Comparison between order of magnitude and numerical estimates of the solute boundary layer in an idealized horizontal Bridgman configuration

1991 ◽  
Vol 113 (3-4) ◽  
pp. 587-592 ◽  
Author(s):  
J.P. Garandet ◽  
A. Rouzaud ◽  
T. Duffar ◽  
D. Camel
Keyword(s):  
Boundary Layer ◽  
Solute Boundary Layer ◽  
Order Of Magnitude

‘Inactive’ motion and pressure fluctuations in turbulent boundary layers

1967 ◽  
Vol 30 (2) ◽  
pp. 241-258 ◽  
Author(s):  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Outer Layer ◽  
Pressure Fluctuations ◽  
Turbulent Energy ◽  
Viscous Sublayer ◽  
Wave Number ◽  
Noticeable Effect ◽  
Frequency Spectra ◽  
Active Motion ◽  
Order Of Magnitude

Townsend's (1961) hypothesis that the turbulent motion in the inner region of a boundary layer consists of (i) an ‘active’ part which produces the shear stress τ and whose statistical properties are universal functions of τ and y, and (ii) an ‘inactive’ and effectively irrotational part determined by the turbulence in the outer layer, is supported in the present paper by measurements of frequency spectra in a strongly retarded boundary layer, in which the ‘inactive’ motion is particularly intense. The only noticeable effect of the inactive motion is an increased dissipation of kinetic energy into heat in the viscous sublayer, supplied by turbulent energy diffusion from the outer layer towards the surface. The required diffusion is of the right order of magnitude to explain the non-universal values of the triple products measured near the surface, which can therefore be reconciled with universality of the ‘active’ motion.Dimensional analysis shows that the contribution of the ‘active’ inner layer motion to the one-dimensional wave-number spectrum of the surface pressure fluctuations varies as τ2w/k1 up to a wave-number inversely proportional to the thickness of the viscous sublayer. This result is strongly supported by the recent measurements of Hodgson (1967), made with a much smaller ratio of microphone diameter to boundary-layer thickness than has been achieved previously. The disagreement of the result with most other measurements is attributed to inadequate transducer resolution in the other experiments.

Trailing-edge stall

1970 ◽  
Vol 42 (3) ◽  
pp. 561-584 ◽  
Author(s):  
S. N. Brown ◽  
K. Stewartson
Keyword(s):  
Boundary Layer ◽  
Complete Solution ◽  
Leading Edge ◽  
Asymptotic Solutions ◽  
Angle Of Incidence ◽  
Trailing Edge ◽  
Boundary Layer Equations ◽  
Order Of Magnitude ◽  
Stall Angle ◽  
Fundamental Equations

A study is made of the laminar flow in the neighbourhood of the trailing edge of an aerofoil at incidence. The aerofoil is replaced by a flat plate on the assumption that leading-edge stall has not taken place. It is shown that the critical order of magnitude of the angle of incidence α* for the occurrence of separation on one side of the plate is$\alpha^{*} = O(R^{\frac{1}{16}})$, whereRis a representative Reynolds number, for incompressible flow, and α* =O(R−¼) for supersonic flow. The structure of the flow is determined by the incompressible boundary-layer equations but with unconventional boundary conditions. The complete solution of these fundamental equations requires a numerical investigation of considerable complexity which has not been undertaken. The only solutions available are asymptotic solutions valid at distances from the trailing edge that are large in terms of the scaled variable of orderR−⅜, and a linearized solution for the boundary layer over the plate which gives the antisymmetric properties of the aerofoil at incidence. The value of α* for which separation occurs is the trailing-edge stall angle and an estimate is obtained from the asymptotic solutions. The linearized solution yields an estimate for the viscous correction to the circulation determined by the Kutta condition.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

2003 ◽  
Vol 125 (1) ◽  
pp. 28-37 ◽  
Author(s):  
Ralph J. Volino ◽  
Michael P. Schultz ◽  
Christopher M. Pratt
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Streamwise Velocity ◽  
Turbulent Shear ◽  
Conditional Sampling ◽  
Freestream Turbulence ◽  
Mean Velocity ◽  
Transitional Boundary Layer ◽  
Turbulent Shear Stress ◽  
Order Of Magnitude

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

Turbine Separation Control Using Pulsed Vortex Generator Jets

2000 ◽  
Author(s):  
Jeffrey P. Bons ◽  
Rolf Sondergaard ◽  
Richard B. Rivir
Keyword(s):  
Boundary Layer ◽  
A Priori ◽  
Vortex Generator ◽  
Reynolds Numbers ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Pressure Distributions ◽  
Order Of Magnitude ◽  
Flow Oscillations ◽  
Vortex Generator Jets

The application of pulsed vortex generator jets to control separation on the suction surface of a low pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high altitude aircraft engines and aft stages of industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets have a 30 degree pitch and a 90 degree skew to the freestream direction. Jet flow oscillations up to 100 Hz are produced using a high frequency solenoid feed valve. Results are compared to steady blowing at jet blowing ratios less than 4 and at two chordwise positions upstream of the nominal separation zone. Results show that pulsed vortex generator jets produce a bulk flow effect comparable to that of steady jets with an order of magnitude less massflow. Boundary layer traverses and blade static pressure distributions show that separation is almost completely eliminated with the application of unsteady blowing. Reductions of over 50% in the wake loss profile of the controlled blade were measured. Experimental evidence suggests that the mechanism for unsteady control lies in the starting and ending transitions of the pulsing cycle rather than the injected jet stream itself. Boundary layer spectra support this conclusion and highlight significant differences between the steady and unsteady control techniques. The pulsed vortex generator jets are effective at both chordwise injection locations tested (45% and 63% axial chord) covering a substantial portion of the blade suction surface. This insensitivity to injection location bodes well for practical application of pulsed VGJ control where the separation location may not be accurately known a priori.

Control of a swept-wing boundary layer using ring-type plasma actuators

2018 ◽  
Vol 844 ◽  
pp. 36-60 ◽  
Author(s):  
Nima Shahriari ◽  
Matthias R. Kollert ◽  
Ardeshir Hanifi
Keyword(s):  
Boundary Layer ◽  
Cross Flow ◽  
Plasma Actuators ◽  
Swept Wing ◽  
Ring Type ◽  
Transition Control ◽  
Turbulent Transition ◽  
Roughness Elements ◽  
Order Of Magnitude ◽  
Induced Velocity

Application of ring-type plasma actuators for control of laminar–turbulent transition in a swept-wing boundary layer is investigated thorough direct numerical simulations. These actuators induce a wall-normal jet in the boundary layer and can act as virtual roughness elements. The flow configuration resembles experiments by Kim et al. (2016 Technical Report. BUTERFLI Project TR D3.19, http://eprints.nottingham.ac.uk/id/eprint/46529). The actuators are modelled by the volume forces computed from the experimentally measured induced velocity field at the quiescent air condition. Stationary and travelling cross-flow vortices are triggered in the simulations by means of surface roughness and random unsteady perturbations. Interaction of vortices generated by actuators with these perturbations is investigated in detail. It is found that, for successful transition control, the power of the actuators should be increased to generate jet velocities that are one order of magnitude higher than those used in the experiments by Kim et al. (2016) mentioned above.

High Rayleigh number thermal convection in a shallow laterally heated cavity

1993 ◽  
Vol 440 (1909) ◽  
pp. 273-289 ◽  
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Rayleigh Number ◽  
Hadley Circulation ◽  
Large Contribution ◽  
The Core ◽  
Vertical Boundary ◽  
Order Of Magnitude ◽  
End Wall ◽  
Lateral Heat

The flow near the end of a shallow laterally heated cavity enters a nonlinear convective régime when the Rayleigh number R , based on cavity height, is of the same order of magnitude as the aspect ratio L (length/height). In the present work the asymptotic structure of the flow that develops in the limit as is R/L →∞ considered for the case where the horizontal surfaces of the cavity are thermally insulated. A model is discussed in which the formation of a vertical boundary layer on the end wall involves an unexpectedly large contribution to the local ambient temperature field. Expulsion of fluid from the base of the layer, and its subsequent return to the core through a horizontal boundary layer, maintains the necessary lateral heat transfer in the cavity. Implications of the model for the flow throughout the cavity are also discussed. The evolution of the end-zones leads to a change in the amplitude of the main Hadley circulation when R = O ( L 12/7 ). Various properties of the solution for this new régime are determined, including the Nusselt number for the lateral heat transfer, which is found to be proportional to L 3/7 . A comparison is made with both numerical and experimental results.

scholarly journals Enhanced effects from tiny flexible in-wall blips and shear flow

2015 ◽  
Vol 772 ◽  
pp. 16-41 ◽  
Author(s):  
Luisa Pruessner ◽  
Frank Smith
Keyword(s):  
Boundary Layer ◽  
Interactive Effect ◽  
Fluid Motion ◽  
Unsteady Motion ◽  
Local Motion ◽  
Second Stage ◽  
Order Of Magnitude ◽  
High Reynolds ◽  
Parameter Values ◽  
Nonlinear Solutions

Fluid motion at high Reynolds number over a flexible in-wall blip (a compliant bump or dip in an otherwise fixed wall) is considered theoretically for a very short blip buried low inside a boundary layer. Only the near-wall shear of the oncoming flow affects the local motion past the tiny blip. Slowly evolving features are examined first to allow for variations in the incident flow. Linear and nonlinear solutions show that at certain parameter values (eigenvalues) intensifications occur in which the interactive effect on flow and blip shape is larger by an order of magnitude than at most parameter values. Similar findings apply to the boundary layer with several tiny blips present or to channel flows with blips of almost any length. These intensifications lead on to fully nonlinear unsteady motion as a second stage, after some delay, thus combining with finite-time breakups to form a distinct path into transition of the flow.

A Theory for Fine Particle Deposition in Two-Dimensional Boundary Layer Flows and Application to Gas Turbines

1982 ◽  
Vol 104 (1) ◽  
pp. 69-76 ◽  
Author(s):  
M. Mengu¨turk ◽  
E. F. Sverdrup
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Particle Deposition ◽  
Fine Particles ◽  
Magnitude Estimate ◽  
Two Dimensional ◽  
Boundary Layer Flows ◽  
Order Of Magnitude ◽  
Dimensional Boundary ◽  
Layer Flow

A theory is presented to predict deposition rates of fine particles in two-dimensional compressible boundary layer flows. The mathematical model developed accounts for diffusion due to both molecular and turbulent fluctuations in the boundary layer flow. Particle inertia is taken into account in establishing the condition on particle flux near the surface. Gravitational settling and thermophoresis are not considered. The model assumes that the fraction of particles sticking upon arrival at the surface is known, and thus, treats it as a given parameter. The theory is compared with a number of pipe and cascade experiments, and a reasonable agreement is obtained. A detailed application of the model to a turbine is also presented. Various regimes of particle transport are identified, and the range of validity of the model is discussed. An order of magnitude estimate is obtained for the time the turbine stage can be operated without requiring cleaning.

Some dynamical effects of heat on a turbulent boundary layer

1970 ◽  
Vol 40 (2) ◽  
pp. 361-384 ◽  
Author(s):  
C. I. H. Nicholl
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Mechanical Energy ◽  
Turbulent Energy ◽  
Strong Discontinuity ◽  
Sudden Increase ◽  
Boundary Layer Turbulence ◽  
Heated Air ◽  
Order Of Magnitude ◽  
The Mean

The dynamical effects of a sudden increase of as much as 100°C in boundary temperature upon fully turbulent boundary layers at low Reynolds numbers in air have been investigated in a wind tunnel. A section of the floor or the roof of the tunnel could be heated, so that the rate of working of gravitational forces on the turbulence could be made to represent either a gain or a loss of turbulent mechanical energy. Techniques of hot-wire anemometry were employed which enabled the instantaneous temperature and the instantaneous velocity to be measured simultaneously at a point in the non-homogeneous turbulent flow field.In the case of a strong discontinuity in the floor temperature, a fine-scale convective structure developed from the highly unstable interface between the heated air just above the surface and the turbulent boundary layer; and the motion in this region was sufficiently vigorous that the mean pressure in the vicinity of the floor was reduced and a local wall jet was generated. The deduced pressure distribution is regarded as evidence of coupling between the free and forced convection modes which may lead to a series of local wall jets downstream of the discontinuity.In the case of a strong discontinuity in the roof temperature, the interface between the heated air and the turbulent boundary layer was stable; and the boundary-layer turbulence, acting to spread this stable gradient over the vertical extent of the boundary layer, was required to do work against the gravitational field. A rate of working against gravity which was an order of magnitude less than the rate of supply of turbulent energy from the mean shear proved sufficient to suppress the turbulence in a very short time.

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