Lars Bergström

Optimal growth of small disturbances in pipe Poiseuille flow

A theoretical study is made of initial algebraic growth for small angular‐dependent disturbances in pipe Poiseuille flow. The analysis is based on the homogeneous equation for the pressure for which the eigenvalue problem is solved numerically. In the limit of small streamwise wave numbers asymptotic results for the eigenvalues are derived. On the basis of the modes of the system, which are all damped, the initial value problem is considered and in particular the largest possible growth of the disturbance energy density is determined following the ideas of Butler and Farrell [Phys. Fluids A 4, 1637 (1992)]. The results show that a large amplification of the disturbance energy is possible. The largest amplification is obtained for disturbances with a small streamwise wave number and with an azimuthal wave number of one. The energy growth is then only due to the growth of the streamwise disturbance component. However, for disturbances of shorter wavelength, the energy growth is also substantial and not only...

On the decay of θ mesons in charmonium models

Abstract We derive a new formula for the decay rate of the ψ mesons by properly treating the dependence of lowest order electromagnetic annihilation on the quark-antiquark bound state wave function. Results are presented for currently used non-relativistic potentials models for charmonium. In all cases substantial corrections to previous estimates from commonly used formulas occur. Our results are of relevance also for hadronic decay via gluons in QCD and weak decay of pseudoscalar mesons.

Relativistic corrections to the electromagnetic decays of heavy quarkonium states

Abstract We present expressions which go beyond the usual non-relativistic approximations for the electromagnetic decay widths of vector and pseudoscalar mesons in the quark model. Numerical calculations within typical potential models for heavy quark-antiquark system shows the corrections thus obtained to be substantial. Possible experimental implications for the ψ and ϒ families are discussed.

Gluonic corrections to the decay rate of heavy quarkonium states into lepton pairs

We calculate one-gluon corrections to the decay rate of heavy quarkonium states into lepton pairs. We find that the correction due to one-gluon exchange is not very significant for charmonium, and for practical purposes only the relativistic kinematical effects need to be taken into account. For theY family the gluonic corrections are, relatively, of greater importance.

QCD bound state calculation of Dalitz decay form factors

Measurements of form factors in Dalitz pair decay of pseudoscalar neutral mesons are suggested as a sensitive probe of their quark structure. Predictions from QCD and vector meson dominance differ significantly at the largest values of the invariant mass of the Dalitz pair.

Nonmodal growth of three-dimensional disturbances on plane Couette–Poiseuille flows

The time development of three-dimensional disturbances superimposed on a variety of mean flow profiles representing plane Couette–Poiseuille flow is investigated numerically. Specifically, with y representing the wall normal coordinate, the mean flow profiles U(y) are represented by U(y)=A(1−y2)+By, where B=1 when 0⩽A⩽1∕2 and B=2A(1−A) when 1∕2⩽A⩽1. For streamwise independent disturbances, which are the most amplified ones, there is an increase of the disturbance peak amplification when the parameter A increases in the interval 1∕10⩽A⩽1∕2. In the interval 1∕2⩽A⩽9∕10, and especially for 9∕10⩽A⩽1, the disturbance peak amplification decreases rapidly when A is increased. For A close to 1, a slight reduction of A will therefore cause a strong increase of the disturbance amplification.

Estimates of recoil nucleon spectra in high-energy hadron-nucleus collisions

Abstract It is shown that recent data on the recoil nucleon spectrum in 10.5 GeV/ c pion-neon collisions are in excellent agreement with a parameter free estimate from the so called tube model for high-energy hadron-nucleus collisions. A simple independent collision model is also treated.

Is the recently published counterproof of the tube model credible

Abstract It is shown that recent data from pion and proton collision with emulsion nuclei at 200 GeV/ c are in perfect agreement with expectations from the collective tube model. The opposite conclusion in publications by Azimov et al. came about because the authors ignored the important effects of target fragmentation.

The effect of the Earth's rotation on the transient amplification of disturbances in pipe flow

In pipe flow, for large Reynolds numbers, Draad and Nieuwstadt [J. Fluid Mech. 361, 297 (1998)] showed that the small Coriolis force due to the Earth’s rotation may affect the mean flow profile of liquids substantially. In this paper, the development of small disturbances superimposed on a laminar mean flow affected by the Coriolis force is investigated analytically. The investigation is focused on the time development and the transient growth of streamwise-independent disturbances since they are the most amplified disturbances without the Coriolis effect included. The results show that the modification of the parabolic mean flow caused by the Coriolis force significantly affects the transient disturbance amplification when the Reynolds number (R) is high. For example, with R=9000 and the Ekman number Ek=5.23, due to the Coriolis effect, the peak value of the transient disturbance amplification becomes about two-thirds of the peak value obtained in the case where the mean flow is unaffected by the Corioli...

Interactions of three components and subcritical self-sustained amplification of disturbances in plane Poiseuille flow

A low-dimensional nonlinear model for the normal velocity (v) and normal vorticity (η) disturbance development in plane Poiseuille flow is studied. The study is restricted to the interactions of a pair of oblique components of the form ei(αx±βz) and the component of the form ei2βz, where α and β are streamwise and spanwise wave numbers, respectively. The disturbances considered are also assumed to be highly elongated in the streamwise direction. Owing to the non-normal properties of the basic equations, the η disturbance is first transiently amplified. Then, if the Reynolds number (R) and the initial disturbance are sufficiently large, the nonlinear interactions lead to a self-sustained process of disturbance amplification at subcritical R. For large R (R≳5000), the threshold disturbance amplitude scales like R−3. The results also strongly indicate that the nonlinear feedback from η to v is crucial for the establishment of the self-sustained process.