Overview of Sensitivity Analysis and Shape Optimization for Complex Aerodynamic Configurations

Auxiliary power units (APUs) are gas turbine engines that provide high-pressure air and electrical power to aircraft systems. They provide primary power while the aircraft is parked on the ramp, starting services for the main engines, and backup power while the aircraft is in flight. Many APUs employ inlet systems which include a “pop-up” door that allow for the capture of freestream ram pressure during flight. This results in increased inlet recovery and a corresponding improvement in the performance of the APU. This APU door, when open, is exposed to airflow instability inherent in the aircraft boundary layer in the aft section of the fuselage, where APUs are typically housed. Additionally, the pop-up nature of the inlet door produces a large region of separated airflow off of the back side of the door. Systematic vortex shedding is frequently a major component of this separated region. As new APU doors are made with less rigid material to save weight, a need to better understand the unsteady aerodynamic excitations of the flow field around the door has arisen, as these new doors may be more susceptible to vibration during flight. Recent advances in Computational Fluid Dynamics (CFD) meshing tools and transient modeling have enabled a CFD study to be performed which will investigate this time-dependent phenomenon. As transient CFD analysis is still a relatively new field for commercial CFD codes, a calibration was needed to verify the accuracy of the CFD predictions and to form any calibration correction terms. Honeywell Aerospace owns a Boeing 757 flight test vehicle which is normally used to flight test propulsion engines. However, this aircraft also includes a pop-up APU inlet door that is similar to most other APU inlet door styles. This APU door was instrumented using high response pressure transducers placed on the forward and aft sides of the inlet door as well as upstream of the door to measure upstream instability. The aircraft was flown at a variety of flight conditions and APU operating points and the transient data was recorded. After the completion of the flight test, a CFD model was constructed of the B757 flight test vehicle. Because aerodynamic instability can be generated anywhere on the aircraft, the entire airframe from nose to tail was modeled. The APU inlet door geometry was also created, meshed and added to the CFD model. This CFD model was run in a transient mode to simulate the exact same flight conditions and APU operating points as were tested during the flight test. Dynamic results in the time and frequency domains predicted by the CFD analyses were compared to flight test data and correlation and calibration factors were derived.

A high-fidelity multidisciplinary analysis and gradient-based optimization tool for rotorcraft aero-acoustics is presented. Tightly coupled discipline models include physics-based computational fluid dynamics, rotorcraft comprehensive analysis, and noise prediction and propagation. A discretely consistent adjoint methodology accounts for sensitivities of unsteady flows and unstructured, dynamically deforming, overset grids. The sensitivities of structural responses to blade aerodynamic loads are computed using a complex-variable approach. Sensitivities of acoustic metrics are computed by chain-rule differentiation. Interfaces are developed for interactions between the discipline models for rotorcraft aeroacoustic analysis and the integrated sensitivity analysis. The multidisciplinary sensitivity analysis is verified through a complex-variable approach. To verify functionality of the multidisciplinary analysis and optimization tool, an optimization problem for a 40% Mach-scaled HART-II rotor-and-fuselage configuration is crafted with the objective of reducing thickness noise subject to aerodynamic and geometric constraints. The optimized configuration achieves a noticeable noise reduction, satisfies all required constraints, and produces thinner blades as expected. Computational cost of the optimization cycle is assessed in a high-performance computing environment and found to be acceptable for design of rotorcraft in general level-flight conditions.

In a previous paper, the author presented a procedure for evaluating piping thermal mixing points using finite element analysis. However, several assumptions had to be made with respect to the thermal boundary conditions. While, at the time, those were thought to be conservative assumptions, there was no comparison with alternate methods of determining the thermal boundary conditions. With recent advances in computational fluid dynamics (CFD), particularly the availability of high-quality commercial code, such a comparison can now be made. In this paper, the author reviews the piping thermal mixing point cases evaluated in the previous paper using CFD as the comparative analysis tool. The temperature fields in the piping thermal mixing points are compared. The calculated stress fields are also examined to determine if the original thermal boundary conditions were reasonable.

Modeling of laboratory and commercial scale hydro-processing reactors using CFD

The results of a National Research Council study on the effect that advances in computational fluid dynamics (CFD) will have on conventional aeronautical ground testing are reported. Current CFD capabilities include the depiction of linearized inviscid flows and a boundary layer, initial use of Euler coordinates using supercomputers to automatically generate a grid, research and development on Reynolds-averaged Navier-Stokes (N-S) equations, and preliminary research on solutions to the full N-S equations. Improvements in the range of CFD usage is dependent on the development of more powerful supercomputers, exceeding even the projected abilities of the NASA Numerical Aerodynamic Simulator (1 BFLOP/sec). Full representation of the Re-averaged N-S equations will require over one million grid points, a computing level predicted to be available in 15 yr. Present capabilities allow identification of data anomalies, confirmation of data accuracy, and adequateness of model design in wind tunnel trials. Account can be taken of the wall effects and the Re in any flight regime during simulation. CFD can actually be more accurate than instrumented tests, since all points in a flow can be modeled with CFD, while they cannot all be monitored with instrumentation in a wind tunnel.

Computational limitations of computers have emerged as a critical barrier to the advancement of Computational Fluid Dynamics (CFD). Consequently, exploring novel accelerator architectures tailored for large-scale CFD applications and closely integrated with CFD algorithmic characteristics holds significant value. Through an in-depth analysis of the finite difference method (FDM) for solving Navier–Stokes (N-S) equations, we propose a specialized accelerator architecture for FDM-based CFD (FAcc). Implemented on a 28 nm process, FAcc integrates 16,384 differential computing cores (FCores). Experimental validation demonstrates FAcc’s capability to solve N-S equations of varying complexities by flexibly configuring boundary conditions. Compared to conventional approaches, FAcc achieves significant acceleration performance, with its programmability underscoring adaptability to high-precision, large-scale CFD simulations. As the first CFD-focused accelerator designed from the instruction set architecture (ISA) level, FAcc bridges a critical gap in domain-specific hardware for CFD, offering a paradigm shift in high-performance fluid dynamics computation.

Computational fluid dynamics (CFD) solutions of the full Navier-Stokes equations along with finite-rate chemical kinetics, are used to numerically simulate the reacting in-bore flowfield for a 120mm ram accelerator projectile propulsion system. Various unsteady flow phenomena are investigated, including projectile starting, obturator discard, and high-velocity unstarting. These simulations illustrate the importance of obturator discard dynamics in achieving a successful starting of the ram acceleration process. However, once started, high projectile velocities can induce an unstart if the combustion wave precedes the projectile under certain conditions. In addition, a CFD code validation case is presented for low pressure, steady flow, hydrogen/oxygen combustion around a wedge as compared to Schlieren/PLIF data from Stanford. Introduction Experimental testing and gasdynamic modeling of the ram acceleration technique for in-bore projectile propulsion has been investigated at the U.S. Army Research Laboratory (ARL) under the Hybrid Inbore RAM (HIRAM) propulsion program. This research program seeks to provide a highly efficient method of achieving hypervelocity (> 3 km/s) projectile gun-launch for use in high speed impact testing applications. The ARL ram accelerator system used a 120-mm (bore diameter) tube which was modeled after the 38-mm system at the University of Washington. Numerical solutions of the Navier-Stokes equations have been obtained at the ARL via computational fluid dynamics (CFD) codes for non-reacting and reacting, two-and three-dimensional flows. These codes are being used to investigate the complex gas dynamic physics of ram accelerator projectile propulsion. A variety of CFD techniques have been brought to bear on this problem, including models for chemically frozen (non-reacting) gas, finite-rate global and multiple step chemical kinetics, and equilibrium chemical processes. Accurate numerical simulation of hydrocarbon-based reacting flow creates a very great demand on computational resources since the number of intermediate species and the number of kinetic steps for typical hydrocarbon fuels are prohibitively large. Global reaction mechanisms based on up to thirty-two steps have therefore been investigated for use in preliminary design studies. Within several hours on supercomputers, viscous and chemically reacting gas dynamic simulations are used to assess the influence of projectile velocity, tube fill pressure and mixture composition, and projectile geometry on species consumption, tube wall pressure and projectile thrust. These studies are being used to seek optimum performance for the 120-mm ram accelerator with minimal gun firings. In particular, significant advances in CFD * Aerospace Engineer, Ballistics &; Weapons Concepts Division. Associate Fellow AIAA. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. • modeling for ram acceleration have been made at the ARE. Recently, ARL CFD models have been modified to simulate the unsteady coupling between the launch gun and the ram accelerator. In the Army experimental gun, projectile transition from the conventional (solid propellant) launcher to the accelerator was made through a transition/vent section. This section decouples the launch gun movement from the accelerator and vents combustion gases from the conventional launcher. Redesign of the ram accelerator for Navy applications has required that the ram accelerator operate without the vent section. The removal of the vent section requires that the CFD code be linked to an interior ballistics (IB) simulation of the launch gun in order to supply the pressure behind the obturator. The IB code must be able to adjust to the pressure on the forward face of the obturator, supplied from the CFD code simulating ram combustion. Simulation of ventless ram acceleration has been accomplished. Reacting Flow Model The chemically reacting hypervelocity flow field around a body is numerically simulated using a computer code based on computational fluid dynamics. The ARL-NSRG2 code solves the 2D/axisymmetric, unsteady, real-gas Navier-Stokes equations including equations for chemical kinetics and diffusion. These partial differential equations are cast in conservation form and converted to algebraic equations using a finite-volume formulation. The conservation law form of the equations assures that the end states of regions of discontinuity (e.g., shocks, flames and deflagrations) are physically correct even when smeared over a few computational cells. Solution takes place on a mesh of nodes distributed in a zonal fashion around the projectile and throughout the flow field such that sharp geometric corners and other details are accurately represented. The Navier-Stokes equations for 2D/axisymmetric reacting and unsteady flow are written in the following conservation form with variables nondimensionalized in a conventional fashion. dU dF_ dG_ fat

The study of rotor–fuselage interactional aerodynamics is central to the design and performance analysis of helicopters. However, regardless of its significance, rotor–fuselage aerodynamics has so far been addressed by very few authors. This is mainly due to the difficulties associated with both experimental and computational techniques when such complex configurations, rich in flow physics, are considered. In view of the above, the objective of this study is to develop computational tools suitable for rotor–fuselage engineering analysis based on computational fluid dynamics (CFD).To account for the relative motion between the fuselage and the rotor blades, the concept of sliding meshes is introduced. A sliding surface forms a boundary between a CFD mesh around the fuselage and a rotor‐fixed CFD mesh which rotates to account for the movement of the rotor. The sliding surface allows communication between meshes. Meshes adjacent to the sliding surface do not necessarily have matching nodes or even the same number of cell faces. This poses a problem of interpolation, which should not introduce numerical artefacts in the solution and should have minimal effects on the overall solution quality. As an additional objective, the employed sliding mesh algorithms should have small CPU overhead. The sliding mesh methods developed for this work are demonstrated for both simple and complex cases with emphasis placed on the presentation of the inner workings of the developed algorithms. Copyright © 2008 John Wiley & Sons, Ltd.

The one- and two-dimensional inviscid Euler equations are formulated in an integro-differential form for shape design sensitivity analysis and optimization. The principal tool employed to derive the performance derivative sensitivity equations is the variational method, which is a continuous alternative to the discrete sensitivity analysis. Along with the sensitivity equations, the co-state equations and their boundary conditions are derived. Thereafter, based on the modal analysis of Von Neumann theory, the stability limits of the co-state equations are investigated. The stability characteristics of the co-state equations are compared with those of the Euler equations. Finally, using the criteria from the stability analysis of the state and co-state equations, some results of shape design optimization and sensitivity analysis for both quasi one- and two-dimensional Euler equations are presented. Copyright © 1999 John Wiley & Sons, Ltd.

Recent Advances in Computational Fluid Dynamics

Parshall flumes were developed in the early 1900s as a simple and inexpensive way to provide accurate flow measurement in open channels. Unfortunately, many Parshall flumes in use today do not follow essential design parameters that prevent them from accurately measuring flow. Two design parameters that are commonly overlooked are: the need to measure upstream head in the correct location and the need to have appropriate entrance wingwalls attached to the flume. With recent advancements in computational fluid dynamics (CFD), numerical modeling has made it possible to accurately model nonstandard Parshall flume designs of varying sizes without extensive financial commitments for physical modeling. This paper utilizes numerical modeling to create a correction procedure for 2–8-ft nonstandard Parshall flumes. Basic background information regarding Parshall flumes and the numerical model used are presented. In addition, the results and correction procedures determined when head measurements are taken at any location (other than the standard design location) on the converging wall or along the longitudinal centerline of the flume with either standard or nonstandard entrance wingwalls.

BackgroundAccurate functional diagnosis of coronary stenosis is vital for decision making in coronary revascularization. With recent advances in computational fluid dynamics (CFD), fractional flow reserve (FFR) can be derived non-invasively from coronary computed tomography angiography images (FFRCT) for functional measurement of stenosis. However, the accuracy of FFRCT is limited due to the approximate modeling approach of maximal hyperemia conditions. To overcome this problem, a new CFD based non-invasive method is proposed.MethodsInstead of modeling maximal hyperemia condition, a series of boundary conditions are specified and those simulated results are combined to provide a pressure-flow curve for a stenosis. Then, functional diagnosis of stenosis is assessed based on parameters derived from the obtained pressure-flow curve.ResultsThe proposed method is applied to both idealized and patient-specific models, and validated with invasive FFR in six patients. Results show that additional hemodynamic information about the flow resistances of a stenosis is provided, which cannot be directly obtained from anatomy information. Parameters derived from the simulated pressure-flow curve show a linear and significant correlations with invasive FFR (r > 0.95, P < 0.05).ConclusionThe proposed method can assess flow resistances by the pressure-flow curve derived parameters without modeling of maximal hyperemia condition, which is a new promising approach for non-invasive functional assessment of coronary stenosis.

The increasing emission of carbon dioxide (CO2) into the environment has a significant influence on increasing global warming. The widely used carbon capture technologies (CCTs), including adsorption, absorption, cryogenics and membrane separation, have been reported previously through both experimental and simulation studies. Simulation studies are particularly considered essential tools for investigating the performance of the process from various aspects, as well as for predicting the complex behavior in any real system without actual experimental setups. This review mainly focuses on the recent advancements in computational fluid dynamics (CFD), modeling and simulation as a transformative tool for optimizing CO2 adsorption processes. The fundamentals of CFD with their role in accurately simulating complex phenomena involved in the CO2 adsorption are discussed. Parametric sensitivity analysis, hydrodynamics, kinetic and adsorption isotherms are thoroughly studied, and critical challenges, including mesh independence, model complexity, and scalability, are also addressed. The future research directions proposed include the integration of CFD with machine learning algorithms, the development of multi-scale models, and the deployment of CFD methodologies to industrial-scale applications.

Chapter 16 - Computational fluid dynamics in pharmaceutical formulation development

Unsteady wind loads due to turbulence inside the telescope enclosure result in image jitter and higher-order image degradation due to M1 segment motion. Advances in computational fluid dynamics (CFD) allow unsteady simulations of the flow around realistic telescope geometry, in order to compute the unsteady forces due to wind turbulence. These simulations can then be used to understand the characteristics of the wind loads. Previous estimates used a parametric model based on a number of assumptions about the wind characteristics, such as a von Karman spectrum and frozen-flow turbulence across M1, and relied on CFD only to estimate parameters such as mean wind speed and turbulent kinetic energy. Using the CFD-computed forces avoids the need for assumptions regarding the flow. We discuss here both the loads on the telescope that lead to image jitter, and the spatially-varying force distribution across the primary mirror, using simulations with the Thirty Meter Telescope (TMT) geometry. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are all estimated; these are then used to compute the resulting image motion and degradation. There are several key differences relative to our earlier parametric model. First, the TMT enclosure provides sufficient wind reduction at the top end (near M2) to render the larger cross-sectional structural areas further inside the enclosure (including M1) significant in determining the overall image jitter. Second, the temporal spectrum is not von Karman as the turbulence is not fully developed; this applies both in predicting image jitter and M1 segment motion. And third, for loads on M1, the spatial characteristics are not consistent with propagating a frozen-flow turbulence screen across the mirror: Frozen flow would result in a relationship between temporal frequency content and spatial frequency content that does not hold in the CFD predictions. Incorporating the new estimates of wind load characteristics into TMT response predictions leads to revised estimates of the response of TMT to wind turbulence, and validates the aerodynamic design of the enclosure.