Three-dimensional Flow Development Research Articles

On the three-dimensional flow development around circular finned cylinders

The three-dimensional flow development around the circular finned cylinders is investigated numerically. Three finned cylinders with constant fin pitch (p), fin thickness (t), and effective diameter (Deff) and a range of diameter ratios (Df/Dr) within 1.25 ≤ Df/Dr ≤ 2.5 are considered in this study. One bare cylinder with a diameter equivalent to the effective diameter of the finned cylinders is also considered. The numerical simulations are performed using the large eddy simulation turbulence model for a free-stream velocity corresponding to the Reynolds number Re = 3900, defined based on the effective diameter (Deff). This study provides novel insights into the flow physics in the channel between the fins and its effect on the three-dimensional flow development. The results accentuate that the three-dimensional flow development around the finned cylinders fundamentally differs from that of the bare cylinder. In particular, the flow separation topology at the surface of the finned cylinders differs significantly from that of the bare cylinder due to flow entrainment between the fins. The distinct flow separation leads to the formation of streamwise edge vortices, which induces downwash flow in the near wake of the finned cylinders. The combined effect of the entrainment between the fins and downwash flow affects the vortex formation length and mean pressure distribution around the cylinder. As a result, the structural loading on the surface of the finned cylinders is distinctively different from that of the bare cylinder with profound dependence on the fin parameters.

Three-Dimensional Development of Coherent Structures in a Two-Dimensional Laminar Separation Bubble

Three-dimensional flow development is experimentally assessed in a convectively unstable laminar separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125,000, an angle of attack of 4 deg, and a freestream turbulence intensity of 0.07%. The flow is weakly excited in a spanwise uniform manner at a frequency matching that of the most unstable disturbances in the natural separation bubble, leaving the base flow unmodified while enabling three-dimensional reconstructions of the dominant coherent structures using phase-locked planar particle image velocimetry measurements. Time-averaged flowfield reconstructions show a strongly two-dimensional topology of the separation bubble for both the natural and weakly excited cases. Analysis of the flow development demonstrates that, for both the natural and excited flows, spanwise-oriented and strongly two-dimensional shear-layer vortices form in the separation bubble upstream of the mean maximum height location. Spanwise undulations develop in the vortex filaments that continually intensify with downstream convection as a result of the streamwise forward sections of the filaments lifting away from the surface. This motion reorients the vorticity of the primary structures from the spanwise direction into the streamwise and wall-normal directions, forming hairpinlike structures above the vortex core region. These findings offer new quantitative insight into the vortex dynamics and breakdown process of the shear-layer vortices in two-dimensional, convectively unstable laminar separation bubbles subject to low freestream turbulence levels.

Fluid–structure coupling mechanism and its aerodynamic effect on membrane aerofoils

Fluid–structure interactions of elastic membrane aerofoils are investigated at Reynolds number $Re=10\,000$ and low angle of attack. The dynamics of the fluid and membrane coupled system are solved using direct numerical simulation (DNS), where the geometry and boundary conditions were applied using a boundary data immersion method. Although membrane aerofoils improve the aerodynamic performance close to stall conditions compared to rigid aerofoils, it has previously been found that membrane aerofoils show lower aerodynamic efficiency at low angles of attack. This study focuses on the coupling mechanism at an angle of attack of 8 degrees, which is below the stall angle. The dynamic behaviour of the coupled system was characterised via spectral analysis in the wavenumber and frequency domain, which allowed the propagating wave nature of the membrane vibrations and their effect on the surrounding pressure field to be clarified. The membrane vibrations are found to introduce upstream-propagating pressure waves that appear to be responsible for a loss in aerodynamic efficiency compared to a rigid aerofoil. Comparison of two- and three-dimensional results reveals that the three-dimensional flow development causes a decrease in the amplitude of the system fluctuations, but the same coupling mechanism is present.

Effects of Pivot Location and Reduced Pitch Rate on Pitching Rectangular Flat Plates

This paper presents the results of an experimental investigation of the unsteady flow about pitching flat plates. Hydrodynamic force and two-dimensional particle image velocimetry measurements are reported for three pivot locations (leading edge, midchord, and trailing edge), reduced pitch rates from 0.022 to 0.39, and in still water, which corresponds to infinite reduced pitch rate. The wing has rectangular planform with effective aspect ratio 4, and the wing pitching motion is from 0 to 45 deg angle of attack. The relation between hydrodynamic force and vortical flow development as a function of pivot location and reduced pitch rate is discussed. Reasonable agreement is found between measured hydrodynamic force and quasi-steady potential flow theoretical results. Several vortical flow features are identified and discussed, including 1) the effect of pivot location and pitch acceleration on formation and evolution of vortical structures, 2) the impact of interaction between vortical flow structures on hydrodynamic force development, and 3) three-dimensional flow development and transient vortex development.

Effects of noncircular air holes on reacting flow characteristics in a micro can combustor with a seven-hole baffle

Turbulent reacting flows in a CH4–air micro can combustor with a baffle plate of seven holes are numerically investigated by the Reynolds Stress Model. In order to examine the effects of baffle configurations on the flow structure, the type of air hole is adopted as circle, square, and triangle. The flame zone strongly depends on the relative positions of air and fuel holes, and the flames are developed to a lobed form depending on the number of air holes. Also, when the circular air hole is changed to square or triangular hole, the development of the central recirculation is more obvious for the reacting flows. Such flow structures promote the decay of the streamwise velocity and increase the turbulent mixing. As a result, the triangular hole baffle gives the shortest flame length. It is recognized that the axis-switching phenomenon is included in the three-dimensional flow development. So to investigate the axis-switching effect on the flow and thermal field, two inlet boundary conditions are tested for the triangular hole baffle. The result shows that the axis-switching flow contributes to the shorter flame length. For the modified inlet without the axis-switching, the flame length becomes twice the original size of the axis-switching condition. Additionally, the combustion efficiency is discussed by the conversion rate and heat loss.

The evolution of a subharmonic mode in a vortex street

The development of a subharmonic three-dimensional instability mode in a vortex street is investigated both numerically and experimentally. The flow past a ring is considered as a test case, as a previous stability analysis has predicted that for a range of aspect ratios, the first-occurring instability of the vortex street is subharmonic. For the flow past a circular cylinder, the development of three-dimensional flow in the vortex street is known to lead to turbulent flow through the development of spatio-temporal chaos, whereas subharmonic instabilities have been shown to cause a route to chaos through the development of a period-doubling cascade. The three-dimensional vortex street in the flow past a ring is analysed to determine if a subharmonic instability can alter the route to turbulence for a vortex street. A linear stability analysis and non-axisymmetric computations are employed to compute the flow past a ring with an aspect ratio ${\sc ar}\,{=}\,5$ , and comparisons with experimental dye visualizations are included to verify the existence of a subharmonic mode in the wake. Computations at higher Reynolds numbers confirm that the subharmonic instability does not initiate a period-doubling cascade in the wake.

Some unexpected effects of wavelength and perturbation strength on heat transfer enhancement by Görtler instability

Convective heat transfer is studied in a laminar boundary layer over a concave wall in the presence of Görtler vortices with constant wall heat flux conditions. Comparison of local Nusselt and Stanton numbers demonstrates that the latter is more accurate in revealing three-dimensional flow development. Analysis of heat transfer with different freestream velocities shows that the Reynolds similarity is not valid in the presence of Görtler centrifugal instability and indicates that the Görtler number based on the Blasius momentum thickness enables one to follow the different steps of Görtler instability development. Analysis of the effects of the Görtler vortex wavelength and of perturbation wire diameter shows that the smaller the vortex wavelength, the faster the apparent transition to turbulence. Moreover, larger wire diameters also cause a faster boundary-layer transition. It has been shown experimentally that in the range of heat loading studied here, the buoyancy force has no effect on Görtler vortex generation and growth.

Numerical simulations of laminar flow over a 3D backward-facing step

A numerical investigation of laminar flow over a three-dimensional backward-facing step is presented with comparisons with detailed experimental data, available in the literature, serving to validate the numerical results. The continuity constraint method, implemented via a finite element weak statement, was employed to solve the unsteady three-dimensional Navier–Stokes equations for incompressible laminar isothermal flow. Two-dimensional numerical simulations of this step geometry underestimate the experimentally determined extent of the primary separation region for Reynolds numbers Re greater than 400. It has been postulated that this disagreement between physical and computational experiments is due to the onset of three-dimensional flow near Re ≈ 400. This paper presents a full three-dimensional simulation of the step geometry for 100⩽ Re⩽ 800 and correctly predicts the primary reattachment lengths, thus confirming the influence of three-dimensionality. Previous numerical studies have discussed possible instability modes which could induce a sudden onset of three-dimensional flow at certain critical Reynolds numbers. The current study explores the influence of the sidewall on the development of three-dimensional flow for Re greater than 400. Of particular interest is the characterization of three-dimensional vortices in the primary separation region immediately downstream of the step. The complex interaction of a wall jet, located at the step plane near the sidewall, with the mainstream flow reveals a mechanism for the increasing penetration (with increasing Reynolds number) of three-dimensional flow structures into a region of essentially two-dimensional flow near the midplane of the channel. The character and extent of the sidewall-induced flow are investigated for 100⩽Re⩽ 800. © 1997 John Wiley & Sons, Ltd.

Three-dimensional turbulent boundary layer on a body of complex shape

Flow and heat transfer problems associated with three-dimensional compressible gas flow past a body of complex shape at a small angle of attack are investigated on the basis of a finite-difference calculation. The results of a numerical solution of the equations of the three-dimensional turbulent boundary layer are presented. The effect of the leading parameters on three-dimensional flow development and heat transfer is analyzed. The characteristic flow regions in the boundary layer are found: lines of divergence and convergence on the surface, separation zones and flow interfaces. The location of the maximum values of the heat flux and friction on the surface is determined, the behavior of the limiting streamlines on the body is described, and the intensity of the secondary flows in the boundary layer is estimated.

Performance Evaluation of Linear Turbine Cascades Using Three-Dimensional Viscous Flow Calculations

The overall performance of two geometrically similar linear turbine cascades is calculated using an elliptic flow program. The increase in the mass-averaged total pressure loss is calculated within and downstream of the cascades and the results show good agreement with the measured values. The buildup and decay of the secondary kinetic energy are also shown; measurements are available for one of the cascades near and downstream of the trailing edge and these are in close agreement with the calculated values. Details of the flow development are also compared with measurements. Calculated velocity vectors near the endwall show the overturning revealed by surface flow visualization and similarly near the suction surface the strong spanwise flow is well calculated. Calculated contours of total pressure loss in cross-sectional planes confirm the important interaction of the passage vortex with the profile boundary layer at midspan. Regions of high loss near midspan are calculated downstream of both cascades; this three-dimensional flow development is followed in the calculations.

Three-Dimensional Flow Development in MHD Generators at Part Load

The three-dimensional behavior of flow in magnetohydrodynamic (MHD) generators at design and off-design loading points was investigated. Faraday as well as diagonally connected channels with insulating or conducting sidewalls were considered. The role of MHD body forces in generating axial vorticity and hence secondary flows in the cross stream has been analyzed. For Faraday channels with strong MHD interaction, the generated vorticity is shown to direct the secondary flow from the cathode to the anode wall along the centerline. For diagonally connected channels near short circuit, the generated vorticity is shown to direct the secondary flow from the anode to the cathode wall along the centerline, and vice versa at near open-circuit conditions.