Post on 26-Jul-2020
Ian Williams, Director of Applied Engineering, NVIDIA
Its all about Conserving Energy How predictive rendering can avoid creating an accidental Death Ray
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Agenda
The Problem
A Quick Physics Recap
Simulating the Death Ray in Iray
Deriving Temperature from Irradiance
Comparing Predictions with Measurement
Conclusions
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The Problem August 29th 2013
Martin Lindsay parked his Jaguar car on Eastcheap, London around noon
2 hours later he returned to find parts of the car had melted
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The Scale of the Problem
“On Tuesday afternoon, I was sent out to see if I could fry an egg in the heat, a task that I presumed was impossible on an overcast September day. But, not only was it possible, I had to run out of the death ray that was slowly cooking my egg, because the thinning hairs on my head started to catch fire. ”
“On Monday, the air temperature in the concentrated beam, reached 69.8C, which in old money is 158F. To put that in context, the world’s hottest temperature was recorded in Death Valley at 56.7C (134F) over a century ago.”
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Death Ray’s Apparently are Fairly Common
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Agenda
A Quick Physics Recap
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Electromagnetic Radiation
Electromagnetic Radiation consists of waves that are synchronized oscillations of electric and magnetic fields that propagate at the speed of light
Produced whenever charged particles are accelerated, and these waves can subsequently interact with any charged particles
EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact
Electromagnetic spectrum: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays
A Quick Recap
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How Does Sunlight Heat Things Up?
Radiation is both absorbed and emitted
Convection
Conduction
Three Main Modes of Energy Transfer
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Governing Laws
The law of conservation of energy states:
That the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed
The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems, and states:
That the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings
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Quantifying Energy Transfer in the Electro Magnetic Spectrum
Radiant Flux per unit area received by a Surface
Units = watts per square metre W/m2
Hemispherical quantity
Irradiance
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Solar Irradiance and Atmospheric Absorbtion
The electromagnetic radiation emitted by the sun incident on the top of the atmosphere
50% in region beyond visible light
40% in visible light region
10% in ultraviolet region
On a clear day approximately 50% of solar energy absorbed by atmosphere
Irradiance
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Agenda Simulating the Death Ray in Iray
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Simulating the Physics of the Death Ray
Walkie Talkie Building and Environment were extensively modelled in detail using AutoDesk 3D Studio Max
Used Iray Renderer to:
Simulate time of day and year using calibrated Sunlight model
Calculate Irradiance incident on specific surface
Iray
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The Model
Movie
20 Fenchurch Street and Surroundings, London
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Time of Day Analysis
Movie
29 August 2013
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Predicted Irradiance from Iray
Movie
29 August 2013
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The Energy Contained in the Death Ray
Direct Sunlight on a clear day has an Irradiance of ~110K Lux or ~1.1 KW/m2
Predicted Irradiance at street level outside the Death Ray was ~98K Lux or ~1.1 KW/m2
Predicted Irradiance at street level inside the Death Ray was ~700K Lux or ~7.5 KW/m2
=6x more powerful than the Sun!
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It Could Have Been Worse...
Slight modifications to the building geometry, in the form of between 5 to 10 degrees more curvature, increased Irradiance from:
~700K Lux or ~7.5 KW/m2
To
~3M Lux or ~32.3 KW/m2
=30x more powerful than the Sun!!!
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Agenda Deriving Temperature from Irradiance
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Calculating Temperature from Irradiance Solar Heat Transfer to a Surface
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Temperature of the Surface
Assuming system is steady state with no phase changes, then from 1st law of thermodynamics the energy balance equation is:
Energy Balance
Ee . α = ε . σ [ Tsurf-air4 - Tair-amb
4 ] + hsurf-air [Tsurf-air – Tair-amb] + k . [Tsurf-air – Tsurf-other] / s
Absorbed Energy Re-Radiated
Energy
Energy transfer through Convection
Energy transfer Through Conduction
Surf -> Atmosphere
Atmosphere -> Surf
Ee = Irradiance (Wm-2) α = Absorptivity ε = Emissivity σ = Stefan-Boltzman constant = 5.670373x10-8 (Wm-2K-4) Tsurf-air = Temperature at air surface boundary (K) Tair-amb = Ambient air temperature (K) Tsurf-other = Temperature at other side of surface (K)
hsurf-air = Heat transfer coefficient between surface and air (Wm-2K-1) k = Thermal Conductivity of surface material (Wm-1K-1) s = Surface material thickness
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Simplifying Things
Assume worst case scenario where energy transfer/cooling through conduction is negligible
Assume radiation from atmosphere back to surface can be considered negligible
Energy Transfer Equation be re-written:
Which equals:
Which is a quartic of the form:
Ee . α - ε . σ Tsurf-air4 - hsurf-air [Tsurf-air – Tair-amb] = 0
ε . σ . Tsurf-air4 + hsurf-air .Tsurf-air – [ Ee . α + hsurf-air .Tair-amb] = 0
a . x4 + b . x3 + c . X2 + d .x – e = 0
a = ε . σ
d = hsurf-air
e = Ee . α + hsurf-air .Tair-amb
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General Solution To A Quartic Equation
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In Our Case…
b = c = 0
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So The Formula Becomes…
Δ0 = 12ae
Δ1 = 27ad2
Q = [ (Δ1 + (Δ1
2 - 4 Δ03)1/2 )/2]1/3
S = ½ [ (1/3a) * (Q + Δ0/Q) ]1/2
q = d/a
a = ε . σ
d = hsurf-air
e = Ee . α + hsurf-air .Tair-amb + ε . σ .Tair-amb4
Tsurf-air = X1 or X2 or X3 or X4
Depending on which root is a +ve real number
Where:
x1,2 = -S ± ½( -4S2 + q/S )1/2 x3,4 = S ± ½( -4S2 – q/S )1/2
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Assumed Values…
Ee = Irradiance (Wm-2) P(W) = Ev(lx) × A(m2) / η(lm/W) , where η = Lumic Efficacy = 0.93 for sunlight α = Absorptivity ε = Emissivity σ = Stefan-Boltzman constant = 5.670373x10-8 Wm-2K-4
Tsurf-air = Temperature at air surface boundary (K) Tair-amb = Ambient air temperature (K) = 300K hsurf-air = Heat transfer coefficient between surface and air (Wm-2K-1) = 10.45 –v + 10v1/2 , where v is velocity of object in air (source engineering toolbox)
= 10.45 Wm-2K-1 for static object
Absorptivity Emissivity
Asphalt 0.93 0.93
Polished Aluminum 0.09 0.09
Magnesium Oxide Paint 0.09 0.09
Chromium 0.2 0.2
Sherwin Williams White Paint 0.89 0.87
Concrete 0.6 0.88
Red Brick 0.94 0.93
Polythene Black Plastic 0.94 0.92
Black Paint 0.94 0.94
Human Skin 0.97 0.97
(source engineering toolbox)
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Calculated Temperatures Ambient Temperature = 27C, 80F
NORMAL SUNLIGHT
INSIDE DEATH RAY
Asphalt 82C, 179F 276C, 530F
Asphalt Light Breeze 57C, 135F 208C, 407F
Black Plastic 82C, 179F 277C, 531F
Black Plastic Light Breeze 58C, 136F 209C, 408F
Chrome 44C, 113F 146C, 295F
Chrome Light Breeze 34C, 94F 82C, 180F
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Melting Point of Plastics
Material Degrees(F) Material Degrees(F)
Acetal (CoPo) 400 PBT 500
Acetal (HoPo) 425 PCT 580
Acrylic 425 Peek 720
Acrylic (Mod) 500 PET 540
ABS (MedImp) 400 Polycarbonate 550
ABS (HiImpFR) 420 Polyetherimide 700
CelAcetate 385 Polyethylene (LD) 325
CelButyrate 350 Polyethylene (HD) 400
CelPropionate 350 Polypropylene 350
EVA 350 Polystyrene (GP) 350
LCP 500 Polystyrene (MI) 380
Nylon (6) 500 Polystyrene (HI) 390
Nylon (6/6) 525 Polysulfone 700
Polyamide-imide 650 PPO 575
Polyarylate 700 PVC (Rig/Flex) 350/325
TFE 600
Source: http://plastictroubleshooter.com/ThePlasticTroubleshooter/melt_temps.htm
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Observations on Calculated Temperatures
Some temperatures seem believable, eg:
Chrome in Normal Sunlight – 44C, 134F
Ashpalt in Light Breeze – 57C, 135F
Some temperatures too high, eg:
Asphalt in the Death Ray – 276C, 530F
Asphalt in a Light Breeze - 208C, 407F
Conclusion energy transfer model is too simplistic
As energy transfer from Irradiance increases
Reality will be greater dissipation through Convection and Conduction
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Agenda
Comparing Predictions with Measurement
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Calibration Scene
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Calibration Scene in Iray
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Physical Measurements
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Comparing Measured and Calculated Temperature Ambient Temperature = 71F, Windspeed ~0 Knots
CALCULATED FROM
IRRADIANCE
MEASURED 3-11-2015 1:05 PM
MEASURED 3-11-2015 1:51 PM*
1 Asphalt (Horizontal) 122F 106F 87F
2 Concrete (Horizontal) 108F 98F 87.5F
3 Concrete (Vertical, ~south) 116F 104F 89F
4 White Painted Steel (Vertical, ~south) 130F 86.5F 76F
5 White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F
6 Black Painted Steel (Vertical, ~south) 131F 99F 80F
7 Black Painted Steel (Vertical, ~east) 114F 92.5F 80F
* Full Cloud Cover for > 30 mins
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Observations Between Measured and Calculated Temperature
CALCULATED FROM
IRRADIANCE
MEASURED 3-11-2015 1:05 PM
MEASURED 3-11-2015 1:51 PM*
Asphalt (Horizontal) 122F 106F 87F
Concrete (Horizontal) 108F 98F 87.5F
Concrete (Vertical, ~south) 116F 104F 89F
White Painted Steel (Vertical, ~south) 130F 86.5F 76F
White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F
Black Painted Steel (Vertical, ~south) 131F 99F 80F
Black Painted Steel (Vertical, ~east) 114F 92.5F 80F
* Full Cloud Cover for > 30 mins
Significant heat loss after full sunlight is obscured by
cloud
- Up to 20% temperature drop
Clearly significant heat transfer occurring through:
- Radiation
- Convection
- Conduction
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Observations Between Measured and Calculated Temperature (cont.)
CALCULATED FROM
IRRADIANCE
MEASURED 3-11-2015 1:05 PM
MEASURED 3-11-2015 1:51 PM*
Asphalt (Horizontal) 122F 106F 87F
Concrete (Horizontal) 108F 98F 87.5F
Concrete (Vertical, ~south) 116F 104F 89F
White Painted Steel (Vertical, ~south) 130F 86.5F 76F
White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F
Black Painted Steel (Vertical, ~south) 131F 99F 80F
Black Painted Steel (Vertical, ~east) 114F 92.5F 80F
* Full Cloud Cover for > 30 mins
Results are expected to be higher since
conduction of heat away from the surface
is being ignored by the model
These are large uniform surfaces so local
heat transfer effects will not occur
Over prediction is approximately ~9-13%
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Observations Between Measured and Calculated Temperature (cont.)
CALCULATED FROM
IRRADIANCE
MEASURED 3-11-2015 1:05 PM
MEASURED 3-11-2015 1:51 PM*
Asphalt (Horizontal) 122F 106F 87F
Concrete (Horizontal) 108F 98F 87.5F
Concrete (Vertical, ~south) 116F 104F 89F
White Painted Steel (Vertical, ~south) 130F 86.5F 76F
White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F
Black Painted Steel (Vertical, ~south) 131F 99F 80F
Black Painted Steel (Vertical, ~east) 114F 92.5F 80F
* Full Cloud Cover for > 30 mins
These surfaces are in very close proximity so
local heat transfer effects will occur and the
model assumes steady-state energy transfer
at a single surface. It doesn’t, for example,
model heat flow from one surface to another
when both are subject to different Irradiance
levels.
The materials for surfaces 4 thru 6 are also
highly conductive (steel) and very thin (surface
4 & 5 < ⅛”, surface 5 & 6 < ¼”) so the impact
of conduction will be increased.
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Observations Between Measured and Calculated Temperature (cont.)
CALCULATED FROM
IRRADIANCE
MEASURED 3-11-2015 1:05 PM
MEASURED 3-11-2015 1:51 PM*
Asphalt (Horizontal) 122F 106F 87F
Concrete (Horizontal) 108F 98F 87.5F
Concrete (Vertical, ~south) 116F 104F 89F
White Painted Steel (Vertical, ~south) 130F 86.5F 76F
White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F
Black Painted Steel (Vertical, ~south) 131F 99F 80F
Black Painted Steel (Vertical, ~east) 114F 92.5F 80F
* Full Cloud Cover for > 30 mins
The above inaccuracies with the
model are almost certainly causing
this under predicition
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Summarizing Temperature Derived from Irradiance
Temperature could be calculated from Irradiance in certain limited situations
Large uniform surfaces with consistent, steady state exposure to sunlight
Most real-world situations extremely complex
Assumptions in energy transfer model easily break down
For more accuracy heat transfer needs to be modelled with significant detail
Heat transfer through/between solids
Multiple surfaces exposed to different Irradiance levels
Etc etc
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Agenda
Conclusions
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Overall Conclusions
Miscalculating design impact at either design or engineering stage can lead to significant and expensive consequences
Knowledge is Power - ability to understand full implications of design choices critical
- Issues may not be observable in required validation points
Interactive rendering combined with calibrated sun model within in Iray facilitated identifying the Death Ray problem literally in minutes
Irradiance prediction within Iray is an extremely good indicator of potential heat problems
Deriving temperature from predicted Irradiance extremely complex challenge
- requires significant research, investigation and calibration
Thank You!