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Newbie lurker and this discussion is way over my head but wayback in the 70s I did my undergrade Co-op at NIH and we were modeling blood shear rate changes at the endothelial walls as it relates to aethrosclerotic plaque build-up. Mostly in the bifurcations at the celiac and superior messinteric(sp) branches. With constant shear cells stayed in alignment with each other but when you ate a big meal or other activity the blood flow ratios changed and cells would become misaligned with each other causing gaps which allowed initial calcification etc. Not sure where that research ever ended up as I went off on a life long non medical field tangent. Perhaps similar mechanism exists albeit simplistic to your discussion.
We had really cool laser dopler anemometers and dye injection flow visualization techniques to give the velocity profiles as blood flow ratios changed.
You know when I trout fish I look for a rock in the middle of a stream and fish the eddy behind it.

cg50in wrote: You know when I trout fish I look for a rock in the middle of a stream and fish the eddy behind it.

That's a very nice image. I think the difference between problems in arteries and vein problems is the velocity and pressure are so different. Basically in the head you have only gravity to move the blood plus whatever is left that got it up there after passing through the capillary beds (heart, lung muscles...).

So the velocity range should be easy to ballpark, given the distances. If turbulence happens I'd be surprised. Do eddies happen in slow moving stuff? I know not much about this stuff, except for real basics. Don't know anything about blood shear so you are a very welcome contributor,. far's I'm concerned. All I can guess is that where veins and venules split must disturb the flow as much as anything, especially if it's ay a sharpish angle. But where there is a straight stretch it can become smoother, and it it's too slow maybe there is not enough momentum and cells pop through the BBB.