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Laser-Doppler anemometry and its application to flow investigations related to the environment of vegetation

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Abstract

Accurate measurements in highly turbulent flows, as they occur in nature, require reliable velocity measuring techniques that permit instantaneous velocity components to be locally recorded. Hot-element techniques and optical methods are available for local measurements of instantaneous velocity and the present paper summarizes the advantages and disadvantages of different techniques when applied to flow fields in the environment of vegetation.

The paper points out the advantages of laser-Doppler anemometry for velocity measurements in highly turbulent flows under laboratory conditions and stresses the reliability of the technique for measurements in polluted air and water flows. The basic principles of the method are explained and developments are described that have yielded optical anemometer systems for measurements of the magnitudes and signs of the instantaneous velocity components. Both quantities have to be known if accurate measurements of the mean flow properties and turbulence characteristics in flow fields with unknown flow directions are required. Electronic data-processing systems for laser-Doppler anemometer measurements are surveyed, embracing frequency analysers, automatic filter banks, frequency trackers, photon correlators and frequency counters. Photon-correlation and counting techniques are introduced as the methods most likely to be employed for the laser-Doppler anemometer.

Laser-Doppler anemometer investigations are described in boundary-layer flows along bean leaves and a metal model of plant leaves. These measurements formed the basis of heat and mass transfer predictions near leaves for a specific leaf position relative to the free stream and with different turbulence properties imposed onto the oncoming flow. These data are presented and discussed in some detail. Measurements are also presented that were carried out to investigate the velocity fields in different flow regimes around a square obstacle in a water flow. These flow properties were needed to understand the different growth rates of sea-weed observed in differing flow regimes.

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Abbreviations

C f :

friction factor

Fr:

Froude number

f :

instantaneous frequency of laser-Doppler signal

‡f :

frequency fluctuations of laser-Doppler signal

f I0 :

frequency of a light wave scattered in the direction of observation {k o } i from an incident beam in direction {k I }i

f 0 :

frequency of unscattered laser light

g :

gravitation constant

H :

height of flow obstacle

h :

water height in open channel

{k I } i :

unit vector in direction of incident laser beam

{k I, 1} i , {k I, 2}:

unit vectors in direction of incident laser beams 1 and 2

{k 0} i :

unit vector in direction of observation of scattered light

{k 0, 1} i , {k 0, 2} i :

unit vectors in directions 1 and 2 of observed scattered light

{k R } i :

unit vector in direction of reference beam

{n} i :

sensitivity vector of laser-Doppler anemometer, {k I, 1 } i }-{k I, 2 } i ={n i for dual beam anemometer

Nu x :

Nusselt number

Pr:

Prandtl number

Re x :

Reynolds number

U m :

mean velocity in channel flow

U ∞ :

free stream velocity

û 1,û 2,û 3 :

components of instantaneous velocity vector

U 1,U 2,U 3 :

components of mean velocity vector

u 1,u 2,u 3 :

components of fluctuating velocity

{û} i :

instantaneous velocity vector

U ⊥ :

velocity component to which the laser-Doppler anemometer is sensitive

Ω :

open channel width

α 1,α 2,α 3 :

direction angles of vector {n} i

λ :

wavelength of laser light

ϕ :

half-angle between the two laser beams

Ν :

dynamic viscosity of flow medium

Μ :

viscosity of flow medium

Τ w :

wall shear stress

ϱ :

density of flow medium

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Authors and Affiliations

  1. Sonderforschungsbereich 80 (Institute of Hydromechanics), University of Karlsruhe, KaiserstraΒe, 12, D-75, Karlsruhe-1, F.R.G.

    F. Durst & M. Zaré

  2. Atomic Energy Research Establishment, Harwell, UK

    G. Wigley

Authors
  1. F. Durst
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  2. M. Zaré
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  3. G. Wigley
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Durst, F., Zaré, M. & Wigley, G. Laser-Doppler anemometry and its application to flow investigations related to the environment of vegetation. Boundary-Layer Meteorol 8, 281–322 (1975). https://doi.org/10.1007/BF02153554

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