Mark McQuilling

Pharyngeal Airflow Analysis in Obstructive Sleep Apnea Patients Pre- and Post-Maxillomandibular Advancement Surgery

The purpose of this study was to evaluate pharyngeal airflow in obstructive sleep apnea (OSA) patients following maxillomandibular advancement (MMA) surgery using computational fluid dynamics (CFD). Computerized models of four OSA patients, pre- and postsurgery, were created using cone beam computed tomography scans. CFD was used to model airflow at inspiration rates of 340 ml/s, 400 ml/s, and 460 ml/s. The relative pressure, eddy viscosity coefficient, and total area-averaged pressure drops were selected for comparison. Results show a decrease in airway resistance of over 90% for three out of four patients. In these three patients, the MMA surgery reduced the constriction along the airway, which resulted in reduced drag and therefore reduced pressure drop required to move a constant volumetric flow between pre- and postsurgery models. CFD analyses on airways of OSA patients provide data that suggest an improvement in airflow following MMA surgery with less effort required for maintaining constant flow.

The road ahead: A white paper on the development, testing and use of advanced numerical modeling for aerodynamic decelerator systems design and analysis

Quantitative engineering analysis of parachutes and inflatables has been part of the routine design process since the days of World War II. But in most cases, the shear complexity in which their flexible structure interact both externally and internally with the surrounding air demands that empirical data be used to either validate or supplement such analysis. Advanced modeling embodied in the techniques of Computational Fluid Dynamics (CFD), Computational Structure Dynamics (CSD) and Fluid-Structure Interactions (FSI) has great potential for diminishing such reliance. But even though its application to aerodynamic decelerator systems (ADS) has been under consideration for the past four decades, progress has been painfully slow and the results rarely integrated into todays engineering design practice. This report aims at discussing why advanced modeling has not reached the level of practical use that has occurred in other aerospace fields. Such lack of progress origins partly from advanced modeling requiring substantial human resources that are not usually associated with parachute programs (expertise in computational methods in particular). Moreover, the extensive experimental database for Verification and Validation needed to support advanced modeling development is missing. This white paper begins with a pedagogical review of the most current implementations of CFD, CSD and/or FSI in the context of ADS applications. This is followed by a discussion of both non-ADS and ADS examples in which advanced modeling has been shown to yield interesting and relevant results. The report also identifies the type of data and measurement techniques that are needed for V&V, as well as the most pressing challenges - both theoretical and empirical - that are impeding progress. The paper ends with a series of recommendations for action items to be considered in the near and long terms.

Effect of the Transient Nature of Flow on Annular Parachute Drag Prediction

The aerodynamics of parachute systems are highly unsteady, resulting from separated ∞ows, vortex shedding, etc., caused by complex interactions between a ∞exible canopy and its payload. However, when dynamically permitted, the ∞owflelds and resulting surface forces have often been approximated using the assumptions and characteristics of rigid bodies and steady-state simulations. For example, results from steady-state simulations have been used successfully to assist the determination of parachute loading and trajectory. They have even been used in the estimation of peak drag and canopy fllling times sustained by in∞ating parachutes. This paper compares ∞ow characteristics and canopy drag obtained from solving the steady-state equations with those calculated with the unsteady equations, using a Reynolds Reynolds-Averaged Navier Stokes ∞ow solver on a rigid concentric annular parachute geometry comprised of two concentric fabric rings. The rings have unequal diameters and are vertically ofiset. Under load, the in∞ated rings not only yield a canopy that is highly porous but one that is characterized by high drag, a result of the cambered proflle formed by the billowing of the rings. The simulations have been performed at two Reynolds numbers of 8:03 ¢ 10 6 and 10:04 ¢ 10 6 . In particular, drag forces and wake characteristics observed in the steady simulations are compared to the unsteady simulations in order to judge the ability of the steady-state equations to capture the behavior noted in the transient cases. Results also include velocity, vorticity, and turbulence contours to help clarify the ∞ow physics resulting from steady and unsteady simulations.

Efficient Fluid-Structure Interaction Method for Conceptual Design of Flexible, Fixed-Wing Micro-Air-Vehicle Wings

Micro-air-vehicle wing designs often incorporate flexible structures that mimic the skeletal and membrane attributes found in natural flyers. Accurate performance predictions for these wing types require coupling of aerodynamic and structural simulations. Such fluid–structure interaction simulations are often performed using high-fidelity, numerically expensive techniques such as computational fluid dynamics coupled to nonlinear structural finite element analysis. Although the computational cost of conducting many conceptual design trade studies with these methods is prohibitive, simplified approaches may lack sufficient fidelity to provide conceptual design insights. This paper summarizes the development, comparison, and application of an efficient fluid–structure interaction method to simulate flexible-wing performance for rapid conceptual design of micro air vehicles. An advanced potential flow model computes aerodynamic performance, whereas a corotational frame and shell finite element structural mode...

Computational Investigation of the Flow Around a Parachute Model

Studying airdrop system aerodynamics is very challenging due to the coupling of aerodynamic forces and structural dynamics.Simplified modelsnot containingthe dynamic systemcoupling canyieldinformation usefulfor fundamental understanding and advanced code validation. This paper presents steady-state results from a Reynolds-averaged Navier–Stokes flow solver simulating flow around a parachute model at a Reynolds number of 365,000over fivepitchanglesof0,5,8,10,and12.Theparachutemodel,rigidandhemisphericalinshape,is similar to that of a ribbon parachute with four ribbons or rings. Pressure results are compared with experimental data to judge accuracy of the simulations. Additional results include pressure and vorticity contours, as well as computational oil-flow visualizations. These results illustrate the complex flow pattern in and around the ribbon parachute model, and they elaborate on existing data available for fluid–structure interaction code development.

The Bi-model: Using CFD in simulations of slowly-inflating low-porosity hemispherical parachutes

The Bi-model aims at simulating the inflation of low-porosity, but apex-vented flat-circular canopies. The most important feature of the model involves the use of CFD to obtain canopy drag data, internal flow data and apex vent flow data in order to inform the parachute-payload’s trajectory equations and the canopy’s growth rates. As such this scheme goes half-way between the high-order modeling of FSI and low-order modeling. But as further discussed here, the Bi-model is only applicable to low-porosity hemispherical canopies that open slowly, i.e., where near-quasi steady state aerodynamics holds, such as during the apex bubble growth stage of the USAF C-9 and US Army T-10/G-12/G-11 families of personnel and cargo delivery parachutes used in current military applications. The Bi-model also exploits the unsteady character of the inflation stage that follows apex bubble growth, namely the Canopy Skirt Spreading stage, by using a model of full mass capture. The latter has been shown to work well for such inflation sequences during which canopy opening is swift. Detailed comparisons are shown with data collected on USAF C-9 emergency parachute and US Army T-10 trooper parachute. . Nomenclature Abot = Bottom partition surface area Amouth = Canopy mouth surface area

Examination of Three-Dimensional Flow over a Chambered Inflatable Wing

Inflatable wings provide a compact alternative for aircraft that need to fit into small volumes before deployment, such as backpack unmanned aircraft or rocket-delivered air vehicles. Common inflatable designs use inflation chambers separated by spanwise baffles, generating a naturally undulating airfoil surface. Early wind tunnel tests of these designs showed evidence that separation over these airfoils was reduced compared to their smooth counterparts. Subsequent efforts to replicate these advantages computationally have proven ambiguous—while under certain conditions the inflatable airfoil may improve the lift-todrag characteristics of the wing, in other conditions the inflatable profile appears detrimental. Currently, these wings are being investigated through a combination of twodimensional and three-dimensional simulations and physical experiments. The initial targets of these comparisons are moderate Reynolds number (~10 5 ) flows, comparable to known flight conditions of inflatable wing UAVs. This places the flows in a naturally transitional regime, presenting additional complications in attempting to reconcile the results for these different investigative techniques.

Computational Study of High-Lift Low-Pressure Turbine Cascade Aerodynamics at Low Reynolds Number

This study uses a Reynolds-averaged Navier–Stokes finite volume flow solver to simulate the flowfields around a two-dimensional linear turbine cascade model at a Reynolds number of 25,000. Three blade profiles have been simulated, including the aft-loaded Pack B, which has a nominal Zweifel loading coefficient Zw equal to 1.15, the midloaded L1M (Zw=1.33), and the front-loaded L2F (Zw=1.59). All three blade profiles are known to be susceptible to varying degrees of laminar flow separation along the suction surface. Turbulence models used, which to the authors’ knowledge have been applied for the first time here, are the Abe–Kondoh–Nagano linear low-Re k-e as well as the Kato–Launder modification. Time-accurate simulations, including fully laminar computations, are compared with experimental data and higher-order computations to judge the accuracy of the results, where it is shown that Reynolds-averaged Navier–Stokes simulations with appropriate turbulence modeling can produce both quantitatively and quali...

Evaluation of RANS Transition Modeling for High Lift LPT Flows at Low Reynolds Number

High lift low pressure turbine airfoils have complex flow features that can require advanced modeling capabilities for accurate flow predictions. These features include separated flows and the transition from laminar to turbulent boundary layers. Recent applications of computational fluid dynamics based on the Reynolds-averaged Navier-Stokes formulation have included modeling for attached and separated flow transition mechanisms in the form of empirical correlations and two- or three-equation eddy viscosity models. This study uses the three-equation model of Walters and Cokljat [1] to simulate the flow around the Pack B and L2F low pressure turbine airfoils in a two-dimensional cascade arrangement at a Reynolds number of 25,000. This model includes a third equation for the development of pre-transitional laminar kinetic energy (LKE), and is an updated version of the Walters and Leylek [2] model. The aft-loaded Pack B has a nominal Zweifel loading coefficient of 1.13, and the front-loaded L2F has a nominal loading coefficient of 1.59. Results show the updated LKE model improves predicted accuracy of pressure coefficient and velocity profiles over its previous version as well as two-equation RANS models developed for separated and transitional flows. Transition onset behavior also compares favorably with experiment. However, the current model is not found suitable for wake total pressure loss predictions in two-dimensional simulations at extremely low Reynolds numbers due to the predicted coherency of suction side vortices generated in the separated shear layers which cause a local gain in wake total pressure.Copyright

A Computational and Experimental Investigation of Flow over an Inflatable Wing

Inflatable wings provide a compact alternative for aircraft that need to fit into small volumes before deployment, such as backpack unmanned aircraft or rocket-delivered air vehicles. The naturally undulating airfoil surface created by the inflation chambers may also provide an opportunity for flow control, as early wind tunnel tests indicated that separation over these airfoils was reduced compared to their smooth counterparts. Subsequent efforts to replicate these advantages computationally have proven ambiguous— while under certain conditions the inflatable airfoil may improve the lift-to-drag characteristics of the wing, in other conditions the inflatable profile appears detrimental. Further, these results have not been satisfactorily compared to experiments to confirm their validity. To attempt to address this, a series of PIV measurements has been taken for both a bumpy and smooth NACA 4318 airfoil at varying Reynolds number and angle of attack. This data is compared to CFD simulations in order to validate the models and gain further insight in to the flow physics. The combined computational/experimental analysis will allow for a better determination of the performance penalty or benefit that occurs due to the bumpy surface. Select three-dimensional simulations will also be used to further illuminate the effects of the bumpy surface on the aerodynamics.