An experimental facility was developed in the 1.23 (48 inch) wind tunnel of the Applied Research Lab. at the Pennsylvania State Univ. to model the ground vortex. The purpose of the facility was to study the effect of various parameters on the location and characteristics of a ground vortex. An experimental investigation was conducted in the tunnel into the formation, stability and strength of the ground vortex for several flow parameters. The design of the facility, special instrumentation and results are summarized

Billet, Michael L.

Walters, Marvin M.

English

NASA Technical Reports Server

N87-24415
SUMMARY OF STOL GROUND VORTEX INVESTIGATION*
Michael L. Billet
Applied Research Laboratory
The Pennsylvania State University
and
Marvin M. Walters
Naval Air Development Center
*This work was accomplished at the Applied Research Laboratory, The
Pennsylvania State University under contract to the Naval Air
Development Center.
207
https://ntrs.nasa.gov/search.jsp?R=19870014982 2020-03-20T11:05:04+00:00Z
S_vmbol
Cp
Dj
Pa
Po
qo
V
vj
V o
V
Vx
V z
V_
Z i
Zle
Zs
List of Symbols
static pressure coefficient
inner diameter of jet
ambient pressure
maximum pressure in the impingement region
reference dynamic pressure, equal to (Po - Pa )
velocity
velocity of jet at nozzle exit
reference velocity, equal to Jzoq--°
velocity at jet centerline
velocity in the x direction
velocity in the z direction
wind tunnel velocity
jet impingement point
vortex maximum penetration point
vortex separation point
Vo
reference jet-to-crossflow velocity ratio = V-_
2O8
Introduction
The impingement of a concentrated circular jet exhaust flow on a ground
plane results in the formation of a wall jet which flows radially from the
point of impingement along the ground surface. Forward motion of the jet
source or the introduction of a counter-flowing freestream interacts with the
wall jet to create a stagnation line and tend[ to roll the wall jet back on
itself forming a horseshow-shaped ground vortex, as illustrated in Figures 1
and 2. Generally taking the shape of an ellipse whose major axis is aligned
with the freestream flow the location of this stagnation line is dependent on
the relative velocity of the freestream and wall jet flows and the injection
angle of the jet exhaust into the freestream flow. The location of the
vortex is nearly coincident with the stagnation line but at a height above
the ground which is also a function of the relative velocities of the jet and
freestream flows. When flow conditions are appropriate for its formation,
this vortex is a major source of the induced flow in the near field.
An experimental facility has been developed in the 1.23 m (48-inch) wind
tunnel of the Applied Research Laboratory at The Pennsylvania State University
to model this ground vortex. The purpose of this facility is to study the
affect of various parameters on the location and characteristics of a ground
vortex.
Previous studies concerning this type of flow are few and very limited
in scope. Colin and Olivari [i] have experimentally determined the location
and established the elliptical shape of the vortex line for one nozzle height.
In addition a dimensionless relationship was proposed between the vortex
separation point and the ratio of wall jet velocity at the impingement point
209
to the cross flow velocity. Parameters such as the presence or absence of a
boundary layer on the ground plane have also been shownto influence the
location of the ground vortex [2,3]. A recent survey on ground effects and
testing techniques is given by Kuhn [4].
A recent effort by Stewart and Kuhn [5] to develop a prediction method
for STOLground effects indicated the need to not only establish the location
of the ground vortex as it varies with parameters but also the strength of the
resulting vortex. As a result, an experimental investigation was conducted in
the 1.23 m (48-inch) wind tunnel into the formation, stability and strength of
the ground vortex for several flow parameters. The intent of this paper is to
summarizethe design of the facility, special instrumentation and results.
Experimental Facility
All tests were conducted in the subsonic wind tunnel. This facility
is a closed-circuit, closed-jet wind tunnel with an octagonal test section
which is 1.2 m (4.0 ft) across and is 4.9 m (16.0 ft) long. The test section
velocity can be varied continuously up to 36.6 m/s (120 ft/sec) and honeycombs
and screens in the settling sections reduces the turbulence level in the test
section to be less than 0.i0 percent of the free stream velocity.
A 76.2 mm (3.00-inch) diameter open-jet was fabricated and inserted
through one side of the test section as shown in Figure 3. The jet features a
16.0 to 1.0 contraction ratio and is equipped with two wire mesh screens and a
honeycomb to reduce turbulence. The 45.7 m/s (150 ft/sec) jet was powered by
a variable speed 3.7 kW (5-hp) blower which injested air from the wind tunnel
at a port far downstream from the test camber.
The test facility was formed by two vertically mounted 2.4 m (8.0 ft)
long wooden panels with circular arc leading edges. The jet tube extended
210
152.4 mm (6.0 inches) through the center of one of the panels. The movable
ground board shown in Figure 4 was designed to facilitate conducting the
various phases of the test program. The ground board was located between
horizontal ceiling and floor inserts and could be positioned at i, 2, 3, 4, or
6 jet diameters from the jet exit plane. At each position it could be moved
and flared to control the static pressure gradient due to the wall boundary
layer growth. In addition, a slot was included on the ground board for
boundary layer control. The ground board was also equipped with
interchangeable .6 m x .9 m (2 ftx 3 ft) window inserts. Three windows were
available for various phases of the test program, i.e., a glass with
fluorescent mini-tufts, a glass window for LDV surveys, and a plexiglass
window instrumented with static pressure taps.
Instrumentation
The jet velocity was monitored via 3.17 mm (0.125-inch) diameter kiel
probe in the plenum section and a static pressure tap in the wall of the jet
tube. The wind tunnel velocity was measured by a 3.17 mm (0.125-inch)
diameter pitot-static probe mounted on the floor insert midway between the
sidewalls outside of all wall boundary layers. For the wall-to-wall flow
surveys a miniature five-hole probe [8] fabricated at ARL/PSU was utilized.
In addition, this five-hole probe was also used to determine the mean velocity
jet characteristics and a single element hot wire anemometer was used to
determine the turbulence characteristics. It is important to note that the
jet characteristics were determined with the ground plane removed and the wind
tunnel separated at points A and B as annotated in Figure 3.
Flow visualization tests to locate the separation line and the forward
extent of the recirculation bubble on the ground plane were conducted with a
211
large matrix of fluorescent mini-tufts on the ground plane. This technique
which was originally developed by Crowder [8] and was extended by Stinebring
and Treaster [9] uses extremely fine fluorescent monofilament fibers [(0.178
mm 0.007-inch diameter)]. These fibers are attached to the window by a tiny
drop of cyano-acrylate glue and illuminated by an ultraviolet light source to
map the flow.
The static pressure distribution on the ground plane was measured via
window insert having static pressure taps. In all, 56 static pressure taps
having 0.787 mm (0.031-inch) diameter holes were used as shown in Figure 5.
All pressures in this test program were measured with individual
transducers which could be sampled electronically. The temperature of the
test environment was determined with a temperature probe and recorded as one
of the input channels. All data were acquired on-line via the VAX 11-782
computer system which permitted on-site graphic terminal display of the
primary and reduced test parameters and later hard copy output of the selected
data.
Details of the ground vortex velocity field were determined by a
five-beam, three-component laser Doppler velocimeter system. The system
measured three velocity components at the crossing of three green beams and
two blue beams by collecting the light scattered by the seeded particles in
the flow. Thermal Systems, Inc., optics with a four-watt Lexal Argon-Ion
laser was mounted on a three-axis traverse as shown in Figure 6. The digital
output of the three counter processors were processed in a VAX 11-782
computer. The computer produces histograms of the measured velocities and
computes the statistics of the flow including the mean velocity and turbulence
intensity.
212
Summary of_Experimental Results
Jet Characteristics
Velocity surveys to measure jet characteristics were conducted at
x/Dj = 1.0, 2.0, 3.0, 4.0, and 6.0 with Vj = 45.7 m/s (150 ft/sec) across its
potential core. The axisymmetric velocity profiles obtained with a 5-hole
probe are shown in Figure 7 for the vertical plane. The surveys in the
horizontal plane are virtually identical since the jet is axisymmetric. Shown
in Figure 8 are turbulence measurements acquired by using hot-wire anemometry
at x/Dj = 2.0. The turbulence intensity at the centerline and at x/Dj = 2.0
was experimentally measured to be less than 2%. The impingement point of the
jet on the ground plane depends on the jet-to-crossflow velocity ratio (V_/Vj)
and ground plane position (x/Dj). The variation of the jet impingement point
with ground plane position for various jet-to-crossflow velocity ratios is
shown in Figure 9.
Test Chamber Characteristics
The ground board location relative to the ground vortex location was
varied longitudinally so that the LDV surveys could be centered at a position
approximately 101.6 mm (4-inches) downstream of the window's leading edge.
Thus, wall-to-wall surveys were conducted with no jet flow at the five x/Dj
locations and crossflow velocities of 4.6, 9.1, 13.7, and 18.3 m/s (15, 30,
45, and 60 ft/sec). The data for 18.3 m/s (60 ft/sec) and x/Dj = 2.0 are
shown in Figure i0 and is representative of the other velocity and location
data. It is important to note that these survey data were obtained with the
boards flared as to minimize the pressure gradient, and as the data of Figure
i0 indicates, a uniform pressure from wall to wall was achieved.
213
Ground Vortex Position
The window instrumented with the fluorescent minitufts was used to
obtain a first-order measurement of the ground vortex location. Photographs
such as shown in Figure ii were utilized to obtain the location of the separa-
tion line and the leading edge of the recirculating region (Zle) on the
ground board as shown in Figures 12-15. Z s and Zle were measured from the
impingement point of the jet. The summary of the vortex position data are
shown in Figures 16 and 17. In addition, data from tests conducted at
Rockwell [i0] are also included in Figure 17 and are shown to be in very good
agreement. Several of the Zle values at V_/Vj = 0.3 and 0.4 were no longer on
the viewing window as indicated in Table 1 which tabulates the various vortex
parameters for each flow condition. Vortex oscillations were also noted and
were most pronounced at V_/Vj = 0.i and 0.2.
Colin and Olivari [I] derived a dimensionless relationship between the
vortex separation point and the ratio of wall jet velocity at the impingement
point to the cross flow velocity. This relationship was derived by assuming
that the energy in the wall jet equals the energy in the cross flow at
the point of separation and is
Zs _,0.9
- 1.03 (1)
Dj
where Z s = vortex separation point and _ = reference jet-to-cross flow
velocity ratio. Using the ground plane pressure data recorded earlier, the
vortex separation point (Z s) was calculated by Equation (I). These values
are compared to theoretical values and flow visualization in Figure 18. Very
good agreement was found although the boundary layer on the ground plane
varied from 12.7 to 95.2 mm (0.5 to 3.75 inches) for the present study.
214
Laser Doppler Velocimeter Data
The 3-component LDV was utilized to measure the vortex velocity field on
the ground plane. Initially, the shape of the vortex was visualized by
micron particulate as they pass through a laser-light sheet. Photographs
indicate that the vortex appears to be nonsymmetrical. In addition, flow
visualizations indicated that very few of the cross-flow seeded particulate
appeared in the core of the vortex structure where as most of the jet seeded
particulate appeared in the core. Additional flow visualization tests are
planned to document these particle trajectories.
Traverses through the center region of the vortex are shown in Figures
19-21 for the case of V_/Vj = 0.i, 0.2 and x/Dj = 3.0. Velocities in the wall
jet region were measured to be approximately twice the velocities measured in
the opposing cross-flow region. This result notes a nonuniform energy
distribution as opposed to the classical free vortex shape.
Summary of Investigation
A test facility suitable for the study of the ground vortex resulting
from a jet impinging on a ground board in the presence of a cross-flow has
been developed. The aerodynamic characteristics of the test chamber and jet
have been determined. Data on the ground plane static pressure distributions
and flow patterns were obtained for many flow conditions. Experimental data
have confirmed Colin and Olivaris model. LDV measurements of the ground
vortex indicates a nonsymmetric velocity distribution. In addition, the
velocity field appears to have oscillations.
The data reported in this paper represents only some of the test results.
More detailed velocity data of the ground vortex are currently being obtained
with the LDV system. In addition more tests are currently being planned to
determine the influence of incoming ground board boundary layer on the vortex
location and characterize the stability of the ground vortex.
215
References
[I] Abbott, W. A., "Studies of Flow Fields Created by Vertical and
Inclined Jet When Stationary or Moving Over a Horizontal Surface,"
RAE CP No. 911, 1967.
[2] Colin, P. E. and Olivari, D., "The Impingement of a Circular Jet
Normal to a Flat Surface With and Without a Crossflow," Von Karman
Institute for Fluid Dynamics, Rhode-St. Genese Belgium, January 1969.
[3] Crowder, J. P., "Add Fluorescent Mini-Tufts to the Aerodynamicists
Bag of Tricks," Astronautics and Aeronautics, Vol. 18, November
1980, pp. 54-56.
[4] Kuhn, R. E., "V/STOL and STOL Ground Effects and Test Techniques,"
NASA No. NAS2-I1912, December 1984.
[5] Schwantes, E., "The Recirculation Flow Field of a VTOL Lifting
Engine," NASA TT F-14912, June 1973.
[6] Stewart, V. E. adn Kuhn, R. E., "A Method for Estimating the Propulsion
Induced Aerodynamic Characteristics of STOL Aircraft in Ground Effects,"
NADC-80226-60, April 1982.
[7] Steward, V. and Kuhn, R., "A Method for Estimating the Propulsion
Induced Aerodynamic Characteristics of STOL Aircraft in Ground
Effects," NADC Report No. 80226-60, Vol. 2, August 1983.
[8] Stinebring, D. R. and Treaster, A. L., "Water Tunnel Flow Visualization
by the Use of Fluorescent Mini-Tufts, The Applied Research Laboratory,
TM No. 83-24, March 1983.
[9] "Treaster, A. L. and Yocum, A. M., "The Calibration and Application
of Five-Hole Probes," ISA Transactions, Vol. 18, No. 3, 1979,
pp. 23-24.
216
(a)
TABLEI. GROUNDVORTEXDIMENSIONS
V_IVj ZilD j ZslD j ZulD j
XIDj = 1.0
0.I
0.2
0.3
0.4
0.00
0.00
0.00
0.00
4.25
2.75
2.25
1.50
7.75
5.25
4.75
3.25
X/Dj = 2.0
0.1
0.2
0.3
0.4
0.00
0.00
-0.25
-I.00
4.00
2.75
1.50
0.00
6.25
4.25
2.75
2.25
XID. = 3.0 (a)
J
0.1
0.2
0.3
0.4
0.00
-0.75
-1.00
5.50
2.00
0.25
7.75
3.75
2.25
1.00
0.1
0.2
0.3
0.4
XID. = 4.0 (a)
J
-0.50 5.75
-1.50 0.75
-2.75 "*" "*
8.25
3.00
1.50
XIDi = 6.0(a)
0.1
0.2
0.3
0.4
-0.5
-3.25
4.50
-2.25
7.50
1.75
• ** ** indicates that the ground vortex was blown
downstreamoff of the tuft window.
217
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Summary of STOL ground vortex investigation

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