This report covers the period forming the first year of the project. The aim is to experimentally investigate the effects of suction as a technique for Laminar Flow Control. Experiments are to be performed which require substantial modifications to be made to the experimental facility. Considerable effort has been spent developing new high performance constant temperature hot-wire anemometers for general purpose use in the Fluid Mechanics Laboratory. Twenty instruments have been delivered. An important feature of the facility is that it is totally automated under computer control. Unprecedently large quantities of data can be acquired and the results examined using the visualization tools developed specifically for studying the results of numerical simulations on graphics works stations. The experiment must be run for periods of up to a month at a time since the data is collected on a point-by-point basis. Several techniques were implemented to reduce the experimental run-time by a significant factor. Extra probes have been constructed and modifications have been made to the traverse hardware and to the real-time experimental code to enable multiple probes to be used. This will reduce the experimental run-time by the appropriate factor. Hot-wire calibration drift has been a frustrating problem owing to the large range of ambient temperatures experienced in the laboratory. The solution has been to repeat the calibrations at frequent intervals. However the calibration process has consumed up to 40 percent of the run-time. A new method of correcting the drift is very nearly finalized and when implemented it will also lead to a significant reduction in the experimental run-time

Watmuff, Jonathan H.

English

NASA Technical Reports Server

NASA-CR-191276
MCATIn_itute
Prog_ss Repo_
92-019 /_/__
F--/
A FUNDAMENTAL STUDY OF
SUCTION FOR LAMINAR
FLOW CONTROL (LFC)
J
Jonathan Watmuff
!
(NASA-CR-191276) A FUNDAMENTAL
STUDY OF SUCTION FOR LAMINAR FLOW
CONTROL (LFC) Progress Report, Nov.
1991 - Oct. 1992 (MCAT Inst.) 7 p
N93-13277
Unclas
G313_ 0131809
October 1992 NCC2-698
MCAT Institute
3933 Blue Gum Drive
San Jose, CA 95127
https://ntrs.nasa.gov/search.jsp?R=19930004089 2020-03-17T10:13:00+00:00Z
A FUNDAMENTAL STUDY OF
SUCTION FOR LAMINAR FLOW CONTROL (LFC)
Progress Report for Period November 1991 to October 1992
By Jonathan H. Watmuff
MCAT Institute
SUMMARY
This report covers the period forming the first year of the project. The aim is to
experimentally investigate the effects of suction as a technique for Laminar Flow Control.
Experiments are to be performed which require substantial modifications to be made
to the experimental facility. Unfortunately insufficient funds were available to fully
complete the fabrication. As a result only slightly more than half of the necessary
modifications were completed this year. All the design work has been performed and
the drawings are complete. Funding for completion of the fabrication and for some
items of equipment next year has been obtained from the Director's Discretionary Fund
so the work-can-now proceed.
Considerable effort has been spent developing new high performance constant tempera-
ture hot-wire anemometers for general purpose use in the Fluid Mechanics Laboratory.
The design was based on results presented in a series of papers by the author. Twenty
instruments have been delivered, at a cost of around $1k each, and all users report that
the performance is as good as, if not better than, commercially available instruments
costing $8k vach, while the noise level is considerably less. The process of writing a
"User's Guide" is approaching completion.
An important feature of the facility is that it is totally automated under computer con-
trol. Unprecedently large quantities of data can be acquired and the results examined
using the visualization tools developed specifically for studying the results of numer-
ical simulations on graphics work stations. The experiment must be run for periods
of up to a month at a time since the data is collected on a point-by-point basis. Sev-
eral techniques were implemented to reduce the experimental run-time by a significant
factor. Extra probes have been constructed and modifications have been made to the
traverse hardware and to the real-time experimental code to enable multiple probes to
be used. This will reduce the experimental run-time by the appropriate factor. Hot-
wire calibration drift has been a frustrating problem owing to the large range of ambient
temperatures experienced in the laboratory. The solution has been to repeat the cali-
brations at frequent intervals. However the calibration process has consumed up to 40_,
of the run-time. A new method of correcting the drift, is very nearly finalized and when
implemented it will also lead to a significant reduction in the experimental run-time.
MODIFICATIONS TO WIND TUNNEL
Leading edge, extension to contraction and side walls
The base flow consists of a laminar boundary layer in a zero pressure gradient. In the
original configuration the test plate was attached to the floor of the contraction. The
influences of the FPG within the contraction and the APG at the exit on the test plate
boundary layer are uncertain. Therefore a 24:1 elliptic leading edge has been built for
the purpose of starting a new layer on the test plate. The leading edge is downstream
of the APG associated with the contraction exit and it is located in the free-stream. An
extension to the contraction floor will combine with the lower surface of the leading edge
to form a slot for the removal of the contraction boundary layer. It is anticipated that
the slot will function passively i.e. an external fan will not be required. The leading
edge and floor extension have been constructed but they have not been tested as a unit
at this stage.
New ceiling, spacers for traverse and side-walls
A new plexiglass ceiling for setting the zero pressure gradient has been constructed by
the model shop. Spacer blocks will be needed to raise the traverse assembly and the
sidewalls to correctly position them relative to the new height of the test surface above
the optics table which forms the base of the test section. Minor extensions are also
required for the existing side walls in order to complete the test section and these two
pieces will be constructed by FML technicians in situ.
Suction surface_ plenum chamber and support
Plenum enclosures are required underneath the porous surface for the purpose of main-
raining a steady pressure difference for the suction. The porous surface was designed
to be as wide as possible to avoid contamination by the boundary layers forming over
solid edges. The porous surfaces are also quite thin (e.g. lmm) so that support must. be
provided at close intervals. These design objectives require the construction of a large
plenum and plate support system to replace the existing solid plate. An important
feature incorporated into the new configuration is the ability to interchange various test
surfaces.
At the outset, a porous surface was to be used that is typical of those currently being
tested on aircraft wings i.e. 0.002" dia. holes spaced on a 0.020" grid. The manufacturer
of this laser drilled material is located in the U.K. The quotation of $10,000 for 0.040
inches thick porous titanium turned out to be less than half the cost of other more
suitable porous materials that are thicker for example. Drawings were prepared for the
suction surface support and plenum chamber assembly. It was recognized that working
with such a thin sheet would be difficult and there would be considerable uncertainty
with many aspects of the assembly procedure. After consultation with the relevant
Ames shops, a small subset of the final assembly has been designed and built for the
purpose of testing the adequacy of the scheme. A method was devised for attaching thin
suet.ion surfaces to the plenum and support assembly using epoxy resin which maintains
a high degree of flatness of the surface.
2
During the courseof the project, Tadjfar and Reda (1991) obtained numerical results
which indicated that a pair of counter rotating longitudinal vortices could form in the
vicinity of a hole when the suction is strong. This observation is of concern because
further downstream, in the region beyond the capabilities of the calculations, these
vortices could defeat the purpose of the suction and even lead to an earlier transition.
Almost nothing is known about 3D disturbances generated by suction holes despite the
potentially negative influence.
On the basis of these findings the objective of the project has been modified to focus
more directly on 3D disturbances generated by suction holes. A porous surface has
been designed which is geometrically similar to those used in LFC flight tests but the
hole size and spacing will be scaled up by a factor of 20, i.e. lmm (0.040in) diameter
holes spaced on a 10mm (0.40in) grid. This scaling will provide enough spatial res-
olution to detect the formation of disturbances in the vicinity of the holes while still
maintaining a surface length-scale to boundary thickness ratio that is representative of
flight conditions. About 15,000 holes will be required in the scaled up porous surface.
Preliminary tests in the machine shop have demonstrated that it is feasible to drill each
hole individually using an NC machine. Added benefits are that the thickness of the
surface can be increased to around 0.080" and the difficulties of procurring material
from overseas are avoided. All the design work is now complete and the drawings are
ready for submission to the machine shop.
ENHANCEMENTS TO EXPERIMENTAL METHODS
Hot-wire calibration drift
Hot-wire calibration drift is a source of great frustration in the laboratory owing to the
large range of ambient temperatures. A major reason for not implementing temperature
drift correction schemes in the past is that the apparatus is totally automated and this
allows calibrations to be performed at frequent intervals. In essence, the soiution to
the drift problem has been to repeat the measurements until the drift check obtained
after the measurements is within a certain percentage (e.g. 1%) of the drift reference
obtained immediately following calibration. One negative aspect of this "brute force"
approach has been that up to 40% of the total experimental run-time is consumed with
calibration procedures. Further, it has been necessary to sift through large quantities
of data and there is evidence to suggest that sometimes the wires can drift away from
calibration and then back to the calibration just before the check is performed i.e. the
scheme is not foolproof.
Platinum and Tungsten are the most commonly used metals for hot-wires. Platinum has
a strong resistance to oxidation but its tensile strength is about 1/5 that of Tungsten.
While the Tungsten is stronger, it suffers from oxidation at elevated temperatures. A
feature available with both Pt-10%Rh and pure Platinum wires is that they can be
supplied with a thick silver coating (g_llastan process) and this greatly helps with
probe construction i.e. manipulation is easier while soldering onto the prongs. Only
the central region of the silver is etched away to provide the sensitive filament which is
located away from the prongs. Pt-10%Rh has a tensile strength about 50% that of pure
Platinum so it was chosen for use with the facility.
Initial experiments indicated that the drift appeared to be a function of some other
variable in addition to temperature. In follow on work the calibration and drift char-
acteristics of five probes were monitored simultaneously. Recently, Platinum coated
Tungsten wire has become available which offers the best advantages of each material
i.e. the strength of Tungsten combined with the oxidation resistance of Platinum. A
2.5pm diameter sample of this material was obtained for one wire and a regular 5pro
Dantec Tungsten wire was used in addition to three 2.5pm Pt-10%Rh wires. The three
Pt-10_,I:lh wires showed considerable scatter when plotted against temperature com-
pared to the other wires. It turns out that Pt-10%Rh has a temperature coefficient of
resistivity about half that of pure Platinum and pure Tungsten. Consequently, for a
given resistance ratio, the Pt-10%Rh wires would be heated to approximately twice the
temperature above ambient than either Tungsten or pure Platinum wires. A plausible
explanation for the scatter is that the Pt-10%Rh wires are subject to aeroelastic de-
flections since the material is much weaker at the higher operating temperature. The
calibration changes as the bow in the wire changes slightly. This idea was supported by
the much reduced scatter observed when the wires were run at a lower resistance ratio.
The scatter in the drift, versus temperature plots that is common to all probes is most
likely caused by fluid property variations with changes of atmospheric pressure. Im-
plementation of barometric measurements into the experimental program have been
completed and tests will be conducted shortly to see just how long a single calibration
remains valid when it is subject to the appropriate correction.
A small bath was constructed for the purpose of electrolytic plating of silver onto the
2.5pm diameter Platinum coated Tungsten wire. The technique is quite simple and
enough material for about six probes can be produced in about 30 minutes. All wires
will be constructed using this material in the future because it offers all the best features
i.e. the strength of Tungsten, the oxidation resistance of Platinum and the convenience
of working with a larger diameter wire for attaching to the prongs.
Multiple probes
Benchmark tests indicated that the double-buffered data acquisition/processing scheme
for mean flow and Reynolds stresses provides an indefinitely sustainable throughput of
60kHz for a normal wire and 25kHz for a crossed-wire. Since these rates are far in excess
of the rates estimated for a single probe in laminar flow control experiments, the higher
level code was enhanced to incorporate multiple probes. The use of multiple probes will
reduce the total experimental run-time by the corresponding factor e.g. measurements
requiring a 30 day continuous run with a single crossed-wire will now take only 5 days
if 6 probes are used. Drawings were submitted to the Machine Shop for construction of
multiple probe holders needed for the traversing mechanism. Modifications were made
to the Dantec probes in the form of a size reduction and stiffening of the prongs and
the installation of 2.5 pm diameter thick silver coated, thin Platinum coated Tungsten
mentioned above.
NEW FML HOT-WIRE ANEMOMETERS
Over the last few years much effort has been spent on the developmentof new high
performanceconstant temperaturehot-wire anemometersfor generalpurposeusein the
Fluid MechanicsLaboratory. The designwas basedon results presentedin a seriesof
papersby the author i.e. Watmuff (1987), Watmuff (1988), Watmuff (1989) and War-
muff (1990a,1990b). Twenty instruments have been delivered, at a cost of around $1k
each,and the performance is equivalent to commercially available instruments costing
$8k each, while the noise level is much less.
Considerable time this year was spent on writing the NASA Technical Memorandum "A
High-Performance Constant-Temperature Hot-Wire Anemometer". The memorandum
has evolved into an extensive publication. The philosophy and ideas behind the design
are discussed in detail. The original basis for the design was a 7 th-order analysis derived
by the author while at Princeton University (see "vVatmuff 1987). Since that the time
the derivation has been "algorithmized" so that the transfer functions can be derived
for a system of arbitrary complexity. This work will be published for the first time in
the memorandum as well as later in a journal publication. The memorandum will also
contain a schematic of the circuit and a parts list with suppliers. Enough information
will be provided to enable the anemometer to be constructed by others. In addition
the memorandum will act as a "User's Guide". The hot-wire anemometer is a difficult
instrument to operate properly and detection of an incorrect mode of operation can
be quite subtle. The memorandum will provide examples of incorrect operating modes
which draw on the analysis mentioned above for an explanantion. The instructions and
the collection of tips should prove valuable to users. A number of researchers outside
the FML have already heard of the anemometer and the author has started receiving
requests for the memorandum even at this preliminary stage.
Invited talks and seminars
(1) A seminar was given at the California Institute of Technology in November 1991.
(2) A talk was given at University of Melbourne, Australia, in September 1992.
(3) A talk was given at CSIRO, Highett, Australia, in September 1992.
(4) A talk was given at Aeronautical Research Laboratory, Fisherman's Bend, Australia,
in September 1992.
PUBLICATIONS
Watmuff, J. H. 1991 "An Experimental Investigation of Boundary Layer Transition in
an Adverse Pressure Gradient". Forty-Fourth Annual Meeting of the Division of Fluid
Dynamics, American Physical Society, Scottsdale, Arizona, November 24-26, 1991
"Experimental and Numerical Study of a Turbulent Boundary Layer with Pressure
Gradients", by Philippe 1R. Spalart and Jonathan H. \Vatmuff. Paper accepted for
publication in the Journal of Fluid Mechanics.
"A High-SpeedCross-WireData-Reduction Algorithm", By Jonathan H. Watmuff. Pa-
per submitted to the Journal of Experimental, Thermal and Fluid Science.
REFERENCES
Tadjfar, M. and Reda, D. C. 1991 "Flow Field Near a Single and a Row of 3D Suction
Holes Inside a Laminar Boundary Layer". Forty-Fourth Annual Meeting of the Division
of Fluid Dynamics, American Physical Society, Scottsdale, Arizona, November 24-26,
1991
\Vatmuff, J.H. 1987 " Higher Order Effects in the Frequency Response of the Constant
temperature Hot-wire Anemometer". ASME Applied Mechanics, Bioengineering and
Fluid Mechanics Conference, Cincinnati, Ohio, June 1987.
Watmuff, J.H. " Increasing the Frequency Response of Constant Temperature Hot-wire
Anemometer Systems for use in Supersonic Flow". Presented at AIAA 26th Aerospace
Sciences meeting, Reno, Nevada, January, 1988.
Watmuff, J.H. " The effects of Feedback Amplifier Characteristics on Constant Tem-
perature Hot-wire Anemometer Systems". Presented at the tenth Australasian Fluid
Mechanics Conference, Melbourne, Australia, December, 1989.
_Vatmuff, J.H. " Tuning the Constant-Temperature Hot-Wire Anemometer". Presented
at the ASME Symposium on Heuristics of Thermal Anemometry, Ontario, Canada,
June 1990.
Watmuff, J.H. " Frequency response of hot-wire anemometers". NASA Tech Brief ARC-
12469, 1990.


A fundamental study of suction for Laminar Flow Control (LFC)

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