Version 7.6 Release Notes Page 1

Version 7.6 of VSAERO

James K. Nathman

Director of VSAERO Development

July 2012

Version 7.6 of VSAERO supersedes Version 7.4. (Version 7.5 was limited to

improvements in the frequency domain module.) This release consists of the updated code with

improvements described below, a Version 7.6 Users Manual, and 60 testcases (three new). Version 7.6 has been tested for 144 cases under the Linux 2.4 and Windows XP operating

systems.

TYPE-8 JET WAKES

Type-8 Jet wakes can now be attached to a thin-surface (Type 3 or 5) patch and

VSAERO will correctly treat the numerics associated with a jet wake. Previously, the user had

to manually specify the base panel.

TYPE-5 SIMPLE PROPELLER WAKE

The propeller normal velocity, wake axial velocity and wake swirl can be determined by

VSAERO from the disk loading, CPA, and advance ratio, ADVRT. This is signaled in the wake

input by providing the patch number of the propeller disk.

DOUBLE PRECISION

Version 7.6 is released in two precisions: 32-bit and 64-bit precisions. As the number of

panels in a model inevitably increases, the ratio of smallest panel to largest panel increases,

which requires more precision. 64-bit operating systems with large amounts of memory are

more common, so the penalty for double precision arithmetic is not as significant as was in the

past. However, some matrix solvers are not yet available in double precision and the 64-bit

version will require a year of service before it can be regarded as thoroughly tested.

MODELS WITHOUT WAKE GRID PLANES

In some situations, the constant-x wake grid planes of older versions are a hindrance. For

example, a deployed leading-edge slat on a swept wing requires many spanwise wing columns

because constant-x wake-panel edges will intersect the wing unless the wake panels have small

span. Version 7.6 allows the wakes to be modeled without wake grid planes. LGRID=.FALSE.

in the TRGRID namelist will signal VSAERO that the wakes will be stored like patches with no

requirement that the panel edges be at the same X location. Such an input is compatible with

gridding programs that output the wake panels like patches. The lb435_nogrid input illustrates a

Version 7.6 Release Notes Page 2

lb435 with wake grid planes

lb435 without wake grid planes

Version 7.6 Release Notes Page 3

solution without wake grid planes. Wake relaxation without wake grid planes is not

recommended.

Wakes for propellers at low advance ratios also can benefit from a different

discretization. At low advance ratio, the thrust of a propeller will cause the slipstream to

contract, but constant-x grid planes can generate a spurious tip vortex trajectory that increases in

radius. With constant-azimuth grid planes (HELREL=2), the spurious trajectory is absent.

Propeller with constant-X grid planes

Version 7.6 Release Notes Page 4

Propeller with constant-azimuth grid planes

FREQUENCY DOMAIN IMPROVEMENTS Oscillatory calculations for high frequencies and Mach Number (kM) now converge faster

with respect to panel number when DIVZ=.TRUE. in the OSCILL namelist. Experience since

Version 7.4 was released has shown that the large number of panels required for convergence at

high reduced frequencies could be improved considerably by a higher-order integration.

Oscillatory calculations (mode=5) are now possible with unstructured, triangular patches

(IDENT=10 & 11). Unstructured patches are still not allowed for mode=6 calculations of the

influence coefficient matrix.

SIMULTANEOUS BOUNDARY LAYER AND WAKE ITERATIONS

Version 7.6 can determine the wake shape and boundary-layer thickness simultaneously with

BLWIT=1 in the BLAYER namelist. Previously, wake rollup was determined with the

circulation from the inviscid solution, then the boundary layer calculated with the inviscid wake

shape. It was possible to restart VSAERO with the viscous solution for further wake shape

calculations. This is unnecessary with Version 7.6. With BLWIT turned on the boundary layer

and wake shape will be determined simultaneously. The lb435_blwit testcase demonstrates this.

A slightly different final answer is obtained from previous results because the boundary layer

and wake shape are compatible with each other.

Version 7.6 Release Notes Page 5

SQUEEZE UTILITY IMPROVEMENTS

TRAILING-EDGE PANEL SPACING

The trailing-edge pressure distribution is often best behaved when the ratio of panel sizes

is 1:3:5 where the smallest panel is at the trailing edge. This is the size distribution generated by a cosine distribution. SURFGEN does not readily have a cosine distribution available.

Version 7.6 remedies this deficit by including the spacing, SQSPAC, in the SQUEEZE utility

(see SQUEEZE namelist input). Besides eliminating gaps and matching panel sizes, SQUEEZE

can now change the panel size distribution with SQSPAC=COSINE.

WAKE SQUEEZE

SQUEEZE has been limited to airfoils defined by a complete section. Wake-separations

defined at the edges of different patches could not be improved by SQUEEZE. Version 7.6

allows wakes (instead of patches) to be squeezed with SQEEZW, instead of SQEEZ for patches. The matching and spacing of the separation-line paneling are available without

restriction as to how the patches involved are sectioned. However, closing an open trailing edge

is not available.

Version 7.4 with Input spacing, NMATCH=2

Version 7.6 Release Notes Page 6

Version 7.5 with SQSPAC=COSINE, NMATCH=2

ERRORS FIXED

The number of wake grid planes was incorrectly counted, which could lead to VSAERO

terminating because the number of grid planes exceeded the limit when the input was within

capacity.

Boundary layer transition when momentum Reynolds number reaches 200 (ITRIP=3) works properly again in the Nash & Hicks boundary layer.

A divide by zero in determining the velocity/pressure on an unstructured panel adjacent

to a structured patch has been eliminated.

Frequency domain calculations for compressible cases now correctly include symmetry

(RSYM=0.).

New Testcases

Lb435_blwit calculates the boundary layer simultaneously with the wake relaxation.

Lb435_nogrid bypasses the constant-x grid plane description of the wake and uses the

input corner points as wake panel corners.

Version 7.6 Release Notes Page 7

Prop5bwi_cal illustrates a simple propeller model where VSAERO determines the

propeller normal velocity, wake axial velocity and wake swirl from the propeller thrust and

advance ratio.