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turbodiesel · 11-22-2015, 12:46 PM

Dear all,

As you all know, a precise frontal area is one key to a precise Cd determination. A precision of 0.01m² is desireable to reduce the uncertainty in Cd to around +/-0.005.

The following applies to Volkswagen Polo Bluemotion model year 2011:
For precise frontal area determination I decided to compare the factory given data with data obtained by the photography method descibed in this thread.

sources
drawings of VW:
volkswagen.de/content/medialib/vwd4/de/dialog/pdf/polo/polo_tup_110127/_jcr_content/renditions/rendition.file/polo_tup_110127.pdf
photograph:
taken with a 12x optical zoom camera (fully zoomed), distance of 160ft

data processing
GIMP Pixel counting method, pixels of drawings due to their high contrast can be easiliy counted using the "fuzzy select tool", the images are outlined manually using the "paths tool", only the left half image was outlined assuming the cars front face is symmetrical
scales were directly taken from drawings
wheel-to-wheel distance measured with folding ruler (uncertainty +/-1cm) for the image scale

results
ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7105-polo-bluemotion-aus-prospekt.png
ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7103-bild-polo-rotiert.png
ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7104-bild-polo-rotiert-overlay-zeichnung.png

The first picture shows the drawings of VW. Note that height and width are not scaled 1:1, which had to be considered when calculating the drag area.
Interestingly the frontal area of the front view differs from the back view, the ground clearance is larger in the back view.
The Pixel-counted frontal area (front view) including the thick surrounding red outline is 2.15 m², excluding the red line it's only 2.04m².

The second image was rotated to align the car horizontally.
The third picture is an overlay of pic 1 and 2.
First sight suggests a good agreement of both pictures. Nonetheless, some significant differences remain:
a) Obviously the red line needs to be excluded from frontal area.
b) The underbody of the car is in the photo somewhat lower and contibutes frontal area that is not included in the drawing.
c) The tyres of the drawing are wider than stock as seen in the right side of the picture.
d) The photograph is not perfectly aligned as seen in the not colinear tyres of left side of the picture. Also the roof of the photo is lower than the drawing, which indicates that the distance the photo was taken is still to low as explained elsewhere in this forum.

The results of the photo evaluation are 1.97m² +/- 0.02m². The error is the pure scale error due to length measurement and does not include any errors in image acquisition.

conclusion
The drawing's frontal area is 2.04m² what reduces by another 0.01m² when the tyre width is adjusted to meet the tyre width of the image.
The frontal area with the photographer's method most likely underestimates the true frontal area.
As a weighted mean value between these two areas a value of 2.02m² is suggested.

Some additional results from photo pixel counting:
Both tyres contribute a frontal area of 0.07m². This could be reduced to almost zero by lowering the ride height of the car.
Both mirrors contribute a frontal area of 0.04m².

Happy modding!
turbodiesel

11-22-2015, 12:46 PM	#6 (permalink)
turbodiesel EcoModding Lurker Join Date: Nov 2015 Location: Berlin, Germany Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts	Comparison between factory values and the photographer's method Dear all, As you all know, a precise frontal area is one key to a precise Cd determination. A precision of 0.01m² is desireable to reduce the uncertainty in Cd to around +/-0.005. The following applies to Volkswagen Polo Bluemotion model year 2011: For precise frontal area determination I decided to compare the factory given data with data obtained by the photography method descibed in this thread. sources drawings of VW: volkswagen.de/content/medialib/vwd4/de/dialog/pdf/polo/polo_tup_110127/_jcr_content/renditions/rendition.file/polo_tup_110127.pdf photograph: taken with a 12x optical zoom camera (fully zoomed), distance of 160ft data processing GIMP Pixel counting method, pixels of drawings due to their high contrast can be easiliy counted using the "fuzzy select tool", the images are outlined manually using the "paths tool", only the left half image was outlined assuming the cars front face is symmetrical scales were directly taken from drawings wheel-to-wheel distance measured with folding ruler (uncertainty +/-1cm) for the image scale results ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7105-polo-bluemotion-aus-prospekt.png ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7103-bild-polo-rotiert.png ecomodder.com/forum/member-turbodiesel-albums-frontal+area-picture7104-bild-polo-rotiert-overlay-zeichnung.png The first picture shows the drawings of VW. Note that height and width are not scaled 1:1, which had to be considered when calculating the drag area. Interestingly the frontal area of the front view differs from the back view, the ground clearance is larger in the back view. The Pixel-counted frontal area (front view) including the thick surrounding red outline is 2.15 m², excluding the red line it's only 2.04m². The second image was rotated to align the car horizontally. The third picture is an overlay of pic 1 and 2. First sight suggests a good agreement of both pictures. Nonetheless, some significant differences remain: a) Obviously the red line needs to be excluded from frontal area. b) The underbody of the car is in the photo somewhat lower and contibutes frontal area that is not included in the drawing. c) The tyres of the drawing are wider than stock as seen in the right side of the picture. d) The photograph is not perfectly aligned as seen in the not colinear tyres of left side of the picture. Also the roof of the photo is lower than the drawing, which indicates that the distance the photo was taken is still to low as explained elsewhere in this forum. The results of the photo evaluation are 1.97m² +/- 0.02m². The error is the pure scale error due to length measurement and does not include any errors in image acquisition. conclusion The drawing's frontal area is 2.04m² what reduces by another 0.01m² when the tyre width is adjusted to meet the tyre width of the image. The frontal area with the photographer's method most likely underestimates the true frontal area. As a weighted mean value between these two areas a value of 2.02m² is suggested. Some additional results from photo pixel counting: Both tyres contribute a frontal area of 0.07m². This could be reduced to almost zero by lowering the ride height of the car. Both mirrors contribute a frontal area of 0.04m². Happy modding! turbodiesel Last edited by turbodiesel; 11-22-2015 at 12:55 PM..