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Skin-Friction and Forced Convection from an Isothermal Rough PlateAubrey G. Jaffer
Digilant2 Oliver Street, Suite 901
Boston MA 02109 [email protected]
Developed are new correlations for skin-friction and forced convection from an isothermalplate having root-mean-squared height-of-roughness q. Corresponding to Prandtl and Schlichtingsfully-developed roughness region, the skin-friction correlation is independent of Reynolds number;the convection correlation is a linear function of Reynolds number (versus an exponent of 4/5 forturbulent convection from a smooth plate).
Measurements taken of a plate having precisely 3 mm of roughness match the convectioncorrelation within 3% at Reynolds numbers from 5000 to 50000. A model addressing the transitionfrom fully-rough turbulent to smooth turbulent convection for this bi-level test plate matches thosemeasurements within 2% from Re=5000 to 80000.
Convection measurements of a plate having 1.03 mm of roughness are within the expectedmeasurement uncertainties of the transition model.
An analysis of sand-roughness finds that its relation to root-mean-squared height-of-roughnessis non-linear. The resulting scale error explains discrepancies between the Prandtl and Schlichtingformula, two recent papers, and the results presented here.
These new correlations and measurements provide a threefold tighter upper bound for theheight of admissible-roughness than is given in Schlichtings book Boundary-layer theory.
Keywords: skin-friction; forced convection; roughness; sand-roughness; admissible-roughness
Table of Contents
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Prior Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Pipe-Plate Analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Rough to Smooth Turbulence Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Sand Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Admissible Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Measurement Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Measurement Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Forced Convection Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Directions for Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Convection from a rough surface is important in modeling weather and the thermal behavior of buildings.The related phenomenon of skin-friction has wide application to the fluid dynamics of vehicles and turbines.
The scope of this work is the total (not local) skin-friction and the total turbulent forced convection fromrectangular isothermal rough plates. There are self-similar roughness profiles for which the total skin-frictionand convection are well-defined and the local skin-friction and convection are not.
All forced convection measured by the apparatus was turbulent.What are the practical implications of not treating laminar forced convection? Reynolds numbers scale
with characteristic-length. At architectural and meteorological scales, laminar forced convection will rarelyoccur. Laminar forced flows occur frequently at scales much smaller than the 305 mm test plate. But atthose scales, isolated flat rough plates are of less interest than pipes, ducts, spheres, and cylinders.
2. Prior Work
In 1934 Prandtl and Schlichting published Das Widerstandsgesetz rauher Platten (The Resistance Law forRough Plates) which brilliantly infers the relation for skin-friction resistance for rough plates from theiranalysis of Nikuradses measurements of sand glued inside pipes. In Boundary-layer theory Prandtl andSchlichting give a formula for fully rough (large Re) total skin-friction coefficient for a rough flat plate:
(1.89 + 1.62 log10
)2.5102 < L/kS < 10
In On the Skin Friction Coefficient for a Fully Rough Flat Plate, Mills and Hang present a formulathey claim to be more accurate on the local skin-friction measurements done by Pimenta, Moffat, andKays. Their total skin-friction formula is:
(2.635 + 0.618 ln
Putting these formulas into similar form:
cf = 0.299
)2.5CD = 0.404
)2.57The roughness in both papers is reported in Nikuradses sand-roughness metric kS , the height of coarse
and tightly placed roughness elements such as for example coarse sand grains glued on the surface. Prandtland Schlichting gives the parameters of Nikuradses sand coatings:
kS grains/cm2 k2S
.08 cm 150 156.25/cm2
.04 cm 590 625/cm2
.02 cm 1130 2500/cm2
.01 cm 4600 10000/cm2
Table 1 Nikuradses sand coatings
A correlation retaining its validity when all lengths are scaled by the same factor is a central tenet offluid-dynamics theory. Because their packing densities scale as the inverse square of grain size, the .08 cmand .04 cm sand coatings should scale correctly as long as the glue thickness is proportional to grain size.Similarly the .02 cm and .01 cm sand coatings should scale.
The two roughest coatings (largest kS values) had packing densities slightly less than a square packing.The other two had packing densities less than half that of a square packing. But comparing the .02 cmand .04 cm sand coatings, their packing density ratio is nearly 1:2, not 1:4. Thus formulas (1) and (2) willhave incorporated a scale error (between measurements of large and small kS) and should not be applied toL/kS values much larger than 1000 (507 is the largest rough pipe R/kS value in Schlichting Fig. 20.18).Section 5 compares these formulas with the present work.
3. Pipe-Plate Analogy
The ratio of the mean height-of-roughness in a pipe to the pipe diameter (its characteristic length) affectsthe friction factor used to compute pressure drops in flows through pipes. The Chilton and Colburn J-factoranalogy (3) relates friction factors to turbulent forced convective heat transfer.
Nu = (f/2)Re Pr1/3 (3)
ASHRAE and Lienhard citing Dittus and Boelter (1930) give correlation (4) for convection in asmooth pipe:
Nu = 0.023 Re4/5 Pr0.3 (4)
Combining (3) and (4) (treating 0.3 1/3) and solving for the Darcy friction factor fD = 4f yields (5):
fD = 8 0.023 Re1/5 (5)
For turbulent flow, Re > 2300, fD in a pipe is governed by the Colebrook-White equation (6):
= 1.74 2 log10(kSR
)= 2 log10
) (6)Figure 1 is a Moody diagram relating fD to Re for rough pipes. It shows that 8 0.023 Re1/5 (5)
approximates the solution of equation (6) with kS = 0 over a wide range.
10000 100000 1x106
Darcy Friction Factor
kS/R = 10-2
kS/R = 10-3
kS/R = 10-4
kS/R = 0
Figure 1When the sand-roughness kS > 0, fD(Re) is asymptotically constant to the right of the fully-rough
trace. Thus Nu in (3) is proportional to Re in rough pipes (versus Re4/5 in smooth pipes).The correlations for laminar and turbulent convection from a smooth flat plate are:
Nu = 0.664 Re1/2 Pr1/3 (7)
Nu = 0.037 Re4/5 Pr1/3 (8)
Like the smooth pipe, turbulent forced convection from a smooth isothermal plate is proportional toRe4/5.
According to Lienhard and Lienhard, a very thin viscous sublayer is responsible for a major fractionof the thermal resistance of a turbulent boundary layer.
At the scale of the viscous sublayer, the inner radius curvature (being perpendicular to the flow) in alarge diameter pipe should be indistinguishable from a flat plate. Because most of the convective thermalresistance occurs close to the surface of a pipe or plate, it is reasonable to expect the turbulent correlationsfor pipe and plate to be related. Combining correlations (7) and (8) with the Chilton and Colburn J-factoranalogy (3) yields:
fD = 8 0.664 Re1/2 (9)fD = 8 0.037 Re1/5 (10)
The top two straight traces in Figure 1 are correlations (9) and (10); they do not lie near the curve fora smooth surface. But the trace for (10) is parallel to the trace for (5), both correlations being proportional
to Re1/5.Although it must have the correct relation to dimensions and parameters, the scale of characteristic-
length is otherwise a matter of convention. The forced-convection correlations are usually monomials in Reand scale easily. With the characteristic-length of a pipe being perpendicular to the direction of flow andthe characteristic-length of a plate being in the direction of flow, it is not unreasonable that scaling may benecessary in order to make correspondences between them.
Scaling the characteristic length R L/9.5 and Re ReF /9.5 in the Colebrook-White equation causescorrelations (9) and (10) to approximate the smooth convection curve in Figure 2. If the mean height-of-roughness , which is perpendicular to the characteristic-length for both pipes and plates, can be correctlyscaled, the result may predict convection for rough plates.
Doubts about how to scale the roughness in the Colebrook-White equation motivated the constructionof an apparatus designed to measure rough plate convection. The experiments described later in this paperfound that scaling the height-of-roughness kS 7.41/9.5 (such that the first term of the argument to log10becomes /L) matched measurements well. The scaled Colebrook-White equation is (11):
= 2 log10(
)ReF > 21850 (11)
Its asymptote as ReF is:
= 2 log10
]2 fc = 116[log10
10000 100000 1x106
Darcy Friction Factor withScaled Colburn Analogy to Flat Plate Convection
fully-rough/L = 10
/L = 10-3
/L = 10-4
/L = 0 8*.664/Re
Figure 21.2 of Schlichting shows that the Schultz-Grunow formula (13) for turbulent skin-friction ofa smooth plate is well matched with experiment:
fc = 0.427(log10(Re) 0.407)2.64 (13)
The close match of this formula to /L = 0 in Figure 2 shows that the scaling by 9.5 works for thesmooth plate.
The scaled Colburn analogy asymptote (14) is the result of combining the scaled Colebrook-Whiteequation asymptote (12) with the Chilton and Colburn J-factor analogy (3):
NuF =ReF Pr
]2 (14)Measurements displayed in Figure 3 show that rough surface convection is a linear function of ReF
down to ReF = 5490. Figure 3 shows that scaled Colburn analogy convection (top trace) is not linear. Itsasymptote (14) is a much better fit to the data.1
1000 10000 100000
Scaled Colburn Analogy Convection
SCA /L = .01asymptote /L = .01
MeasuredSCA /L = 0
smooth turbulentsmooth laminar
The deviation of the measured data from correlation (14) at Re < 5000 can be accounted for by themixing of natural and forced convection, although the experimental uncertainty of the mixed model is largerthan 4% in that range. The deviation at Re > 50000 is addressed in Section 4.
Correlation (14) corresponds to the fully-developed roughness region of Prandtl and Schlichting,but extends to much lower Reynolds numbers. Natural convection prevents investigation of the forcedturbulent-laminar transition in the Convection Machine; measurement of skin-friction drag would be a bettermethodology for those other regions.
1 This may be related to the Colebrook-White equation showing more gradual transitions between roughand smooth turbulent flows than Nikuradses rough pipe measurements.
4. Rough to Smooth Turbulence Transition
The roughness of the bi-level plate is not self-similar; it exhibits different convection behaviors at differentscales. The slope of the upper end of both curves in Figure 6 is 4/5, but with a magnitude possible only ifoperating with a much shorter characteristic length than the plates LF = 0.305 m.
At the leading edge of the plate, the smooth-turbulent boundary-layer depth would be much smallerthan the height-of-roughness; so fully rough convection (14) applies. The boundary-layer depth increaseswith distance x from the leading edge:
Solving for Rex where tur = pv, the peak-to-valley height:
the flow transitions to a regime consisting of smooth turbulent convection (8) from the channels betweenposts at scale LF and smooth turbulent convection (8) from the post at scale LS = 8.3 mm, the length ofthe post in the direction of flow.
Where SS = L/26 is the post center spacing, the longitudinal (parallel to flow) channels between theposts have convection:
0.037 Re4/5 (17)
The top of the posts occupy roughly half of the plate area; their convection is:
NuT /Pr1/3 =
)1/5 0.037 Re4/5 (18)
The (LF /LS)1/5 factor converts the characteristic length from LS to LF .
Both post sides parallel to the flow also experience smooth turbulent convection at scale LS = 8.3 mm,but because the sides are within the channels boundary layer, TF , V and Re are not constant with elevation.The channel velocity profile provides velocity as a function of elevation y, boundary layer depth , and freestream velocity V :
1/7when y ;
By analogy with the derivation of the thermal profile for laminar flow from Lienhard and Lienhard,the turbulent thermal profile would be:
T TwT Tw
1/7when y t;
))1/7when y ;
The temperature profile will affect side convection by a factor of approximately Pr1/21 (y/)1/7
wheny ; and 1 otherwise. In air, Pr1/21 is a reduction by 1.5% (for the post sides convection only).
1000 10000 100000fl
Convection Mode Regions Versus ReF
Figure 4Each vertical slice of Figure 4 represents the regions of convection on the surface of the 1 mm roughness
plate at the indicated value of ReF . The entire surface is in rough turbulent convection of correlation(14) below the ReM trace where (16) is not satisfied. Above the ReM trace, the convection is modeledas a combination of channel (17), post-top (18), and post-side (19) convection. When ReF is larger than40000, there is a small region where the channel boundary-layer height is smaller than the post side heightpv = 2 mm. Post side convection from this region was modeled, but its effect on total convection wasnegligible. For simplicity, the post side model uses formula (19) for all side convection.
The convection for a vertical slice of the post side is:
)1/5)1/74/5 dy= 0.038
Re149/175 Pr1/21 x113/175 9/35pv
The convection from a row of post sides in line with the flow is:
NuS/Pr1/3 = 2
Integrating the local convections for correlations (14), (17), (18), and (19) over distance:
ReM = min
)5/4)Ret = 121 103
NuF /Pr1/3 =
]2 +[SS LSSS
1000 10000 100000
Measurement versus Theory of 3.0 mm Roughness Downward-Facing Plate
Expected UncertaintyRough Turbulent Asymptote
4 Mixed T=11K
The constant Ret adds a degree of freedom to the plate model; but Figure 17 shows that 121000 fitsboth 3 mm roughness and 1 mm roughness data-sets within their estimated uncertainties below Re=80000.Figure 5 shows that the percentage discrepancy from the mixed model is within 2% below Re=80000.
1000 10000 100000
Convection from Rough Plates
L3 Mixed T=11K; 3.0 mm
L4 Mixed T=11K; 3.0 mm
Rough Turbulent Asymptote 3.0 mm
Smooth Turbulent Asymptote
3.0 mm T=11K Measured
L4 Mixed T=11K; 1.03 mm
Rough Turbulent Asymptote
1.03 mm T=11K Measured
While the L4 Mixed and Rough Turbulent Asymptote traces for 3 mm roughness are close in Figure 6,those traces for 1 mm roughness are not. This is because when of the plate was reduced from 3 mm to
1 mm, LS was not similarly scaled.
5. Sand Roughness
Modeling sand grains as unit radius spheres glued to the plate surface in a square array and modelingthe space not covered by a sphere as glue of height 0 g 1, the mean height of the enclosing square cellof area A containing a unit sphere is:
1 x2 + 1)dx+ (A ) g
3 + (A ) g
The arithmetic-mean height-of-roughness Ra and the root-mean-squared height-of-roughness Rq are:
2x1 x2 + 1 y dx+ (A ) |y g|)/A
1 x2 + 1 y)2dx+ (A ) (y g)2
For glue level 1 < g < 2, the grains are more than 50% submerged. Let xg =
(1 (g 1)2) =2g g2 be the radius of the glue opening through which the sphere is exposed. The mean height of the
enclosing square cell of area A containing a unit sphere is then:
1 x2 + 1)dx+
(4 + 3g2 2g3
The arithmetic-mean height-of-roughness Ra and the root-mean-squared height-of-roughness Rq are:
2x1 x2 + 1 y dx+ (A x2g) |y g|)/A
1 x2 + 1 y)2dx+
Figure 7 shows the conversion factor from to kS as a function of relative glue thickness.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Glue Thickness (sand grain radius = 1)
Sand-Roughness ks per Height-of-Roughness vs. Glue Thickness and Cell Area A
Root-Mean-Squared q; A=4.20
Arithmetic-Mean a; A=4.20
Root-Mean-Squared q; A=8.77
Arithmetic-Mean a; A=8.77
Figure 7In order to compare the Prandtl and Schlichting formula (1) with formula (12) of present work,
different kS/ conversion factors (and relative glue thicknesses) must be used at large and small kS values.
The unit cell area for the larger grains is 4.2. With L/ = 150, compare 4cf for L/kS = RaL/ in Table 2and L/kS = RqL/ in Table 3 with fD = 0.0528 calculated from the scaled Colebrook-White asymptote(12):
g y 1/Ra L/kS 4cf0.00 1.25 3.14 47.7 0.0880.70 1.42 4.97 30.2 0.1051.00 1.50 6.38 23.5 0.117
Table 2 Average roughness Ra versus large-grain sand-roughness kS
g y 1/Rq L/kS 4cf0.00 1.25 2.66 56.4 0.0820.87 1.47 4.98 30.1 0.1051.00 1.50 5.64 26.6 0.111
Table 3 RMS roughness Rq versus large-grain sand-roughness kS
fD = 0.0528 is out of the range of possible 4cf values for arithmetic-mean or RMS roughness with g 0.However, there are values of 4cf/2 which are within range for both RMS and arithmetic-mean roughness.
A factor of 2 is introduced in the last step of Prandtl and Schlichtings paper. While it is justifiedfor the local coefficient of resistance, the reason for its presence in the total coefficient of resistance is notobvious. In their example of the use of their formula, Mills and Hang divide CD by 2.
Half the 4cf value from the g = 0.70 row in Table 2 matches fD = 0.0528, as does the g = 0.87 row inTable 3, for arithmetic-mean and RMS height-of-roughness respectively.
The unit cell area for the smaller grains is 8.77. The largest pipe which Nikuradse tested had a diam-eter of 0.101 m. With a coating made from the smallest sand grain size (.01 cm), L/kS = 1014 and thecorresponding fD = 0.0277. As before, there is no glue level g 0 whose 4cf matches fD, but half of 4cfdoes match at g = 0.90 and g = 0.99 in Tables 4 and 5.
g y 1/Ra L/kS 4cf0.00 0.60 2.61 388.6 0.0440.90 1.17 5.65 179.4 0.0551.50 1.57 17.98 56.4 0.082
Table 4 Average roughness Ra versus small-grain sand-roughness kS
g y 1/Rq L/kS 4cf0.00 0.60 2.46 411.4 0.0430.99 1.23 5.65 179.4 0.0551.50 1.57 13.95 72.7 0.075
Table 5 RMS roughness Rq versus small-grain sand-roughness kS
Afzal et al in Turbulent flow in a machine honed rough pipe for large Reynolds numbers: General rough-ness scaling laws fit 5.333 for the RMS to sand-roughness conversion factor and 6.45 for the arithmetic-mean to sand-roughness conversion factor. The mean of the 1/Rq values 4.98 and 5.65 is within 1% of 5.333(and not close to 6.45). This is strong evidence that RMS height-of-roughness is the true metric for surfaceroughness in the scaled Colebrook-White asymptote (12).
In Skin-friction behavior in the transitionally-rough regime, Flack et al measure skin-friction fromgrit-blasted surfaces. They find The root-mean-square roughness height (krms) is shown to be most stronglycorrelated with the equivalent sand roughness height (kS) for the grit-blasted surfaces.
Figure 8 shows that the Mills-Hang formula (2) is nearly coincident with the Colebrook-White asymptote(12) over the range 100 < L/ < 1000; so it is an improvement over Prandtl and Schlichtings correlation(1). At L/ > 1000, the inset in Figure 8 shows that both suffer from the scale error discussed in Section 2,growing to 13% and 18% relative to the scaled Colebrook-White asymptote (12) at L/ = 106.
Fully Rough (Turbulent) Darcy Friction Factor
.5 Schlichting (kS = 5.65 q) .99 glue
.5 Mills-Huang (kS = 13.95 q) 1.5 glue
Scaled Colebrook-White Asymptote
.5 Schlichting (kS = 4.98 q) .87 glue
1000 10000 100000 1x106
6. Admissible Roughness
The amount of roughness which is considered admissible in engineering applications is that maximumheight of individual roughness elements which causes no increase in drag compared with a smooth wall.2
Schlichting develops a formula for kadm from its Fig. 21.9. (Resistance formula of sand-roughened plate;coefficient of total skin friction):
= 100 (21)
A footnote states The estimates performed in this section make no distinction between the equivalentsand height, kS , and the actual height, k, of a protuberance. In the Section 5 model of sand-roughness,the protuberance height is (1 g/2)kS . For Nikuradses large grains that is 0.57 kS ; for small grains it is0.51 kS . So equation (21) overestimates the admissible-roughness height.
The admissible-roughness threshold does not occur in the fully-developed roughness region (Prandtland Schlichting) which contains all the Convection Machine measurements. However, these measurementscan be used to develop an upper bound for admissible-roughness.
For a given RMS roughness Rq, the equal-area bi-level plate has the smallest possible k = 2Rq; whilelarge-grain sand-roughness would have k = 0.575.333Rq = 3.0Rq and small-grain k = 0.515.333Rq = 2.7Rq.
The basis for equation (21) is that UkS/ = Re kS/L is nearly invariant at the intercepts of theconstant kS/L and kS = 0 curves at Re from 10
5 to 108. For the 1.03 mm trace in Figure 6, this would bethe point below which the slope of measured Nu/Pr1/3 is 4/5. Natural convection obscures the slope belowRe=4419; so 4419 is an upper bound.
2 This definition does not imply that roughness always causes an increase in drag; for example in Figure 6the 1 mm roughness curve is less than the smooth-turbulent asymptote from 3000 < Re < 7000.
L< 15 k
L< 30 k < 30
This upper bound for k is a significant reduction from equation (21).
Figure 9 Figure 10Figure 9 shows the rough surface of the 30.5 cm 30.5 cm test plate. Machined from MIC-6 aluminum
to keep its emissivity low, the reflective surface is composed of (676) 8.28 mm 8.28 mm 6 mm postsspaced on 11.7 mm centers. The area of the top of each post is 0.686 cm2, half of the 1.37 cm2 cell. Thearithmetic-mean (also the root-mean-squared) height-of-roughness is thus 3 mm.
The back surface of the plate has 9 precision electronic resistors embedded as heating elements and atemperature sensor. There is 2.54 cm thick thermal foam insulation between the back of the plate and a0.32 mm thick sheet of aluminum. The aluminum sheet also has a temperature sensor at its center.
Figure 10 is a cross-section drawing of the plate assembly.
Figure 11The Convection Machine wind-tunnel (Figure 11) has a 61 cm 35.6 cm cross-section and a 61 cm
depth. It allows the plate assembly to be centered with 15 cm of space on all sides. The fan pulling air
through the test chamber produces a maximum wind-speed of 4.5 m/s (Re = 9 104 for the 0.305 m squareplate). Its minimum nonzero wind-speed is 0.12 m/s (Re = 2300).
The 99% boundary-layer depths  for the wind-tunnel versus the distance x from its front edge are(laminar and turbulent):
lam = 4.92
Vtur = 0.37x
)1/5The boundary layer depths from the plate are similar, although the Reynolds numbers are lower with
its shorter characteristic-length. Figure 12 shows that 15 cm clearance between the plate and wind-tunnelwalls keeps the boundary layers from interacting at airspeeds within its range.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Distance from front edge in m
Wind-Tunnel Boundary Layer Thickness; DH = 0.449 m
laminar V=0.12 m/s; Re=3432laminar V=0.24 m/s; Re=6864
turbulent V=2.0 m/s; Re=57204turbulent V=4.5 m/s; Re=128708
The rotation rate of the fan is sensed by a photo-transistor receiving the light from a LED beinginterrupted by the passing fan blades. Software controls a solid-state relay (supplying power to the fan) tomaintain the fan rotation rate dialed into switches. At rotation rates below 400 r/min, the software pulsespower to the fan according to a phase comparator. Above 400 r/min it servos the duty cycle of a 10 Hzsquare-wave supplying power. It operates over a range of 32 to 1500 r/min.
The correspondence between fan rotations per second and wind-speed is made using an anemometer.The ABM-200 Airflow & Environmental Meter specifies an accuracy of 0.5% of reading from 0.22 m/s to62.5 m/s. At speeds below 5 m/s, 0.5% is much better than can be achieved with hot-wire anemometers,which specify accuracy as a percentage of 10 m/s full-scale.
Figure 13 shows the calibration curve for the empty wind-tunnel. These measurements make clear thatthe 0.22 m/s lower limit must be a misprint; the smallest nonzero reading my unit was able to report was0.66 m/s. Even at 1.25 m/s, the slope is on a trajectory to reach 0 when the fan is still moving air.
For any fan, there must be some rotation rate and wind-speed Vs above which airflow does not increase.That flattening will appear gradually from slower rotation rates. Modeling the stall speed as Vs = 7.5 m/s,equation (23) is the solid line in Figure 13. From 500 r/min to 1500 r/min, it matches measurements witha standard deviation of 0.0075 m/s.
V =[(rS)2 + V 2s
1 (V1500/Vs)2V1500 = 4.17195 m/s
1/sqrt((298.8/r)^2+1/7.50^2)Airspeed vs Fan Rotation 2017110418.4C58.5%100710Pa+90, 90
Fan Rotation, r/min0 200 400 600 800 1000 1200 1400 1600
Figure 13The plate assembly is suspended from six lengths of 0.38 mm-diameter steel piano wire terminated at
twelve zither tuning pins in wooden blocks above and around the test chamber. The wire is sheathed by0.95 mm Teflon tubing where it would otherwise contact the plate.
With the plate assembly in the wind-tunnel, the airspeed is increased in proportion to the reduction ofwind-tunnel aperture by the plates cross-sectional area:
Vp = VAwt
The increase is 7.7% for the 3 mm roughness plate and 7.3% for the 1 mm roughness plate.
The ambient sensor board can be seen at the lower edge of the tunnel in Figure 11. This small boardmeasures the pressure, humidity, and temperature of the air at the wind-tunnel intake.3 The LM35 temper-ature sensor projects into the wind-tunnel test chamber; it is wrapped in aluminum tape so that radiativeheat transfer is minimized. The temperature sensor is powered only while it is being read so that self-heatingdoesnt affect it; self-heating was a problem before it was switched.
Logging of the measurements is performed by a STMicroelectronics STM32F3 Discovery 32-Bit ARMM4 72MHz development board. The custom electronics board which the STM32F3 plugs into, containspower supplies, heater control and drive, and signal conditioning.
A program was written for the microprocessor which takes measurements and writes them to the micro-processors non-volatile RAM, controls the heater, servos the fan speed, and (later) uploads captured datato a computer over a USB serial interface. Every second for 108 minutes the program calibrates and readseach on-chip 12 bit analog-to-digital converter 16 times and sums its readings to create a 16 bit value.
3 It has since been moved to the chamber beneath the wind-tunnel where, although slower to respond toambient changes, it is unaffected by convection from the plate.
8. Measurement Methodology
Previous laboratory measurements (for example ) of forced convection have been performed by startingthe fluid flow and plate heater, waiting for the system to reach equilibrium (as indicated by a stable platetemperature), then recording the measurements of the physical quantities.
The expectation for this wind-tunnel was that it would support wind-speeds up to 10 m/s. The test platewas made massive in order to maintain uniform temperature at high rates of convection. As a consequencethe plates temperature settling times range from minutes in the best case to hours at low airflow rates.
Ubiquitous electronic sensors and digital processing make possible an entirely different measurementmethodology. Rather than waiting for the plate to reach equilibrium before each measurement, the state ofall sensors is captured every second for 108 minutes and compared with simulations of the system, which israrely in equilibrium.
The physical parameters derived from measurements and specifications of both plate versions are:
symbol value value descriptionq 3.0 mm 1.03 mm RMS height-of-roughnessa 3.0 mm 1.01 mm Arithmetic-mean height-of-roughnessAp 0.093 m
2 0.093 m2 Plate Test AreaC 4690 J/K 4242 J/K Plate Thermal CapacityCb 146 J/K 146 J/K Back Thermal CapacityUI 0.075 W/K 0.075 W/K Insulation Thermal Conductancep 0.07 0.07 Plate Surface Emissivitywt 0.90 0.90 Wind-Tunnel Emissivity
Table 6 Physical Parameters
The effective wind-tunnel emissivity wt may differ from the emissivity of medium-density-fiberboard(0.90 ) because the temperatures of the (internal) tunnel surfaces may not be uniform, and the plateexchanges thermal radiation with objects in the room which may have different emissivities and temperaturesthan the tunnel and air.
With its low emissivity (p = 0.07), the radiative heat loss from the rough face of the plate is only35 mW/K, about 3% of the 1.08 W/K expected for V = 1m/s convection. Even if the wt value of 0.9 iswrong by 20%, forced convection measurements will be affected by less than 1%.
The measured dynamic physical quantities are:
symbol units descriptionH W Heater PowerV m/s Fluid Velocity (from fan rotation rate)TP K Plate TemperatureTF K Fluid (Air) TemperatureTB K Back Surface Temperature
Table 7 Dynamic Quantities
The calculated and simulated quantities are:
symbol units descriptionUS(V ) W/K Side Thermal ConductancehR W/(m
2K) Radiative Surface Conductanceh(V ) W/(m2K) Convective Surface ConductanceUP (t) W/K Convective Thermal ConductanceTP (t) K Plate Surface Temperature
Table 8 Calculated and Simulated Quantities
The forced convective thermal conductance of the four sides US(V ) is computed by integrating the localforced convective surface conductance in series with the insulation thermal conductance over the area of thesides.
The short side is not an isothermal surface; it has a 4 mm metal strip and a wedge of extruded polystyrene(XPS) insulation. Each points effective surface conductance will depend on the temperature profile alongthe side.
A point on the side which is near to metal will have a large conductance. Summing the conductivitydivided by the shortest distance to metal at each angle will be roughly proportional to the local conductance(see Figure 14). The conductance through a slab with parallel isothermal faces is well known. The ratio ofthe slab conductance to the slab sum is the factor 0.637, which is used to normalize conductivities calculatedthis way.
With forced air the four sides have three distinct behaviors. The forced air flows parallel to the longdimension on two sides but flows into the windward side and away from the leeward side.
The forced convective component for the two parallel sides averages the local forced convective surfaceconductance in series with that points insulation conductance as computed above.
For the windward side, a similar calculation is done, but the wind speed grows linearly from 0 m/s atthe mid-line to V along each long edge. For the leeward side the wind speed grows linearly from 0 m/s alongeach long edge to V at the mid-line, but with the flow always turbulent. The leeward surface conductanceturned out to be within 1% of the windward surface conductance, so the windward value is used for both.
Convection for the sides uses the smooth turbulent correlation (8), but does not visibly differ from useof the scaled Colburn analogy asymptote (14) with 0.25 mm roughness. The expected convection throughthe sides is 9% of the convection from the rough surface at 4 m/s wind-speed.
Collecting terms having a factor of TP TF into UM (V ), the heat balance equation for the plate inforced convection is:
UM (V ) = US(V ) + h(V )Ap + pwthRAp
H = UM (V ) [TP TF ] + UI [TP TB ] + CdTPdt
To track temperature measurements as functions of time requires characterization of the temperature-time responses of the thermal masses and insulation (a combination of polyisocyanurate (PIR) and XPSfoams). Figure 15 shows the temperature-time response of the downward-facing plate to a 57.5 W step inheat drive applied starting at second 151. The dashed line is the temperature expected with a 15 s delay.Figure 15 and the heating phase of experiments show that a 15 s delay models the system well.
20160813T155911Z 57.5W step response Orientation=+90.00
Time, s150 162.5 175 187.5 200 212.5 225 237.5 250 262.5 275 287.5 300
The temperature-time response of the insulation is best ascertained with the plate facing up so thatthe back is facing down for its minimum convection. Not a step response, this is the response to the platestemperature ramp. The blue trace showing back temperature in Figure 16 is superimposed on a red tracewhich models the insulation as thermal conductance UI = 75 mW/K with a 110 s delay between the plateand the back.
Time, s0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
k=0.026, Ra_U=305278, h=4.71W/(K.m^2), U=0.438W/K, Nu=13.78, Pr=0.73020160812T010335Z upward Convection Roughness=3.00mm; T=22.4+07.1C; 90.00
Figure 16Including the delays for the insulation and plate thermal masses, and isolating TP (t):
TP (t) =H(t 14) + UM (V )TF (t) + UITB(t 110) CdTP (t)/dt
UM (V ) + UI(26)
Solving (26) as a finite-difference equation where dt = t t:
H(V, t) = H(t 14) + UM (V )TF (t) + UITB(t 110)
TP (t) =H(V, t)[t t] + CTP (t)[UM (V ) + UI ] [t t] + C
In equation (27), TP (t) is the previous simulated value (not the measured value).
Forced convection measurements are usually presented as Nu versus Re. In order to obtain Nu, the heatbalance equation is solved for UP = h(V )Ap:
(V, t) = H(t 14) UI [TP (t) TB(t 110)]
UP (t) =(V, t) C [TP (t) TP (t)] /[t t]
TP (t) TF (t)AppwthR US(V ) (28)
In the denominator of equation (28), TP (t) and TF (t) are the 11-element cosine averages of plate andfluid temperature (centered at time t). Averaging is needed so that the derivative doesnt correlate with thedenominator, causing bias.
9. Measurement Uncertainties
The Convection Machine measures heat-flow into the plate from electrical heater dissipation and convec-tive heat flow from the rate of change in plate temperature. For the 3 mm roughness plate, Table 9 computesthe sensitivity of heat flow to each parameter and the product of each sensitivity with the estimated parame-ter bias to yield the component uncertainty. In accordance with ASME Measurement Uncertainty, theroot-sum-squared (RSS) of these uncorrelated uncertainties is 2.0%. Reducing the roughness q to 1.03 mm(pv = 2 mm) raises the combined uncertainty to 2.9% in Table 10.
With a wind-speed of 3 m/s or greater, emissivity and natural convection parameters dont have asignificant effect on the combined uncertainty and arent included in Tables 9 and 10.
Symbol Nominal Sensitivity Bias Uncertainty ComponentT 19.5C -0.163%/C 1.0C 0.163% LM35C Temperature Sensor
T 10.5C 5.759%/C 0.15C 0.864% LM35C DifferentialP 101kPa 0.001%/Pa 1.5kPa 0.820% MPXH6115A6U Air PressureV 3.00m/s 18.454%/ms 15mm/s 0.277% Airspeedb 10.000 4.304% 0.25 1.076% Logarithmic Base
Ret 121000 0.00006% 2000 0.124% Rough-Smooth ThresholdLc 0.305m 11.560%/m 500um 0.006% Characteristic LengthLw 0.305m 213.753%/m 500um 0.107% Plate WidthLs 8.29mm 1236.477%/m 250um 0.309% Block Lengthpv 6.00mm 7065.258%/m 150um 1.060% P2V Height-of-Roughnesskpir 23.0
mWKm 0.060% PIR Thermal Conductivity
kxps 34.0mWKm -0.064%/
mWKm 0.109% XPS Thermal Conductivity
Symbol Nominal Sensitivity Variability Uncertainty ComponentV 3.00m/s 18.454%/ms 7.5mm/s 0.138% Airspeed
1.99% RSS Combined UncertaintyTable 9 Uncertainties for Plate with 3mm of Roughness
Symbol Nominal Sensitivity Bias Uncertainty ComponentT 19.5C -0.210%/C 1.0C 0.210% LM35C Temperature Sensor
T 10.5C 7.501%/C 0.15C 1.125% LM35C DifferentialP 101kPa 0.001%/Pa 1.5kPa 1.074% MPXH6115A6U Air PressureV 3.00m/s 24.165%/ms 15mm/s 0.362% Airspeedb 10.000 0.446% 0.25 0.112% Logarithmic Base
Ret 121000 -0.00003% 2000 0.066% Rough-Smooth ThresholdLc 0.305m -32.377%/m 500um 0.016% Characteristic LengthLw 0.305m 281.685%/m 500um 0.141% Plate WidthLs 8.29mm 9347.066%/m 250um 2.337% Block Lengthpv 2.00mm 2781.195%/m 150um 0.417% P2V Height-of-Roughnesskpir 23.0
mWKm 0.102% PIR Thermal Conductivity
kxps 34.0mWKm -0.112%/
mWKm 0.190% XPS Thermal Conductivity
Symbol Nominal Sensitivity Variability Uncertainty ComponentV 3.00m/s 24.165%/ms 7.5mm/s 0.181% Airspeed
2.89% RSS Combined UncertaintyTable 10 Uncertainties for Plate with 1mm of Roughness
Convection measurements are 35 times more sensitive to T = TP TF than to T (the average of TFand TP ). The offsets in the measurement software for the plate, back, and ambient sensors were matchedto about 0.15 K. In 300 consecutive readings at constant temperature, the variability for the ambient andplate sensors was 0.03 K, and 0.02 K for the back sensor at T = 19C.
The vertical dimensions of the aluminum plates rough surface are correct as far as can be determinedwith spot checks using a caliper depth gauge. The only accuracy specification for the ShopBot PRSstandardCNC (used to machine the rough surface of the plates) is a Positional Repeatability of 0.05 mm. Threetimes this value (150m) is used for estimated bias of pv in Tables 9 and 10.
10. Forced Convection Results
Figure 17 compares the measurements for the downward-facing plates with equations (20) and (14) and theirpredicted uncertainties calculated for each measurement. Below Re=5000 the curves for equation (20) arecorrected for mixed convection, the subject of a future paper.
1000 10000 100000
Convection and Measurement Uncertainties from Rough Plates
Expected Uncertainty 3.0 mm
L4 Mixed T=11K; 3.0 mm
Rough Turbulent Asymptote 3.0 mm
3.0 mm T=11K Measured
Expected Uncertainty 1.03 mm
L4 Mixed T=11K; 1.03 mm
Rough Turbulent Asymptote
1.03 mm T=11K Measured
Figure 18 shows a forced convection experiment (of 6144 s duration) with simulation. In the (upper)temperature graph, the solid trace is the plate temperature, while the dashed trace is the simulation of theplate temperature (taking the back and ambient temperatures as inputs).
Diamonds on the simulated temperature trace mark the beginning and end of the measurement period.The ending temperature difference with ambient is at most half of the peak temperature difference withambient. The simulated plate temperature is reset to the measured plate temperature at the start diamond.
Underneath the temperature graph is a graph of the air velocity versus time for mixed convection runs(and Raleigh number versus time for natural convection runs).
Each round dot in Figure 17 represents the convection measured over the time interval between thediamonds in its experiment. The dots at the left edge are the natural convections (where Re = 0). A graphof each dots run is provided in the supplementary files.
Time, s0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000
6404r/min, V=2.218m/s, Re=42505, Ra_D=3301678, h=25.9W/(K.m^2), U=2.41W/K, Nu=301.920160807T144333Z mixed Convection Roughness=3.00mm; T=22.7+09.9C; +90.00
1000 10000 100000
Mixed Convection from Downward Facing Rough Plate
L4 Mixed; T=4K; q=0.75 mm RMS
L4 Mixed; T=4K; a=0.57 mm AVG
Rough Turbulent Asymptote; q=0.75 mm RMS
Rough Turbulent Asymptote; a=0.57 mm AVG
11. Directions for Further Work
Having equal areas of the plate at low and high elevations created a roughness whose height was the samewhether calculated using root-mean-squared or the arithmetic-mean of absolute-values. While Section 5finds evidence that should be calculated root-mean-squared, a definitive test would be to alter the plate sothat the two metrics yield significantly different values.
The plate will be machined to reduce the high area from 50% to 17.2% of the total plate area. Thenthe arithmetic-mean height-of-roughness a = 0.57 mm and the RMS height-of-roughness q = 0.75 mm.Figure 19 shows the convection expected by formula (20) for the two roughness metrics. At Re=10000 therough turbulent convection with RMS q is 9% larger than convection with arithmetic-mean a. This shouldbe sufficient to determine which is the true roughness metric.
LS is an input to inequality (16), but had the same value, 8.29 mm, for both plates. This modificationwill reduce LS to 2.43 mm, allowing its true effect on ReM and the transition model (20) to be discovered.
It is not difficult to devise a formula for off-axis forced convection; but with only 15 cm space betweenthe plate and the tunnel walls, off-axis flow would not be uniform in the Convection Machine. A significantlylarger wind-tunnel would be required to test it. Also, there is no guarantee that a tractable model could bedevised for the sides with non-orthogonal angles of wind incidence.
Thanks to John Cox and Doug Ruuska for machining the plate. Thanks to Martin Jaffer and Roberta Jafferfor their assistance and problem-solving suggestions.
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