17 research outputs found
Discontinuous transition to shear flow turbulence
Depending on the type of flow the transition to turbulence can take one of
two forms, either turbulence arises from a sequence of instabilities, or from
the spatial proliferation of transiently chaotic domains, a process analogous
to directed percolation. Both scenarios are inherently continuous and hence the
transformation from ordered laminar to fully turbulent fluid motion is only
accomplished gradually with flow speed. Here we show that these established
transition types do not account for the more general setting of shear flows
subject to body forces. By attenuating spatial coupling and energy transfer,
spatio-temporal intermittency is suppressed and with forcing amplitude the
transition becomes increasingly sharp and eventually discontinuous. We argue
that the suppression of the continuous range and the approach towards a first
order, discontinuous scenario applies to a wide range of situations where in
addition to shear, flows are subject to e.g. gravitational, centrifugal or
electromagnetic forces
Investigation on Semi-active Suspension System for Multi-axle Armoured Vehicle using Co-simulation
The objective of the study is to evaluate the performance of various semi-active suspension control strategies for 8x8 multi-axle armoured vehicles in terms of comparative analysis of ride quality and mobility parameters during negotiation of typical military obstacles. Since the cost, complexity and time precludes realisation of actual system, co-simulation technique has been effectively implemented for this investigation. Co-simulation combines advanced virtual prototyping and control technology which offers a novel approach to investigate the dynamics of such complex system. The simulations for the integrated control system along with multi body model of the vehicle are carried out for the control strategies, viz. continuous sky hook control, cascade loop control and cascade loop with ride control and compared with passive suspension system. The vehicle with 8x8 configuration is run on the real world obstacle profiles, viz. step, trench, trapezoidal bump and corrugated road and the effect of control strategies on ride comfort, wheel displacement and ground reaction is presented. It is observed that cascade loop with ride control in semi-active mode offers better vehicle ride comfort while crossing the said obstacles. The improved performance parameters are achieved through stabilisation of heave, pitch and roll motions of the vehicle through outer loop and isolation of vehicle level uneven disturbances through the fuzzy logic controller employed in inner loop
Self-similar shear-thickening behavior in CTAB/NaSal surfactant solutions
The effect of salt concentration Cs on the critical shear rate required for
the onset of shear thickening and apparent relaxation time of the
shear-thickened phase, has been investigated systematically for dilute
CTAB/NaSal solutions. Experimental data suggest a self-similar behavior of the
critical shear rate and relaxation time as functions of Cs. Specifically, the
former ~ Cs^(-6) whereas the latter ~ Cs^(6) such that an effective Weissenberg
number for the onset of the shear thickened phase is only weakly dependent on
Cs. A procedure has been developed to collapse the apparent shear viscosity
versus shear rate data obtained for various values of Cs into a single master
curve. The effect of Cs on the elastic modulus and mesh size of the
shear-induced gel phase for different surfactant concentrations is discussed.
Experiments performed using different flow cells (Couette and cone-and-plate)
show that the critical shear rate, relaxation time and the maximum viscosity
attained are geometry-independent. The elastic modulus of the gel phase
inferred indirectly by employing simplified hydrodynamic instability analysis
of a sheared gel-fluid interface is in qualitative agreement with that
predicted for an entangled phase of living polymers. A qualitative mechanism
that combines the effect of Cs on average micelle length and Debye parameter
with shear-induced configurational changes of rod-like micelles is proposed to
rationalize the self-similarity of SIS formation.Comment: 27 pages, 17 figure
The rise of fully turbulent flow
Over a century of research into the origin of turbulence in wallbounded shear
flows has resulted in a puzzling picture in which turbulence appears in a
variety of different states competing with laminar background flow. At slightly
higher speeds the situation changes distinctly and the entire flow is
turbulent. Neither the origin of the different states encountered during
transition, nor their front dynamics, let alone the transformation to full
turbulence could be explained to date. Combining experiments, theory and
computer simulations here we uncover the bifurcation scenario organising the
route to fully turbulent pipe flow and explain the front dynamics of the
different states encountered in the process. Key to resolving this problem is
the interpretation of the flow as a bistable system with nonlinear propagation
(advection) of turbulent fronts. These findings bridge the gap between our
understanding of the onset of turbulence and fully turbulent flows.Comment: 31 pages, 9 figure
Factors associated with no apparent coronary artery disease in patients with type 2 diabetes mellitus for more than 10 years of duration: a case control study
Flow-Induced Self Assembly in Micellar Fluids with Applications to Nanomanufacturing
Structure and rheology of surfactant solutions are very sensitive to electrostatic interactions and flow deformation. The objectives of this experimental study have been to uncover the essential physics of structure transitions in surfactant solutions under simple shear as well as porous medium flows, and how they are influenced by the presence of added salt First, we study the effect of salt concentration on the shear rheology of ionic surfactant solutions in simple shear flow. It is shown that for a given surfactant concentration, low salt concentrations induce shear-thickening behavior, while higher salt concentrations induce shear thinning. The shear thickening transition occurs due to the formation of shear induced structures (SIS), which are qualitatively different from simple micellar aggregates. It is shown that the shear-thickening transition occurs when a critical amount of strain is applied on a solution that is sheared beyond a critical shear rate. We provide robust scaling laws for the onset of shear thickening, and for the relaxation time of the shear-induced phase λ. Experimental data suggest a self-similar behavior of and λ as a function of the salt concentration such that an effective Weissenberg number for the onset of shear thickening is practically independent of salt concentration. However, SIS are found to be extremely shear sensitive, and instantaneously disintegrate upon removal of applied strain. Based on this understanding, we design an experiment to induce and study SIS in microfluidic channels, which forms the second part of the thesis. For the first time, it has been shown that irreversible SIS can be produced. We discuss the factors responsible for this irreversibility. Moreover, unlike conventional sol-gel processes, the irreversible gels are formed without addition of alcohols, making the process completely bio-compatible. Different surfactants were tested for the robustness of the process. Finally, AFM images of the irreversible structures suggest a highly entangled network of micelles with morphology that is ideal for nano-manufacturing applications
The critical point of the transition to turbulence in pipe flow
In pipes, turbulence sets in despite the linear stability of the laminar Hagen–Poiseuille flow. The Reynolds number ( ) for which turbulence first appears in a given experiment – the ‘natural transition point’ – depends on imperfections of the set-up, or, more precisely, on the magnitude of finite amplitude perturbations. At onset, turbulence typically only occupies a certain fraction of the flow, and this fraction equally is found to differ from experiment to experiment. Despite these findings, Reynolds proposed that after sufficiently long times, flows may settle to steady conditions: below a critical velocity, flows should (regardless of initial conditions) always return to laminar, while above this velocity, eddying motion should persist. As will be shown, even in pipes several thousand diameters long, the spatio-temporal intermittent flow patterns observed at the end of the pipe strongly depend on the initial conditions, and there is no indication that different flow patterns would eventually settle to a (statistical) steady state. Exploiting the fact that turbulent puffs do not age (i.e. they are memoryless), we continuously recreate the puff sequence exiting the pipe at the pipe entrance, and in doing so introduce periodic boundary conditions for the puff pattern. This procedure allows us to study the evolution of the flow patterns for arbitrary long times, and we find that after times in excess of advective time units, indeed a statistical steady state is reached. Although the resulting flows remain spatio-temporally intermittent, puff splitting and decay rates eventually reach a balance, so that the turbulent fraction fluctuates around a well-defined level which only depends on . In accordance with Reynolds’ proposition, we find that at lower (here 2020), flows eventually always resume to laminar, while for higher ( ), turbulence persists. The critical point for pipe flow hence falls in the interval of $2020 , which is in very good agreement with the recently proposed value of . The latter estimate was based on single-puff statistics and entirely neglected puff interactions. Unlike in typical contact processes where such interactions strongly affect the percolation threshold, in pipe flow, the critical point is only marginally influenced. Interactions, on the other hand, are responsible for the approach to the statistical steady state. As shown, they strongly affect the resulting flow patterns, where they cause ‘puff clustering’, and these regions of large puff densities are observed to travel across the puff pattern in a wave-like fashion
Study of glancing and blunt fin shock-boundary layer interactions at low supersonic mach numbers
Experiments have been carried out investigating both sharp fin and blunt fin-induced turbulent boundary layer interactions at freestream Mach numbers of-1 .80 and 2 .47.
Two sharp fin models of wedge angle 10° and 15° and three blunt fin models of different nose diameter were chosen for this study . Surface pressure distributions were measured and surface flow visualization studies were carried out for all the cases . The results show that the interactions at these relatively lower freestream Mach numbers are broadly similar to those observed at higher Mach numbers (> 3.0).
It is suggested that the influence of the blunt nose exists atleast upto 3d in the spanwise or lateral direction
Experimental characterization of transition region in rotating-disk boundary layer
International audienceThe three-dimensional boundary layer due to a disk rotating in otherwise still fluid is well known for its sudden transition from a laminar to a turbulent regime, the location of which closely coincides with the onset of local absolute instability. The present experimental investigation focuses on the region around transition and analyses in detail the features that lead from the unperturbed boundary layer to a fully turbulent flow. Mean velocity profiles and high-resolution spectra are obtained by constant-temperature hot-wire anemometry. By carefully analysing these measurements, regions in the flow are identified that correspond to linear, weakly nonlinear or turbulent dynamics. The frequency that dominates the flow prior to transition is explained in terms of spatial growth rates, derived from the exact linear dispersion relation. In the weakly nonlinear region, up to six clearly identifiable harmonic peaks are found. High-resolution spectra reveal the existence of discrete frequency components that are deemed to correspond to fluctuations stationary with respect to the disk surface. These discrete components are only found in the weakly nonlinear region. By systematically acquiring low- and high-resolution spectra over a range of narrowly spaced radial and axial positions, it is shown that while the transition from laminar to turbulent regimes occurs sharply at some distance from the disk surface, a complex weakly nonlinear region of considerable radial extent continues to prevail close to the disk surface