229 research outputs found
Travelling-waves consistent with turbulence-driven secondary flow in a square duct
We present numerically determined travelling-wave solutions for
pressure-driven flow through a straight duct with a square cross-section. This
family of solutions represents typical coherent structures (a staggered array
of counter-rotating streamwise vortices and an associated low-speed streak) on
each wall. Their streamwise average flow in the cross-sectional plane
corresponds to an eight vortex pattern much alike the secondary flow found in
the turbulent regime
The apparent roughness of a sand surface blown by wind from an analytical model of saltation
We present an analytical model of aeolian sand transport. The model
quantifies the momentum transfer from the wind to the transported sand by
providing expressions for the thickness of the saltation layer and the apparent
surface roughness. These expressions are derived from basic physical principles
and a small number of assumptions. The model further predicts the sand
transport rate (mass flux) and the impact threshold (the smallest value of the
wind shear velocity at which saltation can be sustained). We show that, in
contrast to previous studies, the present model's predictions are in very good
agreement with a range of experiments, as well as with numerical simulations of
aeolian saltation. Because of its physical basis, we anticipate that our model
will find application in studies of aeolian sand transport on both Earth and
Mars
Jump at the onset of saltation
We reveal a discontinuous transition in the saturated flux for aeolian
saltation by simulating explicitly particle motion in turbulent flow. The
discontinuity is followed by a coexistence interval with two metastable
solutions. The modification of the wind profile due to momentum exchange
exhibits a second maximum at high shear strength. The saturated flux depends on
the strength of the wind as
Application of an Energy-Vorticity Turbulence Model to Fully rough Pipe flow
Based on a more direct analogy between turbulent and molecular transport, a foundation was recently presented for an energy-vorticity turbulence model. The new turbulent-energytransport equation contains two closure coefficients; a viscous-dissipation coefficient and a turbulent-transport coefficient. To help evaluate the closure coefficients and provide insight into the energy-vorticity turbulence variables, fully rough pipe flow is considered. For this fully developed flow, excellent agreement with experimental data for velocity profiles and friction factors is attained over a wide range of closure coefficients, provided that a given relation between the coefficients is maintained
Turbulent Friction in Rough Pipes and the Energy Spectrum of the Phenomenological Theory
The classical experiments on turbulent friction in rough pipes were performed
by J. Nikuradse in the 1930's. Seventy years later, they continue to defy
theory. Here we model Nikuradse's experiments using the phenomenological theory
of Kolmog\'orov, a theory that is widely thought to be applicable only to
highly idealized flows. Our results include both the empirical scalings of
Blasius and Strickler, and are otherwise in minute qualitative agreement with
the experiments; they suggest that the phenomenological theory may be relevant
to other flows of practical interest; and they unveil the existence of close
ties between two milestones of experimental and theoretical turbulence.Comment: Accepted for publication in PRL; 4 pages, 4 figures; revised versio
Hydraulic resistance to overland flow on surfaces with partially submerged vegetation
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96239/1/wrcr13661.pd
Reference enthalpy method developed from solutions of the boundary-layer equations
A simple average of local enthalpy over the velocity profile is proposed as the proper definition of reference enthalpy for the purpose of quasi-one-dimensional treatment of compressible boundary layers. Use of Van Driest's nearly exact solutions of the laminar boundary-layer equations shows that this definition produces Eckert's reference enthalpy formulation for the special case of an adiabatic wall. For surfaces other than adiabatic, either Eckert's form should be replaced by that of Young and Janssen, or the coefficient in Eckert's viscous heating term should be slightly modified. A similar analysis was conducted for turbulent flows using Whitfield and High's simplified solutions of the turbulent boundary-layer equations. Dorrance's derivation of reference quantities is also addressed. This work provides a theoretical basis for the empirical reference enthalpy formulas of Eckert and others and supplies practical expressions for the reference enthalpy of both laminar and turbulent compressible boundary layers
The uses of coherent structure (Dryden Lecture)
The concept of coherent structure in turbulent flow is a revolutionary idea which is being developed by evolutionary means. The main objective of this review is to list some solid achievements, showing what can be done by using the concept of coherent structure that cannot be done without it. The nature of structure is described in terms of some related concepts, including celerity,
topology, and the phenomenon of coalescence and splitting of structure. The main emphasis is on the mixing layer, as the one flow whose structure is well enough understood so that technical applications are now being made in problems of mixing and chemistry. An attempt is made to identify some conceptual and experimental obstacles that stand in the way of progress in other technically important flows, particularly the turbulent boundary layer. A few comments are included about the role of structure in numerical simulations and in current work on manipulation and control of turbulent flow. Some recent developments are cited which suggest that the time is nearly right for corresponding advances to occur in turbulence modeling
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Strömungsgesetze in rauhen Rohren
An experimental investigation is made of the turbulent flow of water in pipes with various degrees of relative roughness. The pipes range in size from 25 to 100 millimeters in diameter and from 1800 to 7050 millimeters in length. Flow velocities permitted Reynolds numbers from about 10 (sup. 4) to 10 (sup. 6). The laws of resistance and velocity distributions were obtained as a function of relative roughness and Reynolds number. Mixing length, as described by Prandtl's mixing-length formula, is discussed in relation to the experimental results
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