1,171 research outputs found
Structure of unsteady stably stratified turbulence with mean shear.
No full textThe statistics of unsteady turbulence with uniform stratification N (Brunt–Väisälä frequency) and shear α(=dU1/dx3) are analysed over the entire time range (00 and \it Ri>0.25 respectively, oscillatory momentum and positive and negative density fluxes develop. Above a critical value of \it Ri\scriptsize\it crit(∼0.3), their average values are persistently countergradient. This structural change in the turbulence is the primary mechanism whereby stable stratification reduces the fluxes and the production of variances. It is quite universal and differs from the energy and stability mechanisms of Richardson (1926) and Taylor (1931). The long-time asymptotics of the energy ratio ER(=\it PE/VKE) of the potential energy to the vertical kinetic energy generally decreases with \it Ri(≥0.25), reaching the smallest value of 3/2 when there is no shear (\it Ri→∞). For strong mean shear (\it Ri<0.25), RDT significantly overestimates ER since (as in unstratified shear flow) it underestimates the vertical kinetic energy VKE. The RDT results show that the asymptotic values of the energy ratio ER and the normalized vertical density flux are independent of the initial value of ER, in agreement with DNS. This independence of the initial condition occurs because the ratios of the contributions from the initial values PE0 and KE0 are the same for PE and VKE and can be explained by the linear processes. Stable stratification generates buoyancy oscillations in the direction of the energy propagation of the internal gravity wave and suppresses the generation of turbulence by mean shear. Because the shear distorts the wavenumber fluctuations, the low-wavenumber spectrum of the vertical kinetic energy has the general form E33(k)∝(αtk)−1, where (LXαt)−1≪k≪L−1X (LX: integral scale). The viscous decay is controlled by the shear, so that the components of larger streamwise wavenumber k1 decay faster. Then, combined with the spectrum distortion by the shear, the energy and the flux are increasingly dominated by the small-k1 components as time elapses. They oscillate at the buoyancy period π/N because even in a shear flow the components as k1→0 are weakly affected by the shear. The effects of stratification N and shear α at small scales are to reduce both VKE and PE. Even for the same \it Ri, larger N and α reduce the high-wavenumber components of VKE and PE. This supports the applicability of the linear assumption for large N and α. At large scales, the stratification and shear effects oppose each other, i.e. both VKE and PE decrease due to the stratification but they increase due to the shear. We conclude that certain of these unsteady results can be applied directly to estimate the properties of sheared turbulence in a statistically steady state, but others can only be applied qualitatively
Settling of particles in homogeneous shear turbulence
No full textThe settling of (inertial) particles is studied in homogeneous shear turbulence. A drift velocity perpendicular to gravity is measured due to the interplay between the homogeneous shear turbulence and gravity acting on the particles. We introduce a model to predict and understand this phenomenon
Supplementary_Material – Supplemental material for The Need to Evaluate Risks and Benefits of Pharmacists Independently Diagnosing and Treating Dermatologic Conditions in Canada
No full textSupplemental material, Supplementary_Material for The Need to Evaluate Risks and Benefits of Pharmacists Independently Diagnosing and Treating Dermatologic Conditions in Canada by Alex Derstenfeld, Neil H. Shear and Ivan V. Litvinov in Journal of Cutaneous Medicine and Surgery</p
Sound radiation in turbulent channel flows
No full textLighthill’s acoustic analogy is formulated for turbulent channel flow with pressure as the acoustic variable, and integrated over the channel width to produce a two-dimensional inhomogeneous wave equation. The equivalent sources consist of a dipole distribution related to the sum of the viscous shear stresses on the two walls, together with monopole and quadrupole distributions related to the unsteady turbulent dissipation and Reynolds stresses respectively. Using a rigid-boundary Green function, an expression is found for the power spectrum of the far-field pressure radiated per unit channel area. Direct numerical simulations (DNS) of turbulent plane Poiseuille and Couette flow have been performed in large computational domains in order to obtain good resolution of the low-wavenumber source behaviour. Analysis of the DNS databases for all sound radiation sources shows that their wavenumber–frequency spectra have non-zero limits at low wavenumber. The sound power per unit channel area radiated by the dipole distribution is proportional to Mach number squared, while the monopole and quadrupole contributions are proportional to the fourth power of Mach number. Below a particular Mach number determined by the frequency and radiation direction, the dipole radiation due to the wall shear stress dominates the far field. The quadrupole takes over at Mach numbers above about 0.1, while the monopole is always the smallest term. The resultant acoustic field at any point in the channel consists of a statistically diffuse assembly of plane waves, with spectrum limited by damping to a value that is independent of Mach number in the low-M limit
Turbulent stratified shear flow experiments: Length scale comparison
No full textStratified shear flows are ubiquitous in geophysical systems such as oceanic overflows, wind-driven thermoclines, and atmo- spheric inversion layers. The stability of such flows is governed by the Richardson Number Ri which represents a balance between the stabilizing influence of stratification and the destabilizing influence of shear. For a shear flow with velocity difference U, density difference ∆ρ and characteristic length H, one has Ri = g(∆ρ/ρ)H/U^2 which is often used when detailed information about the flow is not available. A more precise definition is the gradient Richardson Number Rig = N^2/S^2 where the buoyancy frequency N = ((g/ρ)∂ρ/∂z)^{1/2}, the mean strain S = ∂U/∂z in which z is parallel to gravity and suitable ensemble or time averages define the gradients. We explore the stability and mixing properties of a wall-bounded shear flow over a range 0.1< Rig <1 using simultaneous planar measurements of density and velocity fields using Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), respectively. The flow, confined from the top by glass horizontal boundary, is a lighter alcohol-water mixture injected from a nozzle into quiescent heavier salt-water fluid with velocity between 5 and 10 cm/s and with a relative fractional density difference of 0.0026 or 0.0052. The injected flow is turbulent with Taylor Reynolds number between 50 and 100. We compare a set of length scales that characterize the mixing properties of our turbulent stratified shear flow including the Thorpe Length L_T, the Ozmidov Length L_o, the Ellison Length L_E, and turbulent mixing lengths L_m and L_ρ
Mechanics of inhomogeneous turbulence and interfacial layers
No full textThe mechanics of inhomogeneous turbulence in and adjacent to interfacial layers bounding turbulent and non-turbulent regions are analysed. Different mechanisms are identified according to the straining by the turbulent eddies in relation to the strength of the mean shear adjacent to, or across, the interfacial layer. How the turbulence is initiated and the topology of the region of turbulence are also significant factors. Specifically the cases of a layer of turbulence bounded on one, or two, sides by a uniform and/or shearing flow, and a circular region of a rotating turbulent vortex are considered and discussed.
The entrainment processes at fluctuating interfaces occur both at the outer edges of turbulent shear layers, with and without free-stream turbulence (e.g. jets, wakes and boundary layers), at internal boundaries such as those at the outside of the non-turbulent core of swirling flows (e.g. the ‘eye-wall’ of a hurricane) or at the top of the viscous sublayer and roughness elements in turbulent boundary layers. Conditionally sampled data enables these concepts to be tested. These concepts lead to physically based estimates for critical modelling parameters such as eddy viscosity near interfaces, entrainment rates, maximum velocity and displacement heights
Optimizing Anti-Inflammatory and Immunomodulatory Effects of Corticosteroid and Vitamin D Analogue Fixed-Dose Combination Therapy
No full textAbstract: Fixed-dose combination topical therapy with corticosteroid and vitamin D analog provides effective treatment and possible long-term management of psoriasis. The anti-inflammatory and immunomodulatory effects of corticosteroids and vitamin D analogs in treating psoriasis are well investigated; their complementary effects lead to the disruption of the inflammatory feedback loop underlying psoriasis pathogenesis. Recent preclinical data showed that combination therapy is more effective than monotherapies of the active ingredients in preventing activation of resting pro-inflammatory cells, inducing immunomodulation, reducing inflammatory responses by regulating T cell production, and normalizing keratinocytes. The increased understanding of the mechanism of action of fixed-dose combination therapy from preclinical studies is supported by several clinical studies. As the efficacy of topical therapy is correlated with the skin penetration of the active ingredients, new drug delivery systems have been developed. The fixed-dose combination Cal/BD aerosol foam creates a modified supersaturated formulation when applied to the skin, which is maintained for at least 26Â h in the laboratory setting. Clinical studies have demonstrated superior efficacy of fixed-dose combination calcipotriol (Cal) 50Â Âμg/g and betamethasone dipropionate (BD) 0.5Â mg/g aerosol foam compared with monotherapies of the active ingredients. Furthermore, Cal/BD aerosol foam has shown significantly improved efficacy compared with more traditional formulations, such as Cal/BD ointment and gel, in other studies. Calcipotriol also mitigates risks associated with betamethasone dipropionate and vice versa, resulting in the favorable safety profile observed with fixed-dose combination treatment. Recent data also suggest that fixed-dose combination treatment could provide long-term management of psoriasis, although further clinical investigations are needed. Overall, these data support the value of fixed-dose combination therapy of corticosteroid and vitamin D analog and highlight the added potential of innovative drug delivery for the treatment of psoriasis. Funding: LEO Pharma
Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)
No full textDrug reaction with eosinophilia and systemic symptoms (DRESS) is a delayed, potentially life-threatening, hypersensitivity reaction characterized by a widespread, long-lasting skin eruption, fever, lymphadenopathy, hematological abnormalities, and organ involvement. Time to onset and course are relatively long; relapses may occur. Clinical and biological variability make DRESS a challenging diagnosis. Pathogenesis is not exactly known, but probably reflects a complex interplay of drug and viral-related factors in which genetics and abnormal metabolic pathways of drugs play an important role. Although associated with many drugs, DRESS is mainly observed after a limited number of “high risk” drugs. Early recognition, prompt withdrawal of the culprit, and treatment with corticosteroids are the mainstay of management.</p
Fully Turbulent Mean Velocity Profile for Purely Viscous non-Newtonian Fluids
No full textThe characteristic near wall behavior of turbulent flow of purely-viscous non-Newtonian fluids is discussed for both power-law (P.-L.) and Herschel-Bulkley (H.-B.) rheological models. A proper scaling is presented for H.-B. fluids to establish an analogy with power-law fluids with same flow index. To provide reference data for turbulent flow of non-Newtonian fluids, DNS simulations of power-law fluids are conducted in a rectangular channel for a large range of power-law indices ( = 0.5, 0.69, 0.75, 0.9, 1, 1.2). The DNS data show that the mean velocity profile in the viscous and logarithmic layers follow expressions of the form and respectively, where shows a logarithmic dependency on the flow index.Comparison with some experimental data shows the above formulation to be valid for Reynolds numbers (based on shear velocity) as high as 1000
Direct numerical simulation of turbulent flow past a trailing edge and the associated noise generation
No full textDirect numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge (TE). The objective is to investigate the turbulent flow field in the vicinity of the TE and the associated broadband noise generation. To generate a turbulent boundary layer a short distance from the inflow boundary, high amplitude lifted streaks and disturbances that can be associated with coherent outer layer vortices are introduced at the inflow boundary. A rapid increase in skin friction and a decrease in boundary layer thickness and pressure fluctuations is observed at the trailing edge. It is demonstrated that the behaviour of the hydrodynamic field in the vicinity of the TE can be predicted with reasonable accuracy using triple deck theory if the eddy viscosity is accounted for. Point spectra of surface pressure difference are shown to vary considerably towards the trailing edge, with a significant reduction of amplitude occurring in the low frequency range.The acoustic pressure obtained from the DNS is compared with predictions from two- and three-dimensional acoustic analogies and the classical trailing edge theory of Amiet. For low frequencies, two dimensional theory succeeds in predicting the acoustic pressure in the far field with reasonable accuracy due to a significant spanwise coherence of the surface pressure difference and predominantly two dimensional sound radiation. For higher frequencies, however, the full three dimensional theory is required for an accurate prediction of the acoustic far field. DNS data are used to test some of the key assumptions invoked by Amiet for the derivation of the classical trailing edge theory. Even though most of the approximations are shown to be reasonable, they collectively lead to a deviation from the DNS results, in particular for higher frequencies. Moreover, because the three dimensional acoustic analogy does not provide significantly improved results, it is suggested that some of the discrepancies can be attributed to the approach of evaluating the far field sound using a Kirchhoff-type integration of the surface pressure difference
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