If the admonition to exclude ambient light is followed, the flash time
is what counts- the shutter speed is unimportant.  Only while the
flash lasts is there light for the exposure, and the flash is much
shorter than the shutter speed.

Here's another piece of advice. Ignore trolls like this gentleman, and
google the term "high-speed flash sync." Most DSLR/flash combinations
will give you the ability to use flash at any shutter speed now, and the
information in the paragraph above is simply false.

You need to extinguish all ambient light and use the light of the flash alone.
Also, if using automatic flash that might be depending on thyristor circuitry,
it will help to be as close to the subject as possible. The closer you are, the
less flash that is needed, and the shorter will be the duration of that flash.
Flash output levels are controlled by its duration rather than luminosity (in
most instances, because that is easier to control.)

If you don't extinguish all ambient light and depend on flash alone, then the
very slow speed of any dSLR's focal-plane shutter will also use the ambient
light to expose those threads. Causing blurring, as you have seen. If you have
access to a high-quality P&S camera instead, whose flash-sync isn't limited to
last-century's focal-plane speeds, that will also help. One of my P&S cameras,
for example, allows use of its flash up to its highest shutter speed of 1/2400
(some others go higher than this), in perfect sync with full-frame illumination.
If you use a P&S that can do this then you don't need to find ways to extinguish
the ambient lights, which in some/many situations is impossible.

Hi,

I'm wondering if anyone may be able to help me.  I am wanting to
photograph the airflow over a turbine blade being tested in a wind
tunnel, via the attachment of small cotton threads to its surface.
The only way to isolate the blade is to use a stroboscope and I would
like to then take a photo of this, so I have a record of the direction
the cotton threads are being blown in.

I tried today without the strobe, and just using my flash gun.  It
wasn't too bad, but a little blurred still from the slow shutter
speed.  I've tried also using the bulb setting and letting the strobe
pulse a few times hoping there would be enough light, but this wasn't
very successful either.

If anyway has ever photographed this sort of thing and has some clues
on what I should do, I would be very grateful.

Thanks in advance.