Data conversion via Reynolds # - -comments?

[IceMan30]

***Apologies up front for the long, general nature of this post...***

I'm trying to solve an aero problem by converting drag data (Cd at given Mach #s) from a given source to a smaller scale. Somewhat specifically, I'm trying to transpose Cd data for a given rocket body to a smaller cylindrical object. (Sorry, thats all the detail you get)

My approach has been:
Calculate the drag from Cd, using an assumed altitude for rho and speed of sound conversion of Mach.
Next, use the same assumed altitude and appropriate reference length to calculate Reynolds # (RE).
Next, use RE, reference length for smaller object, and sea level (SL) conditions to calculate velocity at SL
Then, calculate Cd using SL conditions and vel I just came up with.


The results trend well when plotted against Mach, which tells me I'm not goofing up too much. But the scale of my results is way off, by 1, maybe 2 orders of magnitude.

**Any thoughts on where I might be going wrong in the process? I'm looking over the details, but would appreciate some confirmation that the method is at least sort of sound.**

Thanks, and Cheers!

 

[rb1957]

why not check the velocity derived from Re with sealevel M ?

not sure what you're trying to work out ... you have Cd vs M (what Re is this valid for ?) ... unless you're calcing Cd from measured drag data ??

 

[btrueblood]

Cd for both bodies should peak at M=1. Sounds like your method does not generate such a curve? It's a tricky problem.

My approach would be: Generate a baseline Cd (at, say M=0.1) for both bodies, using data for similar bodies (cylinders, streamlined or otherwise) at various Re. The low speed Cd for the smaller body should generally be higher, as its Re will be lower. Scale the Cd vs. Mach curve for the larger body to the smaller body (ratio it up), i.e. make the Cd vs. Mach curve that you have correctly estimate the low speed Cd. You'll tend to (probably) overpredict the smaller body's Cd as Mach increases.

In the end, nothing beats a ballistic test.

 

[IceMan30]

@rb1957: Checking derived velocity against SL mach was a good call. Pointed out some errors I made. This led me to come up with a better atm model for the calculations.

@btrueblood: the curve (trend) is good (as is the expected peak at Mach 1), I'm just not getting the scaling I had anticipated.

Overall, I'm having second thoughts about this process I cooked up. I'm not sure the Reynolds regime is close enough to validate the "data transfer." Thats what I'm working through now. I may fall back on empirical data.

Thanks for the input!