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To check my two dimensional CFD calculation I am looking for reference data on the drag coefficient of an open wedge. The geometry is shown below, together with the flow direction.

enter image description here

I have found several sources for drag coefficient data of closed wedges (shown below), but I'm not sure closed wedges are similar enough to open wedges to use that data.

enter image description here

So my question is:
What is the drag coefficient of a two dimensional open wedge placed in the flow as shown in the first image?

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  • $\begingroup$ To be 100% on the drag coefficient you will have to dig out some correlation for the open wedge case (this may not be easy, but it will have been done - unfortunately I don't have a reference to hand). However, depending on your application, you can use the same drag coefficient (DC) as the standard wedge, and argue that the use of this DC will be the pessimistic case. This could be true if, say you wanted to find the resistive force on the object for some flow speed. In this case the use of the closed wedge DC would be pessimistic and justifiable as the DC on the closed case will be smaller. $\endgroup$
    – MoonKnight
    Sep 3, 2013 at 11:03
  • $\begingroup$ this may not be easy, but it will have been done - unfortunately I don't have a reference to hand That's the problem exactly. I'm sure there must be something on it but no I failed to find any references as well. I only found something in the Fluid Dynamic Drag book of Hoerner on the open wedge shown in the first image, but the flow coming from the right. $\endgroup$ Sep 3, 2013 at 12:15

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The drag coeeficient of both of the wedges are the same since the flow velocity and SWEPT area are the same. However, due to turbulence behind the wedge there will be differences in the flow after the object, but the drag coefficients are equal. To be very precise, there are small and negligible in the coefficients which are experimentally derived, but one can ignore those since they are infinitesimal.

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  • $\begingroup$ Turbulent effects are generally part of the drag coefficient calculation, since it is an experimental value. I'm not convinced that this difference in flow wouldn't have a subtle effect on the drag coefficient. As you point out, it may effect the flow behind the object, implying that it is acting somewhat differently with it's surroundings, and therefore may get a different force in return. $\endgroup$
    – JMac
    Apr 24, 2019 at 16:29
  • $\begingroup$ The difference it makes in the flow after it is due to the separation of the flow and the location that is happens. The drag force will be the same. It would change the pressure coefficient as well. Just look at the equation of drag and you will see the important variable in this case is only the swept area which is the same. $\endgroup$ Apr 24, 2019 at 16:38
  • $\begingroup$ The drag equation has more than just swept area. $C_D$ is an experimentally determined variable, and although the front area is the major influence in how much $C_D$ varies; what happens behind the object does actually effect the correct value of $C_D$. For one example, Wikipedia approximates a sphere as $C_D = 0.47$ while for a half-sphere $C_D = 0.42$. I would expect this effect to be much less pronounced when going from an open to closed wedge, because nothing extends behind in either case; but the slight change in turbulence behind can in theory slightly change $C_D$. $\endgroup$
    – JMac
    Apr 24, 2019 at 16:44
  • $\begingroup$ That is true, but as you mentioned it is an experimentally derived variable and it cannot easily be said what the number is but what I can say is that the difference is very small and can be ignored. $\endgroup$ Apr 24, 2019 at 16:46
  • $\begingroup$ I would at very least make that clear in the answer then. Also though, this question is about those specific differences, so even if they are negligible, it seems like it would be important to actually show this in some way, especially given the resource recommendation tag. $\endgroup$
    – JMac
    Apr 24, 2019 at 16:50

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