The experimental contract objective is to provide a complete set of benchmark quality data for the flow within a large rectangular turning duct. The data are to be used to evaluate and verify three-dimensional internal viscous flow models and computational codes. The analytical contract objective is to select such a computational code and define the capabilities of this code to predict the experimental results. Details of the proper code operation will be defined and improvements to the code modeling capabilities will be formulated

Crawford, R. A.

English

NASA Technical Reports Server

GAS FLOW ENVIRONMENTAL AND HEAT TRANSFER NONROTATING 3D PROGRAM*
R. A. Crawford :u_ 8 7 - 1 1 2 2 8
University of Tennessee Space Instit
OBJECTIVES
The experimental contract objective is to provide a complete set of "benchmark"
quality data for the flow within a "large" rectangular turning duct. These data are
to be used to evaluate and verify three-dimensional internal Viscous flow models and
computational codes. The analytical contract objective is to select such a computa-
tional code and define the capabilities of this code to predict the experimental
results. Details of the proper code operation will be defined and improvements to
the code modeling capabilities will be formulated. Separate but coordinated experi-
mental and analytic approaches are in progress to attain the contract objectives.
APPROACH, EXPERIMENTAL
The experimental facility design features modular tunnel components which allow
O .
flow measurements every 15 in the 90 ° bend. The 25.4 cm (i0 in) square cross sec-
tion tunnel is designed with a 13 to i area ratio bell mouth contoured to provide
uniform flow velocity and is powered by a variable speed, six-bladed fan. The tunnel
is designed for incompressible flow and will produce tunnel velocities of 6 to 20
m/sec (20 to 65 ft/sec). These two flow conditions provide laminar and fully turbu-
lent boundary layer profiles at the entrance t_ the 90° bend. Corresponding _eynolds
numbers based on tunnel width of Re_ = 98 X i0 at 6 m/sec and Re_ = 328 X i0 at 20
m/sec are significantly higher than_the Reynolds number values from NASA-CR-3367 by
Taylor, Whitelaw and Yianneskis. Thus the boundary layer thickness in this investi-
gation was also significantly less (1/3) than that reported in NASA-CR-3367. This
investigation provides baseline curved duct flow data which is closer to real turbine
cascade Reynolds numbers and Dean numbers than previously obtained.
The primary instrumentation is designed for non-intrusive flow measurements
utilizing a three-dimensional, laser velocimeter (LV) and wall static pressure gages.
The LV utilizes two color beams and Bragg diffraction beam splitting/frequency shif-
ting to separate the three simultaneous, orthogonal, vector velocity components. The
LV signal processors determine the digital values of velocity from the seed particles
crossing the laser beam probe volume. To improve and speed up digital data acquisi-
tion, the LV processors are designed around an S-100 bus Z-80 microprocessor which
provides on-line, near-real time data reduction. This on-line data as it is acquired
and recorded for off-line detailed analysis. To qualify the measurements as "bench-
mark" data, the LV data will be compared with both pitot probe and hot wire
anemometer measurements for flow conditions which permit comparisons.
RESULTS, EXPERIMENTAL
Following careful checkout and calibration of the experimental facility and
instrumentation the curved duct was surveyed with the 3D LV system at six stations
(inlet, 0°, 30 ° , 60 ° , 90 °, exit). Extensive tunnel flow quality surveys were com-
pleted in the inlet section following "benchmark" calibration of the LV system,
pitot-static, and hot wire anemometer. All three measurement systems were calibrated
against reference standards and were found to agree within ± 1% on the entrance sec-
tion velocity. The LV system was calibrated with a spinning disk reference velocity
*NASA Contract NAS3-23278.
389
https://ntrs.nasa.gov/search.jsp?R=19870001790 2020-03-23T15:05:43+00:00Z
at 20 m/sec. The pitot-static pressures were read on a precision slant manometer
with 13mm of H20 full scale. The hot-wire system was calibrated in a reference noz-
zle flow at 30.5 m/sec. _ Velocity surveys taken 50.8 cm (20 in) behind the bell mouth
exit showed flat velocity profiles ± 0.5% mean velocity outside of the boundary layer.
The mill bed traverse system has demonstrated repeated accuracy of ± 0.1mm on all
three axis movement. Both LV and probe positions are controlled by the computer
driven mill bed.
Development and operation of a satisfactory flow seeding system remained a pro-
blem during the experimental investigation. Phenolic micro-ballons of equivalent 2-5
micron size were successfully seeded in a water-alcohol slurry and in a fluidized bed
air seeder. However the phenolic spheres would become staticly charged and collect
on screens and tunnel walls. The operational problems of using the micro-ballons
were greater than their advantages of small particle lag. Water droplets were
selected because of their good scattering characteristics, clean properties and low
cost. However water droplet seed must be in the 2-5 micron size range to prevent
serious particle lag problems. It is very difficult to control droplet size as temp-
erature, relative humidity and atmospheric dust influence water droplet size. At
high tunnel speeds, 20 m/sec, the influence of centrifugal forces caused some
particle lag problems in the Y-component velocity measurements.
The experimental investigation is complete with 3-D velocities surveys at six
flow stations, 600 spatial points per station, for two bulk flow velocities of 6
m/sec and 20 m/sec. Since each of the 600 point surveys took several days to accom-
plish, the complete data set was non-dimensionalized on bulk velocity to eliminate
small variations in tunnelospeed during the testing. Sample results are sho_n in
Figures i and 3 for the 60 station. Due to the thinness of the entrance boundary
layer the axial and crossflow velocity developments in the turn are driven primarily
by the pressure forces.
APPROACH, ANALYTICAL
The analytical approach involves, first, selecting a computer code capable of
solving the Navier Stokes equations with turbulence models for three dimensional in-
ternal flow, and adapting it to the experimental geometry and flow conditions. After
this, calculations are to be made for laminar flow conditions for unheated flow.
Analysis of these calculations will define the grid size and stretching factors re-
quired for adequate resolution as well as the values of time steps and smoothing
factors required for convergence. Also, any output and graphics capability required
for comparison with data is to be developed during this phase. The adequacy of the
code with respect to the differencing scheme, adaptability and convergence will be
decided in this phase, by comparisons with published experimental and computational
data and by grid sensitivity studies. After the grid refinement studies to determine
grid sensitivity are completed, computations on the curved duct at conditions corres-
ponding to the UTSI experiment will be performed. The analytic results from these
computations will be used in detailed comparisons with experimental data.
RESULTS, ANALYTICAL
The code selected was based on the Beam-Warming algorithm. It is fully elliptic
with all inertial terms retained. Both cross-stream viscous terms are retained, and
only the stream-wise viscous term is neglected. The code, developed by P.D. Thomas,
uses generalized coordinates and was easily adapted to the curved duct case. Exten-
sive comparisons were made with the analytic results published by Briley and McDonald
and with experimental data published by Whitelaw et. al. The comparisons were made
for laminar flow to avoid uncertainties due to turbulence modeling. Grid refinement
comparisons involving doubling the number of points did not significantly effect the
390
downstreamresults for the inlet boundary layer profile selected. If a much thinner
entrance boundary layer is input the fine grid results may differ from the course
grid results.
For the turbulent cases simulating the curved duct experiment, an algebraic eddy
viscosity model was used. The sensitivity to initial boundary layer thickness was
investigated by comparative runs with 13mm(0.5 in), 25mm(i.0 in) and 51mm(2.0 in)
profiles. The 13mmand 25_ solutions were very similar but the 51mmboundary layer
was large compared to the tunnel half height and the solution was different. The
similarity of the two thin boundary layer solutions especially near the end of the
curved duct may indicate an axial numerical diffusion of the boundary layer which
would not occur experimentally.
Comparison of the cross-flow and stream-wise velocities at the 60° station from
the analytic and experimental cases shows a significant difference. Although the
general flow pattern is similar the experimental results showmuch thinner boundary
layers and larger velocity gradients. Figures 1 and 2 comparecrossflow velocities
at the 60° station. Figures 3 and 4 comparestream-wise velocities at the 60° sta-
tion. A significant effort remains to complete analysis of all data and explain the
differences in theoretical and experimental results.
391
8d
1
1
k
EXPERIMENTAL CROSSFLOW VELOCITY, 60 ° STATION
FIGURE i.
PRESSURE WALL _ SUCTION WALL
I
D
I1 / ,' ," -" ." , _ _ - /,lilt 1
Sll I 1 _ , , , _ _, _, -- /. /ll/,Ill
,ll 1 _ , , , , - -- _ _ G//_I/I
,| _ • _ _ _ _ _ ------.,. ---____. .... _,-,
ANALYTIC CROSS FLOW VELOCITY, 60 ° STATION
FIGURE 2
392
^X
1
1
C
0
m
P
U
m
/
Q
8---
1
N
2_
0
-150
S. W_
= I
I t I I I I i , i i
-100
I I I I I I I I I II
i
-50 0
PJ Wm
i I LI I I I I I q I50 100 1 •
X-O. Oj Rodtal Oto_onoe Y _mm)
EXPERIMENTAL AXIAL VELOCITY, 60 ° STATION
FIGURE 3
^
x
I
o
I
C
O
m
P
U
m
/
O
6u_
g
2_n
u
-150
0, Z= 0. 00
z_, Z= -50. 80
• Z=-101.60
0, Z=- 120. 65
I I I II I I I I I I I I I I I I ,_, , /
-50 0 50 100 1_0
S, W,
I i'Ll i I , i , i
-100
X=O. Os Racltal Ote't, anoe Y (ram)
ANALYTIC AXIAL VELOCITY, 60 ° STATION
FIGURE 4
393


Gas flow environmental and heat transfer nonrotating 3D program

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