  A turbulent mean profile for pipe flow is prescribed which closely matches
experimental observations. The nature of perturbations superimposed upon this
profile is then considered. Optimal growth calculations predict two distinct
classes of structures, clearly associated with near-wall and large-scale
structures. Quantitative correspondence of the spanwise wavelength of
wall-structures with experimental observations is very good. The response to
harmonic forcing is also considered, and the linear growth tested with direct
numerical simulation of forced turbulence. Despite the very simple eddy
viscosity assumption, this linear approach predicts well the surprisingly large
growth of outer-scale modes in the bulk flow.
  Un profil moyen turbulent est prescrit dans une conduite cylindrique, en
adequation avec les observations experimentales. Nous considerons ensuite la
nature des perturbations a cet ecoulement synthetique. Le calcul des
croissances optimales predit deux types de structures, associees respectivement
aux structures de proche-paroi et de grande echelle. Un excellent accord
quantitatif est trouve avec les resultats experimentaux quant a la longueur
d'onde transversale. La reponse harmonique est egalement etudiee, et la
croissance lineaire observee comparee a des simulations numeriques directes de
turbulence forcee. Malgre de l'hypothese simple de type `Eddy viscosity', cette
approche lineaire predit efficacement la croissance spectaculaire des modes de
grande echelle au coeur de l'ecoulement.
