# What is the “sweeping velocity” of a turbulent flow?

Background:

While reading Rubenstein and Zhou 1997 I encountered an unfamiliar term, "sweeping velocity".

To quote the authors (pg. 3):

...the sweeping hypothesis makes the decorrelation depend on the sweeping velocity $V$, which is not an inertial range property, but an entirely independent property of the energy containing range...

I'm interested in estimating $V$ because the authors use it to predict the frequency of peak energy production by turbulent flows:

$$\omega \sim V k_{0}$$

Where $k_0$ is the inverse of the integral time scale which characterizes the intertial subrange of the turbulence:

$$\frac{1}{k_0} = \mathcal{T} = \int_{0}^{\infty} r(\tau) \ d \tau$$

(In fact, I believe they describe the entire acoustic energy spectrum generated by the flow as a piecewise function with $k_0$ as the point of discontinuity.)

Question:

What is the sweeping velocity, $V$, and how does it differ from other velocities of a flow (such as mean velocity)? Is there a way to measure $V$ for streams or rivers?

• Hmm... +1 for interesting qn... perhaps it might be the flow velocity parallel to the source – QuIcKmAtHs Jan 7 '18 at 3:26
• Since they mention that the sweeping velocity pertains to the energy containing range it must be the r.m.s. velocity of turbulent flow and not its mean velocity. Mean velocity, more precisely its gradient, is merely an energy source that sustains turbulence. – Deep Jan 7 '18 at 5:53
• Information on "sweeping velocity" can be found by looking at the references provided in the article for "sweeping hypothesis" at pg 5. – Guill Jan 9 '18 at 22:38