of physical reality, thus making the resulting models less accurate. Previous 2D models of the vocal fold structure have utilized a plane strain formulation, which is shown to be an erroneous modeling approach since it ignores influential stress components. ...

Cook, D.D., Nauman, E., and Mongeau, L., (2009) “Ranking vocal fold model parameters by their influence on modal frequencies.” Journal of the Acoustical Society of America. 126(4)2002-10.

The purpose of this study was to identify, using computational models, the vocal fold parameters which are most influential in determining the vibratory characteristics of the vocal folds. The sensitivities of vocal folds modal frequencies to variations model parameters were used to determine the most influential parameters.Adetailed finite element model of the human vocal fold was created. ...

Cook, D.D., and Mongeau, L., (2007) “Sensitivity of a continuum vocal fold model to geometric parameters, constraints, and boundary conditions.” Journal of the Acoustical Society of America. 121 (4), 2247-2253.

The influence of key dimensional parameters, motion constraints, and boundary conditions on the modal properties of an idealized, continuum model of the vocal folds was investigated. The Ritz method and the finite element method were used for the analysis. The model's vibratory modes were determined to be most sensitive to changes in the anterior-posterior length of the vocal fold model, due to the influence of three-dimensional stress components acting in the transverse plane. ...

Cook, D.D., Nauman, E., and Mongeau, L., (2008) “Reducing the number of vocal fold mechanical tissue properties: Evaluating the incompressibility and planar displacement assumptions.” Journal of the Acoustical Society of America. 124(6):3888-96.

The incompressibility and planar displacement assumptions were used to reduce the number of independent tissue parameters required for the characterization of a structural model of the vocal folds. The influence of these simplifying assumptions on the vibratory properties of the model was investigated. The purpose was to provide estimates of the error introduced by these assumptions. The variability in human tissue properties was accounted for through systematic variation of several material parameters. The modal properties of a vocal fold structural model were computed with each assumption and, in turn, were relaxed to determine their respective effects. ...

Julias M., Riede, T., and Cook D.D. (2013) “Visualizing Collagen Network Within Human and Rhesus Monkey Vocal Folds Using Polarized Light Microscopy.” Annals of Otology, Rhinology, and Laryngology 122, pp. 135-144.

Collagen fiber content and orientation affect the viscoelastic properties of the vocal folds, determining oscillation characteristics during speech and other vocalization. The investigation and reconstruction of the collagen network in vocal folds remains a challenge, because the collagen network requires at least micron-scale resolution. In this study, we used polarized light microscopy to investigate the distribution and alignment of collagen fibers within the vocal folds. ...

Cook, D.D. “Systematic structural analysis of human vocal fold models” PhD. Dissertation, Purdue University, August 2009.

Computational models of biological systems can provide detailed information that is not possible to obtain experimentally. Such models are often created and analyzed even though the geometry and material properties are not well understood. This approach introduces modeling errors of undetermined magnitude. An alternative approach is to use parametric variation of all model parameters to determine the validity of modeling assumptions and to identify the most and least influential model parameters. The purpose of this study was to test modeling assumptions commonly applied in phonation modeling and to determine the most influential parameters of the vocal folds. A systematic analysis of the vocal fold structure was performed using finite element models of the vocal fold. Parametric variation of model parameters was a core component of this approach. Simplifying assumptions commonly applied to vocal fold structural models were examined for their effect on vibratory response. ...

Cook, D.D., (2005) “Computational Models of Fluid Flow, Structural Vibration, and Fluid-Structure Interactions of Human Phonation” Masters Thesis, Purdue University.

The purpose of this study was to characterize mechanisms which contribute to human phonation through the use of structural, fluid, and fluid-structure interaction numerical models. The flow of air through an orifice representative of the glottis was simulated using a finite element Navier-Stokes solver. The motion of the orifice was imposed through moving boundary conditions. ...

Research Publications

Cook, D., and Julias, M., "A Survey of Graduate Students as One Component in a Strategy for Departmental Improvement" Submitted to Review of Higher Education

Graduate student attrition results in losses that negatively affect departmental productivity. While many studies have identified factors that influence attrition, few have addressed the methodologies which departments may use to incorporate these findings to improve retention. In this article, we present a strategy for improving retention, one component of which is the need to obtain detailed local data regarding graduate student experiences. Survey results are then considered in conjunction with general research findings to identify areas of improvement that will result in high returns on invested time and resources. The design, analysis, and results of one such survey are reported.

Cook D., Julias M., and Nauman, E. (2014) "Biological variability in biomechanical engineering research: Significance and meta-analysis of current modeling practices." Journal of Biomechanics 47, p 1241 - 1250

Biological systems are characterized by high levels of variability, which can affect the results of biomechanical analyses. As a review of this topic, we first surveyed levels of variation in materials relevant to biomechanics, and compared these values to standard engineered materials. As expected, we found significantly higher levels of variation in biological materials. A meta-analysis was tehn performed based on thorough reviews of 60 research studies from the field of biomechanics to assess the methods and manner in which biological variation is currently handled in our field. The results of our meta-analysis revealed interesting trends in modeling practices, and suggest a need for more biomechanical studies that fully incorporate biological variation in biomechanical models and analyses. Finally, we provide some case study example of how biological variability may provide valuable insights or lead to surprising results. The purpose of this study is to promote the advancement of biomechanics research by encouraging broader treatment of biological variability in biomechanical modeling.

Robertson, D., and Cook. D, (2014) "Unrealistic statistics: How average constitutive coefficients can produce non-physical results." Journal of the Mechanical Behavior of Biomedical Materials. Vol 40. p 234 - 239

The coefficients of constitutive models are frequently averaged in order to concisely summarize the complex, nonlinear, material properties of biomedical materials. However, when dealing with nonlinear systems, average inputs (e.g. average constitutive coefficients) often fail to generate average behavior. This raises an important issue because average nonlinear constitutive coefficients of biomedical materials are commonly reported in the literature. This paper provides examples which demonstrate that average constitutive coefficients applied to nonlinear constitutive laws in the field of biomedical material characterization can fail to produce average stress–strain responses and in some cases produce non-physical responses. Results are presented from a literature survey which indicates that approximately 90% of tissue measurement studies that employ a nonlinear constitutive model report average nonlinear constitutive coefficients. We suggest that reviewers and editors of future measurement studies discourage the reporting of average nonlinear constitutive coefficients. Reporting of individual coefficient sets for each test sample should be considered and discussed as designation for a “best practice” in the field of biomedical material characterization.

Robertson, D., Smith, S., Gardunia, B., and Cook. D, (2014) "An Improved Method for Accurate Phenotyping of Corn Stalk Strength." Crop Science. Vol 54. No. 5, p. 2038 - 2044

Weak stems or stalks in grass crop species increase the likelihood of stalk failure, thereby reducing yield. Three-point bending tests are often employed in selective breeding studies to characterize stalk strength. However, it is hypothesized that the loading setup used during three-point bending experiments may significantly alter test results. To investigate this hypothesis, two different loading configurations were employed in conducting three-point bending experiments of corn (Zea mays L.) stalks. In the first configuration, stalks were loaded and supported at nodes. In the second configuration, stalks were loaded and supported at internodal segments. Significantly higher bending moments were experienced at internodal segments during the node-loaded configuration than was required to fail the same segment during internode-loaded tests. This is because the loading anvil significantly deforms the stalk’s cross section when it is placed on an internodal segment, thereby inducing premature failure. In addition, internode-loaded tests were observed to produce unnatural failure patterns, while node-loaded tests demonstrated natural variability in failure location. While transverse deformation of the stalk cross section cannot be eliminated in three-point bending tests, its effects can be mitigated by placing the loading anvil at nodal locations, which are much stiffer than internode regions. Maximizing the span length of bending tests likewise reduces transverse deformation of stalk cross sections. These results are relevant to selective breeding studies designed to produce lodging resistant crop hybrids.

Robertson, D., Smith, S., and Cook. D, (Submitted 2014) "On Measuring the bending strength of septate grass stems." American Journal of Botany

Premise of the study:

Reliable testing methodologies are a fundamental tenet of scientific research. However, very little information is found in the literature explaining how to accurately measure the structural bending strength of plant stems. It is hypothesized that the most commonly employed loading configuration used in bending experiments (placement of loading anvil at an internodal region of the stem or stalk) may significantly alter test results and introduce errors in bending strength measurements of plant stems