Prediction of Flow Separation in a Diffuser by a Boundary Layer Calculation

1977 ◽  
Vol 99 (2) ◽  
pp. 379-386 ◽  
Author(s):  
Y. Senoo ◽  
M. Nishi
Keyword(s):  
Boundary Layer ◽  
Flow Separation ◽  
Shape Factor ◽  
Limit Relation ◽  
Performance Data ◽  
Separation Point ◽  
Pressure Recovery ◽  
Two Dimensional ◽  
Blockage Factor ◽  
Mean Pressure

From a consideration of flow stability, it is shown that the onset of separation in a diffuser depends upon the local blockage factor. Using performance data for two-dimensional diffusers from the literature, the shape factor of the boundary layer at the separation point Hs is related to the blockage factor Bs at that section, and the formula Hs = 1.8+3.75Bs is deduced as the separation-limit relation. It is proved that this separation-limit relation is also applicable to conical diffusers. Furthermore, a simple theory is derived to evaluate the time-mean pressure recovery in the separated region. Using this method, it is possible to predict whether a separation occurs in a diffuser and to evaluate the pressure recovery.

scholarly journals The Effects of Stratification on Flow Separation

2005 ◽  
Vol 62 (7) ◽  
pp. 2618-2625 ◽  
Author(s):  
Maarten H. P. Ambaum ◽  
David P. Marshall
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Linear Theory ◽  
Flow Separation ◽  
Nonlinear Deformation ◽  
Stratified Flow ◽  
Two Dimensional ◽  
Critical Aspect ◽  
Streamline Curvature ◽  
Scaling Relationships

Abstract Separation of stratified flow over a two-dimensional hill is inhibited or facilitated by acceleration or deceleration of the flow just outside the attached boundary layer. In this note, an expression is derived for this acceleration or deceleration in terms of streamline curvature and stratification. The expression is valid for linear as well as nonlinear deformation of the flow. For hills of vanishing aspect ratio a linear theory can be derived and a full regime diagram for separation can be constructed. For hills of finite aspect ratio scaling relationships can be derived that indicate the presence of a critical aspect ratio, proportional to the stratification, above which separation will occur as well as a second critical aspect ratio above which separation will always occur irrespective of stratification.

Effect of Separation on Partial Cavitation

1988 ◽  
Vol 110 (2) ◽  
pp. 182-189 ◽  
Author(s):  
C. Pellone ◽  
A. Rowe
Keyword(s):  
Boundary Layer ◽  
Calculation Method ◽  
Cavitating Flow ◽  
Experimental Results ◽  
Separation Point ◽  
Two Dimensional ◽  
Flow Conditions ◽  
Partial Cavitation ◽  
Two Dimensional Flow ◽  
Cavitating Hydrofoil

Partially cavitating flow around a hydrofoil in a confined two-dimensional flow is presented. The calculation method, based on the singularities technique combined with a minimisation method, is adapted to open configurations. With this extension, cavity wakes not necessarily merging with the upper-side of the foil can be treated. In the case of subcavitating flow, a boundary layer calculated is made, indicating a separation point downstream of which the flow becomes separated. In this area, an imaginary streamline (wake) is introduced to simulate the effect of separation. The choice of different forms of wake clearly shows the influence of wake form on the value of results. The process is extended to the case of cavitating flow for wakes developing behind the cavity. The method is applied to a test cavitating hydrofoil placed in a tunnel. Several cavity wakes progressively diverging from the foil were tested. The results obtained, compared with experimental results, show the great importance of achieving more accurate modelling of flow conditions behind cavities.

Performance and Design of Straight, Two-Dimensional Diffusers

1967 ◽  
Vol 89 (1) ◽  
pp. 141-150 ◽  
Author(s):  
L. R. Reneau ◽  
J. P. Johnston ◽  
S. J. Kline
Keyword(s):  
Boundary Layer ◽  
Flow Regime ◽  
Flow Characteristics ◽  
Performance Data ◽  
Center Line ◽  
Two Dimensional ◽  
Rational Basis ◽  
Primary Flow ◽  
Wide Range ◽  
And Performance

Performance data and flow characteristics for subsonic, two-dimensional, straight center line diffusers are presented. The four primary flow regimes which can occur are described and presented as functions of overall diffuser geometry. The performance of both stalled and unstalled diffusers is mapped for a wide range of geometries and inlet boundary layer thicknesses. An understanding of the relationships between flow regime and performance leads to a rational basis for diffuser design. The important maxima of performance and their location on the performance maps are presented. Both the range of data and correlations of optima of performance are extended beyond previous results.

On ramped vanes to control normal shock boundary layer interactions

2018 ◽  
Vol 122 (1256) ◽  
pp. 1568-1585 ◽  
Author(s):  
S. Lee ◽  
E. Loth
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Flow Control ◽  
Flow Separation ◽  
Shape Factor ◽  
Incompressible Flows ◽  
Numerical Study ◽  
Normal Shock ◽  
Wall Region ◽  
Literature Study

ABSTRACTA novel vortex generator design positioned upstream of a normal shock followed by a subsequent diffuser was investigated using large eddy simulations. In particular, “ramped-vane” flow control devices with three different heights relative to the incoming boundary layer thickness (0.34δ, 0.52δ and 0·75δ) were placed in a supersonic boundary layer with a freestream Mach number of 1.3 and a Reynolds number of 2400 based on the momentum thickness. This is the first numerical study to investigate the size effect of the ramped-vane for flow control device in terms of shape factor, flow separation and flow unsteadiness. The results showed that these devices generated strong streamwise vortices that entrained high-momentum fluid to the near-wall region and increased turbulent mixing. The devices also decreased shock-induced flow separation, which resulted in a higher downstream skin friction in the diffuser. In general, the largest ramped-vane (0.75δ) produced the largest reductions in flow separation, shape factor and overall unsteadiness. These results and a careful review of the literature study also determined the quantitative correlation of optimum VG height with Mach number, wherebyh/δ~1 is often optimum for incompressible flows while higher Mach numbers lead to small optimum heights, tending towards h/δ~0.45 atM=2.5.

The Influence of Pressure Gradient on Desinent Cavitation From Isolated Surface Protrusions

1986 ◽  
Vol 108 (2) ◽  
pp. 254-260 ◽  
Author(s):  
J. William Holl ◽  
Michael L. Billet ◽  
Masaru Tada ◽  
David R. Stinebring
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shape Factor ◽  
Cavitation Number ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Relative Height ◽  
Velocity Range ◽  
Circular Arc ◽  
Air Content

An experimental investigation was conducted to study the desinent cavitation characteristics of various sizes of two-dimensional triangular and circular arc protrusions in a turbulent boundary layer for favorable, zero, and unfavorable pressure gradients. The roughness height (h) varied from 0.025 cm (0.01 in.) to 0.762 cm (0.30 in.) and the relative height (h/δ) varied from 0.026 to 2.53. Desinent cavitation numbers (σd) were obtained visually over a velocity range of 9.1 mps (30 fps) to 18.3 mps (60 fps) at an average total air content of 3.8 ppm (mole basis). The data for zero pressure gradient were in fair agreement with data obtained for the same protrusion shapes by Holl (1958). The cavitation number (σd) was correlated with relative height (h/δ), Reynolds number (Uδ/ν) and Clauser’s (1954) equilibrium boundary layer shape factor (G) which includes the effect of pressure gradient. The data show that σd increases with pressure gradient. This result was not expected since it appears to contradict the trends implied by the so-called characteristic velocity theory developed by Holl (1958).

Effects of sandpaper roughness and stream turbulence on the turbulent boundary layer

1999 ◽  
Vol 213 (5) ◽  
pp. 507-515 ◽  
Author(s):  
J. C. Gibbings ◽  
S. M. Al-Shukri
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Skin Friction ◽  
Boundary Layers ◽  
Pipe Flow ◽  
Shape Factor ◽  
Turbulent Boundary Layers ◽  
Experimental Measurements ◽  
Two Dimensional

This paper reports experimental measurements of two-dimensional turbulent boundary layers over sandpaper surfaces under turbulent streams to complement the Nikuradse experiments on pipe flow. The study included the recovery region downstream of the end of transition. Correlations are given for the thickness, the shape factor, the skin friction and the parameters of the velocity profile of the layer. Six further basic differences from the pipe flow are described to add to the five previously reported.

Simulation of Viscous Diffusion for Extension of the Surface Vorticity Method to Boundary Layer and Separated Flows

1981 ◽  
Vol 23 (3) ◽  
pp. 157-167 ◽  
Author(s):  
D. T. C. Porthouse ◽  
R. I. Lewis
Keyword(s):  
Boundary Layer ◽  
Flow Separation ◽  
Point Vortex ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
Separated Flows ◽  
Two Dimensional ◽  
Drag Forces ◽  
Boundary Layer Parameter ◽  
Lift And Drag

A numerical method for two-dimensional incompressible viscous fluid flows is tested on the diffusion of a point vortex. It is then applied to the boundary layer to reconstruct the Blasius profile, to demonstrate flow separation, and to simulate turbulence. The significance of Thwaites' boundary layer parameter for flow separation is explained. The Kelvin-Helmholtz instability, which is responsible for two-dimensional turbulence, is represented by the motion of an array of point vortices after an initial disturbance. The formation of the Von Karman vortex street downstream of a circular cylinder is described by computer simulation, and the influence of viscous diffusion is shown. For two different cylinder Reynolds numbers the vortex shedding frequencies and oscillating lift and drag forces are evaluated.

Upstream Separation Point for an Internal Corner in a Two-Dimensional Channel Flow

1963 ◽  
Vol 30 (4) ◽  
pp. 505-508 ◽  
Author(s):  
J. F. Barrows
Keyword(s):  
Boundary Layer ◽  
Mathematical Model ◽  
Finite Difference ◽  
Finite Difference Method ◽  
Turbulent Boundary Layer ◽  
Channel Flow ◽  
Series Solution ◽  
Separation Point ◽  
Two Dimensional ◽  
The Mathematical Model

A solution is presented for predicting the upstream separation point in a two-dimensional channel flow in which an obstacle is present. The paper considers only the separation of a laminar boundary layer but could also be applied to the turbulent boundary-layer case. The mathematical model considers free-streamline theory to get the pressure distribution ahead of separation. Go¨rtler’s series solution and Witting’s finite-difference method are then used to predict separation. The theory is checked experimentally.

The Departure from Equilibrium of Turbulent Boundary Layers

1968 ◽  
Vol 19 (1) ◽  
pp. 1-19 ◽  
Author(s):  
H. McDonald
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Kinetic Energy ◽  
Stress Distribution ◽  
Boundary Layers ◽  
Turbulent Boundary Layers ◽  
Two Dimensional ◽  
Pressure Distributions ◽  
Momentum Integral ◽  
State Examination

SummaryRecently two authors, Nash and Goldberg, have suggested, intuitively, that the rate at which the shear stress distribution in an incompressible, two-dimensional, turbulent boundary layer would return to its equilibrium value is directly proportional to the extent of the departure from the equilibrium state. Examination of the behaviour of the integral properties of the boundary layer supports this hypothesis. In the present paper a relationship similar to the suggestion of Nash and Goldberg is derived from the local balance of the kinetic energy of the turbulence. Coupling this simple derived relationship to the boundary layer momentum and moment-of-momentum integral equations results in quite accurate predictions of the behaviour of non-equilibrium turbulent boundary layers in arbitrary adverse (given) pressure distributions.

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