As part of NASA's High Alpha Technology Program, research has been aimed at developing and extending numerical methods to accurately predict the high Reynolds number flow about the NASA F-18 High Alpha Research Vehicle (HARV) at large angles of attack. The HARV aircraft is equipped with a bidirectional thrust vectoring unit which enables stable, controlled flight through 70 deg angle of attack. Currently, high-fidelity numerical solutions for the flow about the HARV have been obtained at alpha = 30 deg, and validated against flight-test data. It is planned to simulate the flow about the HARV through alpha = 60 deg, and obtain solutions of the same quality as those at the lower angles of attack. This report presents the status of work aimed at extending the HARV computations to the extreme angle of attack range

Murman, Scott M.

English

This research has been aimed at validating numerical methods for computing the flow about the complete F-18 HARV at alpha = 30 deg and alpha = 45 deg. At 30 deg angle of attack, the flow about the F-18 is dominated by the formation, and subsequent breakdown, of strong vortices over the wing leading-edge extensions (LEX). As the angle of attack is increased to alpha = 45 deg, the fuselage forebody of the F-18 contains significant laminar and transitional regions which are not present at alpha = 30 deg. Further, the flow over the LEX at alpha = 45 deg is dominated by an unsteady shedding in time, rather than strong coherent vortices. This complex physics, combined with the complex geometry of a full-aircraft configuration, provides a challenge for current computational fluid dynamics (CFD) techniques. The following sections present the numerical method and grid generation scheme that was used, a review of prior research done to numerically model the F-18 HARV, and a discussion of the current research. The current research is broken into three main topics; the effect of engine-inlet mass-flow rate on the F-18 vortex breakdown position, the results using a refined F-18 computational model to compute the flow at alpha = 30 deg and alpha = 45 deg, and research done using the simplified geometry of an ogive-cylinder configuration to investigate the physics of unsteady shear-layer shedding. The last section briefly summarizes the discussion

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Numerical simulation of the flow about the F-18 HARV at high angle of attack

NASA-CR-197023
MCAT Institute
Annual Report
94-11
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Numerical Simulation of the Flow About
the F-18 HARV at High Angle of Attack
Scott M, Murman
(NASA-CR-197023) NUMERICAL
SIMULATION OF THE FLOW ABOUT THE
F-IB HARV AT HIGH ANGLE OF ATTACK
AnnuaI Report (MCAT Inst.) 7 p
N95-14614
Unclas
G3/02 0030166
August 1994 NCC2-729
MCAT Institute
3933 Blue Gum Drive
San Jose, CA 95127
https://ntrs.nasa.gov/search.jsp?R=19950008200 2020-06-16T10:21:08+00:00Z
Numerical Simulation of the Flow About
the F-18 HARV at High Angle of Attack
Annual Report for NASA Grant NCC2-729
August, 1994
Scott M. Murman
Principal Investigator
MCAT Institute, Moffett Field, CA 94035
INTRODUCTION
As part of NASA's High Alpha Technology Program, research has been aimed
at developing and extending numerical methods to accurately predict the high
Reynolds number flow about the NASA F-18 High Alpha Research Vehicle (HARV)
at large angles of attack. The HARV aircraft is equipped with a bi-directional thrust
vectoring unit which enables stable, controlled flight through 70 ° angle of attack.
Currently, high-fidelity numerical solutions for the flow about the HARV have been
obtained at a = 30 °, and validated against flight-test data. It is planned to simu-
late the flow about the HARV through a = 60 °, and obtain solutions of the same
quality as those at the lower angles of attack. This report presents the status of
work aimed at extending the HARV computations to the extreme angle of attack
range.
The physics of the flow about the HARV changes as the angle of attack is
increased from a -- 30 ° to a = 60 °. At the lower angles of attack, the flowfield is
dominated by the formation and subsequent breakdown of the vortices shed from the
wing leading-edge extensions (LEX). As the angle of attack increases, the flow about
much of the aircraft becomes dominated by an unsteady shedding of the boundary
layers, and the convection downstream of these unsteady waves. In order to better
understand the physics involved with this unsteady shedding, various computations
have been performed using the simpler geometry of a tangent-ogive cylinder at high
angle of attack. These computations not only give insight into the physics involved
with the full-aircraft geometry, they provide experience and confidence with the
numerical code.
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RESULTS AND DISCUSSION 1 \,/_
A variety of cases have been computed for the ogive-cylinder configuration at
high angle of attack. This wide variation in computational studies is possible due
to the simplified geometry of the ogive-cylinder (Fig. 1). A single-zone grid with
approximately 500,000 grid points can provide a highly accurate simulation of the
flowfield, as compared to the multi-zone, several million grid point systems that are
required to simulate the complete F-18 HARV aircraft. Two body configurations
have been computed: one having a 3.5 caliber tangent-ogive portion with a cylinder
portion of 7.0 diameters, and the other having a 3.0 caliber tangent-ogive section
with a cylinder region extending 12.0 diameters. The study has thus far focused on
comparing two angle of attack and Reynolds number combinations, a = 40 ° and
60 °, and ReD = 80,000 and 200,000. Both of these Reynolds numbers are assumed
to maintain laminar flow throughout the computational region. This combination
of different bodies, Reynolds numbers, and angles of attack provides a wide test-
bed for the numerical scheme. This test-bed is further extended by also comparing
computational cases having a lateral plane of symmetry with the same cases com-
puted using a periodic grid set-up (Fig. 2). These two computational configurations
should provide nearly the same results for computations which are symmetric about
the lateral plane of symmetry (this includes all cases computed thus far).
= 40 ° Computations
The cases that were computed at a = 40 ° were generally "well behaved," and
displayed many of the general features of unsteady boundary layer shedding. Of
importance to note were that the computations with the lateral plane of symmetry
and the full periodic grid were identical to the order of round-off error, in terms
of both the convergence history, and the final median value of normal force. This
same behavior was observed in comparing different numerical algorithms. Figure
3 shows the normal force history for an a = 40 °, ReD = 80,000 case using the
3.0 caliber ogive cylinder body computed using two distinct algorithms. The F3D
algorithm uses flux-vector splitting in the streamwise direction while the Beam and
Warming algorithm (B&:W) uses a central differencing scheme in the streamwise
direction with added numerical dissipation. As can be seen from the force history,
both algorithms are showing nearly identical behavior at this angle of attack and
Reynolds number.
Two caseswere computed using the sameogive-cylinder body at 40° angle of
attack, with different Reynolds number, ReD = 80,000 and 200,000. This shows
the difference in shedding frequency and amplitude of the pressure variations as
the Reynolds number is increased. Figure 4 shows the computed normal force
history for the Reynolds number comparison. It is required to compute to a non-
dimensional time of near [ = 200 before the initial transients are removed from
the computational domain, and a regular periodic variation in normal force is ob-
served. The normal force oscillates at a frequency of approximately 145 Hz in the
ReD = 80,000 computation, and 60 Hz in the .ReD --- 200,000 computation. The
amplitude of the oscillation is increased in the ReD -- 200,000 computation. The
frequency or the normal force variation and the shedding frequency are the same
in the ReD --- 80,000 computation, however in the ReD -_ 200,000 computation
the two frequencies are not identical. The shedding frequency at ReD = 200,000 is
about 4 times as great as the frequency of the normal force oscillation. The cause
of this is not known at this time, and further research is planned to understand
the behavior. In order to visualize the shedding of the shear layers from the side
of the ogive cylinder, an animation of streaklines released in the unsteady flowaeield
was created using the UFAT visualization program. A snapshot of this animation
is shown is Fig. 5. The shedding waves are clearly visible as the groups of particles,
and the convection downstream and rolling into the vortex shed from the nose, can
also be seen.
-_ 60 ° Computations
While the cases that were computed at a = 40 ° were termed well behaved, the
computations at o_ -- 60 ° have thus far been pathologically behaved. However, this is
also representative of trends that have been observed in experimental testing, where
even small changes in an experimental set-up can lead to large changes in results in
the extreme angle of attack range. Where at a -- 40 ° the plane of symmetry and
periodic configurations display the same results to the order of round-off error, at
a -- 60 ° the two computations show radically dis-similar behavior. Specifically, the
lateral force coefficient no longer remains at a nominally zero value in the periodic
configuration. This build-up of lateral force influences the other force and moment
coefficients to diverge from their "symmetric" values as well. This behavior is
not consistent between algorithms, as can be seen by the history of lateral force
coefficient shown in Fig. 6. Here, both the F3D and Beam and Warming algorithms
show a build-up in lateral force, but the meanvaluesand amplitudes of motion are
different. In fact, the sideforcesseenin the two computations are of opposite sign.
This behavior hasbeen extensively investigated in order to better understand how
a nominally symmetric algorithm cangenerateasymmetric solutions. Progresshas
beenmade,howeverany results which could be stated now would be conjecture.
F-18 Grid Generation
An ongoing effort has been underway to generate a new grid system for the
F-18 HARV aircraft. This is motivated by several factors, most important being
the computational requirements of simulating the unsteady shedding at the extreme
angles of attack. As has been seen with the ogive-cylinder computations, the com-
putational time required to remove the initial transients from the solution is very
significant. In order to make this feasible for a full-aircraft grid system, the grid
points must be used in the most efficient manner possible. This must be done while
improving the overall grid resolution of the system to provide accurate convection
of the shedding waves downstream. To achieve these goals a grid system based
on "background" cartesian meshes with embedded "viscous wall" meshes is being
developed. An overview of this method as applied to the F-18 HARV forebody and
LEX region is shown in Fig. 7. Separate grids are generated for the forebody and
LEX regions and then embedded in the background, inviscid cartesian grid. Work
is underway to model the entire aircraft in this manner.
Fig. 1. Tangent-ogive cylinder grid configuration.
a. Lateral plane of symmetry. b. Periodic grid.
Fig. 2. Symmetric grid boundary conditions.
10.0-
7.5
C_z 5.0
2.5
0.0
0 25 50 75 100 125
Non-dimensional time
Fig. 3. Normal force history at a - 40 °, ReD = 80,000.
C_z 5.11
_._...........___1--- _°_'_ I...._
0.0 _
0 I00 200 300 400 500
Non-dimensional time
Fig. 4. Normal force history at c_ = 40 °.
Fig. 5. Shcaa'-layer shedding at a = 40 °, ReD = 200,000.
2........................... : .....................................................
C_Y_ 2
3, .............................................................. •..................
6
100 150r
Non-dimensional time
Fig. 6. Lateral force history at a = 60 °, ReD = 200,000.
i ! ! i
Fig. 7. F-18 forebody/LEX grid system.


Numerical simulation of the flow about the F-18 HARV at high angle of attack

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