System for boundary layer control through pulsed heating of a strip heater

A system is described for controlling the transition of laminar/turbulent flow at a surface which comprises a thin narrow strip heater disposed adjacent the surface and extending substantially transversely of the flow of the air stream thereacross, the heater being resiliently held in tension on or in closely spaced relationship to the surface, and a power source operatively connected to the heater for applying pulsed voltage of preselected amplitude and frequency to the heater.

BACKGROUND OF THE INVENTION 
The present invention relates generally to systems and methods for 
controlling laminar/turbulent flow at a surface and more particularly to 
system and method for selectively promoting or retarding the 
laminar/turbulent transition by controlled surface heating. 
In a laminar boundary layer on a substantially flat plate, periodic 
disturbances known as Tollmien-Schlichting waves can develop prior to 
transition to a turbulent boundary layer. A laminar boundary layer on a 
surface is considered stable if the disturbances decrease in amplitude as 
they move downstream along the surface and unstable if the disturbances 
increase in amplitude. If the disturbances increase in amplitude, a 
transition to a turbulent boundary layer eventually results. 
Experiments by Schubauer and Skramstad (J Aero Sci 14:69-78 (February 
1947)) showed that mechanical oscillations of thin ribbons near a surface 
generate disturbances and influence the transition from laminar to 
turbulent flow. More recently, Thomas (J Fluid Mech 137:233-250 (1983)) 
used vibrating ribbons to control or delay boundary layer transition. 
Liepmann et al reported (J Fluid Mech 118:187-200,201-204 (1982)) that 
periodic heating of heater strips could be used to excite laminar 
instability waves in water and to control, cancel or reduce instability 
wave amplitudes. By reducing the amplitude of instability waves, 
transition from laminar to turbulent flow is delayed. Maestrello (AIAA 
Shear Flow Control Conf. Paper 85-0564 (March 1985)) showed 
experimentally that surface heating of a boundary layer led to growth of 
disturbances. 
In the prior art teachings, the laminar/turbulent transition is affected by 
utilizing ribbons spaced away from the surface over which the flow is 
maintained, and by inducing vibrations in the ribbon mechanically or 
electromagnetically. Such arrangements suffer from certain shortcomings, 
particularly in practical application, since mechanical or electromagnetic 
means are required adjacent the ribbon to provide the required vibration, 
and mounting the ribbon in spaced relationship to the surface severely 
disturbs flow along the boundary surface and subjects the ribbon and 
vibrating means to damaging effects of the flow when not in use. 
The present invention solves or reduces in critical importance problems of 
the prior art as just suggested by controlling (promoting or retarding) 
the laminar/turbulent transition at a surface using a heated ribbon (or 
other appropriate strip heater) which is controllably vibrated by pulsed 
heating to excite vibration in the ribbon, which in turn leads to 
sinusoidal disturbances in the boundary layer along the surface. Ribbon 
vibration amplitude and frequency are controlled by selective pulse 
heating and suitable selection of ribbon attachment geometry. In the 
practice of the invention, the ribbon is mounted on or in closely spaced 
relationship to the surface or is flush mounted (recessed) in a groove 
provided in the surface over which the flow is to be controlled. Inducing 
vibrations using pulsed heating according to the invention avoids the need 
for mechanical or electromagnetic vibrating means which necessarily 
interfere with flow at the boundary surface. Flush mounting is especially 
desirable as presenting a configuration least susceptible to equipment 
damage and to interference with boundary layer flow. 
The invention has substantial practical utility in the control of boundary 
layer transition at lifting surfaces on flight vehicles to control stall, 
to increase lift, to reduce lift loss at high angles of attack, to reduce 
drag, to counteract pressure oscillations in cavities such as aircraft 
bays at high subsonic and supersonic Mach numbers, and to improve fuel 
efficiency. Pulsed heating according to the invention can be selectively 
applied or removed rapidly. Minimal interference with normal operation of 
airfoils, lifting surfaces, and the like is realized through use of the 
invention herein. 
It is therefore a principal object of the invention to provide system and 
method for controlling the laminar/turbulent transition of flow at a 
surface. 
It is a further object of the invention to provide system and method for 
controllably heating a surface in the control of laminar/turbulent flow at 
the surface. 
These and other objects of the invention will become apparent as the 
detailed description of representative embodiments proceeds. 
SUMMARY OF THE INVENTION 
In accordance with the foregoing principles and objects of the invention, a 
system is described for controlling the transition of laminar/turbulent 
flow at a surface which comprises a thin narrow strip heater disposed 
adjacent the surface and extending substantially transversely of the flow 
of the air stream thereacross, the heater being resiliently held in 
tension on or in closely spaced relationship to the surface, and a power 
source operatively connected to the heater for applying pulsed voltage of 
preselected amplitude and frequency to the heater.

DETAILED DESCRIPTION 
Substantial background information and theoretical discussions related to 
disturbances in laminar flow and temperature responses of pulse heated 
thin metallic ribbons according to the invention are given in "Boundary 
Layer Disturbances Caused by Periodic Heating of a Thin Ribbon", a thesis 
by Lawrence Kudelka (Milton E. Franke, Advisor), Document 
AFIT/GAE/AA/86D-7, Air Force Institute of Technology, Wright-Patterson 
AFB, OH (1986), which background information and theoretical discussions 
are incorporated herein by reference. 
Referring now to FIG. 1 of the drawings, shown therein is a schematic 
perspective view of a representative system according to the invention. 
FIG. 2 is an enlarged edge view along line A--A of FIG. 1. FIG. 1 is shown 
partially broken away to reveal system components connected on the 
underside of the system. 
In FIG. 1 is illustrated a plate 11 defining a surface 13 across which the 
laminar/turbulent transition of airflow 15 is to be controlled. Plate 11 
is representative in practical application of an aerodynamic lifting 
surface or control element. A thin metallic ribbon 17 was mounted on 
surface 13 of plate 11 substantially transverse of the flow direction of 
airflow 15 as suggested in FIG. 1, and provided a thin region on and 
across surface 13 for controlled pulsed heating according to the invention 
in the control of the laminar/turbulent transition of airflow 15. It is 
noted that strip heaters of suitable configuration may be used in place of 
ribbon 17 of the demonstration system and may be mounted at or in closely 
spaced relationship (i.e., a few thousandths of an inch) to surface 13 as 
would occur to one with skill in the field of the invention guided by 
these teachings. Groove 19 of appropriate size may be provided in surface 
13 for receiving ribbon 17 and providing flush mounting with respect to 
surface 13, this configuration being illustrated in FIG. 2. In the system 
shown in the figures and built and operated in demonstration of the 
invention, ribbon 17 comprised thin nichrome ribbon 1/16 inch wide by 
0.002 inch thick having a resistance of about 8 ohms per foot at room 
temperature. 
In order to selectively excite ribbon 17 (or other appropriate heater) to 
vibration in accordance with the teachings of the invention, ribbon 17 is 
resiliently tensioned along its length by suitable tensioning means across 
surface 13 to take up slack caused by alternate expansion and contraction 
along its length during pulsed heating. Changes in length of ribbon 17 
during pulsed heating was accommodated in the demonstration system by 
extending ribbon 17 around the edges of plate 11 across 1/32 inch OD steel 
tubing segments 23 which provided substantially frictionless edges for 
ribbon 17 to slide on. Corresponding ends of ribbon 17 were connected at 
the underside of plate 11 using axially resilient spring 25, insulator rod 
27 and suitable electrical connectors 29. Power amplifier 31 (Bosen Model 
60B in the demonstration system) was electrically connected between 
respective ends of ribbon 17 substantially as shown in FIG. 1. Connectors 
29 included means to adjust the effective length of ribbon 17 and the 
tension of spring 25 was adjustable so that the vibration frequency and 
amplitude of ribbon 17 during pulsed heating may be selected. Voltage 
pulses at suitable frequency were generated by function generator 33. 
Amplifier 31 provided amplification of the signal from generator 33 to a 
power level sufficient to heat ribbon 17 by ohmic heating. Spring 25 
provided resilient means to take up slack which occurs when ribbon 17 
expands upon being heated, and to maintain substantially uniform tension 
as ribbon 17 decreases in length upon cooling. 
In the operation of the demonstration system, voltages pulses applied by 
generator 33 to ribbon 17 causes small fluctuations in the length of the 
ribbon. The shape and frequency of the voltage pulses applied to ribbon 17 
is not considered limiting of the invention herein so long as sufficient 
heat is applied to and removed from ribbon quickly to cause suitably rapid 
expansion and contraction and the desired vibrational frequency and 
amplitude. Square wave, sinusoidal or other pulse shape may therefore be 
used in accordance with these teachings. Because ribbon 17 is held under 
tension by spring 25, ribbon 17 vibrates and causes disturbances in the 
laminar boundary layer of airflow 15 across surface 13. In order to 
characterize these disturbances as functions of applied electrical pulse 
and ribbon tension and placement, a hot wire anemometer 35 was placed near 
ribbon 17 in the demonstration system. The disturbances appear as 
sinusoidal velocity fluctuations propagated along and across the boundary 
layer, as discussed more completely in the Kudelka and Franke reference, 
supra. 
Tests in demonstration of the invention were conducted on a nichrome ribbon 
17 flush mounted on a plate 11 comprising a piece of fiber-reinforced 
phenolic resin board 8 inches wide by 12 inches long by 3/16 inch thick on 
an aluminum support in a nine-inch low speed wind tunnel having a 36 inch 
long test section capable of a stream velocity of about 67 ft/sec. The 
wind tunnel had sufficiently low free turbulence (less than 1%) to permit 
the detection of velocity perturbations in the boundary layer caused by 
periodic heating of the ribbon. 
Velocity and turbulence in the boundary layer were measured with a hot wire 
anemometer 35 (Thermal Systems Inc (TSI) series 1050 connected to a TSI 
1218-20 hot film boundary layer probe sensor having an operating 
temperature to about 250.degree. C.); the output of anemometer 35 was 
passed through a linearizing module, a signal conditioner with a 500 Hz 
low-pass filter, a digital voltmeter (Hewlett-Packard) and a storage 
oscilloscope to a microcomputer with an analog-to-digital converter. The 
computed mean and standard deviation voltages corresponded closely with 
the digital voltmeter DC and RMS voltages, respectively (within 1%). The 
storage oscilloscope displayed the voltage applied to the ribbon and 
anemometer 35 output (velocity fluctuation). The DC voltage corresponded 
to the mean flow velocity and the RMS voltage to velocity fluctuations 
(turbulence). 
A hot-film probe measured the sinusoidal disturbances induced in the 
boundary layer by pulsed periodic heating of ribbon 17. Vibration of 
ribbon 17 while mounted on plate 11 was observed and measured using a 
microscope having a calibrated reticle. A strobe light was used to 
illuminate ribbon 17 for displacement measurements during pulsed heating. 
The amplitudes ranged from 0.001 to 0.005 inch, the higher amplitudes 
indicating resonant modes of ribbon 17 pulsed under tension. At the 
largest amplitude the periodic heating was set at approximately 180 or 370 
Hz. Other frequencies also excited this particular ribbon, but at lower 
amplitudes. Response of the ribbon was not significantly affected by 
spring tension. 
The tests run using the demonstration system configured as described above 
showed that ribbon 17 can be vibrated at selected frequencies by periodic 
heating to control the growth or decay of disturbances or to cancel 
disturbances by amplitude and phase shifting of the disturbances induced 
by vibration. Any number of ribbons can be used on plate 11 spaced at 
various locations along, and independent of the curvature of surface 13. 
The invention therefore provides system and method for promoting or 
retarding the laminar/turbulent transition of flow at a boundary surface. 
It is understood that modifications to the invention as described may be 
made by one skilled in the field of the invention within the scope of the 
appended claims. All embodiments contemplated hereunder which achieve the 
objects of the invention have therefore not been shown in complete detail. 
Other embodiments may be developed without departing from the spirit of 
the invention or from the scope of the appended claims.