Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence $(Tux¯=0.5–1.0%)$ are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of $Tux¯=5.0%$ and an integral length scale of $Λx¯=3.25 cm$. At a Reynolds number of $ReD¯=U∞¯D/υ=21,000$, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients.

1.
Hiemenz
,
K.
, 1911, “
Die Grenzschicht an einem in den gleichformigen Flussigkeitsstrom eingetauchen geeraden Kreiszlinder
,”
Dinglers Polytechnic J.
,
326
, p.
321
410
. 0002-7820
2.
Goldstein
,
S.
, 1938,
Modern Developments in Fluid Dynamics
,
Clarendon
,
Oxford
.
3.
Kestin
,
J.
, 1966, “
The Effect of Freestream Turbulence on Heat Transfer Rates
,”
, Vol.
3
,
,
New York
, pp.
1
32
.
4.
Kestin
,
J.
, and
Wood
,
R. T.
, 1969, “
Enhancement of Stagnation-Line Heat Transfer by Turbulence
,”
Prog. Heat Mass Transfer
0079-631X,
2
, pp.
249
253
.
5.
Smith
,
M. C.
, and
Kuethe
,
A. M.
, 1966, “
Effects of Turbulence on Laminar Skin Friction and Heat Transfer
,”
Phys. Fluids
1070-6631,
9
, pp.
2337
2344
.
6.
Lowery
,
G. W.
, and
Vachon
,
R. I.
, 1975, “
The Effect of Turbulence on Heat Transfer From Heated Cylinders
,”
Int. J. Heat Mass Transfer
0017-9310,
18
, pp.
1229
1242
.
7.
Traci
,
R. M.
, and
Wilcox
,
D. C.
, 1974, “
Analytical Study of Freestream Turbulence Effects on Stagnation Point Flow and Heat Transfer
,”
Proceedings of the AIAA Seventh Fluid and Plasma Dynamics Conference
, Paper No. 74-515.
8.
Yardi
,
N. R.
and
Sukhatme
,
S. P.
, 1978, “
Effects of Turbulence Intensity and Integral Length Scale of a Turbulent Freestream on Forced Convection Heat Transfer From a Circular Cylinder in Cross Flow
,”
Proceedings of the Sixth International Conference on Heat Transfer
, Toronto, ON, Canada, Aug. 7–11.
9.
Dullenkopf
,
K.
, and
Mayle
,
R. E.
, 1995, “
An Account of Free-Stream-Turbulence Length Scale on Laminar Heat Transfer
,”
ASME J. Turbomach.
0889-504X,
117
, pp.
401
406
.
10.
Van Fossen
,
G. J.
,
Simoneau
,
R. J.
, and
Ching
,
C. Y.
, 1995, “
Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer
,”
ASME J. Heat Transfer
0022-1481,
117
, pp.
597
603
.
11.
Van Fossen
,
G. J.
, and
Ching
,
C. Y.
, 1997, “
Measurements of the Influence of Integral Length Scale on Stagnation Heat Transfer
,”
Int. J. Rotating Mach.
1023-621X,
3
, pp.
117
132
.
12.
Sak
,
C.
,
Liu
,
R.
,
Ting
,
D. S.-K.
, and
Rankin
,
G. W.
, 2007, “
The Role of Turbulence Length Scale and Turbulence Intensity on Forced Convection From a Heated Horizontal Circular Cylinder
,”
Exp. Therm. Fluid Sci.
0894-1777,
31
, pp.
279
289
.
13.
Peyrin
,
F.
, and
Kondjoyan
,
A.
, 2002, “
Effect of Turbulent Integral Length Scale on Heat Transfer Around a Circular Cylinder Placed Cross to an Air Flow
,”
Exp. Therm. Fluid Sci.
0894-1777,
26
, pp.
455
460
.
14.
Sutera
,
S. P.
,
Maeder
,
P. F.
, and
Kestin
,
J.
, 1963, “
On the Sensitivity of Heat Transfer in the Stagnation-Point Boundary Layer to Free-Stream Vorticity
,”
J. Fluid Mech.
0022-1120,
16
, pp.
497
520
.
15.
Sutera
,
S. P.
, 1965, “
Vorticity Amplification in Stagnation-Point Flow and Its Effect on Heat Transfer
,”
J. Fluid Mech.
0022-1120,
21
, pp.
513
534
.
16.
Bae
,
S.
,
Lele
,
S. K.
, and
Sung
,
H. J.
, 2000, “
Influence of Inflow Disturbances on Stagnation-Region Heat Transfer
,”
ASME J. Heat Transfer
0022-1481,
122
, pp.
258
265
.
17.
Bae
,
S.
,
Lele
,
S. K.
, and
Sung
,
H. J.
, 2003, “
Direct Numerical Simulation of Stagnation Region Flow and Heat Transfer With Free-Stream Turbulence
,”
Phys. Fluids
1070-6631,
15
, pp.
1462
1484
.
18.
Zhongmin
,
X.
, and
Lele
,
S.
, 2004, “
Distortion of Upstream Disturbances in a Hiemenz Boundary Layer
,”
J. Fluid Mech.
0022-1120,
519
, pp.
201
232
.
19.
,
W.
,
Sutera
,
S.
, and
Maeder
,
P.
, 1970, “
An Investigation of Vorticity Amplification in Stagnation Flow
,”
ZAMP
0044-2275,
21
, pp.
717
742
.
20.
Wei
,
C.
, and
Miau
,
J.
, 1992, “
Stretching of Freestream Turbulence in the Stagnation Region
,”
AIAA J.
0001-1452,
30
, pp.
2196
2203
.
21.
Wei
,
C.
, and
Miau
,
J.
, 1993, “
Characteristics of Stretched Vortical Structures in Two-Dimensional Stagnation Flow
,”
AIAA J.
0001-1452,
31
, pp.
2075
2082
.
22.
Sakakibara
,
J.
,
Hishida
,
K.
, and
Maeda
,
M.
, 1997, “
Vortex Structure and Heat Transfer in the Stagnation Region of an Impinging Plane Jet (Simultaneous Measurements of Velocity and Temperature Fields by Digital Particle Image Velocimetry and Laser-Induced Fluorescence)
,”
Int. J. Heat Mass Transfer
0017-9310,
40
, pp.
3163
3176
.
23.
Nix
,
A. C.
,
Diller
,
T. E.
, and
Ng
,
W. F.
, 2007, “
Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer
,”
ASME J. Turbomach.
0889-504X,
129
, pp.
542
550
.
24.
Baines
,
W. D.
and
E. G.
Peterson
., 1951, “
An Investigation of Flow Through Screens
,”
ASME J. Heat Transfer
0022-1481,
73
, pp.
467
480
.
25.
Hinze
,
J. O.
, 1975,
Turbulence
,
McGraw-Hill
,
New York
.
26.
Ewing
,
J.
,
Gifford
,
A.
,
Hubble
,
D.
,
Vlachos
,
P.
,
Wicks
,
A.
, and
Diller
,
T.
, 2010, “
A Direct-Measurement Thin-Film Heat Flux Sensor Array
,”
Meas. Sci. Technol.
0957-0233,
21
(
10
), p.
105201
.
27.
Gifford
,
A.
,
Hoffie
,
A.
,
Diller
,
T. E.
, and
Huxtable
,
S.
, 2010, “
The Convection Calibration of Heat Flux Sensors in Shear and Stagnation Flow
,”
ASME J. Heat Transfer
0022-1481,
132
(
3
), p.
031601
.
28.
Raffel
,
M.
,
Willert
,
C.
, and
Kompenhans
,
J.
, 1998,
Particle Image Velocimetry
,
Spinger-Verlag
,
Berlin
.
29.
Westerweel
,
J.
, 1997, “
Fundamentals of Digital Particle Image Velocimetry
,”
Meas. Sci. Technol.
0957-0233,
8
, pp.
1379
1392
.
30.
Eckstein
,
A.
and
P.
Vlachos
, 2009, “
Digital Particle Image Velocimetry (DPIV) Robust Phase Correlation
,”
Meas. Sci. Technol.
0957-0233,
20
, p.
055401
.
31.
Smith
,
T.
,
Moehlis
,
J.
, and
Holmes
,
P.
, 2005, “
Low-Dimensional Modeling of Turbulence Using the Proper Orthogonal Decomposition: A Tutorial
,”
Nonlinear Dyn.
0924-090X,
41
, pp.
275
307
.
32.
Chong
,
M. S.
,
Perry
,
A. E.
, and
Cantwell
,
B. J.
, 1990, “
A General Classification of Three-Dimensional Flow Fields
,”
Phys. Fluids A
0899-8213,
2
, pp.
765
777
.
33.
Samtaney
,
R.
,
Silver
,
D.
,
Zabusky
,
N.
, and
Cao
,
J.
, 1994, “
Visualizing Features and Tracking Their Evolution
,”
Computer
0018-9162,
27
, pp.
20
27
.