Hot wire manufacturing and resolution effects in high Reynolds number flows

Lo studio della turbolenza è di fondamentale importanza non solo per la fluidodinamica teorica ma anche perchè viene riscontrata in una moltitudine di problemi di interesse ingegneristico. All'aumentare del numero di Reynolds, le scale caratteristiche tendono a ridurre le loro dimensioni assolute. Nella fluidodinamica sperimentale già da lungo tempo si è affermata l'anemometria a filo caldo, grazie ad ottime caratteristiche di risoluzione spaziale e temporale. Questa tecnica, caratterizzata da un basso costo e da una relativa semplicità, rende possibile la realizzazione di sensori di tipo artigianale, che hanno il vantaggio di poter essere relizzati in dimensioni inferiori. Nonostante l'ottima risoluzione spaziale degli hot-wire, infatti, si può verificare, ad alto numero di Reynolds, che le dimensioni dell'elemento sensibile siano superiori a quelle delle piccole scale. Questo impedisce al sensore di risolvere correttamente le strutture più piccole. Per questa tesi di laurea è stato allestito un laboratorio per la costruzione di sensori a filo caldo con filo di platino. Sono in questo modo stati realizzati diversi sensori dalle dimensioni caratteristiche inferiori a quelle dei sensori disponibili commercialmente. I sensori ottenuti sono quindi stati testati in un getto turbolento, dapprima confrontandone la risposta con un sensore di tipo commerciale, per verificarne il corretto funzionamento. In seguito si sono eseguite misure più specifiche e limitate ad alcune particolari zone all'interno del campo di moto, dove è probabile riscontrare effetti di risoluzione spaziale. Sono stati analizzati gli effetti della dimensione fisica del sensore sui momenti statistici centrali, sugli spettri di velocità e sulle funzioni di densità di probabilità.

Stability of the flow of a fluid through a flexible tube at high Reynolds number

The stability of the Hagen-Poiseuille flow of a Newtonian fluid in a tube of radius R surrounded by an incompressible viscoelastic medium of radius R < r < HR is analysed in the high Reynolds number regime. The dimensionless numbers that affect the fluid flow are the Reynolds number Re = (ρVR / η), the ratio of the viscosities of the wall and fluid ηr = (ηs/η), the ratio of radii H and the dimensionless velocity Γ = (ρV2/G)1/2. Here ρ is the density of the fluid, G is the coefficient of elasticity of the wall and Vis the maximum fluid velocity at the centre of the tube. In the high Reynolds number regime, an asymptotic expansion in the small parameter ε = (1/Re) is employed. In the leading approximation, the viscous effects are neglected and there is a balance between the inertial stresses in the fluid and the elastic stresses in the medium. There are multiple solutions for the leading-order growth rate do), all of which are imaginary, indicating that the fluctuations are neutrally stable, since there is no viscous dissipation of energy or transfer of energy from the mean flow to the fluctruations due to the Reynolds strees. There is an O(ε1/2) correction to the growth rate, s(1), due to the presence of a wall layer of thickness ε1/2R where the viscous stresses are O(ε1/2) smaller than the inertial stresses. An energy balance analysis indicates that the transfer of energy from the mean flow to the fluctuations due to the Reynolds stress in the wall layer is exactly cancelled by an opposite transfer of equal magnitude due to the deformation work done at the interface, and there is no net transfer from the mean flow to the fluctuations. Consequently, the fluctuations are stabilized by the viscous dissipation in the wall layer, and the real part of s(1) is negative. However, there are certain values of Γ and wavenumber k where s(l) = 0. At these points, the wail layer amplitude becomes zero because the tangential velocity boundary condition is identically satisfied by the inviscid flow solution. The real part of the O(ε) correction to the growth rate s(2) turns out to be negative at these points, indicating a small stabilizing effect due to the dissipation in the bulk of the fluid and the wall material. It is found that the minimum value of s(2) increases [is proportional to] (H − 1)−2 for (H − 1) [double less-than sign] 1 (thickness of wall much less than the tube radius), and decreases [is proportional to] (H−4 for H [dbl greater-than sign] 1. The damping rate for the inviscid modes is smaller than that for the viscous wall and centre modes in a rigid tube, which have been determined previously using a singular perturbation analysis. Therefore, these are the most unstable modes in the flow through a flexible tube.

A Multiscale-Domain Decomposition Method for High Reynolds Number Flows

The high Reynolds number flow contains a wide range of length and time scales, and the flow domain can be divided into several sub-domains with different characteristic scales. In some sub-domains, the viscosity dissipation scale can only be considered in a certain direction; in some sub-domains, the viscosity dissipation scales need to be considered in all directions; in some sub-domains, the viscosity dissipation scales are unnecessary to be considered at all. For laminar boundary layer region, the characteristic length scales in the streamwise and normal directions are L and L Re-1/ 2 , respectively. The characteristic length scale and the velocity scale in the outer region of the boundary layer are L and U, respectively. In the neighborhood region of the separated point, the length scale l<Reynolds numbers RDxi between different cells in Navier-Stokes (NS) equations computations for high Reynolds number flows, an idea of solving the conservation equations for discrete cells was proposed and named the discrete fluid dynamics (DFD) algorithm. Analysis shows that the basic conservative equations for discrete cells are the Euler equations, NS- and diffusion parabolized (DP) NS equations. In this paper, a new multiscale-domain decomposition method is developed for the high Reynolds number flow. First, the whole domain is decomposed to different sub-domains with the different characteristic scales. Then the different dominant equation of all sub-domains is defined according to the diffusion parabolized (DP) theory of viscous flow. Finally these different equations are solved simultaneously in whole computational region. For numerical tests of high Reynolds numerical flows, two-dimensional supersonic flows over rearward and frontward steps as well as an interaction flow between shock wave and boundary layer were solved numerically. The pressure distributions and local coefficients of skin friction on the wall are given. The numerical results obtained by the multiscale-domain decomposition algorithm are well agreement with those by NS equations. Comparing with the usual method of solving the Navier-Stokes equations in the whole flow, under the same numerical accuracy, the present multiscale domain decomposition method decreases CPU consuming about 20% and reflects the physical mechanism of practical flow more accurately.

High Reynolds number research, 1980 : proceedings of a workshop held at NASA Langley Research Center, Hampton, Virginia, December 9-11, 1980 /

Includes bibliographical references.

Inverse Spectral Methods in Acoustic Normal Mode Velocimetry of High Reynolds Number Spherical Couette Flows

Experimental geophysical fluid dynamics often examines regimes of fluid flow infeasible for computer simulations. Velocimetry of zonal flows present in these regimes brings many challenges when the fluid is opaque and vigorously rotating; spherical Couette flows with molten metals are one such example. The fine structure of the acoustic spectrum can be related to the fluid’s velocity field, and inverse spectral methods can be used to predict and, with sufficient acoustic data, mathematically reconstruct the velocity field. The methods are to some extent inherited from helioseismology. This work develops a Finite Element Method suitable to matching the geometries of experimental setups, as well as modelling the acoustics based on that geometry and zonal flows therein. As an application, this work uses the 60-cm setup Dynamo 3.5 at the University of Maryland Nonlinear Dynamics Laboratory. Additionally, results obtained using a small acoustic data set from recent experiments in air are provided.

Flow around a hemisphere-cylinder at high angle of attack and low Reynolds number. Part II: POD and DMD applied to reduced domains

Three-dimensional direct numerical simulations (DNS) have been performed on a finite-size hemispherecylinder model at angle of attack AoA = 20◦ and Reynolds numbers Re = 350 and 1000. Under these conditions, massive separation exists on the nose and lee-side of the cylinder, and at both Reynolds numbers the flow is found to be unsteady. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are employed in order to study the primary instability that triggers unsteadiness at Re = 350. The dominant coherent flow structures identified at the lower Reynolds number are also found to exist at Re = 1000; the question is then posed whether the flow oscillations and structures found at the two Reynolds numbers are related. POD and DMD computations are performed using different subdomains of the DNS computational domain. Besides reducing the computational cost of the analyses, this also permits to isolate spatially localized oscillatory structures from other, more energetic structures present in the flow. It is found that POD and DMD are in general sensitive to domain truncation and noneducated choices of the subdomain may lead to inconsistent results. Analyses at Re = 350 show that the primary instability is related to the counter rotating vortex pair conforming the three-dimensional afterbody wake, and characterized by the frequency St ≈ 0.11, in line with results in the literature. At Re = 1000, vortex-shedding is present in the wake with an associated broadband spectrum centered around the same frequency. The horn/leeward vortices at the cylinder lee-side, upstream of the cylinder base, also present finite amplitude oscillations at the higher Reynolds number. The spatial structure of these oscillations, described by the POD modes, is easily differentiated from that of the wake oscillations. Additionally, the frequency spectra associated with the lee-side vortices presents well defined peaks, corresponding to St ≈ 0.11 and its few harmonics, as opposed to the broadband spectrum found at the wake.

Large eddy simulation of a hot, high speed coflowing jet

Computations are made of a short cowl coflowing jet nozzle with a bypass ratio 8 : 1. The core flow is heated, making the inlet conditions reminiscent of those for a real engine. A large eddy resolving approach is used with a 12 × 106 cell mesh. Since the code being used tends towards being dissipative the sub-grid scale (SGS) model is abandoned giving what can be termed Numerical Large Eddy Simulation (NLES). To overcome near wall modelling problems a hybrid NLES-RANS (Reynolds Averaged Navier-Stokes) related method is used. For y+ ≤ 60 a κ-l model is used. Blending between the two regions makes use of the differential Hamilton-Jabobi (HJ) equation, an extension of the eikonal equation. Results show encouraging agreement with existing measurements of other workers. The eikonal equation is also used for acoustic ray tracing to explore the effect of the mean flow on acoustic ray trajectories, thus yielding a coherent solution strategy. Copyright © 2011 by ASME.

Transient Momentum and Enthalpy Transfer in Packed Beds at High Reynolds Numbers

An experimental study for transient temperature response and pressure drop in a randomly packed bed at high Reynolds numbers is presented.The packed bed is used as a compact heat exchanger along with a solid-propellant gas generator, to generate room-temperature gases for use in control actuation, air bottle pressurization, etc. Packed beds of lengths 200 and 300 mm were characterized for packing-sphere-based Reynolds numbers ranging from 0.8 x 10(4) to 8.5 x 10(4).The solid packing used in the bed consisted of phi 9.5 mm steel spheres. The bed-to-particle diameter ratio was with the average packed-bed porosity around 0.43. The inlet flow temperature was unsteady and a mesh of spheres was used at either end to eliminate flow entrance and exit effects. Gas temperature and pressure were measured at the entry, exit,and at three axial locations along centerline in the packed beds. The solid packing temperature was measured at three axial locations in the packed bed. A correlation based on the ratio of pressure drop and inlet-flow momentum (Euler number) exhibited an asymptotically decreasing trend with increasing Reynolds number. Axial conduction across the packed bed was found to he negligible in the investigated Reynolds number range. The enthalpy absorption rate to solid packing from hot gases is plotted as a function of a nondimensional time constant for different Reynolds numbers. A longer packed bed had high enthalpy absorption rate at Reynolds number similar to 10(4), which decreased at Reynolds number similar to 10(5). The enthalpy absorption plots can be used for estimating enthalpy drop across packed bed with different material, but for a geometrically similar packing.