Airfoil for high efficiency/high lift fan

Airfoil for vehicle fan which operates in low Reynolds number and varying turbulent conditions having a discontinuity formed on the suction surface thereof adjacent to its leading edge to trigger early formation of a laminar flow bubble so that sufficient suction surface remains downstream of the air bubble whereby air circumventing the bubble can reattach to the suction surface for sufficient pressure recovery for high lift and low drag performance.

This invention relates to fans and more particularly to new and improved 
high efficiency and high lift fan airfoils with predetermined suction 
surface discontinuity providing for induced development of a laminar 
separation bubble at a predetermined forward point on such surface so that 
separated air flowing over the bubble can reattach to the suction surface 
for establishment of substantial pressure recovery. 
Automotive engine cooling air fans generally operate in a range of low 
Reynolds numbers and a wide range of turbulence levels and encounter 
excessive separation of laminar boundary air flowing across their suction 
surfaces. This detracts from fan efficiency and air pumping capability. To 
provide for improved fan operation, classical fan airfoils such as NACA-65 
have been employed. However, such fan blade designs have been unable to 
supply the higher lift to drag ratios now desired for surface vehicle 
applications. In such classical airfoil construction boundary air flows 
across a laminar boundary layer region of the suction surface of the 
airfoil and a laminar flow separation bubble naturally occurs. This bubble 
has varying length, which increases as the Reynolds number decreases 
and/or turbulence level of upstream air decreases. The laminar flow 
separation bubble can accordingly grow from its origin to extend across a 
large part of the remaining suction surface so that the air flow 
circumventing the bubble cannot readily reattach to the suction surface to 
provide for the high lift and efficiency needed to meet higher standards 
for vehicle engine cooling fan operation. 
To obtain high lift and low drag, flow reattachment and pressure recovery 
is needed in the shortest possible distance after the flow separation 
bubble. To this end, this invention tailors an airfoil for the 
establishment of a mechanically induced bubble at a predescribed forward 
point on the suction surface. This point is before the transition from 
laminar to turbulent flow and is accordingly in the laminar boundary flow 
region and ahead of the starting point of a naturally occurring laminar 
flow separation bubble. With the positioning of the bubble in a forward 
location in a laminar boundary flow region and with a smooth Stratford 
pressure recovery region downstream of the bubble, a large pressure 
recovery area is provided so that there is reattachment of boundary air to 
such surface and such reattached flow extends across the recovery surface. 
Generally, with this invention, separated flow is quickly blended and 
reattached to the airfoil at optimum points on the Stratford pressure 
recovery region of the suction surface at the end of the bubble to provide 
for high lift and low drag. 
In the development of one embodiment of this invention, a mathematical 
model of a section of the blade surface was made with prescribed velocity 
or pressure distribution which would give the highest lift and lowest 
drag. The model was designed to recognize the existence of a laminar 
separation bubble and to require separation of laminar boundary flow at a 
predetermined upstream or forward position on the suction surface thereof. 
At predetermined pressure points on this surface, pressure recovery was 
required so that flow downstream of the bubble blended and reattached to a 
large area of the Stratford pressure recovery region of the suction 
surface. Accordingly, to maximize lift and minimize drag, the suction 
surface of the mathematical model was designed to obtain a velocity 
distribution recovery a given pressure difference in the shortest distance 
without turbulent flow separation. The blade shape corresponding to the 
optimized prescribed velocity distributions was then calculated from the 
model. 
In a preferred embodiment of this airfoil, a surface discontinuity, such as 
a flat, step, scribe mark, cavity, surface roughness, can be made on the 
suction surface thereof to precisely establish the points of origin of the 
induced laminar separation bubble in the laminar boundary layer region of 
the suction surface and well ahead of the points of origin of a naturally 
occurring separation bubble. In one preferred design, a flat was added to 
the suction surface in the laminar boundary layer region of the calculated 
blade shape to provide the discontinuity to thereby originate the induced 
laminar separation bubble at a precise location and to accommodate part or 
all of reattachment of the separated flow upstream of the pressure 
recovery region. This airfoil design procedure therefore dictates the 
separation point through the discontinuity location and provides for the 
flat geometry in predicting the reattachment points of the separated flow. 
In this preferred embodiment, the flat forms a ramp that makes a 9.degree. 
angle with a tangent to the upstream suction surface of the blade to 
efficiently pump under low Reynolds number flow conditions or under a wide 
range of turbulence levels. Early establishment of the start of the bubble 
provide a control to prevent catastrophic failure (no reattachment of 
separated flow) so that the design is suitable for the entire range of 
engine fan cooling operation including idle. 
A feature, object and advantage of this invention is to provide a new and 
improved fan airfoil section with a suction surface designed to 
mechanically induce the development of a laminar separation bubble at a 
predetermined forward point on a laminar boundary air flow region and in a 
forward point on such surface so that separated boundary air flowing over 
the bubble can subsequently reattach to the suction surface for 
establishment of substantial pressure recovery. 
Another feature, object and advantage of this invention is to provide a new 
and improved airfoil for a vehicle fan having a suction surface 
discontinuity formed in the laminar boundary layer region thereon to 
trigger development of a separation bubble at a predetermined forward 
point thereon allowing detached boundary air sufficient area on the 
suction surface to reattach for pressure recovery for high lift and low 
drag. 
Another feature, object and advantage of this invention is to provide a new 
and improved airfoil section for vehicle fan blading in which a laminar 
separation bubble developed on the suction side of the airfoil is forced 
to occur in an upstream or forward portion of the airfoil section and 
ahead of points at which a laminar flow separation bubble naturally occurs 
so that there is sufficient space on the suction surface for reattachment 
of separated air to provide for high lift and low drag operation for 
improved fan efficiency. 
Another feature, object and advantage of this invention is to provide a new 
and improved airfoil for a vehicle fan in which a surface discontinuity is 
formed on a predetermined portion of the upstream suction surface so that 
a laminar flow separation bubble starts in a laminar boundary layer region 
at the same place regardless of low Reynolds number and low turbulence 
conditions of flow to thereby enable flow reattachment to the suction 
surface downstream of the bubble for high lift and efficiency. 
A feature and object of this invention is to provide a method of improving 
air pumping efficiency of a fan having a plurality of blades each having a 
pressure surface and a suction surface leading rearwardly from a forward 
and radially extending nose portion to a terminal and radially extending 
edge portion defining an airfoil. The method incorporates the steps of 
establishing a surface discontinuity at points on said suction surface 
closely adjacent said radially extending nose portion and in a region 
upstream of a zone of boundary air transiting from laminar to turbulent 
boundary air. The laminar boundary air flowing across said suction surface 
is tripped with said discontinuity to establish a laminar flow separation 
bubble. With early tripping, the bubble terminates well upstream of the 
terminal radial edge of the blade s that laminar boundary air flowing onto 
said suction surface will become detached approxiamte to said 
discontinuity and flow over said bubble and quickly back onto said suction 
surface downstream of said bubble in a pressure recovery region of said 
suction surface thereby providing improved lift and reduced drag of said 
blades.

Turning now in greater detail to the drawing, there is shown in FIGS. 1 and 
2 a multibladed fan assembly 10 designed for use for a land vehicle and 
particularly for inducing air flow through a radiator for engine cooling 
purposes. The fan has a hub 12, a plurality of blades 14 extending 
generally radially from hub 12 and has an outer ring-like shroud 15 with 
an annular bell-mouthed inlet section 16 to provide for smooth 
recirculation flow into the fan blading such as disclosed in U.S. Pat. No. 
4,329,946 issued May 18, 1982 to Richard E. Longhouse and assigned to the 
assignee of this invention and hereby incorporated by reference. 
To improve fan efficiency, airfoils 22 with classical profiles such as the 
profile of the NACA-65 series illustrated in FIG. 3 have been employed in 
engine cooling fans. Such airfoils develop laminar separation bubbles 24 
which start at a plurality of locations at the end of the laminar boundary 
layer region and which grow depending on flow conditions such as dictated 
fan speed. These bubbles cause separation of boundary air flowing across 
the suction side of the airfoil. This separation may begin at point 26, 
for example, immediately before the separation bubble. After flow 
separation or detachment, the air flows over the bubble and usually 
becomes reattached to the suction surface of the airfoil at some point 
downstream of the bubble. At low Reynolds number operation, such as during 
low relative speed due to engine design constraints or engine idle, the 
laminar separation bubble grows to a point where there is limited surface 
remaining for reattachment. When this occurs, pressure recovery is reduced 
so that lift is materially reduced and drag is increased. Under extreme 
conditions which may be termed "bubble busting", the bubble extends across 
the pressure recovery area of the airfoil so that reattachment cannot 
occur and there is substantial performance failure of the airfoil. 
To provide for improved pressure recovery, a new and improved airfoil 30 
shown in FIG. 4 is provided by this invention. In this design, a 
discontinuity in the form of a sharp edged flat or ramp 32 transverse to 
the cord of the airfoil and adjacent to the airfoil nose 34 is provided. 
The flat 32 extends rearwardly from a forward sharp edge 36 that is 
located forwardly in the laminar boundary layer region R illustrated in 
FIG. 4. The fan is inclined at a predetermined angle with respect to a 
line T, tangent to the upstream blade surface. This flat extends 
rearwardly from the sharp edge 36 and smoothly blends into the suction 
surface of the blade prior to the pressure recovery profile so that there 
is only one discontinuity and only a single laminar flow separation bubble 
develops. 
Accordingly, with this invention a single laminar separation bubble will 
develop which has a predetermined starting point as dictated by the 
discontinuity, i.e., the sharp edge 36 off flat 32, and which extends 
rearwardly along the ramp. At a particular point along the ramp the bubble 
will terminate so that the air flowing around the bubble will reattach on 
the suction side of the blade. This action is illustrated in FIG. 5 which 
is an airfoil section made with a flat in accordance with this invention 
and painted with a mixture of titanium dioxide and oil. This airfoil 
section was placed in a wind tunnel and the flow visualization photograph, 
FIG. 5, was made while the section experienced low Reynolds number flow. 
The separation bubble 40 was triggered by the sharp edge 36 of the flat 
during tests in a range low Reynolds numbers and varying turbulence 
intensity conditions including low turbulence operation. This bubble 
terminates on the flat 32 and the air flowing around and over the bubble 
reattaches on the flat and along the Stratford pressure recovery region on 
the suction surface 42 immediately behind the bubble to provide for 
improved airfoil operation for high lift and reduced drag. The turbulence 
shown in FIG. 5 represent turbulent boundary layer reattached to the 
suction surface. In this invention there is no separation once the 
turbulent boundary layer is formed. 
FIG. 6 contains curves C and I respectively comparing pressure loss 
incurred by classical airfoil 22 and the airfoil 30 of this invention for 
Reynolds numbers decreasing from 200,000 to 100,000. At point G, the 
laminar bubble in the classical airfoil starts growing extending into the 
pressure recovery region of the airfoil. Pressure loss subsequently 
increases to a point H, for example, in which there is failure to provide 
appreciable lift and drag is high. In contrast, the laminar separation 
bubble in the blade configuration of this invention illustrated by curve 
I, is controlled by its predetermined downstream location and the pressure 
loss is stabilized so that there is high lift and low drag throughout the 
illustrated Reynolds number operating range. 
FIGS. 7A and 7B illustrate the development of the preferred embodiment of 
the present invention. The curve of FIG. 7A represents the mathematical 
model of blade surface of velocity distribution which gives the highest 
lift and lowest drag. At point A on the suction surface curve S, there is 
forced separation as close to the origin of the suction surface as 
practical. The segment A-B of this curve represents the start of flow 
separation to the recovery region. The curve from point B to point D, the 
trailing edge of the blade, represents the shape of the velocity curve to 
produce pressure recovery in the shortest practical distance. The pressure 
curve P extending from the origin 0 to point D', the trailing edge of the 
pressure surface, was devised to provide for a practical blade design in 
terms of blade thickness including thickness of the trailing edge. This 
surface is also designed to control the amount of turning of air flow into 
the blade and, in conjunction with the suction surface, provides for the 
high lift and low drag. If curve P is rotated counterclockwise 
180.degree., the area formed between curves S and P represent the 
maximized high lift obtained. Using the surface coordinate points from the 
mathematical model, the airfoil section illustrated in FIG. 7B is plotted 
to which the discontinuity is subsequently added to form the shape of the 
airfoil of FIG. 4. The location of the flat is determined from the 
specification of the velocity distribution coordinates of the mathematical 
model and the discontinuity point corresponding to the peak velocity 
location. 
While a preferred embodiment of the invention has been shown and described, 
other modifications will become apparent to those skilled in the art. 
Accordingly, the scope of this invention is set forth in the following 
claims.