Adam Jirasek

Vortex-Generator Model and Its Application to Flow Control

A new vortex-generator model is introduced, the jBAY model, which provides an efficient method for computational-fluid-dynamics (CFD) simulation of flow systems with vortex-generator arrays. The ...

Computational Investigation into the Use of Response Functions for Aerodynamic-Load Modeling

The generation of reduced-order models (ROM) for the evaluation of unsteady and nonlinear aerodynamic loads are investigated. The ROM considered is an indicial theory based on the convolution of step functions with the derivative of the input signal. The step functions are directly calculated using the results of RANS simulations and a grid movement tool. Results are reported for a two dimensional airfoil and a UCAV configuration. Wind tunnel data are first used to validate the prediction of static and unsteady coefficients at both low and high angles of attack, with good agreement obtained for all cases. The generation of the aerodynamic models is described. The focus of the paper shifts to assess the validity of studied ROMs with respect to new maneuvres. This is accomplished by comparison of the model output with time-accurate CFD simulations. The results show that the ROMs can accurately model the unsteady loads in response to slow and fast pitch and plunge motions.

Lessons Learned from Numerical Simulations of the F-16XL Aircraft at Flight Conditions

This thesis covers the field of vortex-flow dominated external aerodynamics. As part of the contribution to the AVT-113 task group it was possible to prove the feasibility of high Reynolds number CFD computations to resolve and thus better understand the peculiar dual vortex system encountered on the VFE-2 blunt leading edge delta wing at low to moderate incidences. Initial investigations into this phenomenon seemed to undermine the hypothesis, that the formation of the inner vortex system depends on the laminar/turbulent state of the boundary layer at separation onset. As a result of this research, the initial hypothesis had to be expanded to account also for high Reynolds number cases, where a laminar boundary layer at separation onset can be excluded. In addition, unsteady transonic computations are used to shed light on a highly non-linear phenomenon encountered at high angles of incidence. At certain conditions, the increase of the incidence by a single degree leads to a sudden movement of the vortex breakdown location from the trailing edge to mid-chord. The lessons learned from the contribution to the VFE-2 facet are furthermore used to prove the technology readiness level of the tools within the second facet of AVT-113, the Cranked Arrow Wing Aerodynamics Project International (CAWAPI). The platform for this investigation, the F-16XL aircraft, experiences at high transonic speeds and low incidence a complex interaction between the leading edge vortex and a strong, mid-chord shock wave. A synergetic effect of VFE-2 with a further project, the Environmentally friendly High Speed Aircraft (HISAC), is also presented in this thesis. Reynolds number dependence is documented in respect to leading edge vortex formation of the wing planform for a reference HISAC configuration. Furthermore, proof is found for a similar dual vortex system as for the VFE-2 blunt leading edge configuration.

Design of Vortex Generator Flow Control in Inlets

This article summarizes the results of an optimization study of a microvortex generator flow control in an inlet. Five parameters optimization was carried out using the classical design of experiment method. Two main objectives were in focus: first, to develop the methodology and skills necessary to conduct a design of experiment optimization study in area of the flow control; and second, to develop the procedures which would be used during design of vortex generator flow control in inlets. New information about the dependency of the vortex generator flow control in inlet on its geometrical parameters were obtained. Several interesting configurations were located. The parameters of optimal settings were then used to set up the vortex generator installation in a generic inlet.

SACCON Static and Dynamic Motion Flow Physics Simulations Using COBALT

With recent advances in computational techniques, turbulent Navier-Stokes solvers are now capable of capturing the unsteady nonlinear aerodynamic behavior that leads to various static and dynamic instabilities of full aircraft. As such, the focus of stability and control (S&C) research has been to effectively incorporate computational fluid dynamics (CFD) into the model development process using dynamic CFD solutions of complete aircraft configurations. This has lead to an innovative approach for modeling aircraft stability and control characteristics: (a) CFD simulations are performed using computational training maneuvers designed to excite the relevant flow physics encountered during actual missions, (b) a mathematical Reduced Order Model (ROM) is built of the aircraft response using system identification methods, (c) the model is validated by comparing CFD simulations against model predictions, and (d) predictions of all flight test points are made using the model to determine the expected behavior of the aircraft. This process would identify unexpected S&C issues early in the design process. This article describes the application of this innovative approach to a problem of the System Identification (SID) of a UCAV geometry. The training maneuver used to produce the training data has been tested in the DNW-NWB low speed wind tunnel which enables directly comparing the predictive capabilities of the CFD and wind tunnel SID modeling.

Improved Methodologies for Maneuver Design of Aircraft Stability and Control Simulations

With many modern fighter aircraft experiencing unpredicted flight dynamics during flight tests, recent research has focused on developing methodologies for incorporating computational fluid dynamics into the aircraft development process. The goal of this approach is to identify configurations susceptible to stability and control issues early in the design process. Previous research has primarily focused on full aircraft configurations, however, to increase the rate of development the current study focused on a two-dimensional NACA0012 airfoil. The two-dimensional NACA0012 airfoil has the advantage of reducing the computational cost by orders of magnitude compared to full scale aircraft simulations, while still providing complicated aerodynamics at high angles of attack. Computationally predicted lift coefficients from a number of newly developed training maneuvers were used to generate reduced order aerodynamic loads models. For evaluation, these models were compared to generated static and dynamic validation data. Methods of improving both the computational training maneuver and the reduced order modeling approach are suggested.

What was learned from numerical simulations of F-16XL (CAWAPI) at flight conditions

Nine groups participating in the CAWAPI project have contributed steady and unsteady viscous simulations of a full-scale, semi-span model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations were computed and compared with flight-test measurements for the purpose of code validation and improved understanding of the flight physics. Steady-state simulations are done with several turbulence models of different complexity with no topology information required and which overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation (DES) and its successor delayed detached-eddy simulation (DDES) have been used to compute the time accurate flow development. Common structured and unstructured grids as well as individually-adapted unstructured grids were used. Although discrepancies are observed in the comparisons, overall reasonable agreement is demonstrated for surface pressure distribution, local skin friction and boundary velocity profiles at subsonic speeds. The physical modeling, be it steady or unsteady flow, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time it cannot be determined how much each part contributes to the whole. Overall it can be said that the technology readiness of CFD-simulation technology for the study of vehicle performance has matured since 2001 such that it can be used today with a reasonable level of confidence for complex configurations.

Plans and Example Results for the 2nd AIAA Aeroelastic Prediction Workshop

This paper summarizes the plans for the second AIAA Aeroelastic Prediction Workshop. The workshop is designed to assess the state-of-the-art of computational methods for predicting unsteady flow fields and aeroelastic response. The goals are to provide an impartial forum to evaluate the effectiveness of existing computer codes and modeling techniques, and to identify computational and experimental areas needing additional research and development. This paper provides guidelines and instructions for participants including the computational aerodynamic model, the structural dynamic properties, the experimental comparison data and the expected output data from simulations. The Benchmark Supercritical Wing (BSCW) has been chosen as the configuration for this workshop. The analyses to be performed will include aeroelastic flutter solutions of the wing mounted on a pitch-and-plunge apparatus.

Prediction of Aerodynamic Characteristics of Ram-Air Parachutes

The focus of this work is on the computational methodology for aerodynamic modeling of ram-air parachutes and increasing confidence and understanding in their concept designs including new parachute control methods. The complex geometries of ram-air parachutes are modeled by two-dimensional rigid airfoil geometries with or without trailing-edge deflections and bleed air spoilers. The aerodynamic forces are then calculated from steady or unsteady Reynolds-averaged Navier–Stokes simulations using Cobalt and Kestrel flow solvers. The effects of the grid size and type, the time step, and the choice of solver parameters are investigated. The flow solvers are then used to study the flow around three-dimensional wings with open/closed ram-air inlets by comparing lift and drag coefficients with available experimental data. The results show that computational fluid dynamics simulations are a valuable aid in understanding the flow structure of ram-air parachutes, which resemble a rectangular wing with open inlets. ...

CFD Modeling for Trajectory Predictions of a Generic Fighter Configuration

A framework to simultaneously solve the coupled ∞uid ∞ow Navier-Stokes equations with the dynamic equations governing the aircraft motion is assessed. The predicted trajectories have been compared against a series of free-∞ight spark range experiments to validate the approach. The model was flred from a gun room with an initial transonic Mach number. Also, the conflguration, the Standard Dynamics Model (SDM), has highly swept slender wings resulting in complex vortical ∞ow under various conditions. CFD validation of unsteady loads subject to these conditions is very challenging. Wind tunnel measurements at low speed are flrst used to validate the vortical ∞ow predictions at both low and high angles of attack. The dynamic efiects from breakdown of slender vortices on the forces and moments of maneuvering aircraft are next discussed. Finally, the CFD model for trajectory predictions is validated by integrating the aircraft rigid-body dynamic equations through time-accurate CFD calculations and comparing the ∞ight path of aircraft against measured free-∞ight range data.